Tyre width and Friction?

[wowf1]

ok so its common for people to talk about 'that car has massively wide rear tyres so it must have more grip'. However, it always seems to me to be the less well-informed people in the car world.

Being a maths and physics A level student in the UK, i have come across the [Friction = mu x Reaction Force] formula. I can grasp from this when relating it to F1 cars in particular that the downforce increases 'R', and and 'mu' changes with the properties of the tyre and road surface.

So why is it that wide tyres are considered to provide more grip (= more friction) when surface area has no bearing on the F=uR formula?

I have thought about real world situations rather than just calculations, but i am still unsure of the benefit of wider tyres to grip?

Anyone who knows please post!

thanks, rob

[Lukin]

That equation is just a basic equation used to explain the basics of friction to high school students. It doesnt take into account so many things, the biggest two of which are Surface Area and Temperature. So your hunch is right (it's true with most things, go with your gut feeling not an equation).

In relation to F1 tyres, there are many factors that effect the grip. The biggest one probably (I think) is the surface area. Common sense tells you the more rubber on the road the better and it's right!

Van Valkenburgh lists these factors as as the ones that have the most effect on the coefficient of friction:
- Temperature
- Slip Angle (Percent and Angle itself)
- Camber
- Load (this thing confused the hell out of me when I started looking at tyres, it still holds a few questions)

However, this assumes for one tyre. If you have a 6 inch tyre and an 8 inch tyre, the effective coefficient of friction (load/traction) might be the same, but one tyre will have a heap more grip!

Sorry if thats obtuse in places! Cheers

[Lukin]

However, this assumes for one tyre. If you have a 6 inch tyre and an 8 inch tyre, the effective coefficient of friction (load/traction) might be the same, but one tyre will have a heap more grip!

Thats bloody wrong! Sorry

The coefficient of friction (COF) is the traction/load. The COF varies with the tyre. If you have 1000 kg of vertical load a 6 inch tyre might produce might produce, say, 750 kg of traction (COF=0.75) but a 8 inch tyre might produce 800 kg (COF=0. due to the extra contact patch.

So what I said in that quote, is crap!

Michelin used a wider tyre in 2003 to get more front grip, as was widely reported at the time.

[wowf1]

yeah that does help and thanks for the fast reply!

I did kind of suspect that we were being fed a simplified equation (or representation of it) at school. As with most equations it seems!

One thing did confuse me a little about your post though, concerning the last few lines about the 6 inch tyre and the 8 inch one. If their coefficient of friction is the same, and R is also the same, surely they should have the same grip?

Or does it all change when you corner and you get grip and re-grip of the tyre? (not sure if you read that racecar-engineering article with the stepped graph?)

thanks, rob

[StiK]

The area is implicit in the F=uR formula because F=Pressure*Area.
Also saying that a wider tire will produce more grip isn't necessarily right.
It's far more complicated! The explanations that Van Valkenburgh gives are far to simplistic and even today the mechanism of grip producing isn't totally understood. A more complicated, mathematical and precise explanation is given in Race Car Vehicle Dynamics!

[Lukin]

Considering the level the question was at, the PVV book is a lot less easier to explain to a 'newbie' that quoting chapter and verse from Milliken. It's where I started and it's good for the basics.

Your right, it's much more complex than basic contact patch, but as far as a basic introductions to the mechanics of grip, its a good place to start.

The first few chapters of Milliken and tyre dynamics are bloody complex and takes a fair bit of re-reading. I heard about a good book that bases itself around the Pacejka's Model that has some good non-dimensional modelling.

[red300zx99x]

ever look at tire load sensitivity graphs comparing 1 tire width to another. I haven't, but my gut feeling is that you will find a better understanding there or in a number of other types of tire graphs.

[Lukin]

Here is a link for some Formula Ford tyres, fronts (6 inch) and rear (10 inch)

http://members.iinet.net.au/~bushfam/ja ... GRAPHS.XLS

As you can see on the cornering forces vs slip angle for different loads, the rear tyres generate around 40% more cornering force than the front tyres at the same pressure and camber.

Im assuming the cornering force is the available traction (for braking and cornerning).

Anyway, hope that helps. As Stik said, its tip of the ice berg, but everyone has to start somewhere.

Cheers. Lukin.

[StiK]

If the left column is the front tire and the right column is the rear one, than the cornering forces produced by the rear and front tires with the same load is almost the same. For the front tire the loads are 100, 200, 300Kg and for the rear tire are 200, 350, 500Kg!
One thing that i wasn´t expecting is the high slip angle that this tires need to creat large cornering forces!

StiK

[Lukin]

Damn

I didnt even pick that up! Im confused now. Of what point are wider tyres when there isnt an increase in cornering performance?

My understanding was that the rear tyres on open wheeled cars were due to the rear-biased weight distribution? I thought that it was ideal to keep the ratio of lateral force to vertical force at the front and rear the same to try and gain a nuetral balance?

Would the front wheels have smaller slip angles than the rear wheels to account for the similar coerning loads?

Sh*t!

Also, the tyre model of lapsim shows there is only a small decline in traction at 10 degrees compared to the optimal slip of around 6-8.

[StiK]

I´ve read the first chapters of Race Car Vehicle Dynamics, but that was so boring that meanwhile i´ve forgotten much of it! Also I´m having a Aerodynamics test on friday and haven´t got the time and the will to re-read that first few chapters!
Peter Wright in the book Formula1 Technology talks a little bit about tires and explains some of the reasons why wider tires are used at the rear.

[Lukin]

I've read a fair bit of Milliken and the tyre stuff was mostly in one ear and out the other. I will ask in another forum as they are all on top of this engineering stuff!

Good luck on Friday!

[red300zx99x]

http://eng-tips.com/viewthread.cfm?qid=102250&page=1

[Guest]

A wider tyre can have more grip because the rubber can be made softer, that is the main advantage with wide tires.

[Lukin]

A wider tyre can have more grip because the rubber can be made softer, that is the main advantage with wide tires.

Every racing series I know has same compound front and rear tyres which makes it redundant having wider tyres. I still reckon it's due to equalising the available traction from each end of rear weight biased cars.

I do believe that wider tyres provide greater lateral grip for any given vertical load (Greg Locock said it in aforementioned thread; and he knows what he is on about).

I still wanna find out the a more iron clad reason for it, but Im in the middle of exams. Afterwards I will be getting Milliken and working through it.