Torque and rangie diffs in a series

[TheRecklessEngineer]

I was having a discussion with a mate at the weekend. I've fitted rangie diffs to my series (v8). The debate was over increased torque through the gearbox.

He said: Because the diffs are a higher ratio, there will be more torque required to drive them, resulting in increased torque through the gearbox and as it is a series box = bang.

I said: The engine produces only so much torque. It cannot produce any more by changing the gearing - therefore the torque through the gearbox won't be any more than normal for a v8 conversion. The only thing that will change is the torque on the half shafts which will in fact decrease, resulting in less likelihood of failure.

Who is correct?

[darkman]

I was having a discussion with a mate at the weekend. I've fitted rangie diffs to my series (v8). The debate was over increased torque through the gearbox.

He said: Because the diffs are a higher ratio, there will be more torque required to drive them, resulting in increased torque through the gearbox and as it is a series box = bang.

I said: The engine produces only so much torque. It cannot produce any more by changing the gearing - therefore the torque through the gearbox won't be any more than normal for a v8 conversion. The only thing that will change is the torque on the half shafts which will in fact decrease, resulting in less likelihood of failure.

Who is correct?

The higher gearing at the diff does req a bit more effort to drive. Its like fitting taller tyres.

[cipx2]

He is.

As you might remeber Power = Torque x Angular Speed.

Gearboxes, transfer boxes, diffs are all mechanic (cinematic) transformers. In theory (that is neglecting internal losses), in any kind of transformer the input power equals the output power.

If you put that together with the fact that you need to transmit the same amount of power to the wheels in order to obtain the same thing (speed/acceleration) as another LR with a higher diff ratio and same wheels, you'll realize that the engine will have to work on lower revs but will have to supply a greater torque (it will work on a higher load characteristic curve).

The engine supplies so much torque but it depends how often and for how long it loads the gerabox with this so much torque. Its not the same thing for a gearbox/TB/diff to drive a car at 40 mph for 100,000 miles or drive it at 80 mph for the same distance.

[GBMUD]

I was having a discussion with a mate at the weekend.

Who is correct?

Your mate, sorry.

Chris

[TheRecklessEngineer]

Hmmm....that's a shame.

I'm not sure I see it at the moment, but i'll chew it over for a bit. I'm sure it will become apparent at some point (probably when gears exit through the side of the gearbox...)

[Night Train]

Your mate is right.

The torque going in the gearbox may only be as much as the V8 will produce but what you have done in fitting the higher ratio Range Rover diffs is increased the load to the output of the gearbox. In other words you have made it much harder for the gearbox to turn the prop shaft.

So when you apply that V8 torque, which is already higher then the series box is designed for, it will have to transmit that torque through its gears and shafts. You said that all you have done is to reduce the torque in your half shafts. Where has that torque gone? It has gone upstream into your gearbox.

As said above, it is like running much taller tyres, or perhaps hanging a heavy trailer on the back and dumping the clutch to try and get your usual performance.

If you are gentle with the throttle then the box may just wear a bit more or faster then it would other wise. If you are hard on and off the throttle then the box will probably give up fairly quickly.

[Turbocharger]

For what it's worth, I think you're right. The peak torque when driving is decided by the engine and gear / transfer box ratio. (I'm ignoring inertial effects of the tyres when they spin/slip and 'grab' at the floor). However, with a higher ratio final drive, you'll see more time at these high torque values are sustained for longer periods (because with a lower ratio, the torque gives higher acceleration, and so torque x seconds equates to higher speed.

If it's peak torque that will break components, I say there's no difference. If the parts can fail through fatigue or wear out through sustained high-torque running, it'll break quicker with a higher ratio eg RR diff.

Settle a debate

[missingsid]

For me this is a little bit paranoid

Both myself and a far more experienced mate have been running Series 1 V8s for years (more than 20).

I have broken a layshaft twice both times when taking it easy.

I am notorious for caning cars and my LR is no exception, as to my mate, he may be a little older, more experienced, more reserved but then again he would drift it with a caravan on the back so maybe not.

If you run 7.50s then I don't see a problem if you are running bigger tyres then yes.

Marc.

[bill van snorkle]

Agree with most of the comments above, and want to add that series gearboxes suffer from flexible layshafts and main shafts. They are particularly vulnerable in the vicinity of 2nd gear ( roughly the middle of the shafts) Excessive use of 2nd gear as would be required when running high ratio diffs or regular heavy towing etc will cause the layshaft on series 1,2 and 2a boxes to fatigue and break, and on series 3 boxes which have more rigid layshafts, the mainshaft flexes and either fatigues and breaks or more commonly causes the bronze distance sleeve that supports 2nd and 3rd gear to break and or seize to the gear and or shaft. You get the idea.

Bill.

[TheRecklessEngineer]

Let me see if I've got this straight....

Peak torque won't change, however the gearbox will be required to cope with a high torque value for longer, resulting in faster wear.

Anyone got an LT77/R380/LT95 lying about? Seriously though - she isn't going to be driven hard or do any serious offroading. If/when the gearbox packs up I'll replace it with something more robust.

[nicks90]

to try and make it slightly clearer and easier for you to visualise....

think of a mountain bike.

set off in 1st gear and your legs pump away quite quickly with very little effort to get to 5mph. By then your legs are milling away like a lunatic.

now try setting off in top gear. You will be holding on the handle bars and yanking down with as much force as you can to just start the bloody thing turning!

total torque experienced at the back wheel wont change, as thats jsut the overall output from the cyclists legs. BUT, the sprocket attached to the pedals will be experiencing considerably more torque in the high gear when setting off.

translating this to your series, you legs are the engine and the front sprocket is the gearbox. So yes, putting higher ratio diffs will put more stress on your gearbox.

[Aragorn]

It will not increase the torque the gearbox sees.

The engine produces torque, the gearbox multiplies it, and these values will remain as they always have been.

What will happen though is that the higher geared output means you COULD put the transmission under more strain, as your holding a specific torque value for a longer period of time.

Example, if you engage second gear and floor it, the gearbox has to deal with the engines torque curve while the vehicle accellerates, now that you've changed the diff ratio, the gearbox will be dealing with the EXACT same torque curve, however the revs will take longer to climb, meaning your applying the torque for longer.

When a gearbox struggles to handle a torque value, the casing and shafts tend to twist, the gears try to push apart, and this force can split the casing of the box, the twisting action can also snap the shafts inside. By applying the torque for longer the chance of this happening increases, however imo its a bit like trying to say if you accellerate in 4th your going to break the box but if you accellerate in third you wont.