Date: Fri, 16 Apr 1999 18:58:26 -0400
Reply-To: EMZ <vw4x4@FYI.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: EMZ <vw4x4@FYI.NET>
Subject: Re: Syncro tire dia. spec. - tolerances? Ask Mr. Science!
In-Reply-To: <C1A47AF64D80D01198AC00A024CC3499852C9C@TA_EXCHANGE1>
Content-Type: TEXT/PLAIN; charset=US-ASCII
NO, That what it was put in there for. But! I would do
whatever I could to easy it's work, by keeing things as close
to the same as possible.
Eric 86-VW4x4
vw4x4@fyi.net 72-240z
Pittsburgh, PA USA 1936-Chrysler
On Fri, 16 Apr 1999, Fitz-Randolph, Douglas wrote:
> >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
>
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