TIRES: Did You Know That...

[cabinetman]

There is a misconception about getting "more rubber on the road" with a wider lower profile tire. The actual footprint of a tire (pattern on the pavement), is determined by the tire width, height, (rim in some circumstances), and the weight of the vehicle. In most cases, the actual footprint is much smaller than a sheet of typing paper (8 1/2" x 11"). A motorcycle tire produces a footprint somewhere around the size of a postcard. Scary thought.

Changing the size of a tire from a factory spec to a lower/wider tire doesn't make a larger print, it only changes in shape. For a taller narrow tire, the patch is longer and and for the wider tire it is shorter but wider. The actual area of the patch in square inches doesn't change, only the shape. This is produced with the weight of the vehicle.

For the difference in driving applications, the most noticeable would be in the stopping forces (braking) applied to the two design examples. The longer patch will afford more friction than the shorter patch. As for cornering, the wider shorter patch will outperform the longer narrow one.

There are also other differences including sidewall height, width, tread design, tire compound and air pressure which affect driving responses.



A THING OF BEAUTY IS A JOY FOREVER - John Keats

[Ed62]

I can't let LOML see this. It's just one more thing to worry about.

Ed

[dpaton]

The actual contact patch is MUCH MUCH smaller than a sheet of typing paper. (my budddy's race bike has about 6sqin of patch peer end) The contact patch on the front tires on my little Honda Civic is only 2 1/2"x5/ 1/2", each (13.75sqin). That's with the normal tire size that came with the car (165/50R14). Upgeading to bigger, wider tires does increase the patch slightly (16" rims and 215/40R16s gave me a patch 2 1/2"x 7", 17.5sqin, +27%), but not a lot. The real changes in handling come from tire compound changes (softer = more grip on dry pavement) and sidewall stiffness (shorter sidewalls = faster cornering), along with air pressure and some simple physics (rim weight and moment arm, and wheel mass).

Unless you race (like I used to, sanctioned events only thank you), there's not much of a reason to worry about your tires for around-town use. If you tow, or deal with tons of inclimate weather, upgrades are always nice, but if it's just a grocery getter that's never asked for much, as long as they're round, rubber, and not flat, the tires are fine, despite the tiny contact patch.

-dave

[crokett]

I just learned last week that they've started putting nitrogen in tires now. It is supposed to help with tire oxidation and rot. Since I've never had a tire blow because it rotted from the inside I am skeptical.

[Timmah80]

The nitrogen also helps to keep tire pressure at around the same psi when the weather gets cold or hot. Nitrogen doesn't react to temperture as readily as compressed air.

[Pappy]

That is the reason for nitrogen use in NasCar and the NHRA. I ran 12.5#'s in the slick on my drag bike. After a burnout and run the pressure would read under 12.75.

[Russianwolf]

As an avid 4x4 driver and offroader I can tell you that the biggest way to increase the contact patch is to decrease the tire pressure. But you can only do this if both the tires and wheels are designed to run at the lower pressure.

It in not uncommon for rock crawlers to run at 5-10PSI which allows the tire to mold itself to the rocks more and gain additional traction. The big trick is bead-lock rims. The rims are actually three pieces that when combined prevent the bead of the tire form losing contact with it so you can't "break the bead" and loose all your air. The tires need to have stronger sidewalls than normal "car" tires so that they can handle the deformation without busting the sidewall.

Now saying all this, If you over inflate your standard tires, you are doing several bad things. 1) the tire isn't designed to hold the amount of pressure (especial as it increases once hot) and could cause a blowout. 2) the extra pressure doesn't allow the car weight to press it down for optimal contact with the road, so in effect over inflating your tires=less contact patch. 3)gas milage. higher pressure mean your tire/wheels weigh more and puts more of a load on the engine, decreasing efficiency. It might not be much, but it is measurable.

[bmyers]

You guys heard of the scented tires? NPR listeners may have heard this but I was blown away that they're actually doing this..

http://blogs.edmunds.com/women/413


Bill

[Black wallnut]

...... 3)gas milage. higher pressure mean your tire/wheels weigh more and puts more of a load on the engine, decreasing efficiency. It might not be much, but it is measurable.

WHAT????? I've always heard that over-inflated tires increase fuel mileage due to lower contact which gives less rolliing resistance. If I'm reading you right you are saying that a passenger tire inflated to 35 psi, which used to be pretty common weighs less than the same tire inflated to 40 psi. Exactly how measureable?

What I have learned by years of expierence dealing with big truck tires is that excessive under inflation builds heat and the tire fails, over inflation does not seem to be as bad but the ride suffers.

With my Samuri the last time I bought tires (6.5 years ago) Les Schwab over inflated them by 20 psi and the ride/ handling was just flat scary! Normal pressure is 20 psi. In the fall when I'm spending lots of time out in the unpaved woods I'll let it get down to 18 psi and have great traction on all surfaces. Keep in mind the little SUV only weighs 2000 lbs.

[Russianwolf]

[QUOTE=Black wallnut;239128]WHAT????? I've always heard that over-inflated tires increase fuel mileage due to lower contact which gives less rolliing resistance. If I'm reading you right you are saying that a passenger tire inflated to 35 psi, which used to be pretty common weighs less than the same tire inflated to 40 psi. Exactly how measureable?

QUOTE]
I might be wrong on that. It's what I've always heard. But as I said, Us 4x4 guys usually go the other way and underinflate them while carrying portable tanks to refill for the ride home.

The big thing is, If your car/tire is recommended for 30psi, try to stay there and not 20 or 40. The tires on my Jeep are rated at 40PSI Max, but they will never seen that much.

I don't have a tire handy thatI can play with. Otherwise I would try a little home testing to see if the weight changes as I've been led to believe.

[Kristofor]

I agree with Black Walnut, the tiny extra mass from extra air in the tires is more than offset by reduced rolling friction.

Lower pressure makes a big difference in traction. Even 4WD low won't get you anywhere in loose enough sand dunes, but drop your tire pressure down and you're back in business (experienced directly in Great Sand Dunes national park a couple years ago).

Kristofor.

[Tequila]

I just learned last week that they've started putting nitrogen in tires now. It is supposed to help with tire oxidation and rot. Since I've never had a tire blow because it rotted from the inside I am skeptical.

The air around us (that gets compressed to fill tires) is already about 79% nitrogen and 20% oxygen. Charging people extra to fill tires with pure nitrogen is a total scam.

[messmaker]

It still seems to me that a wider tire would have a larger contact spot than a narrow tire. What am I missing?