Kent, IMO the damping factor associated with materials like steel is very very low, which in this case is a very good thing. The best way to test the results IMO is on the road as Dave Kirk suggests.

I want to back Dave up here. Not so many years ago now, I was interested in the same thing as Dave but didn't venture as far outside the box as he did. I simply bent up a bunch of wildly s-bent stays and then indeed tested them statically. There was about a repeatable 2-3mm difference between straight stays and the most radical of the S-bends under the same load, depending on the stays used. I have no problem believing Dave's 6mm claim. It's quite a trick to be able to preload the stays properly to work under a given rider's weight without overstressing the joints, but based on Dave's customer's feedback, he obviously has it dialed.

+1

Its been a great thread and extremely informative!

Im just curious as to what the experts have to say about the chainstays on the Big Leg Emma in relationship to effeciency,stiffness etc.

Anyone care to give that a gander?

Thanks in advance,

Steve

the expert here is the user atmo.
what do you think?

Because of this thread I just had to spent last evening ooh-ing and ah-ing over curly-stayed Hetchins over the years. Thanks for the memories. :)

And thanks to the little birdies who gave me some historical factoids on seat stay development and put some things in perspective. The list of possible frame builders just got shorter.

Thanks, Dave. I figured you must have done some of this testing at some point and am glad you responded. Lots of people say they can feel the effect but many swear a carbon post makes a bike feel more comfortable. I feel better knowing the movement can be quantified. I REALLY doubt my Seven Axiom's curved seat stays add any comfort, regardless what the company says but figure yours would if anyone's does due to the magnitude of the bend. I am also interested in audio and it's amazing what people claim to hear as long as they know what is playing. Once the blinders go on all bets are off.

Tim McTeague

will this thread convince richard sachs to start making curvy stays???

Steve, I’m not an expert or bike builder, but will give you my 2-cents strictly from a technical standpoint as it relates to loading (not specifically addressing Big Leg Emma except as noted below) to help you draw your own conclusions.

Chainstays mostly have to resist chain tension in compression (more so on the drive side) and lateral bending due to chain load being off-center. Compression loads due to chain tension are relatively minor (particularly on road bikes due to higher gearing) and can be resisted fine by most stays. However, as discussed by the experts above, when chain tension tries to bend the chainstays laterally, it is important to resist motion in that direction (i.e. – control rear wheel yaw) or the back wheel can wag from side to side like a dog’s tail. And obviously if that happens to an appreciable amount you essentially introduce rear wheel steering which adversely affects the way the bike handles and may also introduce poor drivetrain efficiency (i.e. – flexibility may reduce pedaling power). To make the chainstays stiffer, you can increase their size to make them stiffer in bending (side to side direction), make them shorter, affix the axle end so as to load the chainstays differently, or by increasing the number of stays. Or you can use a combination of all of these.

Beefing up seatstays is another matter that I can’t quite follow. The main function of seatstays from my perspective is to support the rider’s vertical weight, to control rear wheel roll motion, and to provide a mounting place for the rear brake. IMO since straight stays are plenty stiff in compression regardless of how lightweight they are made, I see no advantage in making them heavier unless: 1) they are anticipated to buckle under expected load or; 2) they flex excessively in bending when the rear brake is applied (I’ve heard of this happening more so on tandems). Outside of meeting those functional requirements, making seatstays stiffer is beyond me -- I don’t see how it would improve anything unless taken to unrealistic extremes.


Regarding the Big Leg Emma, placing a horizontal strut inside a tube is an interesting solution, but I honestly don’t get it unless it’s for nostalgic reasons -- seems like a lot of work. If butted tubing was not possible, then I could see it as a means to reinforce a tube locally without introducing added weight throughout the entire length. However, with present tubing technologies it seems that custom butting would be easier and more effective. Do any of you know why it was done back in the 60s? Perhaps some here could shed some historical light on this technique. Personally I would love to learn more about it since I find it intriguing.

some of the coolest frames ever made were the lugged
serottas with the curvy stays and the oval by round top
tubes, and the biconical down tubes. i don't care how
they work (but i do know they work perfectly), they
were and still are state of the art atmo.

I have 25 pair of NOS Serotta curvy chainstays in a box in the shop if you're interested. :beer:

i have a gig.
put them on the frame listserve or frameforum.net
so that folks who need an edge can use them atmo.

OK... well, I'm way out of my area of expertise here, so I'm not sure what you mean by stating that the damping factor is very very low. Further, I have no idea why that would be a good thing.

The point I was trying to make is that I believe you can analyze the rear triangle of the bike as a sping + mass. The spring constant, to first order, is going to be the seat stay. In the case of a straight stay, the constant is so high that no appreciable deflection occurs at the rear axle during typical riding. In the case of a terraplane or dks stay, the constant is intentionally lowered to the point where rear axle deflection occurs during normal riding.

Having said that (assuming its a valid comparison), once you deflect the rear axle upwards, you have stored energy into the spring (stay), and you need to dissipate that energy (damp the system), otherwise the system will resonate.

When you ride the bike, if you hit a sharp edge of pavement or what not, you are essentially applying an impulse or step input to the spring+mass system. The response of the spring+mass is going to either be underdamped, overdamped, or critically damped depending on the parameters of the spring+mass.

In the DKS seat stay, was the elastomer providing the damping for the system or was it providing some other function?

Just trying to understand...

Kent

Thanks for the sound advice, Richard, I appreciate it!

Yo Atmo, I gotta' say, this DWF cat is a boring mofo. All bidness.

Shut up Archi!

any time e-DON. i'm here for ya'.
but remember: i said i liked the total package,
not just the chainstays atmo. gestalt bro.

Not sure if anyone knew, so I just thought I would
add that Bicycle Quarterly recently tested a Trek
spa elastomer road bike with a claimed 1/2 inch
of travel. Both Jan and his fellow tester found it
to actually work/be of use. They felt that even
with wide tires (28c roly poly's) they could still
feel the difference the added travel made in the
ride. The main issue being that there was some
unwanted pogo-ing at times- which may very
well be addressed according to Trek with
different elastomer gauges. So indeed it would seem
that travel can be of use/added comfort for those
who want it.
For anyone who wants to read the article here's
the issue (article bolded):
Vol. 4, No. 4 (Summer 2006)
1 Peter Rich: A Pioneer of American Road Racing
4 Readers’ Forum
5 Book Review: Lightweight Cycles Catalogues Volume I
5 Web Resources: EBykr.com
6 Book Review: "Flying Scotsman" by Graeme Obree
7 Project: Overhauling a 1920s Hirondelle Retro-Directe
8 Riding with Classic Derailleurs – Part 2: Racing Derailleurs
23 Eric Svoboda's Mystery Randonneur Bike
26 To Restore or Not to Restore – That is the (Often Difficult) Question
28 Respecting the Maker's Vision
29 Randonneuring Basics, Part 2: Enjoying the Scenery
30 Spring-Time Adventure in the Cascade Mountains
32 Builders Speak: Mark Nobilette on Making Custom Lugs
34 Test: Ebisu All-Purpose – All Details Considered
38 Test: Trek 2100C – Suspension for Comfort and Speed

torsion
 

It seems that controlling pedaling input and rider weight are just part of the cs/ss/dt's functions. Additional considerations are when there is no rider input but the bike is banked in a sharp turn. Here there are lots of torques and everything needs to resist twisting. Much harder than handling pure compression.

The OS downtubes are there not just to stiffen up the bottom bracket, but to keep the angular deflections down between front and back wheel, along with the chainstays of course.

Just my guesses. Haven't touched a finite element package since school.

Spring-mass systems have no damping and hence will be completely underdamped, like a tuning fork vibrating forever in an ideal sense. Damping in the system comes from the tires, air in the tires, and the rider's body, which is why they tell you not to lock your elbows. Adding materials like elastomers and/or composites will increase your damping, thus reducing the amplitude and settling time to impulse inputs. in theory, at least.

Is anyone else on here having trouble reading
this thread on their browser? (I'm using Firefox)
My computer is displaying the threads rather
haphazardly, that is out of order todays posts
before yesterday posts etc?
Would like to hear if anyone else is having
this trouble. JL

It appears to be in sequence on my machine. I'm using Firefox 2.0.

Kent, I’m not sure I can help, but will gladly express my opinion on the subject.

I was referring to the fact that the natural damping of real engineering materials is quite small. For some steels, it's in the range of 0.0006, which means that for all practical purposes any energy that is stored by a steel spring will be returned. The natural damping of aluminum, for example, is even lower. IMO real material “damping” is insignificant, and should not be confused with what most people report as a smooth or damped ride.

The reason I stated that IMHO low damping is a good thing is a little complicated to express without art (I tried that a couple of weeks ago and was shot down). Anyway, before I even try to describe my reasoning, I want to iterate that in my opinion, road and MTB suspensions are two different animals and shouldn't be compared as one.

On a road bike we are dealing mostly with forced vibration as is the case on an MTB; but IMO most of the interest is in reducing high-frequency low-amplitude buzz associated with rough surfaces; and not that associated with jumping over rocks, logs, etc… which we seldom do on road bikes. Having said that, damping is mostly useful when the forcing frequency is around the range of the suspension's natural frequency (i.e. – damping can limit oscillation amplitude when the system resonates) – but much below and above that it doesn’t buy much if anything. In fact more damping than optimum for a given condition is often counterproductive.

Logically, we can predict that any bike with a low-enough suspension spring constant to do any good (like Terraplane, DKS and others) will have a lower natural frequency that the forcing frequency of a few 100 cycles per second that an accomplished rider will often experience at or above 20 MPH. Because of this we can conclude that no or very little damping will be beneficial. It will allow the wheels to oscillate up and down over the road's imperfections without giving the rider enough time to move up and down himself -- hence what we feel as smoothness or a smoother ride.

Obviously, the lower the spring constant of the suspension, the easier it is for the wheels to ride up and down over imperfections on the road without having enough time to affect the rider to the same degree. Also, lower spring constant lessens shock forces when the bike strikes objects like rocks or pot holes.

If you try to run the math on curved seatstays acting as springs, don't forget that it gets complicated because the springs (i.e. -- seatstays) do not behave as linear or progressive springs as is the case in most vehicle suspensions. The more they bend the lower the spring constant. Where it gets complicated is in the rebound phase because the spring constant increases and you can experience a catapulting effect.