Quick question regarding wheel balance

[FinallyGotOne]

My 911sc has 17 inch c4 cup wheels. 40 and 45 series tires. I noticed a shake at about 75 or so and was thinking a wheel weight came off. The guys at the wheel shop said it was 1 ounce off in the right rear and there was a small bend in the rim on both driver side wheels. But they corrected the back left wheel by a 3/4 ounce weight, and said the fronts were still in balance. Would that make the car shake a bit?? Just wondering if an ounce is enough to throw off the tight feel of the car at speed?

[911pcars]

Originally Posted by FinallyGotOne

My 911sc has 17 inch c4 cup wheels. 40 and 45 series tires. I noticed a shake at about 75 or so and was thinking a wheel weight came off. The guys at the wheel shop said it was 1 ounce off in the right rear and there was a small bend in the rim on both driver side wheels. But they corrected the back left wheel by a 3/4 ounce weight, and said the fronts were still in balance. Would that make the car shake a bit?? Just wondering if an ounce is enough to throw off the tight feel of the car at speed?

Why did they add 3/4 oz. to the LR when they said the RR was 1 oz. off? Missing some info?

Sherwood

[FinallyGotOne]

sure, they added the 1 oz weight to the right rear, and they added the 3/4 oz to the left. They ended up balancing all wheels. My question is mainly can just a 3/4 oz and 1 oz weight cause the shake at 70+ mph?

[Wil Ferch]

A lot less can cause a shake....it depends "where" the weight was and what counter-balancing effect it has on the assembly.. I had a bent steel rim that was radially "out" by about 1-2 mm.....couldn't take the shaking. Got a new rim..... fixed.

[911pcars]

A wheel/tire combo can be statically in balance but dynamically out of balance. Depends on where the correction weights are placed. For example, if the tire machine says, "place X weight here (and perhaps there)", but the balance operator or owner says, "put the weight in back of the wheel so it isn't visible", then that can compromise the procedure.

Your wheels are probably aren't this wide. If instead of placing the weight in the exact location as recommended, the installer places the weight along the correct axial plane, but far from the dynamic balancing location (i.e. owner wants to hide the weight for appearance purposes). On a bubble balancer or other static balancer, the wheel may appear to be balanced, but only statically.


More info here:
Tire balance - Wikipedia, the free encyclopedia

Sherwood

[HFR_Racer]

Most quality shops dynamically balance wheel/tire assemblies. Sherwood is correct that the location of the weight is of importance, but I've never seen a good technician that didn't verify the dynamic imbalance of the wheel/tire assembly after the correction weight was added.

Additional questions here are as follows:

1) Why didn't you straighten the wheel flange prior to spending your hard earned $$ on balancing? Maybe more than just the wheel flange is bent?

2) Did the shop check road force or not? I assume not. If they did then the would've likely detected a source of radial disturbance.

Correction of the dynamic imbalance has nothing to do with road force or harmonics of the wheel/tire assembly.

[RSTarga]

I'd also worry about the incorrect offset of those 964 wheels on a car designed for the fuchs wheels. ABS requires positive offset and the earlier cars had negative offset.

[Flieger]

Other way round, I think. ABS wants negative scrub radius at least. Shallow dish wheels.

[Neel]

Originally Posted by FinallyGotOne

sure, they added the 1 oz weight to the right rear, and they added the 3/4 oz to the left. They ended up balancing all wheels. My question is mainly can just a 3/4 oz and 1 oz weight cause the shake at 70+ mph?

Absolutely!! I've had access to the best balancers on the market.. IMO, tire/rim matching is a must.. Dynamic balance can be achieved without putting weights on the outside of the wheel.. It depends on the limits of the balancer and the tech involved.. Sometimes caliper to wheel clearance can come into play (for wheel weights) The new Snapon/John Beane balancer can acheive a balance within 1/8 ounce, tire wheel/ match etc.. The Hunter Road Force balancer is a good balancer, but I think the Snapon is better.. In the end we start 'splitting hairs'..