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pjbgravely
pjbgravely Reader
8/5/15 6:48 p.m.

I don't know if this is illegal, crazy or genius. I put on wider tires this year 225 summer verses 205 all-seasons. After two events I continue to suffer from under-steer with a very planted rear-end. I was thinking that maybe running the wide tires in the front with the narrow ones in the back to try to get the rear-end to help steer.

I have tried running the rears at max pressure but they would only slide at the end of the day when it was the hottest. It is possible I just need to get crash bolts to get some real negative camber in the front end. Or I just could be a really bad driver.

Any thoughts would be appreciated, especially if the tire set up is illegal.

drdisque
drdisque Reader
8/5/15 7:13 p.m.

Virtually everyone who has tried a setup like that has found it to not be faster. The setup is not illegal as long as you keep the wheels the same.

What tires are you using?

What car is this?

Are you able to left foot brake/try trail braking?

Regardless of your answers, you're probably overdriving the entries to the corners or getting back on the gas too soon (or both).

Plowing the front end relentlessly means you've exceeded the total grip that the car can provide by a wide margin. Tire pressures and compounds and sizes aren't going to fix that. You're going to have to change your driving style a bit regardless of what you do.

pjbgravely
pjbgravely Reader
8/5/15 7:23 p.m.

I forgot, the narrows are on 15's and the wides are on 16's. I am probably over driving, by not braking enough before the sharpest corners. I trail brake and start to throttle as soon as I can, too soon and the car under steers worse.

My best times are when I can get the tail out. That way I can make up for lack of horsepower by not braking as much.

wvumtnbkr
wvumtnbkr SuperDork
8/5/15 7:32 p.m.

What kind of car? I would make the front end grippier. What are your tire temp profiles across the front tires?

pjbgravely
pjbgravely Reader
8/5/15 7:44 p.m.

I forgot the garage isn't in the side panel like other forums. It is a 2000 Mustang 3.8 standard. I don't have a temp gauge but I use chalk to adjust pressure to just above the roll over point. The tires are BF Goodrich Gforce Comp 2s. They are going to have to be my tires for a few years so I will need to learn how to use them. So I don't know how to fix the front end, except possibly by adding crash bolts for more negative camber next year.

Spoolpigeon
Spoolpigeon UberDork
8/5/15 7:54 p.m.

How wide are the front wheels? Some tires don't take well to being pinched onto a skinny wheel.

pjbgravely
pjbgravely Reader
8/5/15 8:10 p.m.

The wheels are 7" which is all that I am allowed, the 225 tires median rim size is 7". I am watching videos of other cars in the same race and it looks like my lack of power compared to the cars weight will never be competitive in my class. This car is shifting into 1st. Unfortunately I think my T5 is too slow to to try that. On the other hand first gear would get the tail out.

https://www.youtube.com/watch?v=L0qzSm_Cdws

Brotus7
Brotus7 HalfDork
8/5/15 8:12 p.m.

Also, what class are you running?

pjbgravely
pjbgravely Reader
8/5/15 8:13 p.m.

GS

wvumtnbkr
wvumtnbkr SuperDork
8/5/15 8:34 p.m.

Get a tire temp probe and learn how to use it. We have dropped 4 to 9 seconds perlap on our chumpcsr by adjusting camber and pressure to even out the temps.

It will tell you soooo much more than chalk.

It may not be a camber issue.

It may not be a pressure issue.

A temp probe will tell you most of what you need to know.

For the current situation, can you cut the front springs or stiffen the swaybar or get more camber? Anything should help if it is a strut front end (I am not sure).

drdisque
drdisque Reader
8/5/15 8:42 p.m.

Bigger rear sway bar (allowed under GS rules) and crash bolts and work on the nut behind the wheel.

pjbgravely
pjbgravely Reader
8/5/15 9:26 p.m.
drdisque wrote: Bigger rear sway bar (allowed under GS rules) and crash bolts and work on the nut behind the wheel.

Actually I have no rear sway bar, I am pretty sure I can add one. Watching the video I see that the Miata was slowing down a lot more than I did, of course it sped up a lot faster and beat me by 11 seconds.

Mr_Clutch42
Mr_Clutch42 SuperDork
8/5/15 10:45 p.m.

The mag has stated that tires handle the best when they're on the same width wheel. Your tires are too wide for your wheels, 225 width fit 8" wheels the best. If there are factory 8" Mustang wheels that you can purchase later and mount the tires on them, will help you get faster. However, like drdisque said, your driving is making the car plow relentlessly more than your tire setup. The fastest local hotshoe told me that, "your job as an autocrosser is to find the limits of the car, and take it there." You need to work with what you have, especially if you're going to keep your car in the stock class. That's why this sport is so difficult, it takes time to hone your skills and us guys often don't have the patience to do so.

pjbgravely
pjbgravely Reader
8/5/15 11:34 p.m.
Mr_Clutch42 wrote: The mag has stated that tires handle the best when they're on the same width wheel. Your tires are too wide for your wheels, 225 width fit 8" wheels the best. If there are factory 8" Mustang wheels that you can purchase later and mount the tires on them, will help you get faster.

Strange, I am looking at the spec sheet for the tires I have right now, and they can go on rims from 6" to 8", with the Measuring Rim Width being 7". The tires fit the rims well. Maybe you are looking at low profile tires.

The next event is a 2 day event. I have decided to run 4 Comp 2s the first day and maybe run 4 all seasons the next. I was hoping for rain but it looks like it will be dry.

For the nut behind the wheel, I am going to look into down shifting into the tight corners, slow down shifts won't matter because I will be braking. Then I can slow down more and still rotate the corner with throttle, I know my 1st to 2nd shifts can be fast.

Trackmouse
Trackmouse Reader
8/6/15 3:09 a.m.

Whoa, wait, what? No rear sway bar? The Miata out accelerated you?!?!? Moar camber? Son, how old are ya?

Get a rear bar. 3.8 rustang should still out accelerate the Miata. You lose in the CORNER. Not the straight. If autocross was about straight lines it would've been termed drag racing.

And did I not read that right? Your car is shifting into 1st? As in, it's an automatic?

foxtrapper
foxtrapper UltimaDork
8/6/15 5:27 a.m.
pjbgravely wrote: Or I just could be a really bad driver.

Never underestimate your abilities to drive poorly!

Before you take umbrage, I speak from personal experience. I was out there, tearing up the course with my CRX. Little old(er) lady ran it in her Saturn wagon, and was over 10 seconds faster than me. She made a lot less noise or smoke too.

Some cars just inherently understeer. Yours may well be one of them. Though usually it's a matter of the driver overcooking.

If you really want to play with tire pressures, consider going well outside the box of normal or sidewall rated pressures. You likely won't go any faster, but you can sure make the car feel different.

wbjones
wbjones MegaDork
8/6/15 7:13 a.m.
drdisque wrote: Virtually everyone who has tried a setup like that has found it to not be faster. The setup is not illegal as long as you keep the wheels the same. What tires are you using? What car is this? Are you able to left foot brake/try trail braking? Regardless of your answers, you're probably overdriving the entries to the corners or getting back on the gas too soon (or both). Plowing the front end relentlessly means you've exceeded the total grip that the car can provide by a wide margin. Tire pressures and compounds and sizes aren't going to fix that. You're going to have to change your driving style a bit regardless of what you do.

you don't even have to keep the wheels the same to stay legal … (if in Street Class) as long as they are no more then +/- 1" up or down from OEM … though you do have to keep the same width and keep the offset the same (or within a 1/4" of OEM)

lots of cars come stock with different size tires … though the rims will be the same …

and on some of the more open for mods classes, lots of top level drivers have run non-symetracal wheel and tire combinations with varying degrees of success

wbjones
wbjones MegaDork
8/6/15 7:16 a.m.
wvumtnbkr wrote: Get a tire temp probe and learn how to use it. We have dropped 4 to 9 seconds perlap on our chumpcsr by adjusting camber and pressure to even out the temps. It will tell you soooo much more than chalk. It may not be a camber issue. It may not be a pressure issue. A temp probe will tell you most of what you need to know. For the current situation, can you cut the front springs or stiffen the swaybar or get more camber? Anything should help if it is a strut front end (I am not sure).

not in GS … he can change one sway bar (front OR rear) … can't mess with the OEM springs … can change shocks, can put in OEM approved crash bolts to gain camber

wbjones
wbjones MegaDork
8/6/15 7:21 a.m.
pjbgravely wrote:
drdisque wrote: Bigger rear sway bar (allowed under GS rules) and crash bolts and work on the nut behind the wheel.

Actually I have no rear sway bar, I am pretty sure I can add one. Watching the video I see that the Miata was slowing down a lot more than I did, of course it sped up a lot faster and beat me by 11 seconds.

re. my earlier post .. you can change/add ONE bar in Street Class

wbjones
wbjones MegaDork
8/6/15 7:23 a.m.
drdisque wrote: Bigger rear sway bar (allowed under GS rules) and crash bolts and work on the nut behind the wheel.

rule of thumb has always been to stiffen the end that DOESN'T do the driving … does this hold true for RWD … it's almost holy script for FWD

jstein77
jstein77 SuperDork
8/6/15 7:27 a.m.

Though you can certainly try to improve your setup, changes in driving technique make more difference than all the legal mods put together. I highly recommend soliciting the help of an instructor or an experienced driver in your group. Have them ride along with you, look for potential areas of improvement, and then drive you through to demonstrate those techniques. That's what I do for novices at every event I attend, and it never fails to improve their times by leaps and bounds.

jstein77
jstein77 SuperDork
8/6/15 7:28 a.m.
wbjones wrote:
drdisque wrote: Bigger rear sway bar (allowed under GS rules) and crash bolts and work on the nut behind the wheel.

rule of thumb has always been to stiffen the end that DOESN'T do the driving … does this hold true for RWD … it's almost holy script for FWD

Stiffening the front of a car that already understeers will lead to more understeer.

wbjones
wbjones MegaDork
8/6/15 7:32 a.m.

there is lots of discussion and even disagreement about that on other forums and with National's level drivers .. but they are also in much higher prep level cars that have had LOTS of suspension mods … (ST, SP, SM, & CAM)

but that was why I was asking about "rule of thumb" for RWD street class cars … all I've ever driven since '76 are FWD … though I have Lemons raced a couple of E30's … I didn't have anything to do with their build

GameboyRMH
GameboyRMH MegaDork
8/6/15 8:16 a.m.

If you have near-zero negative front camber you could probably use some more, only cars with excellent suspension geometry generally do well with little to no front camber.

Adding a rear bar sounds like a good idea too. This will move the handling balance more toward oversteer.

If you're really desperate you could try running a little rear toe-out to help the rear end rotate. This will make the car very skittish though.

Try to stick to a square tire setup, staggered setups are a maintenance PITA.

wbjones
wbjones MegaDork
8/6/15 9:11 a.m.

isn't his car a Mustang ? unless one of the new ones, isn't it a straight axle in the rear … can you do much about toe with that type of rear axle ?

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