relative stiffness among main tubes

[RonaTD]

Hi all - looking at old frame tubing specs, I see that for a long time Reynolds tended to sell sets with .1mm thicker walls in the down tube than in the top and seat tubes. So, a typical classic 531 frame was TT 1" .8/.5/.8, DT 1 1/8" .9/.6/.9, ST 1 1/8" .8/.5. Columbus, on the other hand, was typically .9/.6/.9 all around. Current day it seems that everyone (including Reynolds) just uses the same wall thickness all around.

The effect is that the relative stiffness among the tubes is different in the classic Reynolds frame than in others. I can't find any discussion of this on the web in general or here, though it's possible I needed to use some different search terms. Anyone with some longer history know whether this has been discussed or studied? I've read a pretty exhausting (not necessarily exhaustive) amount on whether "stiffness matters". I'm more interested in the question of whether relative stiffness between the tubes of the main triangle might matter in a frame where some flex is anticipated. Do any builders use stiffness ratios between tubes to tune flex characteristics to the rider, riding style, and where additional gear may be carried?

Ted Durant
Milwaukee, WI USA

[Andrew R Stewart]

Back in the day I doubt that any builders ran the math to decide which wall thickness went where, instead relying on their and others' experience. Mixing tubes was (and still is) done by many builders. This is one reason why some of our mentors didn't apply tubing decals on their builds. Don't forget that tube walls can also be different in the blades and stays and that there's a shape difference between the Continental and the "classic" Reynolds fork blades. Taking the main frame tubes in isolation from the rest of the frame seems more an engineering discussion and not a completed bike one.

My view of choosing tubes involves more than just stiffness, although that is an important consideration. In general I have thought that the focus on stiffness is somewhat lacking in understanding the full picture. But since it is easy to measure stiffness (and that other marketing device...weight) we see it banted about as a holy grail far more than I think it deserves.

Another view I have is that the fork influences the perception of a frame's stiffness more than many will want to admit. But given today's (yes, this is drifting from the OP's question) frequent use of a third party fork on "custom" frames the builder is handing off much of how the complete package will ride to others.

The last bit I will mention is that while it is easy to measure a frame/fork stiffness with proper tooling/devices this is not what the rider experiences. They also feel the geometry, the fit, the wheelset in actual riding.

I look forward to reading other more experienced people give their take on this. Andy

[RonaTD]

Don't forget that tube walls can also be different in the blades and stays and that there's a shape difference between the Continental and the "classic" Reynolds fork blades. Taking the main frame tubes in isolation from the rest of the frame seems more an engineering discussion and not a completed bike one.

Excellent point, not forgotten! I spent some time trying to include that in my original post but didn't want to wander too much from my main question which is about relative stiffness around the main triangle. As someone who really enjoys the round Reynolds blades on my Heron Road bike, I fully appreciate the contribution of the fork to frame feel. I also think chain stays are a very big part of the equation.

Ted Durant
Milwaukee, WI USA

[Andrew R Stewart]

Ted- Given the triangulation of the rear end of a bike I feel that the stays have less effect on frame stiffness/ride perception than most think. But so much of this, as perceived by the common rider, is subjective and influenced by other factors (which the marketing people use to the brand's advantage).

Forks lack this triangulation. In my seeking a better loaded touring frame/fork I have built myself 4 F&Fs. Starting with classic road diameters and wall thicknesses in the later 1970s. Then moving to the thickest walls I could source but still with traditional diameters. Still the bike would "inch worm" on front braking and fall into shimmy mode easily. #3 say continuation of the thicker walls and traditional diameters but with revised geometry to try to reduce the shimmy when loaded. Finally I got smart and went to a 1.125" steerer and somewhat over sized diameters and can stop further efforts as the bike is much more solid. nearly no inch worming and far less shimmy tendencies. Andy

[Mark Walberg]

I agree with Andrew that fork geometry has a very big effect on how a bike rides.
The best way to learn about this is to ride a frame, changing the fork. Easy to do if you build your own forks.

By the way, I recall when I moved to Palo Alto very long ago (1977), and I asked around for someone who could miter some tubes for me. I was pointed to a young Tom Ritchie, living with his parents then. He had a lathe in the garage and cut the tubes for me on it. My tubes in that set were Columbus SP. Tom asked if I would trade him the heavy, thick walled top tube for a thinner wall tube.
I figured that the wall thickness of the top tube was probably not so important and gladly made the trade.
In retrospect, I would trust Tom's opinion more than mine, but I have never done a test of this.

[RonaTD]

Ted- Given the triangulation of the rear end of a bike I feel that the stays have less effect on frame stiffness/ride perception than most think.

If we are talking about "vertical compliance", I agree, chain stay and main triangle tube dimensions, within the relevant range, don't make much difference. Tires and forks rule there. But, to me, ride quality is much more than vertical compliance, and bottom bracket flex is a part of it. I'm very light and not very powerful but it still makes a noticeable difference to me. Regarding bottom bracket flex, several experienced frame builders whom I respect have said they think chain stay dimensions (and not just length) have an important impact on bottom bracket flex, and in my limited experience I have felt that to be true.


Ted Durant
Milwaukee, WI USA