Choice of Diffs for a special

[restoman]

This might be a strange/stupid question but here goes anyway . . .

Diff and rear driveshafts from:

A) Sierra
B) BMW 3 Series
C) MX5

When looking at these diffs none appear symetrical - ie the pinion for the prop seems to be to one side of the diff rather than central. So is this because:

1)the propshaft does not run down the exact centre line of the car but is slightly to one side?
2)the propshaft does run down the centre line of the car and therefore the driveshafts are of unequal length to compensate?

Thank you

[Yazza54]

Freelander diff? Mine looks pretty symmetrical

Edited by Yazza54 on Sunday 5th December 19:31

[ferg]

Certainly on the Sierra the shafts are different lengths.
I'd imagine that this is always the case, bearing in mind how the diff works.....

Incidentally... I saw a Mk1 escort once that had been 'independent-ised' at the rear using an Impreza rear end, seemed quite neat.

Edited by ferg on Sunday 5th December 19:32

[anonymous-user]

You are really better off deciding on a diff to use by it's torque capability and drive ratio, rather than "little" things like halfshaft length. You can easily get custom shafts etc made up. For example the MX5 diff would be no use with an LS engine etc!

I've got an E46 BMW M diff in the back of my Seat, really tough, couple of different ratio's availible from the US, really nice "progressive" limited slip unit, the only down side it's bloody heavy!!


The pinion is never on the centreline of the unit, because it has to drive the crownwheel on one side of the other, and you don't want to have to react the large spool carrier bearing loads along a pointless / heavy casing extension just to get it symetrical looking etc

Edited by anonymous-user on Sunday 5th December 22:33

[Dr Derek Doctors]

We're using Freelander units for both our cars, lightweight and neatly packaged.

[Stubby Pete]

Depends on the engine and ratio required.

If you're going down the bike engine route, E36 BMW is worth looking at if only for the variety of ratios available and the only one (to my knowledge) with a production diff of less than 3:1. LSD available too.

[Sam_68]

Stubby Pete said:

If you're going down the bike engine route...

But bear in mind that a using a tall final drive ratio gives you a tall 1st gear as well as a taller top gear, and 1st gear ratios are often very tall on BEC's anyway...

Less of a PITA on the motorway can therefore mean more of a PITA in traffic.

[Stubby Pete]

Sam_68 said:

Less of a PITA on the motorway can therefore mean more of a PITA in traffic.

Was thinking more of track than motorway but I take your point.

[Sam_68]

Stubby Pete said:

Was thinking more of track than motorway but I take your point.

Yeah, same principle... instead of getting pissed off 'cos you're bouncing off the limiter on the straights, you end up getting pissed off 'cos it's bogging down out of tight corners.

[Stubby Pete]

We'll have to agree to disagree Sam. In my (albeit limited to one BEC) experience, a top end limited to around 120mph (I run 13" wheels and live axle) is much more restrictive than using lower gears exiting corners. At Brands I can get out of Druids in third, dropping down to second or even first would be preferential to bouncing off the limiter on the main straight.

[Sam_68]

Stubby Pete said:

We'll have to agree to disagree Sam.

I don't think we're disagreeing - I was merely pointing out that the limitation of changing final drive is that it takes the whole range of gearing up or down with it, so by making top gear tall enough not to be a nuisance for some situations, you can make first gear too tall to work properly in others.

If you can avoid the 'problem' situations (accelerating from low speed, in the case of a tall final drive), then it isn't an issue, so I agree that on a fast circuit, a very tall first gear will be less of a nuisance than a very short top.

My competition background was mainly in hillclimbing though, where you'll appreciate that the opposite holds true, and of course if you drive on public roads you're going to encounter an even broader operating range...

Unfortunately, bike engines lack the torque to pull widely-spaced intermediate ratios, so the only other way of making them deal effectively with a broad performance envelope would be to fit a secondary gearbox with high- and low-range gears, like the Weisman unit that Gordon Murray specified on the Rocket.

[jason61c]

The cheapest range must be the bmw e36 ones, there's a massive choice of ratios to choose from, the none LSD diff's are so cheap.

The LSD do weigh a little though!