How do tire balancing machines work?

[Leaffan]

I get the physics. A tire on a rim will have an uneven weight distribution that may cause it to wobble at certain RPMs.

But what I don't understand is how the Junior Mechanic who runs the tire balancing machine know were to weight the rim, and by how much. As I've peered through the window at this process it looks like this: wheel goes in machine; machine spins it; it stops; JM reaches into one of several weight buckets and crimps a weight to the rim.

I've never noticed a display indicating how much weight, not have I noticed any markings or laser beam pointing to the location.

So, how does this work?

[Cheesesteak]

Each machine is different, so I will have to tell you about the one I've used, probably now a 15 year old, relatively manual unit. The tire was clamped into the machine, measurements of the wheel size taken and set on the dials. You spin the tire, and when it beeps, the measurements come up. It displays the inside weight needed and the outside weight needed (red LED screen), and indicates where to place each via a light. The placement indicator lights up when the correct location is at a specified position (the top).

You rotate the tire until the outside indicator lights, then attach the weight at the top of the wheel on the outside. Repeat for the inside indicator/weight. Spin the tire again, and it should come up "0.00".

You may not have been able to see the screen, but I'm sure it was there somewhere.

[Leaffan]

Originally Posted by Cheesesteak

Each machine is different, so I will have to tell you about the one I've used, probably now a 15 year old, relatively manual unit. The tire was clamped into the machine, measurements of the wheel size taken and set on the dials. You spin the tire, and when it beeps, the measurements come up. It displays the inside weight needed and the outside weight needed (red LED screen), and indicates where to place each via a light. The placement indicator lights up when the correct location is at a specified position (the top).

You rotate the tire until the outside indicator lights, then attach the weight at the top of the wheel on the outside. Repeat for the inside indicator/weight. Spin the tire again, and it should come up "0.00".

You may not have been able to see the screen, but I'm sure it was there somewhere.

Cool. Thanks.

[DanBlather]

But how does it work? What are the mechanics? How does it know how much weight and where?

[spingears]

Originally Posted by DanBlather

But how does it work? What are the mechanics? How does it know how much weight and where?

Basically it measures the unbalance and calculates the how much and where. Beyond that it gets a bit obtuse.
Maybe a physicist will explain the theory in detail for all of us.

[Cheesesteak]

In addition to being the son of a mechanic, I studied physics in college, let me dust off the cobwebs and give this a wag, I've never actually thought about it before.

The measured inputs to the machine are, IIRC
diameter of the wheel
width of the wheel (inner to outer)
distance from machine body to inner surface of the wheel


The unbalanced spinning wheel will produce forces on the spindle in proportion to these dimensions and the speed at which it is spinning, which would be monitored by the machine. The forces can be shown as a waveform.

If you measure those forces at different places along the spindle, I think you can separate the inner and outer imbalances. The differences would be due to the changes in the length of the 'lever' which will change by a greater percentage for the inner side of the wheel. By closely monitoring the size of the peaks, with the RPM, you can determine the size of the imbalance.

When you move the measuring point from 12" inside the machine to the edge of the machine, the force from the inner will decrease a greater amount than the outer, so you can determine what % of the imbalance is inner vs. outer by the %decrease. Let's say a 50% decrease will be 100% inner (by the geometry) and 40% decrease ,100% outer. If the actual decrease is 47%, then you have 70% inner imbalance and 30% outer.

The raw size of the force will tell you what the total imbalance is, which you then split by the percentage. The waveform tells you where the weights have to be in relation to the rotational position.

[DanBlather]

Wow, thanks. Nicely explained.