Track only, welded vs open diff (LSD not an option for this exercise)

I drove a first gen rx7 for years with a welded diff and was competitive with lsd cars (It7 spec series). I wouldn't call them bad at all, but you have to adjust your driving for it as others have said. 

However, the OP's stated mission is hpde, so I'd think the most fun/rewarding setup is the best choice rather than which is fastest. Given that, I'd start with shorter gears and open diff and only consider welded if you're really struggling to put power down. Unless you've already confirmed that's a problem with the taller gears. 

I ran welded diff for years in my MG race car - no big deal to drive on the track unless it is raining, in which case a dose of caution is advisable.  I eventually went to a Quaife when they became available - nicer to drive but no big change in lap times.  Interestingly, I also ran another car, powered by the same engine (early TVR) and it would equal the MG lap times without an LSD or locker due to the IRS they came with.

I was aware of Porsche using spools but as noted it was for reliability.

A fellow 1200 competitor refers to my driving style as chuck and catch..........and I am decidedly not a fan of welded diffs on road courses.

There is a reason F1 cars have a such sophisticated diffs.

Welded diffs are terrible in the wet and when dealing with traffic.

They are a compromise I'd only run if I absolutely had to. I'd never run one on a track day car.

sevenracer said:

However, the OP's stated mission is hpde, so I'd think the most fun/rewarding setup is the best choice rather than which is fastest. Given that, I'd start with shorter gears and open diff and only consider welded if you're really struggling to put power down. Unless you've already confirmed that's a problem with the taller gears. 

Your thinking aligns with mine. I think until there is inside wheel spin on exit that it is not worth messing with welding it. And at least with the 2.93, I don't believe that was ever an issue.

Even if welding the diff itself is a $0 budget hit, open diff will be the cheaper in regards to wear and tear. It'll also be less likely to suffer catastrophic failure.

How good or bad either will be relative to the other probably depends at least a bit on the rest of the suspension setup, as well as the drivers ability to adapt their technique to it. I'd guess that the car is already probably closer to setup for the open diff than the welded diff. That said, with multiple diffs on hand, as long as it's pretty quick and easy to swap out, I would have a hard time not at least trying a welded one just to find out... Especially if the car was being trailered to the track. Driven to the track would be less enticing.

ShinnyGroove (Forum Supporter) said:

I'll caveat this by saying that I've never driven a car with a welded diff. 

OK- that said, I can't possibly imagine a welded diff putting down decent lap times compared to an open diff. My evidence for this is that there are hundreds of race cara with open diffs, and zero to my knowledge that have welded diffs. Forcing drift mode in every corner sounds very slow to me. 

When I had a welded diff in my RX-7, on pavement it wasn't "drifty" so much as pointy.

Apply throttle and it felt like something was pulling the nose into the corner. Not sliding the back end out, but driving the nose in.  It was addictive and yet I never felt like chasing the point where that characteristic ran out of its talent.

It should be pointed out that all tires slip to an extent when making grip.  So I think what was happening was, the outside tire was providing more torque instead of braking slightly due to scuffing, so this was like a torque vectoring effect.  The inside tire would be scuffing forwards no matter what.

I've owned a Samurai with a very aggressive Detroit-style locker in the rear, and I even drove it on track a couple times in navigational rallies where driving around a track was part of the route...one time on a traditional circuit (got to drive as fast as I wanted around most of the course which was...something ) and one time on a British-rallycross-style course with slippery mud sections. I have some experience driving cars with moderate to aggressive LSDs on track, although the only actual car with an actual spool/welded diff I ever drove on track had a sidewaystastic setup and was never intended to grip corners.

If you don't have a traction problem right now, I'd say leave the diff open, this will just make handling easier and better from a grip-driving perspective. Any diff lock action will resist rotation and for track use, ideally you want only enough to keep the wheelspin in check. Some people add more to fix suspension problems with diff solutions but I'm not a fan of that approach. In sim racing one thing I always try that will improve 3 out of 4 random cars with adjustable diffs and some decel lockup action, is dial down the diff's decel lockup progressively until it's at the lowest possible setting. After that I try the same with coast lockup, and finally I'll try turning down accel lockup, all to get that minimal diff setup that works best on track.

If you're getting a lot of one-wheel-peeling coming out of corners then welding the diff may be worth a try, but there will be a lot of re-learning involved and increased wear on the tires and rear end components.

When I went from an open diff to a clutch-type LSD on my Datsun 1200, it was an enormous revelation. Suddenly I could put down power going around turns (autocross) and I feel it was the single biggest improvement, just behind lowering it.

I've never driven a welded-diff car but I see the attraction: cheap, simple, and reliable are hard to argue with.

These cars don't suffer from 1 tire fire on track.  Until you do, don't weld the diff.  Makes them pigs on turn in and frustrating to drive.

My old Datsun Two Litre Roadster had an ex-race Datsun Racing clutch LSD that was shimmed to within an inch of its life. Not quite locked up but much tighter than a stock spec LSD. Wow, could that thing go around corners! Wow, was that thing a bitch to push around when it wasn't running! All that said, we're talking apples and tangerines in this case. The Datsun was an early 60's stick axle, buggy spring, flexy frame chassis design and the E46 is a 30 year newer IRS, coil spring, much stiffer chassis design.

As others have said, the spool/welded notadiff will work better in a E36 M3tier chassis that unloads the inside tire more than an IRS chassis. Maybe bolting on the biggest rear sway bar will mitigate some of the tendency to push at corner entry?

My friends IT prepared 280z had a welded dif. It was much faster once you learned how to drive it. The issue he kept running in to was it would brake the brackets that held the dif in the car. He eventually made up new brackets that were much stronger that seemed to work fine. It survived a complete season of endurance racing. I lost track of both the car and the friend after that so I don't know how it ended up working in the long run. 

In reply to rdcyclist :

I run a roadster diff in my 1200 and I've taken an opposite approach.

Because of my motorcycle racing background I am not the least bit phased by the car weaving around in heavy brake zones. In fact the welded diffs always make the car feel sluggish to me.

I have the diff set up on the loose side. I rapidly rotate the car on corner entry and the loose diff aides that. It also allows me to unwind the wheel sooner so the car is flatter on exit.

wvumtnbkr said:

These cars don't suffer from 1 tire fire on track.  Until you do, don't weld the diff.  Makes them pigs on turn in and frustrating to drive.

I was going to say something similar, but I'm of two thoughts:

First was: I was running a 330ci witb the automatic trans gear set, and a 5 speed manual (so 4.56 rear gears, I think?).  Even on 245 series 340tw tires, I never had a problem putting power down (unless I was trying to get the back end loose)

Second was:  you got 3 diffs!  Weld one and send it! 

Tom1200 said:

In reply to rdcyclist :

I run a roadster diff in my 1200 and I've taken an opposite approach.

Because of my motorcycle racing background I am not the least bit phased by the car weaving around in heavy brake zones. In fact the welded diffs always make the car feel sluggish to me.

I have the diff set up on the loose side. I rapidly rotate the car on corner entry and the loose diff aides that. It also allows me to unwind the wheel sooner so the car is flatter on exit.

You make a good point. This was many years ago and I knew way more then than I do now so I knew bolting a race third member was going to be the ticket. Also, my Two Litre was putting down about 180 horsies so I had a little extra to burn with the POC chassis. It was a great fun car for a 20yo idiot. Got totalled by a driver in a '60s Mercury blowing a red light in the rain when his brakes got wet. I was very, very lucky to see him at the last moment and stopped so he only hit the front of the car.

Sonic said:

We ran a mini spool in the back of an S10 Lemons car for years.  We also tried a gov lock, some other terrible one I forget, and finally a zexel Torsen from an F body.   In the end, we put the axle with the mini spool back in, it was the easiest to drive and fastest, especially on tracks with uphill corners.   Even after we sold it the next two owners also thought the limited slip would be better so they swapped axles to the torsen, and both of them have gone back to the mini spool, which it has to this day as it is still active.  This has all been over a period of 10 years.   The only time it gave any problems is parking in the paddock.  It would be no good for a street car.   
 

Don't listen to the haters who have never tried it.  For a race only car it is a fine option. 

I also seem to recall Mark Donohue saying something about locked axles in racing applications in his book The Unfair Advantage.  Might have been Detroit Lockers instead of fully 100% locked all the time.  But they were clearly advantageous over open diffs. on a road course.

In reply to A 401 CJ :

Carroll Smith talked about this too and said: "The only reason to run an open diff is if the regulations require one."

Regarding the Detroit Locker: "To my knowledge, the Detroit Locker has no advantage over any other type of differential except the open diff. It is an abortion"..."The best thing to do with a Detroit Locker is to remove the center cam and run it locked. Period. End of discussion."

I think it's easy to overlook the physics of how the tires are each slipping through the different phases of the corner when using one-wheel-peel as the litmus test for open diff. Considering the full range of potential slip for each tire being somewhere between 0% and 100%, changing the differential characteristics changes the slip at each tire. That pretty much has to change how the the car responds to the changing power and cornering loads, even when the inside tire slip is less than 100% and the car doesn't seem to be struggling to put the power down. They key would seem to be in figuring out how to best take advantage of that though.

Admittedly, that's all focusing on lap times over fun... Although, driving a well set up locked diff car does also sound like it could be a rather entertaining experience.

I tried a Detroit Locker.  I wanted to like the Detroit Locker.  I played with different springs and everything.  The concept is sound.

In practice, unless the road is smooth, it can unlock under power.  Then it rips itself up.  I ruined a NASCAR spec Locker by rallycrossing it with a naturally aspirated 13B.

The neat thing is how much it talks.  And under almost any amount of load, when cornering on pavement, the locker stops talking.  This means that both wheels are turning the same speed even when cornering at like 10% throttle.  So really it has no advantage over a spool other than weighing more and being less reliable, which sure don't sound like advantages.

Pneumatic tires get traction by slipping to some degree, a spool is definitely not the end of the world.

Yeah Detroit lockers have no place on track. They can be useful for offroad vehicles where they can unlock when making tight low-speed turns on high-traction surfaces like pavement and rocks, which not only reduces the need for multi-point turns but saves a lot of wear on your axles and tires. The one on my Samurai never unlocked under power...it doesn't unlock much in general.