Date:         Sat, 29 Oct 2005 16:30:28 -0400
Reply-To:     The Bus Depot <vanagon@BUSDEPOT.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         The Bus Depot <vanagon@BUSDEPOT.COM>
Subject:      Re: Oh Boy! Tire Thread! (was RE: Michelin LTX/MS 215/75R15
              tires...)
In-Reply-To:  <20051029191423.85530.qmail@web60319.mail.yahoo.com>
Content-Type: text/plain; charset="us-ascii"

> This is what I was able to find in Federal Regulations:
> http://www.gpoaccess.gov/cfr/index.html
>
> S4.2.2.1  Except as provided in S4.2.2.2, the sum of the maximum load
ratings
> of the tires fitted to an axle shall not be less than the GAWR of the axle
system
> as specified on the vehicle's certification label required by 49 CFR part
567. If the
> certification label shows more than one GAWR for the axle system, the sum
shall
> be not less than the GAWR corresponding to the size designation of the
tires
> fitted to the axle.
>
> S4.2.2.2  When passenger car (P-metric) tires are installed on an MPV,
truck, bus,
> or trailer, each tire's load rating is reduced by dividing it by 1.10
before determining,
> under S4.2.2.1, the sum of the maximum load ratings of the tires fitted to
an axle.
> 
> S4.2.2.3  (a) For vehicles equipped with P-metric tires, the vehicle
normal load on the 
> tire shall be no greater than the value of 94 percent of the derated load
rating at the
> vehicle manufacturer's recommended cold inflation pressure for that tire.
>
>(b) For vehicles equipped with LT tires, the vehicle normal load on the
tire shall be no
> greater than 94 percent of the load rating at the vehicle manufacturer's
recommended
> cold inflation pressure for that tire.
>
>
> Note that Paragraph S4.2.2.3 refers to 'vehicle normal load' not 'vehicle
maximum
> load' or GVWR.


Tim, I do see your point that the 6% "safety margin" mentioned in 2.2.3
appears to apply to the vehicle's "normal load" rather than GAWR - although
the de-rating if you use passenger car tires (rating divided by 1.10)
specified in 2.2.2 still applies to the GAWR.  The difference is that the
normal load does not include the maximum permissable amount of cargo. It is
the sum of the van's curb weight plus the weight of all "accessories"
(which, in the case of a camper, may be substantial) plus the weight of 3
occupants. But not any additional cargo.

I don't know of an "official" VW spec for this.  So let's err on the side of
the most lenient possible spec, and forget about the 94% safety margin
entirely. Then we end up with a more lenient requirement of about 1500 lbs
(the approximate GAWR of a Vanagon; slightly less for 2wd, slightly more for
a Syncro) for a light truck tire, or 1650 lbs for a passenger car tire
(since 1650 / 1.10 = 1500).  These numbers are arguably lower than VW felt
was safe, as VW themselves never used tires rated this low on any Vanagon,
Eurovan, or T5 (with 14", 15", or 16" tires).  But even using these more
lenient (perhaps even too lenient) requirements, a passenger car tire rated
at 1510 lbs (such as that on the GoWesty site) would still fall
significantly short. So while the numbers change slightly, the conclusion
remains the same.

- Ron Salmon
  The Bus Depot, Inc.
  www.busdepot.com
  (215)  234-VWVW

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