Diff-specific bunfight thread

[honitonhobbit]

I just love Bill..

What in gods name is the point of 500bhp in an off road vehicle? All it says is "Who ate all the pies" when it came to vehicle building.

In a real world application it's light weight plus oodle sof BE torque with a reasonable range of torque spread form low to high rpm added to sensible axles/driev train

Which D44 - for christ sake there are so many it would take half a wet weekend to count them and then you would miss out the hybrid versions and the amalgamations. Same goes for D33,35,60, 70, 80 and whatever.

Rockwells - The "I need a big truck as my P*n*s is so small" axle. Just use the front axle off a New Holland 4x4 - it's tougher. cheaper and lighter and you get 4x4x4

Check this out for common sense http://www.engbergracing.com/spider/

Just right, all you need

[SOA 93]

Which D44 - for christ sake there are so many it would take half a wet weekend to count them and then you would miss out the hybrid versions and the amalgamations. Same goes for D33,35,60, 70, 80 and whatever.

As this topic is about diffs not axles I used D44 as a generic term, I suggested the D44 as a compromise between weight and strength, it is available in the uk " stuffed under the back of diesel Jeep cherokee" , this was suggested as part of a hybrid axle so what vehicle it came off is not important as the diff centres and driveshafts would be upgraded.

The BMW option has me a little puzzled, on one hand there is talk of portal box's reducing torque to the axle shafts which I can understand, but then some suggest 300bhp capability through a BMW axle that runs on about 22" tyres, what happens when you stick 36" tyres on them?

[Hybrid_From_Hell]

All it says is "Who ate all the pies" when it came to vehicle building.

A real tech thread I thought .................as I read tho it

................Then I got to HHs Post

PMSL

Superb

Nige

[bill van snorkle]

Any diff that is going to have hub reduction of 2:1 or more and is also expected to cover reasonable distances on the road with 36 to 40'' diameter tyres would probably need to have a ratio of no more than 3.5:1, be happy to run upside down without frying the pinion bearings,have difflock availability and have a reasonable parts backup in Europe.

If many LR owners in Europe are now successfully running 35'' tyres on strengthened Ashcroftised LandRover diff/axle assemblies with some of the OTT horsepower and torque figures I see bandied about on these forums, then it is not unreasonable to expect them to survive a 12 to 15%(40'') increase in tyre size with a 2:1 reduction gear attached to the ends.

The best straight out diffs I have ever put under a Landrover were the early fully floating Salisbury style Transit van diffs, cut and remodelled to attach Landey hubs, swivels etc. They were very strong and beautifully designed and finished with proper thread adjustable side races. They also gave more ground clearance than a Rover type diff. Unfortunately the highest ratio available was 4.623:1 so not really suitable for portals unless one was building a tractor.

bill.

[Astro_Al]

Yes Bill - the ratio thing is bang on - that's partly what led me to Ford 9s.

Regarding the power figures. I essentially agree with the comments. However, a regular LS is going to be 350 to 400, and I plan on a little fettling of my own, both for the sake of personal interest / grin factor, and also because I want to use it to investigate a few things for something else. If you start with a baseline of say 400 from a crate LS2, I really don't think building an axle to take 500 is in any way excessive??? Its not so much that I think I need 500 for its own sake. Anyway, there is already an engine bunfight thread, this is a diff bunfight thread. Savvy?

Honitonhobbit - yeah. Not so different. That's essentially pretty much it, just an ali block V8 and some slightly beefier axles to cope. I'm not sure where all the hugemongous tank comments fit in - the idea of a hybrid axle is to save weight, surely?!?

Thanks for all the suggestions so far. Its still pretty up in the air. I'm liking Dan's G-wagen suggestion - I'll defo look into that. Anyone know how they compare to Toys / Nissans etc. Might be good to use something with a genuine locker if I can.

Cheers, Al.

[dirtydiesel]

I'm not following the 30% thing...?

Torques are products of ratios, no?

What am I missing?

Al.

Has someone spiked my coffee this morning? It doesn't make sense. 2:1 is 2:1. If you try to balance a 10 lb load attached to one end of graduated lever and you place the fulcrum at the 1 ft mark, you will need to push down at the 2ft, mark

(a ratio of 1:1) with a force of 10lb to acheive a state of balance.Disregarding the weight of the lever material, at the 3ft mark you have a 2:1 mechanical advantage so should only need to push down at this point with a force of 5lb to acheive a state of balance. That is half the force that my hands need to apply=half the stress. A pair of gears are just continuous rotary levers and the same principal applies surely ? Discounting frictional losses,with a 2:1 ratio gearbox you put 5ft lb of torque at 100 rpm in one end and you get 10 ft lb out the other end at 50 rpm. and vice versa? I'm confused.If the 30% number hadn't come from Dan I would be more sure of myself.

Bill.

I think i may have miss-worded what i said.

Allthough with a 2:1 reduction the cv's and half shafts are seeing half the torque, they (the shafts and cv's) are not as a result twice as strong (only around 30%).

For example;

If you have an axle that can stand 500bhp, if you add 2:1 portals to the ends, it does not magically become capable of withstanding 1000bhp.

It does however give the diff and shafts better protection from backloads from the tyres, but they still carn't stand the 1000bhp.

This little gem was brought to my attention by a higly experianced gearbox designer and engineer, he did give some very good reasons for this but i was to dense too absorb them.

Thanks for all the suggestions so far. Its still pretty up in the air. I'm liking Dan's G-wagen suggestion - I'll defo look into that. Anyone know how they compare to Toys / Nissans etc. Might be good to use something with a genuine locker if I can.

Don't know how they compare but i doubt anything would see off the rear diff, might be a little weedy on the shaft diameters for mogs though.

[simonr]

I think i may have miss-worded what i said.

Allthough with a 2:1 reduction the cv's and half shafts are seeing half the torque, they (the shafts and cv's) are not as a result twice as strong (only around 30%).

I'm afraid, I agree with Mr Dirty here!

Think about what generally breaks diffs & shafts? It's not the raw torque from your gearbox, but more generally allowing a wheel to spin then stopping it suddenly.

The deceleration suffered by the diff will double, but this is mitigated by the torque at the wheel trying twice as hard not to slow down.

I was under the impression that, in coping with shock loadings, the strength was closer to the square root of the reduction in gearing - so by halving the ratio, you get a 1.41 x increase in strength. It was a long time ago that I looked at this kind of thing- so I may have got a bit muddled - but i'm very sure that if you halve the final output ratio, it is not sensible to assume you also halve the peak input torque experienced just because some of the energy involved in the failure comes from that 'stored' in the rotation.

30% would seem a reasonable rule of thumb (assuming I'm not mistaken) - with a bit of a safety margin built in!

Si

[dollythelw]

whats stopping you using indy diffs in beam axles?

[Astro_Al]

Personally, nothing.

I think someone mentioned it earlier, but if you can make a hybrid axle, I think you can figure a way to put the diff you want in the middle - its only a housing.

Isn't this nap-time for you Jez?

Al.

[dollythelw]

no sleep till the ferry and tonights gonna be a long one

[Astro_Al]

Man - take a leaf out of my book.

Calories expended on vaporbuilding this month: zero. (I'm on a low-fab diet at the mo...)

Must be nearly there!?

Al.

[dollythelw]

not close enough for my liking, if we get out before 3am tonight we will be doing well

[bill van snorkle]

I'm afraid, I agree with Mr Dirty here!

Think about what generally breaks diffs & shafts? It's not the raw torque from your gearbox, but more generally allowing a wheel to spin then stopping it suddenly.

The deceleration suffered by the diff will double, but this is mitigated by the torque at the wheel trying twice as hard not to slow down.

I was under the impression that, in coping with shock loadings, the strength was closer to the square root of the reduction in gearing - so by halving the ratio, you get a 1.41 x increase in strength. It was a long time ago that I looked at this kind of thing- so I may have got a bit muddled - but i'm very sure that if you halve the final output ratio, it is not sensible to assume you also halve the peak input torque experienced just because some of the energy involved in the failure comes from that 'stored' in the rotation.

30% would seem a reasonable rule of thumb (assuming I'm not mistaken) - with a bit of a safety margin built in!

Si

My head hurts from trying to visualise all that Si. I am now really struggling to maintain confidence in my puny 1:56:1 portals ability to preserve my diffs and halfshafts. Surely once a spinning wheel grabs and stops or slows, and doubles the diffs decelleration, the influence of the wheel trying twice as hard not to slow down does not apply anymore, as it has stopped or slowed ,and was the cause of the decelleration of the diff? In my simple example above, whereby you put 5 lb ft of torque at the input shaft of a 2 :1 reduction gearbox and get 10lb ft at the output shaft, does it not also follow that if a spinning wheel at the output shaft suddenly grabs and applies 10 lb ft of resistance, then via what is then 1:2 step up gearbox this resistance is reduced to 5lb ft at the upper axle shaft or in other terms, half the torsional shock loading that a non hub reduction halfshaft would be subjected to in the same circumstance? As an aside, to a small degree the slight cushioning effect of the lubricant film between the hub gear teeth, splines etc should also reduce effect of shock load of a suddenly decellerating wheel compared to a direct drive axleshaft.

Bill.

Good luck Jez.

[biasbilt]

I reckon everyone is lookiing at this from the wrong direction, as you're all starting at the wheel and workiong back to the diff, but that is the wrong way to look at things in my opinion. The diff will see torque from the engine, multiplied by the gearbox. This will then be passed through the half shafts to the portal boxes to the wheels and hence to the ground. Assuming that no component breaks (i.e. is unable to withstand the torque being applied) the wheel will turn and the vehicle will move forward (again assuming there is sufficient friction bertween tyre and ground).

Where it will get tricky is that for driving around on tarmac, only a little torque is required, so the components never see peak torque, before the vehicle starts moving forward. When a wheel is in the air and spinning and then drops and grips instantly, the components will then see the full torque applied by the engine, and will either survive or grenade depending on their strength. If they are all strong enough, remembering that the ratios will multiply torque, all components will survive.

Assumptions for the sake of the arguement, the engine has 200 lbft, it drives through 4:1 ratio in the gearbox, 3.5:1 in the diff and 2:1 in the portal box and 36" tyres:

There will be an application of 200x4=800 lbft on the diff, 800x3.5=2800 lbft on the halfshafts and portal boxes, 2800x2=5600 lbft at the hub. Due to the size of the tyres (36" = 3') 5600/3=1867 lb effective force pushing the axle forward (in each corner assuming equal torque splits front/rearand side to side). This will give 7468 lb of force pushing the vehicle forward.

If you are driving on a flat grippy surface, you can assume that the coefficient of friction between rubber and ground = 1, so all that force will translate into forward drive = acceleration, but the torque is applied gradually as the engine speeds up from tick over to full power, with the associated gradual increase in torque. If you are offroad on mud and a gradient, the force may well overcome the (far lower) friction between rubber and mud/slippery rock etc, and the tyres will just spin, so there will be limited or no force applied = stuck. If a wheel suddenly grips, the effective torque will spike to the maximum for each component (as generated by the engine), but will not exceed that amount (where would it come from). Provided that they can all deal with that shock load (and I don't know how that affects things in terms of teeth seperation etc), there should be no problem.

Hope that sort of makes sense, I kept being distracted, but the long and short of it is, if the diff can take the torque being applied by the engine, it will not be the weakest link. Flame away!

Toby

[Astro_Al]

Ok, some good stuff rolling in. Thanks guys.

I'm still after some tech on the whole 30% thing. Are you guys thinking of max shear stress or something? I'm just trying to figure out where that is coming from. I don't necessarily think its wrong though... Is it a derivative of the tube vs bar argument or something?

Biasbilt - great post. Couple of points:

Agreed - the source of maximum/all torque is the engine (though rarely operating at max torque of course).

I'm not sure about a coefficient of 1 between the ground and a tyre of an offroad vehicle under 'heavy' acceleration - you'd certainly have some slip I guess tyres wiggle and squirm and do all kinds of weird things - especially with big lugs, deep sidewalls etc, but the idea makes sense compared to mud etc.

Regarding the sums you did - wouldn't the tyre moment 'length' be the radius (i.e. 1.5' rather than 3')? But whatever - neglecting that and losses in the system (20% ish?) I agree. Your numbers seem low - there's no transfer box in there, maybe thats it.

Sorry - just to clarify in the interest of complete retentiveness - torque increases from tickover to max torque, not max power (but I know you know that ).

The problem is that people are discussing peak torque which in fact could be much higher than the calcs suggest - I suppose (correct me if I'm wrong) that peak torque could actually occur when no lockers are engaged and you have, say a couple of wheels in the air, one slipping and the final one with excellent traction - all your torque is going to be routed to that single wheel, so it can easily exceed the figure for the 4-wheels in contact case. *pop* Or am I smoking crack again?

The more wheels you have with some traction, the safer your driveline will be (in an unlocked situation). Am I right?

There is also rotational inertia I guess - if its all spinning merrily away at high revs, its gonna be difficult to stop when you gain traction. I guess this is the source of the torque spike people talk about. It will certainly increase the torque figures we come up with in the sums.

I suppose I should actually dig out some number and do the calculations, since I already have the boxes, portals and a target engine output...

Keep it coming / point out the holes in my thinking. Any more centre section inputs?!?

Al

[Mark90]

I suppose (correct me if I'm wrong) that peak torque could actually occur when no lockers are engaged and you have, say a couple of wheels in the air, one slipping and the final one with excellent traction - all your torque is going to be routed to that single wheel, so it can easily exceed the figure for the 4-wheels in contact case. *pop* Or am I smoking crack again?

I think you've been smoking that crack again.

With no lockers engaged and a couple of wheels in the air you'll not be generating torque at the wheel with good traction. However if all diffs locked then you could transmit the full engine torque (minus loses) to the one wheel with traction, or at least as much torque as that wheel can take before traction is broken.

There is also rotational inertia I guess - if its all spinning merrily away at high revs, its gonna be difficult to stop when you gain traction. I guess this is the source of the torque spike people talk about. It will certainly increase the torque figures we come up with in the sums.

I think this is where you'll develop the high torque figure, not from engine derived torque. How many people bust diffs/shafts/CV's when applying torque from the engine to tyres that are gripping well enough to build up torque in the drive train? I suspect not many. Compare that to how many people bust stuff with spinning wheels.

[bill van snorkle]

Would not an engine develop peak torque only when there is sufficient resistance,ie load to allow it to do so? one would hope that wheel spin would occur well before the engine was asked to develop maximum torque if the vehicle was in low range first gear. even in my own case with just a puny 2 1/4 litre petrol engine developing a paltry 115 lb ft of torque operating in low low crawler with an overall reduction of 278 :1, I could theoretically develop enough torque ( almost 32000 lb ft) to turn some of the strongest axles into shrapnel, but hopefully traction will always fail before the situation gets anywhere near that stage .

Bill.

[biasbilt]

Al

"I'm not sure about a coefficient of 1 between the ground and a tyre of an offroad vehicle under 'heavy' acceleration - you'd certainly have some slip I guess tyres wiggle and squirm and do all kinds of weird things - especially with big lugs, deep sidewalls etc, but the idea makes sense compared to mud etc."

That's called making an assumption - even a soft rubber slick will slip by a certain percentage.

"Regarding the sums you did - wouldn't the tyre moment 'length' be the radius (i.e. 1.5' rather than 3')? But whatever - neglecting that and losses in the system (20% ish?) I agree. Your numbers seem low - there's no transfer box in there, maybe thats it."

You're right, the tyre length would be the radius, not the diameter, and I did forget the tranfer box, although the principle remains the same.

"Sorry - just to clarify in the interest of complete retentiveness - torque increases from tickover to max torque, not max power (but I know you know that )."

I was rushing

"The problem is that people are discussing peak torque which in fact could be much higher than the calcs suggest - I suppose (correct me if I'm wrong) that peak torque could actually occur when no lockers are engaged and you have, say a couple of wheels in the air, one slipping and the final one with excellent traction - all your torque is going to be routed to that single wheel, so it can easily exceed the figure for the 4-wheels in contact case. *pop* Or am I smoking crack again?"

The manufacturer will assume that you do have all driven wheels on the ground and will therefore share the torque load through all driveshafts, so with equal traction on all wheels and open diffs front, centre and rear, each shaft will take 1/4 of the max torque, but with locked diffs, you can easily put over half the available torque through one wheel (vehicle diagonally across a steep slope, with little weight on the front end or rear uphill wheel, lots of weight on the downhill rear wheel, and bang goes a shaft - we've all been there!). Therefore we need to be looking at components that can take that load, exactly as you're doing.

"The more wheels you have with some traction, the safer your driveline will be (in an unlocked situation). Am I right?"

Correct - sharing the load. If you're unlocked, the max torque you can apply through your driveline is equal to the load applied by the friction between the tyre and ground and the effective length of the lever (RADIUS of the tyre). This is why even a standard Rover diff is able to survive, because there is no locker/limited slip in the axle, and also the reason the LR will not fit from the factory, as they know that their axles wouldn't be able to survive.

Hope that all makes sense, trying to get replies into each part of the message - Al's are in quote marks.

Toby

[simonr]

There is also rotational inertia I guess - if its all spinning merrily away at high revs, its gonna be difficult to stop when you gain traction. I guess this is the source of the torque spike people talk about. It will certainly increase the torque figures we come up with in the sums.

That's what I was talking about. Changing the ratio makes this kind of failure more likely even if the chances of a failure from the straight torque from the engine is reduced.

The trouble with any gearbox is you have backlash. This means that if you stop the final output gear dead, one might imagine that all the rotational inertia would be taken up and absorbed by the final drive. However, in some circumstances, the final gear will stop, the second to last will tale up the slack and slam in to it and so on. As you move backwards through the chain, the rotational kinetic energy increases (because although the mass is reducing, the speed is increasing. The KE is proportional to the square of the speed, but only the product of the mass. This means that gearboxes are more likely to fail closer to the input than you might expect.

Si

[Astro_Al]

I think you've been smoking that crack again.

Jeeez... You're not wrong... Umm, it was very early, I hadn't had my wheatabix, I banged my head, I forgot to write 'NOT' after what I said, it fell behind the fridge, my dog ate it...

>"I think this is where you'll develop the high torque figure, not from engine derived torque. How many people bust diffs/shafts/CV's when applying torque from the engine to tyres that are gripping well enough to build up torque in the drive train? I suspect not many."

Well, you hear of it in old series trucks pulling away from traffic lights sometimes. (i suspect its more fatigue-based though...).

Al. Fully compus-mentus, honest.

[dollythelw]

try chucking a shock load absorber into the drive line - a la shaft driven motorbike..?

[Astro_Al]

try chucking a shock load absorber into the drive line - a la shaft driven motorbike..?

Yup - good plan. Use to have one on my boat for misalignment - rubber donut jobber. But how long would it last in this application?!?

Rubber gears instead? Where's Mr Moglite when you need him?

Al.

[bill van snorkle]

Yup - good plan. Use to have one on my boat for misalignment - rubber donut jobber. But how long would it last in this application?!?

Rubber gears instead? Where's Mr Moglite when you need him?

Al.

What about an overload slip clutch on the propshafts like on some tractor pto driven equipment,ie brush slashers etc ?

Bill.

[dollythelw]

the bike method is similar to a UJ just minus the spider kept in contact by the mother of all springs, in the event of a nasty spike they ride up on the ramp face, low tech and keeps you safely out of the reach of the rubber princess...?

[dirtydiesel]

I think this is where you'll develop the high torque figure, not from engine derived torque. How many people bust diffs/shafts/CV's when applying torque from the engine to tyres that are gripping well enough to build up torque in the drive train? I suspect not many. Compare that to how many people bust stuff with spinning wheels.

I have broke lots of shafts and cv's in this manner.

Normally by having the entire weight of the car on 1 front wheel, usually on full lock then giving it a bit of throttle.

See's off cadburys stuff in a heartbeat