Re: Split/Different Front and Rear Cold Tire Pressures

[Wade Garrett]

On 8/18/20 12:26 PM, Chris K-Man wrote:
> Split Cold Tire Pressures: Front-to-Rear
>
> I understand the technical reasons for why some car makers specify different cold tire pressures for the front and rear axles of certain models. My question concerns those vehicles (except for the obvious: large SUVs and work vans) for which different pressures are specified, vs vehicles for which a single cold pressure is recommended all around(all four wheels):
>
> IE: Makes and models with less of a Front/Rear GAWR(Gross Axle Weight Rating ie: less than 55/45) - Audi, BMW, Mercedes, Subaru, etc., might specify up to a 5psi difference for the Front vs Rear axle pressures, where as makes with a significantly higher Front/Rear weight split(60/40) - Chevrolet, Ford, Honda, Hyundai, Toyota, etc., recommend a single pressure
> value for front and rear Axle - in particular front-wheel drive models.
>
>
> Typical late-model European sports sedan or wagon recommended cold tire pressures: F/R: 32/36psi
>
> Typical late-model domestic or import front-wheel drive sedan or wagon cold pressures: F/R: 30 to 36psi, all tires.
>
>
> My theory: The former are built for a more discriminating, enthusiast driving demographic whom will pay more attention to such things as different front/rear axle tire pressures, while the latter are built for the blue-collar masses, whose main priority is economical transportation in reasonably well equipped, reliably built vehicles, and live a busy lifestyle where it is easier to remember one PSI number - a compromise I'm sure - to set all of their tires to.
>
> What's your feeling behind this difference?
>
Ya' got way too much time on your hands, dude ;-)

--
Why is it that the people who want more government control over your
life are the same ones who want you to be disarmed?

[AMuzi]

On 8/18/2020 11:26 AM, Chris K-Man wrote:
> Split Cold Tire Pressures: Front-to-Rear
>
> I understand the technical reasons for why some car makers specify different cold tire pressures for the front and rear axles of certain models. My question concerns those vehicles (except for the obvious: large SUVs and work vans) for which different pressures are specified, vs vehicles for which a single cold pressure is recommended all around(all four wheels):
>
> IE: Makes and models with less of a Front/Rear GAWR(Gross Axle Weight Rating ie: less than 55/45) - Audi, BMW, Mercedes, Subaru, etc., might specify up to a 5psi difference for the Front vs Rear axle pressures, where as makes with a significantly higher Front/Rear weight split(60/40) - Chevrolet, Ford, Honda, Hyundai, Toyota, etc., recommend a single pressure
> value for front and rear Axle - in particular front-wheel drive models.
>
>
> Typical late-model European sports sedan or wagon recommended cold tire pressures: F/R: 32/36psi
>
> Typical late-model domestic or import front-wheel drive sedan or wagon cold pressures: F/R: 30 to 36psi, all tires.
>
>
> My theory: The former are built for a more discriminating, enthusiast driving demographic whom will pay more attention to such things as different front/rear axle tire pressures, while the latter are built for the blue-collar masses, whose main priority is economical transportation in reasonably well equipped, reliably built vehicles, and live a busy lifestyle where it is easier to remember one PSI number - a compromise I'm sure - to set all of their tires to.
>
> What's your feeling behind this difference?
>

Feeling?? Engineers actually know something about this.

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

--
Andrew Muzi
<www.yellowjersey.org/>
Open every day since 1 April, 1971

[Heron]

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

[thekma...@gmail.com]

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.

[AMuzi]

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.

[Xeno]

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.

--

Xeno


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

[Xeno]

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

[Xeno]

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.

[thekma...@gmail.com]

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?

[Xeno]

On 20/8/20 7:24 pm, thekma...@gmail.com 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]

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.

[Chris K-Man]

On Thursday, August 20, 2020 at 8:55:40 AM UTC-4, Xeno wrote:
> On 20/8/20 9:50 pm, Xeno wrote:

______
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.

[Chris K-Man]

On Thursday, August 20, 2020 at 8:55:40 AM UTC-4, Xeno wrote:

> On 20/8/20 9:50 pm, Xeno wrote:

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.

[Chris K-Man]

On Thursday, August 20, 2020 at 8:55:40 AM UTC-4, Xeno wrote:

> On 20/8/20 9:50 pm, Xeno wrote:

Another source dor those Escort pressures: https://tirepressure.com/1983-ford-escort-tire-pressure

[Xeno]

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.

[Chris K-Man]

On Thursday, August 20, 2020 at 11:29:36 PM UTC-4, Xeno wrote:
> On 21/8/20 10:52 am, Chris K-Man wrote: l

________

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?

[Xeno]

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.

[Chris K-Man]

____
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.

[Chris K-Man]

___________
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..!

[Xeno]

The suspension design and/or compensatory factors are an unknown. All
most mechanics have to go on are steering and suspension specs,
including tyre pressures, so it is very hard to second guess what was
behind the motivations of the engineers when they designed the
suspension of those cars. FWIW, the factory recommended tyre pressures
are what is considered safe. They have to make allowance for the
*average driver* and most of those don't really have a clue on vehicle
handling.

[Chris K-Man]

_________
Xeno:

Actually, this car does all the things a car does, quite well, with the pressures set according to the b-pillar tire and load placard(32psi all) or optionally, 1-2psi cold above it.

At least with the recent model Perellis I had put on it several months a go.

Honda Accords have had a decades-long reputation for driveability and handling superior to, at least, its chief competition, cars like Camry, Taurus, Altima, and later on, Sonata. Mine is from the last generation to feature full control arm front end, and although I wish I hade siezed upon Accord sooner, better late than never.

So after experimenting with front and back differentials - more pressure front, more pressure rear, I've concluded that for drivability, handling, and comfort, Honda chose their pressures wisely for this trim level of Accord. Going 1-2psi over 32(especially for winter months), is as much as I'd recommend.

[Xeno]

>>>>>>>> Nothing astonishes Noddy so much as common sense and plain dealing..

The manufacturers do a lot of *empirical* test track evaluation of
handling with a lot of test equipment attached to the car. It is during
this that they work out what the best (read: safest) tyre pressures are.
They do not rely on what *feels best*.

[Hank Rogers]

Is that how Ford arrived at the pressures for their older
SUV's/Vans ? I remember they had firestone tires, but were running
them damn near flat.

There was a big stink over it, but it's been a long time.

[Xeno]

I doubt it. My guess was they tested/evaluated with different type of
tyres and, at the last minute, couldn't get sufficient supply. So they
went with a less than ideal type of tyre and had to rejig the tyre
pressures to compensate for the different handling characteristics. I
bought a new late 90s Mazda run out model once and it had very
underwhelming tyres on it. At the recommended inflation spec, the
handling wasn't all that good. I experimented and found a better
balance. My guess, at least on that car, was that they stuck whatever
tyres they could get cheap in order to get rid of the last of the model.
The replacement model was a totally different car. The car I had
previously was identical except for the wheels/tyres and handled
brilliantly. I was disappointed with the late 90s replacement so went
with Toyota ever since. No hassles at all now.

[Chris K-Man]

_____________
Re: "Safest" vs "feels best"

I wasn't disputing you on that. It just happens that what is listed on my car's tire & load placard is as close to optimal as I believe this car can achieve. It was more squirrely and unpredictable during the weeks when I ran a front or rear inflation bias. It wasn't meant to be run with more pressure in the front or in the rear. BMW's models and Subarus were. I just had to find that out via conservative experimentation.

It runs fine with 32-34psi cold in all four tires, it feels like... a car! Albeit a grippy and responsive one, with 50-series 17"s at the EX trim level, and reasonably stable in a straight line, for a ten year old example.

I would not recommend what the Mibbies^^ do - running 40psi and up, toward the maximum cold pressure stamped on its tires. I've been there, with cars I drove many years ago, and would never return to such practice, nor understand why people insist on doing that.

^^Mibbie = (M)ore-(I)s-(B)etter person!