Shawn---There's nothing wrong with "disagreeing", when one has a valid
point to put forth.
I believe some differences might arise as to when "slow" is judged.
Wide, sticky tires will certainly slow a vehicle designed for ultimate
top speed on a straight track, as a Bonneville race car. So we see
skinny, semi-pneumatic or even hard rubber treaded wheels on these, for
less rolling resistance and a lower CD.
Then we see vehicles that must stop, turn and accelerate quickly that
like the opposite, so we have the wide, sticky gumballs, as on Formula
One, and the roundy-round cars. This must add to better lap times, or
they wouldn't be using them.
I must admit that the theory of a 90 percentile of the 195's does sound
like it would have certain advantages, and give less squirm. I don't
know if this tire on the appropriate rim would be faster in a slalom, or
a road course, than say a wider tire, everything else being the same.
Mitch offered that his wider (215) tire on the 7 inch rim scrapes the
(front) fender lip during turns when the suspension is compressed. (His
car is also lower than most) One more point for narrower tires?
Dick Taylor
205x70-15x5.5
69th percentile
Shawn wrote:
I hate to disagree with you Dick but it is quite possible to "over-tire"
a car by going with more rubber than you need. Your traction will
increase with the wider tire but you will be slower due to the increased
mass and to much stick um (believe it!). This will actually make your
car slower. (maybe that is safer for some)
According to the Tire Rack tire guru who writes the Tire Tech
articles for Grassroots Motorsports (forget his name write now, not at
home), your optimum tire width for performance should be based on a
percentage comparison between cross section (tire width) and wheel
width. With optimum performance when your wheel width is in the 90%
range of your cross section. The 85% range is real good as well but not
optimal. This has to do with side wall flex. The wider the wheel under
the tire, the more broad of a shoulder of support for the tire and side
wall. Of course the shorter the side wall the less flex as well but
shorter side wall tires require wider wheels in the first place. As
always there is a trade off. On a 7 inch wide wheel, a 225 cross section
yields a 78% shoulder, lots of flex, a 205 tire would yield an 85%
shoulder, a 195 would hit into the magical 90th percentile.
195's will hit the magical 90% area but will probably result in a larger
tire to fender clearance or much smaller rolling diameter to screw the
speedo readings up even more. So to wrap this up. The 205/65's should
offer the best all around performance and appearance solution. One
last item because someone else will be sure to point this out as well.
Side wall flex will have a direct corelation to ride comfort. The more
flex, the more your tires help soak up bumps and road irregularities.
The less flex, the better the handling but you also get more NVH (noise,
vibration & harshness). So if you wanted more comfort with more stick
the 215 & 225 tires will get you there. Just a little slower.
Shawn J. Loseke
Fort Collins, CO
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