tire tuning

[IheartSTI]

Me and my friend Morgan figured out how to get rid of the awd understeer quite easily. It was acually an accident that turned out good. Somehow he poped both of his rear tires so he got some cheep tires for it and sence they where not nearly as sticky it made the rear follow the front. The car handles much better with the cheep tires in the rear. There is plenty of other ways to get rid of the understeer but this might be a good idea for someone who does not want to spend a lot of money on tires when there just going to get runed when racing.

[MattR]

You'll kill a subaru by doing that, but on a 2wd car, its fine.

[AtomicLabMonkey]

Originally Posted by MattR

You'll kill a subaru by doing that, but on a 2wd car, its fine.

No it won't - what he was describing is no different than typical tuning with tire pressure at an auto-x. Higher pressure in the front, lower in the rear, so the fronts grip more and the rears roll over and grip less.

[IheartSTI]

why would it mess it up? there the same size, it just does not push nearly as bad. and if u want less understeer you put more pressue in the rear then the front.

[MattR]

Originally Posted by AtomicLabMonkey

No it won't - what he was describing is no different than typical tuning with tire pressure at an auto-x. Higher pressure in the front, lower in the rear, so the fronts grip more and the rears roll over and grip less.

The tire size needs to be nearly identical to that of the front, or the center differential will not be happy. The odds of a different make and model tire being the same are slim, unless you go out of your way to match the overall circum. to that of the front tires. I suppose it can be done, but you need to make sure the tire's actual size is correct.

[Kevin M]

Originally Posted by AtomicLabMonkey

No it won't - what he was describing is no different than typical tuning with tire pressure at an auto-x. Higher pressure in the front, lower in the rear, so the fronts grip more and the rears roll over and grip less.

Yes it will- possibly. Mismatching tires is likely to create a larger rolling diameter discrepancy, which WILL burn out the center diff relatively quickly. Also, the large difference in friction would create a lot more wheelspin in the rear than in the front, which is even worse.

I would highly advise against not keeping all 4 tires as close to identical as possible on a Subaru. There are other ways to take out understeer, but mostly you just have to drive it out. It is possible to tune some power oversteer in (Dean's car for example) but it will never be all that tail happy. You just have to not overwork the front tires, that's all there is to it.

[MattR]

Just be careful, tire "size" does not always match up from tire to tire...In that I mean, an Azenis Sport in a 225-45-17 is significantly larger in diameter than a Toyo Proxes fz4 in a 225-45 17. They are published as the same size, but not the same. The only way to know for sure is to pay attention to the overall diameter.

[IheartSTI]

it is....we made sure before they where put on. i was just saying there a harder compound so the rear is looser

[Kevin M]

Damn, that reply took me too long. Good job Matt.

[MattR]

Originally Posted by scoobanader

it is....we made sure before they where put on. i was just saying there a harder compound so the rear is looser

Sweet, that should be cool...Its probably a lot of fun to drive like that, with the tail hanging out. What brand cheap-o tire did you guys use?

[MattR]

Originally Posted by BAN SUVS

Damn, that reply took me too long. Good job Matt.

I don't reply much in the tech forum, but when I do...look out!

[Kevin M]

Originally Posted by scoobanader

it is....we made sure before they where put on. i was just saying there a harder compound so the rear is looser

What is the exact diameter of each different tire at the recommended pressure? Even .1-.2" can be cause for concern. The cheaper tires will also probably deform more readily at higher air pressures because they are not made to ever go higher than ~32-35 psi (not the same as approaching failure, but deflection will be greater).

[Kevin M]

Oh, by the way- going to all 4 cheapos makes it just as easy to get tail happy- ask those of us who have gone back to RE92s after S03s or Azenis Sports or MXs...

[MattR]

Basically, from day one when I got into Subaru tech...I've been told to stay away from any mis-matched tires, so I'll stick with that to be on the safe side, any chance of harming the diff is not one I want to take.



IBScottsaysAWDburnouts?

[IheartSTI]

dont you want more pressure in the back tires then the front. so that the rear kicks out and follows the front tires?

[Kevin M]

Originally Posted by scoobanader

dont you want more pressure in the back tires then the front. so that the rear kicks out and follows the front tires?

Most of us probably do that, although we all run identical tires front and rear of course. I run 38 front/41 rear on my S03s usually.

[sperry]

Originally Posted by scoobanader

dont you want more pressure in the back tires then the front. so that the rear kicks out and follows the front tires?

Tuning for under/oversteer via air pressure depends on the tire.

On normal street tires, let's use the RE-92s as an example, the biggest problem with grip at the limit comes from side-wall roll over. Basically, the tire deforms and you end up driving on the sides of the tires where there's not much grip. That's the sound you hear when the front tires are sqeeling and then they start to vibrate really bad. To increase overall grip on a "street" tire, you up the air pressure, because it helps to prevent roll-over.

On performance or race tires, let's take some Azeniz Sports as an example, the side wall of the tire is already very stiff. Adding air pressure to these tires actually reduces traction because it reduces the footprint of the tire on the ground.

So if you're running softer sidewalled street tires, you'd actually lower the rear pressures to get the car to rotate. Less traction in the rear = more oversteer. When I was racing on my RE-92's I'd run 44psi front 40psi rear, or there abouts. The car still had a ton of understeer, but this was about the best I could get it and still keep from destroying my sidewalls.

Now that I've got coilovers, swaybars and pseudo-race tires, My air pressures are primarily determined by tire temps. Idealy right after coming off course, you should have Inner, Middle and Outter tire temps (measured across the treadblock) that are equal. This mean that the entire treadblock is gripping as much as possilble. Using a combination of camber adjustment and air pressure, you can get pretty close. Then you use the adjustability of the suspension components to set under/oversteer. (I will also fine tune by changing rear air pressures a pound or two between runs.... more pressure = looser, less = tighter.)

[AtomicLabMonkey]

Originally Posted by MattR

The tire size needs to be nearly identical to that of the front, or the center differential will not be happy. The odds of a different make and model tire being the same are slim, unless you go out of your way to match the overall circum. to that of the front tires. I suppose it can be done, but you need to make sure the tire's actual size is correct.

Well, I said that assuming the tires would be the same dimensions.