OEM Spec on LSD differential torque

#21

are the “side gears” referenced in the instructions directly connected to the hub where we test?
(sounds like the instructions are for testing a diff on a bench partially dismantled)
bruce

Steve D wrote:

[quote]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.[/quote]

#22

leggwork wrote:

[quote]are the “side gears” referenced in the instructions directly connected to the hub where we test?
(sounds like the instructions are for testing a diff on a bench partially dismantled)
bruce [/quote]

From page 33-40 of the shop manual… #10 is the clearest use of the term “differential side gear” in the LSD section of the rear axle manual.

#23

Here’s another one showing the prescribed “slip torque” test:

#24

Steve D wrote:

[quote]Here’s another one showing the prescribed “slip torque” test:

[/quote]

Good find Steve.

I infer from Steve’s pic that the BMW test is the same as ours. Not exactly the same, but the differences don’t matter. Which brings us back to, why is the OEM spec so low? I don’t buy KishG’s “typo” answer, that’s too easy.

If I had to guess, it’s just a conservative #. There’s various justifications for that, one of which would be warranty replacements.

#25

Mine measured 30 ft-lbs(~41 Nm), about mid range of the specified 30 - 50 Nm.

#26

rrroadster wrote:

You’ll feel it when the diff needs a rebuild. You’ll get wheel spin in slow corners. It won’t feel like oversteer exactly, but you’ll see rpms climb for no good reason and you’ll have to back off the gas a little in order for the slipping to stop. I’m not sure what was slipping when this happened to me…tires?..diff? But it forced me to go easy on the gas on slow turns.

#27

Ranger wrote:

[quote]rrroadster wrote:

You’ll feel it when the diff needs a rebuild. You’ll get wheel spin in slow corners. It won’t feel like oversteer exactly, but you’ll see rpms climb for no good reason and you’ll have to back off the gas a little in order for the slipping to stop. I’m not sure what was slipping when this happened to me…tires?..diff? But it forced me to go easy on the gas on slow turns.[/quote]

Yeah, that was what made me check it, it was engaging waay too easy the other weekend. It was my first time driving an E30 and I wasn’t sure what was what. Turns out it was the old R888s I was running on. They were so bricked out that there was absolutely zero pick up from the track on them.

#28

rrroadster wrote:

[quote]Ranger wrote:

[quote]rrroadster wrote:

You’ll feel it when the diff needs a rebuild. You’ll get wheel spin in slow corners. It won’t feel like oversteer exactly, but you’ll see rpms climb for no good reason and you’ll have to back off the gas a little in order for the slipping to stop. I’m not sure what was slipping when this happened to me…tires?..diff? But it forced me to go easy on the gas on slow turns.[/quote]

Yeah, that was what made me check it, it was engaging waay too easy the other weekend. It was my first time driving an E30 and I wasn’t sure what was what. Turns out it was the old R888s I was running on. They were so bricked out that there was absolutely zero pick up from the track on them.[/quote]

My diff breaks when cold at 30-34ftlbs, IIRC. I would imagine it’s less then that when it’s cold. I don’t sense it slipping right now, but since it’s so much weaker then my diff that was swiped, I figure it’s just a matter of time before it needs a rebuild.

#29

Mine breaks away at 53 ft/lb warm. I think the reason the measurement is so much higher on the car than in the manual is because the friction of turning the driveshaft and driven gears in the transmission is multiplied by the lsd gear ratio and must be overcome for the lsd to break-away.

#30

turbo329is wrote:

???

I guess you’re talking about an auto?

The LSD test is a test of diff clutches. What does that have to do with the driveshaft and transmission.

Congrats on the excellent condition diff.

#31

I meant in the shop manual not as compared to the manual transmission. For the lsd to break away it has to turn the driveshaft, a few transmission gears and at least three bearings. Maybe I’m overestimating the friction this would cause in ft/lbs but that amount would be multiplied by 3.73 and added to the breakaway tq of the lsd when measured at the wheel. The diagram from the shop manual shows the lsd mounted on a bench without the driveshaft attached so it would purely measure the lsd clutch breakaway.

#32

turbo329is wrote:

Are you sure the driveshaft turns when you test LSD? The times I did the test I wasn’t looking at the driveshaft so I can’t say for sure. But based on the basic design of an LSD diff it seems like one wheel ought to be able to turn and the driveshaft not turn.

#33

positive. For one wheel to turn without the driveshaft turning the other wheel would have to turn in the opposite direction, but that would only happen on an open diff. I just realized that I can test the actual clutch breakaway by jacking up both sides and using a tq wrench to find the amount of tq required to turn both wheels forward and the driveshaft. Then subtract that from the rulebook test. Though my tq wrench starts at 20 ft/lbs so I would need a smaller one.

#34

Jacking up both wheels and using a torque wrench on one side doesn’t accomplish anything, because the spider gears (and consequently the LSD clutches) are not being asked to rotate.

The whole point of the exercise is to create an environment where the spider gears (and clutches) are forced to rotate, in order to determine the amount of resistance the clutches provide to prevent the rotation.

The way we’ve been checking diffs so far is to put the car in neutral, take off the parking brake, jack up the RR wheel only so the LR wheel is still on the ground, then put a torque wrench on the center nut and see how much torque it can withstand before the wrench doesn’t click, and instead makes the wheel turn. More is better, up to the limit of the rules.

There’s a lot of argument that this method still leaves several important variables, but it’s easy to do and should be relatively consistent from car to car. Haven’t heard of anyone yet tossing a higher number than the rule allows.

#35

I understand how to measure it according to the se30 rules. I was speculating that if you subtracted the tq required to turn both wheels while they are off the ground from the tq using the se30 measurement you may arrive at a tq value within the shop manual bench test range.

#36

I know I’m late to the game here, but I found this at E30tech.com

http://www.metricmechanic.com/pdfs/metric-mechanic-differential-booklet.pdf

Page 6 on the PDF for the medium 188 case differentials.

Hope that helps?

#37

MM: so their diffs engage sooner but with less max load transfer capability?

#38

Im going to CMP this weekend with my club, THSCC, for 3 days. B) This will be my last shake down before Comp Skool in July (finally!).

I’ll measure my diff breakaway right after a session to see what it measures hot. I would expect to see a difference due to heat expansion of materials (more preloading of shim/clutch stack = higher break away torque).

#39

Well, I didn’t measure right after a session. I checked about 15 minutes after. The result was the same as cold.

While not at full operating temp, I doubt if the diff had cooled all that much, so I guess that dispells my theory about break away torque increasing with temperature.

#40

FYI
I just finished replacing the clutch packs on the car last night.
Measuring cold, I got around 50-52lbs of break away torque.