Tayloring an ZF-LSD into a Fiat 132 axle
Posted: March 22nd, 2007, 4:08 pm
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
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