U-joints are not as strong when run at angle and the slip joint may not have enough travel and push too far into the transmission.

Some feel spring wind-up can contribute to traction problems.
https://www.youtube.com/watch?v=k2eXvhdAviM

An entire industry provides products intended to combat leaf spring wind up.
This is an early one.
http://tractionmaster.com/

Thanks for your reply. It's because the difference among traction forces of front and rear wheels are bigger than the nominal value, right?

No ... that video is a rear-wheel-drive car with leaf-spring rear suspension (which is the type most prone to axle wind-up - and you can see the leaf spring in the video doing just that). It's got nothing to do with the front wheels, which are completely unconnected here.

The issue is "wheel hop". The axle winds up, the geometry of the suspension pulls the wheel upward, the wheel loses traction and spins, the axle unwinds and that tramps the wheel downward, the tire regains grip but that winds up the axle. This sets up a big oscillation, and if the amplitude of that oscillation gets big enough, you can certainly start breaking things. You can see the axle hopping and tramping up and down together with the leaf springs winding up and unloading in that video.

All sorts of add-ons have been invented to combat this, usually called "traction bars" or "slapper bars" or some such thing.

Other types of rear suspensions which don't have as much compliance in the axle-twisting direction are the proper way to fix this.

Torsional oscillations in the driveline can destroy engine mounts, and half shafts, typically. The reason that doesn't happen very often is that people like me cure it at the prototype phase. The analogous issue with FWD cars was very common in the early turbo years.

Cheers

Greg Locock


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Hooning around in a 4000 lb car with 325 HP, on patchy ice over asphalt, will break a diff pin and distort the diff case.

The parts cost ~$300, way back when that was a lot of money.

Mike Halloran
Pembroke Pines, FL, USA

So help to understand. Let's consider a tractor, thereofore a vehicle with part-time 4WD transmission, no central differential and front axle lead. On turning, front wheels needs to run faster than rear ones and therefore there is a kinematic mismatch with the axle speed. Thus front tires will slip, there will a breaking force on the front wheels and higher is the vehicle speed and higher is this braking force, right? This kinematic mismatch generates higher stress into transmission gear teeth?

@Greg,
if the statement above is true, how torsional oscillation may be generated?

thanks

And, once again, we see the benefit to explaining the WHOLE situation, not just a piece of it.

Driving around in a 4x4 vehicle with the front and rear axles locked (no center diff and no slippage allowed) will indeed cause the rear wheels to push and the front wheels to drag when going around a corner because the front wheels travel a longer path than the rear ones do. Something has to give. Hopefully, one of the tires starts slipping. If the front-to-rear interconnection has compliance in it, it will store up some energy in the form of winding up or loading up whatever has compliance until such time as either a tire loses grip ... or something breaks because you are doing this on a very grippy surface that doesn't allow tire slippage to release the strain before something breaks.

On a low speed vehicle like that, axle tramping and oscillations as shown on the drag-race car shouldn't be of concern.

No part of the drivetrain in a tractor is very springy, so it shouldn't experience any torsional oscillations. The majority of the turning stresses would be after the the transmission or start at the front/rear power splitter, but the transmission would experience a little extra stress as the engine applies enough power to force a wheel or wheels to slide or give.

I think you would be surprised by the angular wind-up in the half shafts. And of course the torsional stiffness of the tires contributes.

I'd expect a very strong mode consisting of the mass of the tractor driving back and forth on the stiffness between the front and rear contact patches.

Cheers

Greg Locock


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

Greg - having watched a RWD car that was fitted with a drag racing spool (no differential action whatsoever) chirp, chatter, and hop its way around a fairly tight turn, I'd absolutely expect analogous behavior when the complete lack of differentiation is between the axles instead.

Norm

@LionelHutz: I believed tractor transmission was rather stiff do to high modules.

@BrianPetersen: I'm sorry for not being clear enough, I knew only that wind up and from your answers I realized there are many other types of wind-ups

Axle wind-up can reduce peak dynamic loads in the rest of the drivetrain, as long as the stress levels in the axles are not excessive. Excessive torsional displacement along the axle spline tooth faces can cause over stressing of the teeth.