Stan Heinricher

40-some years ago, when I was riding 400 miles a week in preparation for weekend races, I rode a Fausto Coppi 60 cm road frame made of light Columbus tubing. My friends said the frame was ‘whippy’, but it also was very comfortable to ride for hours at a time — that is, not much vibration was transmitted thru the frame. Then, to have an opposite kind of experience, I bought a 60 cm frame made of heavy Columbus road tubing. I discovered that it ‘beat me unto death’ and was not suitable for 50-mile training rides even on the relatively smooth roads in eastern Florida. So, I used this new, stiff, frame for criterium and short time trials only. It gave me a 20-second advantage in a 10-mile time trial over the Coppi frame. I think it sprinted faster, too. And so, to wrap up this long narrative, I do think that stiffness matters as does shock absorption when it comes to comfort over the long haul.

AlanO

First of all, a bicycle frame is not a spring, it is a complex system made up of many different pieces, each with its own properties. More importantly, none of the models that you learn in your undergrad physics or engineering classes are true, they are all approximations. There is no such thing as an ideal spring, springs have mass, they do not have linear restoring forces. If you enclose a real spring in a vacuum and launch it into space it won’t continue to vibrate forever. It may oscillate for a long time but not forever. A real beam which is deflected will not continue to oscillate, it’s not an ideal beam. In fact, a beam is much further from ideal than a simple spring. The more complicated a system becomes the less it behaves as ideal. Every piece of a bicycle frame interacts with every other piece. None of the pieces are ideal, they have properties which contribute to the behavior. That’s not to say that ideal models aren’t useful as a first approximation in a simple scenario but they are inadequate for describing real physical phenomena as you depart further from the ideal.

Kapusta

You seem tonbe stuck in a false dichotomy that something is either
a) an ideal spring, or
b) so heavily damped that it dissapates most energy in the first cycle.

AlanO

I said no such thing.

livedarklions

So you're saying hope doesn't spring eternal?

Kapusta

Well, I never said anything about a bike frame being a perfect spring (in fact I pretty well ruled that out when I said a frame has LOW hysteretic loses rather than NO hysteretic losses), so why did you bring it up and go on at length about it?

You keep saying that a frame is not a spring, but when asked what the difference is, you just say it is made up of different parts. Well, on a steel frame those parts are all steel, all with relatively efficient spring characteristics. Not perfect, but relatively efficient.

To say that something "Mainly" dissipates energy (which I assume means most of the energy) through hysteretic loss in just a single compression is some pretty heavy damping. Where exactly is all this damping happening? In the welds?

PeteHski

As far as torsional, lateral and vertical stiffness are concerned, a metal bicycle frame is a very stiff, undamped spring. But a CF frame could also include a lot of damping, often designed into the layup e.g. Bianchi CV. Modern CF frames are so stiff in torsion and lateral flex as to make any energy losses when sprinting insignificant - especially among similar modern bikes you may be considering back-to-back. Vertical compliance is what people really notice on modern bikes and even that is less significant now we are moving toward wider, low pressure tyres.