Why do some bike hold speed better than others?

[skiezo]

What makes some bike hold speed better than others?
On my gravel bikes my Pivot vault holds better/easier than my T Labs Ti. Weight is within 14 ozs .
My Ti Desalvo seems to hold speed better than my steel AR but the AR get the nod on comfort and long rides.

[NateFrentz]

If everything is maintained well, then it's probably the stuff that affects your positioning on the bike / aero. You could try replacing the chains plus all bearings in the BBs and wheels in the "slower" bikes to eliminate those sources. How different are the tires, tubes, and psi?

[Ken Robb]

Heavier wheels are probably slower to get up to speed but may provide a flywheel effect to maintain speed on rolling terrain?

[prototoast]

Rolling resistance from the tires is probably the biggest factor.

[Fat Cat]

Weight and friction rule the world.

[Ken Robb]

Originally Posted by Fat Cat

Weight and friction rule the world.

Are you a marriage counselor?

[cmg]

Deep rims coupled with light tires and tubes hold speed better. It's really mixture of fit and wheels.

[Mark McM]

Originally Posted by cmg

Deep rims coupled with light tires and tubes hold speed better. It's really mixture of fit and wheels.

This effect is often exaggerated. It is true that effective inertia of mass at the periphery of the wheel is double the inertia of the non-rotating parts of the bike. But mass at the periphery of the wheel (rims and tires) is just a small portion of the total inertia. Take for example, a rider that weighs 170 lbm wearing clothing, shoes and helmet that weighed 6 lbm, and that the weight of their bike is 20 lbm, for a total mass 196 lbm. Further, the rims + tires on this bike weight 4 lbm, and since the effective inertia of rotating mass is double, the effective inertia of the bike is 200 lbm. Now let's increase the weight of each rim + tire by 1/2 lbm, or 1 lbm for both wheels, which increase the total effective inertia by 2 lbm, for a total effective inertia of 202 lbm. This is just 1% increase the total inertia. On any given day and on any given road, the rider's power output, or total drag (rolling resistance plus aero drag) may vary by more than that, so on a good day the rider may be able hold speed better on the lighter wheels, and on a bad day they won't be able to hold speed as well on the heavier wheels.

Or consider this: A full water bottle weighs about 2 lb. this adds the same inertia as adding 1 lb. to the rims + tires. When was the last time you heard someone remark, "boy, my bike sure does hold speed better when I've got a water bottle on the bike!", or, "I'm going to be riding rolling terrain today, I better make sure my water bottles are full so my bike can hold speed better!"

[Talrand]

99-101% placebo.

[jimoots]

Assuming both bikes are well maintained, tyres and aerodynamics is the answer here.

[Black Dog]

Originally Posted by Talrand

99-101% placebo.

Agree

[cmg]

Originally Posted by Mark McM

This effect is often exaggerated. It is true that effective inertia of mass at the periphery of the wheel is double the inertia of the non-rotating parts of the bike. But mass at the periphery of the wheel (rims and tires) is just a small portion of the total inertia. Take for example, a rider that weighs 170 lbm wearing clothing, shoes and helmet that weighed 6 lbm, and that the weight of their bike is 20 lbm, for a total mass 196 lbm. Further, the rims + tires on this bike weight 4 lbm, and since the effective inertia of rotating mass is double, the effective inertia of the bike is 200 lbm. Now let's increase the weight of each rim + tire by 1/2 lbm, or 1 lbm for both wheels, which increase the total effective inertia by 2 lbm, for a total effective inertia of 202 lbm. This is just 1% increase the total inertia. On any given day and on any given road, the rider's power output, or total drag (rolling resistance plus aero drag) may vary by more than that, so on a good day the rider may be able hold speed better on the lighter wheels, and on a bad day they won't be able to hold speed as well on the heavier wheels.

Or consider this: A full water bottle weighs about 2 lb. this adds the same inertia as adding 1 lb. to the rims + tires. When was the last time you heard someone remark, "boy, my bike sure does hold speed better when I've got a water bottle on the bike!", or, "I'm going to be riding rolling terrain today, I better make sure my water bottles are full so my bike can hold speed better!"

Does your calculations consider that it is the rider providing the torque to spin it all? He's the one that feels the effect not the person observing from the sidelines.

[fourflys]

Originally Posted by cmg

Does your calculations consider that it is the rider providing the torque to spin it all? He's the one that feels the effect not the person observing from the sidelines.

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..

[marciero]

Originally Posted by fourflys

.. 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..

Thats because it has a greater resistance to changes in velocity from an applied force. So yes it slows down more slowly. But it takes no more force to keep the more massive flywheel spinning at a given rate than it does it does the less massive one- you only need to counter the resistive force.

[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.