DeTomaso Mailing List: February 1997, Message #103
     



From: Andy Poling <andy@realbig.com>

Subject: Re: Pantera Tires (mixing different sidewalls)

Date: Fri, 14 Feb 1997 13:59:38 -0500 (EST)



On Fri, 14 Feb 1997 MikeLDrew@aol.com wrote:

> Lots of people will tell you you should never mix tires of different aspect

> ratio.  Well, we do it all the time with GT5/Group 4 cars; I think the rules

> are different when you're dealing with mid-engined cars with wider rear

> wheels anyway.



I think that their concern can be generalized to "mixing tires with different

sidewall stiffnesses".  This doesn't necessarily mean tires with different

aspect ratios, but is more concerned directly with the height of the

sidewall and its construction/stiffness.  E.g. a wide low aspect ratio tire

may have the same sidewall height as a narrower higher aspect ratio tire.



I can see the potential problem: bizarre transient response.  I consider

transient response to come from three elements: spring rates, spring damper

(we call them "shock absorbers") rates, and the tire sidewalls.



Consider for a moment an exagerated but not-so-imaginary Pantera with short

stiff sidewalls on the front tires and tall flexible sidewalls on the rear

tires.



When you enter a turn, the tires start generating side forces.  However,

before those side forces even reach the wheel to start changing the car's

direction of travel, they must travel through the tire sidewalls from the

tread.



Viewed as mechanical systems evaluated for their damping wrt side forces,

short stiff sidewalls tend to act overdamped and tall flexible sidewalls

tend to act underdamped.



The short stiff sidewalls on the front tires of our imaginary Pantera take

little time to "settle" into their loaded posture and therefore start

transmitting side forces to the wheel almost immediately.



The tall flexible sidewalls on the back of our imaginary cat take much

longer to start transmitting the side forces to the wheel.  This is because

they are more flexible (they allow more motion of the tread wrt the bead)

and they are taller (again they allow more motion of the tread wrt the bead).



The upshot of all of this, as the imaginary car enters a turn, is that the

front end starts tracking the turn sooner, while the rear lags in it's

response - continuing straight on for longer.



This behavior (along with the lower frequency of resonance of the tall

flexible sidewalls, which may tend to produce some oscillation after the

initial overshoot) produces a wiggle of our imaginary car's rear end at

every transition.  This is what those nay-sayers are concerned about.  Under

the wrong circumstances it could be quite scary, and it's almost certainly

not the fast way around a track.  :-)





> 3)  With all the hand-wringing about the lack of suitable tires for 15-inch

> wheels, I'm surprised nobody has mentioned running treaded race tires.  My

> 427 Cobra came with Goodyear Blue Streak Sports Car Specials, bias-ply

> super-sticky roadrace rubber.



Just remember that Officer Friendly (in the US anyway) would like your tires

to have DOT approval if he catches you driving on the street.  That excludes

most racing rubber, with the exception being tires such as the BFG R1,

Yokohama RS and RS-II, and Hoosier Autocrosser - tires designed for showroom

stock road racing and stock class autocrossing (both of which require DOT

approved tires).



Also, though we are probably not talking about driving in inclement

conditions, none of the tires (that I mentioned) developed for racing has

anyuthing more than a vestigial tread pattern.  They're basically street

legal slicks.





> 5)  Hate to break it to you all, but 10-inch Campys are dropping in value as

> people become concerned about the lack of tires available for them.  Doesn't

> matter if they're early or late, most people seem to be buying a complete set

> of four wheels and tires (8's and 10's) for $1200 or so; I'd imagine a bare

> set of 10-inch wheels is worth something less than $1000 used.  New, they

> still cost a fortune, $1300 each or so if I recall.



I like this - it's good news to me.



-Andy



'91 Miata: "ZIP"               '96 A4Q: "ZOOM"              '84 RZ350: "ZING"