Technical questions about road force balancing

chrispitude · #1 (**permalink**) 12-11-2005, 04:52 PM

I guess this should really be titled "Questions for DJ"...

Does tire pressure affect the balance? I had a set of new tires mounted and balanced on a set of 17" BBS wheels for my BMW. The shop aired up the tires to the max sidewall pressure (44psi) and the Hunter machine reported significant road force measurements. He called me back to have a look, thinking my wheels or the tires from Tire Rack might be the issue. I asked them to air down the tires to the car's recommended pressures (33/36psi) and the road force numbers decreased significantly.

This makes sense to me intuitively too. As the tire pressure increases, any variances in the tire shape should be transmitted more directly to the measurement drum. If the tire is aired down, the tire would be softer and allow more cushioning.

Should tires be set to their normal operating pressure before a road force balance?

- Chris

DJ · #2 (**permalink**) 12-11-2005, 05:14 PM

Tire pressure usually doesn't affect tire balance, but tires should be set at the pressure they will normally be used at when a road force balance is done. A pound or two either way won't matter, but 44 lbs when they are used at 33-36 lbs sure will.

chrispitude · #3 (**permalink**) 12-11-2005, 05:37 PM

Hi DJ,

Thanks! I have another question along the same line.

Does rotational speed affect the road force balance? The reason I ask this is that on a wheel/tire combo, you have a fixed mass (metal wheel) and a dynamic mass (tire) which has an energy storage and damping component to it.

Thinking about it, I would think that the force profile presented to the drum would be different at 10mph than it would be at 80mph, due to the frequency-dependent behavior of the wheel/tire combo. My question is, do the Hunter machines target a particular speed? Can they be programmed to optimize for a specific cruising speed, say, 70mph?

This one is a little more out there. Thanks.

- Chris

DJ · #4 (**permalink**) 12-11-2005, 09:22 PM

Quote:

Originally Posted by chrispitude

Hi DJ,

Thanks! I have another question along the same line.

Does rotational speed affect the road force balance? The reason I ask this is that on a wheel/tire combo, you have a fixed mass (metal wheel) and a dynamic mass (tire) which has an energy storage and damping component to it.

Thinking about it, I would think that the force profile presented to the drum would be different at 10mph than it would be at 80mph, due to the frequency-dependent behavior of the wheel/tire combo. My question is, do the Hunter machines target a particular speed? Can they be programmed to optimize for a specific cruising speed, say, 70mph?

This one is a little more out there. Thanks.

- Chris

Nope. They rotate the tire at a very slow speed while measuring the force variation and have no provision for "targeting" a particular speed.