Hoosier SM7 Autocross Questions
First of all I realize that these are not the ideal autocross tire, however they are available and inexpensive so I’m running them! Anyone here autocrossing these on a FWD strut-suspension car? A Mazda BG chassis based car would be even better (90-94 Protégé/323, 91-03 Escort, MX3). I’m curious what you’re running for front camber, caster, and toe, and in particular front tire pressure.
I’m racing a ’95 Escort GT, turbocharged, with more than enough power and a variety of suspension upgrades. Everything feels pretty balanced on the street, on street tires. The past few events I ran NT01’s all around and fought against understeer/lack of rotation. My alignment was far from ideal, so before yesterday’s event I set my camber to ~-2 degrees, maxed out my caster (camber plates flipped 90 degrees), and set a slight toe-out up front. I also raised the rear coil overs slightly (two turns of the collar, nothing extreme). Unfortunately this didn’t seem to help much, so halfway through the event I swapped SM7’s onto the front of the car only, with NT01’s still on the rear. The controllable throttle lift-off oversteer that these cars have in stock form was back! My times immediately improved, and I’m looking forward to trying this mismatched tire setup again at the next event.
I ran the SM7’s at about 37-38 psi hot, but I seem to be running the outside edge of the tire a bit harder than I’d like. Trying to decide which to try first - more tire pressure, or more negative camber? How much pressure is too much? I’m sure a wider wheel would also help (running 15x6 miata wheels), but that’s not in the cards until maybe next season.
Tl:dr – Recommended SM7 tire pressure and preferred static negative camber for Mazda BG chassis autocross car, please?
IMO, more negative camber.
More negative camber if possible. All you can get. 35 psi should be fine, the narrow wheel isn't helping you any there, so some roll is gonna happen.
Enter the corners a bit slower and use the grip of the hoosiers to get onto the power earlier and longer.
I can answer some of that, I think. SM7 tires are 205/50-15, and he already said he was running 15 x 6 wheels. My guess on class would be CSP since the Hoosiers aren't legal for Street or ST classes.
The Hoosiers have very stiff sidewalls, so I would think they wouldn't need very much pressure. If you hadn't already said 37 wasn't enough, I would have recommended 34.
Thanks for the tips so far! Lets see:
Yes, SM7’s come in any size you want, as long as it’s 205-50-15. I’m running 15x6 NB1 wheels, mostly because all of my 2nd hand racing tires happened to come attached to them :)
Technically it would be an SMF car if I were running with the SCCA. I run with a small private club so they grouped the SCCA classes by PAX index into just 6 classes; more competition that way.
This is my first season of autocross & I’m driving a powerful car, so I am almost certainly over-cooking corner entry… I suppose this alone could be a good chunk of the problem. Perhaps braking a little earlier and working on cleaning up my driving would be a good idea vs. making more adjustments? Driver mod as they say...
With my current setup I can adjust either camber or caster (by flipping the camber plate 90*, which is what I’ve done). My -2 degrees of camber came from extended lower control arms via a longer ball joint. It was suggested to me that lots of caster would allow me to run less static negative camber, since the caster results in camber gain as you turn the wheel. True?
My concern about too much static camber is that at -2 I’ve already noticed a significant drop in forward traction vs. my previous factory spec alignment. In a stock ford escort this wouldn’t be a big deal, but I’m trying to put down about double (and capable of triple) the stock power. I'd really hate to waste whatever advantage that power might afford me, especially since it's part of why I'm in such a ridiculous class. That's what I get for building a street/drag car 10 years ago and then deciding to take it autocrossing...
I ran 2.5 degrees of negative camber in my turbocharged Sentra (270 whp) and I always thought I could use a little more. Forward traction wasn't really much of an issue since it had a Torsen LSD. If you have an open diff, I could see this as more of a concern for you. In that case, I would recommend investing in an LSD.
On my ZX2SR I nearly doubled the caster. Ended up with 2.5 degrees neg.camber and ZERO toe.
I tried in and out toe and zero seemed to work best. I forget the pressures but it is what Hoosier said for the A7. My temperatures were even. Bigger rear bar.
Yeah, forward grip is gonna suffer with the ultra stiff sidewalls and fwd weight transfer. Do you have a LSD in the car?
Autocross really is about cornering speed (see Miata) and that's what Hoosiers do well. Adjust your driving accordingly.
-4 degrees of negative camber is what my Miata waddles around on. But it lets me run 25-24 psi air pressure which really helps the grip levels.
Back up your corners a bit. It's educational to monitor tire wear on an endurance race team. Same lap times over a two hour stint can show dramatically different tire wear. We had one guy complaining about understeer, and he was grinding the edges off. Convinced him to use the brake pedal to slow the car, instead of hurling it into the corner..."But I don't like to do that!"..
I see you are running coil overs. What is your rate balance front to rear ? Going stiffer in the rear and or a bigger rear bar will loosen things up.
The car does have an lsd (Quaife). At 15 PSI wastegate pressure, which is all I'm running at the moment, I should be making approximately 250 lb ft. With boost control hooked up the car is tuned to run 23 PSI, where it made 325 lb ft before I upgraded the cams. My torque curve in either case is pretty steep, so traction has always been a challenge. I may try getting a lighter wastegate spring & tuning my boost control to ramp up boost more gradually. Dyno Graph
I forgot to mention, the car is a little more front heavy than your typical escort as I'm running a heavy (but strong!) gearbox out of a camry. Definitely not needed for autocross, but it was a lot of work & I'm not about to undo it. I'm sure that's not helping.
A little more detailed info on the current suspension setup:
-Tokico zx2 s/r struts, nothing special
-Coilover sleeves 300 lb front / 200 lb rear
-Front camber plates
-Stock 25mm front sway bar from a ZX2 (thicker than the stock GT bar)
-Aftermarket rear sway bar, 22mm I think. I considered trying to get my hands on a 25mm bar, but I've also seen people snap lateral links with those...
-Poly bushings everywhere
-Widened front track & added camber via RX7 balljoints as I mentioned above
Overall I'm thinking this should put me relatively close to iceracer's setup, depending what springs you are running? I have no idea what my actual caster setting is, I just maxed out the adjuster plates on both sides.
I have a deposit down on a set of “real” coilovers which will allow me to adjust the height “properly” i.e. without losing suspension travel. The springs rates the manufacturer decided on were 275 front, 225 rear. I'm worried the front may be a little soft, but maybe not, we'll see.
Streetwise, what exactly do you mean by “back up your corners”? Brake earlier?
