This is my First AWD vehicle. I've always run a lower front and a higher rear air presure, yet the EVO asks for the opposite. I am currently running equal front and rear at 32psi

Would like to hear what others are running. What would you suggest for a bit looser rear end. maybe a 32 front 36 rear?

Thanks

 

If you want more oversteer, you'd increase the front relative to the rear. Probably 34 front, 32 rear would be a good starting spot.

 

WSB

When I used to race Go-Karts we would add presure to the end we wanted to loosen.. Is this your understanding of all cars or is your suggestion to lower presure to the axel I want to loosen an AWD thing? Thanks

 

Oversteer occurs when the rear tires saturate their grip prior to the front tires saturating. To give a passenger car tire more lateral grip, you increase the pressure (and thus the cornering stiffness), within a small margin. This increase is pretty minimal, but it's enough to affect the over/understeer balance without significantly altering the longitudinal grip. I wouldn't go under 30 or over 36 (cold pressure) with the Evo's tires, so you'd work within that range to balance. There are numerous other ways to change the balance, of course, such as changing anti-sway bars, spring rates, etc; none of these are as easy to work with as tire pressure though. In the end, it's all the same goal: controlling when the tires saturate.

I'm not 100% certain of this, but in the situation of a go-kart, increasing the pressure is going to drastically decrease the contact patch, thus taking away grip. Putting in extremely high pressures, say 42 lbs (cold), would have a similar effect in a car (not to mention totally saturating the tires' longitudinal grip under moderate acceleration and braking).

 

Claudius: Since I'm Evo-less, I'll trust your judgement on the ideal tire pressures. But on every car I've ever seen, riding on radial tires, decreasing the rear and/or increasing the front tire pressures leads to more oversteer. The only thing I can see that would affect this would be the AWD system, but I don't see how that would alter the physics of the tire-road interaction.

 

With the exception of the BFG Comp TA R1s from a few years ago, every tire I have autocrossed with responded to air pressure changes such that more air in the front (or less in the rear) resulted in more oversteer (or less understeer). Which is why I find it odd that Cladius is saying it's the other way around.

Anyway, with the alignment as delivered by Mitsubishi, I am running the factory recommended pressures on the street. At my last autocross though, I used 42f/35r warm on my last run and it felt pretty good, and it was the first time I actually got the car to rotate at all. Prior to that, I was using the same front pressures, but higher rear pressures. Once I get the thing aligned and start running race tires though, I will have to start all over...

JW

 

Thanks I'll give the raised front lower rear presures a try.. Good luck with your autocrossing !!

 

I agree with WestSide, for street driving at least. The method I use came from the toyota performance handbook years ago. Start by setting the pressure based on sidewall wear, raise /lower pressure until you get even wear on the outer edge of the bead when cornering. Then change by a few psi either way to get the feel you want. Also the evo is 60 40 weight split so when it's all said and done, you should have a few psi more up front than in the rear.

I'm just talking out of my ars here but it seems that allowing the tire to role just a little more at the rear will increase the slip angle and decrease grip at turn in resulting in a little oversteer

I would think that over inflating the rear enough to reduce the contact patch will also work, though it will cause premature center wear.

I think I'm running 32F 30 rear cold. they were 34 front 34 rear from the dealer.

 

mad - Increasing the tire pressure increases sidewall stiffness, which in turn increases lateral grip. That's the whole basis of my post, which is why I still can't figure out Claudius' reasoning.

Also, you should never be in the range where you're significantly reducing contact patch with pressure, as this would take well over 40 lbs cold in most wide tires. The situation where you're going to lose contact is more likely from misalignment.

 

I agree with claudius on the tyre pressure changes. I think this has to do with how the car handles weight transfer. If you have more pressure in the rear, the weight transfer is less to you have less grip and vice versa

 

Weight transfer isn't a property of tire pressure, though. Sprung mass, CG height, primary roll stiffness (springs), auxillary roll stiffness (suspension geometry, anti-sway bars, etc) are what determines weight transfer.

 

Not an assumption. Within the range of properly inflated tires, this is how tires work. There are other factors (particularly how the suspension controls the tire's camber as it travels vertically), but what I said is true.

More of an estimation based on the tires I've seen utilized on sports cars (since that's what we're talking about here).

What I said is true of any tire on any car. If you can generate that much grip at 29 PSI, you can generate more at 32 PSI because you'll have a higher cornering (sidewall) stiffness. The converse of this is that, as you say, the approach to those (higher) limits will most likely be more drastic and a bit less predictable. Also, tires have diminishing gains with increased pressure, so you can't take this to extremes and expect 50 PSI to work handle well.

All this banter still leads me to say that if you want oversteer, the front is going to have a higher pressure than the rear, unless your suspension already has a significant amount of oversteer designed in.