Tayloring an ZF-LSD into a Fiat 132 axle

[TR-Spider]

One thing you certainly need for spirited driving is a limited slip differential (LSD).

Short history: On my '82 Fiat 124 Spider I had to go back to the early model's axle, because that was the only one I could find a reasonably priced LSD. It was good fun to drive, but unfortunately the small size of the diff and bearings made an annoing high number of bearing failures (lasted max. 1 year). The reason is not only the size of the bearing, but also the distance between them: with the LSD locked and grippy racetires, there is a huge momentum on the diffbody, which then seems to kill the bearings. First I suspected excessive heat generation as the problem, but it was not.

So something needed to be done, and I decided to go for the Fiat 132 axle, which is said to be the toughest of the Fiat lot. Well at least it was designed for 2L and Diesel engines, while the old one was designed for a 1.4L 70hp engine...

But, the LSD? I could not find one for reasonable money, so I decided to go for a ZF clutchplate type, as used in the BMW 6-cylinder engines. They are widely available and you can pick a complete one from a scrapjard for about 200 Euros (did that 2 times until now, on each one the oil looked like new, despite 100000+km's on).
And, most important, I found a small workshop which deals with these diffs, offers upgrades etc. We agreed to increase the number of clutches from 2 to 4, which increases the locking rate from 25% to 45%. Also we agreed to use Porsche Sintermetal frictionplates, which offer increased lifetime. Future option is to increase the locking rate to 75% by changing the ramp angles from 45‚° to 30‚°, we will see.

So, with the BMW diff out, a trialfit in the 132 axle looked promising, but more detailled look revealed that the casted housing of the diff could not be safely reduced in diameter neither to suit the 132 crown wheel nor to accept the 132 bearings.

So the workshop offered me to swap the BMW housing for a Alfa (156 or 164? they also used ZF clutchplate type internals) housing made of steel, strong enough to reduce the diameter. We decided to ignore the 8/10 holes mismatch for the crownwheel fixation and said that the new 8 hole topcover will be rigid enough ( I trusted the workshop man, he is racing autocross and should know when things break or not). This casing was then modified. On the bottom of the steel housing a tube was weld in to accept the 132 bearing. On top a new topcover was made, also to accept the 132 bearing, see fig1:


Next figure 2 shows all parts disassembled and fig.3 after assembly. Funny how small things can nearly kill a large project: it was impossible to find longer bolts in 12.9 grade, until I remembered the Fiat flywheel bolts of the smaller engines could do...

Of course you have to get the power to the wheels, so you need to connect them to the diff. Unfortunately all car manufacturers have fun by using very individual toothes, both number and shape...so it was impossible to find a matching gear to the Fiat halfshaft toothing. As an interim solution we decided to drill out BMW halfshafts, reduce the diameter of the Fiat halfshafts end and press them together, combined with a small welding, fig4. Will see how that lasts...but new halfshafts out of single blocks are under manufacturing.

Figure 5 shows how it looks now with the diff installed.

The adaptation of the axle itself was done by welding on the fixation for the upper sruts and the panhard rod. Relatively easy if you have a friend who owns a plasmacutter for the sheetmetal stuff, thanks Peter! Axle fits into Fiat 124 Spider without trouble, only the bend in the panhard rod needs to be increased to clear the bigger crownwheel.

The first driving was a bit frustrating due the banging noises caused from excessive (for a road car) preload. After reducing the preload by decreasing the clutchplate thickness, the breakaway torque is now 30-35 Nm (was 60Nm before). This made the noise go away...and it still applies the lock in a very smooth fashion.
So I am looking ahead for the first trackday now...see how it lasts.

Thomas

[per]

Needless to say I'm very interested in a similar solution... :)

[Guy Croft]

Thomas, you are so impressive!

GC

[sumplug]

There is an alternative. Use a Ford LSD as it has the same splines and nearly fits straight on. Just needs a little modification to fit.
Iam well impressed with this conversion though.

Andy.

[TR-Spider]

Impressions with the new diff from the first track day:
it was great fun to drive through and out of corners and sideways in the wet bits...
until at the end of the day an attempt of rapid acceleration from standstill (for a timed slalom) tore loose the shrinked/welded BMW bushing from one of the halfshafts...nothing broken really, just turned from a tight fit to a loose one, so the car did take me home with minimum torque applied...time to get the new halfshafts made!
So it looks like the (cheap) solution with the shrinked/welded BMW bushing can not be recommended. (Maybe the reason of failure is that the tolerances were too loose?)

Use a Ford LSD as it has the same splines

Andy, that is for a 125, what splines does it have? I have 3 Fiat axles in my garage and they are all different...
124 <1978 D=24.5mm 22 tooth
124 >1978 D=26.0mm 24 tooth
132 <1978 D=28.5mm 26 tooth
Lengthes of the halfshafts differ also, so you cannot swap them.
Would be interessting to know the size of the 125.

[Guy Croft]

Thomas, maybe you upset the metallurgy of the half-shaft by welding it?

GC

[sumplug]

Thomas.
Ive asked Andrew, the owner of the 125 for some pictures. And i hope he can measure the splines. He has a leak on the seal, so it will need the cover plate off soon. 125 and 132 are very similar[132 is basically a rebodied 125].
I know the Lada boys have used the Ford Escort mk1 LSD in the Lada axle with a 6mm spacer plate made up to line the splines up, and a bit of casing grinding, but it all fits nicely together.

Andy.

[TR-Spider]

Guy, I don't think the welding was the problem.

As you can see on the upper scetch, the welding was only thought as a "helper" all the torque should be transferred by the shrink fit (90mm long).

As you can see on the photo the weld was sheared off at the side of the sleeve.

During the making, the workshop reported me a problem with one of the shalfts, they were not fast enough so the sleeve got stuck at 2/3 of the way due to fast heat transfer between sleeve and shaft. They had to cut it off and make a new one, maybe they upset the tolerances there...
For such a long sleeve it would be a better solution to have two diameters, as scetched below, that should allow a much easier fitting.

[Guy Croft]

Thomas, hi

FWIW I'm sure you can do the torsional and hoop stress calculations as well as I can. Frankly I doubt that you could generate anywhere near enough hoop stress in the outer section to transmit torque on the scale you need. And even if you could, you'd never be able to put them together.

I have to say it's asking too much to rely on guesswork there, though I admit it is easy to say that after the event! As for the weld, a fillet weld on that small scale would not add any strength and especially when it has no penetration at all on the LH section. I think - as I say - with hindsight - that a component of that type and small size probably has to be a one-piece part.

GC

[sumplug]

I would look at reducing the BMW end down from near the groove, to match the Fiat, weld it up again, then sleeve it so its well held.
I'am just wondering how much custom halfshafts would be?

Andy.

[Acki]

Hmm I hope you can drive good with the LSD. Maybe a ATB could be better...
I think the shafts will brake, be careful there! It can hurts you, the metall is not very strong of a Fiat when the shaft work like a "mixer". Some guys told me horror's story from hill racing...

/edit: Ups the shaft are broken already... I don't read right.... sorry!

[4v6]

Would it be possible to drill through the shaft and apply a high tensile pin (welded in) of some kind to help give some security to the two parts and prevent them rotating out of fit?

[TR-Spider]

Hi Guy

Frankly I doubt that you could generate anywhere near enough hoop stress in the outer section to transmit torque on the scale you need

...I must admit you are right!
I just did the calculations myself, and I am now amazed that I could drive around a whole day...

So now I am waiting for the new one-piece halfshafts from 42CrMo4 to be finished, they already have been turned down to the basic shape, just need to cut the teeth, harden them and try again...

[1969race125]

Thomas.
Ive asked Andrew, the owner of the 125 for some pictures. And I hope he can measure the splines. He has a leak on the seal, so it will need the cover plate off soon. 125 and 132 are very similar[132 is basically a rebodied 125].
I know the Lada boys have used the Ford Escort mk1 LSD in the Lada axle with a 6mm spacer plate made up to line the splines up, and a bit of casing grinding, but it all fits nicely together.

Andy.

Sorry for the delay in replying - I hadn't seen this posting.

As Andy mentioned, I intend to post some more information and pictures of the Ford LSD / Fiat rear axle conversion when I get a chance. In the meantime I can confirm that the axle spline is 22-tooth, which matches the old English Escort (/Anglia/etc.) spline.

Andrew.

[TR-Spider]

To finish a long story...

today I finally mounted my new custom-made halfshafts.

They are made from 42CrMo4, the outer geometry is an exact copy of the Fiat one, the spline (BMW fitting to the diff) is cut in and afterwards induction hardened. Shaft diameter is increased compared to the Fiat one. While waiting I used the time and made me copies of the brake adapter plates from a strong light alloy to replace the ridiciously heavy cast iron Fiat ones. Somewhat amazing, on the late 124 Spider /132 the original front brake adapter is from 3 mm sheetmetal and the rear from 15 - 20 mm (!!) cast iron, I have no idea why...

While it became clear that the new halfshafts will take their time, I went to a high-class welder and discussed the repair of the broken old ones. We then welded them on two locations each, with an u-groove on the end surface and on the old location (old weld removed). Meanwhile the repair allowed me 500 km's of joyfull driving, The only things I avoided were heavy starts from standstill and I did not do trackdays with sticky tires. So now I have a spare set, which I hopefully don't need.

Thomas