FWD: Tire PSI question

mrsoul55 · Post by **mrsoul55** » Tue Jun 07, 2005 1:42 pm

I need some clarification from you all on setting the tire pressures for a FWD car. I had Mark Pilson figure out "good" pressures to run in my Civic (mini-minivan) a while back and I'm happy with the pressures he told me to run. He said a high front and low rear pressure is ideal so I'm running 51f/22r. But I've read other opinions and they say low front and high rear for a fwd car. Apparently both opinions remove some of the understeer so why would one be better than the other?

Post by **thrdeye** » Tue Jun 07, 2005 1:54 pm

high - makes contact patch smaller, lessening grip, but (IMHO) makes car a bit unpredictable.

low - makes tire roll onto sidewall, lessening grip. I run lower pressures in the rear as well.

I think it is easier to take air away than to put air in and a tire that is lower pressure wil be more predictable than one that is higher pressure.

mrsoul55 · Post by **mrsoul55** » Tue Jun 07, 2005 2:00 pm

thrdeye wrote:high - makes contact patch smaller, lessening grip, but (IMHO) makes car a bit unpredictable.

low - makes tire roll onto sidewall, lessening grip. I run lower pressures in the rear as well.

I think it is easier to take air away than to put air in and a tire that is lower pressure wil be more predictable than one that is higher pressure.

So then why run high front and low rear if both cases lessen grip?

Post by **thrdeye** » Tue Jun 07, 2005 2:09 pm

maybe I didn't make that clear......the pressure you're running in the front is probably the ideal pressure for that tire. Were you to inflate it any more, it would lessen the contact patch of the tire, causing understeer

scottgib · Post by **scottgib** » Tue Jun 07, 2005 2:19 pm

51!!??

What kind and size of tire? What width rim? That is the largest differential I have heard of. I think Mark generally knows his stuff, but is wrong once every couple of years. I don't know if this is one of those or not.

If the max lateral grip in the rear is had at say 32, then to get the car to rotate you can obviously raise or lower the pressure. Which is best may depend on the suspension and type tire. Softer is going to give some spring action which may be good (unless you play pogo stick like Ted V once did in a jetta). The tire provides a spring action with no shock damping. With a very soft tire on some rough courses you could bounce just when you don't want to. A tire may give more warning about loosing all grip at different pressures. You want a little rotation, not snap spin.

With my new Hankook 215/45-16's on good clean pavement I am running 38-39 f and 33 r. I have an adjustable rear sway bar and get a good amount of rotation. These pressures roll only to the indicator mark on the tires when running on relatively good pavement. This is less pressure than I had to run on Azenis of the same size, but the Azenis were on 1/2" more narrow rims and without camber plates.

With past cars, I found going up in the rear (vs max grip) gave better predictabilty and control, rather than letting the tire roll over on the sidewall to loose some grip. I am not sure there is a right answer to this question, and it may depend on driving style. I'll only argue with Pilson about this when I am able to out drive him. Most likely I will die of old age before that happens.

mrsoul55 · Post by **mrsoul55** » Tue Jun 07, 2005 2:28 pm

scottgib wrote:51!!?? What kind and size of tire? What width rim? That is the largest differential I have heard of. I think Mark generally knows his stuff, but is wrong once every couple of years. I don't know if this is one of those or not.

If the max lateral grip in the rear is had at say 32, then to get the car to rotate you can obviously raise or lower the pressure. Which is best may depend on the suspension and type tire. Softer is going to give some spring action which may be good (unless you play pogo stick like Ted V once did in a jetta). The tire provides a spring action with no shock damping. With a very soft tire on some rough courses you could bounce just when you don't want to. A tire may give more warning about loosing all grip at different pressures. You want a little rotation, not snap spin.

With my new Hankook 215/45-16's on good clean pavement I am running 38-39 f and 33 r. I have an adjustable rear sway bar and get a good amount of rotation. These pressures roll only to the indicator mark on the tires when running on relatively good pavement. This is less pressure than I had to run on Azenis of the same size, but the Azenis were on 1/2" more narrow rims and without camber plates.

With past cars, I found going up in the rear (vs max grip) gave better predictabilty and control, rather than letting the tire roll over on the sidewall to loose some grip. I am not sure there is a right answer to this question, and it may depend on driving style. I'll only argue with Pilson about this when I am able to out drive him. Most likely I will die of old age before that happens.

The tires are Kumho V710's 205/50/15 on a 15X6 wheel. Stock (read very soft) suspension on my car.

steverife · Post by **steverife** » Tue Jun 07, 2005 2:30 pm

Scott, did you spray the 'kooks on Saturday? Did they get hot? If so, did they loose grip?

As far as pressures, I'll leave that those that know what they are talking about.

MARKP · Post by **MARKP** » Tue Jun 07, 2005 2:34 pm

The Honda mini-minivan also has a higher CG than a BMW-Mini and no camber. The GS Mini guys are running similarly high pressures.

scottgib · Post by **scottgib** » Tue Jun 07, 2005 2:38 pm

Yeah well, I am not sure I do know what I am talking about.

I did not spray the Hankooks and got my best time at their highest temp. Relative to others that I usually compare times to, I seemed fast. This could have been the course rather than the tires, but I think the tires are at least as good as the same size Azenis and are probably better (certainly than the older version). They seem to get stickier with higher temp and started acting a little like R compounds, just with much less contact patch than most R's due to the tread.

Post by **thrdeye** » Tue Jun 07, 2005 2:45 pm

Yeah, Scott stomped me on Saturday.

BTW, do I win any sort of award for having the most posts on teh board?

steverife · Post by **steverife** » Tue Jun 07, 2005 2:49 pm

scottgib wrote:Yeah well, I am not sure I do know what I am talking about.

I did not spray the Hankooks and got my best time at their highest temp. Relative to others that I usually compare times to, I seemed fast. This could have been the course rather than the tires, but I think the tires are at least as good as the same size Azenis and are probably better (certainly than the older version). They seem to get stickier with higher temp and started acting a little like R compounds, just with much less contact patch than most R's due to the tread.

You haven't ever ran the Kumho MX's have you?

scottgib · Post by **scottgib** » Tue Jun 07, 2005 3:38 pm

Have not run MX's. Haven't heard of any top drivers using them although they are popular. I have seen and been beaten (badly) by a WRX on shaved FK-451's. I think this tire (and the car that beat me) won STX nationally last year (maybe wet pavement).

Doug is our street tire expert. He would know.

Post by **thrdeye** » Tue Jun 07, 2005 3:58 pm

Tommy Hoppe won on the FK 451's in the wet last year.

scottgib · Post by **scottgib** » Tue Jun 07, 2005 4:10 pm

That was his car but not him that I had a problem with in Greenville SC. I think Tom sold it after nationals.

Don't tell Shawn!

Dentspeed · Post by **Dentspeed** » Tue Jun 07, 2005 5:03 pm

Scott, which Hankook model is that you are running. Do you have any rain experience with them, and if so how are they?