Date: Mon, 23 Oct 2006 11:12:10 -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: What tires for 91 Westy?
In-Reply-To: <20061022135155.85847.qmail@web60315.mail.yahoo.com>
Content-Type: text/plain; charset="us-ascii"
> Could you give us the VWOA or DOT links showing
> how you came up with the "99" load rating?
According to Title 49, Part 571,of the U.S.D.O.T regulations for Federal
Motor Vehicle Safety Standards, a suitable tire must meet both (not just
one) of the following criteria:
"S4.2.1.1 The vehicle maximum load on the tire shall not be greater than the
applicable maximum load rating as marked on the sidewall of the tire.
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.
AND...
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."
This means that the U.S. D.O.T. recommends a 6% safety margin for all tires
(the vehicle's normal load should be 94% of the tire's capacity), but if it
is a passenger-car (P-metric) tire, it should first be derated by 10% before
this safety margin is applied.
"Normal" load is hard to estimate; it would be arbitary unless you weighed
your vehicle with all typical contents, including 3 occupants, cargo, gear,
etc. (The D.O.T. defines the "normal load" as including 3 occupants, but
provides no specific guidelines on cargo.) However since a "normal" load
will almost certainly not exceed the GAWR (gross axle weight rating) of the
Vanagon, that number could be used. It might be argued that using the GAWR
as a "normal" load is a bit conservative (unless you overload your vehicle),
but when it comes to tire safety it's smart to be conservative, and the GAWR
is a known value (since it is quoted by VW).
Let's take the least conservative approach first; the bare minumum. Say we
ignore S4.2.2.3 (the recommended safety margin) entirely, and use a tire
that just barely meets S4.2.1.1 specs (handles the vehicle's GAWR) with no
safety margin at all. This would require a light-truck rated tire that is
rated at 1433 lbs (load index 93) for a 2WD Vanagon, or 1521 lbs (load index
95) for a Syncro. If it is a passenger-car (P-metric) rated tire, and
therefore must be derated by 10% first, this requires a capacity of 1577 lbs
(load index 97) for a 2WD and 1674 lbs (load index 99) for a Syncro.
If we do allow the 6% safety margin that the D.O.T. recommends (calculated
based on the GAWR), the requirement for light-truck tires is now 1525 lbs
(technically load index 96, but just a few lbs above 95) for a 2WD and 1618
lbs (load index 98). For passenger-car tires, it would require 1678 lbs
(load index 99) for a 2WD and 1780 lbs (load index 101) for a Syncro.
This is consistent with VW's requirements. On my '89 Westy, the sticker
advises a 97R Reinforced tire if using a 205-70R14 size. (It does not
specify a load index for the 185R14, but does specify a 6PR tire, meaning an
light-truck or reinforced tire.) I think it's noteworthy that VW chose to
specify "reinforced" in the spec for the 205-70R14 - "97R Reinforced," not
simply "97R." Thus they stop short of saying that _all_ tires with a 97
load index are acceptable, but do specify that sidewall-reinforced ones
rated at 97R are acceptable. This 97R rating matches the D.O.T.'s minimum
requirement without any safety margin (but again, only if a
sidewall-reinforced tire is used). Also, of course, the tire must handle
VW's recommended tire inflation. This alone disqualifies most non-reinforced
passenger-car tires for use on Vanagons, even if they would otherwise meet
the load requirement.
- Ron Salmon
The Bus Depot, Inc.
www.busdepot.com
(215) 234-VWVW
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