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Date:         Sat, 24 Oct 1998 00:13:18 -0400
Reply-To:     David Katsuki <dkatsuki@WORLD.STD.COM>
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         David Katsuki <dkatsuki@WORLD.STD.COM>
Subject:      Re: tire pressure - How to calculate
Comments: To: YauMan Chan <YauMan@CCHEM.BERKELEY.EDU>
Comments: cc: vanagon@VANAGON.COM
In-Reply-To:  <s62f1ede.029@collchem.cchem.berkeley.edu>
Content-Type: text/plain; charset="us-ascii"

Good analysis! I would just add that although the pressure formula is self-correcting, the effect of low pressure is that the outsides of the tread will wear faster, since the tread doesn't flex evenly, and since more weight is coupled in from the sidewalls. With overinflation, the center of the tread wears faster.

Dave 87 Wolfsburg 90 Westfalia 75 air compressor

At 12:02 PM 10/22/98 -0700, YauMan Chan wrote: >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 = 30 psi and for the read is 1485/40 = 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!) > >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=27psi and rear is 1485/45=33psi. > >>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 > >>>> David Marshall <vanagon@VOLKSWAGEN.ORG> 10/21 9:37 PM >>> >So, what is correct? On my Vanagon there is 16" wheels with 225/60 tires >on it with a max pressure of 44psi. The only thing that I can see relating >to pressure is info on 14" wheels... >


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