Without a doubt Kas. Thats the reason in SCCAs GT classes, there is a
weight penalty when going up from 13 to 15 rim sizes. Despite the
drawbacks in rotating and unsprung weight, the tires run cooler which allows
the use of a softer compound tire. This isnt an issue in classes that
require DOT legal tires, since they vary rarely are manufactured with
different compounds.
It also should be a null point because the cars with 14 wheels have a
lighter minimum weight than the TRs, which I think would cancel out any
advantage to running a larger diameter tire.
I think the issue to eastern/midwest TR racing community is the compound and
tread design differences between the two tire sizes. The manufacturers build
current treaded DOT-R tires in a 205/55 size which is legal under the
vintage organizations rules (205/55 radials have the same overall diameter
as a 205/60 bias-ply such as the Street TD), but the same tire model /
compound (including the Street TD) are constructed in 205/50 sizes, which
are not legal (sidewall is too short). The tires that are constructed in a
15 vintage legal size either have narrower tread widths (Vintage TDs and
Bluestreaks- 5.50 tread width) or are too hard, get greasy, and heat cycle
out way before the tread is used up (Speedsters and A048s). Oddly enough,
all these tires are much more expensive and dont last as long as a modern
205/50/15 treaded R-DOT tire.
Here is a handy calculator that shows how the sizes stack up..
http://www.1010tires.com/TireSizeCalculator.asp?action=submit
I know I might sound like a broken record to some and most of you guys
arent racing to win, but its pretty fustrating for everyone involved to
watch your car get outbraked, and the passing car leave 3ft of track width
open on exit, while your car is dropping wheels just to keep up.
Any thoughts?
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