There is an article in the current issue of Vintage Bicycle Quarterly that uses a finite element analysis to look at the four different types of flex that occur at the bottom bracket... and in each case, when the pedaling load decreases at the dead spot, the frame springs back and this either rotates the cranks or pull the chain - thus turning the rear wheel. This, of course, helps you move the bike, effectively smoothing your pedal stroke. You put energy into the frame when you have plenty during the downstroke, and you get it back when you are at the dead spots. The finite element analysis found that the total amount of energy going into and out of the frame was very small. However, as one reviewer (Hank Folsom of Henry James lugs) pointed out, the model did not allow the rider to adjust their pedal stroke to the bike's particular flex characteristic.
Basically, if taken at face value, the article would imply that within reason, the amount of flex does not matter at all. However, that does not match my experience.
The more I ride different bikes, the more I realize that for me, some bikes require adjustments to my pedal stroke. The Peter Weigle Randonneur (lightweight OS tubing) I tested for VBQ felt OK for the first 200 miles, then I finally got in sync with the bike by decreasing my cadence a bit, and it was outstanding. When I got back on my customary Singer (lightweight standard diameter tubing), I bogged down on the first hill until I realized that I was using a one gear higher than usual. It took me a few miles to readjust my cadence. More recently, I rode a Rene Herse with OS down tube, and it took me about 70 miles until the bike became part of me. Then it flew. I am not sure what I did differently, so it must have been subtle.
On some bikes, the adjustments are too much for me to make. I rode one bike from heavy-gauge OS tubing that felt fine at a full effort and 50 rpm. I can't do that for more than a few seconds... as I usually pedal around 90-110 rpm.
Jan Heine Editor/Publisher Vintage Bicycle Quarterly c/o Il Vecchio Bicycles 140 Lakeside Ave, Ste. C Seattle WA 98122 http://www.vintagebicyclepress.com
At 8:40 PM -0400 7/15/06, Ken Freeman wrote:
>I'd think that energy return only is not lost if it contributes to
>propulsion, i.e. it aids the next power stroke in some way. Anybody know if
>that's actually the case? If it isn't, or it's only partially returned to
>the power stroke, then some types of frame flex imply a loss of pedal
>energy.
>
>Ken Freeman
>Ann Arbor, MI
>
>-----Original Message-----
>From: classicrendezvous-bounces@bikelist.org
>[mailto:classicrendezvous-bounces@bikelist.org] On Behalf Of
>Philcycles@aol.com
>Sent: Saturday, July 15, 2006 11:30 AM
>To: classicrendezvous@bikelist.org
>Subject: Re: [CR]Was 753, now is stiffness desirable?
>
>
>In a message dated 7/14/06 9:32:30 PM, heine94@earthlink.net writes:
>
>>That is an interesting story. It is something I always am concerned
>>about when I test bikes. If a bike works for me, the whimpy guy who
>>likes flexible frames, it may not be appropriate for others with
>>different power outputs or different pedaling styles...
>
>If I may comment, While there is no free lunch you do get the energy used in
>deflecting the frame back minus the hystersis-internal material
>friction-loss, which in metal is minimal but pretty high in rubber. The
>deflection attributed to rubber will be returned mostly as heat in the tires
>but the deflection in the frame will be returned mostly intact when you
>finish deflecting-when the sprint is done.
>Phil Brown
>Way too technical way too early in San rafael, Calif.