With regard to the 18" vs. 19" debate. I noticed on the European Mag shootout that the "best handling" car award went to a turbo running 19's. That was a surprise considering the feedback that you usually hear. I believe that it was Cargraphic's car, but I am not sure.

 

Hi John. The reason the front wheel is completely unloaded
in the theoretical perfect corner is that the rear inside is too.
The maximum cornering force you can get is when the centrifugal
force operating outward at the cg of the car, times the vertical
distance from the road-tire surface to the center of gravity is
equal to the transferred weight on the outside wheels, times the
horizontal distance from the center of the tire contact patch to
the center of gravity.
If only one wheel is lifting off, then you are absolutely correct
that the lifting end is overstiff, either oversprung, or over sway
-barred.
Joe
Joe

 

Code Red,

The Cargraphic car was running 18" Cargraphic forged stars wheels at the track with the Pilot Sport Cups. 19" was only for show, not go. They knew they couldn't compete with the 19".

 

The 19" wheels is one of many variable that went into the handling setup. It could very well be that the car would have handled even better with 18". Likewise, one can setup a Turbo with 18" race rubbers in such a way as to render it practically undriveable.

Stephen
'01 Turbo Ti Upsolute
1stephen1.com

 

Joe,

Not wanting to beat a dead horse here...but I am trying to learn...how can what you say about both the front and rear inside tires being unloaded in the perfect corner coexist with the fact (or at least so I have always read) that there is a net loss of traction when you transfer weight from the inside to the outside tire. Is that not the basic reason why softening the anti roll bar at one end of the car causes that end to gain traction?

 

Back to a simple starting place; what initial tire pressures would the experts recommend for a car with a PSS9 set up with factory 18s? The PSS9s helped a bunch on the on understeer exiting a corner but not so noticeable on entry into a corner. Great thread!

 

Hi John, no problem. In the theoretical maximum cornering,
there is enough traction available from the fully-loaded
outside tires to roll the car, hence the full liftoff of the insides.
In any circumstance with less traction, you would still be
carrying some weight on the inside wheels, and you would
want both insides to bear some, not just one.
Joe

 

re: back to the simple case, What tires are you running?
If you're running stock tires, start with the recommended
pressures, and play with changing one end by 3psi at a time
to see how it feels. If you were running different tires, the
pressure might differ significantly. I've seen the same 996tt
require 34psi front and 37psi rear, or 44psi front and 46psi rear,
depending solely on what brand of tire. (this is an extreme
case).
Joe

 

Joe, I see. Sounds like time to begin lowering the CG...or widening the car's track

 

Oh, yes! Widening the track and/or lowering the center of gravity
is a "Good Thing". Formula 1 cars have strict minimum weights
they must meet, but they often complete the car at hundreds of
pounds lighter than that, and then they tack on precisely the
minimum needed extra weight by bolting on a plate of solid
Tungsten to the chassis under the driver cocoon. They even alter
the position of this plate depending on the course. For shorter,
tighter courses they move it a little forward to get enough
static load on the fronts for them to stick in slow corners where
aerodynamic downforce isn't practical.
Watch the WRC races on speed channel. Look at the navigators
in the cars. They aren't all midgets (though it helps ) but for
center-of-gravity reasons their seats are purposely mounted so
low in the car that they can only see forward near enough to
keep track of where they are in their course notes. The driver
needs to see the apex of the current corner etc, so he gats to sit
high enough for good close-up vision, but the navigator is
practically bolted to the floorpan.
Joe

 

I agree it's all about manipulating the contact patch. I find when I go the the track, my tires 'crown' and I loose grip as they get hot and the pressure increases from normal street pressures. So my statement that grip will increase when pressure is reduced is coming from that perspective.

I want to buy a pyrometer to be able to compare outside/inside edges with the center of the contact patch to determine exactly what's happening. Anyone know of a good online source?

Mike

 

Tyson, I was just checking to if you are there.

 

Thanks Joe, I've got Stock Conti Sport II's. Thanks for the advice. How do these tires stack up? What is a practical procedure for testing if you're not at the track?

Craig

 

The best way to do it is to start by contacting the tire maker,
and getting their recommendation for pressures for the
track, given the weight and weight distribution of your car.
If you're lucky or persistent enough to get to talk to someone
in the sports/competition department of your tire vendor, they
may also have initial camber recommendations. From there,
you would be best to find a parking lot or other flat area where
you can chalk out a 100' circle, and drive around it steadily
as fast as you can, noting which end wants to give up first.
Once you've done it for a few minutes, pull over toward your
helpers with pyrometers, and turn your front wheels so the
outside front tire sticks out. The helper should take your outside
middle and inner temp as quickly as possible, with another
helper writing it down. Then you'll know what you might need
to do. or at least have the raw info to infer what tp do.
If you're in the SCCA or other car club that does autocrosses,
they sometimes have test-and-tune days with a skidpad and
ready helpers for tire temp taking etc.
Joe

 

Thanks. I'm going to search online for a pyrometer, any good sources out there?
Craig