Contact patch

[Q45tech]

"“This is the main reason why hot pressure is so critical,” Forthofer says. “The contact patch area is roughly the load on the tire divided by the air pressure (actually about 90 percent of that, but it varies with tire construction). For instance, a tire with 500 pounds of load on it at 35 psi will have approximately 14.3 square inches of contact patch area. If we increase that pressure to 36 psi at the same load, the area becomes 13.9 square inches, which is a reduction of 2.8 percent. From this information, you would think that lower pressure is always better. That is true only until the tire does not have enough pressure in it to support a stable contact patch. Once you go below that pressure, the contact patch gets distorted and very inefficient, which causes the corner speeds to slow down. There is also the concern of sidewall flex. Going too low can be risky for the sidewalls, and the manufacturer should be consulted for the minimum pressures to prevent this.”

http://www.valvolinecup.com/Ne...19274

Always on the look out for quotes to help people understand that larger tires don't necessarily have a larger [AREA] contact patch............

[Nismo_Freak]

Always on the look out for quotes to help people understand that larger tires don't necessarily have a larger [AREA] contact patch............

Assume we have a 0 toe, 0 camber in a fully laden situation with no bump in suspension geometry.

Take this as an ideal launching point, weight has already transfered and the suspension has zeroed out, and is no longer in travel.

Assuming we have 800 lbs. of load on each of the rear tires at this point, ceteris paribus.

Would a 275/40/17 tire of the same make/model have more traction than a 245/45/17 tire. Difference in sidewall being 0.25mm which makes them assumingly the same in terms of sidewall flex.

Why would it provide more/less/equal traction in this situation.

My only guess is that the contact patch does not change in area because of the way load is distributed over the tire. Wider tire has more area to distribute load so the contact patch distorts to form more of a rectangle than a square, however when more load is applied, you are still working with a round object, the 275 in my opinion would gain area faster than the 245. Am I right in assuming this or is it more along the lines of the 245 will gain area as fast as the 275 but the 275 in the end offers more usable area.

Area meaning the contact patch.

[Exar-Kun]

I'm going to roll that article into the tire FAQ...thanks dennis!-chet

[C-Kwik]

While this is good info to consider, it is also important to consider heat distribution across the tire's tread. The wrong pressure can easily overload a portion of the tread with too much heat and cause poor handling as the tires get hot. But it is important to consider the tire's hot pressure as racing generates much more heat and pressures can jump significantly.

[Nismo_Freak]

While this is good info to consider, it is also important to consider heat distribution across the tire's tread. The wrong pressure can easily overload a portion of the tread with too much heat and cause poor handling as the tires get hot. But it is important to consider the tire's hot pressure as racing generates much more heat and pressures can jump significantly.

Heat distribution is not a factor in this case... all is ceteris paribus.

Plus we don't have any alignment settings causing excessive build-up of head in any one part of the tire. Nor is there cornering load to apply heat on the outside edge.

But yes, heat plays a large factor in getting the full contact patch down. Pyrometer's RULE!

[C-Kwik]

As much as I like discussing the little things, I also think it's important to see the big picture as well. Teh reason I mentioned it is because many may look at contact patch only while not putting the right focus on what happens in a turn. And unfortunately, all things rarely ever remain the same(ceteris paribus).

[Q45tech]

The point was to remind people that larger wider tires don't have a larger bigger [in area ] contact patch.

Every story and ad you see seems to promote that untrue idea.

[aither]

What you and the article are saying makes sense. However, I'm still wondering why I have continued to notice that wider tires on wider wheels, with everything else being the same, seem to provide more grip when cornering. The only analogy I have been able to think of is to take a rubber soled shoe. Try sliding it from the side, then try sliding it from the end. When there is more rubber lined up with the force vector, there seems to be more resistance to that force (friction, I assume). I'm still not quite sure that wide tires to a better job of providing friction when cornering than narrower tires provide.

[Nismo_Freak]

What you and the article are saying makes sense. However, I'm still wondering why I have continued to notice that wider tires on wider wheels, with everything else being the same, seem to provide more grip when cornering. The only analogy I have been able to think of is to take a rubber soled shoe. Try sliding it from the side, then try sliding it from the end. When there is more rubber lined up with the force vector, there seems to be more resistance to that force (friction, I assume). I'm still not quite sure that wide tires to a better job of providing friction when cornering than narrower tires provide.

It goes back to what I was saying...

What I pretty much gathered is that when the tire is rolling down the road it will have the same contact patch as a smaller tire, just in a more rectangular shape. It's when weight is transfered to the larger tire it gains contact patch roughly at the same rate as the smaller tire but since the smaller tire is indeed round you have only so much gain in contact patch. The wider tire in the end will have more grip simply because it has more surface area to be applied.

[candela]

gosh I love reading this stuff...

[aither]

It goes back to what I was saying...

What I pretty much gathered is that when the tire is rolling down the road it will have the same contact patch as a smaller tire, just in a more rectangular shape. It's when weight is transfered to the larger tire it gains contact patch roughly at the same rate as the smaller tire but since the smaller tire is indeed round you have only so much gain in contact patch. The wider tire in the end will have more grip simply because it has more surface area to be applied.

I've been thinking about that as well, but I'm wondering if there is something more to the question of grip. It is still a good explaination.

[crzycav86]

I was thinking that wider contact patch is better because the tread doesn't deform as much as a square patch..

Alan: That's a great explanation, one I haven't heard before. props 2 u

[Q45tech]

"One of the most common swaps is to change the standard tyres for ones with a lower profile and wider section. However, a tyre swap like this will change the shape of the contact patch of the tyre on the road. Changing to the wider tyre will give a contact patch that is shorter fore/aft, but is wider. The change in width is easy to understand - a 225 tyre is nominally 20mm wider than a 205 so of course the contact patch is wider - but what about the contact patch shortening in the fore/aft direction? This occurs because - despite widespread belief to the contrary - only the internal air pressure and the weight of the car acting through the wheel determine a tyre's contact patch area. Nothing else affects contact patch area- not tyre construction, nor tyre size! As a result, if the contact patch is wider, the patch must also be shorter, so that the same area is maintained.

The change in the shape of the contact patch resulting from fitting a tyre of increased width is likely to reduce ride comfort. This is because a tyre with a shorter contact patch will ride up and over bumps more abruptly than one with a long contact patch, that can "flow" over the bump will a less rapid distortion. Of course, a tyre with a longer contact patch (and so one that is narrower) will also miss bumps that a wider tyre will catch! A tyre with a wide, short contact patch will also have reduced self-aligning torque (the propensity for the wheel to wish to track straight). This is obvious in cars with suspension that makes them prone to tramlining - on the same car, wider tyres will tramline more than narrower tyres. The widening of the shape of the contact patch will, however, improve turn-in, as the shorter contact patch takes less time to "re-arrange" itself when establishing the cornering force. This is because the individual blocks of tread can only change their direction after they lift off the road - and a shorter contact patch means that each tread block is in contact with the road for less time.

The wider, shorter contact patch resulting from the fitting of wider tyres gives cornering gains, but can also result in reduced braking and acceleration performance. However, the use of stiffer sidewalls and softer tread rubber of high performance tyres usually results in some improvements to everything!"

http://www.autospeed.com/cms/A_0277/article.html

[aither]

wow! That is the best explaination I have heard so far. Thank you!I noticed the article focused on turn in, but what about steady state cornering, such as on a skid pad? Why does a wider tire usually perform better mid corner, when there is a lot of force for the tire to resist in a lateral direction?Also, I don't know why, but a wider tire in the rear (215 vs. 195) feels grippier mid-drift.

[Q45tech]

The problem is many of the less expensive wider tires have started to use harder and harder rubber compounds, so much of the potential gain is lost trying to make them last.

I'll race anybody thru twisties using my 60 series 15" under 200 treadwear index tires against any 18-20" in 35-50 series with hard 400+ compounds.

Tire compound is much more important than size. Besides the extra wheel weight puts you in a serious disadvantage.