Half-shaft and diff torque ratings.

[JimK]

This is partly just curiousity, but cans anybody find out what torque the standard half-shafts and diff are designed to withstand?

I've asked a trader selling a couple of different hardened types for the same infomation, and I'm curious how much stronger they are.

As far as the diff goes: My maths tells me that a 4.22 diff will take more engine torque than a 3.9 but pass on more to the half-shafts. According to my maths, anyway.

If I know what the drivetrain torque ratings are I can calculate what the engine can put out before something breaks.

[rayofleamington]

As far as the diff goes: My maths tells me that a 4.22 diff will take more engine torque than a 3.9 but pass on more to the half-shafts. According to my maths, anyway

I've never seen torque ratings for the original half shafts anywhere
A 3.9 diff will give LESS torque to the half shafts than a 4.22 (ratio 3.9/4.22)
People often quote the Minor axle capability in terms of bhp (over 100 bhp is seriously pushing it) however without a torque figure, this is all being measured in units of cheese IMHO.

I hope you strike lucky and find a torque rating, however bear in mind that impact loading (eg a snappy clutch) etc.. will have a major effect - it's not 'just' the powertrain torque that's the problem
Also bear in mind that a standard 1098 will occaionally snap a half shaft. As you raise the torque, you exponentially increase the probability of failure (until it reaches 100%). The basic probablility curve is often called an S-N curve which concentrates on the cyclic loading. That all gets a bit deep, so basic rule of thumb (based on cheeses) is that a non-tuned 1275 should be fine even with a 4.55 diff. Heavily tuned A-series (1300cc+ with good gasflowing) is too much for the standard axle.
Somehwere inbetween and a 3.9 diff - it's in the grey zone :?

[JimK]

A 3.9 diff will give LESS torque to the half shafts than a 4.22 (ratio 3.9/4.22)

That's what I thought. Gearing down (as in the diff) multiplies the torque on the output.

People often quote the Minor axle capability in terms of bhp (over 100 bhp is seriously pushing it) however without a torque figure, this is all being measured in units of cheese IMHO.

Absolutely, it's meaningless. As I remember, power is proportional to torque multiplied by engine revs.

I hope you strike lucky and find a torque rating, however bear in mind that impact loading (eg a snappy clutch) etc.. will have a major effect - it's not 'just' the powertrain torque that's the problem

That's why this is mostly for curiosity's sake. I have a tendency to sidestep the clutch, though I haven't done it in the Minor yet

Also bear in mind that a standard 1098 will occaionally snap a half shaft.

I've heard that.

As you raise the torque, you exponentially increase the probability of failure (until it reaches 100%). The basic probablility curve is often called an S-N curve which concentrates on the cyclic loading.

I remember some of that analysis stuff from working on QA for rocket motor cases. Customers don't like fatigue cracks in those...
Is that your profession?

That all gets a bit deep, so basic rule of thumb (based on cheeses) is that a non-tuned 1275 should be fine even with a 4.55 diff. Heavily tuned A-series (1300cc+ with good gasflowing) is too much for the standard axle.

I'm going to be running an A+ turbo. Don't know how far to turn the boost up, but it's looking sensible to leave it off for the time being.

Somehwere inbetween and a 3.9 diff - it's in the grey zone :?

If I didn't have a 5spd I'd change the diff, but I think I'll leave the current one there. I have enough to buy as it is...

[plastic_orange]

With a highly tuned MGB engine, I snapped about 8 halfshafts and a few 3.7 diffs in about 12 years. When I ran out of Riley halfshafts, I used Minor ones and never broke 1 (I think) - I then changed to Ford Capri 3 litre.
This is probably due to some residual strength being left in the Minor shafts as they are possibly understressed in a standard Minor.
The worst diff explosion I have experienced was my wife's mildly tuned 1098 with 3.7 diff - needed complete new axle - teeth went through it (half shafts were ok though).
If you are in any doubt that your engine could be too powerful for the Minor axle, I would recommend upgrading to Ford or similar - cost - probably around £100 and it isn't too hard to do.

Pete

[IslipMinor]

Started off with 1380 and 100bhp at the flywheel, 3.9 diff and 'hardened' half shafts. No 'dropped' clutch starts (what for?), but lots of enthusiastic driving (incl. a number of track days), and no broken half shafts in about 15,000 miles.

Power now around 105bhp, same 3.9 diff, but LSD and Yokohama A021R tyres (VERY sticky when warmed up). First corner from home after fitting the LSD, 200m then climbing LH turn at x-roads, the RH shaft broke!

Then fitted Peter May's EN24 rather special half shafts and 10,000 miles on with no problems.

The crownwheel and pinion are the same, as are the pinion bearings.

[plastic_orange]

Mine always broke when turning at a junction - usually right.

Pete

[JimK]

Started off with 1380 and 100bhp at the flywheel, 3.9 diff and 'hardened' half shafts. No 'dropped' clutch starts (what for?), but lots of enthusiastic driving (incl. a number of track days), and no broken half shafts in about 15,000 miles.

I've got a 1275 turbo. Not sure what output, but I suspect a naturally-aspirated 1380 will have the same or less torque than a blown 1275?
One of the reasons I wanted a blower was more torque at lower revs.

Power now around 105bhp, same 3.9 diff, but LSD and Yokohama A021R tyres (VERY sticky when warmed up). First corner from home after fitting the LSD, 200m then climbing LH turn at x-roads, the RH shaft broke!

I've driven with A021s, they're the ones with interlocking diagonal cuts? More like cut slicks, I've no idea how they are road legal. An unholy nightmare in the wet, especially in a 200bhp car...

Then fitted Peter May's EN24 rather special half shafts and 10,000 miles on with no problems. The crownwheel and pinion are the same, as are the pinion bearings.

So, EN24 half-shafts and standard diff? Hmm. I'd like to keep the Minor axle if possible but those half-shafts are more expensive than a Escort axle.
But if I change axle I'll probably have to change my nice black wheels.

What to do? :-/

[IslipMinor]

JimK,

The A021R's have a reasonably conventional tread pattern, the ones you describe I think are the A032R's?

In the damp you need a bit of care, but full wet they are ok.

[plastic_orange]

Just get the Escort axle redrilled to 4 inch pcd - any machine shop can do this.

Pete

[bmcecosse]

At the end of the day - the torque through the transmission is limited to what the tyres will transmit to the road surface. Sticky tyres will therefore load up everything rather more than was intended by Issigonis !

[JimK]

The A021R's have a reasonably conventional tread pattern, the ones you describe I think are the A032R's?

You're right, I had a look. I did a trackday, and they watered part of the circuit to make it fun...

[Peetee]

At the end of the day - the torque through the transmission is limited to what the tyres will transmit to the road surface

Thats what i thought.

A 3.9 diff will give LESS torque to the half shafts than a 4.22

I'm not so sure that's true. Torque is a product of the resistance of the wheels to turn. A longer ratio diff would increase the resistance in much the same way as a larger wheel or a start in second gear would.

[ian-s]

The weak part of the diff is the star wheels and housing. The housing wears allowing the gears to spread farther apart and lessening the toth contact area until the teeth break. A lot of Clubmens and Formula junior cars which use BMC final drives have the crown wheel modified to use a Ford diff and star wheels , unfortunately then you need special halfshafts with ford splines. its expensive but the only way to get to use all the different ratios that BMC supply with the strength of the Ford star wheels.
It may be that a limited slip would do the job on its own as it has new star wheels and I suspect a steel casing.

[Innovator]

When I ran A Series it was only gearboxes that went. I then fitted a B Series axle and the planet / star gears kept dropping off the pin.

There is a fundamental design error on the Minor half shafts that could be removed if you get custom after market shafts made. Or standard shafts could be modified if you then get them heat treated.

The splines on any shaft (the same applies to threads) should be on a larger diameter than the rest of the shaft. Or put another way, the root diameter of the splines should be on a larger diameter than the rest of the shaft.

So what this means is that by removing material you can make the shaft more resistant to fatique. Cracks will always start at the point of highest stress and this is normally at a change of section (ie the splines, especially the run out of the splines).

I would just fit the Ford axle and have peace of mind.

John

[Peetee]

I'm not an engineer but was led to believe that any severe angle (the inner cuts on the splines) in such a metal should be radiussed or chamferred to relieve stress.
Here's an example of a half shaft failure

This was on a modified 948!!

[Innovator]

Exactly. The shaft should the diameter of the spline roots in the area where the splines start to taper up. This will remove this stress raiser and give a more even distributon of stress.


This site has some excellent discussions on shaft, spline and gear design. specifically here http://www.pirate4x4.com/tech/billavist ... dex1b.html which will explain the above oncept. Scroll down the page for an excellent drawing of shaft spline design.

John