Actually the this string 'measuring' thing was only to compare wear on tires of same brand, size, model, etc. (I think someone mentioned that their tires wear more on one side that the other.) And to do that they don't have to be on the car as long as you measure all of them off the car in the same manner at the same tire pressure. :)
Bill
    -----Original Message-----
    From: Fitz-Randolph, Douglas <dfrandolph@TALKAM.NET>
    To: vanagon@GERRY.VANAGON.COM <vanagon@GERRY.VANAGON.COM>
    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 

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