Bicycle frame flex and power loss

Factual Questions
1

When pedaling a bicycle, the rider mostly pushes down on the pedals, which means there is very little pedaling force when the pedal is at the top or bottom. So pedaling force is not constant but varies periodically. Now, a bicycle frame is not perfectly rigid, so chain tension causes it to flex sideways. It flexes more when the pedal force is at maximum, and it un-flexes at the minima. The question is, if the frame behaved like an ideal spring (perfect elastic deformation only), would this flex result in power loss? Or is the energy stored in the deformed frame recovered (used for propulsion) at the end of each stroke?

I’m discussing this on another message board and most seem to think that once the energy is used to deform the frame, it’s lost forever. I’m not so sure.

By the way I’m not asking which bikes or materials have more/less frame flex. This is purely an academic question.

2

well a pure spring certainly doesn’t dissipate energy BUT that doesn’t mean your pedaling efficiency will not suffer, in fact i am sure your pedaling efficiency WILL suffer. that is, your stroke will be longer due to flex and thus you will expend more energy on the stroke, however when the frame springs back your muscle will not be able to absorb that power back from the spring, because muscles dont work as either generator or motor, only as motor.

that said, the bigger problem is the shock absorbers in bike’s suspension - those really eat a lot of power, that is the reason why many expensive bikes do NOT have rear suspension. front suspension is not as bad because front wheel is further from the pedals.

3

Once the frame flex energy has been expended, it cannot all be returned to the rider, however some of it can.

Tendons act like springs, prove this by darwing both feet together and jumping vertically just once, and see how far you reach.

Now jump up and down on the spot a few times and measure your peak leap.

You will find you exceed the single leap, and this is partly because when you land, your tendons, such as the achilles, stretch to absorb the impact and on subsequent leaps this energy can be reused, assisisting your muscles.
The timing is important, if you are not properly warmd up you can cause yourslef lots of damage, if you kick too hard you can cause lots of damage, and if you leap too late, the stored energy will be dissipated before you can reuse it.

A certain amount of flex in a frame is desirable, this help absorb some of the imperfections in road surfce and helps give the rider some ‘feel’ as to what the bicycle is doing, which can be vital in downhill mountain biking or on short circuit road racing(criteriums).

On longer events frame flex helps, too stiff a frame is tiring.

An example of how frame flex helps a bicycle track better when cornering hard in my personal experience.
Forks usually have a curve in the blades, and this allows them to flex in a certain axis, and this helps them ride road surfaces better, one fram I had came with dead straight forks(a trend which has become common among racers).
When I was desceding on country roads which are often not of the best quality, I would find that on fast sweeping bends those straight forks were so stiff that they would cause the bicycle to ‘crab’ sideways putting me off the optimum line round. Turning in to counter this seemed to make the crabbing worse and made the rear wheel skip sideways too.
As soon as I changed to curved fork blades, everything was fine.

If you look at pro cyclists you will see that althought their forks are straight, they are raked quite a lot, that is they are further away from the vertical than say the curved fork blades, and I strongly suspect straght blade machines also have a longer wheelbase,

gotta go for now windows i just about to crash yet again!!!

4

Dontcha just love XP!!

Anyways, sprinters, those specialist fast finishers prefer very stiff frames, and you will find on the track on the shorter events such as the Devil and persuit that these are stiff frames also.
The shoes the riders wear will have extremely stiff soles, possibly with steel reinforcing rods built into their construction.

Power delivery is immediate a stiffer frame, in a sprint it is not the highest speed achieved that is most important, it is what is called the ‘jump’, how quickly a rider can accelarate to top speed, flexy frames reduce the sharpness of this jump giving other riders just enough time to respond.You can gain a few yards if you jump away well, and this is often all that is needed to win, the second across the line might be closing the gap, might well be going faster, but the few yards gaing by sharp accelaration can be decisive.

Frame flex is only one component, the axles will flex, the shoes will flex, the chainset flexes quite a lot(more then the frame), if you are sprinting then you would be wrenching the bars, and these will flex along with the handlebar stem, and to add all this, the tyres also flex up and down and from side to side.

Typically, tyres do not act as perfect springs and absorb energy, other components such as flexing handlebars may not return stored energy in a direction that is useful or their flexing may reduce control of the machine causing the rider to put in less power.

5

I was thinking that as the frame unflexes, the energy stored will be returned not to the rider, but tranferred to the drivetrain. Where else can the energy go?

6

if you consider your muscle working in reverse ( eccentrically ) as a shock absorber, then that energy may very well go into heat generated inside your muscle :slight_smile:

7

Nope. Straight or curved matters very little compared to design parameters and material, most likely you had other differences in that frame other than just a fork that gave you trouble while descending.