View Full Version : The mystery of the round black magic thingy, aka the tire

11-13-2008, 04:08 PM
Were going to keep in Laymanís terms here not scientific stuff or formula.

Traction: hey thatís what tires are all about right?* Part of the traction a tire gives is simply the CF (coefficient of Friction) that a particular compound gives. This can vary slightly.* There is virtually NO difference in CF of any compound normally used in street tires.

* Most of the differences in traction of various rubber compounds comes from the hardness of the rubber. This is measured by indenting the rubber with a spring loaded devise and measuring the depth of the indentation. This ďDepthĒ is then transcribes on a Durometor Scale There are several scales, the one that covers tires it the ďDĒ Scale. A given tire rubber compound will measure from around 50 to 100 D.* The higher number the harder.* You wonít find the number on any tires but most car tires have a Wear factor number that runs from 100 to 1000 with the higher* being the longest wearing. This number is not solely a hardness number alone, but thatís most of it.

* * Why the hardness of a tire relates so much to the traction limit of that tire is because most of the grip of a tire on the road is from the rubber deforming around the road surface irregularities.* Not the CF of the rubber.* So the softer the rubber the more it deforms, the more grip.* Itís not hard to imagine that there are real limits to how far you can go in either direction.* Too soft and the tire wears rapidly or chunks, too hard and it doesnít grip at all.* BTW this is why motorcycles can lean past 45Deg.

Ok up to now we havenít talked about tire construction.* This is also related to traction. (Well hell everything about a tire (except white walls) is related to traction).
There are three fundamental types of tire constructions.* The cross Bias, The Belted cross Bias. And the Radial.

The difference lies in the way the strengthing cords are arranged (all tire have cords)
* Cross Bias:* this is the oldest type.* The cords run from one bead or edge to the other on an angle and each layer crosses the first in the opposite angle.* This produces a tire that flexes uniformly across it entire surface.

* The Bias belted:* Constructed as the Cross Bias, with the addition of a stiffer circumferential bead or Breaker.* This give a tire with Dual flexing rates. The breaker stiffens up the tread to reduce it deformation under load, but the side walls remain more flexible

* The Radial:* In this tire the cords are laid at a 90 degree angle in respect to the centerline of the tire and there is no crossing, Then a strong Breaker (usually steel Mesh) in laid under the tread, virtually eliminating any tread flexing at all.* The side wall must then be very flexible.

Slip Angles:* This is the angle of the difference between the direction the wheel is pointed in and the actual direction of travel of the machine.* Or a fair estimate of the degree of sliding.

* First let me say that this is not a racer thing or something confined to hard riding near the limit of adhesion.* Every tire on every machine generates a slip angle when turned. A couple of forces cause this.* The natural histories of the rubber (you know, the spring back) and the construction of the tire cords.* Bias tire have more self aligning torque than radials and conversely generate greater slip angles.

* Tire pressure can increase /decrease the slip angle of a given tire
Watch NASCAR?* When you hear the commentators talk about the carís pushing, or is loose/tight. Then he went up/down Ĺ lb. in the rear to fix it.* What they were doing is adjusting the slip angle of the tires with tire pressure.

OK on any vehicle:* a rear with a greater slip angle than the front will tend to tighten a given turn. If this is severe the thing could be un rideable.* This is the reason that youíll always be warned that it is un wise to mix tire brands. Especially Radial and Bias types.

There are a few thousand more points that affect how tires perform, but they are off into the area we donít need to get to.

Think about all of the above the next time youíre buying tires or have a handling problem, some of it may help.