Sudden on-track handling issues, help me diagnose them

I need some input from the hive mind here, especially as I'm not exactly a suspension guru.

Car in question is a Miata with FM frame rails, FM VMaxx coilovers running 205/50 15 Federal 595 RS-R high performance summer "not quite trackday tires". Re the tires, I've run these before with decent success but in a cooler climate.

First session of the day, everything is fine. Second session about 1 1/2h later and slightly warmer outside temperatures - still "normal" or even "cool" for here, but warmer than the first session and we have a bit of sun on the track.

About halfway through the session I noticed that the rear of the car felt "iffy" through the set of high-speed corners at the end of the start-finish straight. In a Miata even I have to barely slow down to set the car up for the corners and once set up you accelerate through them. The track has a crest in between the first two of the three "kinks" but it's reasonably smooth. On the track map below, I'm talking about the squiggly part in between turns one and two:

This time I noticed that the rear felt as if it was getting very close to snap oversteer as I was making the minute course corrections necessary. There was definitely an exaggerated "pendulum" effect at the back. To me, as a non-suspension engineer it felt like the rear suspension wasn't able to accommodate the change in load and the rear was about to put in an overtaking attempt. It almost felt like the suspension was binding at some point.

Oddly enough the car felt OK through turns 2-4 and especially 5 (which is uphill and over a little bump that gets the car light and one that you will have a tiny slide in if you go fast enough) only for the rear to properly let go in the very slow right hander between turns 8 and 9 on the above map. Full pendulum effect, too, as in slide - swear - grip - slide -grip - slide - grip - wipe brow. When I got the car back under control it felt as if I had had a major loss of pressure in one of the rears, so I limped back into the pits, only to find that the rear tire pressures were OK and within 1 psi of the hot pressures in the other session. I had noticed that the fronts were feeling a bit greasy (presumably due to the heat) in turn 8 but other than that the car had felt OK at that point.

Back in the pits, the tires felt like they had about the same amount of heat in them (sorry, forgot to borrow an IR thermometer), no signs of "proper" overheating of the tires like discolouration or excessive rubber snakes on the tire. In fact, the tires seem to be wearing rather well and even. No fluids on the track in that spot either, at least as far as I could tell. Plus, given that the car didn't feel "right" earlier in the lap I don't think it had anything to do with the track condition. The wheel nuts were tight, too.

The car was OK for the last two sessions of the day but I toned it down a little 'cos I try to limit myself to a single brown trouser moment per track day.

My working theory on the day was that I overheated the tires, but I'm not sure if that explains the "suspension feels like it was binding" moments. I'm also going to check over the suspension for any signs of binding/bottoming out/blown seals/missing bolts before the next outing.

Any suggestions what else I need to check?

Sounds like you might have killed a bushing somewhere in the rear subframe. That could be anything from the suspension arms to the subframe, though.

What kind of Tire pressures? And are they shaved or full tread?

Coming from an analytic standpoint (not experience) I would check the rear in this order:

Lugnut torque, eccentrics, bushings, wheel bearings, rear subframe brace, a-arms themselves (for rust, cracks), subframe mount bolts.

My bet is that your eccentrics are slipping/bent.

I had something similar on one of my 96 Miatas at Lime Rock. One of the eccentric bolts on the rear suspension was just loose enough to let it slip under load.

racerfink wrote: What kind of Tire pressures? And are they shaved or full tread?

Tires a full tread - well, more like 3/4 tread now. Tire pressures hot were F 35, R 33, equal on both sides when the above loss of handling occurred. I bumped the rears to 35 hot for the last session but that didn't seem to be making that much of a difference.

Warren v wrote: Coming from an analytic standpoint (not experience) I would check the rear in this order: Lugnut torque, eccentrics, bushings, wheel bearings, rear subframe brace, a-arms themselves (for rust, cracks), subframe mount bolts. My bet is that your eccentrics are slipping/bent.

Lugnuts were checked between sessions (I always take my torque wrench), as usual the lugnuts needed nipping up between sessions but only in a minimal way. We're talking less than 1/8 turn to get them up to 75Nm again.

I'll inspect the eccentrics (which are about a year old all around) and the bushings as soon as I can and will retorque the rear subframe brace as I had that off. The funny thing about that brace was that I felt its bolts were a bit on the loose side when I took the brace off to fit the exhaust so that sounds like it's worth another look.

I'll also get the alignment checked and make sure that the sway bars don't appear to be binding - maybe even rewrap them in teflon tape and regrease them (they're new FM sways with the polyurethane bushes they are sold with).

DaewooOfDeath wrote: Sounds like you might have killed a bushing somewhere in the rear subframe. That could be anything from the suspension arms to the subframe, though.

I'll check them over, they're polyurethane that are only about 18 months and maybe 4000 miles old with three PDXs on them. I need to regrease them anyway so it might be a good time to inspect them. Oh, and the subframe mounting bushes are still the originals so they probably warrant a closer look.

I've been told that those eccentric bolts need a crazy amount of torque, something along the lines of the biggest available guy with the longest available breaker bar. Any time I had a Miata aligned, I would mark the position of the the eccentrics with paint before leaving the alignment shop.

Also check the shock for any oil leaks. A bad shock doesn't allow you to control the grip very well.

Not a suspension engineer, don't play one on TV, didn't stay at a Holiday Inn Express, but given the fact that your rear suspension appears to be functioning normally under steady load, but abruptly changing grip levels in back and fourth transitions, any chance you're getting a "bind and release" in the rear sway bar? (End link/bushing issue?)

BoxheadTim wrote:

racerfink wrote: What kind of Tire pressures? And are they shaved or full tread?

Tires a full tread - well, more like 3/4 tread now. Tire pressures hot were F 35, R 33, equal on both sides when the above loss of handling occurred. I bumped the rears to 35 hot for the last session but that didn't seem to be making that much of a difference.

Doesn't sound out of line on tire pressure, but I remember running the 13hr at VIR in a Spec Miata one time using some Goodyear tires we got for free. They were shaved, but they neglected to tell us they were "street" compound. Worst tires I've ever tried to race on. Lock-up was very easy to do, push on entry, and loose on exit, and a very thin line between control and sideways.

Being on treaded "street" compound tires might give some weird feedback in addition to the other suggestions.

BoxheadTim wrote:

DaewooOfDeath wrote: Sounds like you might have killed a bushing somewhere in the rear subframe. That could be anything from the suspension arms to the subframe, though.

I'll check them over, they're polyurethane that are only about 18 months and maybe 4000 miles old with three PDXs on them. I need to regrease them anyway so it might be a good time to inspect them. Oh, and the subframe mounting bushes are still the originals so they probably warrant a closer look.

Sometimes there are manufacturing defects. I crushed a lower control arm bushing in my car about a year ago when the bushing only had about 1,000 miles on it. It exhibited a lot of the same symptoms you're describing.

I'm thinking you might have it right with the overheated tires. I don't know that particular rubber or how it behaves with heat. Definitely inspect the rear suspension for shock leaks.

If it was a slipping eccentric, they usually stay slipped.

Will do. I know from experience with these tires that their wet grip is nothing to write home about and that they let go in the wet fairly suddenly so it might well be that they generally behave like that when transitioning from grip to Oops.

The longer I think about this, the more I'm convinced it was something heat related; if I had managed to break or bend something, it should have affected the following two sessions also.

Another thought is that maybe the two "incidents" were unrelated; I might actually have managed to drive the car at the edge of grip between turns one and two, with it letting me know that things were about to get hairy if I was pushing it any harder.

Either way, I have to climb underneath it.

And yes, I think I need to plan for another set of tires for next year.

Got the car back out on track yesterday. I had checked everything over (no leaks from the shocks, no damage or lose components) but didn't find any damage to anything.

It was cooler this time, plus I got of the HF infrared thermometers (hey, they were on sale).

I got the distinct feeling of the tires getting slightly greasy when I was pushing the car through my possibly favourite section of the track (turns 5-7 on the above map), but I never got the "unsettled" feeling between turns 1 and 2. Highest tire temp I measured (when the tire started feeling a little greasy) was around 130F so everything points at me overheating the tire.

So, in conclusion it looks like some better tires are in order.