I just purchased some Yokohamas AVS es100's. 245/45/16 for the back & 225/50/16 for the front. Tires pressure is 40 psi (Max. 44 psi) all around. I'm experiencing an understeer effect. Any help is appreciated.

Dave

89 GT/ Black & Tam

 

Try running a stagger between your front and rear tires. I would recommend about 38 front and 34 rear as a starting point. To reduce your understeer you can either raise the front pressure (there by increasing the contact patch) or reduce the rear tire pressure a little more. 1-2 pounds from here is a lot.

When you raise the tire pressure a pound it has the same effect has increasing the spring rate by probably 10-15 pounds.

I have a pretty modified suspension, but found that my 85 liked a stagger as noted above.

The key is to make small changes at one end at a time and keep track of your cold pressures then for reference once you get the feel you like.

Let us know how you do !

Ken

 

Huh?? Increasing tire pressure has always made the contact patch smaller, at least here in the northern hemisphere.

 

So far i've considered going with my vehicles specs of 44 psi in the back & 36 psi in the front. However the tires have a maximum of 44 (Cold I assume) psi.

I reduced the fronts to 36 psi and noticed some reduction of the understeering, but still not grabbing the road to my satifaction. I will reduce the back to 38 psi first, then try the front @ 34 psi.


Dave

87 Carrera / Blk & Grey (sold)
89 GT/ Black & Tam
03 Mini / Blk & Red

 

You know, these aren't the stickiest tires. I use them primarily because they are a good tire for open road racing and like them overall, and you can't complain about the price. But these are not Pole Position S03s. I think 34 upfront is too low for even tire wear and you give up some steering sharpness. I would leave the back at 40 and front at 36. If you want to more neutral handling you can fiddle with rear alignment, rear droplinks or put wider rims on the front.

 

I have Kuhmo's on wife's S4. Stock sizes both front and rear. I run them at 32 front and 36 rear. Seems to work fine for me. If I were going to run the car on the track, I would start at 34 in front to increase turn in a bit and experiment from there. I have tried the 40 and 44 psi pressures and find the car skates all over the place and has no grip. Also, the ride becomes unbearable.

I have used these pressures for many years and with multiple sets of tires. I have never had any unusual wear problems except when alignment was screwed up. Just my personal experience.

 

Dave I have the ES100's on my '85. I settled on 40 psi front and 28 psi rear....but bear in mind that I have sizes 225/45-17 front and 275/40-17 rear. It handles what I would label as "neutral" in this configuration. I set it up like this to control power-on oversteer.

N!

 

Gents:

Here's a link that may be helpful to you. Raising the pressure would reduce the contact patch only if you get to a point of overinflation where you are running on the center of the tire. I actually chart my tires with a pyromter and have them pretty well optimized for street and for DE and autocross.
I'm running the Bridgestone RE750's on both the 928 and my 911. For Autocross and DE events I'm running the new Kumho V 710 . 295/40 17 on 11 inch rim in the back and 275/40 17 in the front on 9.5 inch rims. The Kumhos like 40 PSI in front and 34 rear. This is with Eibach springs and externally adjustable Koni Shocks, Ott rear drop links and an adjustable Weltmeister front sway bar.
I'm undefeated in the Improved class in our Atocross series that runs in Ohio and Western PA for the last 3 1/2 years.

http://www.rallylights.com/other/stuning.htm#tires

Regards,

Ken

 

Funny you should mention that ... I have Pole Position S03's on my S ... and .... I run 32 to 33 all round. Car is nimble, solid, sticky, balanced, just the way I like it.

The reason Porsche specify large pressures at the rear is that they made rear-engined cars. That's my opinion. My 911 specifies (and needs) 36 rear, and 29 front, because a LOT of weight is on the rear, but the front is light as a feather. So, in front you need lower pressure for more grip, and at the rear you need higher pressure to withstand the added weight.

A 928 is 50-50 balanced. You do not need higher pressure anywhere. Case-in-point: Some "racer-types" like MORE pressure up front, because of the amount of weight transfer that happens in their driving, forward under braking. They also like to have less pressure in the rear so as to keep the rears grippy under same conditions. YET:::: the factory say the rears need to be 44 and fronts 40?? So ... am I right?

Generally-speaking, higher pressure == less grip (within high and low points mind you), and lower pressure == more grip. In my 911, the tyres were 32 all round and I drove like that a long time. She used to understeer horribly. I woke up and got the fronts down to 29 and the rears to 36, and she is VERY neutral now.

My Euro 928S2 was overinflated on the left to 77 up front and 44 rear, but the right was underinflated a bit. Guess what handling she exhibited ...... right-turns would lose it badly. Left turns were fine.

 

Vlocity- how did you manage to fit a 275/40-17 on the front? Any rubbing? Any pix?

N!

 

Hey Norm:

I bought these wheels new in the box from Kim Crumb when he was getting ready to make a jump up to 18 inch wheels and tires. No major rubbing issues, I have an occasional scrub on the inside under full lock, but that only happens at really low speeds. i'll take some pics the next time I have them on the car. Heres a link with some photos at the bottom of my 928 at an autocross.

http://www.morpca.org/IRAC.html

I can actually get a 315 under the rear and have run those for DE but it is too much tire on the back for autocross and I just couldn't get the car to turn.

I know it is counter intuitive to believe that MORE pressure equals more grip....but it is a fact. The only time less is more is if you are rock climbing or running in sand. (Like I do with my Jeep) And we do that to promote flex in the sidewall.

With the stiff sidewalls we have on all of our performance tires, more pressure will increase the contact patch for that tire. I recall an article about the insane tire pressures that they ran on the rear of the Bonneville S-4. This was to increase grip because of wheel slippage they were having on the salt at 160 MPH !

Our cars are not 50-50 weight balanced....especially once they have been put on diet and much of the weight comes off the rear of the car in comparison to the weight drop in the front.

Also keep in mind that the factory pressures in my case were based on 225/50/ 16 on a 7 inch rim front and rear. Once you change wheel size and tire size the factory specs are useless.

Regards,

Ken

 

Hmmm well, increasing tyre pressure will increase only the sidewall firmness, which can give you increased grip only in cornering. In a straight line, higher pressure == lower grip. It's different also for steering wheels versus nonsteering and driving versus trailing wheels.

But in general, higher pressure == smaller contact patch == less grip == more slide.

 

Ok Henirich.......believe what you want to believe.....
Here's a link to the results of one of last years autocross events where I had FTD.

http://www.morpca.org/Results&Lists/...04Results.html

Of interest you will note two other 928s which were 10 seconds slower than me on a 60 second course. One of those cars is an 84 Euro... gutted interior and on R compound tires.

As a visual aid your example makes sense, we've seen the rounded nature of a tire as we pump it up....with it off the car....but what happens once you put 750 pounds on it ? With load the tire deflects, the air pressure is used in conjunction with the sidewall stiffness to balance the amount of deflection. This is also why there are optimum rim width and tire combinations. You really want a wheel a little wider than the tire so that you end up with an inverted "triangle" so that the inside of the tire will not lift during cornering. We've seen the posts of whether you can put a 275 on a 7 inch rim? Well you probably can...but it will not optimize the cornering. So if I run 10 pounds of air pressure all the way around my car will be faster? No. Higher frictional losses, more scrubbing, less feel for turn in. In the same manner I am not suggesting that 45 PSI all the way around is better, but most of the examples I see, people have the wrong pressures on the wrong axel.

As another comment let me ask all of you.....what is the major component that is built into all production cars? Understeer. This also explains a lot of the reason why the recommended pressures may not be accurate.

This thread afterall was about reducing understeer.

By increasing the tire pressure you are forcing the inside edge of the tire to remain in contact with the road surface while you are turning. You are also stabilizing the sidewall to prevent rollover. The higher pressure will help reduce the deflection of the load under cornering.

Here's another link to the tire rack with another chart similar to the one I posted earlier.

http://www.tirerack.com/tires/tirete....jsp?techid=58

It gives an example for our front engine rear wheel drive cars as I have outlined before. Higher front pressure lower rear pressure for optimum performance. In a discussion with one of the Kuhmo Tire technicians last year he gave me a rule of thumb for there tires at 2 PSI per 100 pounds for the axel you were working with. So if you have 1800 pounds on the front of your car and 1600 on the rear that would equate to 36/32 for their competition tire. He told me to then raise the pressure until the tires "skate" indicating that you have now reached an overinflated point and lost your maximum contact patch.

The lone exception for raising pressures beyond the norm (as I have done by charting and testing with a pyrometer) is running in the rain where as the link indicated 6-10 PSI OVERINFLATED will reduce the contact patch.

My apologies to everyone. Yes I really mean it. LOL

Ken

 

Ken, no offense is intended but this is an important SAFETY point:

Your experiences on an autocross track cannot change the age-old laws of automobile physics.

Again, no offense, but go to google, type in the following: "contact patch inflation traction" and read the billion articles written by men greater than you or me. Or, read the several driving handbooks like:

Vic Elford: Porsche Performance Driving Handbook

 

Heinrich:

No offense taken. I certianly don't expect to change your mind and I'm pretty sure that you won't change mine. I just know through experience what has worked for me with my setup. YMMV I guess is the adage of the day.

Back to the original post however, I would challenge you to raise your front tire pressure and drop the rear and then disagree that it does not change the balance of the car more toward oversteer and away from understeer.

Have a good day, I gotta get back to work......

Ken