LSD

[Doug]

I was wondering how inportant limited slip is for autocross (I'm new). We were looking at BMW Z 4, only th M has LSD. Also 1& 3 series BMW and Boxster are non LSD are they anygood for autocross?

[nc4me]

Limited slip diffs are important for autocross. When your driving on the street at normal speeds you don't need it. However, when you're pushing the car hard around a turn and you try and apply power you'll just spin the inside rear wheel. I had an open differential last season and swapped to a factory limited slip differential this season and it does make a difference.

It's up to you on how competitive you want to be and if you're going to race with any other organizations(different rules).

[Loren]

Slightly different opinion here:

Yes, it's generally considered good, but not a requirement.

It really depends on how much power you have and how much grip.

Example: 1.6 Miata with an open diff and really grippy tires... the inside wheel won't spin much due to the grip and not enough power to break it loose.

An LSD also changes the oversteer profile of the car. Without an LSD, if you power a little too aggressively through a turn, you spin the inside wheel... but unless you apply LOTS of power, the outside wheel is going to stay planted. So, while you might not be putting down all the power that you could with an LSD, at least you're still pointed in the correct direction!

Same car WITH an LSD, if you apply too much power through a turn, instead of the inside wheel spinning, you get BOTH wheels spinning and the back end of the car launches sideways.

So, IF the car is set up properly and needs it, an LSD is a plus. But, it is not outside the realm of possibility to set up a non-LSD car to work very well, too. I would trust BMW to be able to set up such a car pretty well from the factory.

A Boxster is a fantastic autocross car, but it's got the engine in the back and some weird Porsche technological magic behind it... don't try to compare it to "normal" cars.

[treekiller]

given my "style" I put a limited slip in my Z3 and was noticeably quicker. when at the local autocross on a 60 second course I was a complete second behind the national B-stock drivers, by just installing the factory optional diff and doing nothing else to the car I made up most of that second. Now a 1.9 Z3 had a lot of torque (compared to even a 1.8 Miata) and an awful rear suspension design.

I beleive a torque biasing diff is a must have for optimum performance in front wheel drive, in rear wheel drive it's a toss up like Loren said, a clutch type diff, that will make both wheels spin (given enough power). However a Gleason/Torsen Zexel/torsen Helical or Quaife. (all do roughly the same thing) can allow the wheels to spin at different torque loads and speeds. this is why the drift kiddies HATE biasing diffs, they offer too much grip, and are "too good" now if you like to use throttle induced over-steer then by all means the clutch type is for you.

I won't get into the technical details of each design (torsen type 1,2,3) however what a 2 way Diff can also do is transfer braking loads more evenly between wheels. (less likely to lock up one rear) and actually improve braking performance, More so when trail braking etc.

I see no condition where an open diff is superior to even a viscous LSD.

[Doug]

Thanks for the info. Does adding limited slip change class in FAST or SCCA?

[Loren]

Thanks for the info. Does adding limited slip change class in FAST or SCCA?

FAST doesn't care what kind of diff you have.

SCCA does. If an LSD was available as a stand-alone option on your car, you have nothing to worry about. If it was only available as a part of a package, then you must have the whole package to run with the LSD in stock class. Beyond stock class, I think STS is the only other class that has any limitations on LSD.

[blacksheep-1]

FWIW, check with these guys they built a unit for my MG midget that I raced in the SCCA, reasonably priced, crude and efficient.I was very happy with it and you can install it w/o taking the rearend apart.

http://www.phantomgrip.com/" onclick="window.open(this.href);return false;

[Miata GT]

.

Same car WITH an LSD, if you apply too much power through a turn, instead of the inside wheel spinning, you get BOTH wheels spinning and the back end of the car launches sideways.

I will comment rather than disagree with Loren's point; on my Miatas without LSD I noted the inside tire would unload and spin its tire more easily than a similar car with LSD, which would 'take a set' in a hard corner. As far as losing traction with both wheels, it is nice to have that option when appropriate.