Achieving Rear Rotation for Autocross - Mazda3 Forums : The #1 Mazda 3 Forum
 
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post #1 of 8 (permalink) Old 05-11-2012, 02:28 PM Thread Starter
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Achieving Rear Rotation for Autocross

OK, I've got almost a year under my belt with this car, and I've had some very good results so far (H Stock Street Tire).

I'm running (at this time) the 225/45/17 Hankook RS-3's, at about 50# pressure front and rear. I have the Corksport Rear Sway bar, usually set at middle.
I get fairly even front tire temps at that pressure. Tried lower in front, outer edge wore more, middle not so much.

Front alignment toe is +1/16" out, rear is 1/16" in. Front camber is all that pushing and shoving on the bolts allows, about - one degree. Rear is about - two degrees.

This is a 2008 2.3 S Hatchback. I run at about 1/2 tank of gas, with the spare out.

I've never been able to get the rear to rotate easily. I can do so with great attention and effort (breaking into the corners). I'd like it to begin rotation by
charging up to a corner, turning and lifting (initiating rotation) without the braking. I've made this happen in other FWD cars, but not this one.

I've read tales here about some experiencing "wild and sudden oversteer", but I can't even get close. Once in a test event I left the spare in, and did experience
noticeably more oversteer......but I don't want the weight in the car.

So, what's next? I'd think the Corksport rear bar would make it happen, but not yet. I'm trying for only the second time setting it at full stiff (#3) next Sunday.

I'm also thinking of trying to LOWER the rear pressure some.

Thoughts? Comments? Opinions?

Appreciate all perspectives and experiences.

Scott Meyers
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post #2 of 8 (permalink) Old 05-11-2012, 02:48 PM
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Re: Achieving Rear Rotation for Autocross

Optimal tire wear generally isn't optimal performance.

Personally, I'd drop the front pressure quite a bit, add more rear pressure and ditch the toe in on the rear.

In my experience, if you get the front grip right, the rear will follow.

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post #3 of 8 (permalink) Old 05-11-2012, 06:48 PM
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Re: Achieving Rear Rotation for Autocross

Steve is right, I think 50 lbs in the front is too much. Thinking back when I ran the SS tires, I was around 40-45lbs with no rollover on the sidewall. I think 45#'s up front will increase front grip and thus increase rotation slightly. With a camber challenged car you are going to get more wear on the outside shoulder if the pressure is right. Just the nature of the beast and you should be flipping your tires on the rims halfway through their life cycle if you are using a set exclusively for AX. Going to super high front pressures in order to try to achieve even wear and make up for the lack of camber is actually hurting grip, not helping.

The car is sensitive to rear toe adjustments. I run 1/16" toe out in the rear. Try that and I guarantee you'll see more rotation than now. With the rear bar and alignment my car rotates nicely and I simply adjust rear pressure depending on site, surface, and how much rotation I want.

Tire grip in relation to PSI is on a bell curve. There is the optimum psi for grip then on either side of that grip is progressively less. The key for stock fwd is to find the optimum grip up front (which is probably less than 50 psi in my experience) then dial in the desired amount of rotation with rear pressures. Optimum rear grip happens at a much lower PSI than the front since the tires don't have nearly as much weight on them and because they don't see the same lateral loads, they are simply along for the ride and don't do much work. From my experience rear grip is highest at around 30 psi...maybe even a little less. The closer I get to 30 psi, the more rear grip I have. So, in order to reduce grip I can either hike up the rear pressure north of 30 psi or go extremely low and set pressures south of 30. I personally don't like the idea of running 20-25 psi in my tires to reduce grip so I increase pressure to decrease rear grip.

My advice to you Scott, would be to drop the front pressure to 45 lbs and do 1/16" total toe out in the back. If you are taking it to an alignment shop, that is about -.07 on each side. My guess is that you'll have too much rotation with 50 lbs in the rear and then you can start dropping rear pressures and dialing in rotation from there.
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post #4 of 8 (permalink) Old 05-11-2012, 08:52 PM
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Re: Achieving Rear Rotation for Autocross

I've only done two events, though both were at the same site.

My 09 3i is bone-stock with GY Eagle GTs on all 4(400 treadwear rating).

At the first event I had a lot more rotation than I expected based upon everybody needing more on this forum. I was at less than 1/4 tank(I was at about 1/8 tank after the event was over), started with 40psi front and rear, increased the front to 45, then to 49psi... kept rear at 40 the whole time... I kept increasing the front to try and get even tire wear. I had what I think is pretty darn reasonable rotation the whole time. I ran in the 2nd heat at this event.

Then at the 2nd event(last weekend), I went straight to 49psi front and rear... thinking I would get more rotation.... but it was actually worse than at the first event. I didn't mess with pressures at all because I wanted to just focus on driving. The only significant difference was that I had about 3/8 tank full of gas this time. I ran in the 3rd heat at this event.

I seriously doubt that was the only reason, but I'm going to try harder to race at below 1/4 tank from now on.

I think running in the 3rd heat had some impact as well, because the course would be more cleared off, more rubber deposited on the ground for more grip, etc.

I will be doing an at-home alignment on mine as soon as we close on our house in a few weeks... I hope to have a car that rotates reasonably well at the next event.


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post #5 of 8 (permalink) Old 05-12-2012, 10:51 AM Thread Starter
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Re: Achieving Rear Rotation for Autocross

[quote author=Adam3s link=topic=209952.msg4228361#msg4228361 date=1336776482]
Steve is right, I think 50 lbs in the front is too much. Thinking back when I ran the SS tires, I was around 40-45lbs with no rollover on the sidewall. I think 45#'s up front will increase front grip and thus increase rotation slightly. With a camber challenged car you are going to get more wear on the outside shoulder if the pressure is right. Just the nature of the beast and you should be flipping your tires on the rims halfway through their life cycle if you are using a set exclusively for AX. Going to super high front pressures in order to try to achieve even wear and make up for the lack of camber is actually hurting grip, not helping...............


My advice to you Scott, would be to drop the front pressure to 45 lbs and do 1/16" total toe out in the back. If you are taking it to an alignment shop, that is about -.07 on each side. My guess is that you'll have too much rotation with 50 lbs in the rear and then you can start dropping rear pressures and dialing in rotation from there.
[/quote]
Given that this is my first car ever without front camber access, I still stare at the front outside edge of the tires and just shake my head. Stupid SCCA rules.......

How do you know when to turn the front tires? I don't want to find myself staring at holes forming in the outer edge. Right now, with the 1/16" front toe out, the tires
show stress on the outer edge, but the rest seems to be wearing fairly evenly.

I'll try the lower front pressure tomorrow, along with the #3 setting on the Corksport rear bar. I only tried the stiff setting once before, then went back to the #2 setting.
Later I'll redo the rear toe and try just a little toe out, as you suggest. No time to do it now.

Anyone find the stiff rear bar helps the front tire wear? Others claim it does.

Thanks for the ideas.

Scott Meyers
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post #6 of 8 (permalink) Old 05-12-2012, 02:41 PM
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Re: Achieving Rear Rotation for Autocross

[quote author=mazda3az link=topic=209952.msg4228474#msg4228474 date=1336834275]

Anyone find the stiff rear bar helps the front tire wear? Others claim it does.

Thanks for the ideas.
[/quote]

It makes sense in theory... if you stiffen up the car(either front or rear)... it will have more roll resistance... meaning it won't pitch as far over on the edges of the tires. The rear being stiffer will make it oversteering, front being stiffer would have a more profound effect but will induce understeer.

Technically, if you installed some 100,000lb roll resistance front and rear bars, you'll have nearly no tire edge-wear, but it will drive like it's on ice the second you hit a tiny imperfection during a turn. :P You'll probably go through end links pretty quickly too...


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post #7 of 8 (permalink) Old 05-13-2012, 03:33 PM
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Re: Achieving Rear Rotation for Autocross

Thats way too high of a tire pressure. 50psi isn't going to make the tires last longer, the car has no camber, just live with it.

Why are you running toe in? That's not going to help rotation.

I would lower the front tire pressure, stiffen up the rear sway bar, get some adjustable shocks, and toe out the rear.


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post #8 of 8 (permalink) Old 05-13-2012, 07:00 PM Thread Starter
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Re: Achieving Rear Rotation for Autocross

[quote author=justint5387 link=topic=209952.msg4228709#msg4228709 date=1336937599]
1.) Thats way too high of a tire pressure. 50psi isn't going to make the tires last longer, the car has no camber, just live with it.
2.) Why are you running toe in? That's not going to help rotation.
3.) I would lower the front tire pressure, stiffen up the rear sway bar, get some adjustable shocks, and toe out the rear.[/quote]
At that higher pressure I was trying to "engage" the middle of the tire more. I believe I mentioned that the tire temps looked very good at that front pressure. Today I tried a little lower pressure (46#) in front, a little more front toe out (1/2 turn above the daily 1/16" out), the rear bar at full stiff, and results seemed to be....good. Don't have all the numbers yet, but scored high against my "benchmark" competitors.

The rear has very small toe-in; I was shooting for zero but missed by a smidge. With that large Corksport rear bar, I was thinking I wouldn't need much if any toe-out in the rear, counting on the big bar to produce the results sought. Many articles stress using the least possible toe-out to achieve the desired effect (especially if driven on the street)

I have the Koni FSD's; good enough for Burns, good enough for me.

EDIT: Redid the rear alignment some; going out tomorrow (5/20) armed with 3/16" toe out in the rear, and the rear bar set on full still.
THAT should do something.......

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