Racy frame design - long chain stays - WHY?

[compulsive]

I am neck-deep in trying to learn all I can about frame geometry and came across some comments in an article that surprised me.

The author likely isn't quoting Darren Baum but the numbers are precise enough that it seems fairly credible. I have underlined and highlighted the bits that caught my attention.

In Darren’s opinion, longer chain stays help deliver a better quality ride. When you climb, the bike has more traction. When you go through a corner, the bike trails further and you can hop on the pedals earlier. The intended use of the bike is a big consideration however.

When chainstays started getting shorter throughout history (late 60′s, early 70′s), it was when riders started getting more powerful and the technology didn’t exist to make the materials strong enough for the desired stiffness. Therefore the chainstays were designed shorter in order to make the bike stiffer. A good bike was considered one that you could barely fit a Tally-Ho cigarette paper in between the rear wheel and the seat tube. This design had nothing to do with handling. It was all about making the bike stiffer. People started identifying this small rear triangle as a “race bike”, and therefore a race bike must handle better. This never changed as materials progressed.

These days the materials exist to make a long chainstay that is still very stiff. However if you have a longer rear end, the bike naturally needs to be manufactured with more material and therefore will be heavier. These days in the industry there is a race for the lightest spec’d bike. What does Baum do with their bikes? They recommend making the rear end as long as acceptable by the customer.

If you’re flexible and can bend forward, 412mm is what Baum will recommend. If the rider sits more upright, the chainstay might go as long as 420mm. If the rider is really tall (i.e. over 6’3″), and the femer is very long, 430mm might be required. The reason for this starts to relate back to seat tube angle and pedalling technique, however I think we might leave that one for another discussion.

Since I am in the process of spec'ing a custom frame now is a good time for me to understand the pros and cons of chain stay length! I am 6'2" tall with less-than-average flexibility but I have never ridden a frame with chain stays longer than 415mm. Wouldn't 430mm chain stays give a slow handling bike, partly due to the long wheelbase?

Any insight would be appreciated.

[e-RICHIE]

What doe your vendor suggest for a meaurement? And PLEASE, for the sake of clarity, realize that the blueprinted measurements will differ from those you take with a tape measure once a bicycle is made. A 410mm blueprinted stay can yield a 420mm result on a finished unit. All in all, I personally like longer, ESPECIALLY on frames larger enough to fit someone 6'3" atmo. On my road frames, I'd spec a 420mm stay that, once delivered, would look a cm longer.

I am neck-deep in trying to learn all I can about frame geometry and came across some comments in an article that surprised me.

The author likely isn't quoting Darren Baum but the numbers are precise enough that it seems fairly credible. I have underlined and highlighted the bits that caught my attention.



Since I am in the process of spec'ing a custom frame now is a good time for me to understand the pros and cons of chain stay length! I am 6'2" tall with less-than-average flexibility but I have never ridden a frame with chain stays longer than 415mm. Wouldn't 430mm chain stays give a slow handling bike, partly due to the long wheelbase?

Any insight would be appreciated.

[compulsive]

Thanks for the reply. I am still going through the numbers with the builder but we are currently @ ~415mm.

I can't quite grasp how a spec'd/blueprinted 410mm chain stays can end up at 420mm. What am I missing here? Isn't the final product (here 420mm) the only relevant number and why would it differ from the millimeter-perfect blueprint?

What doe your vendor suggest for a meaurement? And PLEASE, for the sake of clarity, realize that the blueprinted measurements will differ from those you take with a tape measure once a bicycle is made. A 410mm blueprinted stay can yield a 420mm result on a finished unit. All in all, I personally like longer, ESPECIALLY on frames larger enough to fit someone 6'3" atmo. On my road frames, I'd spec a 420mm stay that, once delivered, would look a cm longer.

[Will Neide]

Thanks for the reply. I am still going through the numbers with the builder but we are currently @ ~415mm.

I can't quite grasp how a spec'd/blueprinted 410mm chain stays can end up at 420mm. What am I missing here? Isn't the final product (here 420mm) the only relevant number and why would it differ from the millimeter-perfect blueprint?

410-415 is a good range, IMO. Stiffness is going to be driven more by the choice in chainstay diameter/wall thickness with-in that given chainstay length than the difference in 5mm of overall length. I've raced a 430mm chainstay'd bike many times on the road (crits and road) and never had an issue with slow steering.

Not sure about the deviation from spec though. If you expect something to be 415 from the center of the BB to the center of the dropout, I would expect a tolerance of around a mm, if that.

[abbeyQ]

What doe your vendor suggest for a meaurement? And PLEASE, for the sake of clarity, realize that the blueprinted measurements will differ from those you take with a tape measure once a bicycle is made. A 410mm blueprinted stay can yield a 420mm result on a finished unit. All in all, I personally like longer, ESPECIALLY on frames larger enough to fit someone 6'3" atmo. On my road frames, I'd spec a 420mm stay that, once delivered, would look a cm longer.

Do you build off horizontal chain stay length? I can't see how else your chainstays would grow.

[veryredbike]

I think that e-richie is talking about the difference between the drawing and the physical stay. On the drawing it's a straight line through the center of the bike (center of BB to center of axle). When someone who bought his bike measures it, they do it from the crank to the dropout... which is necessarily longer because it makes up the same distance front-to back but with a path that ends up 65mm from the center. Based on the tone, I'm guessing that the client then calls him and complains that he built their bike wrong because they're not measuring it correctly ;-)

Is that about right? If not, give the word and I'll say my 50 atmo's.

[e-RICHIE]

Thanks for the reply. I am still going through the numbers with the builder but we are currently @ ~415mm.
I can't quite grasp how a spec'd/blueprinted 410mm chain stays can end up at 420mm. What am I missing here? Isn't the final product (here 420mm) the only relevant number and why would it differ from the millimeter-perfect blueprint?

Do you build off horizontal chain stay length? I can't see how else your chainstays would grow.

veryredbike is correct (I think...). The fixture/tooling is set up at a measurement which is the virtual center line. When a bicycle is built and you measure from the bb axle along the stay to the qr part, you get a different measurement. Back to the OP, which method or measurement is he obsessing over atmo?

[P K]

It seems like you're talking about 2 legs of the triangle created from the bb back to the dropouts. One is the longer hypotenuse, from the outside along the stay, and the other shorter from center of bb to center of dropouts.

[e-RICHIE]

It seems like you're talking about 2 legs of the triangle created from the bb back to the dropouts. One is the longer hypotenuse, from the outside along the stay, and the other shorter from center of bb to center of dropouts.

Yes - and frame set-ups and blueprints are typically the latter (or - the virtual center line) atmo.

[Will Neide]

I think that e-richie is talking about the difference between the drawing and the physical stay. On the drawing it's a straight line through the center of the bike (center of BB to center of axle). When someone who bought his bike measures it, they do it from the crank to the dropout... which is necessarily longer because it makes up the same distance front-to back but with a path that ends up 65mm from the center. Based on the tone, I'm guessing that the client then calls him and complains that he built their bike wrong because they're not measuring it correctly ;-)

Is that about right? If not, give the word and I'll say my 50 atmo's.

Yea, but the BB width is 68mm (34mm for half), and if you do all the calculations, a 415mm CS would be roughly 417mm taking the angle into account. This is without the crank installed, which would only push your ruler further towards parallel if it were installed. Any customer with enough skills to use a ruler would be happy with 417, but probably not 425.

[e-RICHIE]

Yea, but the BB width is 68mm (34mm for half), and if you do all the calculations, a 415mm CS would be roughly 417mm taking the angle into account. This is without the crank installed, which would only push your ruler further towards parallel if it were installed. Any customer with enough skills to use a ruler would be happy with 417, but probably not 425.

If I set up my Bike Machinery Hydra, a tool I consider for professional use only (sic), and the virtual length of the chainstay is 410mm, and then I measure the actual frame that comes out when complete (using a rule laid across the stay from the bb to the outside of the hub axle), the distance is 420mm. That's all I have to offer.

[afwalker]

Is this that pesky The Law of Cosines ?
Sort of the Pythagorean theory on steroids, solve the length knowing 2 sides and some angles?
It is a great mental exercise to get rid of some cobwebs in the old brain, although do Millennium kids even know what a cosine is? No, not your cousin:)
cheers
andy walker

[compulsive]

Got it - crystal clear now! I hadn't taken into account the extra distance added measuring a built bike on an angle with the BB+crank and QR fitted.

So back to my original question, for an aggressive race-type frame for a tall rider, are there any really strong arguments for going with short (~410mm) vs long (~430mm) chain stays?

If I set up my Bike Machinery Hydra, a tool I consider for professional use only (sic), and the virtual length of the chainstay is 410mm, and then I measure the actual frame that comes out when complete (using a rule laid across the stay from the bb to the outside of the hub axle), the distance is 420mm. That's all I have to offer.

[e-RICHIE]

So back to my original reply -
It depends which measurement you are using, the blueprint OR OR OR the one yielded when the bicycle is assembled atmo.
PS who's the builder and why would you not simply trust him?

Got it - crystal clear now! I hadn't taken into account the extra distance added measuring a built bike on an angle with the BB+crank and QR fitted.

So back to my original question, for an aggressive race-type frame for a tall rider, are there any really strong arguments for going with short (~410mm) vs long (~430mm) chain stays?

[compulsive]

So back to my original reply -
It depends which measurement you are using, the blueprint OR OR OR the one yielded when the bicycle is assembled atmo.
PS who's the builder and why would you not simply trust him?

The numbers I was suggesting (~410mm and ~430mm) were the blueprint numbers.

I didn't mention the builder because the question really wasn't meant to doubt their approach or 'throw them under the bus', I just wanted a better understanding of why a highly regarded builder (Baum) might suggest chain stays that are ~15mm longer than why my builder spec'd :) Perhaps the end result wouldn't add up to much and I understand that the geometry needs to work as a complete picture, not just one measurement.

These have been my first posts on this forum and I am very impressed with the fast and educational feedback - thanks!

[e-RICHIE]

Like I wrote, longer is better. longer will play more friendly with shallow seat angles, wider tires, 11 speed cassettes, cross over gears, flimsy chains, and with the combination of misalignments that your frame will have. If yours doesn't have misalignments, count yourself lucky. And when I say "longer" I only mean within the range you already mentioned.

The numbers I was suggesting (~410mm and ~430mm) were the blueprint numbers.

I didn't mention the builder because the question really wasn't meant to doubt their approach or 'throw them under the bus', I just wanted a better understanding of why a highly regarded builder (Baum) might suggest chain stays that are ~15mm longer than why my builder spec'd :) Perhaps the end result wouldn't add up to much and I understand that the geometry needs to work as a complete picture, not just one measurement.

These have been my first posts on this forum and I am very impressed with the fast and educational feedback - thanks!

[compulsive]

Longer is better is now etched into my mind

I don't doubt that this custom frame will be less than millimeter-perfect but it should still carry me down the road as fast as my legs and lungs will allow.

Like I wrote, longer is better. longer will play more friendly with shallow seat angles, wider tires, 11 speed cassettes, cross over gears, flimsy chains, and with the combination of misalignments that your frame will have. If yours doesn't have misalignments, count yourself lucky. And when I say "longer" I only mean within the range you already mentioned.

[Gattonero]

Like I wrote, longer is better. longer will play more friendly with shallow seat angles, wider tires, 11 speed cassettes, cross over gears, flimsy chains, and with the combination of misalignments that your frame will have. If yours doesn't have misalignments, count yourself lucky. And when I say "longer" I only mean within the range you already mentioned.

From a non-framebuilder: put into accounts chainstay length, there are in fact small things that would easily make the FD working tits up, and that is just one example

[ron l edmiston]

--- educated with a smile at a cr gathering in greensboro, years back --- to "trust the man who wears the star.."

uncle milty, miss kitty & ronnie

[steve garro]

There is effective CSL and there is actual CSL, and there is cut length.
Three different concepts.
- Garro.