A really dumb question about grip

I was at a school camp when I was about 14 or 15. A small group of us were talking to one of the camp leaders about cars (what else?) when we got on to the subject of grip.

The camp leader, a guy in his late 30’s by my estimation, stated quite convincingly that skinnier tyres were more conducive to better grip because they transferred the weight of the vehicle to the road via a smaller contact patch. A fatter tyre had to distribute that weight over a larger contact patch and so would have had a smaller amount of pressure-per-square-inch (or centimetre).

Now, I know that wider tyres are more conducive to better grip. I know it from personal experience and simply by observing the wheels and tyres fitted to vehicles that are designed primarily for high performance.

And yet the simple logic of his argument still sticks with me to this day.

Has anyone got a simple, layman’s terms explanation as to why his argument doesn’t work? Why do fatter tyres offer more grip than skinny tyres when the pressure per-unit-of-area on the contact patch would be greater for the smaller tyre?

TedY, you’re the physics guy. Your time to shine.

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31 Comments

  1. Paraphrasing from the Internets. Haha

    Tire width has no direct relation to the amount of grip generated; it is a secondary factor, and the width basically relates to cooling potential and so the Tire compound that can be used. The size of the contact patch has no bearing on the amount of grip generated at all, apart from the extreme of where the compound is getting so hot that it no longer acts as a solid (and therefore doesn’t follow Amonton’s Law). The Tire pressure has no direct bearing on the level of grip (apart from aquaplaning), but it does have a bearing on the heating and cooling characteristics of the Tire. Having a lower Tire profile gives improved handling through reduced sidewall stress and improved contact patch shape stability.

    1. So which part of the tyre does have something to do with grip – width doesn’t, pressure doesn’t, contact patch doesn’t – what does?

      1. I think your camp leader’s logic would apply to tyres designed for adverse conditions. Echoing other’s comments, tyres designed specifically for winter conditions are usually thinner and have a M+S (= mud and snow) rating stamped on their sidewall. The narrower width should allow the weight of the car to bear down on loose, wet or slippery surfaces, affording more traction in these conditions. It’s also correct that the tread pattern plays an important role in water dispersal. Winter tyres as you’d know may be studded for increased bite.

        Conversely, dune buggies have fat tyres to spread load on soft sand.

        Rubber compound is another factor as it affects the rate at which the tyre warms up and consequentlythe way it moulds itself to the contour of the road surface.

        Slightly off topic but related is the fact that early (2-stroke) Saabs were fitted with freewheel because of engine-lubrication issues on a trailing throttle. The freewheel capability was, however, retained into the V4 era because of its apparent benefit in winter driving. If the car goes into an (understeer) slide on a slippery road, the driver was advised to take his foot off both accelerator and brake (ie employ freewheel), point the steering in the intended direction of travel and the car should come out of the slide. That takes some nerve to do.

        One Tasmanian winter in the mid 90’s I had occasion to put the theory to the test: I was driving a patient from the West Coast to Hobart for a doctor’s appointment. The car was a front wheel drive 4 cylinder, a Toyota Camry I think. I was travelling pretty slowly on a long descent, anticipating ice. Well in advance of an approaching right hand bend I began gentle braking + engine braking but the car went into an understeer slide and was heading for a ditch on the opposite side of the road. Beyond the ditch was a solid rock face. Remembering the above advice I put the car in neutral (to emulate freewheel), took my foot off the brake and aimed the steering wheel in the direction of travel. To my utter surprise, almost on the apex the car came back under control and we safely negotiated the bend. Something I won’t forget in a hurry.

        Of course any Scandinavians or Canadians reading this are probably having a good old chuckle at the simplicity of it all but it was quite a revelation to me at the time.

      2. Sapan’s essentially got it here. Here’s my recollection from high school physics. We tested coefficient of friction with a spring scale pulling an object across a desk to measure the amount of force required to move the object. When the footprint of the object was changed, but the weight was the same, the force required to move the object was unchanged: a larger foorprint spread the weight but had more surface for drag, and one cancelled the other.

        In the real world, with tires, there are so many other factors. I bet that on a glass smooth surface with slick tires and sidewalls with no deflection and no suspension, the tire width might indeed be irrelevant, but certainly the narrower tire would have no advantage. More to the real world, and there are so many things to consider. I would disagree, beyond a hypothetical situation, that pressure makes no difference. Take a care equipped with modest tires, and you can influence the under/oversteer characteristics fairly dramatically with tire pressure manipulations. Then there is the whole notion of “stiction.”

        Back to the camp counselor statement–according to his premise and basic physics, tire width should have no impact one way or the other.

  2. Friction = Grip

    The more area touching the road, the more friction, so therefore better grip.

    When I go 4wd’ing you often let the pressure down on the tyres, this is done so you have more contact with the ground, and less weight per square centimetre of ground.

    Hope this helps

    1. Was thinking of an experiment.

      Try pushing your finger as hard as you can down on the desk, then try dragging it accross the desk. It should slide along.

      Now put the palm of your hand on the desk and push down with a bit less force. You’re hand shouldn’t slide as easy as your finger.

    2. Now that’s a theory that makes sense, even to a dumbass like me!

      Tried the experiment, didn’t work so good. I liked the theory better when I didn’t have to back it up with an action 🙂 My palm was going all over the place, singing “try again, fool!”

      It still makes sense in my mind, though.

  3. Well the skinny tire theory works in one case….look at Rally cars driving in snow….skinny tires to push the weight of the car thru the loose snow to where they can find grip….

    1. Like a sharp knife cuts…

      The thing in snow is this: a wider tyre floats above the snow, which is good in very, very deep snow. A skinny tyre pushes (or tries to push) the snow to its sides, quite possibly allowing contact with the pavement, dirt, gravel or, at least, compacted snow below the the fluffier surface snow.

  4. Let’s look at two tyres, one skinny and one fat; both slicks.
    F1 cars have skinnies to steer, and fatties to propel.
    Now, treaded tyres… (Treads come to life when deformed)
    WRC cars use skinnies with complicated tread or spikes in the snow, and fatties with little tread patterning on tarmac.
    So it stands to reason that in most cases, skinnies are preferred for steering and fatties for propulsion (remember “in most cases”). You want constant grip (hook up) with the tyres that propel; usually fatties, and you want the steering tyres to be nimble, not dragging; usually skinny. ie. staggered tyre setup on higher end RWD cars.
    Now, can someone come up with a solution with the above data?

  5. Bigger contact patch = more friction = more grip. It therefore means that the car can “lean” on the fatter tyre more before it loses grip (body roll). This weight transfer puts more pressure on the outer tyre, increasing the contact patch, and increasing friction more than a skinny tyre could cope with, assuming the same compound, tread and conditions…but there is always a point where the tyre stops generating enough friction to overcome the g-force that the car exerts on it by trying to continue in a straight line (momentum etc).

    Now the other factor for road cars is tread pattern. In order to dispel water effectively you need a nice open tread pattern. This reduces contact patch and therefore a wider tyre helps you achieve good water displacement AND a good contact patch.

    In snow, you need skinny tyres to dig into the snow and increase the pressure rather than fat tyres that skate over the top.

    Downside of fat tyres – tramlining, vague steering (on FWD especially) and unsprung weight.

    A 96 works best with skinny tyres because it rolls too much for fat, if you give it too much grip it will pa taket. A 9-5 works best with wider tyres because the suspension geometry and roll centre is so different. And it weighs twice as much…

  6. Talking of tyres, there’s no such thing as ‘grip’ 😉 As Brendan says, it’s about friction, mass and surface area. Different combinations give better results under different circumstances. Everything is a compromise.
    Some extremes:
    o F1 car generate huge levels of ‘grip’ via downforce, which increases ‘mass’
    o A tracked vehicle spreads it’s weight so that it doesn’t sink into soft ground but has loads of friction
    o A racing bicycle has skinny tyres to keep friction down
    And so on…
    Anyways, here is the scientific proof, courtesy of Chris Harris: http://www.youtube.com/watch?v=HPh90yNX-mY

  7. F=μR

    More friction equals more grip, as Brendan said.

    Trouble is the ‘idea’ we have of Grip is not really what the Mathematical idea is. For a body to slide on a surface the μ value suddenly drops, as it is the coefficient of friction(or drag in this case). If you are on ice, then μ is really small. Near zero. On perfect bitumen it can be near 1.

    If the car tyre starts to slip in a lateral direction and stop the back of the car sliding out (not something one gets very often in a SAAB btw…) then the μ reaches threshold value proportional to the surface characteristics. If there are ‘marbles’ on the track this affects things greatly, and oversteer results.

    If the tyre is hot, slicked and surface is dry then the stickyness factor of the tyre improves the μ value to greater levels. A wider tyre in this situation then allows more sticky surface to meet the road, in a lateral manner to contradict the centrifugal forces at work around the axel concerned.If you have ever driven a go kart hard and stopped. the thing literally gets stuck to the road and you have to ‘pop’ it to get it moving again. Hot rubber generated by lots of lateral friction under high cornering loads, melting the rubber to a near glue situation.

    Get the car to its limits too and hope like heck the μ value stays high for you, and a wider tyre will help out. A skinnier tyre will grip to its maximum under lateral forces applied and the μ value will be reached earlier than with the wider tyre.

    The interaction between track and ambient temperature, tyre pressures, side wall profile ratios and more importantly spring rates and damper rebound settings, all go into the mix in a multi variable setting. Change one variable and they all affect the system differently. Which is the dark art of setting up a decent race car, just managing all that.

    A really skinny tyre on good bitumen at high speed under high lateral cornering forces will do well, up to a point. The wider tyre has more lateral resistance possibility in this situation (where we want the car to feel like it is in fact Grippy) and potentially a higher μ threshold.

    Look at the F5000 cars. Massive rear tyres. Today’s F1 cars are arguably under-tyred, and a good thing too. That way we see more crashes and the entertainment value goes up.

    It’s all in the μ.

  8. Swade,

    Here are your answers:
    http://autospeed.com/cms/A_108915/printArticle.html

    Short version:
    Contact patch size is a function of tyre pressure and vehicle weight, and nothing else.

    Wider tyres generate less heat. That’s why they are used on cars with higher top speeds and/or weight.

    I used to be on a (non-automotive) mailing list that had an F1 engineer as a member. His golden rule was “always use the lowest tyre pressure that you can.”

    1. I forgot to directly answer your question.
      “Why do fatter tyres offer more grip?”

      They don’t per-se, all things being equal.
      That being said fatter tyres heat-up less, and this allows you to run a softer compound and/or lower pressures. Softer compounds generally have more friction, and lower pressures have a larger contact patch, either of which will generate more grip on dry roads.

  9. Check out Nissans strange “DeltaWing” they entered in this years LeMans. It has two “skinny” tires mounted very narrow in the front giving the car a very sharp nose. A Bat-mobile for the race track… 🙂
    I have no idea how that car works in corners, but evidently it works, and works good. Pity it was pushed off the track in the race.
    http://en.wikipedia.org/wiki/DeltaWing

  10. Yes, all of the above have it correct — friction is the crucial element. So for tires with the same formulation (identical compounds, tread, etc.), the one with the greater contact patch wins.

    The ‘skinny tire’ argument is so compelling because the ‘pressure per square inch’ principle is sound for so many other applications where only vertical force matters — cutting, structural supports, the cylinders in our engines. It’s also a portion of the reality for tires. However, for tires, lateral forces are so important that other measures begin to dominate the equation.

  11. Most of it has been said.

    There is the big myth that wider tyres “put more rubber on the road”. They don’t. All they do is make the contact patch shorter and wider. To keep the overall diameter the same as the original, the tyre will be low profile with shorter sidewalls. This will help keep the contact patch flatter and more in contact with the road. It will make steering more predictable and it may improve roadholding, but only insofar as a tyre that rolls over the edge of the tread increases distortion and adds heat which won’t help friction.

    The different width of tyres has effects in different situations. A narrow tyre is possibly best in snow and rain, as it’s clearing less material to get the tread on the road. That’s all.

    If there is no hard surface below (mud and sand) then reducing tyre pressures will increase the size of the contact patch and reduce pressure on the ground. This will help to prevent the tyre digging in to soft ground. That’s all. Nothing to do with increased grip.

    I think that wide tyres and performance is also largely a myth too. Performance cars fitted bigger wheels to get them over bigger brakes. That led to the development of lower profile tyres to keep the overall wheel and tyre size within the wheel arch. Then people found that you needed more width to prevent too much sidewall flexing. Then the idea that the lower wider tyres were the performance element, rather than the brakes, became common. No reason why, but tyre marketeers have been loving it ever since.

    It’s all engineering really.

    The only other thing to add to all this is that on a stable rough surface, there will be a measure of deformation and the shear strength of the rubber to add to the pure friction.

  12. I see you already have some good answers, but since you asked, I (Ted Y) will add my simple 2 cents here:
    The classic approximation for ideal materials behaving in a linear fashion is:
    F= μ x mass of object x g
    where μ is the coefficient of static friction, g is the gravitational constant, and F is the tangential force required to move the object. For ideal materials, the approximation holds pretty well up to a point, and μ is independent of surface area. For example, if a tire patch area happens to be twice as large as another, each square inch of that area will only require half as much force to make that square inch of the object (tire) slide, however, there will be twice as many square inches contributing to the total force required to make the tire slide so it makes no difference.

    However, under heavy stressful loads, the rubber behaves differently and the coefficient becomes smaller for various reasons. For one, rubber molecules can start scrubbing loose, and the rubber will deform under heavy load, temperature affects μ, etc. The larger tire, with a larger patch area will not be affected as early as the more heavily stressed skinny tire.

    Actually, the tire patch area is pretty much independent of tire size. It’s mostly just a function of tire pressure and body weight. Roughly, area = weight / pressure. However, you can use lower pressures in larger tires to increase the patch area and reduce the stresses on the contacting rubber, so the larger tire will perform better under stress, on smooth hard surfaces anyway. That all changes on snow, however, Saab used tires almost as skinny as bicycle tires in winter racing. I guess the skinny tires cut down into the snow better and could hold a corner better.

    Well, that’s how I see it, but I’m sure there are better, more thorough explanations.

  13. So, Swade….it was not such a dumb question after all! 🙂

    As the others have stated, there are many factors involved combining the formulation of the tire compound, temperature, tire pressure, vehicle weight, road conditions, etc. And that is one reason why there are so many types of tires available.

  14. Something I react to with fat tires is:
    * Higher sound->less comfy ride
    * More problem keeping a straight line on bad/tracked tarmac-> less comfy
    * More vibrations and more bumpy ride on anything other than silk tarmac->less comfy

    On the originaly mounted winter tires on my xwd 9-5 the grip was much worse on loose snow roads than the fwd 9-3 I used to drive before.

    So, everything boils down to priority. If you need that extea grip and dont care about the ride. Go for the fattest there are, but usually an everyday car or a long drive car, the comfyness is important. At least for me.

    My next set of tires will be skinnier

  15. I think the thing the pressure per given footprint conversation neglects is that downward pressure isn’t actually terribly important to determining lateral grip. If it were, heavy trucks would be the best cornering vehicles simply because they had the most “grip” (coming from contact point pressure).

    And that’s wherein lies the issue. “Grip”, as defined as something to increase by us car guys, is actually the amount of resistance to lateral slippage. Although, strictly speaking, not part of “grip”, the ability of a vehicle to transmit power to the ground without slipping also gets lumped into this idea of “grip”.

    A wider tyre is more resistant to lateral slippage than is a narrower tyre because the wider contact patch allows the transference of weight from side to side to be spread out across a wider area. Similarly, a narrower sidewall helps keep the contact patch engaged with the ground for a longer period of time because it deflects less than a tall sidewall. Finally, a tyre with a narrow sidewall will lose adhesion less dramatically than one with a tall sidewall because the tall sidewall acts as a sort of energy storage device. As the sidewall deflects, it absorbs kinetic energy. When grip is lost, that kinetic energy is released and the springing action can dramatically upset other suspension and steering components already under stress.

    Of course, a higher sidewall’s ability to absorb energy actually results in a smoother riding tyre. Narrow sidewalls as, by design, very stiff, so as to prevent damage to the wheel and to prevent the sidewall from deflecting enough to break the bead. Taller sidewalls are softer and, along with their ability to absorb energy, can provide for a softer ride. The “floaty” feeling often associated with American cars of a certain vintage is as much the result of tyre design as it is of suspension design.

    Just my 2¢…

  16. Well, it’s not a stupid question at all.

    I seen a lot of answers to this question over years, and most are wrong.

    Actually, according to classic mechanics wider tires shouldn’t matter, because the friction coefficient is supposed to be a constant, i.e. the bigger area means lower force per area giving a constant sum. This actually is true, or close to, for most cases. For flat areas. But it doesn’t apply that well to tires.

    Next thing, as pointed out above, with spikes on ice rally, smaller area is beneficial as that increases the depth into the ice of the spikes which gives a better vertical area for the spikes to use.

    On tarmac, smaller area doesn’t give any more depth to hold on to but on coarse road, the opposite, more area gives rubber tire more vertical area to cling on to. However, that in itself doesn’t necessary give more than marginally more grip as the force downwards is lower as well if it wasn’t for the fact that rubber sticks a bit, i.e. an adhesive effect, even at near zero pressure, coarse road or not. (In the dry, that is.) This sticky property is very obvious on competition go-cart tires. Or differently put, the friction coefficient of tire rubber isn’t constant, it decreases with higher pressure.

    The very same sticky effect also gives higher losses, and thus fuel consumption. That’s why the top speed decreases (slightly) with wider tires, and eco tires has worse grip.

  17. By “Grip” do you mean traction. Traction is dependent on the surface and weather conditions. That’s why there are snow tires. Racing slicks have the most traction on say formula 1 surfaces. And, Michelin might say all “rubber” tires are not created equal. An American politician might say “grip” is in the compound stupid. Some time on http://www.tirerack.com might convince you that “grip” varies widely even on tires that are the same size. So, give me a Michelin Pilot Super Sport 225/45R17 over a Goodyear Assurance Fuel Max 205/55R16 any day. You can look it up.

  18. Bigger tires work better because they look cooler. End Of Story.
    It’s just like an airplane — if it looks good, it flies good.
    But if the tires are too big — think monster truck – they are just stupid.

  19. The coefficient of friction between the rubber and the ground decreases with increased contact pressure. This is one factor with narrow tires.

  20. I worked for The City Water Dept. In the winter, it was still necessary to drive out to and in the well fields. The boss had a bigger is better mentality. He fitted the 4wd truck with big ol fat knobby tires. It had very little traction in the snow constantly getting stuck. A tire expert wised him up and installed normal snow tires (not wide) on the truck and it never got stuck again and traversed the snow beautifully. You could see the deep imprint in the snow that those wonderful standard snow tires left behind.

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