what exactly causes an axle to break?

acannell · Oct 17, 2010

what are the actual physics that cause an axle to break?

yes the sight of a jeep spinning its wheels at full throttle in mud and not moving sure LOOKS like sometihng is going to break....but why exactly?

seems like when the axle is under the most stress is when its seeing the most torque input and the highest resistiance to turning on the output....wouldnt that be in the lowest gear while gassing it uphill with HIGH TRACTION? (i.e. not mud)

Chainer · Oct 17, 2010

It's not usually the spinning that breaks a shaft, it's the stopping. It shock loads the shaft when the tire spinning slams into something or gets traction and abruptly is doing 1 mph again.

Gaius · Oct 17, 2010

When the torque load exceeds the axle shaft, it breaks. This can be from too much torque when the tire is bound up, or from spinning then stopping suddenly.

acannell · Oct 17, 2010

check this out, does this make sense:

for an 89 stock jeep in 1st gear, with tcase in LO, and 3.07 diff:

engine outputs ~200 ft lbs pretty much anywhere, very flat torque curve.

BA10/5 tranny is 3.39 in 1st
NP231 tcase is 2.72 in LO
diff is 3.07

so 3.39 * 2.72 * 3.07 = 28.30 ratio

So that 200 ft lbs of torque is 5660 ft lbs, which is what the axle sees.

According to an axle breaking strength table I have from a book:

The dana 35 1.16" carbon steel axle "yield strength" is 4160 ft lbs.

UH OH

Anyone have any comments on this? I'm pretty sure the math is right, but the overall logic of it may not be sound.

stroked88 · Oct 17, 2010

your gearing doesnt actually multiply your torque like that.

acannell · Oct 17, 2010

88xj 4.0 31" said:
your gearing doesnt actually multiply your torque like that.

Jesus. Thats like the guy on the corvette forum telling me chevy used porous aluminum for the oil pans, and thats why you see oil stains sometimes.

What I'd like to know is, is there at least a little voice in your head telling you that you don't really know something, but you post anyways, or is there no little voice at all?

Please explain how it multiplies your torque then. And also please explain how I can lift my jeep with a jack but I cant lift it with my hands.

SleeperXJ · Oct 17, 2010

I believe torque, like horsepower is measured at the wheels.

acannell · Oct 17, 2010

SleeperXJ said:
I believe torque, like horsepower is measured at the wheels.

Awesome!

Any other ideas folks?

You see, a gear is kind of like a lever.

So how come you cant start from a stop in 5th but you can from 1st? Hmm.....

AIbandit · Oct 17, 2010

acannell said:
88xj 4.0 31" said:

your gearing doesnt actually multiply your torque like that.

Click to expand...

Awesome!

Any other ideas folks?

You see, a gear is kind of like a lever.

So how come you cant start from a stop in 5th but you can from 1st? Hmm.....

He means gearing increases your torque but not with that formula.

acannell · Oct 17, 2010

AIbandit said:
He means gearing increases your torque but not with that formula.

How else could it possibly increase your torque besides multiplying it according to the gear ratios?

AIbandit · Oct 17, 2010

SleeperXJ said:
I believe torque, like horsepower is measured at the wheels.

You can also measure torque and horsepower at the flywheel but it doesnt take the parasitic drag from your drivetrain into consideration.

AIbandit · Oct 17, 2010

Flywheel torque x gearing = tire torque (tiresize would be somewhere in the gearing too)

So yes if you had 200ft lbs at your flywheel(but you don't) you would put down x gear ratio to the pavement.

acannell · Oct 17, 2010

AIbandit said:
Flywheel torque x gearing = tire torque (tiresize would be somewhere in the gearing too)

Thats exactly what I said originally, except I was determining axle torque, so tire size was irrelevant.

AIbandit · Oct 17, 2010

What your saying is tire torque effects tire torque with the gear ratio. Which doesn't make much sense because it's already tire torque.

kastein · Oct 17, 2010

Assuming a frictionless, "spherical horse" world, you are correct. This is why I noted that locking or gearing a d35 axle is a waste of time and money...

I know people who have broken a d35 on: 32s on a gravel road pulling a U-turn, plenty of breaks on 31s, one of my friends locally blew up a d35 on his DD with stock 225s on pavement, and another guy blew one up with 215s on pavement. It just is not an axle worth beefing up - sure, you can make it stronger, but you can also junkyard an axle that is stronger than a beefed up 35 for $150 instead of spending 700 bucks on shafts, truss, etc.

PS - that peugeot ("pukegoat") transmission is also well known for barfing out its innards under torque. The AX series units are far superior.

AIbandit · Oct 17, 2010

fine minus the tires but either way your calculation is incorrect.

AIbandit · Oct 17, 2010

acannell said:
89 stock jeep in 1st gear, with tcase in LO, and 3.07 diff:

engine outputs ~200 ft lbs pretty much anywhere,

NOT AT THE FLYWHEEL... The end.

acannell · Oct 17, 2010

AIbandit said:
NOT AT THE FLYWHEEL... The end.

ahh, i think i see the mixup

by "engine outputs 200ft lbs pretty much anywhere", i meant "anywhere in the torque curve of the engine", not "anywhere in the drivetrain"

yes it does output around 200ft lbs at the flywheel...the 4.0 I6 is rated ~220ft lbs at the flywheel

acannell · Oct 17, 2010

acannell said:
ahh, i think i see the mixup

by "engine outputs 200ft lbs pretty much anywhere", i meant "anywhere in the torque curve of the engine", not "anywhere in the drivetrain"

yes it does output around 200ft lbs at the flywheel...the 4.0 I6 is rated ~220ft lbs at the flywheel

also, some interesting relevant links:

crawling ratio calculator:
http://www.4lo.com/calc/gearratio.htm

list of axle strengths:

xcm · Oct 17, 2010

what is this math good for?

what exactly causes an axle to break?

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