Wide Tires and Grip

[Kitdy]

Well, I have read some on Wikipedia, talked to some friends, including one in physics and two in engineering, I was formerly in engineering and none of us have a conclusive answer.

Do wider tires give you more grip?

What is the advantage of wider tires?

As I was taught in high school, and according to Wikipedia:

"This approximation mathematically follows from the assumptions that surfaces are in atomically close contact only over a small fraction of their overall area, that this contact area is proportional to the normal force (until saturation, which takes place when all area is in atomic contact), and that frictional force is proportional to the applied normal force, independently of the contact area (you can see the experiments on friction from Leonardo Da Vinci)."

This means that friction is independent of surface area.

What the hell is the difference between grip and friction?

From the rolling friction article on things that affect the coefficient of rolling friction: "Dimensions - rolling resistance is related to the flex of sidewalls and the contact area of the tire[4]. For example, at the same pressure wider bicycle tires have less flex in sidewalls and thus lower rolling resistance (although higher air resistance)[4]."

I have seen another thread somewhere else on the net where they argued this and did not read it was long and my friend did not find a consensus - I am not sure I will find a consensus here.

What the hell is going on?

EDIT: Sorry if this is a repost.

[fpv_gtho]

Generally, if you take the same compound/construction tyre and run the same pressure you get more grip from a wider tyre. Pressure has alot to do with it though, as lower pressures give you a bigger contact patch which is what really determines the grip.

[pimento]

The short answer is.. there is no short answer. Wide tyres generally give better grip in the dry, but can give worse grip in the wet due to not being able to clear water from the tread as efficiently. Wider tyres can also give rise to steering issues, due to having to overcome more friction to turn the wheels against the momentum of the vehicle. Hence powerful RWD cars having wider rear tyres than fronts. Basically it depends on... everything. Conditions, suspension geometry, compounds, vehicle mass... blah blah blah.

[Kitdy]

Originally Posted by fpv_gtho

Generally, if you take the same compound/construction tyre and run the same pressure you get more grip from a wider tyre. Pressure has alot to do with it though, as lower pressures give you a bigger contact patch which is what really determines the grip.

What is grip? Friction? Contact area does not increase friction.

[acfsambo]

I agree with pimento. With RWD thing, look at F1 cars. They have thinner front wheels than the back, due to pimento's reason.

Also if you have too much pressure in the tyres they will have a smallwe contact area which means less resistance, but if its under pressure (by too much) you will basicaly be running on your side walls and the middle of the tread will have less contact. For a road car you need balance but it different for every race car and track as well as the conditions, so there is no conclusive answer.

Kidty. Contact area does increase friction. Think about it, if there is more matter in conact with the road, more friction will be produced. Put the weight of your hand on the end of one finger then drag it alo gthe desk. Then do the same with your whole hand on the desk. There is more resistance, more heat therefore more resistance.

[pimento]

Originally Posted by Kitdy

What is grip? Friction? Contact area does not increase friction.

Sure it does.. more area in a friction per unit area situation as tyres are (extremely broadly speaking) would increase it.

[Kitdy]

Originally Posted by pimento

Sure it does.. more area in a friction per unit area situation as tyres are (extremely broadly speaking) would increase it.

Are you saying surface area increase friction? Not according to physics or physicists or what I have been taught.

Force of friction is dependent on two things: mass, and coefficient of friction. This has been experimentally verified.

[pimento]

Originally Posted by Kitdy

Are you saying surface area increase friction? Not according to physics or physicists or what I have been taught.

Force of friction is dependent on two things: mass, and coefficient of friction. This has been experimentally verified.

No, I'm explaining it poorly. I mean that a greater area can 'use' more frictional force (really don't know how to explain that properly) to resist slipping to a greater extend than a smaller area and hence provide more grip.

Not being a physicist and not having done anything using physics terminology since high school (8 years ago) undermines my terminology somewhat on the subject... and my ability to explain what I mean properly.

[Kitdy]

Originally Posted by acfsambo

Kidty. Contact area does increase friction. Think about it, if there is more matter in conact with the road, more friction will be produced. Put the weight of your hand on the end of one finger then drag it alo gthe desk. Then do the same with your whole hand on the desk. There is more resistance, more heat therefore more resistance.

It may seem wrong, but physics does not agree with this. I suppose the line of reasoning would be that if you have small surface the force is more concentrated and heats up more - the same energy is lost (I think this may be correct). Also, friction is more than just heat.

Originally Posted by pimento

No, I'm explaining it poorly. I mean that a greater area can 'use' more frictional force (really don't know how to explain that properly) to resist slipping to a greater extend than a smaller area and hence provide more grip.

Not being a physicist and not having done anything using physics terminology since high school (8 years ago) undermines my terminology somewhat on the subject... and my ability to explain what I mean properly.

I fail to understand.

This is complex. My amateur interest in physics and engineering is not helping me.

[hightower99]

Matra has posted a link to a very interesting website that has pretty much all the information you would want about wheels and tires.

Basically the size of the contact patch is determined by tire pressure. Too much pressure and you end up reducing the contact patch conversly if you don't have enough tire pressure then you again end up reducing contact patch size.

Because the size of the contact patch is determined by tire pressure that means that it doesn't matter how wide the tires are the contact patch will be the same, however the shape and behavior of the contact patch will be changed by tire width.

It is possible to increase the size of the contact patch with a wider tire you just need to lower the pressure.

The advantage of using wider tires is that you can reduce the pressure and the tire will still maintain proper shape.

With a larger contact patch greater force can be supported (more G's)

Edit: Heres the link: Car Bibles : The Wheel and Tyre Bible Page 2 of 2 scroll alittle less than half way down and read from there...

Here is another good site that talk about the effects of the shape of the contact patch: Tyres - Page 5

[pimento]

Originally Posted by Kitdy

I fail to understand.

This is complex. My amateur interest in physics and engineering is not helping me.

It's a tough thing to comprehend, especially if you start trying to think about everything at once. The easiest way to look at it is to isolate it purely to contact area, all else being equal all the time. Firstly, look at putting the engine's power down to the road. If you look at each square centimetre of tyre in contact with the road as being able to transfer x bhp before slipping, then doubling the contact area could theoretically mean that 2x bhp can be transfered. This would be defined as giving more grip.

This model can be applied to cornering as well, each unit of contact providing x units of resistence to sliding sideways, double the contact then you have 2x units of resistence.

I know it's not that simple, it's just a way to describe what I was referring to as the 'use of friction'. I'm off for a walk now, I'll check in later on.

[clutch-monkey]

i have more grip with 255 width tires than with 225 [/thread]

to be fair the 255's have stiffer sidewalls as they are on 17" rims

[pimento]

My Volvo on 17s has more grip than my Mum's on 16s...

...to be fair, I have an LSD.

[fpv_gtho]

If you want a good, simple example check out any half decent mud terrain tyre. Theyve usually got about an inch or 2 of tread still on the shoulder and sidewall of the tyre, yet when you travel on the road with 35-40psi in them that part never touches the ground. When you get offroad though and drop the pressure down to 15-20psi, its now touching the ground.

Also, when you run your tyres with not enough pressure the shoulder of the treadface usually wears alot quicker than the centre. However, if you were to run optimal pressure the centre may wear quicker. Especially if you did alot of burnouts.

Sports cars/race cars run alot of camber to utilise the whole tread face whilst cornering and prevent the outside shoulder from overheating, which usually means the outside edge doesnt touch the ground when unloaded whilst the inside edge gets worn quickly.

[Kultag]

I read many times that the area of the contact patch = weight on the given wheel / tire pressure. Okay. But than a rally car with something like 125 mm wide spiked snow tires that have a stamp-sized contact patch has a tire pressure of 30 or 50 bars?