4 pinion (twin pin) Diffs. The Facts.
Turbo Mini Forums

I was tempted to put this together when i read a recent post, and was bored with waiting for some files to download!



MiniSpares (Tran-X) diffs are identifiable by the pins not being at 90degrees to each other - more like 80/100degrees...
The cheaper ones (no names mentioned!) are at 90 degrees.

The cheaper (90degree) ones are inherently weaker because of the way the planets engage on the pinions. Picture a pair of course-cut gears meshing; because of the course nature, only one tooth is ever in contact, and the loading lands on either the flank, or the tip of each tooth on each gear. This generates high stresses and loads.

(Back to the diff) Due to the equal number of teeth on the pinions (the outputs) if you add in another pair of planets onto a stock diff, and position them at 90 degrees to the stock pin, the planets will always mesh the pinions at exactly the same point - i.e. the tips on all the planets will be loaded at exactly the same point relative to the pinion(s).
So, by moving the second (on a twin pin diff) pin around by [[n/360]*0.5] degrees (where n is the number of teeth on the pinion) relative to the original pin, you will have a loading situation where the planets on one of the pins are loaded on the tips, the planets on the other pin will be loaded on the flank (the strongest scenario). This also runs smoother, as the speed of the planets relative to the pinions are more consistent (gear forms that look like ?mangle cogs? speed up and slow down relative to each other depending on the coarseness of the tooth form).

That hopefully made some sense to some of you, but suffice to say there is a big difference in the strength of these...
Whilst in theory 90degree twin pin diff is undoubtedly stronger than a standard diff, an offset 4-pin is the real key to an unbreakable drive train... You won't break one of these!

Edited by TurboDave16V on 22nd Mar, 2005.

good info Dave,

Team Racing

Jim, was there a picture of your diff from your gearbox build?

yea, hold on I'll dig it out

Team Racing

Team Racing

Thats just what i thought Dave

Best 1/4 mile 13.2 seconds @116 mph
First 5 port miniturbo to make over 200 bhp on a carb?
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http://www.mattwoodsphotography.com

Not that easy to see the offset but then it's only +/- 10 degrees.
How many miles do you have on that Diff Jim ?

Two things are infinite: the universe and human stupidity; and I'm not sure about the the universe.

you can clearly see the offset in the top pic! .. interesting reading :)

turbo 16v k-series 11.9@118.9 :)

Denis O'Brien.

in the photo the diff was new, I only gave it around a thousand miles before I replaced it with an LSD

Team Racing

Interesting and probably right. I would have thought an involute section tooth would have transferred the loads smoothly. It isn't to do with minimising backlash is it?

Just a thought.

Bugger off, I'm getting there.

I think a decent involute probably would take the load - but these are more like mangle wheels aren't they! LOL!

dave, whats the minispares part number for that diff?? Is it the Evolution one?

yes it is the evolution

Team Racing

A tip for fitting:
It's basic engineering stuff, but I'm sure someone will over-look it at some point...

Note the 'tangs' attached to the thrust washers under the planets. you need to make sure these are bent so they are home in the slots in the diff cage. If you don't do this, they could spin, which will be far from good - probably damaging the diff after a short time.

I'm sure we did this to yours after the pic Jim?

On A engines, those bronze washers didn't have those "tangs". All A differentials I've taken apart has been seriously worn on this point. Behind the thrust washer that is. A+ are much better catered for as the washer stick, which makes the washers wear instead of the differential cage.

So, Mark Dave's words..

I'm sure we did dave

Team Racing