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Sheldon Brown talking about BB stiffness... Huh?(12 posts)

Sheldon Brown talking about BB stiffness... Huh?hayaku
May 25, 2002 4:58 PM
"This is mainly related to the stresses generated by the forces you create from pedaling. Any frame will flex around the bottom bracket a bit in response to pedaling loads. This flex can be felt, and many riders assume that it is consuming (wasting) pedaling effort. Actually, that's not the case, because the metals used in bicycle frames are very efficient springs, and the energy gets returned at the end of the power stroke, so little or nothing is actually lost. While there is no actual loss of efficiency from a "flexy" frame, most cyclists find the sensation unpleasant, and prefer a frame that is fairly stiff in the drive-train area."

I don't get it, everyone knows that a weak BB kills a sprint. What is he talking about? Can anyone clarify it for me?
re: Sheldon Brown talking about BB stiffness... Huh?gtx
May 25, 2002 5:33 PM
everybody knows... that Sean Kelly was a great sprinter, and he rode a Vitus...
Succinct to the max!!! (nm)Kerry
May 25, 2002 6:16 PM
me, or Keith? ;) (nm)gtx
May 25, 2002 6:31 PM
You!!!! (nm)Kerry
May 26, 2002 3:16 PM
Great site, Thanks... nmrwbadley
May 25, 2002 9:40 PM
re: Sheldon Brown talking about BB stiffness... Huh?bic
May 25, 2002 9:32 PM
It's all about marketing. Sound like someone who is knowledgeable about everything and someone will listen. I'm sorry, but I am leary of anyone who has answers for everything like Sheldon seems to. Gee what about the human body and it's ability to flex, and I don't mean like a gymnest etc. Gosh, maybe we sould look for cycling shoes like those silly spring shoes of the 70's. Store it all up and save for that last sprint!!!
Actually...Trent in WA
May 26, 2002 9:53 PM
Speaking as an obsessional bicycling junkie who reads almost everything he can find, online or off, about the sport, Sheldon Brown is one of the most knowledgible people around when it comes to cycling--he's been in the business for around thirty years, is regularly cited by Bicycling on technical matters, and has the expertise to back up most of what he says. Much of what he says on his website goes against the conventional wisdom and marketing shibboleths of the contemporary bicycle industry, but he does know what he's talking about.

Can't we all just get along? The Unified Field Theory:Me Dot Org
May 26, 2002 8:25 AM
What if both parties were right? What if a bike were torsionally rigid and vertically compliant? In other words, what if it flexed in a vertical plane but not on the horizontal plane?

If you mash a pedal and the bike frame twists laterally, I can't see that "the energy gets returned at the end of the power stroke", because the bike has to spring back horizontally. If all the energy springs back vertically, there would much less loss.

But loss, alas, there still would be. A steel spring may be efficient, but it still loses energy. The spring of a "shock" absorber could just as rightly be called an "energy" absorber.
no free lunchgtx
May 26, 2002 9:16 AM
all bikes flex in the horizontal plane, but the standard two triangle design really doesn't allow any significant compliance in the verticle plane.

As someone stated on a frame building forum, a vertically compliant frame is a broken frame. If you want real verticle compliance, get something like this:

Otherwise, comfort comes from fit, geometry, saddle, tires (width and inflation) and (maybe) fork.

I agree that a flexy frame can be irritating while sprinting (mostly from front der rub) but I doubt there is any significant power loss.
this is a good questioncyclopathic
May 26, 2002 1:39 PM
while there's no doubt that additional friction loses associated with frame flex (FD rub, spring losses etc) are minimal, the biggest question is how flex affects pedaling?

- does it make it more or less efficient?
- is the energy "accumulated" during downstroke converted used to propel bicycle or absorbed by leg muscles?
May 27, 2002 8:47 AM
Although I'm open to the idea that power loss is minimal (and am well aware of folks who have been excellent sprinters--and others excellent climbers--on flexy frames), it's hard to buy the "(nearly?) perfect spring" theory. We know that force from the return cannot be driving the rear wheel directly. Can it be driving your crank/pedals/feet? Maybe some--after all, you often feel the give and return through your feet--but how is this supposed to work as an aide to both feet as you turn the pedals? The notion that it's assisting even one foot in a purely efficient way seems dubious. Assisting both?

Again, I don't know that there's much of anything to worry about here, I'm just wondering about the story purporting to explain why that's so.