Importance and effects of rotational weight

[RojoRacing]

So it's rather common knowledge that your wheels and tires are the best place to loose weight because they need to be spun up to speed. It takes longer to accelerate up to speed with heavier wheels but once up to speed maintaing speed is about the same. This is why TT bikes who just focus on a steady speed on a flat course run heavier more aero wheels but road racers who have to climb and sprint go at light as possible. So on a TT bike we accept the weight penalty with having to spin up a disc wheel because it'll be more aero once up to speed.

So does adding 1lb of weight to the front non drive wheel slow down acceleration or climbing as much as adding 1lb to the rear drive wheel? You driving the rear wheel so I see how the weight hurts there but I fail to gasp if the front wheel which is just being forced to roll by the frame has equal effect.

The reasons I'm asking all this is a CruzBike being front wheel drive is an odd bird. Can we run a heavier rear disc at less of a penalty vs a rear wheel drive bike?

This sounds like a stupid question based on simple physics that should be obvious to someone like be but I fail to have confidence in my thoughts around the matter.

 

[ccooper]

No difference. Adding 1 lb, in the same relative location, will add the same amount to the moment of inertia, whether front or rear wheel. Whether you drive the wheel directly at the cassette, or spin it from the rim, it takes the same energy.

 

[RojoRacing]

Yeah that's basically what I was thinking but I couldn't think of a way to prove it to myself. I guess testing out my new disc wheel covers on the back of the vendetta during this weekends very mountainous 200 miler would probably be a bad idea.

 

[JOSEPHWEISSERT]

No difference. Adding 1 lb, in the same relative location, will add the same amount to the moment of inertia, whether front or rear wheel. Whether you drive the wheel directly at the cassette, or spin it from the rim, it takes the same energy.

The CruzBike forum: Smarter than Google.

 

[ratz]

The CruzBike forum: Smarter than Google.

Well.... faster with an answer at least.

 

[Balor]

More than that, moment of inertia of spinning wheel does NOT apply to climbing, this is a woefully popular misconception, only accelerations and braking (and it would make it easier to maintain speed).
When it comes to acceleration/deceleration, rim/tire weight is about 2x the effect of non-rotating weight. (But given the weight of rims/tires compared to weight of entire bike and the rider, the answer would be 'unimportant and negligible')
I think this misconception is so widespread because weight of tires (all things being equal!) directly affect their rolling resistance. Lightening the tires usually result in drop of rolling resistance, and it is a constant boost.

 

[RojoRacing]

I do agree that most people probably misunderstand the effects of rotational weight on a climb or at least agree with the idea without understanding why. I agree accel and deccel are where that extra weight comes into play but when are we truly accelerating? You accelerate as you pick up speed pedaling or coasting it doesn't matter. How about when you maintaining speed? On flat ground while maintaining a steady speed your not accelerating so the difference in rotational weight doesn't really matter. When your climbing your also maintaining a steady speed/effort so again rotation weight shouldn't matter right?

He's the question I'm purposing. While climbing a steep hill your fighting gravitys pull to stop you more then on flat ground. I believe on a climb your having to accelerate against gravity a small amount just to maintain speed. In fact I believe the faster you would coast to a stop the more constant acceleration you would need.

If that doesn't make sense to you think of it this way. Imagine pedaling but not in perfect circles so there would be massive dead spots during each rotation. During these dead spots on a very steep climb you would be decelerating and when you started your power stroke again you'd then be accelerating back up to speed to maintain a steady speed. We're talking tiny amounts here but after a million pedal rotations it adds up. Now the same idea on flat ground and you'd lose almost no speed during your dead spot and this not need to accelerate to maintain constant speed.

Now the above example if for pedaling in squares to exaggerate the issues so you can focus on it. So does the same theory apply if you pedal in near perfect circles? How many people actually pedal in perfect circles. Remember you dead spot doesn't need to be completely dead, it can simply be a lower area of power and you'd still be decelerating in that moment.

These are just some random thoughts I just had while trying to get my mind off life. Let me know what you think.

 

[Balor]

Yea, nobody pedals in perfect circles. But overal effect would still be the same, because net effect of slower acceleration and slower deceleration would would add up to pretty much zero.
In fact, higher weight would be preferable in case of "square pedalling", because aerodynamics is highly nonlinear... but effect is truly negligible.
Lighter weight is preferable for "climb/descent" cycles however for same reasons of aerodynamics, however.
You climb at slow speed, accumulating potential energy and than releasing it as you descent.
However, even if you do not pull the brakes at all, aerodynamics would eat up much of you hard-earned watts on the way down - doubling the weight would half the climbing speed, but come nowhere near to doubling speed of descend.

 

[RojoRacing]

I hadn't through about the heavier wheel carrying the momentum and reducing the deceleration during the dead spot thus canceling the need for acceleration. Good catch.

 

[Balor]

Yup, wheel acts as a, well, flywheel
Though, since rpms are nowhere near true flywheels (20000-50000 rpm, composite construction, magnetic bearings, etc) - it is a rather lousy one.

 

[super slim]

Jason, I think you are correct, and Q rings decrease this angular acceleration.

The worst case for acceleration changes with each 1/2 cycle, would be a DF rider standing on the pedals with only short power strokes, and turning the front wheel!