-----Original = Message-----
From: Fitz-Randolph, Douglas <dfrandolph@TALKAM.NET>
To: vanagon@GERRY.VANAGON.COM <vanagon@GERRY.VANAGON.COM&g= t;
Date: Friday, April 16, 1999 2:32 PM
Subject: Re: Syncro = tire dia. spec. - tolerances? Ask Mr. Science!>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