Different front & rear tire pressures

D

Dave Botsch

The subaru manual for the '05 Impreza Sport wagon says to keep the front
tires at 32 psi and the rear tires at 29 psi. I've always heard that
keeping the front and rear tires at different pressures is bad and even a
difference of 3 psi can greatly, greatly increase the liklihood of
hydroplaning and such.

Even of more interest is that all 4 tires were measured this morning at
30.5 psi. I haven't touched the tire pressure since the car was bought a
few months ago, so, this is how it came from the dealer.

Can anyone offer some insight?

Thanks!
 
Dave Botsch said:
The subaru manual for the '05 Impreza Sport wagon says to keep the front
tires at 32 psi and the rear tires at 29 psi. I've always heard that
keeping the front and rear tires at different pressures is bad and even a
difference of 3 psi can greatly, greatly increase the liklihood of
hydroplaning and such.

Even of more interest is that all 4 tires were measured this morning at
30.5 psi. I haven't touched the tire pressure since the car was bought a
few months ago, so, this is how it came from the dealer.

Can anyone offer some insight?

Thanks!

--
********************************
David William Botsch
dwb7 at cornell.edu
********************************

The STi recommends 36 in the front, and 31 in the rear. The Subaru tech who
works on it (and also races) told me that it is to balance the car properly
and reduce understeer. The camber angles are different on the front and
rear wheels to improve handling. In addition, most of the weight (the
engine) is in the front, so the increase in front tire pressure is intended
to reduce the spring rate and balance the car overall.

They told me that matched front and rear pressures is not bad on Subarus in
general, but the car will have a tendency to understeer. Many dealers want
that since understeer is much easier to drive out of than oversteer. If
there is too much pressure in the front tires, the car will oversteer
though.
 
Tires are supposed to sit on the road a certain way...too much air and the
center of the tread is weighted more so the center wears faster and there is
also less traction. Too little air and the edges of the tread wear plus
sidewalls squirm and the tires run hot. Since the weight on the front and
rear axles is different the air pressure has to be different as well.
My pickup exaggerates this. The engine, trans, and cab are all on the
front and an empty box is on the rear...I need to run 35 in the front and
only 28 in the rear to get good tire wear and handling. TG
 
Dave said:
The subaru manual for the '05 Impreza Sport wagon says to keep the front
tires at 32 psi and the rear tires at 29 psi. I've always heard that
keeping the front and rear tires at different pressures is bad and even a
difference of 3 psi can greatly, greatly increase the liklihood of
hydroplaning and such.

These are the same numbers as the tire sticker for my '04 WRX
(P205/55-R16 tires). Many front-engine cars come with similar
recommendations for more pressure in the front tires, as they
have more weight on them. I'd imagine the recommendations for
a car with a near 50/50 weight split (think mid-engine) might
be different.

Hydroplaning risk is a combination of tread depth, tread design,
tire pressure, tire width, and weight on the axle. I've even
heard that the risk of hydroplaning (for front-engine cars) is
reduced if tires with deep grooves are kept on the rear. However,
that conflicts with the usual desire to rotate tires such that
they will be replaced as a set of 4.
Even of more interest is that all 4 tires were measured this morning at
30.5 psi. I haven't touched the tire pressure since the car was bought a
few months ago, so, this is how it came from the dealer.

Strange things happen.
 
Dave said:
The subaru manual for the '05 Impreza Sport wagon says to keep the front
tires at 32 psi and the rear tires at 29 psi. I've always heard that
keeping the front and rear tires at different pressures is bad and even a
difference of 3 psi can greatly, greatly increase the liklihood of
hydroplaning and such.
Even of more interest is that all 4 tires were measured this morning at
30.5 psi. I haven't touched the tire pressure since the car was bought a
few months ago, so, this is how it came from the dealer.
Can anyone offer some insight?

My rule of thumb for years has been rwd, start with 2psi higher in the
back, fwd 4psi higher in the front,. awd, equal pressures all round; then
set pressures to match your driving style and how you want the car to
handle from there. Obviously don't exceed the tyre manuf recommended
pressures though. If you find a setting you like, accept any higher tyre
wear that results as a penalty for enjoying yourself! Years back we ran an
Austin 1800 with 42f, 26r and almost completely removed any understeer -
mind you, the resulting liftoff oversteer was rather entertaining,
especially on a wet road ....
For what it's worth, my 91 Liberty (Legacy) wgn ran 32f, 35r under most
conditions for 200000+kms and my 99 OB ltd runs 36 all round & is
reasonably neutral. Cheers
 

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