I had what seems like the same issue arise last event. Felt like the clutch was slipping as I went over inside curbing, my first thought was diff was wearing out until I reminded myself that I didn't think that was possible.

See examples at 1:52, 2:04 and 2:35



Based on how the Torsen operates, it makes sense to assume that the inside wheel was spinning due to one or a combination of the usual suspsects; less grip on the painted curb, momentary loss of contact with the ground while transitioning on/off curb, etc.

Except that I was there a month prior, drove the curbs the same way, and it didn't do that once. Only obvious variable changed was that this time I was on Hoosiers instead of the usual RC-1s. But I would've expected a grippier tire to have the opposite effect..

Clutch slipping, or diff opening up in corners?
 

Gripper tire means more roll and is the Hoosier grip level really higher on paint? :)

Clutch slipping, or diff opening up in corners?
 

A Torsen with e.g.TBR of 2.4 can transfer 2.4 times the torque of the tire with the least grip.
What's 2.4 times zero? ;)

With too much rear bar for the available grip I have experienced inside "wheel-hop" in specific corners with Torsen, but most often only smooth wheelspin.

+1. There are certain corners where my rear inside lifts when I curb it. I get revs there. Torsens do, indeed, operate like an open if you drive hard enough to lift that inside wheel.

This.

I started having inside wheel spin on 205 Hoosiers in two specific corners at one track. It has now shown up on 225 rivals at multiple tracks now that ive installed door bars.

OSGiken is sitting in my garage waiting for fresh fluid so I can throw it in.

Fair point. Even with the current ~900lb springs.

I may be underestimating the extra roll from the hoosiers, and the effect of bouncing over the curb at apex.

On previous coilovers the curbs used to upset the car, so I drove accordingly (avoided big curbs) to keep the suspension settled and keep traction. Now I hardly feel the curbs, the car just soaks them up and settles on the other side without drama. I didn't at first think that the issue was the inside tire struggling for grip (it's probably hanging in the air like in my sig pic) because it felt like I was just gliding right along.

-Ryan

Example in this vid at 6:08.

https://www.miataturbo.net/race-prep...42/#post892866

I had this same experience running a Kaaz in 1.5 way configuration. Lift the inside rear and it spins up - even if bouncing over a kerb.

I don't have the invoice to clearly identify the clutch. I think it was an ACT "extreme" PP and an ACT organic clutch package. I believe that it has more than a dozen or so track events on it.

The car is 11# boost; 240 rwtq.

A hot-shot MX-5 driver, Kenton Koch was at the race. He thought that the clutch packs might be slipping. I was just unclear at that point if the MSM had a Torsen or a clutch-pack diff. By the discussion and links above, it sounds like it is a Torsen. ... Apparently, I need a new clutch.

Any recommendations on a track-only clutch set-up consistent with my low-moderate power levels?

As far as the suspension; Yes, it does use a Spec Miata Eibach sway-bar package. I am using 850#/400# rates and I do still need to use the middle setting in the rear bar in order to get the car to point and to turn in a corner. How all of you can run that big front 1.25" RB and no rear bar is a mystery to me... It seems like you'd never get the car to rotate(?)... I'd like to learn more about it.

Another factor is that I did not have enough rebound control in the shocks, which allowed for quite a bit of bounce and "hop" in the corners. This could have kicked it off the clutch; Or, it could be lifting a tire and messing with the Torsen as previously described.

Thank you for trouble shooting.

I've run an ACT Extreme clutch since 2008. I highly doubt you're slipping it. Based on the vids you are lifting a wheel enough to spin it, and you said you have a torsen. There's your answer!

Before you start throwing parts at the car loosen/remove that rear bar and try to duplicate the result. That would be the cheapest way of knowing for sure.

Less rear bar, more rear spring. I'm currently at ~650lb in the rear. I'm taking the less bar/more spring approach a bit further to cope with downforce. Note that this is higher than I'd recommend for more standard setups, but should give you a reference.

Also, check your ride height - rake improves rotation. Many times when someone has the right list of suspension parts but the car pushes, it's because they have the ride height set to level.

-Ryan

Its not your clutch. Its the inside rear wheel spinning. You can either put in a clutch pack diff that you'll probably have to setup, or just drop the rear bar and run more spring.

With 850/400, you never would. ;) Try 550# rear springs and a stock rear sway.

I am sorry that I skimmed and made it sound like the clutch is to blame. I tried to correct myself in my second post in this thread. After reading more and watching your video, it's fairly obvious it's your suspension setup causing this issue, NOT your clutch. Please, listen to the few posts above this one.

Easiest solution would be to remove the rear bar. Handling might be negatively effected, however I think you'd notice your problem disappear. Then work your way back to a well balanced car you like, without reintroducing the problem.

Yes, thank you for your responses. My son once told me "dad, I fixed the car and now it won't run". Yep, that's what I did for this race weekend. No time for test and tune as Oregon SCCA no longer offers practice sessions. I had bumped up the spring rate for this race, hoping to get rid of some of the wallowing in the corners. Obviously, the Bilstiens couldn't control the spring... and gave a bumpy ride. Hard on the kidneys, but it had me belly laughing at a few points... and I was still able to have a lot of fun dicing with old friends.

If I understand the "best practices" for a 2400# miata NB race suspension setup for PIR it would be something like: RB 1.25 front bar; 14mm MSM rear bar; 850# front rate/550# rear rate. Adjustable shocks with attention payed to increasing the rear shock travel.