Question about how ZF transaxles are mounted

[Jim Rosenthal]

If you look closely at original GT40s, and the way that the bellhousings are made, you see that the original small bellhousing, which is just big enough for the multiplate clutch that was fitted to the original cars, has ears projecting from the lower curvature of the bellhousing. These ears accommodate bushings through which are mounted bolts that run through the lower crossmember at the rear of the monocoque. Because I wanted to use a larger single-plate clutch, I opted for a bellhousing made by ERA which does not have these ears. ERA furnish a set of side mounts with their cars, which help distribute the load of the weight of the transaxle down into the rear crossmember. Otherwise the entire weight of the transaxle would be borne by the ears of the transaxle top plate. This doesn't seem like a good idea to me.

ERA's crossmember is flat at the contact point with the side mounts, so the cushions of their side mounts bear on a flat surface and the load is thereby applied to that surface, helping to hold the transaxle up, easing the load on the top plate mounts. Unfortunately in a Mark I monocoque, the analogous surface of the rear crossmember is NOT flat like it is on an ERA moncoque, so (although they may be willing to let me buy a pair of their mounts) it's not clear to me how they will work. However, it seems clear that I need to figure out a way to support the ZF transaxle from below, and distribute some of the load of its weight into the rear crossmember.

I would like to know how other GT40 car builders whose cars can use ZF transaxles (or RBT or ZFQ, which are functionally the same thing) deal with this issue. I have some time to figure out what to do, since the car is still months away from actually being started and run, but this seems like a good time to solicit advice on this. Not being the sharpest tool in the shed, I have been mulling over this for a while, and worrying about the wrong thing, as usual: I thought that the lower ears were intended to stabilize the transaxle against torque reactions forces caused by the axles turning and driving the car forward. While that may be true to some extent, I think the real reason for the lower mounts is just to take the vertical weight of the transaxle off the upper mounting plate ears. The design of the ERA side mounts makes that clear; I didn't work it out on my own.

Most of the people on this forum are far more experienced than I am at figuring this sort of thing out, so I would appreciate input from all on this, especially the folks that are using the same combination- SBF and ZF or ZF type box. All comments welcome, though.

And, hey, at least I didn't start it up, drive it around, and discover this problem by breaking something- better to see it in advance.

 

[David Briggs]

Safir makes a version of the original GT40 bellhousing that has those mounting ears.
It is designed to work with a -2 ZF/RBT and a 6 bolt 302 block.
It comes w/a small flywheel and a McLeod dual disc clutch.
I have that set up on my SPF and it works very well.
I used a Mcleod hydraulic t/o bearing and a small Powermaster geared starter.
Their flywheels are all "o" balance and for most of us ordinary mortals a 28 oz or 50 oz weight needs to added to the flywheel.
Dave

 

[Jim Rosenthal]

Are you able to drive the car on the street etc? The reason I went for the bigger clutch was that I wanted the car to be street-driveable and the multiplate clutches don't seem to last as long in street use. They are clearly better for track use; no question on that.

 

[David Briggs]

I had some concerns initially because the whole assembly is the same weight as just the flywheel on my old setup and a lot of dual disc clutches seem to chatter.
But it all works very well, the clutch is very smooth and the car is so light that the smaller flywheel/clutch assembly still launches ok.
The 6 mounting points make the car very responsive to the throttle.
It is worth the price of admission.
Dave