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Re: [T3] Tire sizes


On 13 Feb 2002, at 20:47, Jedediah Millheiser wrote:

> any idea how well 205s would work on 5.5 inch rims. Ive seen at least 1 in
> VWT running 205/50r15s on stock 5.5 inch rims. 

The 205s may well work okay on 5.5" rims, but wider rims would work better. 
Note that 5.5" is NOT the stock rim size. Running extra wide tires on narrow 
rims will degrade performance.

The stock rims are 4.5" wide and work optimally with a 165mm tread width. 
Going to 205s on 5.5s increases the tread width by 40mm and the rim width 
by 25mm, which puts you at the margins of acceptability and well off the 
optimum. Tire manufacturers have charts that show acceptable rim widths for 
each of their tires. I have a very old Michelin tire guide, circa 1980, 
somewhere, which shows for each tire: recommended rim width in bold, and 
acceptable width in standard type. 

It is interesting once you understand what they are trying to accomplish with 
this. You really want the sidewalls to stay pretty much "parallel" so that the 
tire profile is a rectangle. Thus when the tire deforms with side loads, the 
contact patch stays flat on the road. Once you squeeze a wide tire onto a 
narrow rim, the profile is a trapezoid and the contact patch "tilts" with side 
load and quickly loses advantage.

-
Jim Adney
jadney@vwtype3.org
Madison, WI 53711-3054
USA

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