Why do some bike hold speed better than others?

[Mark McM]

Originally Posted by fourflys

I often take what this guy says with a grain of salt.. anyone who have ever replaced a standard clutch flywheel with a lightened racing one can tell how much rotating inertia matters to keep that circular thing spinning longer.. all else the same, I'm certain a 2,000 gram wheelset will hold inertia better than a super lightweight climbing set at 1,000 grams.. and that light set will probably feel better going up a 12% climb..

Firstly, the inertia of an engine freewheel only serves a purpose when the clutch disengages the engine, and that's because the rotational inertia of the engine is relatively low compared to its power/drag. The freewheel keeps the engine turning more smoothly when there is no load. When the clutch engages the engine, the inertia of the vehicle is so high that no freewheel is required.

Secondly, an engine rotates much faster than the wheels (rear differentials are speed reducers), so the relative inertia of the engine freewheel is much higher than for the wheels.

In contrast, on a bicycle, the wheels are never disengaged from the forward motion of the bicycle, and they turn slower than an engine does, so their "freewheel effect" is minimized compared to an engine freewheel.

As far as climbing on a bicycle - lots of tests and studies show that what matters when climbing is total mass, not inertia, so an 88,000 bike + rider with 2,000 wheels (90,000 grams total) will climb just as fast as an 89,000 bike + rider with 1,000 gram wheels (same 90,000 grams)*


*Actually, for two bikes of the same total weight, the one with the lighter wheels may actually climb slightly slower. When climbing at slow speeds in small gear ratios, speed will oscillate with pedaling power pulses. The bike with the lighter wheels will have a lower effective inertia and therefore will have slightly larger speed oscillations, resulting in slightly increased average drag, resulting in slightly slower speed. But the speed difference is in the minutia.**

**So why do lighter wheels climb faster? Because we typically er don't add weight to the rest of the bike to make up for the wheel reduction of lighter wheels. Replacing heavier wheels with lighter wheels, all else being equal, will result in a lower total weight, and it is total weight that matters when climbing. If instead the non-rotating weight were reduced, it will have the same effect as a reduction in rotating weight.

[unterhausen]

I wish I didn't know that there was a longstanding and bitter argument on the rec.bicycles.tech usenet group about the benefits of light wheels. Almost as disappointing as the digital signal processing groups, which were dominated by trolls. I don't think I stepped into the middle of it, but it used to come up on searches about wheels, so I read a lot of it. I feel confident that nobody is going to feel good after reprising that argument here.

[deluz]

We need Jobst back to answer these kind of questions

[unterhausen]

Jobst would no doubt gently explain how you were wrong in the most life-affirming manner possible, just like he did on rec.bicycles.tech.

[54ny77]

Motors!

[marciero]

I think much of the confusion here is because what it means to "hold speed" was not defined. If the OP is talking about the force required to maintain a constant speed on the flat, then inertia is not a factor. The sum of the forces is is zero. None of these forces involve mass or rotational inertia (which is equivalent to mass). So the velocity does not change. Its Newton's second law.

On the other hand if you are talking about holding speed as you coast, when aero drag and friction create a net non zero force, there is hence a change in velocity, with the more massive bike slowing down more slowly. As an approximation, if you neglect rolling resistance, and assume aero drag is proportional to velocity, one can show that the velocity decays exponentially, with rate of decay inversely proportional to mass. The initial decelleration, for example when you stop pedaling, is also inversely proportional to mass, but that also decays exponentially, and again with rate inversely proportional to mass.

[HenryA]

Too much pie.