contact patch vs weight discussion

[ADAM]

hi guys,

ok....so is there an "ideal " tire size(width) for a given car?
or
is the tire size(width) determined by the weight of the given car?

i am having a discussion with someone, and i just cant seem to agree that just slapping on huge tires on a car is the way to go....the weight of the car and the weight vs contact patch must have some bearing? if you use a smaller tire and increase the weight on that tire..the tire contact patch increases....no? or is it the SHAPE of the contact patch that we are interested in?

but then i look at the corvettes..with 315's..and they are moving? then i look at my car with 225's?

my question......what is the relationship of weight vs tire width and the contact patch?

anyone have any in depth info into this?

[Jay]

I have often considered the same question. In theory it is pretty simple to understand. Optimise the dimensions of the tire.

Changing the dimensions of the tire will alter the traits that it exhibits. For example a smaller diameter wider tire will have a contact patch that is wider and not as long vs a narrower taller tire. Other factors to consider are rotational mass, rolling resistance, heck even wind resistance.

The “TRUE Optimum” size could only be calculated using a whole schite load of interrelated mathematical expressions for the one track, one car, one day! Each expression involving a different variable (contact patch, rotational mass, Rolling resistance, wind resistance) Not very practical for us to calculate. (Uggg the math….I would rather spend a day in a crappy Nissan)

For our purposes you want the widest tire that will fit in the car (Biggest contact patch) and the smallest diameter that will fit over your brakes (to reduce gear ratios)…. All other considerations are negligible!


ooops.... after re-reading your post I see that we are talking about two separate issues. I think you are talking about PRESSURE... Not tire pressures but the normal force down divided by the area of the contact patch.

I think that the "Physics of Racing" article addresses your question Adam......I think Krispy has the link somewhere.

Fram

[ADAM]

i have read over that article(s) and all of the formula i think confuse the issue..

[Marsh]

I'm not sure exactly what your asking, but I suspect your not quite certain either.

There are 3 primary things to consider when choosing a tire size. (There are actually many, but only 3 that you need think about here).

They are:

1st

The SHAPE of the contact patch and temperature. The size of the tire will have no effect on contact patch at all. None. This is determined by the weight of the car and the pressure in the tires. Think of it this way. If there is 40 psi in you tires and your car weights 2000lbs, then you will need 50 sqin to hold up the car (at 40 psi) and that's exactly what you'll get.

Having a long skinny contact patch will lead to some problems. The easiest to understand is the side wall distortion and it's effect on load distribution across the contact patch. A road racing tire has pretty strong side walls, and if you run low pressure in your tires they will being doing a lot of the work needed to hold your car up. This is bad for the side wall, and for traction. The side wall loading will cause a higher pressure at the edges of the contact patch and low pressure in the middle. Even with the correct pressure the same thing happens with a skinny tire. The tire is round, but the road is flat, so in order to fit the road the side walls must deflect in the middle of the patch. That's pretty obvious, you can even see it in a radial street tire. The wider the tire, the shorter and wider the contact patch will be and the less deflection there will be, so better load distribution and better traction. Wider is Better!

The same is true for diameter. A smaller diameter tire will need to deflect more to achieve the same contact patch shape and size. So larger is better, but that will have some big costs.

But there are some other minor side effects. One is that your steering will become more vague. There are two things that make your steering go straight when you let go; Mechanical trail and Pneumatic trail. Mechanical trail is determined by the suspension geometry (caster), but Pneumatic trail is determined by the distortion of the tire. The shorter the contact patch the small the amount of pneumatic trail. Pneumatic trail also goes away at higher slip angles. This is how you "feel" the tire going away. If your tire has less to begin with, it will be harder to feel the limit, plus the steerig will be lighter in general.

2nd

The next thing to think about is temperature. Assume for a second that we are looking at two tires with identicle diameters, but differing widths, runningthe same pressure. Now consider a small portion of tread (say a square inch or less) of each tire. Since the contact patch is longer on the skinny tire, our portion of tread will be on the road longer than it will on the wide tire. If you are travelling at the same speed and one patch is longer, it will take longer to travel through the patch. Since the heat of the tire comes from friction with the road. The ellements of the skinny tire will get hotter than on the wide tire. This isn't exactly rocket science and you probably knew this, but it was important enough to be worth mentioning.

Keep in mind that a smaller diameter tire will run slightly hotter because it will have less "cooling time" while it's off the ground.

3rd

Weight. Obviously a wider tire will weight more... duh! It will also have a higher polar moment of inertia. Plus don't forget that's unsprung mass. So adding weight to a tire is the worst weight you can add. Rotating unsprung mass is BAD. But width isn't everything here. You can fight this with a smaller diameter. A smaller diameter has less polar momentum (even if it weights the same!) and, for the the same width, is lighter.


So in conclusion you want to go with the widest tire you can fit. Going with the smallest diameter you can fit will reduce the negative side effects of weight and temperature reduction, but will also loose you some of the advantage in shape. But never the less, I believe it worth while. If you can not get this tire to reach opperating temperture than reduct the tire width. Remember the patch size doesn't change, so temperature is usually more important. Don't run a wide tire then drop the pressures to rediculous numbers in order to heat them up. This will ruin your contact patch.

[Bad Karma]

Originally posted by Marsh
The SHAPE of the contact patch and temperature. The size of the tire will have no effect on contact patch at all. None. This is determined by the weight of the car and the pressure in the tires. Think of it this way. If there is 40 psi in you tires and your car weights 2000lbs, then you will need 50 sqin to hold up the car (at 40 psi) and that's exactly what you'll get.



It's not *entirely* true, as the size of the tire will determine the SHAPE of the contact patch, but the overall size of the contact patch will remain identical. I'm pretty sure Marshall knows this, as he mentions it later in his comments. I just wanted to point out that the tire does actually affect the contact patch on some level.

A 245/45/16 tire will have a wider, but shorter contact patch than a 235/40/17 tire, simply due to a wider section width.

There is one more critical factor to remember when choosing tires. People often focus on compounds, section width, etc, etc. However, your tires are the un-sung heroes of your suspension, and can often be the difference between having the ability to put power down in a corner, or not.

A lot of people will tell you that PLUS 1 sizing of tires / wheels is a performance benefit to your car, but this is not always the case. On a rough track, like SMP or Cayuga, having a taller, softer sidewall tire can help with putting power down in corners like turn 2 at Cayuga, or coming out of the Carousel. The tire can actually provide some compliance to the suspension (if you're running a race-stiff suspension such as mine). Some sidewall flex is a good thing on a track like this....


As to overall sizing of tires, there is minimal functional benefit going to 17, 18, or 19 inch wheels beyond clearance for bigger brakes. If your 15 inch wheels already clear the brakes on your car, then run a 15 inch wheel / tire combo, as it's likely to be MUCH cheaper than larger tires. I go through about 4 sets of Toyo RA1s per season, and dropping down to 16's would save me about $1000 per year on tires alone...

Pat