Half shaft geometry question

[Slider79SC]

I am thinking about moving my engine and trans forward in the race car by a 1/2 or full inch. Currently the rear engine mounts are gone due to a tube frame rear and I could move things easily at this time.

My question for all of you is the angle of the half shafts. Will this cause any issues (binding) of the CV joints? Excessive wear? Anything you can think of?

Thanks,

[Pazuzu]

Shouldn't, even the 911 shafts are designed for more movement than that from the suspension cycling up and down. People take the 930 shafts and adopt them for high speed 4WD trucks running off-road races, where they're sitting at 20+ degrees at full tilt.

[burgermeister]

Higher angles will cause more wear. But, you're not expecting 200K miles out of a set of race car CV's anyway, so it probably won't matter.

[GaryR]

I moved my engine back about 1 inch in my 930 when I added a G50, the angle change was negligible.

[gestalt1]

i was thinking of doing this on my 68 912. i'm converting to a 6 cylinder so i'm cutting the 4 cylinder mounts out and adding the 6 cylinder mounts in. it would be easy to weld the new mounts in forward 1/2"-1". i would need to re-drill the holes or make an adaptor plate for the trans mount also. how far forward can the engine be moved before it hits the shock crossmember? is this really noticeable in weight distribution?

[Grady Clay]

Good question and good idea.

The sequence of changes are (roughly):

SWB has axles approximately straight from transmission axle flange to stub axle. This is why they could get away with such short axles and small CV joints.

In ’69 with the LWB, the transmission stayed the same fore & aft but the stub axle was moved to the rear 51 mm. This required larger CV joints to accommodate the angularity and a much longer axle to reduce angularity. This was done by moving the inboard CV joint closer to the centerline and the outboard CV joint farther out.

Starting with the ’72 915, the transmission allowed the axle flanges to be moved toward the engine by 10 mm. This reduced the angularity. At the same time Porsche made the CV joints thinner, effectively making the axle longer again. The change from the thin 108 mm CV joints of ’72-’76 to the smaller 923 100 mm CV joints of ’76-’86 probably didn’t change the effective length.

This is basically the configuration of your ’79 911SC.
There is still angularity to the axles, just not as much as in ’69-’71.

If you move the transmission 25.4 mm (1”) toward the front, you will increase the angularity. You can calculate the degree.

If you do this, I recommend you use the 108 mm CV joints from the ’85 and later 911s. This axle is the same as the 928S and uses the same CV joints as the Turbo. These big CV joints will tolerate more angularity. Another benefit of these is the tin end cap to retain the CV joint grease.

All this goes out the window if you use the 930 Turbo trailing arms. The dual roller wheel bearings require a much shorter axle than used on a 911.

Best,
Grady

[Slider79SC]

HI Grady,

Well let me throw a wrench in all of this.....

The transmission is out of a 79SC, BUT, the car is actually a 68' 912 that has had the trailing arm mounts modified like the original RSR's did back in the day to extend the wheelbase. (see picture)

It still uses the short steel trailing arms and I assume the original style hub flanges from those subsequent years. Again, ala RSR.

So now what would you suggest for axles? If you would like to come over and take a look at the car and give me some suggestions I would love it. I am in Arvada.

Thanks,

Jeff

[MBEngineering]

HI Jeff
you will loose strait line speed, BHP and through the corners CV joint over heating/wear.

regards mike