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Old 06-16-2009, 07:22 PM   #1
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understeer

Hi,
Could someone please help me out with my car's understeer. My current set up is the following:
2002 boxster s
HR springs with M030 factory setup
rear tires: 275/35/18 30 PSI
front tires: 245/40/18 25 PSI
new Sumitomo HTR Z III Tires x4
7mm HR spacers on all 4 sides.

Handles great, but when pushed, there is severe tire grinding understeer. The rear never lets loose.

Any ideas on a easy inexpensive fix for this? Also what causes understeer (in novice terms)

Thanks for the information,
James

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Old 06-16-2009, 07:52 PM   #2
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If you're looking to reduce understeer, you need to increase front end grip. This can be done by reducing front tire pressure, reduce the stiffness of the fromt sway bar or reduce the front spring rate. You could instead reduce rear traction by doing the opposite of what I mentioned above to the rear. 1st thing is to make sure your tires are in good shape and there's nothing loose or broken in your suspension.
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Old 06-16-2009, 08:10 PM   #3
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Try easing off the throttle when you're in a situation where you want the back end to break loose. Just a little weight transfer forward should be enough to get the rear end loose. I found that I was bulldozing the front tires instead of easing up and letting the back end rotate.

You might also try adding a couple of PSI to the rear tires...
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Old 06-16-2009, 08:29 PM   #4
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thanks for the information guys. I will try the tires pressure change, and see how that goes. I dont think that the stock front sway bar is adjustable so I am stuck with that as is.
James
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Old 06-17-2009, 11:08 AM   #5
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Understeer ("push" in NASCAReez) is corrected by balancing the car. The Boxster 29Front- 36 Rear contributes to the front bias which keeps the rears planted longer than the fronts and staves off oversteer. Like my old 911 had in spades.

In simplest terms at the limit an understeering car will go off the track nose first, and one that oversteers ("loose" in NASCAR) will go off tail first

To correct for understeer you want to INCREASE front tire pressure and/or decrease rear tire pressure. I run 30-35 on the street. And believe 32-35 is safe in an experienced drivers hands for 8-9/10ths canyon carving.

Some say you should go more negative in rear camber rather than adjust pressure down, but I think for a street car that is sometimes tracked that the tire wear would be intolerable for most.

James, I'd say your 25psi front pressure is not safe and is probably below the manufacturers minimum recommended pressure. You could drive the tire off the rim. I did not check Sumitomo's recs, but I'd go to 30F-35R immediately and work from there.

Your other specs look basically neutral to f-r balance.

Tire temp is a very good indicator of which tire or part of the tire is scrubbing the hardest. Hot fronts indicate understeer. Pyrometers are useful for measuring, but touch and chalking the sidewalls are good enough for casual tracking.
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Old 06-17-2009, 11:29 AM   #6
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Quote:
Originally Posted by 70Sixter
To correct for understeer you want to INCREASE front tire pressure and/or decrease rear tire pressure.
Technically, this isn't correct.

Assuming all pressures are within +/- 5 or so psi of the level that the factory recommends, increasing pressure reduces grip, increasing slip at that tire. If the poster is experiencing understeer, there is insufficient grip at the front axle. This calls for a decrease in pressure at the front or an increase in pressure at the rear.

Check out this web page, specifically, the table near the end:

http://en.wikipedia.org/wiki/Car_handling

Last edited by blue2000s; 06-17-2009 at 11:47 AM.
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Old 06-17-2009, 12:31 PM   #7
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Quote:
Originally Posted by blue2000s
Technically, this isn't correct.

Assuming all pressures are within +/- 5 or so psi of the level that the factory recommends, increasing pressure reduces grip, increasing slip at that tire. If the poster is experiencing understeer, there is insufficient grip at the front axle. This calls for a decrease in pressure at the front or an increase in pressure at the rear.

Check out this web page, specifically, the table near the end:

http://en.wikipedia.org/wiki/Car_handling
Both of you are correct and neither of you are correct. Here's the deal - for a given tire, car setup, track surface and temperature there is an ideal tire pressure for maximum grip. Raising or lowering the pressure away from that ideal will produce less grip. There is no easy answer like, "if the car understeers, reduce front tire pressure"

A driver must find an ideal pressure through experimentation. Once found both for the front and rear you can ADJUST the balance slightly if you find the car unsersteers more for a given surface on a given day. But you CANNOT cure handling imbalances with gross pressure adjustments.

I will agree that 25psi is far too low for at-the-limit cornering with any street tire.

Here is the order I would attack the problem:

1) If you are relatively new to this then get an experienced driver to drive the car at the limit. Ask them what they feel the overall balance is like. You may be surprised that they do not think the car pushes like a pig but is relatively neutral. If that's the case then fix the driver, not the car.

If the experienced driver confirms that the car unsdersteers a lot then:

2) Set all the pressures at 35psi. This is a good safe starting pressure

3) Get an alignment!!!! You need maximum front negative camber and maximum camber. Also try to reduce rear camber to -1.5. If the rules of your race series permit then lenghten the slots on the front top strut mounts which can allow you to get 2 degrees negative. This will really help at the expense of increased tire wear. For more agressive turn in go with toe out in the front. Also make sure rear toe is 0. (if you like to live dangerously think about 1/8" toe out in the rear - but watch out for snap oversteer!)

4) Even out tire widths - why run a wide rear tire and smaller front? Lots of track Boxster drivers use a balanced setup of 245s all around.

5) Install a GT3 front swaybar. While it goes against most of the general guides you see on the web, a big front bar will significantly reduce body roll. Since our cars have McPherson strut design suspensions, body roll has a drastic effect on camber (this is unlike most racecar suspensions which have much more friendly camber curves). By reducing body roll you increase front grip more than the increase front roll stiffness reduces it. An added benefit of a bigger front bar is it allows you to put down power better on corner exit which can be problematic without a LSD.
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Old 06-17-2009, 01:07 PM   #8
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Lots of information.. but I promise tire pressure will not correct this problem. I use these terms to help people out:

1. Course adjustment (adjusting the anti roll bars if adjustable, changing the camber or toe)
2. Fine adjustment (tire pressures and shock settings)

You need a course adjustment, and since you don't have an adjustable bar there are really only TWO things you can do.

Put the rear tires on the front (increases grip with larger tire contact patch and decrease rear grip with smaller contact patch)

Or disconnect the front anti roll bar

You need a course adjustment to "discover" your car. These recommendations by others are all "true" but you will drive yourself nuts trying everything you read here. Disconnect the front bar or swap the tires. Then prepare yourself for the change


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Old 06-17-2009, 01:10 PM   #9
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To add to what renzop stated, changing the tire pressure does two things:

1) Changes the tire spring rate, in which case they behave as blue200s mentioned , lower pressure= lower spring rate and more grip at that end.

2) Changes the tire profile & stiffness. Too low, the tires roll over and overheat the shoulders of the tire. Too high and they crown too much, reducing the contact patch and overheating the center of the tread. Some cars need high front pressures to prevent the tires from rolling over (probably not a Boxster problem)

The best way to maximize total grip would be to fit the widest tires possible, and use an adjustable sway bar or spring rates to adjust the balance. Of course your alignment and tire pressures have to be optimized. The only reason you would ever go with narrower tires would be that the rules require it, such as with spec Boxsters.

Oh yeah, as he also mentioned,driver technique can also induce under/oversteer

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Old 06-17-2009, 04:55 PM   #10
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Ok. I see. Sort of. Awful confusing. I will adjust tire pressures like suggested and go from there. the next step wil be a GT3 sway. I am getting the GT3 lower control arms anyway, so I wil be able to adjust camber more.
Thanks.
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Old 06-17-2009, 06:26 PM   #11
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A GT3 front bar will NOT help this situation and neither will tire pressures. The GT3 bar on full soft is not softer than your stock S front bar, so what benefit do you think will be gained by going to one? You need more front grip. That is the bottom line.

You can do any of one of these.

Decrease front spring rate
Decrease front anti roll bar rate
Increase rear spring rate
Increase rear anti roll bar rate

Run the car square (same size front and rear tires)

I make changes in 25lb increments with springs until I get the cars dialed with the front GT3 anti roll bar set in the middle and a 4 way Tarett set on second from full soft. You don't have the rear anti roll bar option or the easy option of changing springs out. So.. remove a front drop link and drive the car. This effectively removes spring rate from the front of the car and will make the nose turn in and the rear feel loose. This costs you NADA

Tire pressure changes only effect the car in a couple of LB's of spring rate. You need a big change (Course adjustment) then .. then fine tune with the tire pressures.

If you were close to me.. I'd show you in 15min how this is all done, with a single clover leaf highway on/off ramps


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Old 06-17-2009, 06:42 PM   #12
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JHandy,

I know what track you are running. Have you met Fedrick in the BoxsterS yet at your local Austin track? Have you seen him POLE VAULT ahead in lap times recently? Ask him who helped him dial in and setup his ill handling BoxsterS He had never owned a water cooled Porsche until this his current S. He owns a shop in San Antonio that specializes in early air cooled 911's


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Old 06-18-2009, 02:21 AM   #13
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JHandy,

I autocross. Those are low speed events vs HPDE which have much higher speeds. Things that work for me (like the big GT3 bar) might not work for you.

I suggest that you go with 35psi all around. And then the critical thing is to get someone else to drive the car and let them tell you whether the car understeers too much.

In high speed events understeer is the best friend a novice has. If you listen to everything you read here, you might make your car dangerously loose and end up leaving the track backwards when you run out of talent.

It's something that's easy to happen - a novice overdrives the car into corners and gets understeer because he is going too fast, he adjusts the car until it doesn't understeer in the slow corners and all of a sudden it wickedly snap spins when he lifts a tiny bit in a fast corner.

SO make your changes slowly, one at a time. Get opinions from experienced track guys. Get someone to drive your car.

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