The Physics of wheels..

[Chescadence]

Ok, so we've all heard the good old' "1lb on the wheels/tires counts of 2lb on the frame"..

This can be quite easily calculated, total Kinetic energy of a wheel is the linear translational energy + the rotational energy, so:

Ktrans= 1/2xmv^2 and Krot= 1/2xIomega^2 - If we for arguments sake say that a wheel is a cylindrical shell with all it's weight at the rim (Not exactly true but..); we could say the Inertia is I=MR^2. We know Omega (Angular Velocity)= V^2 / R^2. We can then adjust this formula to work out the overall kinetic energy for a wheel and substitute in out I and Omega values:

Ktrans= 1/2xmv^2 , Krot= 1/2xMR^2xV^2/R^2 = MV^2/2 (After simplification, R^2's cancel each other out and are not important in this sum and 1/2xMV^2 is the same as MV^2/2)

Lets assume that our bike wheel has a mass of 1.5kg and is moving at 20kph= (20x10^3/360) = 5.555555ms^-1

KEtrans = 1/2x1.5x(5.5555555)^2 = 23.15 joules , KErot= (1.5X5.5555555^2)/2 = 23.15 joules

KEoverall = 46.30 Joules. From this we can deduce that the KE for the wheels is twice that trans/linear Energy if the weight was on the frame due to the lack of rotational KE - Not exaclty x2 due to the spokes etc. So essentially, if we put 1.5kg on a wheel you'd require nearly twice the energy to power it than if it was on the frame.


So lighter wheels would make acceleration easier for sure, but at a constant speed would the wheels KE still "seem" x2?




I'm studying Meds so i'm by no means a physicist so i'd be interested in anyone with greater expertise than me.. Hope people got the gist anyway.

 

[coffeejo]

I'm studying Meds so i'm by no means a physicist so i'd be interested in anyone with greater expertise than me.. Hope people got the gist anyway.

 

[oldroadman]

Summarises what we more or less know, lighter wheels are a big difference on the flat, and help on climbs. But can be skittish on descents. Going up long climbs everything needs to be light, right down to the 6.8kg weight minimum limit. I won't be surprised if this goes to 6.5 or so soon, then the mechanics can stop carrying bits of lead around to drop in frames which come up below the minimum!

 

[Drago]

When it comes to suspension performance a pound of insprung mass is equivalent to 6 of sprung mass, another reason to rejoice in light wheels.

Tells us what most of us knew anyway - light bikes are good, light wheels especially so, just as long as they're strong enough

 

[Chescadence]

Summarises what we more or less know, lighter wheels are a big difference on the flat, and help on climbs. But can be skittish on descents. Going up long climbs everything needs to be light, right down to the 6.8kg weight minimum limit. I won't be surprised if this goes to 6.5 or so soon, then the mechanics can stop carrying bits of lead around to drop in frames which come up below the minimum!

Lightness is important in all the aspects you mention; however, that could be weight saved from either the frame or the wheels or you or whatever. Considering "rotational mass" specifically rather than overall mass, I fail to understand how apart from extreme acceleration from top end track cyclists the importance of rotational mass in normal road cycling; surely weight reduction from anywhere on the bike will do the same job?

 

[oldroadman]

Lightness is important in all the aspects you mention; however, that could be weight saved from either the frame or the wheels or you or whatever. Considering "rotational mass" specifically rather than overall mass, I fail to understand how apart from extreme acceleration from top end track cyclists the importance of rotational mass in normal road cycling; surely weight reduction from anywhere on the bike will do the same job?

In road racing speed is not constant, and good races are a series of steady state and full on efforts (into the red zone), where accelaration plays a big part in getting into the right moves, on the right wheel, getting a placing in the final. Light wheels help a lot....!
The numbers are interesting, and explain some things well enough, but in the end the actual performance is what counts.

 

[black'n'yellow]

So lighter wheels would make acceleration easier for sure, but at a constant speed would the wheels KE still "seem" x2? .

You need to consider 'decelleration' as well. Then it starts to make sense...

Overall 'system weight' is what really matters, as opposed to the weights of components - even rotating ones. The bike (and rider) need to be accelerated as a single mass - not just the wheels.