Centimeters & Degrees - Help me solve the equation

[InspectorGadget]

Once upon a time I succumbed to the siren song of a beautiful 56cm Wilier frame at a price that was too good to pass up. Unfortunately, like a pair of shoes that are on sale, it doesn’t really matter how cheap they are if they don’t fit your feet. I thought I could make the Wilier work, but a 56cm is just too big for me. I knew better then and I haven’t forgotten that lesson and thus this thread.

Bike fit gurus, help me understand how bikes A, B & C listed below might or might not fit me compared to my Colnago.

My Colnago:
90mm stem with a lot (35mm) of spacers
Top Tube: 545
Seat Tube: 550
Head Tube: 126
Head Tube Angle: 73 (estimated)
Seat Tube Angle: 74

A.) bobswire's Seven Axiom - Will the steeper seat tube help overcome the longer top tube, or is it just too long???
Top Tube: 558
Seat Tube: 550
Head Tube: 148 (I won't use as many spacers under the stem)
Head Tube Angle: 73
Seat Tube Angle: 75

B.) athenasoar's IF SSR
Top Tube: 550 (effective)
Seat Tube: 480
Head Tube: 135.5 (again, fewer spacers)
Head Tube Angle: 73
Seat Tube Angle: 73.5

C.) Hampsten Crema - semi custom (craigslist)
Top Tube: 545
Seat Tube: 525
Head Tube: 116 (heaven forbid, more spacers?)
Head Tube Angle: 72.5
Seat Tube Angle: 73.5


Thanks for educating my ignorant self.

[Smiley]

Ur Nago could fit you better so start with getting a fit measurement from a fitter using a size cycle mimicing the sizing of each of these bikes. That will tell you which one will work best.

next u omitted Stand Over at the TT where the head tube meets which is key to the sapecer count. Now let the peanut gallery chime in.

[Dave]

The simple rule that's not perfectly accurate, but adequate most of the time is simple. Each degree that the STA is steeper will increase the reach by about 1cm. The more accurate calculation is (CosA-cosB) times the c-c frame size.

The head tube angle is rarely of concern.

Using the rule above, bike B has .5 degree less STA, so that reduces the reach to be nearly identical to your Colnago.

Head tube lengths are straightforward, unless you're comparing frames with different type of headsets. Frames with integrated headsets have headsets that only contribute 8-20mm to the stack height, while conventional headsets are in the 25-35mm range.

[InspectorGadget]

next u omitted Stand Over at the TT where the head tube meets which is key to the speacer count. Sorry, stand over where the top tube meets the head tube is 800mm/31.5 inches.


Fow what it's worth, I have been fit on the Colnago. I'm not saying it's perfect, but I have been through the process.

[InspectorGadget]

Using the rule above, bike B has .5 degree less STA, so that reduces the reach to be nearly identical to your Colnago.What about Bike A?

[Mark McM]

A.) bobswire's Seven Axiom - Will the steeper seat tube help overcome the longer top tube, or is it just too long???.

The steeper seat tube angle will push the top tube forward (relative the BB), so for the same saddle setback (rearward offset of the saddle behind the BB) the steeper seat tube angle would lengthen the effective cockpit length (reach to the handlebar). So, it would make the problem of the longer top tube worse, not better. For the same handlebar reach, you'd need a stem approx. 25mm shorter than your current stem.

In the case of B), the slightly shallower seat tube angle will pull the top tube backward, decreasing the cockpit length. For a nominal 550mm seat tube length, each degree change in seat tube angle changes the set back by about 10mm, so the 0.5 degree shallower seat tube angle will have a similar affect on cockpit length as decreasing the top tube length by 5mm, so with the same saddle setback position this frame would have approx. the same reach as the Colnago.

In the case of C), which as the same seat tube angle but 5mm shorter top tube as B), for the same saddle setback this bike would shorten the reach by 5mm.

As far as handlebar height, there's not quite enough information to go on, as in addition to the headset length, headset stack height, spacers and stem angle, the BB drop and fork length also affect saddle-to-handlebar height difference. But assuming that they have the same BB drop and fork length, than as you imply, the increased head tube length of B) could be compensated for by removing some spacers under the stem, and the decreased head tube length of C) would have to be compensated for by a more spacers or a stem with a more upright angle.

Of course, there are other geometry factors that can affect handling, so these would have to be considered as well when determining if a frame fits you well.

[peanutgallery]

Bike A is probably too big and Bike C may be too small. Just keep the IF and send me the Seven as it would be a good fit

[cody.wms]

The simple rule that's not perfectly accurate, but adequate most of the time is simple. Each degree that the STA is steeper will increase the reach by about 1cm. The more accurate calculation is (CosA-cosB) times the c-c frame size.



I use this as a general thumb, too. So, here is the effective top tubes IF the above bikes magically had uniform 74 degree seat tube angles:

Colnago- 545
Seven - 568
Indy Fab- 545
Hampsten - 540


but, yeah, this is only part of it. BB drop, fork length, head tube length, etc. have alot to do with it. IMO, since you hold onto bikes for so long, and know what you like, I'd get something built for you.

[TAW]

Your Colnago has a different way of measuring than some of the other choices as well. A 55 Colnago is measured from BB center to top of seat post, thus a
55 has a 54.3 top tube.

[InspectorGadget]

In other words, Bob's Seven is not going to make its next home in my garage. Too bad, really.

Thanks to all you rock stars, I appreciate your help.

[ultraman6970]

I'm with cody... I almost all the time use the top tube to compare them but another way to compare them is the reach of the bike.

As stepper the seat tube angle the longer the reach, somebody mentioned it already.

Some people uses the stand over, well u can't compare bike using the stand over because te stand over is in relationship with the BB height, as higher it is the higher the top tube and higher the stand over, besides new bikes now a days use really short front tubes so the TT gets lower than what it suppose to be and if you go using the stand over you could end like 2 sizes bigger than the size that should be.

Either way all the bikes you have in there are around the same size. The only one that I would be careful with is the axiom, that frame is like 5 mm to 1 cm longer in reach than the other ones. The axiom should be the fastest one of all tho, but i would be careful to ride a century with it also, well is titanium so it would not be a problem.

A picture of your colnago bike could help also.

Hope this helps.

[Smiley]

Sorry, stand over where the top tube meets the head tube is 800mm/31.5 inches.


Fow what it's worth, I have been fit on the Colnago. I'm not saying it's perfect, but I have been through the process.


Inspector,
The nago is stock so yes they made this bike fit u :)

[Dave]

What about Bike A?


I give people enough info that I assume that can figure out other comparisons without further help. Using the info that I gave, a 1 degree steeper STA would add another 10mm to the already longer TT length.

I did forget to mention all of the other complications that come with head tube length comparisons. They don't take into account the fork length or the BB drop. Most often, stock frames have a BB drop that's close to 7cm, but there are exceptions, like Serotta, where 8cm is used quite often. If the BB is lower, then the head tube can be shorter and still produce the same saddle to bar drop.

About your Colnago. Based on the stem length and spacers, the head tube is too short and the reach is too long. The stem angle wasn't mentioned.

[cody.wms]

C.) Hampsten Crema - semi custom (craigslist)...
Head Tube: 116 (heaven forbid, more spacers?)


the Crema is designed around 57mm reach brakes, and has a longer fork, making the head tube shorter for a given effective handlebar-to-ground reading (effectively making the head tube longer than its nominal size).

[eddief]

how a steeper seat tube angle creates longer reach? it seems to me so logical that if the seattube is more upright (let's say 74 compared to 72.5) then the rider would be closer to the headtube = shorter reach? 74=more straight up, 72.5=lean more backward=longer reach?

help.

[Dave]

how a steeper seat tube angle creates longer reach? it seems to me so logical that if the seattube is more upright (let's say 74 compared to 72.5) then the rider would be closer to the headtube = shorter reach? 74=more straight up, 72.5=lean more backward=longer reach?

help.


Frame comparisons, done properly, always assume that the rider is in the SAME position relative to the BB. A rider should not be forced to adopt a different position by a frame. The saddle is moved further back on the seatpost, if the STA is steeper, to produce the same setback, relative to the BB.

[InspectorGadget]

how a steeper seat tube angle creates longer reach? it seems to me so logical that if the seattube is more upright (let's say 74 compared to 72.5) then the rider would be closer to the headtube = shorter reach? 74=more straight up, 72.5=lean more backward=longer reach?

help.
I got this one.

If you plug the same length top tube between the seat tube and head tube, the steeper angle seat tube is going to push the steer tube further away from the saddle and vice versa.

I'm sure there is a better way to explain it, but that's what I took from the explanations above.


Dave - I posted the thread because I needed remedial frame geometry and fit lessons.

[Mark McM]

how a steeper seat tube angle creates longer reach? it seems to me so logical that if the seattube is more upright (let's say 74 compared to 72.5) then the rider would be closer to the headtube = shorter reach? 74=more straight up, 72.5=lean more backward=longer reach?


Think about the classic carrot and the stick -

A boy is sitting on a donkey, but the stubborn donkey doesn't want to move. So the boy ties a carrot to the end of a stick and waves it in front of the donkeys face. The donkey trots forward trying to get the carrot, but as much as he keeps trotting forward, he can never reach the carrot. Why? Because the carrot is attached to the stick, which is held by the boy, who is sitting on the donkey's back - no matter how much the donkey moves forward, the carrot is always a a stick-length away.

In the case of the bicycle, the handlebars (which we are trying to reach) is the like the carrot, and the top tube is like the stick. When you steepen the seat tube angle, the saddle moves forward - but no matter how much the saddle moves forward, the stem and handlebars are always a top-tube-length away.

But it gets worse - we want to the saddle to be positioned fore-aft relative to the bottom bracket and pedals, so if the seat tube angle gets steeper, we have to slide the saddle back on the rails to compensate and keep the saddle in the same place with respect to the BB/pedals. So the net affect is that rotating the seat tube to a steeper angle just pushes the handlebars further away from the saddle.

[InspectorGadget]

I was right, there is a better way of explaining it.

Very nice Mark! :beer:

[cody.wms]

Ok, here is my not to scale drawing:

http://farm6.static.flickr.com/5027/5735072742_7a67fa6a55_z.jpg (http://www.flickr.com/photos/52811793@N02/5735072742/)
Untitled (http://www.flickr.com/photos/52811793@N02/5735072742/) by Cody Wms (http://www.flickr.com/people/52811793@N02/), on Flickr

Two bikes, both end up effectively the same. On paper, one has a 54 top tube (and a 72 degree seat tube angle) the other a 52 tt (and 74 degree seat tube). But the saddle is in the same place, and the reach (what matters) is the same.

If you have two bikes with 55 top tubes, the steeper seat tube angle will result in a longer reach (IF the saddle stays in the same place). Moving the saddle forward to replicate a reach number will change how you sit on the bike, sometimes dramatically.

And this is one if a dozen things we need to nail down to begin to compare two bikes. To me, this is why I bought a custom (A Hampsten Steve is currently prepping for shipment). A builder can take your fit info and build something that works for you, and with you.

[John H.]

none of them fit you that well.
Can't really go longer given you are on a 90mm stem.
Can't really go lower either.
If anything you need same length or shorter bike with as tall or taller front end.

[eddief]

Ok, here is my not to scale drawing:

http://farm6.static.flickr.com/5027/5735072742_7a67fa6a55_z.jpg (http://www.flickr.com/photos/52811793@N02/5735072742/)
Untitled (http://www.flickr.com/photos/52811793@N02/5735072742/) by Cody Wms (http://www.flickr.com/people/52811793@N02/), on Flickr

Two bikes, both end up effectively the same. On paper, one has a 54 top tube (and a 72 degree seat tube angle) the other a 52 tt (and 74 degree seat tube). But the saddle is in the same place, and the reach (what matters) is the same.

If you have two bikes with 55 top tubes, the steeper seat tube angle will result in a longer reach (IF the saddle stays in the same place). Moving the saddle forward to replicate a reach number will change how you sit on the bike, sometimes dramatically.

And this is one if a dozen things we need to nail down to begin to compare two bikes. To me, this is why I bought a custom (A Hampsten Steve is currently prepping for shipment). A builder can take your fit info and build something that works for you, and with you.

so it starts to make sense when "reach" is a constant and when combined with "setback." as in, "i need my reach to be this long and i need my setback to be this distance. what combo is gonna get me there?"

a diagram is worth a thousand words and then the extra 50 words of explanation helps too.