learn me: LSD's and braking behavior
 

Background: five years ago I built a turbo Miata and started doing track days. At some point along the way I replaced the open diff with a Torsen. At the time I didn't notice much of a difference, probably because I was a shitty inexperienced driver. Followed the MT.net circle of life, sold the turbo street car for a dedicated track car.

Current state: I've done many track days, bought a 1.6L Spec Miata a year ago, recently got my comp license and started racing in SCCA. I'm a mid-pack regional SM driver in an underprepared car, so my driving is no longer total shit although I have lots of room for improvement. It was clear that the stock VLSD needed to go, and I recently replaced it with the MazdaSpeed clutch-type differential in a 1.6L carrier/gears.

Last night I took the car to a track night to test/tune and noticed several things:

- better power on track out (duh, that's why I did it)
- easier to set the front end and rotate the car with trail braking (which surprised me)
- squirrely rear end under hard braking, it feels like I have more rear bias. It might be related to the fact that I had new front rotors, but they were pretty well bedded after the first session. I locked up a rear wheel in the third session.

Which leads to my question. It's fairly intuitive that an LSD will help power out of corners, but the changes to braking behavior with an LSD are not discussed as often. So how do LSD's affect vehicle dynamics under braking? Or is this all in my imagination?

I can't imagine the LSD would have anything to do with braking. If I am wrong I would love to hear why. I have always used the theory of if you are locking up tires you are over brake padding for your grip level of your tires. I know people have bias controllers but I have never used them. The only thing I can think that may have happend with your situation would be when taking all of your rear end items out you may have messed with the rear toe somehow. This could be a pretty unlikely scenerio but those cam style bolts in the lower control arms and get kind of weird sometimes. If you are toe'ed out or in that can explain the unsettled feeling.

I have heard other people comment that LSD's can affect braking, but I don't know exactly how. It's a Spec Miata so it has the factory proportioning valve, no adjustable bias. I was on Hoosier SM7.5's with 15+ heat cycles. Currently have DTC-60 pads in the front and GLOC R8 in the back. Regarding alignment, I had a race shop corner weight and set the car up before my last event, and my recent work wouldn't have done anything to change it. I can go to a less sticky pad in the rear, but I'm confused as to why the brake bias would change so dramatically from event to event. I'm trying to rule things out so I can get an action plan.

Between the last event and this event, the things that changed that could affect braking are:
- addition of LSD
- new front rotors (felt properly bedded after the 1st session)
- deletion of self-adjusting parking brake mechanism from rear caliper pistons
- brake bleed

If someone can tell me with authority that LSD's have no affect on braking behavior, I'll turn my attention to the brakes themselves.

I'm happy to subscriibe to this. Maybe your sm7's are heat cycled out? I am switch to carbotech xp10s this year. Dtc 60s were great but they were very much like an on off switch with my wilwood setup. I love this forum. There is so much real info on here.

Based on no personal experience, but I thiiiiiink the RX7 clutch type LSD is 1.5 way (?? big asterix, maybe the Mazda Motorsports one is 2-way? would explain the difference more) so preload on the clutch packs should absolutely alter the locking behavior of the LSD under deceleration, which would in turn change the trail-braking/corner entry behavior of the car (super tangential, but this is why you generally have entry-mid-exit adjustments for diff settings in things like F1/LMPx/GT3/GTE cars).

Anyway my basic understanding is that a 1-way LSD will only lock under accel, but 1.5 and 2 way LSDs will both, to varying degrees (i.e. more with 2 way), lock under decel ¯\_(ツ)_/¯.

Edit: of course there's a ~8 year old engineering explained video going over it (if you feel like going deeper):

Cats for you sir. That gives me something to research further, thanks.

Anytime dude I blame iRacing

I know in the Porsche 911 world a locking diff under braking really stabilizes the rear end. With an open diff you'd get a lot of waggle. I think in that car the rear brakes act like a parachute because of weight distribution and large tire sizes (315/335)

While a locking diff under braking would also help a Miata I don't expect the differences to be as drastic. Under hard braking the car stands on it's nose a bit and the rear isn't doing much. The rear never felt nervous to me in braking zones but then again I have an OSG.

After doing a lot of reading on this I think the most likely culprit is the new front rotors. Intellectually it seems like the LSD would only serve to stabilize the rear during straight line braking and shouldn’t really be adding any rear bias.

Mazdaspeed clutch type if the same as the RX7 is 2 way, so expect locking under braking. Trail braking will induce slide-age. Same as downshifting, test in a well spaced area until you're comfortable with the change.
Squirrely rear under hard braking sounds correct.

Most would recommend going to a 1.5 way for this reason, as the lockup under decel can be aggressive, but I say if you've got it, learn to use it!

Is the rear locking up during corners when you are loading the outside rear tire and unloading the inside rear tire? Or does this happen during straight line braking? Since you swapped out the front rotors, did you bleed the brakes after doing this work? If you introduced a small air bubble to the front brakes, your clamping pressure on that caliper will be reduced, "shifting" your bias rearwards. I use the "empty bottle and 1/4 in. clear tubing from homedepot" trick for bleeding brakes.

2-way diffs DO have a strong lockup during braking and acceleration. They had a mild lockup when coasting in gear (engine braking). They are more stable when you have rear slip angle (vs a 1.0 or 1.5 diff), with strong lockup in trail braking. Ideal for drifting, they're the preferred clutch type for extremely aggressive driving styles due to their predictable on/off throttle lockup behavior. I don't like an aggressive lockup on deceleration, some lockup is nice though.

I did bleed the whole brake system thoroughly so I don’t think it’s trapped air. After a little more research I think that the increased rear bias under straight line braking is from engine braking locking up the rear; I’m going to try getting on the clutch sooner in the brake zone. The behavior during trail braking is really nice, I adjusted to that quickly.