There are a few myths surrounding bike tyres and grip, and for sure the tyre companies are not going to give anything away.
The first is that the bigger the contact patch area then the higher the grip. Area surprisingly has nothing in theory to do with grip!
Why – because grip is simply weight x coefficient of friction.
The formula for friction (grip) is Fr = μN
- Fr is the force on the tyre caused by cornering, and brakes or accelerating. When this exceeds μN – the tyre slides away.
- μ is the coefficient of friction between the tyre and road.
- N is the force pushing the tyre directly down onto the road, which is mostly equal to the bike and rider’s combined weight. (This varies a bit with undulations in the road and suspension movement, which is why a well set up bike handles much better – but you will likely potentially lose grip over bumps which is obvious to most riders).
The inevitable question is then why do we have such wide tyres? The answer is threefold we believe – to have a carcass that can take the torque from the engine, braking or cornering forces ; to dissipate heat; to reduce wear so softer grippier compounds can be made to last.
Rear tyres are wider as they have to cope with a higher sustained loads than a front tyre. You accelerate hard as you can all the way down the straight, but only brake usually for the last couple of hundred metres when almost all the load will be on the front. The maximum load is limited, as bikes can flip when braking and loop when accelerating (unlike cars). However cornering speed is mainly dependent on tyre grip, which is the obvious payback for having grippier tyres, and hence the development of dual compound tyres with softer shoulders. Hope this makes sense.
Old bikes with far lower power had similar sized tyres front and back, and really skinny ones if anyone has ever seen a 50cc Kreidler close up.
Mohr’s Circle in it’s simplest form shows the theory of tyre grip. You use grip up on cornering, and also when braking or accelerating at the same time, but as long as the combined forces stay inside the circle you have grip, outside the circle you don’t. Clearly in this example the red arrow showing the effect of a high lean angle where the cornering force is nearly at the edge, so any serious braking or accelerating and the tyre will slide. With good modern tyres, the grip ends between 50 and 60 degrees of lean (assuming you have the ground clearance and more with race slicks), but Mohr’s theory does indicate that you can accelerate and brake quite hard at medium lean angles, but this ability reduces quickly as you get towards the limit.
A second myth is that you get more grip at 45 degrees than you do upright. You don’t, as above grip is coefficient of friction x weight, and weight is a constant (bike and rider). It might feel as if you have more grip, but that is just the horizontal forces compressing the suspension at high lean angles (which some riders also confuse with tyre slip).
The third myth is ‘standing it up onto the fat part of the tyre’ when coming out of corners, it’s just that the more upright you can get the more grip is available to accelerate rather than corner. This technique requires an initial slower exit as a trade off with getting on the gas a bit harder and earlier.
Tyres are designed to slip (drift not slide) before they let go, which is where rider feel comes in. Slip angles are typically up to about 6 degrees in ‘normal’ riding, and either tyre or both can drift sideways in the same way a car oversteers or understeers. You need a lot of experience and confidence to get this far, and it would be only sensible on a track. Watch closely how Josh Brooks often rides when he is really on it.
Tyres also distort significantly under load, looking kidney shaped at high lean angles, or ‘s’ shaped when cornering and braking or accelerating, with a steep temperature gradient from front to back of contact patch, which can also move forwards and backwards as the tyres distort, slowing the steering marginally, and ‘shortening’ the effective wheelbase.
There is more information in Motorcycle Dynamics by Vittore Cossalter ISBN-13: 978-1430308614, which you can get on Amazon and elsewhere. Chapter 2 is on tyres. Great book – but heavy on the maths.
It is usually safer to just use the rear brake if you need to slow on corners, as this stabilises the bike and is recoverable if you lock the rear wheel. In theory the front brake would be more effective, but it can sit the bike up and make you run wide, and if you lock it when leant over you’ll very likely drop it, so needs a higher degree of skill and practise, which is risky on the road.
British Superbike School 
You should be careful with your assertion that “grip is independent of area”, which is straight from physics text books but applies ONLY to “smooth bodies”
Tyres do not conform to this law!
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Tyre grip is in reality extremely complex. The point I was trying to make is that it is primarily not the size of the contact patch that determines the level of grip, but the weight on the front or rear tyre, which a rider can influence. Wider or larger diameter tyres also don’t necessarily increase grip levels, but tend to slow the steering. There is clearly an optimum tyre size.
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“A second myth is that you get more grip at 45 degrees than you do upright. You don’t, as above grip is coefficient of friction x weight, and weight is a constant (bike and rider)” – this statement is not strictly true, the Mass is constant but Weight isn’t – Mass is in units of kgs and weight is in units of Newtons – Mass x Acceleration. When the motorcycle is upright the Acceleration is 1G when banked at 45 degrees the Acceleration is 1.44G and at 60 degrees it is 2G. So when a motorcycle banks and is on a constant velocity, both tyres will experience higher forces which lead to greater instantaneous grip than when upright. If you were relying on the equation given we would never get above a mu value of unity.
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You are correct. Good point. Problem is the UK’S police training manual, Roadcraft which is sold to the public, states you get 50% more grip at 45 degrees of lean. Be great if you did.
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The flip side being your vector of weight is no longer pointed directly at the ground but in a direction somewhere between out the bottom of the bike and vertical. Without sitting down and doing the maths I would think that your weight in the vertical plan would not change. How these forces affect “grip” though I have no idea
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Hi Jordan, you are right, the weight in the vertical plane doesn’t change, and this with the friction between the tyre and the road gives the amount of grip available. As the bike leans, the centrifugal force increases also acting on the mass of the bike and rider, and as you say the resulting vector moves until it overcomes the grip. In reality it’s far more complex. There is a diagram in John Bradley’s book which shows this.
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Errrr… Not wishing to totally dispel your post but I studied Physics and Applied Mathematics as a large part of my degree course. One of my theses was specifically related to factors affecting the coefficient of friction on accelerating bodies. While you are definitely correct that the force (N) is a significant factor, the coefficient of friction is an equally important variable. The coefficient of friction is SIGNIFICANTLY increased with higher contact surface areas:
“The major force in friction of mainly flat areas is the van der Waals force between the molecules in the two layers being close to each others, which also are affected by the deeper layers in the materials.”
Obviously we’re not talking about flat areas here but similar rules apply.
I think what you have done is attempted to apply theoretical physics to a real world situation… Always a recipe for disaster.
Theoretical physics (although not Amonton’s law of friction – it was a separate theory developed by John Rennie) states a specific difference between “Real contact area” and “Apparent contact area”. Given that tyres are specifically designed to deform, this difference becomes much more relevant, since the real contact area will differ from the apparent contact area.
Heat, etc. of both surfaces will also have a massive effect on the coefficient of friction.
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I agree. It does seem sensible that grip varies with surface area, but it’s probably not that simple a relationship.
Tyre pressures make a big difference, but that’s probably more related to the additional heat generated through carcass flex than contact area.
The theory we quote I was previously told refered to ‘smooth bodies’. This was confirmed by my engineering training with Ford regarding friction and surface finish being independent of area.
I guess the tyre companies have the data – but understandably they’re not sharing.
If you can find any theory or data specifically on tyre grip we’d very much like to see it.
Thanks for your comments.
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