Monday 26 March 2007

Something is not right?

Since posting the diagram of the rear wheel alignment I've been thinking it wasn't quite right. The total toe-in seems ok but the offsets at the front are unequal. All the measurements where taken relative to the chassis centre line with no reference to the centre of the rear axle - and on reflecton I also wasn't that happy about how I had determined the centreline of the chassis at the front - oops!

So - here we go again. First thing I did was to take a number of measurements from various points on the front suspension to the centre of the visible end of the hub driveshaft. These measurements were all to wthin less than 1mm from side to side so I can be reasonably confident that the rear axle is "square". I then set up the aluminium box section (clamped saquare across the chassis) over the rear hubs as close as possible and marked the projected faces of the hubs:
I then marked the centre of the hubs on the box section and on the chassis. I then spent some time measuring between a number of reference points to determine the physical centre of the chassis and marked that also:

Hey presto nearly 1.5mm difference!? The centre of the hubs being offset towards the off side. This actually does make some sense as there are more disc, camber and diff shims on the offside compared to the nearside (about 50 thou' or 1.27mm - this gives an centreline offset of 0.64 - the remaining 0.63 must just be normal manufacturing tolerances)).

Armed with this invaluable information I moved to the front of the chassis and carefully marked the centre point of the front crossmember relative to the wishbone pivots. I then set up the box section with the chassis centre line marks lined up. I could the project the laser forwards to measure the projected face of the hubs relative to thier marked positions on the box section from the rear - thus measuring the actual toe in of each wheel directly:

I repeated this three times - each time setting everything up from scratch! The end results are shown in the diagram below but are:

Toe in - 0.6mm total (almost equal between rear wheels).

Line of thrust 0.25mm off centre toward the nearside.




RIGHT CLICK ON IMAGE FOR A LARGER VERSION

So that's the end of that saga. I thought I'd finish off the back end by fitting the shocks and dampers -however I hit two snags:

1) The lower shock shaft safety washer (jaguar item) has too big an outer diameter and fouls the adjustment valve on the damper.

2) The upper mounting bolts appear too short and when done up no thread protrudes beyond the end of the nyloc insert in the nut.

I shall have to enquire about these.

P.S. They really must make millimetres a bit larger so I can stop using a magnifying glass!


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