Rear Suspension And Diffs

Add details on the rear suspension fitting and set-up, toe out mods etc etc…………….

Identifying and choosing a Diff

If you have a diff, you need to know what type and spec it is before fitting it. If you got it directly from the donor or as part of a donor package you might find this tricky.

The first thing to do is measure across the top fixing point, this will tell you if you have 7" Sierra bracket or a 7.5" Granada diff. (Assuming your ford based of course)


Then you need to know the gearing ratio, this can make a big difference on how you car performs, if the ratio it too high, you will find first gear gets you up to about 10mph before you need to change gear and are revving too high on the motorway at 70mph, too low and you might struggle to pull away depending on your torque and gearbox of course.

Ford diffs have a metal tag attached to them which gives a part number and gearing ratio, this is normally found at the back of the diff. If this is missing which is quite common, another way to be sure of the ratio is to drain the oil from the diff and remove the backplate, you will then find the ratio stamped on the crown wheel as shown below:


alternatively if you don't want to drain the diff you can spin the prop flange and measure the revolutions of the axle flange.

Typically a diff ratio of 3.54:1 will give good acceleration but high revs at high speed, a lower ratio such as 2.88:1 will give less acceleration and more relaxed top speed cruising. Its very hard to advise but the link below can be a useful way of gauging the performance and revs/speed of your combination:

Diff and gearbox speed calculator

(multiply kph by 0.62 to get mph)

If you want to understand how a differential works, take a look at this video.

Mods to Diff carrier

A problem with the diff carrier from a Sierra or Granada is that it is mounted on some fairly soft rubber inside its own carrier bracket, when you put a lot of power through one of these diffs the rubber allows more movement than there should be, or would be with the donor car. (Also I think the donor has extra fixings) This can be seen if you run the car with the back wheels off the ground and the handbrake slightly on (such as if bedding in new brakes), the diff moves a great deal and can even "clonk" onto the underside of the car when pulling away fast and vibrate quite violently.

This can be prevented by adding additional brackets to the diff, people have used various designs, below is one such example, simple steel angle has been cut and welded to form a very strong bracket that fits squarely on both fixing points. There are existing holes in the chassis that can be used for the front fixing point, the fixing to the diff can be made by using the existing bottom fixing with small headed "cap head" bolts, as these can be done up with no problem.

Some pictures: (both were taken from behind the diff)


The brackets shown above were shaped by making a series of cuts into the steel angle on only one side at a time using a thin (1mm) angle grider blade. Then either the cut was opened and a small triangle of metal added, or a triangle removed the the cut closed and welded, then painted and fitted.

If you are putting a very large amount of power through the diff, eg 400bhp, it becomes possible that not only will the diff move a lot but the brackets at the top of the diff can become bent or broken To resolve this they can be strengthened by welding additional bracing into them and replacing the standard ford rubber bush with a solid steel bracket as shown below:


These were added to a sumo with a fairly high performance Chevy engine that had previously broken the brackets, after this mod the diff was solid and stable.

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