Open or LS Differential...

SBSerpent · #1 (**permalink**) 01-10-2021, 05:40 PM

...on the BMW M3 E36 that comes standard with our cars?

spdbrake · #2 (**permalink**) 01-10-2021, 05:51 PM

It can be either depending on what was optioned when the Backdraft was spec'd out.
To tell what you have there should be a metal tag attached to your rear diff cover.
having an "S" in front of the ratio is the LSD Id. Pics here.

https://differentials.myshopify.com/...p-differential

SBSerpent · #3 (**permalink**) 01-31-2021, 12:13 PM

Quote:

Originally Posted by spdbrake

It can be either depending on what was optioned when the Backdraft was spec'd out.
To tell what you have there should be a metal tag attached to your rear diff cover.
having an "S" in front of the ratio is the LSD Id. Pics here.

https://differentials.myshopify.com/...p-differential

I can't see any tag.

When both wheels are off the ground and the tranny is in neutral, spinning one wheel does not effect the opposite wheel (i.e. the opposite wheel does not spin). However, spinning either wheel in a forward direction results in the drive shaft rotating in the same direction. Is that an indication of a LSD?

bobcowan · #4 (**permalink**) 01-31-2021, 03:26 PM

Put it in neutral, and get both tires in the air. Spin one tire.

If the other tire spins in the same direction, it's LSD.

If the other tire spins in the opposite direction, it's open.

https://itstillruns.com/identify-lim...l-7805308.html

SBSerpent · #5 (**permalink**) 01-31-2021, 05:33 PM

Quote:

Originally Posted by bobcowan

Put it in neutral, and get both tires in the air. Spin one tire.

If the other tire spins in the same direction, it's LSD.

If the other tire spins in the opposite direction, it's open.

https://itstillruns.com/identify-lim...l-7805308.html

As I noted in my post, the other tire did NOT spin at all.

That being said, I went out and did the true test - a burnout in the street. Only the passenger side left a mark. That's disappointing, especially on a car like this.

Gaz64 · #6 (**permalink**) 01-31-2021, 06:02 PM

The easier way is to jack up one side, select neutral.
If the wheel turns easily, you have an open diff.
An LSD takes a lot more effort to turn, the "breakaway torque" can be about 70 ft/lbs.
Gary

patrickt · #7 (**permalink**) 01-31-2021, 06:07 PM

Quote:

Originally Posted by SBSerpent

As I noted in my post, the other tire did NOT spin at all.

That being said, I went out and did the true test - a burnout in the street. Only the passenger side left a mark. That's disappointing, especially on a car like this.

Just replace it with a LSD. That's actually kind of a fun project.

SBSerpent · #8 (**permalink**) 01-31-2021, 09:06 PM

I'm just scratching my head as to why there was no spinning of the other wheel when I turned the opposite wheel.

patrickt · #9 (**permalink**) 02-01-2021, 05:57 AM

Quote:

Originally Posted by SBSerpent

I'm just scratching my head as to why there was no spinning of the other wheel when I turned the opposite wheel.

Well, that's a good question. The only time I can ever remember that occurring with a car of mine was when a gear was broken. But just putting it up on a lift and popping the cover off will tell you. Maybe it was just having a bad day.

joyridin' · #10 (**permalink**) 02-01-2021, 07:28 AM

Quote:

Originally Posted by SBSerpent

I'm just scratching my head as to why there was no spinning of the other wheel when I turned the opposite wheel.

Are you sure it doesn't have a helical style LSD instead of a clutch type? I am not sure if you bought the car new, but if not, maybe somebody had one installed?

Doing a burnout is not a good way of checking. If the clutch plates or the cones are worn in the LSD, both wheels may not spin anyhow.

patrickt · #11 (**permalink**) 02-01-2021, 08:05 AM

Quote:

Originally Posted by joyridin'

Are you sure it doesn't have a helical style LSD instead of a clutch type?

Dang, I don't even know what a helical style limited slip is, but I did post this thread on checking the breakaway torque on my Powr-Lok, for what that's worth.

Breakaway Torque on ERA's Powr-Lok

t walgamuth · #12 (**permalink**) 02-01-2021, 08:10 AM

The opposite side caliper might be sticking enough to keep it from turning easily.

mln385 · #13 (**permalink**) 02-01-2021, 04:16 PM

BDR's come standard with an open diff unless otherwise ordered and paid for.

FredG · #14 (**permalink**) 02-01-2021, 05:48 PM

Do the modern limited slips have the same annoying ratcheting effect when making a slow sharp radius turn that my 66 vette had?

Fred

SBSerpent · #15 (**permalink**) 02-01-2021, 08:17 PM

Did some research on a helical style LSD. The proper way to test the wheel rotation is to have both drive wheels off the ground and the transmission in gear. Spinning one wheel will result in the other wheel spinning in the opposite direction.

bobcowan · #16 (**permalink**) 02-01-2021, 08:18 PM

No. Your old 'Vette had a locking gear. They're famous for that ratcheting action in the corners. A limited Slip is a lot different.

SBSerpent · #17 (**permalink**) 02-07-2021, 05:08 PM

So this is what I did. Lifted the rear end so that both wheels were off the ground.
1) with the transmission in gear, I spun one wheel in one direction and the other wheel spun in the opposite direction
2) with the tranny out of gear, I spun one wheel and it did not affect the motion of the other wheel
3) with the tranny still out of gear, I spun the driveshaft by hand and both wheels spun in the same (forward) direction simultaneously

Took the car out for a drive today and I did feel both tires let loose a couple of times on hard acceleration.