Contour Enthusiasts Group Archives Forums General Dyno and Drag For gods sake help me

Ummm.. Could you explain how lowering the tire pressure (within reason of course not talking about dropping it to 5psi which will shred your sidewalls) reduces the size of the contact patch?

It reduces wheel hop and allows the tire to adhear to the pavement better!

-Andy

When you drop the air pressure in your tires that means there is less pressure pressing the rubber to the pavement.

An underinflated tire will only press the outside edges to the pavement and lose the center contact patch. Therefore you will have less traction to get you moving.

Plus once moving the underinflated tires will drastically increase rolling resistance which will slow you down the rest of the track. Top end especially...

We are taking street tires here. Low profile at that!
Not drag slicks. They are designed differently and normally have much more sidewall.

So lowering your pressure will not garner better times.

Also lower pressure tires will not help with wheel hop unless you mean there is a far smaller threshold of traction (since they have so much less) and go right to wheel spin vs staying in the "wheel hop zone" longer.

NOS :rolleyes: (its all stock) what bottle! :p

Hmm, never heard that explanation before. Sounds reasonable though. However i have to say that the correct tirepressure will vary from brand to brand, car to car, and alignment / rideheight.

Personally i run 28 up front and 25 rear with 205/55/16 Poentenza RE-71's on E1 rims. The RE-71's seem to preffer less pressure than the good years that were on the rims when i got them. I arived at this by observing the wear patterns and reducing pressure until i was getting just the slightest bit of wear on the outer edge of the tread under hard cornering. and then fine tuned based on steering input. I'm sure the pressure's will change when i replace these with 215/50/16's.

I have seen plenty of overinflated tires and the associated wear patterns (bald in the center) but i have never seen worn on both outside edges. Could be because most people tend to overinflate.

Thanx for the info Demon, I'll have to keep that in mind next time i adjust the pressure.

Demon,
Tell me why it is such a common practice and why I've run both ways and have put down better numbers with lower tire pressure in back to back runs! BTW I pump up my rears to 37psi!

-Andy

That's quite odd. Myself and several others have the exact opposite results.
Rolling resistance does increase with lower tire pressure so you are losing performance as well as traction.

We are talking low profile radial tires! Not drag slick made to run at low psi levels. There would be no wrinkling of the sidewall either.

CB99SVT in the drag radial post says he runs at the stock 32psi and gets his best times. Those are on "drag radials" as well!

Read a few magazine, a few books, etc... They all will state exactly what I posted.

Bruce -

As for running at a low 25psi. That will really hurt performance, mileage, and likely tire life.

I'm for tuning the suspension the other way. Increase pressure until abnormal center wear would become apparent.
This way you keep the most contact patch on the road.
It stays to the road better (more air pressure).
The sidewalls are stiffer (Better handling/less rollover).
Less rolling resistance (better performance & mileage)
Much stabler & more predictable handling.
Better road & steering feel.

What demon says is true. Every time I go fourwheeling, I lower the pressure in my swampers.
On some steep climbs I will go back on foot to look at where I had traction problems. Almost every time there are two deep ridges on ether side of the tire track with the middle almost un disturbed, indicating that there is not enough pressure pushing in the middle of the tire to make a full contact patch. But, this is a little more extreme than car tires, about 5 psi, same concept though.

I was shocked to that I had to drop the pressues this low to get the "feel" i was looking for. In my turbo miata I ran 31/29 and 33/31 f/r with 205/50/15 RE-71's on pannasport fs15's and that car bairly weight 2500 lbs with me in it!. I'll try do some messin around with it this weekend and see where the pressures end up.

But, FWIW, i've put 6,000 miles on the re-71's at the above mentioned pressures and the rears were bairly worn, but VERY evenly (i checked them when i rotated the tires two weeks ago). The fronts went through about 70% of the tread (lack of traction with the stock shocks/springs) with the inside edge showing the most wear and the center and outside showing uniform wear.

And these are cold tire pressures by the way.

I understand your points Demon and they do make sense. I'll have to check it out again and see if something has changed or if I was doing something wrong. I was talkin with a friend, GTP Owner/Cyc3killer, and his point was that for a car that makes torque it is imperative to get a good launch and top end isn't a big concern, so the softer tires make the difference. On our cars HP is critical and the drag created at higher speeds probably would hurt more than help.

-Andy