Thread: Reverse Cut Ring & Pinion / aka High Pinion - front Rover diff

Originally Posted by uninformed

I must be having one of those moments. ...


If its 0.04 more revolutions per input over 4.11 I would have thought that the difference?

No, the travel on the wheel will be in inverse proportion to the diff ratio for the same number of turns of the prop shaft. So if we turn the prop shaft 4.11 turns to do one complete turn of a wheel on that axle, the same number of turns on the 4.14 axle will have turned a wheel on that axle 4.11/4.14 of a turn, which as a percentage = 99.28%, or, equivalent to having a tyre with an effective diameter of 31.77" if the first axle tyre is 32". Difference is a little under a quarter of an inch.

I think your calculations assumed that the diff was stepping up rather than down.

John

So a practical solution could be to run new tyres on the low ratio diff 'end' of the vehicle and half worn tyres on the high ratio diff 'end' of the vehicle and replace (also rotate) tyres two at a time.

Deano

Originally Posted by DeanoH

So a practical solution could be to run new tyres on the low ratio diff 'end' of the vehicle and half worn tyres on the high ratio diff 'end' of the vehicle and replace (also rotate) tyres two at a time.

Deano

It doesn't actually work like that. The distance a tyre rolls in a turn depends on its effective circumference, not its measured radius. As the tyre rotates, for any reasonable combination of load and pressure, the bit in contact with the road is flattened out of a circle. Now any tyre you are likely to use has a steel belt below the tread - this, while it will flex, will neither compress nor stretch to any measurable extent, so the effective circumference of the tyre is determined by this belt. Since the layer of rubber vulcanised to the belt is also constrained by the belt, the effective circumference of the tyre is a little outside the belt, probably for most tyres close to the base of the tread. And does not depend on how much tread is outside that, the tread being compressed longitudinally as it comes in contact with the road, to accommodate the inextensible belt.

Hence the state of wear of the tyre makes little difference, until you wear off enough tread that you cut into the rubber that behaves as part of the belt. But the effective radius will vary with make or model of tyre, as each manufacturer has their own ideas.

John

Mike, I thought that you were running 4.12 front and 4.11 rear with the 35's. Other than increase in strength, why chase the 4.14 for the front and leave the rear at 4.11? Curious that's all.

Diffs apart signals lockers to me , soon to be unstoppable.

cheers MLD

MLD, you are correct! It seems now all issues are sorted and i should have it back tomorrow morning. I chose the 4.14 as it was the strongest and the extra outlay was minimal, although to get the full advantage the diff would also need to be pegged..., i'll save that for a rainy day.

Bill from GBR replied to my email last night in detail. I've asked for his permission before i share it here... but long story short, there would be minimal if any extra wear in the drivetrain with this combination. I'm leaving to Cape York on the weekend so i'll need to get a few heat cycles into the gears before i leave. I'll also do a diff oil change when i hit the QLD border... it wasn't supposed to come down to the wire like this so fingers crossed... not ideal i know.

Well then, I'll stand corrected. I still wouldn't do it because of my mechanical paranoia but the figures look good. Have a great trip!

Thanks Davo, can't wait. First trip to the tip and hopefully not my last.

Bill from GBR kindly emailed me back and gave me permission to post his thoughts for anyone interested:

Running slightly different ratios (within reason i.e. less than 1%) is what I would describe as neutral. I think it is on the website but some manufacturers in the past deliberately ran slightly different ratio's because at high speeds in 4WD (assuming the t-case is locked), the vehicle handled and steered better at high speeds on dirt roads. You would run the slightly higher ratio in the front so the front tires slightly over ran the rears. A Land Rover is a full time 4WD so on road, what is happening is that the planetary gears in center diff are very very slightly turning. A 4.11 and a 4.14 are different by .07%. I suppose one could calculate how many revolutions per kilometer (or even if it is one complete revolution!) but the math is beyond me or maybe more accurately, the motivation is beyond me. So "in theory" , yes you are wearing the thrust washers in the center diff very slightly more but the question is - is this significant? My opinion is that it isn't. Remember that the center diff is almost constantly in motion anyway. Everything from turning in curves, depressions in the road such as potholes and differences in tire pressure. Oh and lets not forget that Rovers run significantly different tire pressures front to rear. My Defender 130 tire pressures are recommended at 28 front and 47 rear. Come to think of it, a slightly higher ratio in the front may be compensating for this and your planetary gears are actually turning less! I'd have to think about this for a while to say for sure but again the motivation is beyond me at present.

Having said all of that, what some people may be confusing or extrapolating is that LT230 transfer case are very very strong but are vulnerable to one thing. If you high center the car so that either your front or rear tires are completely off the surface and if your center diff is unlocked and you sit there mindlessly revving the engine, you will smoke the center diff within a minute or two. What is happening is that all of the power is going to the axle off the ground and the planetary gears in the center diff are spinning a million miles an hour. The center diff is splash lubricated so what happens is lubrication goes away and they get really hot, really fast and then they melt down. This is a lot different than the gears doing a partial revolution every kilometer or so.

Hope this helps and enjoy your new gearing.

Best regards
Bill
GBR