Thread: max engine NM for LR HD axles and CV's

New 24 spline chrome moly shafts break around 8500 Nm. With 40:1 gearing that means 200 Nm at the engine can break a shaft through a locked differential in first gear low range. That is why the limit is traction and not engine torque.

My thinking is misplaced then? I thought that there was more mechanical sympathy at the axe hub the lower the gearing is.

I'll get to the simple Q i'm tossing around - is it possible to put a 700nm diesel engine in a defender that has lockers, HD axles and CV's and not be eating axles and CV's?

Good question I think.
I don't really know the answer, but I reckon its harder than it first looks.

It's a dynamic system when some wheelslip is considered.

I do not know know the maximum traction available on dry traction asphalt, but you may approach the specified 200nm figure mentioned above, if you drop the clutch
in first on such a surface. This action increases the torque applied by transferring fast spinning engine momentum to the drive line which then amplifies it.

But don't most diff/axle/cv failures occur when the a spinning wheel finds traction? This transfers at least a good part of the kinetic energy of the fast rotating wheel/axle into the driveline.
Effectively turning your driveline in a spring that its capturing at least some of the energy of the fast spinning wheel.
If the amount of energy transferred exceeds exceeds the amount that can be stored, at any point in that driveline - then the driveline will
break at the weakest point.
The amount of energy that can be stored in a function of the ductility of the metal, amount and shape (stress risers, etc.) of material.
That is the why normally short axles fail first as well as cvs.

I think this would be a quite difficult question to answer, and will be a function of traction, vehicle weight, Tyre pressures, engine torque, wheel, driveline and engine momentum, drive line toughness and maybe probably suspension

Still people who spare the right foot, don't drop the clutch, seems to have a good run with drivelines in my observations.

Clive

I think Clive is on the money.

I have HD CV's in my P38 which I have never broken to date, I do only run 235/85R16 wheels though. A mate of mine runs 12.5" wide tyres and has done CV's. Though he has a manual and I have an auto box, he does not drive like an idiot. The only reason a CV breaks is indeed to much grip or shock loading, which again comes back to a wheel gripping all of a sudden.

In other words, normal driving, not popping the clutch (for so far as the clutch can handle the torque!) the driveline components should be just fine. It's when heavy laden and wheeling on uneven, grippy terrain that things go snap.

So, I think the engines torque is not the limiting factor here because axles snap when rock crawling with 200nm engines as well without stalling the engine, it's the shock loading that get's to them. Since you did not stall the small engine, and certainly won't stall the bigger engine the problem is still down to grip Now, if you were to go street racing with huge tyres and a massive engine, you could exceed the grip/torque figure but I suspect the tyres will spin before that happens, unless again, you have massive wheels. My wheels, since they are only 235 wide spin before anything else breaks.

Cheers,

-P

Axle choice is more tied to the size of tyre you want to run and the way you are planning to drive off road. You want the axle size and strength to match the possible traction available. The engine size is not really that important other than it makes it easier to shock load the parts when driving fast and hard.

To understand the math. That 8500 Nm on a 32" tyre is around 2300 kg of tractive force you would need more than 3000 kg of load on the tyre before you could reach that traction force on a high traction surface. So, it is hard to do. Most people break the axles by shock loading. Spin a tyre in the air and drop onto a surface causing it to stop quickly. If you never drive hard (like in a race) on standard tire or close to it, they won't break.

On the other hand, if you are planning to race on 40s, then get a real axle with 40 spline shafts.

Thanks to all. at least it is feasible to put a high torque engine in a touring vehicle provided you don't treat it like a drift car. Off road is another issue. To proclaim we don't spin tyres off road is like saying you have never played with myself. It happens and there is no point denying it. Controlling your right foot can be a challenge when the red mist depends over the situation. Worse case is GU front diff with RCV CV's.

May I ask what engine you're considering MLD?
Don.

The rest off the drivetrain would explode though.