From: joe@sybase.com (Joseph Weinstein)
Newsgroups: rec.autos.sport,rec.autos.tech
Subject: Re: Tires : Slicks vs. Treaded: Help Please
Date: 13 Oct 92 16:42:53 GMT
Organization: Sybase, Inc.

Every once in a while, this issue becomes new again. Here is something I
wrote about tires. I am a successful autocrosser in a street-prepared class.

Relevant summary: Slicks are better (in the stated conditions), more rubber on the
		road is better, and high school physics formulae miss the boat.


A discussion on slicks vs. treads, wide tires, contact patches and how high school
physics doesn't cut it.

Abstract:

1 - Why slicks and wider tires stick better (when it's dry).

2 - Why the contact patch is *not* proportional to tire pressure.

3 - Why, in dry conditions, the tire tread patterns *hinder* traction.

4 - Why narrow and/or deeply treaded tires work better in snow or
    standing water.

5 - Why normally obtained tire temperatures only matter much on dry
    surfaces with racing tires under racing conditions.

(For the following, acceleration means either 'straight-ahead' or
'cornering traction'.)

1 - The Tire Contact Patch
--------------------------
The tire surface is not a single sticking or sliding block, such as
would be amenable to the elementary physics of friction. Even a racers
or dragsters slick tire should be seen as a dense collection of indiv-
idual contact points between rubber and road. Under acceleration each
of these points is at a different phase of a stick-slide-regrab cycle.
At a very localized level what one contact point is going through relates
more or less smoothly to neighboring ones, but over more distance across
the contact patch points at all phases of this cycle will be found.
What happens macroscopically to the tire supplying traction under accel-
eration depends on the number of contact points in the 'stick' portion
of their cycles, the resistance each of these points can supply, and the
number of points where the sliding has ceased due to the currently holding
crop of points. These 'slide-ceasing' points were holders that eventually
let go. The rubber is returning to an unstressed state over the road
irregularities that had held them. If there are enough points coming into
a hold state these sliding ones will return to a stasis with the road
where a new road point can take hold. Where tire width comes to play is
that the number of contact points increases with tire width, balanced by
the fact that for a given rubber compound and load, there is a limit to 
the shallowness of the contact point intrusion, after which the ability
to resist is lessened.
    A visual analogy would be tires with grooves in their surface which
roughly match grooves in the pavement. There is a component of lateral
resistance to movement proportional to the number of engaged grooves.
The limiting effect of width is that there is an imperfect match between
tire grooves and road grooves that require a minimum load per square
inch to force a given tire groove maximally into the road groove.

	|			      |
	|			      |
	|  _   _   _   _   _   _   _  |
	|_| |_| |_| |_| |_| |_| |_| |_|
   _   _   _   _   _   _   _   _   _   _   _   _   _   _   _   
|_| |_| |_| |_| |_| |_| |_| |_| |_| |_| |_| |_| |_| |_| |_| |_|


Practically this means that wider is better up to a point, after which it
doesn't help, and can hurt to the degree that the tire compound requires
heat to soften to allow good deep contact point intrusions. At this point
wider tires can begin to supply more stick again if the tires are changed
to softer rubber. Also, in cornering applications, I suspect that the
fact that the inside of the tire travels on a different radius than the
outside of the tire will also limit the value of additional width.

2 - The modern radial tires are so stiffly walled that these tires keep
the same contact patch size over a wide range of pressures and loads.
If a tire were more like a balloon, the tire contact patch would be
directly related to the air pressure in it. If the tire were solid the
contact patch would be constant. In reality it is nearer to the latter.
Air pressure affects "spring-rate" supplied by the tire, and resistance
to rollover onto the sidewall. In some racing tires (like the BFG R1s I
run) rollover won't even happen till pressure is in the single digits.
The only way to determine optimal tire pressure in this case is to use
a pyrometer to normalize temperature across the tire.

3 - On dry surfaces, any tread pattern is worse than useless for heavy
acceleration. The blocks deform and roll over. This takes even more
rubber off the road, and allows more lateral movement. The edges of
the blocks wear unevenly and under truely sporting loads may actually
tear off the tire. This is called 'chunking' and is why racers shave
treaded tires down to a few 32ths of tread depth.

	|			|
	| FULL TREAD TIRE	|
	|			|
	|// \--/\  \--/\  \--/\  \
	 ^\  \  ^\  \  ^\  \  ^\  \
	 | \_/  | \_/  | \_/  | \_/
---------|------|------|------|------------
	 |      |      |      |
	 ------------------------------- will tear here

	|			|
	| SHAVED TIRE		|
	|  ____   ____   ____	|
	\__\   \__\   \__\   \__\
-------------------------------------------

4 - On wet or snowy surfaces it's a different game because you have a
film or thick coat of slippery stuff to penetrate before you get to
lock up with any of the 'grooves' in the pavement. The gaps between
tread blocks that were worse than useless in dry conditions, now
supply passages that allow large quantities of water or snow to be
squeezed out of the way, and allow the faces of the blocks to reach
the road for grip.  Because snow is so much more bulky than water
(on well drained roads) there may be too much snow to squeeze between
the blocks unless the majority can be squeezed beyond the sides of
the tire. A good rain tire has maximally wide blocks with just
enough gap to take the water to displace for the block faces to
fully engage the road. These dimensions vary with the amount of
water on the road and the speed of the tire travel, so real tires
leave safety margins on the side of more gap space for water at
the expense of tractive capability. A good snow tire might look
more like a truncated narrow V, to ensure cutting through the snow
down to the road. Of course this tire would be best in deep snow
and increasingly poor as the snow became shallower, allowing more
tire to get to the road than this tire had to offer.

5 - Tire temperature only figures much with racing tires with soft
rubber designed to further soften and stick better when heated.
These tires will at best last only a few thousand miles of road use.
(That doesn't stop me from using my race tires in the street.
T/A R1 204 road race compound)

--
Joseph Weinstein  Sybase - 1650 65th Street, Emeryville Ca 94608
		  joe@sybase.com   sun!sybase!joe   510-596-3620
Disclaimer:	  Opinions, I disclaim thee, I disclaim thee, I
                  disclaim thee!