OEM Spec on LSD differential torque

#1

I found a spec in a OEM shop repair manual. To my surprise, it specs 30-50NM. That converts to 22-36ftlbs. I though sure that it was going to be more like 50-55ftlbs. Odd.

#2

Well… Then a legal diff should pass the 60 ft-lb breakaway test with ease. :wink:

I’m going from memory here, but doesn’t the goober-in-the-field method described in the rules use the lug nut as the place force is applied? What is the conversion from the force at the lug nut to the force at the hub? I’m no engineer and I can’t intuit whether the lug nut location should require more or less force to break the LSD free.

#3

The goober-rules state “center hub nut” so there should be any conversion.

#4

Depends on the nut you choose and it’s location in relation with the wrench and the hub. If torque wrench handle, lug nut and hub are in this order in a straight line, the measured torque is lower than the true hub torque. If the wrench passes over the hub to a nut on the opposite side of the hub from the handle, the measured torque will be higher.

If the distance from the handle to the nut is the same as the handle to hub distance the reading should I think be accurate. If I get some time I’ll do some statics calculations to confirm or refute this conclusion.

Matt

#5

Breakaway torque is on the center nut and it is 65lbs.

#6

So if we can’t change the diff mechanics or fluids from stock, why the 181%-of-factory-spec rule? Are we comparing specs apples to apples?

#7

Re. where you measure ftlbs. For god’s sake people we’re talking about torque here. The length of the lever arm is the “ft” in ftlbs. If you are not measuring it from the axis of rotation you do a little math to compensate for the distance to the axis.

It’s not a matter of where you measure it. If you measure it at a lugnut and fail to compensate for the difference in lever arm than you are just plain wrong.

It’s like putting monster truck tires on and saying “it doesn’t necessarily change the speedo reading, it depends on where you measure it”. No. It’s just a matter of measuring correctly.

#8

Ranger wrote:

Did that manual specify that value as for a new diff or could it have been a wear limit?

#9

jlevie wrote:

[quote]Ranger wrote:

Did that manual specify that value as for a new diff or could it have been a wear limit?[/quote]

Better yet, did it say how to make the measurement?

RP

#10

Scott,

I’m curious. Do you ever just watch TV or read a Romance novel?

How do you find time in between changing engines, compression tests, installing the most intricate oiling system on an E30, attending court, actually going to the track to sit down and read information like this?

#11

ukrbmw wrote:

[quote]Scott,

I’m curious. Do you ever just watch TV or read a Romance novel?

How do you find time in between changing engines, compression tests, installing the most intricate oiling system on an E30, attending court, actually going to the track to sit down and read information like this?[/quote]

Actually, I read a lot. Mostly tho, I’m just curious about everything. Except maybe romance novels. And how if we just give the government everything we produce, it will take care of us.

Moderation is for pussies and you can sleep when you’re dead.

Ah…Maybe when I lose my second wife I’ll get a clue and find new charms in moderation.

#12

rsafreno wrote:

Who says it 65ftlbs? Support your assertion. I thought it was closer to that too, but where does that number come from?

#13

jlevie wrote:

[quote]Ranger wrote:

Did that manual specify that value as for a new diff or could it have been a wear limit?[/quote]

It did not specify. I would interpret the #'s as “an acceptable range”.

#14

Patton wrote:

[quote]
Better yet, did it say how to make the measurement?

RP[/quote]

You’d have to look at the manual, it’s hard to describe. The manual is at ftp:\FileSharing.BrasselerUSA.com, which is our ftp site.

The diff repair pages talked about a test technique using some kind of ABS testing system that, if I understand correctly, the car sits on like a dyno. In a whole different section of the book it mentions the diff Nm spec.

#15

The current limit was set based on actual impound testing in the SE/MA last year with one wheel on the ground & the other jacked up, IIRC.

#16

My old diff, swiped by the evil Molitor, tested at ~50ftlbs. A lot more then my current 30ftlbs diff.

#17

Ranger wrote:

[quote]rsafreno wrote:

Who says it 65ftlbs? Support your assertion. I thought it was closer to that too, but where does that number come from?[/quote]

I think he is referring to the SE30 rule. For everyone’s info, here’s the section of the shop manual. I’m not good at German-to-Broken-English-to-English translation, but it looks like the same test procedure as our ruleset (except ours is “hot” and oem is presumably “cold” ).

Steve D.

#18

And the rule was written without prior knowledge of the, what appears to me, vague BMW info.

Carter and others did random testing.65ft-lbs was suggested (yep, I know who–but I was not completely informed), and so it was written.

Then it gets confuseing 'cause noone has data as to the effects of temp and lube oils (lube oil selection cannot be enforced). Do a search on this subject and you’ll see the discussion.

So, the rule is, and it shall be.

I think it deserves further data as the penalty for exceeding 65 is a DQ. But the rule is, and I’ve not got a diff that exceeds the 65# in either car, at least cold and with 75w90 Mobil 1 oil it doesn’t.

Regards, Robert Patton

#19

Metric Mechanic has a pdf that indicates 55ftlbs, which is in line with what I expected to find in the OEM manual.

#20

error in oem manual most likely.