fine spline VS coarse spline

[snell40]

hey guys is a fine spline front diff assembly stronger than a coarse spline assembly?
im putting a later model front diff assembly from a rangie at work into mine because the shocks mount outside the coils so then i can put lifting blocks under the coil plates
cheers dave

[defmec]

both are weak ,24 spline is stronger,the shock mounts on all rovers are in the coils ,it probably has the twin shock turrents put on it with the bolt on bracket on the axle housing

[Hobie18]

both are weak ,24 spline is stronger,the shock mounts on all rovers are in the coils ,it probably has the twin shock turrents put on it with the bolt on bracket on the axle housing

Not quite correct, it is my understanding that classic rangies which were fitted with air suspension had the shocks mounted outside of where the coils/airbags would have been

I think the diff's pre about 87 had stronger cv's but these were coarse spline diff centre's (10 spline)

[KiwiBacon]

I swapped from 10 spline to 24 spline a couple of years ago, here's what I found.

The axle size is basically identical but the design of the 24 spline axle ends means they're less likely to fatigue to death (small cracks which grow and grow until it finally snaps). In a single twist until death there will be very little in strength between the two axles, but long term the 24's will last longer than 10's.

The 24 spline diffs had a much better design on the spider gears, basically it's a lot harder to break a tooth or two off a later series diff.

If you're upgrading for strength it doesn't matter, you simply buy a locker with a stronger centre and the shafts to match. If you think one of them won't be strong enough, the other won't be much different.

For reference I'm comparing 1985 rangerover 10 spline with around 500,000km to 1997 discovery 24 spline with 80,000km.

[ISUZUROVER]

In theory the 24-spline axles should be stronger as they are 1.24" diameter vs 1.1" diameter for the 10 spline, and the load is spread over 24 spots instead of 10.

However, Landrover switched from steel to cheese when going from 10-24 spline in the RRC/disco. Tests by ashcroft in the UK have found that OEM 24-spline axles break at the same stress as 10-spline (or less!).

However, aftermarket (properly designed) 24 spline axles are significantly stronger than both OEM and aftermarket 10-spline axles.

[KiwiBacon]

In theory the 24-spline axles should be stronger as they are 1.24" diameter vs 1.1" diameter for the 10 spline, and the load is spread over 24 spots instead of 10.

Those diameters you've mentioned are the spline outer diameters. The actual axle diameter is the same for 10 and 24 spline, hence the same ultimate strength in tests like the Ashcroft one.

[ISUZUROVER]

In theory the 24-spline axles should be stronger as they are 1.24" diameter vs 1.1" diameter for the 10 spline, and the load is spread over 24 spots instead of 10.

Those diameters you've mentioned are the spline outer diameters. The actual axle diameter is the same for 10 and 24 spline, hence the same ultimate strength in tests like the Ashcroft one.

Are you saying the 24-spline (RRC/disco) axles are necked down to below the root diameter of the splines???

That is certainly not the case for 24-spline sals axles.

[KiwiBacon]

Are you saying the 24-spline (RRC/disco) axles are necked down to below the root diameter of the splines???

That is certainly not the case for 24-spline sals axles.

Here's a solidworks model of the long side rear 24 spline disco axle. I haven't drawn in the splines themselves, just left them at the major diameter.

[ISUZUROVER]

Here's a solidworks model of the long side rear 24 spline disco axle. I haven't drawn in the splines themselves, just left them at the major diameter.

Not sure what that is supposed to show??? Sure I can see it is necked down, but not the diameter compared to a 10-spliner.

[KiwiBacon]

Here's a solidworks model of the long side rear 24 spline disco axle. I haven't drawn in the splines themselves, just left them at the major diameter.

Not sure what that is supposed to show??? Sure I can see it is necked down, but not the diameter compared to a 10-spliner.

The main diameter of that shaft is identical to the overall diameter of a 10 spline shaft. The 24 spline shafts are only bigger at the spline and that step up where the hub seal runs.

[snell40]

thanks heaps guys...really helpful,
cheers dave

[PSI250]

there is also some info on splines / axles in the bible thread at the top of this section which may help.

[red90]

OK....

All 10 spline and late model 24 spline stock axles are useless crap. Is this clear?

As are the stock 2 pinion differentials and the late model CV joints.

The only "half decent" stock shafts are the Rover axle "HD" shafts and the Salisbury shafts. Neither of these neck down and they are made from something resembling steel.

Today, anyone that cares should purchase Chrome-Moly shafts. They do not cost very much.

[red90]

And I was wondering. I don't have any late model 24 spline shafts around. If time allows, I can try and hook up with a friend that has some to take measurements...

What I have.

10 spline end of 10/23 front halfshaft


23 spline end of same shaft


24 spline Salisbury rear shaft

[red90]

To see how big the shaft needs to be, I plugged in Mal Story's numbers....


Maybe someone can check if I screwed up, cause the Polar Section he states for the 24 spline gives a shaft smaller than the minor diameter, which can't be right.

[KiwiBacon]

OK....

All 10 spline and late model 24 spline stock axles are useless crap. Is this clear?

They're not completely useless. If you stick to stock size wheels and drive them knowingly you'll be fine.

[red90]

[They're not completely useless. If you stick to stock size wheels and drive them knowingly you'll be fine.

Not in my experience.

[snell40]

wow thanks for the details...24 spline is definitely the way im goin then
cheers dave

[red90]

OK, as a learning exercise... I drew the section in AutoCAD (hopefully correctly...) and it gives a polar section of 0.324, which matches the strength of a 30.0 mm shaft.

[ISUZUROVER]

OK....

All 10 spline and late model 24 spline stock axles are useless crap. Is this clear?

They're not completely useless. If you stick to stock size wheels and drive them knowingly you'll be fine.

Stock size wheels on what?

Basically the same (inner) axles were used on Discos, RRCs and 110s. 110s run 32's. I know plenty of people who drive with lots of mechanical sympathy, but have broken 10 spline and 24 spline axles and diffs offroad.

If you don't drive anything more difficult than a dirt road then I agree with your statement.

[KiwiBacon]

Stock size wheels on what?

Basically the same (inner) axles were used on Discos, RRCs and 110s. 110s run 32's. I know plenty of people who drive with lots of mechanical sympathy, but have broken 10 spline and 24 spline axles and diffs offroad.

If you don't drive anything more difficult than a dirt road then I agree with your statement.

Stock size on rangies and discos is 29" (205rR16, 225/75R16, 235/70R16). Due to the guard design you can't go much taller or wider in a tyre without cutting and/or lifting. No such problem with the 110's.

How much traction your offroading has is a big factor in what you break. AFAIK in my vehicles lifetime it's broken cross-pins, bent axle casings (x2) and bent rims but never broken an axle or a CV. My current 24 spline hasn't been in an ugly situation yet.

[ISUZUROVER]

How much traction your offroading has is a big factor in what you break.

No arguments here - I am sure there is a lot more mud in NZ than over here (even after all the rain we have had).

[KiwiBacon]

How much traction your offroading has is a big factor in what you break.

No arguments here - I am sure there is a lot more mud in NZ than over here (even after all the rain we have had).

Clay is the real bugger. Get that on your tyres or even your shoes and you're going nowhere. That's how we get more people popping spider gears and cross pins (spinning one wheel) than breaking shafts.

[red90]

I've seen bone stock Rangies and Discos break shafts and diffs driving easy to moderate trails without being aggressive.

[KiwiBacon]

I've seen bone stock Rangies and Discos break shafts and diffs driving easy to moderate trails without being aggressive.

Just to put things back into perspective
Here is ashcrofts well known twist to failure of landrover half-shafts.


We've got failure at roughly 3300 ft-lbs of torque, that's roughly 4,500Nm.
To create that torque at the contact patch of a 29" tyre you have a traction force of roughly 12kN. That's 1.2 ton of drive force on one wheel.

If you're driving on a high traction surface with lockers then such numbers are not difficult to acheive. But with 29" tyres, no lockers and no sandstone you can't produce that much drive force steady-state, you have to introduce some good shock loads, dropping spinning wheels back to earth kind of thing.

[red90]

A single load to failure test is not the complete story. Low cycle fatigue is what kills shafts.

And sorry, but 1.2 tons is pretty easy when the vehicle weights 2.5 tons..... Go up a steep hill, hit the throttle...

3300 lb-ft / 40:1 gear reduction = 82 lb-ft from the engine.

And exactly what is unclear about seeing multiple failures on stock vehicles driving easy to moderate trails??? Is real life knowledge not what are called facts? If you have not seen this, you must not actually do any off roading.

I remember back when I was in the Brisbane LR club, they were selling plastic drive flanges so that WHEN your stock one piece shaft broke, you could drive home without getting dirt into your bearings.

[KiwiBacon]

A single load to failure test is not the complete story. Low cycle fatigue is what kills shafts.

Both types of failure happen in offroading situations. As I've already mentioned the design of the 24 spline shafts is far superior regarding fatigue life. Fatigue starts with yield, the ashcroft plots show again the 24 spline has an advantage with a higher yield point.

And sorry, but 1.2 tons is pretty easy when the vehicle weights 2.5 tons..... Go up a steep hill, hit the throttle...

3300 lb-ft / 40:1 gear reduction = 82 lb-ft from the engine.

Not at all, because you haven't got the traction to do that. With an open rear diff the torque is limited to the side with the least traction, you need each rear wheel to have 1.2 ton of traction, that's 2.4 ton total thrust which requires friction coefficient greater than 1 and the front wheels doing absolutely nothing. You simply can't get that much traction (unless you've got sticky tyres on concrete). It can only be done with a serious shock-load or repeated shock loads.

With a locker it's certainly doable, because you can get 1.2 ton of traction on one wheel if the ground is hard and your tyres are clean. But anyone who's running a locker in high traction situations with stock shafts probably hasn't thought it through.

The whole "I go harder because I break more stuff" argument is a poor one.

[red90]

I'm not saying I go harder and we are not talking about me... I have lockers and big tyres.

I'm saying I personally have seen stock trucks driven sanely break shafts and diffs. These are facts, not theory. I've seen street only driven trucks break diffs. Keep in mind that they are all V8s here...

The traction at those levels is easy to get. Hills that stall out trucks with 100 lb-ft in 1st gear are easy to find. You fail a climb on a rocky ledge, the tires spin and grab, easy and common. I suppose if you drive around on flat mud, you will never stress anything.

[KiwiBacon]

The traction at those levels is easy to get. Hills that stall out trucks with 100 lb-ft in 1st gear are easy to find. You fail a climb on a rocky ledge, the tires spin and grab, easy and common. I suppose if you drive around on flat mud, you will never stress anything.

You cannot get 2.4t of traction offroad as you'd need friction coefficient of 1 (aka perfect traction). Clean tyres on a dry tarmac or concrete surface have friction coefficient around 0.8. This means if you have the whole 2.4 ton vehicle sitting on the back wheels on dry concrete they will both lose traction with around 1.9 ton of thrust. That's still not enough to break axles.

For reference I have a 2.3t (empty) rangerover with a 400kg turbo diesel in the front. Around 500Nm of torque. The tracks here are dry and a mix of rock and gravel. The rocks here don't have enough traction to break stock axles that stay on the ground, neither does giving it the boot up hill. I've done plenty of both with the old 10 spline and even with around 500,000km of use they never broke. The average climb here is gravel interspersed with schist rock, on any climb you'll be slipping up through the gravel and catching tyres on the rocks.

If I fitted lockers then all bets would be off. If I wanted to go out and break an axle I'm pretty sure I could (drop spinning tyres back to earth is a good way). But I don't and I haven't.

So how many 10/24 spline shafts have you personally broken and how did they happen?

[red90]

OK, firstly, I'm a mechanical engineer and I don't need to be told the math. A coefficient of friction of 1.0 is not "perfect traction". There is nothing magic about 1 and higher is quite possible. I can guarantee you can easily produce greater force at the wheel surface than the weight on the tyre when off road. The wheels just need to sit behind a small ledge. A lug just needs to catch an edge. The low pressure tyre wrapping around a rock. There are many instances to allow the tyre to be able to force beyond the weight on it. Everytime you fail a hill climb on steep, high traction surfaces, the wheels spin and can grab and catch rocks and ledges with loading that is very high. A little jolt into the plastic range and there goes another 5% of the life of the shaft.

10/24...good lord, those snap like preztels. I was following an 88 a couple of years ago and bang on a tiny incline, just driving along the trail. The fatigue adds up and they fail whenever. I thought we had been talking about late model 24 spline shafts. I don't "personally" run stock shafts, so have broken none of them.... As clearly stated, this is other people in stock vehicles. I can think of six failures of people I know personally, 3 in Australia and 3 here in Canada. Just normal moderate off roading. Normally when they fail they are not doing anything special as the failure mode is cumulative. It is usually a toss up if the crappy cross pin on the diff will break first or the shafts.

Just this last weekend, my friend snapped a Salisbury rear axle in half. Now, he is on bigger tyres than stock and who knows the life history of the shaft. But it broke while doing nothing special, just driving slowly through a drift onto a road following me. This was during a weekend on corn snow where there was no traction available to speak of.

All I can guess is you've never driven on anything close to high traction and steep. It is probably best if you don't go wheeling with Ben or myself.