Split/Different Front and Rear Cold Tire Pressures

On 20/8/20 7:24 pm, wrote:
> Xeno:
>
> My particular mid-size front-wheel drive specifies 32psi, front and rear.
> It already has 'light'(easy) steering due to its wide, 50-series low profile
> tires.
>
> So for a while, I took it upon myself to experiment with adding 2psi to
> the heavier axle(the engine) and removing 1-2psi from the lighter rear
> axle. So I had a set up of 34front, 31rear.
>
> While the back end seemed more planted, the steering actually became
> more 'dartier' than ever on the highway, and I found I had to make more
> corrections thn ever to stay in a lane!

Indeed. What you did by inflating the front tyres to a higher pressure
was to *reduce* the slip angle at the front. That makes the car behave
exactly as you described - "dartier". Actually, a more appropriate term
is *precise*. The problem is that you have upset the designed in
*balance*. You would find, if you pushed it harder, that you might get a
tad more oversteer than before. That you might find more than a little
unsettling.
The point to the manufacturers recommended tyre pressure setting is that
it was determined by a lot of *factory testing* and works in conjunction
with suspension and steering design. If you want to vary that, and don't
understand the nuances of steering and suspension design, vehicle
handling and the like, then be prepared to expect the unexpected.
>
> During the third week, I reset all tires back to 32psi cold, and the car
> calmed down, and actually drive as intended. I'm actually running
> 33psi cold all around now, because the weather here is starting to
> get cooler, and handling is still fine.
>
> So for my specific car, a 56/44 split weight front wheel driver, using the
> same front/rear pressure - as specified - actually works!

You will find that it is the design of the steering and suspension that
counters that seeming unbalance in handling. IOW, the 56/44 weight
difference front to rear has been compensated for in the steering and
suspension. In some other cars, a difference in tyre pressures F to R is
the manufacturers solution, especially in FWD cars.

My suggestion is that you do some study into slip angles, their
influence on handling and what influences slip angles. It is a very
complex thing to discuss and you need quite a deal of knowledge of
steering and suspension systems before you can move on to vehicle
handling. This understanding of slip angles however is vital to your
understanding of vehicle handling. The manufacturer of your vehicle has
designed the *basic* handling to be *safe* with a degree of understeer
built in because they have to assume not all drivers have the requisite
skill to operate a vehicle that handles differently.
>
> Can BMW and Audi do different front/back pressures simoly because
> their models' axle weights are closer to 50/50?
>
As I stated, you need to first look at the steering and suspension
design. That will tell you what the manufacturers goals are. Cars are
not designed from the *tyre* up.

--

Xeno


Nothing astonishes Noddy so much as common sense and plain dealing.
(with apologies to Ralph Waldo Emerson)