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  #1 (permalink)  
Old 07-13-2010, 06:48 AM
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Default Measuring wheel allignment and tollerance?

I am currently fine tuning my suspension and chassis allignment before moving on to the motor.

I am taking some measurements as i have been told that more often than not the RCM mounts are in the wrong spot.

The procedure i have used is as follows:
1. Set up a string line on each side of the car.
2. Allign the string line with the rear wheel by measuring the distance from rim to string on front and back of rim.
3. Allign the front wheel with the string line.
4. repeat on other side.
5. Square a line (with the string) from the centre of each hub and mark the string line where they intersect. i.e. inline with the centre of the hub.
6. Measure the distance between the marks on the string.

When i do this i get one side 3mm longer than the other. Seems to be acceptable tollerance to me.

As a final check i measured the distance between the 2 string lines at each end of the car. This came out to be 29mm difference. In other words my string lines tapered in at the front by 29mm over the length of the car (well a bit longer than the car).

If the lines are not perfectly parallel then my distance between wheels is not accurate bacause the centre of the hub will move front to back because of the distance to the ball joints. As i have shortend the control arms at the rear, although i used a jig i made, i cant guarantee they are both 100% straight.

Has anyone got a more accurate way to measure that doesnt involve specialised tools. I just want to make sure it is close enough so it can be fixed with a wheel allignment when the time comes.

Cheers.
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  #2 (permalink)  
Old 07-13-2010, 06:59 AM
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Default

Thinking more about it, this is only about 2mm toe in on the wheels if they are equal or 4mm if it on one wheel.

Do you think these numbers are reasonable? I think that i am probably close enough that i will be able to get it right when i can put it on a proper wheel alignment machine.
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Old 07-13-2010, 07:14 AM
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sounds well close enough to drive to a good wheel aligner (read suspension shop, not a tyre shop)
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Old 07-13-2010, 09:59 AM
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You want to measure each wheel against the frame, not against another wheel. If everything is referanced to the frame, there is less margin for error.

Get two pieces of fishing line, about 12-18" long. Tie a weight to one end of each string - I use a washer. Tape the strings to the top of the round tube - one in front and one in the rear. The strings should hang down the same side of the tube, and not touch the ground.

Use a laser level ($15 at Ace) and cast a line that intersects both strings. Now you have a laser line that is perfectly aligned to the frame. Take all your measurements off that laser line.

Sometimes it's easier to do the same thing on both sides, so you have two laser lines to measure from.
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Old 07-13-2010, 10:40 AM
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The chassis centreline should be established first by finding the centre between the inner pivots of the front lower control arms and then the inner pivots of the rear suspension lower control arms, am not too familiar with Jags. Permanently mark these positions for later reference.

Using these two points you can run a string or a laser between them to determine the vehicle centreline. Measure off this line to accurately place your sidestrings.

As Bob says an aluminum tube front and rear sitting so that the side strings are at wheel centre height works well. You can notch the tubes so the string sits in the same place each time, for repeatability.

Cheers
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Old 07-13-2010, 04:13 PM
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irs or solid axle?
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