>Measure by = wrapping a string around the outside of the tire...>
>Ahh... but don't = forget you have to do that with the tire ON the car...>
Thanks Bill and = Brent. Is this a Friday thing? Or maybe it's some new list member = initiation rite? :)
Seriously... = assuming the tires you buy will be of uniform dimensions if they are = the same brand, model and size, you can figure all this out without = driving on strings, etc. After all, unless we're doing speedo = calculations, it isn't the actual dimensions we care about - only the = differences - right? So your average, brand new 195/75R14 tire has a = diameter of 648 mm, a circumference of 2036 mm and revolves 790 times = per mile. If it doesn't, it'll at least be close enough that the = difference is irrelevant. Every mm of diameter +/- accounts for +/- = 3.1417(pi, you know) mm of circumference. Each +/- 2.58 mm of = circumference accounts for approx. +/- 1 revolution per mile. = "Approx." because as the circumference increases, the mm to = generate another revolution increases as well - and = vice-versa.
So it seems to me = that if my tires are approx. 648 mm in diameter, a difference in tread = depth (sampled from various points on each tire, of course) of 1 mm x 2 = (for each side of the tire) would give me tires with diameters of 648 = and 646 mm respectively. Which would yield circumferences of 2036 and = 2030 respectively. Which would mean that the tires were rotating 790 = and 793 times per mile respectively.
Now... if I had to = take into account tire load and its influence on revs per mile *so = would someone with brand new tires* - so I don't see the relevance in = considering it. I'm figuring - and I'll admit perhaps incorrectly - = that the difference in recommended tire pressure front to rear most = compensate for whatever effects load might have on the = tires.
So will a difference = of 3 revolutions per mile toast my VC? I sure don't know. Anyone who = does - please let me - and everyone else - know!
-----Original Message-----
From: Brent Christensen =
[SMTP:bpchristensen@MINDSPRING.COM]
Sent: Friday, April 16, 1999 1:49 PM
To: vanagon@GERRY.VANAGON.COM
Subject: =
Re: Syncro tire dia. spec. - =
tolerances?
Syncro tire dia. =
spec. - tolerances?Ahh... but don't forget you have to do
that with the tire =
ON the car - the weight of the car will change the
rolling =
circumference of the tire... (This is the way you have =
to
calibrate a bicycle =
odometer in order to be accurate) Then of course, you'd
want to make sure =
you used a string with very low elasticity, since you
could get an =
inaccurate reading my stretching it taught...
I think my wife =
would think I'm crazy if she saw me out in the driveway
laying down string =
under my tires and carefully driving over it, then
marking it, then =
backing up and measuring it.
Brent =
Christensen
'89 GL Syncro =
Westy
'91 Taurus SHO (For =
Sale)
'95 Cherokee =
Sport
----- Original =
Message -----
From: =
Davidson
To: =
vanagon@GERRY.VANAGON.COM
Sent: Friday, April =
16, 1999 9:43 AM
Subject: Re: Syncro =
tire dia. spec. - tolerances?
The measurement you =
want to compare is NOT the tread... it's the
circumference of =
the tire near the center (diameter is too hard to
measure).... =
Measure by wrapping a string around the outside of the tire,
mark the spot on =
the string where it meets itself.... then see if that's the
same spot for the =
other tires... or how far off it is....
Bill
90 Westy =
Syncro
-----Original =
Message-----
From: =
Fitz-Randolph, Douglas <dfrandolph@TALKAM.NET>
To: =
vanagon@GERRY.VANAGON.COM <vanagon@GERRY.VANAGON.COM>
Date: Friday, April =
16, 1999 9:31 AM
Subject: Syncro =
tire dia. spec. - tolerances?
Brent Christensen =
wrote: ...My understanding is that all tires for a Syncro
need to be within =
2.5 mm.
Is this the correct =
spec?...
I'd really curious =
if anybody knows this spec *for sure*.
The tires I've been =
running for about 800 mi. were purchased slightly used,
with 2 of them =
having about 1-2mm less tread than the other 2. In
retrospect, I =
probably shouldn't have bought them, but the difference seemed
so small at the =
time... If it's going to end up messing up my drivetrain
though, I'll =
replace them.
The manager of my =
local highly reputable tire shop said the difference -
hardly noticeable =
- wouldn't cause a problem - he seemed pretty
knowledgeable about =
various awd systems (Subaru, Volvo, etc.) - and he
certainly had an =
easy opportunity to sell me a new set of tires, didn't he?!
The local VW dealer =
would only say "They all have to be the same size and
treadwear" and =
didn't know anything about acceptable tolerances - not much
help.
I know all about =
the whole replacing all 4 (or 5) if one is bad thing, but
how close do they =
really have to be? The 1-2 mm difference only translates
into several =
rotations per mile - it seems as if turning while driving
normally would =
account for almost that much rotational difference on its
own. (Would you =
eventually ruin your VC if you drove the same direction
around a racetrack =
for miles and miles? Probably not, but I'm not an expert.
And assuming =
500 laps at Daytona won't blow the VC, would it make sense to
match the treadwear =
left to right rather than front to back? Hmmm...)
Any VW Syncro =
engineers out there?
Doug =
Fitz-Randolph
Yarmouth, ME
dfrandolph@talkam.net
'90 Syncro