Date:         Thu, 27 Oct 2005 12:30:41 -0700
Reply-To:     John Bange <jbange@GMAIL.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         John Bange <jbange@GMAIL.COM>
Subject:      Re: Oh Boy! Tire Thread! (was RE: Michelin LTX/MS 215/75R15
              tires...)
In-Reply-To:  <6da579340510271229p5371ab23wdcfd74727dd19b20@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1

> Now, with the current thread lauding the 215/75R15 LTX, at a diameter of
> 27.7, I took another look at the HydroEdge spec sheet: Michelin offers the
>
> HydroEdge as a 215/65R16 rated 98T with a max load rating of 1653@44...at
> a
> diameter of only 26.8.
>
> Now...the $64 question: will this tire measure up?


Thing about tires is that it's not a simple yes/no thing. It's fairly easy
to (for example) say why a 1.5mm copper wire should always be fused at 16
amps or less because electrical stuff generally has maybe three significant
variables and behavior is highly predictable. Tires, though, are always a
bit of a wild card. Often designers have to choose a mid point between two
or more mutually exclusive ideals. For example, a solid hard rubber tire
will never have catastrophic deflation failure, but at the cost of pretty
much all handling and controlability above 10mph. A soft slick tire will
stick to the road like glue, but might go to pieces the first time you drive
over a 2x4 on the freeway. Tires ALWAYS embody some sort of compromise.
Given that, the best anyone can do is optimize for safety under expected
conditions. The guidelines offered by tire or car manufacturers or the
DOT/NHTSA are premised upon some chosen degree of safety under maximum rated
load and in the worst conditions. Common sense: it'd be astoundingly naive
to give ratings assuming all vehicles are loaded to 80% capacity or less and
never drive over uneven roads on a 110 degree day, right?
That said, you should be able to "fudge" the ratings somewhat based upon the
particular conditions you expect. Static weight is usually the variable of
primary concern, but various dynamic varibles can affect things too. It's
probably safe to say that an EMPTY van on tires that are only 20lbs below
recommended capacity is OK, just as it's probably safe to throw a 10lb box
of drywall screws into a van that's exactly at capacity already. But what
about tires rated 40lbs under spec? or 80lbs? Even if you're within weight,
at some point an under rated tire will (for example) probably not be hefty
enough take the sidewall flex, which is undoubtedly one of the
considerations the manufacturer had in choosing the recommended tire specs.
But then again, if you're on 16x7.5 rims, for which your "silver sticker"
has no tire recommendation, what specs do you use?
It's hard to say, really. Heck, four thrown truck treads held on with
packing tape is perfectly adequate if all you're doing is rolling forward
eight feet in the garage; and even OVER rated tires on stock 14" rims won't
hold if you go "drifting" down a mountain road at 70mph. All anyone has to
go on is the ratings. The ratings are obviously going to be conservative,
but there's no real way to know HOW conservative. Lucas at GoWesty is
confident that the HydroEdge tires he sells are well within the "engineering
hedge", and as a licensed professional mechanical engineer I'd have to value
his opinion to some dgree. The fact that there have been no reported
failures over many hundreds of thousands of miles certainly lends weight to
Lucas' assertion that abstract rating calculation is no substitute for real
world testing. The question you gotta ask yourself is whether you think this
leaves enough margin for your comfort. There's no way to know unless you
test the tires to the point of failure, and no one has done that. What it
comes down to is a judgement call. I have the GoWesty 16" wheels with
Hydroedge tires on a NON-Westy that never gets more load than a couple
hundred pounds plus 2 people and, with an automatic transmission and stock
2.1 wasserboxer, it rarely sees the high side of 65mph. I'm confident that
they'll be fine. If I had a Westy or carried more weight, I'm not sure I'd
be as comfortable.



--
John Bange
'90 Vanagon
"We'd tell a monkey how to peel a banana, if he said he was peeling one in a
Vanagon."