rec.autos.simulators

by **ttjjekk11923** » Fri, 19 Mar 2004 13:38:44

Got a question regarding the topic at hand.

I was watching the TV show on Speed TV "Tuner Transformation". After
they fixed up the Mazda Rally/Ice car, they took it for a spin. The
driver said it was understeering so he took some tire pressure out of
the rears.

My thoughts .. If the car is in an understeer condition, wouldn't you
add tire pressure to the rears? This would stiffen the back end and
would attract more weight transfer to the rear of the car when the car
is in motion and thus the understeer would be reduced?

by **Kevin Kin** » Fri, 19 Mar 2004 13:49:12

if you drop both rears you soften up the back and increase the weight
transfer speed to the front, and you also give the rears more grip to help
it turn.

by **fredrickso** » Fri, 19 Mar 2004 14:10:32

yes

by **Tex** » Fri, 19 Mar 2004 22:55:48

The guys on that show are idiots; I wouldn't believe a word they say about
anything. As soon as I heard one of them say that 'drilled brake rotors
make you stop better,' that was it. BTW-that line was on their first show,
not the one with the rally/ice car.

by **Haqsa** » Sat, 20 Mar 2004 07:33:24

Understeer = (front slip angle - rear slip angle). That's an old simplified
definition, but good enough for this explanation. Reducing the rear tire
pressures makes them deform more in a corner, which increases the amount of
slip angle for a given force. More slip angle in front means understeer,
more slip angle in the rear means oversteer. So reducing rear tire pressure
will reduce understeer or add oversteer.

by **B Forbe** » Sun, 21 Mar 2004 01:47:50

Wrong.

by **Larr** » Sun, 21 Mar 2004 05:29:33

In a rear wheel drive car, you are correct. It would be an odd change.

But the rules are different with FWD cars, not the least of which is I can't
stand to drive the damned things.

-Larry

by **Haqsa** » Sun, 21 Mar 2004 09:02:27

Prove it.

by **Larr** » Sun, 21 Mar 2004 13:24:14

MOM! He's looking at me!!!

:)

-Larry

> Prove it.

> > Wrong.

by **Haqsa** » Sun, 21 Mar 2004 14:06:10

Indeed. A one word denial is a sure sign of a 13 year old. I shouldn't
even have answered, but too late now.

by **Jason Moy** » Sun, 21 Mar 2004 17:57:53

Doesn't decreasing tire temperatures increase the amount of grip being
provided by them, up to the point where they're no longer making
efficient contact with the road surface?

If that's the case, I'd expect that softening the rear tires would add
more grip to the rear, which would cause understeer.

Likewise, if tires act like really stiff springs, I'd expect the same
behavior to occur.

Not that I'm arguing you're wrong, but it's certainly the opposite of
how I've always learned to deal with tire pressures.

Jason

by **Jason Moy** » Sun, 21 Mar 2004 18:03:14

That makes no sense.

If you add grip to the rear, you're tightening the car. If you add
grip to the front you're loosening it. If lowering the tire temps at
the rear adds grip at that end, then you'd be creating more understeer
(or removing oversteer).

Jason

by **B Forbe** » Mon, 22 Mar 2004 04:24:05

Any sim I have ever used gives added oversteer when adding rear tyre
pressure. It's fundamentally the same as stiffening shocks/springs. Prove it
for yourself!

> Prove it.

> > Wrong.

by **Haqsa** » Mon, 22 Mar 2004 05:18:54

What I said has been known and documented for many years. Here are a few
links from a google search. Read them for yourself, they all say the same
thing I said.

http://rogerkrausracing.com/overundr.html
http://mgaguru.com/mgtech/tires/sp104.htm
http://platz.com/pca/germany/blatt/Blatt%208.htm
http://thundervalleyracing.com/features/tech/wolfsteer.html
http://www.tirerack.com/tires/tiretech/competition/racepres.htm

If you think you are experiencing the opposite in a sim then perhaps you
have your definitions of oversteer and understeer mixed up, or perhaps there
is something in your driving technique which is causing the unusual
behavior.

by **Haqsa** » Mon, 22 Mar 2004 05:31:14

It's not directly related to the tire temperatures. It does seem counter to
common sense but it's well documented - lowering tire pressure increases
grip but also increases slip angle. I have experienced this in real cars
and it can be duplicated in any sim. Grip and slip are not directly
related. Grip is mostly a function of the contact patch, slip is mostly a
function of sidewall deflection. Grip is how hard you can push on the tire,
slip is how much it deflects under load. Increasing tire pressure reduces
the deflection of the sidewall, which reduces slip. It usually reduces grip
also, but it depends on where you started from.

I think part of the confusion might be that understeer/oversteer is often
assumed by people to have to do with which end of the car loses grip first
as you increase cornering force. That is incorrect. The SAE uses a
definition that I could not possibly recite from memory but has to do with
the rate of change of turn radius vs. steering angle. The easiest way to
understand it is to go back to the simplified bicycle handling model that is
used to teach vehicle dynamics concepts. The understeer number is defined
as front slip angle - rear slip angle. When it is positive (front slipping
more than rear) you have understeer. When it is negative (rear slipping
more than front) you have oversteer. In practical terms what this means is
that as speed is increased an understeering vehicle will require more
steering angle for the same radius turn, whereas an oversteering vehicle
will require less. The SAE definition states that in more formal
mathematical terms, but that is essentially what it means.