Hi guys,

I didn't know what topic to post this on, but I figured the autocross guys would know the best, I'll post this on Brakes/Suspension, too..

I have 15x7 with some generic crappy 215/60/15 P-rated tires.

I acquired 16x8 rims and I'm planning to put 245/45/16 W-rated Kumho's on these.

For a while I have been tempted to put 255s in the back.

What I am worried about is whether I will get understeer having larger tires in the back? How much of a difference does 1 cm make anyway?

I reckon Camaros naturally significantly oversteer (might be ), so maybe putting slighltly wider tires in the back is going to compensate for that..BTW, I have no rear spoiler.

How much of an improvement are 255 over 245 in the back with respect to drag racing/straight line performance?

What will the end result of putting 245 in front and 255 in the back be? Understeer? If so, by how much..

What pressures do you guys keep your tires on?

Any clues, personal experiences or war stories are helpful..

Thanks!

 

it is important to run teh same size tires on all four corners in our cars if you plan to explore teh limits of traction. if you hae a larger tire inteh rear the excess traction at the rear of the vehiclke will prevent you setting a proper drift on entry and being ablew to steer by throttle at teh apex.

later
tim

 

Gotcha! Thanks NJSpeeder, very insightful!

Would putting a higher profile in the back, but same diameter & rim change traction?

 

Another guy told me that he thinks profile 45 would look too skinny on a car like mine (which is for the most part stock)..
Again, what combo I'll settle for depends on what end result traction I would get...

 

a taller tire would have the same effect as a wider tire on the rear of trhe car since it adds contact area as well. ti just adds a front to rear surface area to the tire instead of a side to side surface area.
a 45 profile tire can be of great help too. since there is les side wall there is less sidewall flex. if the side wall of the tire is noit flexing and the surface of the tire is not seeing any block shift(the tread blocks of the tires will actually squirm aroudn under full load, a trained or experienced driver can actually learn to feel this) it allows all of the deflection of the change in direction to be controlled by teh suspension. the more of teh movement you put in the suispension the more predictable and controlled cornering can be.

later
tim

 

I'd go with all four the same size. Two reasons: 1. Most of our cars don't have the power to weight ration of an LMP, GT1 or even TransAm series car, so its not worth it. 2 Rotating your tires is easier, especailly if you use the tires on the street as well as the track.

F-Bodies are designed with massive understeer. Most passenger cars are. Understeer is a much easier handling condition to recover from than oversteer, so most cars understeer when pushed too hard. F-bodies are very nose heavy, so they don't get good traction at launch, but a nose heavy car has a tendancy to 'push' in the turns. A tail heavy car like an early Porsche 911 will go in to an oversteer condition when you are brakeing and turning. Power-on oversteer is something else entirely, it looks good, but at that state, you have exceeded your rear traction andc can't launch out of turns.

For autocross, the speeds are slow enough that aerodynamics don't come in to play. A wing that would work at the 30-40 mph speeds of autocross would have more surface area than my mattress, so worry more about mechanical grip than aero stuff.

If you really want to get in to setting up and figuring this kind of stuff out, check out the 'Physics of Racing' series of articles. They're written by a PhD physicist who races Corvettes. Good Stuff.

Oh, yeah, the tires. If you are going to use the tires as your everyday tires, don't go too nuts. Get a good 245 with either 50 or 45 section, Kuhmo's are good, although I run Dunlop Sport5000's right now.
I'm spending my tax refund on some 275/45 Hoosier slicks and 16x10 NASCAR rims, so I'll be hooking up like a **** this summer

 

Wow, thirdgen are some nice forums. So little replies yet each one very constructive. I read through PHOR a month ago, and I was pretty impressed.

That got me interested in autocross and I'll probably post some questions about that later on.

To summarize here's what my current decision is:

245/45/16 Kumho's on all 4 16x8 rims

Why? (feel free to correct me on these..)

1) Same size on all tires to be able to enable/control eventual drift in aggressive turns
2) With my current ghettosetup, I don't need a superstrong hookup as I can barely make a 'peglegged' peel w/ my 215/60 P-rated goo. So even for pure drag/off the line performance 245/45 is enough for an engine of my size with my amateur mods.
3) Even on all 4 same tires, I'll have understeer b/c I have so much weight in front by default. No significant mass transfer occurs with a car of my strength; I can't accelerate fast enough (not enough jerk) so this becomes significant.

Theoretical question: Would putting sandbags in the back help? If I'm not mistaken, my curb wt is 3200lb or so, so probably adding more mass will make my current setup even sh!tt!er..

Another question (a little OT): How much straight line is there on autocross courses? OK, if avg. is 30-40, what are top speeds on your casual autocross track?

Recently I got interesed in aerodynamics of things.. I'm sure spoilers don't do much below, say 55... I'm wondering how steady can a good spoiler keep you at higher speeds than that and what suicidal attempts do you actually have a chance of surviving?

Would CMC qualify as an autocross event? Probably not? What chances does a total rookie like me have of practicing/entering autocross (say, in Chicago area), except for 'practicing' under the joyous supervision of the local Police Dept..

And, finally, how much of a tire does drifting waste?

Sorry for the long post, I'm just very curious and thanks for helping out with choosing the tires..

 

Okay, easy answers first.

Straightaways: most autocrosses don't have any true straight aways of more than about 100 feet. Autocross is a contest of handling skill and driving technique more than outright power, but there are often 'power slaloms' where the gates are pretty far apart. Most courses also have a 'stop box' right after the finish line, where you have to bring the car to a complete stop before you are allowed to leave the course. The exceptions to this are events held at airports or real race tracks. At locations like these it is often common to have 'flying finishes' where the course routes you out on to a long straight or runway, giving you time to slow down.

Top speeds: On most courses, you won't hit more than 40 or 50 miles an hour, depending on the site. A course in a mall parking lot may be pretty slow in terms of top speed, compared to a course on the Jefferson Circuit at Summit Point. Recently, I did a course set in the infield of a local stock car oval. The course featured a long sweeper up the banking leading to a flying finish on the front straight. It was a 3/8 mile oval, not Talladega, but I saw about 65 mph going through the finish lights.

Availability: Unless you have private access to about 10 acres of pavement or so, your only chance for practice will be to go to an actual autocross event. Check with your local SCCA region and any local car clubs for their schedules. Most events will get you 3 to 5 runs for about $20. The season is just getting started, so there may be a driver's school in the coming weeks. Most events are open to anyone. When you show up, don't be afriad to talk to people. If people know you're a rookie, they will be glad to help you get where you need to be. Always look for the F-body guys first


Ballast: I have noticed when I hit an on-ramp at speed when I have my tool box in the trunk, (60lbs) that the car seems less inclined to plow the front tires when I really push it. I don't race like that though. I do usually leave my spare and jack bolted in the trunk, and I like to have my fuel between full and 1/2 tank, to keep the rear weighted. Take as much weight off of the front as you can to get the front to rear ratio as close to 50:50 as possible. I believe a V-8 F-body is abou 65:35 front:rear. 30 lbs is about 1 percent. Taking off the A/C is worth about 2%, moving the battery maybe another 1-2% Aluminum Cylinder heads take about 40lbs off of the motor. You get my drift.....

Getting a Posi-Trac rear end will help you hook up better, as will higher ratio gears, 3.42, 3.73 will give your car more scoot off of the line, but give up some cruising capability (but not much)

Last but not least, Don't worry about being slow. Just be cool, figure out how to get around the course, and speed will come to you.