Bike stiffness - can you have too much?

Discuss light weight issues concerning road bikes & parts.
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spytech
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by spytech

so here is a video of someone standing on the chain/seat stays. usually some frames you can flex them with your hands. but check out the video.

http://www.youtube.com/watch?v=BdC6z1dkKnU

the frame barely moved, is this too much? or do you just use wider rim with bigger tires to balance it all out?

what are the thoughts on this, i like to read what the community has to say.

bricky21
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by bricky21

Yep, there has to be a point where frame stiffness is too much. At the moment we only have theories to go by.

It used to be that in golf everyone(most anyways) used the stiffest shafts they could buy. Why? because all the pros used stiff shafts. It wasn't until launch monitors came out that people could be shown with science what shaft yielded the best results for the individual. That is a bit simplified, but the point being once we can be shown with science the flexing and unflexing(is that a word?) of a frame under an individual we will be able to choose appropriately stiff equipment.

eric
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by eric

That video is not a good way to measure frame stiffness, although it's cool that it doesnt break.

Back in the old round metal tubing days you could have a stiff frame which beat you up, or a good riding frame which was flexy. All they could tune was tubing wall thickness (and frame geometry to some extent) so there was a direct tradeoff between stiffness and ride. The Vitus 979 glued aluminium race bike was famed for riding well and for being really flexy.

These days with CF, not only can they make tube shapes that are not round, they can adjust the layup thickness and fiber orientation everywhere on the frame. This allows a wider range of ride quality for a given stiffness, and allows adjusting stiffness in some directions more than others. We're closer to the famed "laterally stiff yet vertically compliant" trope that is used by magazine reviewers.

However stiffness and ride quality are not completely decoupled. A really stiff frame is still going to ride harsher, as a general rule, than a not so stiff frame.

Many people put too much credence to "power transfer". The power absorbed by frame flex is very small since the frame is essentially a spring. Flex in one direction is returned in the other. (before you say that CF isn't a spring, the rear spring on Corvettes is made from CF, and so are the springy bits on compound bows).
One of the ways that I know I am reading material from marketing dweebs instead of bike engineers is when they say "power transfer".

I think that you can have too much stiffness for some applications. If I was a 190 lbs crit monster I would probably disagree, but I'm a 146 lbs skinny climber who does long rides and races. Since I'm also 6' tall, many frames my size are made for heavier riders and thus are stiffer than I need. If I ever get a custom frame built I'll get it made with thinner or smaller diameter tubing so it's appropriately (not) stiff for my weight and riding style.

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spytech
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by spytech

eric wrote:Many people put too much credence to "power transfer". The power absorbed by frame flex is very small since the frame is essentially a spring. Flex in one direction is returned in the other. (before you say that CF isn't a spring, the rear spring on Corvettes is made from CF, and so are the springy bits on compound bows).
One of the ways that I know I am reading material from marketing dweebs instead of bike engineers is when they say "power transfer".


The look keo blade pedals use carbon as a spring...
Image

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rmerka
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by rmerka

Laterally stiff, vertically compliant...Someone had to say it :mrgreen:

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HammerTime2
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by HammerTime2

That's the opposite of what women want.

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DMF
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by DMF

Also, if you are a 2.000watt crit monster, actually getting the power to the ground may prove a challenge as the rear wheel skips everywhere. Not an issue on the track, but very well a possible issue on anything less than perfect tarmac... Flex can aid traction.

And before some jackass (as always) says it, lowering the tire pressure may solve the issue, but not without creating issues elsewhere.

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andy2
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by andy2

eric wrote:
Many people put too much credence to "power transfer". The power absorbed by frame flex is very small since the frame is essentially a spring. Flex in one direction is returned in the other. (before you say that CF isn't a spring, the rear spring on Corvettes is made from CF, and so are the springy bits on compound bows).
One of the ways that I know I am reading material from marketing dweebs instead of bike engineers is when they say "power transfer"..


Carbon is a very efficient spring, yes. But it takes work to compress/distort that spring and the return force is not magically transmitted back into the chain.. and translated into forward propulsion, it simply springs back. Can we please let this old myth die.

/a
rolobikes

ttakata73
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by ttakata73

HammerTime2 wrote:That's the opposite of what women want.

Props to the funniest reply I have read this year.
I literally just LOL'ed.

I believe too stiff would be bad.
If the frame and wheels didn't flex some when leaned over in a turn a bump in the road would throw the bike off course or even eject the rider.
A skinny road tire can only absorb so much impact so I think frames have to account for this.

lactatehate
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by lactatehate

I own said frame.

Have a read about it under the Gallery section :D

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