Racecar Engineering Mar/Apr 2005 wrote:
It seems plainly obvious that wider tyres equals more traction but in reality it is far from this simple
Complex processes of molecular bonding and mechanical interlock are at play when a tyre is in use and sliding results in a combination of both shearing and dragging processesThe wider the better?Why are wide tyres better?It has been recognised for about 40 years now that wide tyres provide more grip, at least when we are not limited by aquaplaning. One might suppose that this effect would be well understood by now, on a theoretical level as well as a practical one, yet the matter seems to be receiving a lot of attention from various authors lately. This seems to be due in part to the need for mathematical tyre models to be used in computer simulation.
On the face of it, one might wonder why there is any controversy about this at all, and also why it took people until the 1960s to try wide tyres. More tyre — more rubber on the road. More rubber on the road — more traction, right? Why wouldn’t this be obvious?
Essentially, there are two reasons it wasn’t obvious. First, according to Coulomb’s law for dry sliding friction, friction is independent of apparent contact area. It depends on the nature of the substances in contact and the normal (perpendicular) force. Second, a tyre’s contact patch area theoretically doesn’t vary with its width. If we widen the tread, the contact patch gets shorter, the area theoretically staying the same.
Let’s consider each of these notions. Coulomb’s law applies quite accurately to hard, dry, clean, smooth surfaces. However, a tyre tread is a soft, tough, sometimes tacky substance in contact with a hard, rough surface. When two hard, smooth surfaces are in contact, they actually touch only at a small percentage of their apparent or macroscopic contact area. Friction depends on molecular bonding in the small microscopic contact zones. As normal force increases, the microscopic contact area increases approximately proportionally, and consequently friction is directly proportional to normal force.
With rubber on pavement,however, there is not only the usual molecular bonding but also mechanical interlock between the asperities (high points) of the pavement and the compliant rubber. Sliding then involves a combination of shearing the rubber apart and dragging the asperities through it as the rubber reluctantly oozes around the asperities. The interface somewhat resembles a pair of meshing gears. With gears, when we increase the size and number of teeth in mesh, we increase the force required to shear off the teeth. It would be reasonable to expect a similar effect with the interlock between the tread and the pavement.
With increasing normal force, this interlock gets deeper, as the asperities are pushed further into the rubber. However, we might reasonably expect that at least beyond a certain point, the asperities are pushed into the rubber to pretty nearly their full depth, and further increase in normal force does not proportionately increase the mechanical interlock. With greater macroscopic contact area, it should take a greater normal force to reach this region of diminishing return. A tyre typically does show characteristics that would match this hypothesis. It will often have a range of loadings where its coefficient of friction is almost constant and where friction force is almost directly proportional to normal force. Above this range, the tyre exhibits much greater load sensitivity of the coefficient of friction. The curve of friction force as a function of normal force goes up almost as a straight line, then begins to droop at an increasing rate. Of course, the contact patch does not remain the same macroscopic size as load increases. It grows as we add load. Nevertheless, this contact patch growth is not enough to keep the coefficient of friction constant.
The contact patch growth is interesting in itself, and a bit counter- intuitive. A tyre can be considered a flexible bladder, inflated to some known pressure, and supporting a load. If such a bladder is extremely limp when deflated — like a toy balloon — and we inflate it, place it on a smooth, fiat surface and press down on it with a known force, the area of contact with the surface is equal to the normal force divided by the pressure
A = Fn/PIf a tyre approximates this behaviour, then it follows that the contact patch area depends only on the load or normal force and the inflation pressure. If we make the tyre wider, then at any given load and pressure the contact patch doesn’t get bigger, it just gets wider and shorter. Accordingly, much discussion of the reasons a wide tyre gives an advantage focuses on reasons we might expect a wider tyre to yield greater lateral force than a narrower one, assuming similar construction and identical pressure, tread compound and load.
One theory is that a tyre is primarily limited by thermodynamics. It generates drag when running at a slip angle. The drag times the speed equals a power consumption, or rate of energy flow. This energy is converted into heat. For the system to be in equilibrium, the heat must be dissipated as fast as it is generated. Even short of the point of true equilibrium, the tread compound needs to be kept below a temperature where it softens to the point of being greasy rather than tacky. If the contact patch is shorter, that means that each square inch of tread surface spends less time getting heated and more time getting cooled.
Also, when a tyre is operating near its lateral force limit, the front portion of the contact patch is ‘stuck’ to the road and the rear portion is a ‘slip zone’ in which the tread moves across the pavement in a series of slip-and-grip cycles. The slip zone grows as we approach the point of breakaway. Beyond the point of breakaway, the entire contact patch is slip zone. The slip zone generates less force and more heat than the adhering zone. A shorter, wider contact patch is thought to have a larger adhering zone and a smaller slip zone at a given slip angle, and wider tyres are also known to reach peak force at smaller slip angles. Therefore, a wider tyre is not only better able to manage heat, but also generates less heat at a given lateral force. This all makes sense, but it fails to explain why wide tyres give more grip even when stone cold.
There is little doubt that they do. If you have a street car with four identical tyres, and you replace the rear tyres and wheels with ones an inch wider, using the same make and model of tyre, with no other changes, the handling balance will shift markedly toward understeer. You will see this effect at all times, from the first turn to the last — surely this effect is not coming from heat management? Paul Haney explains this by the larger adhering zone theory described above. The tyre makes more efficient use of its contact patch, even if the contact patch isn’t actually larger.
As much sense as the above theories make, they ignore some real world effects that have a bearing on the situation.
A classic example of the ‘bigger is better’ tyre theory the hugely successful Lotus 72 First of all, the degree to which tyres follow the
A = Fn/P rule varies considerably.A very flexible tyre, at moderate load, may have a contact patch as large as 97 per cent of theoretical, whereas a fairly stiff tyre may be well below 8o percent.
We are all aware of run-flat tyres currently being sold, which will hold up a car with no inflation pressure at all. As
A = Fn/P approaches infinity. If A does not approach infinity, and the tyre does not go flat, the contact patch area as a percentage of theoretically predicted area approaches zero.
One might suppose that the effect of carcass stiffness would be significant mainly in street tyres, with run-fiats being an unrepresentative extreme. Yet I have seen dramatic differences in carcass rigidity in different makes of racing tyres intended for the same application. The Formula SAF car run by the University of North Carolina Charlotte uses 10in wheels. Hoosier and Goodyear both make 6in nominal-width tyres for the application. The stiffnesses of these tyres differ dramatically, with the Hoosiers being much more flexible than the Goodyears. The Goodyears are so stiff that they will support the front of the car (without the weight of the driver) with little visible deflection, even when completely deflated — run-flat racing tyres! So how closely do these tyres approximate
A = Fn/P in this load range? Not very closely at all.
My point here is that tyre stiffness — vertically, laterally, and otherwise, is not purely a function of inflation pressure, so it is a bit risky to try to infer contact patch size from pressure and load. Therefore, we don’t necessarily know that two tyres differing only in width will have the same contact patch area at the same inflation pressure and load, or even that tyres of the same size do. Anyway, if it is approximately true that
A = Fn/P , it follows that a wide tyre will have greater vertical stiffness, or tyre spring rate, than a narrow one, at any given inflation pressure. It will also have a smaller static deflection at a given load, which is why the contact patch is shorter. The flip side of this is that for a given static deflection, or tyre spring rate, a wide tyre needs a lower inflation pressure. Consequently, if we compare wide and narrow tyres at similar static deflection, or tyre spring rate, rather than similar pressure, they will have similar length contact patches and the wider one really will have more rubber on the road, just as we would intuitively suppose from looking at them.
As we make a tyre wider, not only does vertical stiffness increase for a given inflation pressure, so does the tension in the carcass due to inflation pressure. A tyre is a form of pressure vessel. We may think of it as a roughly cylindrical tank, bent into a circle to form a donut or torus. Borrowing from the terminology of pressure vessel design, we may speak of ‘hoop stress’in the walls the tensile stress analogous to the load on a barrel hoop. For a given inflation pressure, the hoop stress is directly proportional to the cross-sectional circumference, or mean cross-sectional diameter. When the carcass is under a higher pre-load, the tyre acts stiffer laterally. This effect can easily be seen in bicycle tyres. A fat bicycle tyre will feel harder to the thumb than a skinny one, at any given pressure. If we try to inflate a mountain bike tyre to the pressure we’d use in a narrow road racing tyre, the tyre will expand its bead off the rim and blow out. So when we compare narrow and wide tyres at equal inflation pressures, the wider one will be stiffer laterally as well as vertically, and it will achieve this at no penalty in contact patch size.
Finally, there is the question of tread wear. As we have noted, if the contact patch is longer, it has a larger slipping zone near the limit of adhesion, and it also spends a greater portion of each revolution in contact with the road. Not only do these factors influence how hot the tyre runs, but also how fast it wears. Therefore, assuming good camber control, a wide tyre should last longer than a narrow one, with similar tread compound.
The astute reader will see where I’m headed. If we need to run a given number of laps or miles on a set of tyres, then with wider tyres we can trade some of the inherent longevity advantage, and run a softer compound. Okay, summing up, what does a wider tyre get us?
- It runs cooler, and/or
- it makes more efficient use of its contact patch by having a greater percentage adhering, and/or
- it can run at lower inflation pressure and therefore actually have a larger contact patch, and/or
- it can have greater lateral stiffness at a given pressure and therefore keep its tread planted better, and or it can use a softer, stickier, faster-wearing compound without penalty in longevity.
Note that most of these effects in turn play off against each other. We can only blend and balance them, and get a tyre that is somewhat cooler running, has a somewhat lower operating pressure and somewhat larger contact patch, has somewhat greater lateral stiffness, and survives long enough with a somewhat stickier compound, all at the same time. That would explain an improvement in grip, wouldn’t it?
In theory, a wide tyre with a similar tread compound should last longer than a narrow one, allowing teams to use a softer compound. However, driving style can affect this drastically.. 
racecar engineering rocks 
