Wider tyre, bigger contact patch

[Tony]

The general understanding is that the contact of our tyres on the road is roughly the same whether on narrow or wider tyres. This is because the weight applied on them is the same.

 

But as a result, narrower tyres have greater contact front-to-back (see picture), while wider tyres have less (see right picture). So narrower tyres actually allow our cars to accelerate better in a straight line.

 

Don't you think your diagram of the tyre footprint looks under-inflated for the narrower one? With proper inflation, having such a contact patch would mean accelerated wear.

 

The way to increase contact patch with the road with the same size tyre, will be to deflate your tyre.

 

Food for thought, Pressure=Force/Area , Weight=Mass x Gravity (acceleration) = force.

Having narrow tyres with the same weight, and properly inflated tyres, will just mean the car exerts greater pressure on the road.

E.g. Taking a pin, and a tooth pick, assuming of the same weight and pressing with the same force on them onto table. The pin will have a larger surface area in contact with the table than the tooth pick?

 

Taking the convention of front/back contact as length, and tyre width as width: Having a larger contact patch due to increase in length, wouldn't mean you are able to transmit more forward movement. However, with a wider tyre width to increase contact patch, it would translate to more forward motion.

 

Let's not go into the specifics of minute changes. Let's just say that having wider tyres will lessen the likelihood of wheel-spin, and, tyre width should increase in conjunction with power of the car.

 

2 examples:

 

Let's put the same car with same power, for e.g. a typical WRX - race these 2 cars - one with 215 width, one with 155 width (motorcycle tyre width). Which do you figure would accelerate faster? My assumption is that the one with very narrow tyres would be wheel-spinning away, trying to launch at the same rpm from standstill as the one with 215...

 

On the other hand, having an underpowered car with extremely wide tyres, you would be facing greater resistance due to increased width (contact patch), an also a greater weight disadvantage from the heavier tyres.

 

Highlighting that the width of the tyres should be matched ideally to the power of the car for greater effect, say 225 is better than 215, or 235 is better than 215 for cars like the WRX.

 

 

The example using snow - is totally irrelevant to what I'm saying. There are other rationale for using narrow tyres on snow. An ice skater would have better control with blades, rather than a wide piece of skates. The key is to create a higher pressure from the smaller surface area (due to blade having very narrow width). Under high pressure, ice melts to form a layer of water immediately beneath the blades. This allows the skater to 'dig in' to the ice. They are not really "aqua-planing" when skating...they do have a certain amount of control. Less "drifting".

 

Rallying on snow.... wider tyres will be less likely to have the necessary pressure to "dig in" to the snow. It is disadvantaged on 2 grounds - 1st, more drifting, 2nd, it won't melt the snow fast enough to expose more compact snow/ice/hidden tarmac beneath. Wider tyres will mean they still running on loose snow (less dense/compact)

[dazz33]

Would this also be the case in regards to tyre size and sidewall construction.( softer or harder sidewall ).... I.E so the tyre can deform in corners and increase that contact patch or does it stay the same no matter what.?

[Tony]

Good question...... I'm impressed

 

The sidewall height or "aspect ratio" equals the sum regarding the pneumatic slip angle. The slip-angle percentage within the contact patch equals a sum relative to lateral force -V- saturation grip limits..... Now i've got you

 

Now i'll stop playing with your mind..... In real language the sidewall height bears little toward the contact patch size, in fact the contact patch is unaware of the tyre aspect ratio.

[CIH]

even in extreme cases like runflats ?

[Tony]

even in extreme cases like runflats ?

 

Another argument methinks!!... The RFT saturation limits are argumental, i'm still in the experimental stages researching the RFT so currently my opinion is it's pants, but a more ethical paper will follow soon.

[Bazza]

i dont get it

 

why would a narrower tyre have a larger back to front contact path than the wider one ?

[Tony]

i dont get it

 

why would a narrower tyre have a larger back to front contact path than the wider one ?

 

Depends on the drive wheels and power. Point to remember here is the wider tyre -V- the actual contact patch within the scrub radius is minute compared to the actual width measurement?..... Most people think another 20mm width means another 20mm contact, this is not the case. Between the camber and the kingpin we establish the scrub radius, this is where all the cars weight is concentrated, you can add 50mm either side but it will not add to the scrub radius, and as said additional width can be detrimental to performance.

[HighlandPete]

..... you can add 50mm either side but it will not add to the scrub radius, and as said additional width can be detrimental to performance.

 

This should be a good topic and discussion, as so many think big/wide is always better.

 

On the reference to RFTs and the stiff sidewalls having a different influence, will be interested in your observations.

 

One thing to think about, comparing more supple sidewalls to RFTs, is the distortion of a tyre during acceleration, cornering and the impact on the contact patch. I sense from having inked prints under tyres, that the contact patch for a RFT is even shorter in some dynamic situations than normal tyres, as statically it is so. Then pressure setting is more critical as RFT sidewalls can take/transfer more of the load, than a more supple tyre.

 

Good topic.

 

HighlandPete

[Tony]

Normal tyres apply a slip angle to the lateral saturation grip limits, this is the transition of longitudinal grip to lateral grip. The RFT does transition but does not communicate the grip limit meaning saturation is without warning.

 

Personally i'm at early stages with the RFT contact patch -V- normal tyres because the denial of the slip angle opens a completely new set of values yet to be researched.

[Sagitar]

This should be a good topic and discussion, as so many think big/wide is always better.

 

In the midst of all this esoteric stuff it is easy to forget the simple classical equation for the force to overcome friction between two surfaces in contact.

 

It is:

 

F (the force required to overcome friction) = W (the load perpendicular to the contact plane) x mu (the characteristic coefficient of friction for the two surfaces in contact).

 

The key thing to note is that this equation need take no account of the size of the contact patch. The unit pressure resulting from W is inversely proportional to the size of the contact patch and so e.g. doubling the size of the contact patch halves the unit pressure and they cancel each other leaving F unchanged.

 

The force resisting sliding between tyre and road does not result entirely from simple friction between two plane surfaces.

 

There is some mechanical interference between the elastic material of the tyre and the conformation of the road surface. The extent to which the tyre material distorts into the interstices of the road surface will depend upon a complex set of variables but there comes a point where increasing the area of the contact patch any further will reduce the unit pressure at the contact surface to a degree that will compromise the level of mechanical interference between tyre and road.

 

Under wet conditions, the effect of movement of the tyre over the road surface will generate hydrodynamic pressure in the fluid film between tyre and road. The unit pressure generated is a function of the rate of movement between the two surfaces. The total separating force is equal to the unit pressure multiplied by the contact patch area. So increasing the contact patch size will encourage the onset of separation between the surfaces and the consequent reduction in coefficient of friction. The effect is largely mitigated by grooving the tyre surface to allow leakage of the hydrodynamic pressure, which is why racing drivers put on "wets".

 

Enough has already been said about tyre pressures and load carrying so I will limit myself to just saying that there is a nice balance between contact patch size and the tyre pressure necessary to carry the axle load and the effects that this has on comfort and the ability of the side walls to deal with variable lateral forces created by road surface features parallel or close to parallel to the axis of travel. It's also arguable that more rubber in a wider tyre spreads the wear, but that's a complex argument too.

 

There is a case for a bigger static contact patch for high performance motoring because of the increase in the statistical probability that some part of the patch will remain in contact with the road when the tyre itself is subject to the flexing and oscillations resulting high speed motion and changes of direction. Here too, when wear is less of an issue, it is possible to use exotic materials such as high hysteresis elastomers to give high levels of mechanical interference with low unit pressures or to use aerodynamic assistance to increase the value of W. Conversely, a bigger tyre means greater unsprung weight and a lower natural frequency for the unsprung components which will decrease the statistical probability etc.

 

On balance, it seems to me that for domestic motoring, wider is almost never better, though it may be just as good, and that low profile tyres make little practical contribution other than to fashion and fancy.

 

But if wide, low profile tyres are what floats your boat, then hey, it's your money to spend as you like . . . . . .

[Sam@TDi]

Sagitar that's spot on, as usual

 

As Sagitar has eluded to, the chemical make up of the tyre is a huge variable determining the exact exchange mechanisms by which "grip" is generated, and without factoring it in whilst you're thinking you can't really work with this subject.

[CIH]

Sagitar that's spot on, as usual

 

As Sagitar has eluded to, the chemical make up of the tyre is a huge variable determining the exact exchange mechanisms by which "grip" is generated, and without factoring it in whilst you're thinking you can't really work with this subject.

 

Tricky to calculate as, presumeably, such things are considered trade secrets ?

[Sam@TDi]

Absolutely correct, in fact it's near impossible to model accurately even if you are privy to all the required information.

 

It's for that reason that you'll find tyre testing rigs at all of the tyre manufacturers research bases, whats more tyre testing rigs are being carted about by all the top NASCAR, F1, ALMS teams etc. The chassis engineers simply test the tyres on a simulated appropriate surface and then map out the force responses in the form of an industry standard carpet graph, they map out the tyre as completely as possible, THEN they decide how best to exploit the tyre to achieve their goals.

 

Working without accurate tyre data necessitates extensive real world experience (and/or data) with the tyre, or at least tyres of similar design and chemistry.

 

If you find yourself with neither accurate tyre data or any real world experience with the tyre then you'll find you need a great deal of luck.

[CIH]

"I'd rather bt lucky than good"

 

Reminds me of the success Nissan enjoyed in the late 90s with the BTCC Primeras. Alot of it was put down to them establishing an unusually close relationship with IIRC Micehelin, who suplied most of the teams at the time.