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tires: 275rear, 245front: bad for autox?

Old 08-20-04, 09:20 AM
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tires: 275rear, 245front: bad for autox?

I'm pretty new to autox. I've driven a few events in my FD on my regular street tires: 275 in back, 245 in front. It wasn't until I drove my miata at a few events that I began to suspect that the FD was not quite as well balanced as the miata. This could be for one of 3 reasons:

1) I'm too much of a novice to know what a well-balanced car feels like
2) the power of the FD compared to the miata makes me *think* it is less balanced
3) the wide rear tire / narrow(er) front tire combination upsets the balance a little

If the answer really is (3), then the fix is fairly simple - new wheels & tires. However, I'd like a little advice before I drop the cash.

Thanks for any advice
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Old 08-20-04, 09:42 AM
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I run a stagger on the street and have tried it in Autocross and it wasnt good.

I run the same front and rear and find the car is less prone to understeer.

I would start with the tires.
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Old 08-20-04, 11:12 AM
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First off, there is nothing wrong with running staggered set-ups many people, if not most, do. When you say yur car does not seem balanced, what do you mean? Is it understeering, oversteering, transitions poorly, lacks crisp turn in, What??? What tires are you running (and at what pressures), what are your alignment settings, what sway bars, dampers, spring rates? With all do respect to Jim, we need to be sure that your existing set-up is maximized before you go changing it. What size rims do you have front, and rear(width, and diameter). Is your car modded? Does it make stock power, or more? Is it set up sequential, or in TTC? Do you plan on running a set for the track, and one for the street, or the same one all the time. If you answer each of these questions, I, or one of the other experienced racers can properly direct you. It is true that we can say the ultimate tire wheel set-up is "x", but you may have a fine set-up that needs tweaking. Also, since you are a new driver, you are probably doing a variety of things that make the car feel unbalanced, when in fact it may be fine.

I do not see how anyone can give you a good direction without these answers. Let us know, we will get you squared away. Regards, Carl
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Old 08-20-04, 03:14 PM
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Carl: Thanks for the candid comments. I actually avoided trying to give too much specific information since I'm not really after the perfect setup (I'm still too much of a novice to be worried about that). I really just wanted to know if my tire setup was obviously wrong (I race radio control cars, and with those things tires more than any other setup variable have the greatest effect on handling).

But since you sound willing to give more detailed advice , here's more about my setup

- tires are dunlop SP8000 running 35psi front & rear
- all suspension settings are stock R1
- Rims - 17" front and rear. I don't remember the width off the top of my head.
- Engine - see signature. It is still sequential. Boost is kept at 10psi.

And in reply to some of your other questions:

- by unbalanced, I mean very slight understeer with a tendency to loose the tail if I try to correct too much. This is compared to the miata, which is stock R-package except for a stiffer front sway bar.
- If I buy another set of rims, they will probably be stock rims and I'll run race tires. I'll keep the 17's for the street (unless this is a bad idea?)

Thanks for any help you can give
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Old 08-20-04, 03:52 PM
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First, if you are willing to spring for wheels, and tires, then I think the stockers are an excellent choice to start with. The advantages are they are readily available for ~400.00 a set, you can resell them if you decide to upgrade later for about what you pay for them, and you do not have to worry about fitment. I would use a 245-45-16 Kumho V700 Victoracer on those rims. This will give you an immense improvement over what you are running now.
As to the slight understeer, that is most likely primarily driving style. you can counteract that by lowering the pressure in the rears, this will also help with the end "coming around" on you. When you say "correct too much", that tellsme you were going too fast, and then you over corrected, this is a problem with you, not the car Using an R compound tire will make the car more predictable, and the break away more linear. In other words you will be able to maintain a higher slip angle without having to make abrupt corrections. If you'ld like, you can call me, and I can go into greater detail. So, lower the pressure to ~32 hot in the rear, and slow down a little with your current set up, or buy the R compounds and the stock rims as above. This is the tip of the proverbial iceberg, but it will get you started. You can call me at 707-696-2705, and I will spend a little more time if you like. Have fun, Carl
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Old 08-20-04, 04:14 PM
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Originally Posted by Carl Byck
When you say "correct too much", that tellsme you were going too fast, and then you over corrected, this is a problem with you, not the car
Indeed

Thanks alot for the advice. I won't get too deep just now, but it sounds like I wouldn't be wasting my money on a new set of wheels & tires. And now I know where to come when I need some more setup advice.

Again, thanks.
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Old 08-23-04, 12:42 AM
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too wide in rear >> understeer.
too narrow in rear >> oversteer.

Since you're experiencing both at an autocross, it's probably due to driving. At your next event, try to find someone who is experienced in powerful RWD cars, ask them to drive your car and evaluate your setup. It's also fun to see their lines and technique.

To learn for yourself how the car behaves, do a skidpad test: drive it around a circle, and try to increase your speed without changing the radius of the circle, until you start to lose traction in either the front, rear or both at once. For a bit more 'real-world realism,' try this for different sized circles, then add in some random stomps on the brakes and full throttle so you can see how the car behaves under those circumstances.

Don't do this any place except a large clear expanse of asphalt, far far away from innocent bystanders.
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Old 08-23-04, 01:44 PM
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Thanks Scotty. I've done the skid pad test. As I increase speed, I have to continually increase steering input in order to keep a contant radius. At some point, when the steering wheel is turned quite alot, any decrease in throttle allows the front tires to bite and I oversteer (this usually flattens a few cones!).

Does this sound like my rear tires are too wide?
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Old 08-23-04, 07:10 PM
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Originally Posted by LAracer
Thanks Scotty. I've done the skid pad test. As I increase speed, I have to continually increase steering input in order to keep a contant radius. At some point, when the steering wheel is turned quite alot, any decrease in throttle allows the front tires to bite and I oversteer (this usually flattens a few cones!).

Does this sound like my rear tires are too wide?
When you say increase input, it sounds like you mean the front tires are not maintaining grip, and you are countersteering to deal with this. Notice I said the fronts are not maintaining grip. It is true that you can decrease the amount of grip the rear has to get to neutral, but I would think you would want to work towards more grip not less. What your talking about is what I spoke of in my first response, which is the max slip angle for the tires you are running. Assuming everything else was maximized, I would be working on producing a bit of throttle induced oversteer to counteract the slight understeer. This is not neccesarily the slow way around, done right, you will get a lot faster. Again, without an experienced driver running the car, it is hard to say, it could still be driver input that is the weaker link. Try going in a little slower, and getting on the throttle a little sooner. this should do two things, first, it will decrease understeer in the first part of the turn, second, it will allow you to apply more throttle mid turn forward, giving you better exit speed, and counteracting the understeer with a little left foot. Watch some JGTC FDs, they are not set up nuetral by any stretch. Carl
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Old 08-24-04, 01:59 PM
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Originally Posted by Carl Byck
When you say increase input, it sounds like you mean the front tires are not maintaining grip, and you are countersteering to deal with this. Notice I said the fronts are not maintaining grip. It is true that you can decrease the amount of grip the rear has to get to neutral, but I would think you would want to work towards more grip not less. What your talking about is what I spoke of in my first response, which is the max slip angle for the tires you are running. Assuming everything else was maximized, I would be working on producing a bit of throttle induced oversteer to counteract the slight understeer. This is not neccesarily the slow way around, done right, you will get a lot faster. Again, without an experienced driver running the car, it is hard to say, it could still be driver input that is the weaker link. Try going in a little slower, and getting on the throttle a little sooner. this should do two things, first, it will decrease understeer in the first part of the turn, second, it will allow you to apply more throttle mid turn forward, giving you better exit speed, and counteracting the understeer with a little left foot. Watch some JGTC FDs, they are not set up nuetral by any stretch. Carl
I agree: more grip in the front would be better than removing traction in the rear, but I'll probably never match the grip of the rear 275's. In any case, I'll take your driving advice - brake sooner accelerate sooner.

Thanks
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Old 10-08-04, 01:53 PM
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Just thought I'd share something I learned at the last autox; and this will probably be very obvious to anyone who hangs out in this forum, but I'm sharing for the benefit of noobs like me. I started playing with the tire pressures a bit (I usually keep about 31psi in all 4 tires). This time I started with 30.5psi in front and 33psi in rear. The back end was sliding all over the place. Definitely no pushing. So I lowered the rear to 32psi. Much more balanced. When I took medium-speed turns I could slide the whole car around controllably (alot like the miata). I talked to another RX-7 owner who uses sticky tires (the same width all round) and he runs more pressure in the front than the rear. So it looks like I'm compensating for the increased width of the rear tires by running higher pressure in the rear than the front. Maybe I'll play with dropping the pressure all around - I just need to keep an eye on the sidewall.
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Old 10-08-04, 09:45 PM
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Since I autrcross and track BOTH an FD and a Miata I can tell you that your feeling that the Miata was "better balanced" is normal. The Miata is an incredible little roadster and the way it is balanced and the ease in which it can be driven at its limit is about as good as you can get in a street car. The FD will have higher limits of performance but it won't "feel" as balanced or tossable as a Miata. The FD is designed for all out performance and much heavier than a Miata. This makes it feel more cumbersome.. especially to a novice.

-John
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Old 10-08-04, 10:49 PM
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i had 225/45 and 275/40 and my FD handled great.... never autox'd it though. i was running kumho 712's up front and nitto 555R II road race tires
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Old 10-09-04, 08:00 AM
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Originally Posted by vosko
i had 225/45 and 275/40 and my FD handled great.... never autox'd it though.
Where did it handle great then? Did you go to a racetrack?
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Old 10-09-04, 12:56 PM
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Originally Posted by DamonB
Where did it handle great then? Did you go to a racetrack?
Damon so you should have even width for track & auto-x??
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Old 10-09-04, 04:24 PM
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Originally Posted by Fatman0203
Damon so you should have even width for track & auto-x??
Not at all. You can make either work if the car is set up for it but there's no need for the rears to get wider than the fronts until you're making big horsepower.
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Old 10-10-04, 02:20 AM
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Originally Posted by DamonB
Not at all. You can make either work if the car is set up for it but there's no need for the rears to get wider than the fronts until you're making big horsepower.
What do you consider BIG Hp for the lightness of the FD? Anything over 375? 400?
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Old 10-11-04, 04:51 PM
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Originally Posted by Fatman0203
What do you consider BIG Hp for the lightness of the FD? Anything over 375? 400?
Probably much less than that. I'm making 355 rwhp and I can easily overpower my 315mm Victoracers.
Throttle modulation is key here. More tire = more grip = less modulation required. Just be sure to adjust the front to be consistant with the rears to keep the car balanced.

Chances are pretty good that the car was well balanced with the stock(ish) configuration. Probably a little biased towards understeer because people liek their road cars like that. Increasing the rears will give you more grip, but you'll need to change either the front widths too or otherwise adjust the suspension to dial out the oversteer you're going to get with the wider rear tires.
Just remember that tires and suspension configuration are heavily dependent. One affects the other and vice versa.
And don't forget that driving style plays a huge part in it too. Most of the time heavy understeer is a result of too much speed (ie, driver brain fade) than the overall car handling. Likewise with too much understeer (ie, you mashed to go pedal a wee bit too firmly).
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Old 10-15-04, 04:18 PM
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Originally Posted by scotty305
To learn for yourself how the car behaves, do a skidpad test: drive it around a circle, and try to increase your speed without changing the radius of the circle, until you start to lose traction in either the front, rear or both at once. For a bit more 'real-world realism,' try this for different sized circles, then add in some random stomps on the brakes and full throttle so you can see how the car behaves under those circumstances.
oh MAN is that fun and it sure teaches you where the car wants to go with any drastic input. definitely not the technique to try on off-ramps where you could see a curb or tree up close in a split second.
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