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Need Input: Drive Shaft design for BMW E36 with LS2 and T56

Need Input: Drive Shaft design for BMW E36 with LS2 and T56

(OP)

I have a BMW E36 with an LS2 V8 and T56 trans swapped in.

I am looking for input or correct formulas for calculating critical speed, spec out correct material dimensions/properties for my application.
Also I want to consider any other design factors that I may have overlooked.

I bought a kit for the swap that included an aluminum 3" diameter drive shaft. I've had this drive shaft repaired once and recently twisted the tube beyond repair.

Now I'm in need of a new drive shaft and plan to design it properly for my application.

The car makes 485rwhp/434rwtp. The current rear gear ratio is 3.23.

Drive shaft length is 43.5 inches U-joint center to center. Due to space constraints 3 inch diameter tube is max.
It's an IRS rear so the diff. is stationary.

The drive will run up to 7500 rpm when the car is on the race track. (possibly higher if I change to a shorter rear gear ratio)

The original DS that came with the swap kit has a
1350 U joint at the trans end and 1310/1350 conversion U joint at the diff end.
The configuration is like Style D in this link.
http://www2.dana.com/expert/wc.dll?hvtss~decrits~w...

Any help with this would be greatly appreciated.



RE: Need Input: Drive Shaft design for BMW E36 with LS2 and T56

I'd just go to the local driveshaft shop, give them the specs you've cited, and ask for a shaft with a 3" >steel< tube, balanced to, say, 9000 rpm.

Mike Halloran
Pembroke Pines, FL, USA

RE: Need Input: Drive Shaft design for BMW E36 with LS2 and T56

You may need to go to a graphite wrap, 3 inch propshafts are OK for taxis but not great for higher torques, and high rpm.

One alternative might be a torque limiting cal for the engine in first gear.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Need Input: Drive Shaft design for BMW E36 with LS2 and T56

(OP)
How can I move this post to the Automotive Drivetrain section?

Thanks for the replies but here is what I am looking for.

1. formulas to calculate drive shaft critical speed. I have found a few but they seem to vary, some consider u joint/shaft configuration, u joint angles, some do not.
2. formulas to determine required drive shaft torsional strength for an automotive application

RE: Need Input: Drive Shaft design for BMW E36 with LS2 and T56

Just a reminder, if you your drive shaft is no longer your weakest link, something else will be... I'm guessing the trans, if it took you a while to brake the current one, I'd probably just replace it with the same and get a spare. If you broke it without trying, get a steel one, and start budgeting for a new trans and rear end.

Good Luck! Sounds like a fun car!

RE: Need Input: Drive Shaft design for BMW E36 with LS2 and T56

Critical speed for a metallic drive shaft of a given length is mainly a function of tubing diameter. Aluminum's lesser stiffness essentially cancels out it's weight advantage. Adding wall thickness only lowers critical speed (slightly).
Online calculators and Even the SAE driveshaft manual's critical speed formula and tables assume too much stiffness of the transmission and differential mounts.

What type of u-joints are you using? Cardan type joints create a secondary couple under load that can excite the shaft's bending critical while running at half the actual critical rpm. CV joints cause no secondary couple. Not sure about those big 3 legged rubber types.

Torque capacity increases ~directly with increased wall thickness.

RE: Need Input: Drive Shaft design for BMW E36 with LS2 and T56

as Tmoose says, the simple calculation with the ends of the tube pin jointed overestimates the resonant frequency. It is exactly the same equation as for a beam,

http://www.ewp.rpi.edu/hartford/~ernesto/F2013/SRD...

Of course one option would be to install a 2 piece propshaft.

If you have access to finite element software you could build a model of the driveshaft, mass it up with the UJs, and put in representative rigid models for the diff/axle and powertrain, and linear elastic mounts. There is no particular reason why that would not give an accurate frequency, if you know ALL the numbers, and even a half-assed guess would be better than nothing. ideally you'd use a flexible model of the engine and gearbox, the first resonance of that may be a bit close to shaft whirl, but I can't remember if that is a significant effect (I haven't worked on single piece drivelines for 15 years).


The torque capacity of a tube is easy, yield strength in shear*area of metal*radius of tube for a thin walled tube. How much of that you are able to use is another question. Of course you don't actually know what loads you are putting in either, the maximum torque is probably a factor of 2 times gear ratio*max engine torque, if not more.

The long and the short of it is that for a 500 hp engine with 6000rpm and conventional gearing I'd want a 4.25" single piece propshaft, so you are definitely pushing it.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Need Input: Drive Shaft design for BMW E36 with LS2 and T56

My 100 HP (gross rating) '52 Ford uses a 1310 U-joint, I can't believe that hasn't shattered yet in your car. As ameyerrenke says, if you fix the DS, something else becomes the weakest link.

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