December 26, 2020. Drive shafts.

In a change from working on electrics, the axles for the drive shafts arrived in the mail, so I decided to work on those. As I’ve probably said before the C5 Corvette outer CV joints have the same splines as the Porsche 930 inner CV joints I am using.

Design considerations

The choice of components in the car and their layout has led to some unusual design challenges that the drive shafts need to work within. Often with mid-engined cars people will use an inverted Porsche G50 transmission or another type where the outputs from the differential are at a position above the input shaft from the engine. This enables the drive axles to be near horizontal at normal ride height, here is an example:

Credit to: Thomas’ Gardner for the picture.

Apart from minimizing the angles that the CV joints need to work at this approach also reduces the plunge or difference in length that the axle has to absorb as it goes through its range of motion i.e. assuming it is horizontal at normal ride height then the extra length needed for +10 degrees movement is the same as for -10 degrees.

Porsche G86 (Audi 012) transmission

In the my case I have used a transmission from Porsche 986 (Boxster). Which as a mid-engined car already would if inverted would drive in the car in the wrong direction.

As you can see from the picture above when connected output from the transmission is below the height of the crankshaft in the engine. Therefore as the engine and transmission are going to be mounted as low as possible in the chassis the drive shafts have to accommodate a situation where they are at their lowest angle at full droop and this only increases with bump in the suspension, similarly the axle is at its shortest at full droop and only gets longer as the suspension is compressed.

The picture below shows the situation as per the cad model, in reality at full droop the axle is near horizontal at static ride height it is about 7 degrees up and at full bump it is 13 degrees up. Also allowances need to be made for flex in suspension bushings and movement of the engine on its bushings.

Porsche 930 CV joints

In the off-road / dune buggy community Porsche 930 CV joints have become the de facto choice, due to them being able to operate at angles of up to 25 degrees and be extremely strong. They are also a plunging type joint to take up the difference in length required through the suspension travel. They also happen to have the same splined axle requirements of the Corvette C5 with many aftermarket axles being made to fit the 930 joint so these are used for the inner CV joints.

Corvette C5 CV joints

These are not a plunging type and due to the small clearance for the outer CV boot, no movement could be allowed for at that end. Therefore all of the plunge would need to be dealt with at the inner end.

Measuring, repeatedly…

After measuring the axles lengths repeatedly through the full range of motion that the damper is capable of (which is more than can be used with the bump stop installed. I could see approx. 20mm of plunge would cover the entire range of motion which the 930 CV can do on its own. Allowing another 10mm of axle length beyond the full droop position gave about 30mm of plunge and with the bump stops installed approx. only 50% of that will be used so there is an allowance for flex and movement in the system.

Making the things

The axles were supplied by Pacific customs who will sell individual EMPI Chromoly axles rather than only as a pair the same length like most suppliers, which is good as I needed two different lengths. A 27″ and 26″ shaft were ordered. The 26″ as the preferred 25″ length was not available.

There was some information I’d been unable to find out before they were delivered, like exactly how long the splined sections were and if they were a sliding or push fit into the joint.

There is 3″ of splined section at the end of each shaft which for the shorter shaft was sufficient to have enough left once it had been cut down to length.

The quoted length of the shaft is the overall length the clip grove at the end being 1/8″ in from the end, and they are a sliding fit into the CV joints.

In the picture above you can see the C5 outer end that needed to be replicated in the new axle, in the OEM version the axle is constrained on the outer end by the clip thats pushed through the joint, that comes up against a shoulder on the shaft. With no shoulder on the new shaft a clip was to be fitted in a grove to restrain the movement.

My industrial band saw cut the shafts to rough length easily enough and the scrap end was used to test cut the clip grooves.

Here you can see the cut off piece with a spiral lock clip fitted. Note the visible outer layer of hardening on the shaft.

Angle grinder to the rescue

Whilst facing off the cut ends was able to be done with a carbide tool, cutting the grooves needed further thought, I ground a high speed steel tool in an attempt to cut the grooves, but this was destroyed by the hardened shaft. Not having a carbide tool the correct width for the groves then instead the 4″ angle grinder was mounted to the lathe as per below…

A 1mm thick was used with the lathe counter rotating to the grinder at 90 rpm. The 1mm disk cut a slot approx. 1.3mm wide, which is what was required. As unlike normal lathe tools the disks wear quickly repeated cutting an measuring was required to get to the correct depth. Above shows one of the outer ends completed.

With the clips installed and the shaft inserted into the outer end it looked like this…

The clip thatwas pushed through the joint was held compressed by 2 halves of a aluminium tube held together with a hose clamp. The first side only took 3 or 4 attempts to install. Whereas the second side degenerated into a frustrating greasy mess of a fight with the thing, the 40c+ temperatures in the garage were not helping either. So this was left alone for the day.

Eventually it surrendered and the second shaft was completed. Another problem I needed to overcome was the extremely small gap I’d left in the design between the outer CV joint and the bottom of the damper.

I’ve used the type of CV boot shown below, these are extremely flexible and I hope these can operate in the narrow gap as shown above.

One other thing that was need was a deep socket to do up the nuts at the end of the drive shafts, as the one I’d just bought for the job was too short it was cut in two and lengthened with short piece of scaffold pole.

In assembling the drive shafts a lot of grease was packed into the joints, some of which stayed in the joint, but I made this a really messy job overall.

With the drive shafts completed the the remaining work on the rear suspension could also be completed. lower A arm bushes were greased, wheel bearing and parking brake bolts were tightened down as were the upright to A arm tapered studs.

Also the brake calipers have been installed.

What remains to be done is replace the springs with some heavier items, set the toe on uprights and some small work on the brake tubing.