Titanium always a little flexy?

[bicycletricycle]

Over the years I have really come to enjoy a nice modern stiff steel bicycle. It started with an IF built Tournesol with double oversized tubes and now I have two Chapmans built similarly. I like the way the bikes handle, they just feel more stable. I know that a million variables go into this and this language is inexact but I have ridden a lot of bikes and that is how I feel about it.

Anyways, I know have 2 nice titanium bikes, A Tournesol and a Spectrum. Both are 2nd hand customs and I do not know the tubing specifications but they are both built with relatively large tubes. Both are also much Flexier than my steel bikes.

Question- What would it take to make a titanium frame that is as stiff as my oversize steel frames? I know some modern titanium frames have very large main tubes (especially down tubes) but on a large tired bike it seems like it might be tough to fit a large enough chain stays in.....

Anyways, what say the braintrust?

[Peter P.]

Steel is denser than titanium, which is why a smaller diameter steel tube provides a greater resistance to flex than a same-sized titanium tube.

But that resistance to flex ramps up quickly with small increases in diameter.

The problem may be the availability of titanium tubes in precisely the correct diameter to provide that equivalent road feel. Builders are likely selecting from "standard" tube sizes offered by the manufacturer. And I sincerely doubt the titanium frame builders are provided with the material numbers necessary to make a comparison, and tube testing would prove too costly.

Also, changing wall thickness offers minimal changes in tube stiffness so don't dwell too much there.

As I see it, your only alternative is to have a frame built with tubes larger than what you currently have until you are satisfied. That could prove to be an expensive search if you go through multiple iterations.

[Ken Robb]

You could order a custom Legend from Serotta as stiff as you want--------OOPS! Never mind--too late. :-)

We used to have posts from guys who thought their Legends were too stiff. Mine was perfect--a 60cm with just a little "snappy flex".

[NYCfixie]

Any metal (Aluminium, Steel, Titanium) can be made to be flexy/cushy/comfortable/etc. or stiff/harsh/uncomfortable/etc. as you want with a "custom for you" build. You answered your own question by stating you do not know the tube specs of your 2nd-hand customs.

My Lynskey R255 rode great for fun Sunday rides but was "flexy" in the BB area under my fat arse. I gave it to my sister-in-law (she and are are within a mm of each-other for reach and saddle height) who is at least 50 pounds lighter than me and she just loves it because she finds it neither stiff or flexy but rather neutral.

My Seven Axiom SL was custom built for me (and my fat arse) in all the right ways and I find it to ride like a dream. I had my sister-in-law try it when she was out East a few months back and she hated it. The fit was fine but she found it much too harsh (we use the same wheels and tires).

I had a '89 Cannondale road bike which rattled my teeth even at my young age when I owned it as new. I have ridden newer, similarly fat tube, Cannondales and they are so much better. It is amazing what they can do with Aluminium these days.

And, talk to IF. They can build you a sweet comfortable riding bike or a stiff race bike that will rattle you teeth both made out of steel.

It is not the metal material as much as what tube size, wall thickness, and how they put it together that matters.

[cadence90]

Great question.

It would be very interesting to read what Seven, Spectrum, and other experienced builders who offer custom stiffness-tuned frames in both steel and titanium, have to say about this.
.

[zank]

My last steel bike had a max DT (34.9 mm) and TT (31.8 mm), 31.8 mm ST, 30/16 mm oval tapered chain stays, and 15.9 mm tapered seat stays.
My last alumimum bike had a 42 mm DT, a 34.9 mm TT, a 34.9 ST, 30/16 oval tapered chain stays, and 19 mm tapered seat stays.
My ti bike has a 38.1 mm DT, a 34.9 mm TT, a 34.9 ST, 22.2 mm chain stays, and 19 mm seat stays.
The ti bike feels like it has the stiffest drivetrain. I remember when I went from my steel bike to my aluminum bike and the standing stiffness of the aluminum bike was the biggest difference. Both felt similar otherwise. I felt the ti was even a bit stiffer out of the saddle and in a sprint (my paltry 1200 watts). The non-tapered stays must play a role. Same wheels and tires (Belgium plus with 28 mm Pro Ones at 60 psi). I think the ti is a little smoother on our horrible roads, but I think the rims and tires play a big role in that. Corners solid. I can't feel any wiggle. Feels similar to the Max.

There are plenty of tubes to pick from in all three materials. You can make a great bike with any of them.

[bicycletricycle]

I am sure that the main tubes of a titanium frame can be made to be as stiff as one wanted. The chain stays seem to be the limitation to me, especially if you want big tires and low Q factor cranks. The space in between the chainrings and tire/fender is very constrained.

I guess a machined yoke of some kind could help.

[bicycletricycle]

Thanks for the detailed response, did you have to dimple the 7/8” ti chain stays?

My last steel bike had a max DT (34.9 mm) and TT (31.8 mm), 31.8 mm ST, 30/16 mm oval tapered chain stays, and 15.9 mm tapered seat stays.
My last alumimum bike had a 42 mm DT, a 34.9 mm TT, a 34.9 ST, 30/16 oval tapered chain stays, and 19 mm tapered seat stays.
My ti bike has a 38.1 mm DT, a 34.9 mm TT, a 34.9 ST, 22.2 mm chain stays, and 19 mm seat stays.
The ti bike feels like it has the stiffest drivetrain. I remember when I went from my steel bike to my aluminum bike and the standing stiffness of the aluminum bike was the biggest difference. Both felt similar otherwise. I felt the ti was even a bit stiffer out of the saddle and in a sprint (my paltry 1200 watts). The non-tapered stays must play a role. Same wheels and tires (Belgium plus with 28 mm Pro Ones at 60 psi). I think the ti is a little smoother on our horrible roads, but I think the rims and tires play a big role in that. Corners solid. I can't feel any wiggle. Feels similar to the Max.

There are plenty of tubes to pick from in all three materials. You can make a great bike with any of them.

[mt2u77]

Young’s modulus for Ti is roughly half that of steel. To get an equal deflection for a given load, you have to roughly double the area moment of inertia (I.e. make the diameter bigger and/or walls thicker— diameter changes are by far more impactful).

Turns out to be a 20-25% increase in outer diameter assuming wall thickness stays constant. (Roughly. Back of the napkin here.)

The equation for area moment of inertia for a tube is: I = pi(D^4 - d^4)/64. Big D= outer diam, Little d= inner diam.

I’m sure this gets much more complicated in real life depending on the type of loading/twisting, joints, etc., but this is a decent first order approximation.


Sent from my iPhone using Tapatalk

[zank]

With titanium, I ovalize rather than dimple. Titanium likes soft bends. Anything resembling a crease is prone to cracking.

[93KgBike]

I always thought titanium was for smaller frames, for that reason.

If you're big like me, or bigger, then steel for road bikes. It's the best.

For mountain bikes, I'm still partial to aluminum.

Although if I where still a young racer, which I'm not, carbon fiber makes sense.

Oh, in answer to your question, it would take more titanium, and/or larger diameter tubes.

[bicycletricycle]

With titanium, I ovalize rather than dimple. Titanium likes soft bends. Anything resembling a crease is prone to cracking.

I wonder if you ended up ovalizing them down to 16 millimeters? If so, perhaps some increased stiffnessin the main tubes could compensate for the decreased stiffness in the stays. I know some brands are using 1” stays but they still have to be cold worked into a less ideal shape to fit.

[cadence90]

My last steel bike had a max DT (34.9 mm) and TT (31.8 mm), 31.8 mm ST, 30/16 mm oval tapered chain stays, and 15.9 mm tapered seat stays.
My last alumimum bike had a 42 mm DT, a 34.9 mm TT, a 34.9 ST, 30/16 oval tapered chain stays, and 19 mm tapered seat stays.
My ti bike has a 38.1 mm DT, a 34.9 mm TT, a 34.9 ST, 22.2 mm chain stays, and 19 mm seat stays.
The ti bike feels like it has the stiffest drivetrain. I remember when I went from my steel bike to my aluminum bike and the standing stiffness of the aluminum bike was the biggest difference. Both felt similar otherwise. I felt the ti was even a bit stiffer out of the saddle and in a sprint (my paltry 1200 watts). The non-tapered stays must play a role. Same wheels and tires (Belgium plus with 28 mm Pro Ones at 60 psi). I think the ti is a little smoother on our horrible roads, but I think the rims and tires play a big role in that. Corners solid. I can't feel any wiggle. Feels similar to the Max.

There are plenty of tubes to pick from in all three materials. You can make a great bike with any of them.
Excellent, interesting comparison, thanks much.

Are you building in titanium also now? I thought I had read that here somewhere, but now I can't find any mention of it.
.

[cadence90]

With titanium, I ovalize rather than dimple. Titanium likes soft bends. Anything resembling a crease is prone to cracking.

Some years ago I did a bunch of production drawings for Jim Kish, mtb and road chainstays and seatstays, of various sizes, tapers, and S-bends. Besides getting the dimensions correct, of course, the major criteria was to stay absolutely within the ovalization parameters. No creases, dimples, crimps, etc. allowed at all. :no:

Ovalized look a lot more elegant, too, imho. :)
.

[Kirk007]

To each his own, and there's so many factors that could come into play here. I'm 6 3" currently 220 and I've had a ti bike that I thought was too stiff (Lynskey 29er). I currently have a Spectrum made for me that I ride interchangeably with a Peg Marcelo and a Hampsten carbon bike. All ride very well. The Peg seems harsher which I would interpret as stiffer. But the Spectrum and Carbon are not flexy in the least. When I was at the barn Tom's comment was to the effect that he could make a ti bike as stiff as I wanted. Anyway each bike feels just a bit different but none feels faster than the other. This summer will see which I feel better on at the end of 4-6 hour rides.

I don't buy that you can't get a ti bike that is suitably stiff for a big guy and big tires. My Spectrum takes Compass Bon Jons easily. The only thing it gives up to a steel bike is that it's lighter. And the Spectrum rando frame that I have for sale feels about the same - smooth not flexy.

I just picked up a second hand Hampsten by Moots YBB and we'll see how that feels as its older and the tubes aren't as stout as the Spectrum but first ride impression with the YBB active and 35mm Bon Jons it seems like a good frame for gravel and dirt in particular.

[RyanH]

What size do you ride? I ride small frames and my Litespeed is as stiff as most carbon during a sprint. I found the Indy Fab Crown Jewel to be less compliant, less stiff and much heavier than the Litespeed, which is kind of like what's the point?

[Louis]

Steel is denser than titanium, which is why a smaller diameter steel tube provides a greater resistance to flex than a same-sized titanium tube.

We need to differentiate between density (weight (or mass) for a given volume of material) and Young's modulus (aka E, stiffness in terms of the ratio of stress to strain).

Steel has higher density and E than Ti, and one might argue that the two tend to be positively correlated, but they are different properties. When we're talking stiffness that means we're talking geometry and E. When we're talking weight that means we're talking volume and rho (the traditional symbol for density).

[cadence90]

To each his own, and there's so many factors that could come into play here. I'm 6 3" currently 220 and I've had a ti bike that I thought was too stiff (Lynskey 29er). I currently have a Spectrum made for me that I ride interchangeably with a Peg Marcelo and a Hampsten carbon bike. All ride very well. The Peg seems harsher which I would interpret as stiffer. But the Spectrum and Carbon are not flexy in the least. When I was at the barn Tom's comment was to the effect that he could make a ti bike as stiff as I wanted. Anyway each bike feels just a bit different but none feels faster than the other. This summer will see which I feel better on at the end of 4-6 hour rides.

I don't buy that you can't get a ti bike that is suitably stiff for a big guy and big tires. My Spectrum takes Compass Bon Jons easily. The only thing it gives up to a steel bike is that it's lighter. And the Spectrum rando frame that I have for sale feels about the same - smooth not flexy.

I just picked up a second hand Hampsten by Moots YBB and we'll see how that feels as its older and the tubes aren't as stout as the Spectrum but first ride impression with the YBB active and 35mm Bon Jons it seems like a good frame for gravel and dirt in particular.
We have very similar builds and bicycles.
6'-1", 200#.
Colnago Extreme-Power (extra-stiff);
Kish 3/2.5 straight-gauge ti road;
Moots Pscyhlo-X YBB 3/2.5 straight-gauge ti cross;
Pegoretti Marcelo Spirit.

All have ~58 tts. The Kish is the only custom of the bunch, and I asked Jim to build a slightly stiffer frame (same geo) as a Columbus Foco TIG, which he did. The Marcelo is indeed the stiffest, esp. in the back, but not harshly so (apparently the BLE is a beast even for larger riders). None of the four are what I would call "flexy" in any way. The YBB is obviously the softest, even on lockout, but it is not flexy, and is a very versatile, very comfortable and responsive frame, same uses as you; gravel, o-r, general commuting.

I would concur that an experienced titanium builder should be able to closely replicate the stiffness characteristics of bicycletricycle's steel Tournesols and Chapmans.
.

[bicycletricycle]

For those who asked, I’m 6 foot 3 and 220 pounds.

[fogrider]

I have assembled a few bikes over the years, 2 steel, a titanium, aluminum, scandium, and 2 carbon bikes. These are the ones I've kept...a builder can make any material stiffer or not, but there are other factors to consider for a frame. For the weight, carbon is the stiffest, then scandium, aluminum, titanium,then steel. But I don't have air hardened steel frame, which would be more in the area of titanium.

Sent from my Moto G (5) Plus using Tapatalk

[bicycletricycle]

Here is an interesting article with a chart full of data. Not a lot of perfect apples to apples comparisons but it does look like and is acknowledged by the tester that the rear triangles struggle to keep up with steel and are compensated for with stiffer front triangles.


https://www.sheldonbrown.com/rinard_frametest.htmlP

[Clean39T]

Since I read this not too long ago, I'll post it here...apropos as it is...

-------

From Dave Kirk RE: Chainstay Stiffness -

http://kirkframeworks.com/resources/technical/chainstay-stiffness/

This is a favorite subject of mine.

I’ll say up front that I have no idea what bike you might like best. I’ll leave that alone.

First I’ll say something that you won’t hear from many builders…….curving or bending the stays into any configuration other than straight will make the stays flex more. There are very few absolutes in life but this is one of them. Curved stays flex more than the equivalent stay without the bend. They won’t flex a lot more……just a little. It depends on the type of bend and the degree of bend. One thing is for sure, the bent stay can never be stiffer due to it’s bend. The biggest reason to bend a stay is to allow for better clearances with tire and chainring. This is a bigger deal on mountain bikes than road but true nonetheless.

In any given material, stay diameter is the biggest factor in determining the stiffness. The bigger the diameter, the stiffer the stay. The problem is that if you make the stay bigger in diameter you run into clearance issues with both the chainrings and tire. So….. most stays are about the same in this regard. One can use huge stays but then you need big, deep dents to allow for the clearance room and the dents go a long way to negating the gains in stiffness made by the larger diameter stay.

There is a current trend in frame design to control BB flex with a big-arse downtube. This may feel like a good thing at first and it will certainly help with front derailleur rub but in the end it does not make for a stiffer bottom bracket. It’s often overlooked but it’s the chainstays that make the drivetrain stiff. A big downtube might be good for other reasons but it doesn’t do squat to make for a stiff drivetrain. A Slingshot frame is a great example of this. It’s simple when you think of it. The energy goes into the frame at the BB and goes to the rear wheel………and the only thing between the BB and the wheel is the chainstays. This is one of the big issues in designing a good Ti bike for a big or heavy rider. Titanium is much more flexible than steel. The way to make it as stiff as steel is to make the diameter much larger. But this has it’s own clearance issues. So they make the stays oval (making them stiff in the vertical direction and soft laterally…….not ideal) or they put in huge dents which has it’s own issues. So most Ti builders compensate for the wimpy stays with a huge downtube. And like I stated above this has little effect on true drivetrain stiffness. This is one of the reasons that many larger folks feel that Ti bikes don’t have the snap they are looking for and why it’s fallen out of favor with many racers.

When pedaling a bike out of the saddle chainstays are asked to do a number of different things. They undergo compression which is easy for almost any stay design to deal with as the loads are low. They undergo a torsional (twisting) load which most stays deal with fairly well regardless of shape. Larger diameter helps a good bit with the torsion. The other thing that happens to stays is that they see a lateral bending load as the BB is pushed from one side to the other. This is where ovalized stays can get in trouble. An oval tube has a major diameter and a minor diameter. There is a pretty good rule of thumb that addresses how an oval tube will flex compared to a round tube. The oval tube will flex about the same as a round tube that has the same diameter as either the major or minor diameters of the oval. In other words if you have an oval tube that is 30mm by 17mm in cross section it will flex about the same as a 30mm round tube in one direction and about the same as a 17mm tube in the other direction. So if you think of oval chainstays you in effect get the lateral stiffness of a rather small round tube.

For all the above reason I feel that a round stay is best. You get the most bang for the buck in every direction. You get good clearances with minimal denting and you get a nice stiff stay and drivetrain.

Wow…that’s more writing than I thought it would be. Thanks for sticking with me.

Dave

[Clean39T]

This bike rode like a freakin' rocket (Merlin Works TR 6/4 Ti):

https://uploads.tapatalk-cdn.com/20170827/b211093bd0c414accc7a728ff936db9b.jpg

https://uploads.tapatalk-cdn.com/20170816/87fd578b1f846d4d44cde579cdab0409.jpg

https://uploads.tapatalk-cdn.com/20170816/0b22b8d1e29db036ca458212816849cd.jpg

It had the most aggressively manipulated tubes I've seen or ridden.

Fatal flaw for me though was the rim clearance in the back for running deep carbon tubulars and the size overall. I could get over the first, but it was just a bit too short front to back to hold onto it.

If I ever see one in the XXL (61cm), it'll be bought post-haste.

Or I could just ask TK to replicate...don't know if some of the tubing is available anymore though...

[macaroon]

Interesting stuff from Dave Kirk. I guess what he's saying is Titanium is maybe not the best material for a big, powerful rider, unless you don't mind compromises in tyre/chainring clearance?

Why don't we see more chainstay yokes in the road cycling world? They certainly offer more clearance. Are they stiffer? Weaker? More prone to failure?

https://www.blazingbikes.co.uk/media/catalog/product/cache/1/thumbnail/9df78eab33525d08d6e5fb8d27136e95/s/h/sherpa-yoke.jpg

[weisan]

https://uploads.tapatalk-cdn.com/20170816/0b22b8d1e29db036ca458212816849cd.jpg


Clean pal, you have super long big toe...

[fa63]

My last steel bike had a max DT (34.9 mm) and TT (31.8 mm), 31.8 mm ST, 30/16 mm oval tapered chain stays, and 15.9 mm tapered seat stays.
My last alumimum bike had a 42 mm DT, a 34.9 mm TT, a 34.9 ST, 30/16 oval tapered chain stays, and 19 mm tapered seat stays.
My ti bike has a 38.1 mm DT, a 34.9 mm TT, a 34.9 ST, 22.2 mm chain stays, and 19 mm seat stays.


Nerd alert: going to do some math :)

The bending stiffness is the product of the area moment of inertia (I) by the Young's Modulus (E). For the three scenarios above, if we assume a constant wall thickness of 1 mm for the tubes, and using typical values of E for each material, you get the following EI ratios with steel being the baseline:

- EI-Al / EI-steel = 0.34
- EI-Ti / EI-steel = 0.92

So the math suggests that in this scenario, the aluminum chainstays should have about 1/3 the bending resistance of steel ones, while the Ti chainstays have about the same stiffness.

Of course, I think my assumption that the wall tube thicknesses are constant is probably inaccurate. I imagine steel tubes have the smallest thickness, followed by Ti then aluminum.

Zank - could you share the wall thickness information by any chance?

[zank]

Sure.
Chainstays - steel - 0.8/0.6 butted, al - 1.4, ti - 0.9
Seatstays - steel - 0.7, al - 1.2, ti - 0.9
Seat tube - steel - 0.8/0.6 butted, al - 1.2 bottom/1.0/1.6, ti - 0.9
Top tube - steel - 0.8/0.6/0.8, al - 1.2/0.9/1.2, ti - 0.9
Down tube - steel - 0.8/0.6/0.8, al - 1.8/1.2/1.4, ti - 0.9

[bicycletricycle]

The steel stay I see not round, which dimension did you use? 30 or 17? Also that titanium stay is probably ovalized as well.


Nerd alert: going to do some math :)

The bending stiffness is the product of the area moment of inertia (I) by the Young's Modulus (E). For the three scenarios above, if we assume a constant wall thickness of 1 mm for the tubes, and using typical values of E for each material, you get the following EI ratios with steel being the baseline:

- EI-Al / EI-steel = 0.34
- EI-Ti / EI-steel = 0.92

So the math suggests that in this scenario, the aluminum chainstays should have about 1/3 the bending resistance of steel ones, while the Ti chainstays have about the same stiffness.

Of course, I think my assumption that the wall tube thicknesses are constant is probably inaccurate. I imagine steel tubes have the smallest thickness, followed by Ti then aluminum.

Zank - could you share the wall thickness information by any chance?

[zank]

It's ovalized for about 5 cm section or so where the tire and rings pass. Returns to 22.2 before the BB.

[bicycletricycle]

Yes, this is what is suspected. The ti tubes need to be bigger to be as stiff as steel but the chain stays can’t get much bigger so titanium has a limitation there.


The stays on that Litespeed are cool looking. It looks like they sacrificed some tire clearance for larger stiffer stays.


Since I read this not too long ago, I'll post it here...apropos as it is...

-------

From Dave Kirk RE: Chainstay Stiffness -

http://kirkframeworks.com/resources/technical/chainstay-stiffness/

This is a favorite subject of mine.

I’ll say up front that I have no idea what bike you might like best. I’ll leave that alone.

First I’ll say something that you won’t hear from many builders…….curving or bending the stays into any configuration other than straight will make the stays flex more. There are very few absolutes in life but this is one of them. Curved stays flex more than the equivalent stay without the bend. They won’t flex a lot more……just a little. It depends on the type of bend and the degree of bend. One thing is for sure, the bent stay can never be stiffer due to it’s bend. The biggest reason to bend a stay is to allow for better clearances with tire and chainring. This is a bigger deal on mountain bikes than road but true nonetheless.

In any given material, stay diameter is the biggest factor in determining the stiffness. The bigger the diameter, the stiffer the stay. The problem is that if you make the stay bigger in diameter you run into clearance issues with both the chainrings and tire. So….. most stays are about the same in this regard. One can use huge stays but then you need big, deep dents to allow for the clearance room and the dents go a long way to negating the gains in stiffness made by the larger diameter stay.

There is a current trend in frame design to control BB flex with a big-arse downtube. This may feel like a good thing at first and it will certainly help with front derailleur rub but in the end it does not make for a stiffer bottom bracket. It’s often overlooked but it’s the chainstays that make the drivetrain stiff. A big downtube might be good for other reasons but it doesn’t do squat to make for a stiff drivetrain. A Slingshot frame is a great example of this. It’s simple when you think of it. The energy goes into the frame at the BB and goes to the rear wheel………and the only thing between the BB and the wheel is the chainstays. This is one of the big issues in designing a good Ti bike for a big or heavy rider. Titanium is much more flexible than steel. The way to make it as stiff as steel is to make the diameter much larger. But this has it’s own clearance issues. So they make the stays oval (making them stiff in the vertical direction and soft laterally…….not ideal) or they put in huge dents which has it’s own issues. So most Ti builders compensate for the wimpy stays with a huge downtube. And like I stated above this has little effect on true drivetrain stiffness. This is one of the reasons that many larger folks feel that Ti bikes don’t have the snap they are looking for and why it’s fallen out of favor with many racers.

When pedaling a bike out of the saddle chainstays are asked to do a number of different things. They undergo compression which is easy for almost any stay design to deal with as the loads are low. They undergo a torsional (twisting) load which most stays deal with fairly well regardless of shape. Larger diameter helps a good bit with the torsion. The other thing that happens to stays is that they see a lateral bending load as the BB is pushed from one side to the other. This is where ovalized stays can get in trouble. An oval tube has a major diameter and a minor diameter. There is a pretty good rule of thumb that addresses how an oval tube will flex compared to a round tube. The oval tube will flex about the same as a round tube that has the same diameter as either the major or minor diameters of the oval. In other words if you have an oval tube that is 30mm by 17mm in cross section it will flex about the same as a 30mm round tube in one direction and about the same as a 17mm tube in the other direction. So if you think of oval chainstays you in effect get the lateral stiffness of a rather small round tube.

For all the above reason I feel that a round stay is best. You get the most bang for the buck in every direction. You get good clearances with minimal denting and you get a nice stiff stay and drivetrain.

Wow…that’s more writing than I thought it would be. Thanks for sticking with me.

Dave

[fa63]

I calculated the bending stiffness of an hollow oval section about the axis parallel to the bending force, so the smaller dimension controls in this case.

The steel stay I see not round, which dimension did you use? 30 or 17? Also that titanium stay is probably ovalized as well.

[fa63]

Sure.
Chainstays - steel - 0.8/0.6 butted, al - 1.4, ti - 0.9
Seatstays - steel - 0.7, al - 1.2, ti - 0.9
Seat tube - steel - 0.8/0.6 butted, al - 1.2 bottom/1.0/1.6, ti - 0.9
Top tube - steel - 0.8/0.6/0.8, al - 1.2/0.9/1.2, ti - 0.9
Down tube - steel - 0.8/0.6/0.8, al - 1.8/1.2/1.4, ti - 0.9

Thanks! So in this case, we have the following (assuming average 0.7 mm thickness for the steel tube):

EI-Al / EI-steel = 0.61
EI-Ti / EI-steel = 1.14

So your perceptions of the Ti frame having the stiffest drivetrain appear to be backed up by the numbers :) As another point of reference, for the aluminum tubes to have the same stiffness as the steel ones (with a 1.4 mm wall thickness), the minor oval axis dimension would have to be about 20 mm instead of 16 mm.

That said, this is a gross over-simplication of a complex process, because drivetrain stiffness does not come only from the chainstays (though Dave Kirk argues that chainstays are the primary factor).

[zank]

Interesting. So it can't just be chainstays since the aluminum felt spunkier than the steel bike in sprints and out of the saddle in general, especially on the cross course.

[happycampyer]

Were any of them BB30/PF BB30? I would think the oversized bottom bracket shell would add to the overall stiffness of the drivetrain. BB30/PF BB30 has its drawbacks, but I suspect that for titanium it would be beneficial for a bigger rider.

[fa63]

If I run the numbers for the downtubes based on the information you provided, I get (using the wall thickness at the BB junction):

EI-Al / EI-steel = 1.27
EI-Ti / EI-steel = 0.81

So aluminum wins the downtube stiffness wars. I imagine CS and DT are the primary contributors to drivetrain stiffness, though I don't know by what proportion.

Interesting. So it can't just be chainstays since the aluminum felt spunkier than the steel bike in sprints and out of the saddle in general, especially on the cross course.

[zank]

All 68 mm bsa. Steel was a 38.1 mm shell. Al and Ti were 41.4 mm. All Ultegra 6800 cranks.

My personal opinion? There is no easy answer. This type of exercise has been done for decades. I think makers/companies need to have a position for their marketing, especially if there is an element of the equation they are doing differently. A lot of what is sold and marketed as "better" is really a company trying to sell something that makes their process easier. BB30 comes to mind.

There's only so much space between a tire and chainring. Maybe e-stays are the answer :)

[fa63]

I am not sure that a large BB shell alone would make things stiffer. It does provide a larger platform for welding, and if you weld larger tubes to it, then sure but otherwise, I am not sure it adds much.

Were any of them BB30/PF BB30? I would think the oversized bottom bracket shell would add to the overall stiffness of the drivetrain. BB30/PF BB30 has its drawbacks, but I suspect that for titanium it would be beneficial for a bigger rider.

[Mzilliox]

My perceptions of my bikes in order of stiffness.

Most stiff to least
Ti Heretic
Goodrich UOS steel
Seven carbon/ti
Lyon steel

My old mini max zanc was stiffer than all these bikes.

why stiffness? is this desirable?

[Kirk007]

why stiffness? is this desirable?

The crux of the question. Or for me to rephrase I'd say when is a certain level of stiffness optimal and I think the answer to that begs the questions of for whom and for what?

[Mark McM]

As has been noted, there is more to tube stiffness than just E (Modulus). Tube geometry (diameters, wall thickness, etc.) also plays a large role. If you made from a titanium frame from solid bars the same diameter as hollow frame tubes, it would be very stiff (although it would also weight a lot!).

But there are other tube shaping used in bicycle frames than just diameter and thickness, and bending and crimping - there is also tapering and butting. The most efficient shapes in terms of stiffness/weight will put more material where the stresses are the highest and less material where the stresses are the lowest. This is the purpose of tapering tubes (and to a lesser extent, butting as well). The highest stresses in chainstay are at the BB, and the lowest are at the dropout, so chainstays are often tapered from BB to dropout. (Seatstays are sometimes also tapered for the same reason.) Producing a tapered and butted tube is relatively easy in steel and aluminum (and also carbon fiber), but not so easy in titanium. So if you compare chainstays of equal weight, a non-tapered non-butted chainstay of titanium may be more flexible than tapered and butted chainstays of the other materials.

Note: There are similar factors at play in fork blades as well. Similar to chainstays, the hightest stresses in fork blades are at the crown, and the lowest are at the dropuouts, so fork blades are usually tapered and butted. Since this is more difficult to do in titanium, it is difficult to make a titanium fork with similar stiffness/weight as a steel, aluminum or carbon fiber fork.

[eddief]

or whatever you want to call it. My Carver ti rides smooth as heck and out of the saddle climbing for my 5'11, 190 lbs never seems any less than how I'd prefer it to be.

https://photos.app.goo.gl/DM2aK5uCz62pvxTPA

[Hellgate]

I don't consider my Davidson flexy at all. Rides great. It's semisloping, 57cm square in a traditional measurement. Granted I'm 6', 164lbs.

In general frame performance comes down to tube selection and geometry, for the most part.

Sent from my ONEPLUS A5010 using Tapatalk

[uber]

I don't know if there is a difference between absolute stiffness vs observations of how the bike rides over rough terrain or responds to larger power efforts. My Spectrum from 14 years ago had that supple Ti ride, but was subject to speed wobbles and never felt as responsive as some of my steel bikes. I had a custom Eriksen from 2014 that Kent built with 1" chain stays, 44mm head tube (27.2 seat tube) that felt uncomfortably stiff to me. Harsh over rough roads. It felt stiffer than my Marcelo or BLE that had enormous chain stays. Dave Kirk is one of the most respected frame builders, and I am sure he is 100% correct. If my observations don't line up with his comments, it is my perception not a challenge to his post.

[HenryA]

Put me on the side that says maybe the bike does not need to be that stiff. And while I’m at it, I’ll say that a lot of this characteristic we are discussing comes down to feel and personal preferences developed over time and experience.

The feel part is completely subjective and can be influenced by the rider’s experience and perceptions during the ride. Those feelings, while valid, are not necessarily accurate - after all, any reasonably modern well made frame will be fully functional, while perhaps feeling slightly different. Within the category of “reasonably modern well made frames” I think the feeling of stiffness makes no difference in the actual performance of the bicycle. Not as measured with scientifically designed instrumentation.

I own a Ti Serotta MTB. Dave Kirk’s comments indicate that the bigger tires on an MTB make it even harder to design in stiffness than a road bike. I think that is the case. My experience is that it is plenty stiff and rides nicely. Some of that comes from the material and some if it comes from relatively squishy tires. As an example though, the bike never feels squirrelly and I have no problem with the front derailleur grinding on hard out of the saddle efforts.

I have also ridden several Legend Ti Serottas over the years they were made and found the ones made for a big guy to be great frames. Extremely great. I can say the same for my Colorado II and my CIII. Extremely great bikes of their kind.

Using the “standing hard effort” as a measure, only the Colorado II would give me much flex that I could notice. I think this bike is about at the limit of flex I would choose. My next bike was the CIII and I specifically asked for it to be a little stiffer in the bottom. It was and is. Both bikes ride great.

Of the half dozen Legends I rode (and not nearly as much as my own bikes) the ones built for a big guy were maybe a little stiffer than my old Colorado II but softer, plusher feeling than my CIII. Really nice frames that I always wanted but could never justify for the extra cost. Still want one if I find the right one.

One of the best riding bikes I ever rode was a Serotta Ottrott. That hybrid frame is something special. The ones I rode were made close to my specs and were just magnificent in all regards - comfy, responsive, crisp and smooth. I write about Serottas a lot in this post as that is where I have deepest experience.

After writing all the above to form some basis for what I think makes a great bike here goes:

1. I want the tires to stay in contact with the road at all times. Super stiff frame and super stiff wheels are not good ways to have this when you’re on a bike that is built right. Probably not ever.

2. I want the bike to ride without harshness. For just riding along, a little soft gentle flex is pleasant to me.

3. When pressed (as in a sharp fast criterium turn or similar) I want the bike to track like its on rails. No skipping, no darting, nothing but subtle perfect response to my input. (See No. 1)

Various combinations of the above factors can get the job done for me. Each combination will have its own flavor. But keep in mind that its only “flavor” and you’ll have preferences and none of them are absolutes. Those preferences may change over time, sometimes even one day.

BTW: Dave Kirk gave a great explanation.

[avalonracing]

This bike rode like a freakin' rocket (Merlin Works TR 6/4 Ti):

It had the most aggressively manipulated tubes I've seen or ridden.

If I ever see one in the XXL (61cm), it'll be bought post-haste.

Or I could just ask TK to replicate...don't know if some of the tubing is available anymore though...


Sounds like you and I have something in common. I had the Works CR 3/2.5 and in size large (57.5) and I agree that the bike had an incredible ride. And I always used to think that the tube manipulation was fantastic. Giant ovalized tubes, huge chainstays. Yet the bike was smoooooth. I've also thought about contacting TK and having him make me one with a slightly steeper STA but I also wonder if the tubes can be replicated as he was working with Litespeed to get these tubes made and I haven't seen any Sevens (who I believe he was more recently using) with this kind of tube work.

[bicycletricycle]

I think you would have to change the ti number to represent the minor axis of the squished section. I think the stay is only as stiff as it’s narrowist section. Especially if that narrow section is in the area with the highest load


Thanks! So in this case, we have the following (assuming average 0.7 mm thickness for the steel tube):

EI-Al / EI-steel = 0.61
EI-Ti / EI-steel = 1.14

So your perceptions of the Ti frame having the stiffest drivetrain appear to be backed up by the numbers :) As another point of reference, for the aluminum tubes to have the same stiffness as the steel ones (with a 1.4 mm wall thickness), the minor oval axis dimension would have to be about 20 mm instead of 16 mm.

That said, this is a gross over-simplication of a complex process, because drivetrain stiffness does not come only from the chainstays (though Dave Kirk argues that chainstays are the primary factor).

[zank]

Wouldn't that imply that it is not flexing anywhere else along the length though?

I also wonder how ovalizing affects twisting.

[bicycletricycle]

Wouldn't that imply that it is not flexing anywhere else along the length though?

I am assuming this is in reference to my above post? As pointed out already all of the numbers we have been using are inherently in accurate for a variety of reasons. The chain stay is more highly loaded closer to the B.B., if the bike has a chain stay bridge than I bet a large amount of the load would concentrate right behind the bridge in the same place the stay is normally the narrowist for clearance reasons. This area is probably the best place to increase lateral stay stiffness and it is also the place with the smallest amount of space to do so. Making the tube larger diameter behind this area has increasingly diminishing returns.

[zank]

Great points. That all makes a lot of sense.

[bicycletricycle]

I agree with this statement. Stiffer does not equal better. If the bike works and you like it than that is all that matters really. I don’t think that frame stiffness has a huge relation to absolute measurable performance. But we have preferences for a variety of reasons and those can be important to individuals.


Put me on the side that says maybe the bike does not need to be that stiff. And while I’m at it, I’ll say that a lot of this characteristic we are discussing comes down to feel and personal preferences developed over time and experience.

The feel part is completely subjective and can be influenced by the rider’s experience and perceptions during the ride. Those feelings, while valid, are not necessarily accurate - after all, any reasonably modern well made frame will be fully functional, while perhaps feeling slightly different. Within the category of “reasonably modern well made frames” I think the feeling of stiffness makes no difference in the actual performance of the bicycle. Not as measured with scientifically designed instrumentation.

.

[fa63]

I must have missed the post about the Ti tube being ovalized near the BB junction; that would indeed change the outcome. I am sure the rest of the tube contributes some to the stiffness as well, but indeed the stress concentrations are near the fixed ends.

I think you would have to change the ti number to represent the minor axis of the squished section. I think the stay is only as stiff as it’s narrowist section. Especially if that narrow section is in the area with the highest load

Twisting (torsion) is controlled by the polar moment of inertia (J); generally speaking as ovalization increases, J decreases.


I also wonder how ovalizing affects twisting.

[zank]

I know all that. I want numbers man!
I'm totally teasing. Unless you want to run them for fun :)

[fa63]

Haha, I will crunch some more numbers later :)

Edit: here is a nice summary figure I found with simple-to-implement formulas. Beta (β) is defined as b/a, and polar moment of inertia (J) can be calculated as J = Ix + Iy.

https://farm2.staticflickr.com/1760/40909041870_30cb2b4079_c.jpg

Reference:

Novitskaya, Ekaterina & Ruestes, Carlos & Porter, Michael & Lubarda, V.A. & Meyers, Marc & Mckittrick, Joanna. (2017). Reinforcements in avian wing bones: Experiments, analysis, and modeling. Journal of the Mechanical Behavior of Biomedical Materials. 10.1016/j.jmbbm.2017.07.020.

I know all that. I want numbers man!
I'm totally teasing. Unless you want to run them for fun :)

[bigbill]

My two main bikes are a study in contrast. One is a Pegoretti Big Leg Emma and the other is a Serotta Fierte Ti. Both have threaded bottom brackets and Campy 11 with UT cranksets. I bought the Serotta to replace the steel Argonaut I lost in the Southwest fire. It wasn't expensive and I figured it would be a nice summer bike since it wouldn't be affected by daily drench of sweat. It rides well but is not as snappy as the Pegoretti, but it wasn't built to be that way. It was a production Ti frame in standard sizing. After riding it for almost 2 years, it's growing on me and is actually a great all day bike.

In 1994 I decided to drink the Koolaid and get a Litespeed. I went from a Raleigh 753 to a Litespeed Classic. It was not a good swap. I had to change my riding style and adapt to a more flexy frame and it was never that good. A few years later I sold it on consignment in San Diego and bought a Merckx MX Leader.

[verbs4us]

Modulus of elasticity aside...I'm not sure what is meant by "flexy." What does the OP have in mind? My perspective is a bit weird, since I went from a 1974 steel bike to a 2015 Ti bike, skipping over many generations of design and metallurgical development. One thing I've noticed is that once you reach a high quality frame build, it is difficult to tease apart wheel/tire effect from frame effect. Highly inflated tires on stiff wheel make for an unforgiving (but efficient on some roads) ride. In general, I found that the steel frame tends to dance a little on rough roads, whereas the Ti absorbs bounces and keeps the tires on the pavement. Does this smoothing out the ripples mean it is more flexy? Both are stiff laterally--no chainrub on the front der when pounding it; no discernible deflection of the bottom bracket, or shimmy-ing on braking. My Ti has almost the same road feel as steel, but a little smoothed out.

[Clean39T]

I decided not to bid on this...but was very tempted:

https://www.ebay.com/itm/Spectrum-Cycles-Titanium-Road-Bike-61cm-Tom-Kellogg-Campagnolo-Record-/273259333759

https://i.ebayimg.com/images/g/f0YAAOSwJ7dbFFtb/s-l1600.jpg

We coulda been maternal twins :banana:

It went unbid at $1200.

[adrien]

Interesting read.

I did a lot of research on this (without math!) when it was time for the second custom bike, and spent a long time chatting with Kevin (Firefly) about how stiff I wanted it, and where.

My main bike at the time (still ride it a lot) was an Ira Ryan lugged steel bike, which Ira built a few years back to be all-day comfy. It is, and has some flex built in especially in the back end. Makes it great on dirt, and a very adaptable ride. It's Spirit and Zona, and I'll keep it forever.

For the Firefly, I wanted a stiffer front end, a stiffer BB area and back end, and a more locked in feel. When I rode with them we decided to go with a more oversized head tube and oversized chain stays.

Three years in, the Firefly is both incredibly stiff but also very smooth. The butting and tube manipulation and shaping is fascinating, and I never detect any real flex, but at the same time I feel very fresh after a few hours given its stiffness.

As a reference -- 6'3, 215, significant muscle mass.

So I think ti can be made as stiff as you may want it. For me, my ti bike is much stiffer than my steel bike...

[bicycletricycle]

One on my main "tests" for "measuring" "torsional rigidity" is as follows. Hold onto the bars tight while sitting in the saddle. Try to flex the saddle side to side with your rear while countering that flex with your bars. The two Ti bikes I have really move around a lot when I do this, they also feel a bit "loose" when cornering or changing direction quickly. This would be in comparison to my big tubes steel bikes.

It is not really a huge problem but I don't prefer it.


Modulus of elasticity aside...I'm not sure what is meant by "flexy." What does the OP have in mind? My perspective is a bit weird, since I went from a 1974 steel bike to a 2015 Ti bike, skipping over many generations of design and metallurgical development. One thing I've noticed is that once you reach a high quality frame build, it is difficult to tease apart wheel/tire effect from frame effect. Highly inflated tires on stiff wheel make for an unforgiving (but efficient on some roads) ride. In general, I found that the steel frame tends to dance a little on rough roads, whereas the Ti absorbs bounces and keeps the tires on the pavement. Does this smoothing out the ripples mean it is more flexy? Both are stiff laterally--no chainrub on the front der when pounding it; no discernible deflection of the bottom bracket, or shimmy-ing on braking. My Ti has almost the same road feel as steel, but a little smoothed out.

[Brian Smith]

Thanks! So in this case, we have the following (assuming average 0.7 mm thickness for the steel tube):

EI-Al / EI-steel = 0.61
EI-Ti / EI-steel = 1.14

So your perceptions of the Ti frame having the stiffest drivetrain appear to be backed up by the numbers :) As another point of reference, for the aluminum tubes to have the same stiffness as the steel ones (with a 1.4 mm wall thickness), the minor oval axis dimension would have to be about 20 mm instead of 16 mm.

That said, this is a gross over-simplication of a complex process, because drivetrain stiffness does not come only from the chainstays (though Dave Kirk argues that chainstays are the primary factor).

You'd be better off choosing a wall thickness for the calculations that is closer to the higher value, rather than the average, as the bending loads of interest between the wheel and the chainstay are largest where the chainstays join the BB.

Also, regarding the chainstay stiffness needed to avoid tire rub (the point of the OP) does result chiefly from the chainstays. Added frame rigidity due to the other tubes joined at the BB could actually exacerbate the problem of a chainstay flexing under "transverse" bending loads rather than remedying its effect.

[Brian Smith]

Question- (snipped) I know some modern titanium frames have very large main tubes (especially down tubes) but on a large tired bike it seems like it might be tough to fit a large enough chain stays in.....

Anyways, what say the braintrust?

A willingness by the builder to employ a part, in titanium, into the traditional space, that is heavier, not lighter, than its steel counterpart.

[cmg]

I have a Lynskey R230 and a Kish custom both TI. The R230 has an asymmetrical non-drive chain stay (larger diameter that doesn't match drive side) and an ovalized down tube. It also has a shorter reach than the Serotta legend I had at the time. Sold the Serotta. The Kish just has larger tubes in general. When Kish built it I was coming off a steel Cinelli, really a noodle and a Colnago MIX that was super stiff so those were my references. large diameters will increase stiffness. Somebody in the riding group showed up on a Colango c60, really large carbon tubes, do that with Ti it will be stiff.

[duff_duffy]

This one is not flexible, huge downtube helps;). Will be on the classifieds soon!

[hobbanero]

My Moots RSL, with oversized Ti tubes, is pretty stiff. Too much, in some ways. Relative to my more conventional Independent Fabrications, the Moots loses a lot of the magic Ti ride quality. I decided if I am going to have stiff, I might as well go carbon and save some weight (at the cost of long term durability).