E39 tire pressures
Charles Baker
I have three questions to address the forum regarding tire pressures for
an E39, 528i. The owner's manual shows different tire pressures for
European vs USA/CDN. Why?
The manual also recommends 33 psig front and 41 psig rear. Why? With
less than one percent difference in weight front to rear, what is the
advantage/reason?
Given all the above are there any responsible observations as to tire
wear vs performance for any of the above combinations? I can attest to
the fact that the car rides *much* nicer with lower pressure, but what
kind of tire milage or other factors is one sacrificing for comfort?
Chuck Baker
cebake...@wco.com
James M. Blaschak
European tire pressure specs assume the vehicle will be driven on the
autobahn. The tire needs to be a little stiffer at higher speeds to
reduce excessive heat generation. North American tire pressure specs
assume 70 (yawn) mph.
Tire pressure specs are determined by measuring the consistency of
temperature across the tread of all four tires after a hard stop. If
the tread is hotter in the center, the tire is over-inflated. If the
tread is hotter toward the sidewalls, the tire is under-inflated. In a
hard stop, the weight of the car is exerted more on the front tires than
the rear. The weight squeezes the front tires tire and increases the
momentary pressure inside the tire during the stop. That's why rear
tire pressures need to be higher.
Safety is the overriding factor for determining tire pressures. The
time when you need to have the maximum amount of rubber on the road is
during a hard stop.
Brett Shortland
You are only telling half the story. Cars not only have to stop and
go, they also have to turn. Lower pressures, whilst making the ride
smoother, also increase sidewall flex which decreases lateral
roadholding. The reason most manufacturers recommend lower pressures
on the front tyres than the rear tyres is to introduce understeer
characteristics to the handling. It has been determined that cars
which understeer are far easier for Joe Average to control than those
that oversteer.
As for tyre pressure specs themselves, most manufacturers, BMW
included, generally specify higher pressures for high speed travel and
also when the car is heavily loaded. As for different pressures for
Euro spec cars, I can only suggest that Americans probably tend to
prefer softer suspension and that the pressures are more for comfort
than any other reason.
Brett Shortland
-----
Mail to: sho...@omen.net.au
-----
"James M. Blaschak" <jbla...@scram.spam.ameritech.net> did say:
Charles Baker
Thanks to Messrs. Blaschak and Shortland for their reasoned responses, and
so I apologize for having left out an important piece of data: the factory
recommended pressures are 33 front, 41 rear for USA/CDN and 29 front, 33
rear for the rest of the world. I just assumed that everyone had an owner's
manual :-), and my respondents assumed that Europe with higher driving
speeds would have higher pressures, and that US drivers wanting comfort
would "require" lower pressures. Just the opposite is the case, and my
question still stands, "why?"
Thanks again,
Chuck Baker
Lew
I read somewhere that the seemingly high recommended tire pressures are for
EPA gas mileage purposes.
--
Lew
Delete "RemoveThis' from my address for an EMAIL reply
Charles Baker wrote in message <35798B5E...@wco.com>...
Big Kay
Lew schrieb in Nachricht <6lc7o4$j...@bgtnsc03.worldnet.att.net>...
Hi !
I had to find out, that low tire pressure can cause an accident.
How did I find out?
I ditched my first car, because i had to brake and a tire blew up.
Why?
The tire was on low pressure and heating up from the extensive work caused
by the lack of air.
Yes, it was nice and comfortable to ride on soft tires but it backfired on
me. It was too much to handle for the tires and i ditched my old Ford.
Kristian
Brett Shortland
That would certainly stand to reason.
Increasing the tyre pressures would result in less internal friction
from tyre squirm and less rolling resistance.
However, if the American cars I have driven are any guide, my original
comment abouttyre pressures still stands ;-)
Brett Shortland
-----
Mail to: sho...@omen.net.au
-----
"Big Kay" <k_h...@muenster.netsurf.de> did say:
Er...@my-dejanews.com
Really interesting question, and puzzling, too.
Here is my interpretation:
1) OEM tires are not necessarily the same for US and Europe. In particular,
there is likely an all-season OEM option in the US. Manufacturers could also
differ. 2) BMW likes to have a homogeneous pressure value for a given market,
regardless of the manufacturer. The weakest manufacturer generally governs
the overall choice. BMW tests all the tires they homologate. 3) Typically,
all-season tires (and US tires) put let emphasis on high speed endurance than
European ones, hence to pass the same test they would need more pressure. 4)
This is compounded by typical BMW rear axle geometry that imposes substantial
camber at max load. Camber is very, very aggressive to tire high speed
endurance 5) BMW used to (might not do anymore) adjust pressures by
substantial increments (2.0 bar, 2.5 bar, 3.0 bar, 3.5 bar), which could
explain the amount of the jump 6) IMHO, BMW would not play games with the
inflation pressure for the sake of improving a rolling resistance result at
the expense of comfort... For safety purposes, they will change pressures,
but not for that... 7) Likely consequences of higher rear pressures: more
center rib wear, harsher ride... Handling consequences depend on specific
tire (the relationship between cornering power and pressure is a curve that
increases till a certain pressure, entirely dependent on the tire design, and
decreases afterwards). 7) If you know for sure you have a European tire set,
you could reasonnably consider the European pressure recommendation.
In article <35761908...@wco.com>,
ceb...@wco.com wrote:
>
> I have three questions to address the forum regarding tire pressures for
> an E39, 528i. The owner's manual shows different tire pressures for
> European vs USA/CDN. Why?
> The manual also recommends 33 psig front and 41 psig rear. Why? With
> less than one percent difference in weight front to rear, what is the
> advantage/reason?
> Given all the above are there any responsible observations as to tire
> wear vs performance for any of the above combinations? I can attest to
> the fact that the car rides *much* nicer with lower pressure, but what
> kind of tire milage or other factors is one sacrificing for comfort?
>
> Chuck Baker
> cebake...@wco.com
>
>
-----== Posted via Deja News, The Leader in Internet Discussion ==-----
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P993140
>I read somewhere that the seemingly high recommended tire pressures are
>EPA gas mileage purposes.<BR>
><BR>
>--<BR>
>Lew<BR>
You are right!