Wheel Weight Placement

[Travel'n Man]

I went yesterday to get a little "wobble" out of my rear end of my '72 yesterday - I was wobbling at 55mph+ . I pulled the rear wheels off and just took the wheels to my tire man (I don't like the air wrench so I take mine off and torque them on myself). We got the rear mounted tires and rims out of my X5 and he got one wheel on the spinner and started spinning. On the inside setting (the one that tells where the weights to be placed on the inside of the rim only) said it was dead on - the tech changed the setting for both the inside and the outside weights and it still read off. I was a little puzzled so we removed that wheel and put the other on. He gave it a spin and I was a full ounce off of the inside setting, he changed the setting to read both inside and outside and I was 2.50 ounces off. I had him add 1 ounce to the inside and just got back from test driving after I torqued my lugs nuts back on.

My wobble is not starting until 70+ mph now and would like to get it out. I'm sure if I have them pull all the weights off and change the weight location to the inside and outside mounting of the weights (instead of just the inside placement on the wheels) that it should correct it.

Has anybody else gone through this challange of weight placement on your wheels, and does it make that much of a difference for perfect balance?

[Arne]

After 20+ years in that business, I can say that using weights on both the inside and outside of the rim is the best way to balance a tire/rim assembly.

However, a skilled technician with modern equipment (or even semi-modern, my experience is over 10 years old now) should be able to get a reasonably accurate balance job without putting weights on the front of the wheel, especially on the relatively narrow rims we use on our S30s.

Generally, if you are not putting weights on both edges of the rim, you want to switch the balancer to "static" mode (not "dynamic"), which should give a single reading for the entire wheel. Most people will then put that weight as close to the center of the rim as possible, but with experience, a good tech will compare that reading with the dynamic reading, and may place the weight more toward the front or back of the rim as the dynamic reading indicates.

Current balancers are even more helpful, as the tech can indicate the size and offset of the rim, and then indicate where on the rim area he wishes to place the weight and let the balancer compute the rest. Some even allow you to identify the location and number of any spokes, and will make allowances to attempt to hide the weights behind those spokes.

Frankly, if I am reading your description of what happened on the balancer correctly, I have doubts that your tech is experienced with this technique, and also that his equipment is somewhat dated. Might be time to find a different tire shop.

[jmortensen]

I haven't been in the tire business for close to 15 years but I did bust tires for a couple years and I have a different opinion than Arne. There is no way that you can put weights on the inside or center of the rim only and get a dynamic balance. A static balance is simply inferior and will never be as good as a dynamic balance where weights are used on the inside and outside of the rim.

I've had several vehicles that were very sensitive to tire balance, including my Z which has very few rubber or poly bushings in it anymore, mostly heims joints. I also worked at a Porsche shop and those cars were very sensitive to balance issues as well. My boss there was kind of a hardass, but he would absolutely refuse to balance tires statically, and I didn't blame him a bit for it. I didn't want comebacks because some weenie didn't want weights on the outside of his wheels. In fact I really dislike rims that don't have an outer lip to put weights on. Yeah, you can stick them on, but the hammer on kind mount more securely, and I'm a function over form guy.

The best system is the roller/cutter setups where they put xxx lbs of weight on the tire, spin it on a roller while weighted and then shave the tire so that it is true and THEN balance the tire. Those machines are hard to find because they're expensive, but that's the best tire balance method I've come across.

http://en.wikipedia.org/wiki/Tire_balance

http://www.fourwheeler.com/techarticles/wheels/129_0210_4x4_truck_tire_truing_balancing_shaving/index.html

[Walter Moore]

Actually the situation that Michell describes is exactly what you would expect from a wheel with high couple imbalance.

It is impossible to correct couple imbalance by applying weights in only one plane.

To illustrate couple imbalance imagine this situation: Take a "perfectly" balanced wheel, and apply a large, say 3 ounce weight somewhere on the curb side of the wheel. Then turn the wheel over, and apply another 3 ounce weight on the car side, at a location that is exactly 180 degrees from the first weight. You now have a wheel with 3 ounces of pure couple imbalance.

If you take this wheel to your dynamic balancer and check it in single plane (static) mode it will call for ZERO weight. If you switch to dynamic mode it will call for two 3 ounce weights, exactly across from the weights that you installed.

If you put this wheel on your car and drive it at highway speeds (which I do not recommend) it will shake the fillings in your teeth. (My apologies if you have perfect teeth, it is only an expression.)

As Arnie mentioned, the two correction planes do not have to be on the inside and outside flanges, but when correcting couple imbalance the further apart the planes are in space the smaller the correction weights that will be required. Most new cars have "flange-less" or "soft-face" wheels without a weight flange on the curb side. (For styling, not to make the car ride better.) On these wheels one correction plane is located on the inside of the wheel, just behind the spokes, and the other is either on the inside flange (if present) or on the inside of the wheel near the flange. Flange-less wheels are corrected with adhesive weights.

Another thing to consider when you install your wheels on the car is to make sure the back side of the wheel and the square-up surface on the hub (or drum) is clean and flat.

One of the demonstrations that we do in car plants to try to convince the maintenance staff to keep the tooling clean is to take a perfectly balanced wheel with clean tooling and spin once to prove that it is in fact balanced. Then we unlock the tooling, place a business card under the wheel, lock the tooling, and try again. The resulting weight call-outs are usually eye opening. Often the wheel will be uncorrectable!

Lighter cars, like the early Zs, and cars with smaller tires (that is the outside diameter of the tire, not the wheel diameter) will always seem to be more sensitive to imbalance. That is because any given road force will have more effect on a lighter car (F=MA) and the force generated by imbalance is proportional to the square of the rotational speed. (F=M*(omega)^2)

Class is now over. Go get your tires balanced!