On Thursday, August 20, 2020 at 8:55:40 AM UTC-4, Xeno wrote:
> On 20/8/20 9:50 pm, Xeno wrote:
> > On 20/8/20 7:24 pm, tcom 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.
> >
> A follow up to my point above. Have a look at this link.
>
> http://www.tyre-pressures.com/bycar/ford/escort/1983
>
> FRONT TYRE PRESSURE REAR TYRE PRESSURE
> 23 PSI / 1.6 BAR 29 PSI / 2 BAR
>
> The above vehicle is a front engine front wheel drive car.
>
> From what is the norm, you would expect to see the reverse of the
> above, the higher tyre pressure at the front to compensate for the extra
> weight of the power unit. So what is the *manufacturer's aim* with that
> variance from the norm? It's simple, the car manufacturer has found
> themselves with a car that has a tendency to *oversteer*. Understeer is
> safer for the average driver so they have *increased* the understeer by
> lowering the tyre pressures the front which, in turn, increases the slip
> angles there. They have maintained a high pressure at the rear keeping
> the slip angles there the same as before. By doing this they have
> created an imbalance of slip angles favouring the front wheels. With the
> higher slip angles at the front, the Escort will, for want of a better
> term, run wider at the front.
>
> It's quite clear that the chassis engineer couldn't get what he wanted.
> Who knows why, maybe the bean counters dictated a smaller spend so
> compromises were made. To me though, the solution used above is little
> better than a kludge. What is does show is that you should follow the
> manufacturers specified tyre pressures, especially if you don't
> understand the outcome if you go your own way. This is the reason
> manufacturers put a tyre placard on the car.
> --
>
> Xeno
>
>
> Nothing astonishes Noddy so much as common sense and plain dealing.
> (with apologies to Ralph Waldo Emerson)
______
Th way you and that article explained it makes more sense.

And by the way, another Ford I used to own - a 1996 Contour(That's Mondeo elsewhere in the world) also specified a lower front axle pressure than rear. Probably for the same reasons. F: 31, R: 34 to be exact. After the first year of keeping my tires at the maximum cold pressure on the tires(35psi I believe - this was the late 1990s), dropping them down to what was on that Contour sticker proved a revelation when I drove it afterwards! I still regard it as one of the best driving and handling cars I ever owned.


Since those years and that experience, I have never looked any place else for the correct cold tire pressure for anything I ever owned or drove, than on that little placard stuck to the car frame, fuel cap, or glove compt. lid.