Date: Thu, 28 Feb 2002 07:35:58 -0800
Reply-To: George.Becker@SCE.COM
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: George Becker <George.Becker@SCE.COM>
Subject: 15" Wheels/Tire Pressures
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What tire pressures are correct for 15" wheels?
I've been running 215/60/15's on my '87 Westy for a number of years, but
I've never been positive about the correct tire pressures. I've recently
lowered the pressures to be F29 R34 based on an assumed F45/R55 weight
distribution and some trust in the archive post below. According to the
formula, I could even lower the pressure some, but I'm skeptical.
>>> Posting number 72406, dated 22 Oct 1998 12:02:31
Date: Thu, 22 Oct 1998 12:02:31 -0700
Reply-To: YauMan Chan <YauMan@CCHEM.BERKELEY.EDU>
Sender: Vanagon Mailing List <vanagon@vanagon.com>
From: YauMan Chan <YauMan@CCHEM.BERKELEY.EDU>
Subject: Re: tire pressure - How to calculate
Comments: To: vanagon@VANAGON.COM, vanagon@VOLKSWAGEN.ORG
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The correct tire pressure has nothing to do with the diameter of the =
wheel.. It can be determined by the Width of the tire and the weight of =
the van (+load). Here is a sample calculation:
The typical 2WD Vanagon (non-camper) is about 4400 lb wet! The weight =
distribution of most rear engine vehicle is 45/55 (45% weight carried by =
front wheels, 55% by rear) I guess it's probably 50/50 for synchros.
Assume you carry a load of 1000 lb (7 passengers at ~ 150 lb each - =
1050lb) So the total weight carried by the tires is 5400 lb with 2430 lb =
in front and 2970 lb rear (45%/55%) So EACH front wheel carries 1215 lb =
(2430/2) and EACH rear wheel 1485 lb (2970/2) (see why reinforced side =
wall tires are recommended!!)
Now for the stock 207/xx-14 tires, the WIDTH of the tire is 205 mm which =
is about 8 inches (205/25.4) The typical tire make a "footprint" of about
=
5 inches long This means that the area of surface contact is about 40 sq =
inch (8x5) so the tire pressure for the front wheels is 1215/40 =3D 30 =
psi and for the read is 1485/40 =3D 37 psi (hey pretty close to vanagon's
=
manual of 32psi front and 36 psi rear - they must be using a different =
weight distribution ratio!) =20
So for your tires which are 225mm (8.9in) wide, your contact surface is =
close to 45 sq inch. So the front is 1215/45=3D27psi and rear is =
1485/45=3D33psi.
From this calculation you realised that the pressure formula is self =
correcting. If you put less pressure in the tire, it will flatten out and
=
increase the contact surface area...until the area is big enough to hold =
up the van. Thus, the more pressure the lest contact surface (more =
slipping but better gas mileage) and the less pressure the more road =
contact!) That's why big Catapillaf earth moving truck with 24" wide tire
=
requires only 6psi and skinny bicycle tires requires 100-90psi.
Yau-Man Chan
87 GL
George Becker
e-mail: George.Becker@sce.com
