Split/Different Front and Rear Cold Tire Pressures

Ya' got way too much time on your hands, dude ;-)

Feeling?? Engineers actually know something about this.

Try running a Corvair with same F/R pressure some time. Just don't try cornering!

Don't try cornering in a swing axle Corvair, period.

AMuzi:

re: Corvair

The Corvair had a Front/Rear weight difference that definitely warranted the big difference between recommended front and rear cold tire pressures.

The cars I'm talking about are somewhere in between that extreme, and the other: rear- and all-wheel drive sports sedans with almost no(less than 55/45) front-rear weight bias. The last time I checked, a typical BMW F/R weight split is like 51/49%. Yet recommended front/rear pressures differ by 5psi.

My Honda Accord, for example, is F/R: 54/46% axle weight split. Some 'econoboxes' venture toward 60/40, yet for the Accord and those cars, a single pressure figure, for all tires, is specified on the door frame placard.

And all those have different suspension designs, rates, tire sizes, front end geometry, body roll or lack thereof and so on.

You're isolating one very small aspect of 'auto handling' which is a very large area.

Varying the tyre pressure will vary the slip angles at which the tyres run. The *specification* of that variation, ie. different pressure F & R, will ensure the car runs understeer rather than oversteer. Manufacturers design cars to oversteer and, depending on the suspension design, tyre pressure variation is the way to ensure it.

Make that; "Manufacturers design cars to *understeer*".

Yep, way too complex an issue to be isolated to one specific aspect. You need to look at the car as an entity - and that would, of necessity, be from an engineering perspective.

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!

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!

Can BMW and Audi do different front/back pressures simoly because their models' axle weights are closer to 50/50?

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.

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.

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.

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.

______ 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.

And yes Xeno - I happen to be a big fan of understeer, or at least of steering with a little more 'heft' to it, compared to the video game feel steering of most cars built since 2015 or so.

Another source dor those Escort pressures:

My reference was to the UK Escorts. We never saw those here. Instead we had rebadged Mazdas to fill that market niche. The lower front tyre pressures, as far as I am aware, were on the small engined models. That may in part explain why they needed a lower pressure at the front - the reduced engine weight reduced front slip angles and madethemalittle too precise in the front axle. As always, it is the attached tyre placard that is the final arbiter on tyre pressures for a given vehicle.

I would not have known about the Escort tyre pressure variation had it not been for a reference in a text book I have had for many years and used in teaching; Car Suspension at Work: Theory & Practice of Steering, Handling & Roadholding. Jeffrey Daniels.

A brilliant book and one of the best at covering the nuances of, as the title states, steering, handling and roadholding.

Another almost as good is; Automobile Suspensions: Colin Campbell

Both date from the 80s but cover the topic very well without delving too heavily into the underlying engineering principles.

If you are interested in those aspects of cars, I suggest you get hold of those two texts. Likely they will no longer be available new but used texts will surely be available.

________

Shucks, of course!

You can tell if a car is made for Amerukens because the tire pressures specified are all duhh saim! lol

No wonder Yankees don't really know what it means to really DRIVE, vs just getting from point A to point B. Everything is dumbed down for U.S. use or consumption.

During the 1970s and '80s, many portable radios for U.S. markets had only AM/FM bands, whereas for most other world markets they also featured shortwave bands, weather, etc. Frustrating, for one who really understands those bands and lives in America!

On my 2010 Honda, do you think it's safe to try an understeer configuration for a week? IE F31, R33psi, and if I don't like it, just go back to factory 32/32?

For sure. Not a problem. The difference is that you are now *aware* of the variation and, hopefully, know what to expect from the changes that you make. That is the most critical thing.

Before you push things, just get familiar with the car with the *new balance* first.

____ Ohhhh, I'm not a pusher, lol! Except when I had to veer around that doe standing right on the exit ramp as I got off for work early the other day! Just standing rock still. Probably terrified. This Accord was built for suddent maneuvers like that. 50 series 17s.

I swerved to the right of her, with inches to spare between my left mirror and her butt, and my right mirror and the guardrail. Scared the crap out of me! As I know what a deer hit can do to body work.

___________ Xeno:

So, for three days of combined highway and local street traveling, I tried the following cold tire pressures, similar to what was specified on my 1996 Ford Contour(U.S. Mondeo):

Instead of the 32psi Rear and Front on the pillar sticker for my 2010 Honda, I decided to test drivability at 34 rear and

31 front, cold.

On local streets, it seemed okay, but on highways, the car began to get as squirrelly as it did with 33 in front and 31 rear! I had a hard time keeping it in a lane, and it seemed to want to drift left more than usual.

I guess this generation Accord really like the same cold tire pressures front and rear, whether it be 30psi for the LX,

32psi for the LX & LX-P, or 35 or 40psi for the Accord owner one block over..!