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Tyres - Gripping Stuff!


nikpro

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Following on from the post I made on Anti-Roll Bars (Anti-Roll Bars - The Theory) I thought it would be appropriate to cover the tyres on the vehicle. These posts are made without going into to much technical detail and I am sorry if it is teaching the engineers here to 'suck eggs'; they are intended for the enthusiast to give them a basic understanding and knowledge of how the suspension on our vehicles work.


‘Without any shadow of doubt the tyres we fit will have a bigger effect on the cars handling than any other component!’

That’s a pretty bold statement to start with but the reason it’s true is that the tyres are the only part of the vehicle that are in contact with the ground at any given time; they are therefore the only limiting factor to the maximum grip we can achieve; everything else in respect of suspension geometry and vehicle set up is done in order that we can make best use of this grip from the tyres.

Because the tyres are in contact with the ground we need to examine how they work and I think it would be fair to say that at least everybody understands that the grip they achieve on the tarmac is down to the ‘friction’ between the tyre and road surface.

So to examine grip we first need to examine ‘Friction’.

Friction:

Definition: Friction is “the resistance an object encounters in moving over another”

There are several different types of Friction but we are only interested in ‘Sliding Friction’ so we need to look at the equation for this:

Sliding Friction = µ (Load)

Where:
µ = coefficient of friction.

What is the Coefficient of friction?

The Coefficient of friction is a number that represents the friction between two surfaces; for a relatively smooth surface like glass this number will be low and for a rough surface like sandpaper it will be high.

Going back to our Friction equation you will note that there is no mention of surface area! This really is counter intuitive but if a 1cm block of wood is pressed onto a surface with 10kg’s of load it will take the same force to move that block as it would a 10cm block with the same load applied.


……..But surely wider tyres give us more grip – if not, why do we fit them?

Well, this is where tyre construction comes into Play. Tyres don’t conform fully to the ‘Sliding Friction’ equation. Tyres also rely on a phenomenon known as ‘molecular effects’ to add to their co-efficient of friction.(The more 'molecular effects' the 'stickier' the tyre.) I don’t need to explain what molecular effects are – all you need to know is that this means that the area of contact also becomes a factor in the Friction equation. So, yes, in certain circumstances wider tyres can increase grip and the contact patch area is very important.


Going Over Old Ground:

From our Load Transfer description in the ARB post (ARB's) we know that as the load is increased or decreased on a tyre it gains or looses grip but not at a linear rate (Traction Curve). We have also been told that a loaded wheel on an axle will gain grip at a slower rate than the unloaded wheel looses it. Why is this? This is because the Coefficient of friction for a tyre actually changes! It is dependant on numerous factors such as its loadings/deformation/temperature but the reason it gains or looses grip at different rates on an axle is its slip angle – oh My God; what the Hell are Slip Angles?


Slip Angles

For anyone not interested in vehicle suspension this is a term that they may have heard of and often frightens them to death!
I will try to explain what they are in relatively simple terms, so here goes:

Without going into to much detail, it’s the angular difference between the direction the rim is facing, and the actual path of the rolling tire. It has to do with the fact that the tire is elastic and twists. So the contact patch, because it bears all the weight of the vehicle and has friction with the road surface, will resist when you turn the steering wheel. The end result is that the contact patch will not turn as far as the rim does.

I like to think of the relationship between the tyre’s slip angles and its coefficient of friction as the ‘Tyres DNA’.

As you increase the Load on a tyre you will also increase the slip angle (amount of twist) in that tyre

To understand the relationship between the co-efficient of friction and slip angles we will look at two different tyre curves:

17688.jpeg

It can be clearly seen from above that the Race Tyre’s coefficient is much steeper and the ultimate grip available from it will be higher.
A Race Tyre is designed to act in a more linear fashion as load is applied and gains grip quickly as slip angles increase.

However, the biggest difference between the two is at the top of the Curve, although the Race Tyre’s grip will be higher it’s ‘operating range’ in relation to slip angles is far narrower. This basically means that the Road Tyre will give safer handling as it stays at it’s ultimate grip over a wider range and does not reach this peak in such an abrupt manner – you will often hear people who have run with slick tyres say ‘I had loads of grip one second and then it just went!’

......I'm still confused; how does this explain how the loaded wheel gains grip at a slower rate than the unloaded wheel looses it?

Let's use the Race Tyre as an example; as you enter the turn the Loaded (outside) Tyre very quickly builds it's slip angle to it's ideal operating range (approx. 8deg.). As you continue the 'turn in' more load transfers onto that tyre. (More Load = More slip angle) so the tyre continues to increase it's slip angle, to say 14 deg.

Looking at our friction equation:

Sliding Friction = µ (Load)

We can see Load has increased but the coeficient of friction (µ) hasn't changed much; this means the tyre has gained some grip.


The Unloaded Tyre not only looses Slip Angle (Less Load = Less Slip Angle) and is therefore less efficient (µ is lower) but it is also loosing Load. We therefore have two 'reductions' in our friction equation compared with the 'one gain' on our loaded tyre, hence on an axle the Loaded Tyre gains less grip than the unloaded tyre looses it.


The above is just a ‘brief taster’ on an immensely complex subject but I hope it has helped explain how a tyre grips.


So in Summary we have the following:

(1) Tyres are the most important part of a vehicle’s suspension.
(2) Tyres rely on Sliding Friction to Grip.
(3) Sliding Friction is the product of the load on the tyre and it’s coefficient of friction.
(4) A tyre, unlike many other surfaces has a constantly varying coefficient of friction dependant on several factors but includes Slip Angles.
(5) As Load changes on the tyre so does the Slip Angle it generates (An increase in load will increase the slip angle and vice-versa).
(6) A Tyre is designed to give it’s optimum lateral grip over a range of slip angles


…and finally………

Huston….We have a problem!

Remember the Caveat in the ARB post where I said the following:

A Westfield is a lightweight Sportscar that quite often runs tyres designed for heavier vehicles - they sometimes don't act 'normally' and quite often increasing the roll stiffness can increase overall grip as the vehicle does not put enough vertical load into the outside tyre

Well now we can maybe describe a particular scenario:

Imagine you decide you are going to Run Slick Tyres on your Westfield for a track day. Let’s just consider for a moment what these tyres may have been designed for? Just for this example, let’s assume it is for a single seater Race car. The static weight of the two is fairly similar so there shouldn’t be a problem; however a large number of single seater race cars are using ‘Aero’. The effect of this is far more load is put into the tyres than you can possibly imagine and is something we haven’t got on our Westfields.
Trouble is, the tyre designer didn’t account for this and constructed the tyres so that they achieved an optimum slip angle for grip with a much higher load than we can generate in our cars, there could, however, be a solution!
We know from the ARB post that Load Transfer generally reduces grip across the axle, but how about in this situation we encourage load transfer so we can get the ‘slick’ to operate at it’s optimum slip angle for grip – hey presto the car corners quicker!

I suspect this is the reason why you will see several different people say that there car was quicker when they stiffened the suspension springs/ARB’s above normally accepted values – they have encouraged Load Transfer to get the slick tyre operating within its optimum slip angle range.

(N.B. all posts in this series are available as PDF's to WSCC Members F.O.C. - PM me your email address if you would like a copy)

 

Information for the above articles taken from the following sources: Milliken & Milliken Race Car vehicle dynamics, Optimum G, Dan Kozakewycz blackartdynamics.com, Smithees Race Car technologies, Herb Adams Chassis Engineering, Alan Staniforth Competition Car Suspension.

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b****y hell wuv are you bored or summat ,if you want summat to do pop around ours there is a mini that needs building

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All well and good, Fraser, but I would have thought you might at least have told us something about tyre temperatures, and pressures, and the relationship to the tyre's coefficient of friction.

;)  ;)  :D  :D  :D

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the reason for this is because the Coefficient of friction for a tyre actually changes! It is dependant on numerous factors such as its loadings/deformation/temperature

Temps/Pressures are vehicle specific where as Slip Angles aren't.

Temps/pressures don't explain why Load transfer causes the loaded wheel to gain grip at a slower rate than the unloaded wheel looses grip, where as Slip Angles do.

HTH

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