Transmission Input Shaft Material

[Nicholas R]

Does anyone know what material is used for the input and output shafts of the manual transmission? (Renault UN1) I'm guessing its something like AISI 4140, probably hardened or oil quenched but I'm not sure. I've been searching but so far I cant find any information on it.

[DMC5180]

You might ask TOBY at DMCNW, since he makes the Performance Upgraded Couplers.

[WelmoedJ]

As I have read and understood the gearbox is not a UN1 but a 369.

[DMC5180]

As I have read and understood the gearbox is not a UN1 but a 369.

You would be correct! It is a Renault 369 series and is in the UN-1 family. They share many internal components. I'd assume the grade of steel used in either transmission to the same.

[David T]

The problem with the input shaft is not so much the materiel it is made out of, rather that they pined the coupler. They drilled a hole to stick a pin through the coupler and the shaft to hold them together. That created a weak point that concentrates the stresses and when you put too much stress on the shaft it breaks at that point. Improving the materiel won't get you much. The answer (as you know) is a one piece shaft with no coupler. You don't need any exotic steel for the shafts. BTW, they are probably case hardened and hollow-ground. It is not easy or cheap to fabricate transmission shafts. Never mind the cost of the steel. The transmission was designed to handle a certain level of power. If you increase the power handling capacity of the input shaft figure you will soon find the next weak link. My guess is the crown wheel in the final drive.
David Teitelbaum

[Nicholas R]

To clarify, the reason I need to know the material is because I need to know the maximum tensile and yield strength of the the shafts. This will allow me to calculate the minimum required shaft diameter for a desired safety factor at any given torque as applied by the gears. At second gear where the combined bending moment is the highest, the shaft is about 1.1". I need to calculate whether or not this is enough for different torque inputs.

http://img254.imageshack.us/img254/5849/equation.png

I know everything but the yield strength (sy) and the endurance strength (s'n). I cant get those unless I know the material. I can guess the material but if the info is out there I want to find it.

I know about the coupler. I've talked to the GT40 replica guys quite a bit about what the transmission can and cannot take; which is surprisingly a lot. Several said they run 400hp regularly on their stock UN1's with no issue.

Either way I need to do the math.

[Nicholas R]

I think I got what I needed. It looks like the shafts are either AISI 4140 or 4340.

[David T]

Does your math take into account the varying profile of the shaft? Does it also include a large safety factor for shock loads? Talk to Lotus people and you will see that even though you can uprate the transmission, a steady diet of more than 350 HP, or full acceleration starts will significantly shorten the life of the transaxle. I say transaxle because you cannot separate the final drive form the transmission. Whatever you put into the transmission you have to also handle in the final drive. The crown gear will start shedding teeth. The UN-1 in the Lotus Esprit V-8 is on the hairy edge of being overloaded. Much more than 350 HP and it could not last long enough to have a warranty. And that was a hand-built, massaged piece with better heat treat and finely machined parts. In fact the early V-8's had their torque limited by the ECU in 1st and 2nd because of concerns of longevity. Later models had the High Torque Upgrade to increase torque in 2nd at lower RPM's.
David Teitelbaum

[Nicholas R]

Does your math take into account the varying profile of the shaft? Does it also include a large safety factor for shock loads? Talk to Lotus people and you will see that even though you can uprate the transmission, a steady diet of more than 350 HP, or full acceleration starts will significantly shorten the life of the transaxle. I say transaxle because you cannot separate the final drive form the transmission. Whatever you put into the transmission you have to also handle in the final drive. The crown gear will start shedding teeth. The UN-1 in the Lotus Esprit V-8 is on the hairy edge of being overloaded. Much more than 350 HP and it could not last long enough to have a warranty. And that was a hand-built, massaged piece with better heat treat and finely machined parts. In fact the early V-8's had their torque limited by the ECU in 1st and 2nd because of concerns of longevity. Later models had the High Torque Upgrade to increase torque in 2nd at lower RPM's.
David Teitelbaum

Yea I reluctantly disassembled my input shaft to pleasantly find that under 3rd and 4th gear the shaft is quite a bit larger in diameter, as well as polished, which is nice as 4th gear applies the highest bending moment on the shaft (assuming you take 4th to 5500rpm).