# Friction and tire width? **URL:** \ **Category:** Factual Questions\ **Created:** [May 23, 2005, 10:13am UTC](https://boards.straightdope.com/t/friction-and-tire-width/305066 "2005-05-23T10:13:21Z")\ **Posts on this page:** 16\ **Page:** 1
**Author:** ![Dog80](https://avatars.discourse-cdn.com/v4/letter/d/6bbea6/32.png) [@Dog80](https://boards.straightdope.com/u/Dog80)\ **Post date:** [May 23, 2005, 10:13am UTC](https://boards.straightdope.com/t/friction-and-tire-width/305066/1 "2005-05-23T10:13:21Z")
IIRC, friction is not related to the contact area between two objects (this [wikipedia article](http://en.wikipedia.org/wiki/Friction) verifies it). So what is the advantage of putting wider tires on a car? ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [May 23, 2005, 10:51am UTC](https://boards.straightdope.com/t/friction-and-tire-width/305066/2 "2005-05-23T10:51:25Z")
My understanding is that the larger contact area provides less force per unit of contact area. Less force means less deformation and breakage of the rubber. ---
**Author:** ![max\_power](https://avatars.discourse-cdn.com/v4/letter/m/db5fbb/32.png) [@max\_power](https://boards.straightdope.com/u/max_power)\ **Post date:** [May 23, 2005, 11:05am UTC](https://boards.straightdope.com/t/friction-and-tire-width/305066/3 "2005-05-23T11:05:05Z")
I think this statement is incorrect: > [@](#): > > It is interesting to note that, contrary to common belief, the force of friction is invariant to the size of the contact area between the two objects The equation given is a first order approximation in which the frictional force is proportional to the normal force. However, I’m pretty sure the real world situation is more complicated than this… witness F1 mandating that “slick” tyres must have grooves to reduce grip. I tried googling but couldn’t find any cites. ---
**Author:** ![Marcus](https://avatars.discourse-cdn.com/v4/letter/m/85e7bf/32.png) [@Marcus](https://boards.straightdope.com/u/Marcus)\ **Post date:** [May 23, 2005, 1:32pm UTC](https://boards.straightdope.com/t/friction-and-tire-width/305066/4 "2005-05-23T13:32:12Z")
Assuming constant air pressure and car weight, wide tyres will have exactly the same contact area as narrow tyres. What differs is the shape of the contact patch - wider and shorter for wide tyres, narrower and longer for normal tyres. Contact patches are an oval shape. I have always understood that wide tyres have greater lateral hold than narrow tyres, allowing for tighter cornering without slipping. In addition, they are less “round”, and will hold flatter to the road (maximising contact patch size) when the car is tilting into corners, and the suspension is trying to keep the car level. ---
**Author:** ![Anachronism](https://avatars.discourse-cdn.com/v4/letter/a/e56c9b/32.png) [@Anachronism](https://boards.straightdope.com/u/Anachronism)\ **Post date:** [May 23, 2005, 3:25pm UTC](https://boards.straightdope.com/t/friction-and-tire-width/305066/5 "2005-05-23T15:25:36Z")
> [@Marcus](#): > > Assuming constant air pressure and car weight, wide tyres will have exactly the same contact area as narrow tyres. This would be true if rubber where infinitly flexible, but it is not and steel belts even less so. The size and shape of the contact patch has a lot to do with the construction of the tire in addition to tire pressure and tire width. Rubber is more or less exempt from the ‘friction is not related to the contact area’ rule. The [first page](http://www.insideracingtechnology.com/tirebkexerpt1.htm) from a ‘rubber friction’ search on google does a decent job of explaining it. If I may attempt to sum up what the page says - Rubber has friction benifits that other materials don’t like deforming to fit the shape of the road surface (Mechanical keying) and adhesion (like tape). ---
**Author:** ![Marcus](https://avatars.discourse-cdn.com/v4/letter/m/85e7bf/32.png) [@Marcus](https://boards.straightdope.com/u/Marcus)\ **Post date:** [May 24, 2005, 1:06pm UTC](https://boards.straightdope.com/t/friction-and-tire-width/305066/6 "2005-05-24T13:06:12Z")
Good point, **Anachronism** , I hadn’t thought of that. My thought experiment was in that ideal world of physics populated by all thought experiments. ---
**Author:** ![rwjefferson](https://avatars.discourse-cdn.com/v4/letter/r/a6a055/32.png) [@rwjefferson](https://boards.straightdope.com/u/rwjefferson)\ **Post date:** [May 24, 2005, 2:20pm UTC](https://boards.straightdope.com/t/friction-and-tire-width/305066/7 "2005-05-24T14:20:39Z")
> [@wikipedia](#): > > The coefficient of friction… It is interesting to note that, contrary to common belief, the force of friction is invariant to the size of the contact area between the two objects. The coefficient of friction may be invariant, but the total friction clearly increases. There is no question that wider tires hold better than thin ones in non-lubricated conditions. Peace through Liberty rwjefferson ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [May 24, 2005, 7:36pm UTC](https://boards.straightdope.com/t/friction-and-tire-width/305066/8 "2005-05-24T19:36:39Z")
If the total friction increases, and the normal force (= the weight of the car) stays the same, then by definition the coefficient of friction has increased. Invariant coefficient of friction is just an approximation for physics textbooks, and in the real world it can be a more or less good approximation, depending on circumstances. In principle, it can depend on the contact area, the contact shape, the speed, the distribution of pressure over the contact area, the ambient temperature, the total normal force, and probably a bunch of other factors. ---
**Author:** ![notsocleverusername](https://avatars.discourse-cdn.com/v4/letter/n/bc8723/32.png) [@notsocleverusername](https://boards.straightdope.com/u/notsocleverusername)\ **Post date:** [May 24, 2005, 8:56pm UTC](https://boards.straightdope.com/t/friction-and-tire-width/305066/9 "2005-05-24T20:56:51Z")
> [@kanicbird](#): > > My understanding is that the larger contact area provides less force per unit of contact area. Less force means less deformation and breakage of the rubber. I think you nailed it…let me see if I can expand on that a little more…bear with me because most of this is from memory. Since a wider tire provides a larger contact area, there is less force acting on each square inch of the tire (versus a thinner tire). And since there is less force, the tire manufacturer can make the tire using softer materials. These softer materials naturally have a higher coefficient of friction, or in simpler terms; it makes the tire “sticky”. This of course makes your car perform better when accelerating, braking, and turning. Now you might ask, “why don’t the tire manufactures used the softer materials on the thin tires”. Well, they could and you would likely see similar perforance… but then the tires would not last as long (remember, a thinner tire has more force acting on each square inch of the contact area, thus the tread would wear out faster). ---
**Author:** ![notsocleverusername](https://avatars.discourse-cdn.com/v4/letter/n/bc8723/32.png) [@notsocleverusername](https://boards.straightdope.com/u/notsocleverusername)\ **Post date:** [May 24, 2005, 9:12pm UTC](https://boards.straightdope.com/t/friction-and-tire-width/305066/10 "2005-05-24T21:12:38Z")
[QUOTE=max power] I think this statement is incorrect: However, I’m pretty sure the real world situation is more complicated than this… witness F1 mandating that “slick” tyres must have grooves to reduce grip. QUOTE] I’m not too familiar with F1 racing rules, but I imagine they would mandate the grooves to keep a level playing field amonst all the teams. Assuming my logic is correct in my previous post, then adding grooves to the F1 tires would not necessarily reduce grip…it may have that effect becuase it forces them to use a harder material in the tire (other wise, if they stuck with the same soft material, the tire would not last as long, forcing them in the pit more often). I can imagine that if they didn’t have that rule, then a team, whose sponsor has really deep pockets, could develope a super sticky, long lasting tire that would allow them to consistantly destroy their competition. So like I said, maybe F1 has that tire rule to keep the competition close. ---
**Author:** ![AskNott](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/asknott/32/5790_2.png) [@AskNott](https://boards.straightdope.com/u/AskNott)\ **Post date:** [May 25, 2005, 3:00pm UTC](https://boards.straightdope.com/t/friction-and-tire-width/305066/11 "2005-05-25T15:00:59Z")
F1 cars use grooved tires, but only when it’s raining or when rain is imminent. If the rain starts during a race, you’ll see cars diving into the pits to change from slicks to rain tires. ---
**Author:** ![coop](https://avatars.discourse-cdn.com/v4/letter/c/87869e/32.png) [@coop](https://boards.straightdope.com/u/coop)\ **Post date:** [May 25, 2005, 3:26pm UTC](https://boards.straightdope.com/t/friction-and-tire-width/305066/12 "2005-05-25T15:26:05Z")
Sorry **AskNott** , but according to [this](http://www.formula1.com/insight/rulesandregs/13/511.html), you are incorrect. > [@](#): > > The dry-weather tyres have four grooves and the spacing and depth of these grooves must conform to strict specifications. :wally ---
**Author:** ![Dog80](https://avatars.discourse-cdn.com/v4/letter/d/6bbea6/32.png) [@Dog80](https://boards.straightdope.com/u/Dog80)\ **Post date:** [May 25, 2005, 9:00pm UTC](https://boards.straightdope.com/t/friction-and-tire-width/305066/13 "2005-05-25T21:00:15Z")
I am no huge F1 fan, but if I recall correctly, FIA’s (Fédération Internationale de l’Automobile, european motorsports governing body) reasoning behind this restriction was that modern F1 cars have (once again[sup]1[/sup]) gotten too fast and therefore safety for drivers and spectators is compromised. Reducing the tire grip (by putting grooves) was seen as a simple measure to force the cars to slow down. The F1 tire makers (Bridgestone and Michelen) devised a dirty trick to get around that regulation. They would make tires with grooves allright, but the tire surface was made by some ultra-soft compound designed to wear out after about a couple of rounds. Beneath that layer was the real tire; a groovless slick providing full traction. 😉 I believe that FIA has amended the regulations, so using this trick is no longer possible. [sup]1[/sup] Turbochargers in F1 were banned by FIA during the 1980’s for the same reason ---
**Author:** ![brad\_d](https://avatars.discourse-cdn.com/v4/letter/b/50afbb/32.png) [@brad\_d](https://boards.straightdope.com/u/brad_d)\ **Post date:** [May 26, 2005, 12:56am UTC](https://boards.straightdope.com/t/friction-and-tire-width/305066/14 "2005-05-26T00:56:04Z")
F1 dry tires are required to have grooves now - this has been the case since the late 90’s. [Here](http://www.grandprix.com/jpeg/phc/pmon05/sun/webber1-lg.jpg) is a photo taken at last Sunday’s (dry) Monaco GP. 2005’s go-slow rule for tires has been to require that a single set last for the entire race; except for a few circumstances, tire changes during a race are no longer allowed\*. The idea is to force Michelin and Bridgestone to use harder, less grippy (but more durable) rubber compounds. Some teams/cars are dealing with it better than others. Ferrari’s Bridgestone issues notwithstanding, two Michelin-shod teams (McLaren and Renault) displayed rather opposite success in saving their tires last week. Race winner Kimi Raikkonen’s McLaren looked like it could have gone another race distance on its tires, while Fernando Alonso was having trouble keeping his Renault on the road by the end 'cause his tires were _shot_. **Dog80** , you’re correct; multiple-compound tires are illegal now in F1. - - The rules are written in an attempt to permit tire changes in the event of a flat or something like that, but to disallow them in normal circumstances. And they seem to have _basically_ succeeded. ---
**Author:** ![AskNott](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/asknott/32/5790_2.png) [@AskNott](https://boards.straightdope.com/u/AskNott)\ **Post date:** [May 26, 2005, 3:13pm UTC](https://boards.straightdope.com/t/friction-and-tire-width/305066/15 "2005-05-26T15:13:36Z")
\*\*Coop,\*\*you could have simply shown me I was wrong without being so rude. ---
**Author:** ![samclem](https://avatars.discourse-cdn.com/v4/letter/s/a9a28c/32.png) [@samclem](https://boards.straightdope.com/u/samclem)\ **Post date:** [May 27, 2005, 12:43am UTC](https://boards.straightdope.com/t/friction-and-tire-width/305066/16 "2005-05-27T00:43:03Z")
**Coop**. We try to avoid using the putz smiley in forums outside of the Pit. Especially if you’re aiming it at another poster on the Board. You’re fairly new, so no big deal. Now you knowl. **samclem** GQ moderator