LSD's and braking dynamics

My Spec Miata came to me with the stock viscous LSD, which was tired and acted like an open differential.  Tracking out of tight turns I had lots of inside wheel spin.  In the last month I've done a bunch of upgrades to the car, including addition of a clutch-type LSD (the MazdaSpeed if anyone cares).

Last night I took the car on the track for the first time since doing the work.  Unsurprisingly, it was much better at putting down power on track out, and it was easier to throttle steer around fast sweepers.  A little surprising to me was that the car felt quite a bit more tail-happy under braking.  On a positive note, during trail braking it felt easier to set the front end and get the car rotated.  On a less positive note, during threshold braking it felt a little squirrely, as if there was more rear brake bias. I even locked up a rear wheel, which you Miata guys know is pretty difficult to do with a stock brake package.  

So on to my question: are these braking characteristics inherent to clutch-type LSD's?  Or is it more likely that it's related to something else I did?  I did put on new front rotors which were slippery at first, but they were pretty well bedded by the end of the 1st session.  I also deleted the rear parking brake auto-adjusting mechanisms inside the caliper pistons, but I can't imagine how that would affect braking power.  Right now I have Hawk DTC-60 pads in the front and GLOC R8's in the rear.  I'm thinking about going to a less sticky pad in the rear but I want to understand the problem better first.

It's been a while since I've worked on the rear brakes of a Miata but I seem to recall the adjuster having the ability to effect the balance of the car. I am not sure what deleting the auto adjuster physically does to the system.

If the rear brakes are not doing equal work that will tend to make the car squirm around.

If the rear pads are a more aggressive compounds then the fronts that will cause the issue.

The car should actually be more stable with the LSD as the wheels are locked together to a certain degree.

Generally-speaking, a clutch-type LSD with fresh clutches (or higher preload) *should* increase braking stability, not decrease it. 

If the tighter LSD is better at transmitting drive torque, it is probably also better at transmitting engine braking, and you may have a wigglier rear end because you're carrying more longitudinal slip on 1 or maybe even both rear tires, which makes them take more slip angle for a given turn.

In reply to AngryCorvair (Forum Supporter) :

He said threshold braking so that tells me no turning involved, there should be no slip angle coming into play.

In reply to AngryCorvair (Forum Supporter) :

It's not better at transmitting drive torque, though.  It's better at keeping the rear wheels close to the same speed, everything else derives from that, good and bad.

If you were braking into a corner, then I can see the diff making things squirrely as it will be adding a level of braking/tire slip to the equation.

If it's squirrely in straight line braking... did you realign the rear suspension to the same specs after the diff install?

Sounds like you're braking while you still have your foot on the gas.

Tom1200 said:

It's been a while since I've worked on the rear brakes of a Miata but I seem to recall the adjuster having the ability to effect the balance of the car. I am not sure what deleting the auto adjuster physically does to the system.

If the rear brakes are not doing equal work that will tend to make the car squirm around.

If the rear pads are a more aggressive compounds then the fronts that will cause the issue.

The car should actually be more stable with the LSD as the wheels are locked together to a certain degree.

The reason I removed the rear parking brake adjusters is that they can tighten during a session and cause extra drag, reportedly costing up to 2hp. After they're rwmoved the hydraulic pressure applied to the pistons is the same as before. It's pretty standard protocol to remove them from SM's and to use one step "colder" pad in the rear than the front to increase rear braking, since the stock Miata brake proportion is pretty biased to the front.  

Pete. (l33t FS) said:

In reply to AngryCorvair (Forum Supporter) :

It's not better at transmitting drive torque, though.  It's better at keeping the rear wheels close to the same speed, everything else derives from that, good and bad.

If you were braking into a corner, then I can see the diff making things squirrely as it will be adding a level of braking/tire slip to the equation.

If it's squirrely in straight line braking... did you realign the rear suspension to the same specs after the diff install?

It's squirrely in straight line braking. My last car had an adjustable prop valve and it's the same feeling as when you have too much rear bias- the rear brakes start to lock before the fronts. Alignment is same as before. 

FMB42 said:

Sounds like you're braking while you still have your foot on the gas.

Nope, typically the clutch is disengaged and I'm also managing a heel/toe shift. 

In reply to ShinnyGroove (Forum Supporter) :

Do the rules allow you to install at Tilton valve that would allow you to take off some off the rear brake?

Tom1200 said:

In reply to ShinnyGroove (Forum Supporter) :

Do the rules allow you to install at Tilton valve that would allow you to take off some off the rear brake?

Nope, all the rules permit is different brake pads. Which I will probably do, I just wanted to solve the riddle of whether the LSD has anything to do with it first. 

I wonder if you may have stagger in the rear tires that you never noticed with the open diff, although you'd think if that were the case that you'd notice other issues.

Very strange!  But it's also good that you're in-tune with your chassis enough to notice these things.

In reply to ShinnyGroove (Forum Supporter) :

I've never heard of a clutch LSD doing that. I'm no engineer but I don't see how the diff could do this.

The only other thing I can think of is the newly installed front number pads are not working 100% relative it the rear.

To clarify were the rear pads installed at the same time as the diff?

unrelated to your question, but something That's making me question things...

I was under the impression the mazdaspeed NB had a Torsen Type 2 diff....

or are you referring to a mazdaspeed racing parts clutch type diff?

Yeah this is the Mazdaspeed racing LSD that used to be sold by Mazda Motorsports. It's since been replaced by the Cusco, which is also a clutch type. 

After reading up on this last night, it seems like the diff is an unlikely culprit. Must be that the new front rotors aren't biting as aggressively as the old rear ones. I think my next step is going to be replacing the rear rotors as well.

Engine braking is adding more brake bias to the car. 

dean1484 said:

Engine braking is adding more brake bias to the car. 

The more I think about this, I definitely think it's a factor. In the two main braking zones where I noticed it the most, I was coming down from the rev limiter and I was well into the braking zone before I used the clutch to manage a heel/toe shift. May be something I need to work around with my driving. 

It is just another thing you can use to tune how the car drives. I like a car with more engin braking as you get more lift off rotation/oversteer. Some people don't like it and want a more neutral car off throttal and want the rotation initiated either by braking or steering inputs or both.   Add a adjustable proportioning valve and it will let you tune it to your liking. 

Here's what can be seen to be causing it a bit; clutch types depend on the jacking loads of the spider gears to engage the clutch plates. If power is applied to the diff, the spider gears force outwards and that is the "pressure plate" effect on those clutches.  It works under acceleration and it also works under braking. so engine braking is transmitting pretty well to the inside wheel as it will give the same jacking force. If you are turning at all, its making the inside tire want to go the same speed as the outside. Added, it is effectively applying most of the engine power to that wheel, because its the slower wheel. 

So, instead of (engine braking + brakes) you have (engine braking, + brakes + differential force from outside wheel which travels faster)

Its going to be a bit different of a driving style and may necessitate playing with your pad stagger. 

My mustang has had burnt out diffs that were essentially open and freshly rebuilt with a stronger packing order than stock as well as some time with an auburn roadracers diff. The rear gets a lot more active with the diffs on decel because of that effect.  It wants to act more like a solid rear axle when forces from the driveshaft come into play the stronger your diff is. .

In reply to Apexcarver :

Backwards, though.  The outside wheel is being overbraked, not the inside.

If you're at the threshold of traction, then if the outside tire falls off the traction peak, then the higher braking load transfers to the inside, because at that point the right side has less traction, but the braking force from that brake is still being transferred to the left side via the diff...

Pete. (l33t FS) said:

In reply to Apexcarver :

Backwards, though.  The outside wheel is being overbraked, not the inside.

If you're at the threshold of traction, then if the outside tire falls off the traction peak, then the higher braking load transfers to the inside, because at that point the right side has less traction, but the braking force from that brake is still being transferred to the left side via the diff...

Doh'  I should know better than to post before I've had my coffee.

You have it right. 

Engine braking does effectively add more rear brake bias.  There are a few ways around it: Either downshift later so you don't get the revs as high off throttle to reduce engine braking, or adjust brake bias to be more forward. 

I'm glad I asked- this all makes total sense and is consistent with what I felt while driving. Thanks guys!

This is going to fall under do not try this at home; I don't normally share this technique as it's a carry over from my motorcycle road racing days but I will in this case as the technique applies.  

I typically complete my downshifts just before the turn in point, if I really have the car on the edge (squirming around under braking) I preload the throttle pedal slightly to bleed off some of the engine braking and get the car to squat.  I typically use the technique in late braking maneuvers.

In the same way you blip / squeeze the throttle for a heel and toe down shift you simply apply a small amount of pressure on the throttle pedal to balance out the car.

Why bother with all of this? An extra bit of rear brake bias makes it easy to rotate the car rapidly at the expense of making the car really edgy under threshold braking, so the technique allows you to keep the ability to rotate the car aggressively but still keep the car settled under threshold braking.

There are some serious down sides to this; it puts a lot of heat in the brakes and most drivers are likely to get this wrong. If you do get it wrong you're going to loop the car at higher speeds.

For 99% of drivers it falls under don't try this at home.