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Rim run-out compensation

Motorsport Wheel Alignment Fundamentals

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Discussion and questions related to the course Motorsport Wheel Alignment Fundamentals

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Hi all,

i have two questions concerning the string alignment:

- modern car alignment systems perform a rim run-out compensation before starting the actual measurement. Is there a possibility /necessity to do the same when using a string alignment procedure?

- if there is a difference in camber and/or caster values (left to rigth), positioning the strings using measurements to the hub would generate an error. So before setting up the strings, camber and caster values should be the same on the left and right side. Is that correct?

Thanks in advance,

Thomas

Hi Thomas,

I know the runout compensation you're talking about - for anyone else who may not be familiar with it - it allows you to check for bent rims that would otherwise give you incorrect wheel alignment measurements. This involves having the car jacked up and rotating each wheel in 4 steps, by 90 degrees each time. The sensor on each wheel is on a rotating fixture, to even though the wheel rotates, the sensor does not. By comparing the alignment of the sensor as the wheel is rotated, you can see if there is a significant runout or not.

Really, this procedure applies to anything that could be bent or buckled - the hub as well for example. I don't see any reason you couldn't do exactly the same thing with a string alignment setup, measuring the difference as you rotate each wheel. It would just be helpful if you had the style of stings that hang off the car so that you can easily jack it up like this and not change your sting reference.

On your second point, to be accurate, you do need to have symmetry in the suspension. To be totally rigorous, if you have things like adjustable suspension arms, these should be jigged to be equal side to side, as well as things like adjustable strut tops, before you setup your strings. This is what we're basing our reference off after all.

As Tim said, this applies to any wheels and should be done prior to establishing a base setting for the entire alignment settings. I theory, there should be negligible runout, or axial variation, but in practice some wheels are susceptible to being slightly bent from things like hard kerbing.

You can get a good idea of this from simply spinning the lifted wheels on the vehicle, but to be more precise use a DTI to establish the four points Tim mentioned, these will be the vertical and horizontal planes for the measurement points, and should be marked on the wheel assembly for later correction of the measured values.

In the field, unless you're checking for damage, you should have all the current settings noted and you can adjust from those datum points.

I'm not sure what your last part means - it looks like you're saying you can't check the suspension geometry unless it's already correct?

Hi Tim, Hi Gordon,

thank you for your reply. I am currently building a alignment system, the holder for the strings will be attached to the car. So taking 4 measurements should not be a big deal. I will also try to use a dti to get an idea, how much my rims are bent....

The second question was about positioning the strings parallel to the center line of the car. If there is a difference in caster or camber, the strings would not be parallel (If you use measurements between the strings and the hubs to align the strings). So initially, the suspension must be symmetrical....

As far as I can see, procedure should look like this:

- suspension arm, strut tops etc. symmetrical

- measure camber and caster, adjust symmetrical

- run the strings and align them

- ......

Thomas, your procedure looks correct to me. Another option is to add a reference onto your chassis for the centerline, or record a reference measurement from the sills to the strings.

You could do the procedure you mentioned above, then record the distance from the strings to some convenient references on your chassis (if there are any). This way, once you have a reference, you can use this to set your strings parallel with the car centerline each time instead of the suspension itself. Again, this method relies on having good quality references which may or may not be practical for your case.

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