Method of comparing rear springs independent of tyre size and pressure.

I needed a method of comparing the ride height of the chassis regardless of the type of wheels and tyres fitted to the two cars being compared. This exercise was so that I could judge one set of springs against another but it could also be used for other purposes, such as the effect of a load of touring luggage etc. Getting under the car and measuring the distance between say the bottom of the chassis and the axle is not easy unless you have a pit or you can raise the car on a lift that lefts under the wheels. Even then you have to be sure all 4 wheels are on a level plane. After a bit of thought I came up with the following method.

The following measurements are from my car which is fitted with the older type 5 leaf rear springs.

Fuel about one quarter of a tank. No other load on board.

The car must be on an absolutely level surface. Before measuring I tried to bounce the car a little on its springs to get the dampers to settle. I discovered it is actually not easy to get any movement but I probably got enough to do the trick.

First I measured the rolling radius of the 195/60 R15 tyres by measuring from the hub centres to the ground. A small point made with a marking pen on the centre of the wheel nut would probably help. They averaged to 292 mm I did not worry about pressure but it was probably about 24 psi. It is not really important for the calculation I have in mind. There was actually 2 mm difference between left and right (slightly different pressure??).

The height of the lower chassis rail measured near the wheels was average 174 mm (there was 2 mm difference left and right due to the tyres, in other words the springs were amazingly equal)

By deducting the 174 from 292 we get 118 mm which is the height difference between the centre of the axle and the bottom of the chassis rail. This can then be compared to another car that has been measured using the same system.

Peter

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