Tire Balancing w/Trim Rings

[Mark Milchuk]

This is for a 1968 Corvette.
Just bought brand new 15x8 Kelsey-Hayes Ralley Wheels.
Just painted them this weekend. Now need to get tires.

Going to buy BFGoodrich Radial T/A
Was going to get Size 215/70/15.

Would anyone chosse a different size and why:
225/70/15 ?
245/60/15 ?
255/60/15 ?

Will need to get tires balanced. I have read that balancing
with Trim Rings on Ralley Wheels has problems. Because Trim
ring sits flush against wheel there is no room for the tire weight.
The Trim Rings I have have flaps(grips) all the way around the ring
not just in four places like the original Trim Rings.
Can the tire weight be put on the back side of the wheel?
Is there a correct technique for where the weight goes and how
to balance the tire?

Thanks for the help,
Mark

[Joe L.]

Re: Tire Balancing w/Trim Rings

Mark-----

I would recommend using either the 215-70-15 or the 245-60-15. Either size will fit with no problems and will provide a close-to-original tire OD, so your speedo calibration will not be affected.

Using the style trim rings which you have usually results in no interference with wheel weights. The original style rings with 4 clips sometimes does cause wheel weight interference. You don't need to even consider having the wheels balanced with the trim rings installed. A very satisfactory balance can be achieved without the trim rings installed at the time of balancing.

Have the wheels balanced by a shop with a Hunter GSP 9700 balancing machine. That's all you need to know about balancing.

[Duke W.]

Re: Tire Balancing w/Trim Rings

If you're running 15x8s on a '68 I would recommend running one of the three V-rated 225/70R-15 or 235/70R-15 police pursuit tires. Details in the next issue of The Corvette Restorer, and the background is in the current issue.

Conventional clip on wheel weights should fit within the trim rings. Any modern off-the-car static/dynamic balancer properly operated should yield proper balance. The Hunter 9700 has the additional capability of measuring radial force variation and wheel runnout, and shops will charge extra for this service.
The 9700 is also a balancer, but unless you want the radial force variation measured and optimized, it won't balance any better than other common balancing machines.

As far as speedo accuracy is concerned the OEM size or 215/70R-15 (775 revs per mile) likely create a slight error on the high side - 2 to 4 percent; 225/70s (760 revs per mile) will negate most or all of this and 235/70s (744 revs per mile) could result in low readings of about 2 percent, but consider that over the tread life of the tires the revs per mile will increase by about two percent, so a slightly low reading will decrease with tread wear.

Duke

[Chuck S.]

Re: Tire Balancing w/Trim Rings

Mark, for judging, balance weights should only be applied to the back side of the wheel since they were balanced that way at the factory. You should get point deducts for weights on the outside of the wheel.

If you ask the tire people to balance your tires using only weights on the back side of the wheel, they can do it. The disadvantage is that the balance won't be as perfect as modern balancing machines can make it, and the tire guys will probably make sure you realize that fact. The balance will at least be comparable to the factory balancing.

If you want to drive on performance tires, I believe I would buy (in fact, I have bought) a second set of cool retro performance wheels, mount some H or V rated performance tires like Duke suggests, and get them balanced to the gnat's whisker. Use the rally wheels, trim rings, and repro tires for judging only.

[Duke W.]

Re: Tire Balancing w/Trim Rings

A tech who knows his trade and equipment should be able to balance the tire with only weights on the inside. This is known as a "static balance" and should not result in any wheel shake. Balance weights on both sides of the tire means the tire is balanced both statically and dynamically. In mathematical terms dynamic balance means that the angular momentum vector is coincident with the axis of rotation. If its not coincident, but cocked off at a slight angle, the road wheel can wobble a bit, which typically causes the steering wheel to wiggle back and forth at high speed. Static unbalance or excess radial force variation causes the steering wheel to shake at typical freeway speed.

I have my Cosworth Vega aluminum wheels only balanced statically to avoid marring the outside face, and there is not sufficient clearance between the wheel and caliper to allow stick on weights anywhere in the wheel drop center. I've never had balance problems except with the OEM Goodyear radials that were about as round as a head of lettuce.

Duke