Shifter linkages: Rods or Cables?

The Roaring Forties uses a rod setup to shift the Audi gearbox. I would think the only application for a cable linkage would be a sequential gearbox, such as the ZF or the new Quaife unit.
Actually, you have the choice of either on some boxes.

Porsche,uses a shift rod from the factory, but a cool looking cable set up is available from Renegade Hybrids.

Renault, I have also seen some with a rod/linkage and some with cable.
I'm sure someone will be quick to correct me here if I'm wrong, but I'm fairly sure that RF use cable on the center shift cars with LH drive (Audi box).
GT40 Australia uses both rod and cable. Their
cars are designed for the Porsche G50, which
shifts from the front, now rear, via a push/pull and rotating shift rod mechanism. The push pull action is accomplished by a large Morse cable. The rod rotation is done via a system of rose jointed rods, cams, and shafts. It sounds complex, and is to a degree, but shifts like a gated shifter. It is very tight, and has virtually no play whatsoever.


On the Renault box you can use either system. GTD always supplied cable systems. These were improved by using heim joints instead of the knuckle joints, which sometimes broke! They need proper setting up but once done, worked very well. Before I had telemetry but when I was under a speed gun with graph print out, my gear changes with cables were about 1/3 second at best, average of 1/2 second. The cables will stretch over time and knuckle joints can fail. Heat sheilding the cables under the exhaust was important. It was easy as GTD had done all the development work and supplied it all ready to go, bolt it on and drive away!

I now have a solid link system that Frank fitted. It also has it good points and its bad points. I mainly chose it to get a first forward gear stick action as with cables on standard GTD you pulled back for first.

It also provides good and fast gearshifts. However the Renault box only goes so fast so compared to a well set up cable system it is only just a bit better in terms of speed of gear change. But the cable system needs to be well set up. I think that many are not that well set up. Therefore the solid rod system allows consistant, easier and faster gear changes. Although you can go quicker than the box can cope with!

Unless you have play in the rods! You need to make sure that the rods can't turn in the helicopter joints (glue and big bolts is a cure) and that grit can't get into the slider joints (often these are heim joints and not linear bearings). Grit can seize the system up solid and leave you stranded. Keeping it well greased also helps.

Getting the solid rods past the suspension arm is also an issue whereas cables just flex around them. Dog legs in the bars or holes in chassis rails have been used to date. Consider carefully if you cut holes in chassis rails. Extra bracing is required as a minimum?

You may need to brace the Renault gearbox on the solid bar system. This is from the belly of the box going straight backwards to the rear chassis member. This prevents the fore and aft motion of the box which happens more than you would like to think it does!

I think that the only design let down on my rod system is that the rear 4 inch sliding tube at the gearbox end is too far away from the centre line of the gearbox. I think it needs to be as close to the box casing as possible to reduce issues of the gearbox moving with the engine whilst the gear change system is fixed to the chassis. A re-rout of the system to inside the engine bay may overcome this. However it works as it is and reliably as well.

Now none of the above is difficult to overcome so really the choice is yours as to what system you want. There are second hand cable systems out there but no solid rod systems that I know of.

I like my rod system because it is first forward, has a narrow gate, less likley to break at a joint and looks good. Its weaknesses are that it still requires the same level of maintenance as the cable system previously used and is at risk of grit jamming it solid. It also cost more to install.

Finally on the rod system there are not a lot of alternatives to how the rear set up could be designed. Space and connections to the actual gear box are limiting factors. However having seen a number of gear stick arangements on other race cars (not GT40 replica) there are many, many ways to make the front system with reduced amounts of moving parts.

I am sticking with my rod system but one day I may experiment with some design changes to make it simpler than it currently is.


[ November 18, 2002: Message edited by: Malcolm M ]