RE: [T3] Tire Sizes

["Bryan \"bc\" Castles" <bc@version3.net>]

Sometimes the technical and the real world just don't match up.  I bow to
your superior knowledge Jim, However (it's always in there isn't it?) I
disagree in the sizes that we are currently talking about.  I ran the stock
tires, and I ran my 195's.  The tires that were on there were in pretty good
shape, so it was purely by choice.  I now get much better dry traction, much
better wet traction, and can corner the car like it's a different one all
together.  You'll notice though, that when changing the tread width, I
dropped the sidewall size down to account for the trapezoid effect you were
speaking of.  In this case, the tires to perform MUCH better than stock.

To side with you (because it is a nice things to do) when I squeezed the
205's on my stock '67 rims, it did a lot of weird $hit until I switched to
those 5.5" rims on the back.  So there you go, you're still right!

Bryan Castles
Arlington, TX
USA

'64 Sunroof Beetle
'70 Bus (Ex-Westy)
'73 Square
www.version3.net <http://www.version3.net>



-----Original Message-----
From: Jim Adney [mailto:jadney@vwtype3.org]
Sent: Monday, April 08, 2002 7:28 AM
To: TYPE3@vwtype3.org
Subject: Re: [T3] Tire Sizes


On 8 Apr 2002, at 8:47, Bryan Castles wrote:

> I have a 195 series on my stock rims...   and I had a 205 on my 67 some
> years ago.  I have had a shop or two tell me that a 205 won't go on the
> stock rim.  I think that is the biggest though, assuming you can find a
shop
> that will work with you on it.  I currently have 195's all the way around,
and
> they don't rub anywhere.

We can consider the tires to consist of 4 segments: tread, rim, and 2
sidewalls. If we view the cross section we can see that these 4 segments
make up a geometry that is essentially a parallelogram if the tire is
mounted
on the proper rim.

The beauty of this parallelogram is that when our car corners, the side
force
on the tread forces it sideways but it stays parallel to its original plane.
This
keeps the tire contact patch constant. The standard 165mm tread is the
correct width to match our 4.5" rims.

If we mount a bigger tire on this same rim we now find that the
parallelogram
has been replaced with a trapezoid which no longer maintains the parallel
contact patch with the road. Instead, it tends to lift the inside portion of
the
tread, reducing the contact patch at the exact moment when you could use
it the most.

Of course there's a range of rim widths which given tire will be happy with,
but there is still an optimum and straying too far from the manufacturer's
recommended rim width will make your handling worse.

Clearly there's nothing here that says you can't mount a really wide tire on
our rims, but I believe that you'll find that a 185mm tire is about the
widest
tire that anyone recommends that you use on a 4.5" rim. If you want to get
the best out of wider tires you really need wider rims to match.

-
*******************************
Jim Adney, jadney@vwtype3.org
Madison, Wisconsin, USA
*******************************

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