Low Profile Tires are DANGEROUS

Student Mechanic: Go the f*ck away.

What you smoking?

There's lots of ways to get around this. Jack up the air pressure. make stronger tires.

The only reason people ought to avoid lower profile tires is because they were invented by the tire manufacturers to enable them to sell more rims to replace all the bent ones caused by going over standard sized chuckholes in the road. (they also look stupid, but that's another thing)

Ted

You're full of it and not even consistent in your own post. So is it 10 MPH or 45 MPH? Quite a difference.

Matt

"hydroplanning?"

Isn't that what they did before they built Hoover Dam?

FWIW-- my wife bought low profiles for her 96 Concorde about a year ago. With the new rims, they looked sharp as hell, and handling in good conditions was really good. BUT-- first snowfall ( and we had a LOT last year) they were horrible. She wasn't into having two sets of wheels/tires for different seasons, so she gave them to her son for his Cavalier.

Whether or not it's "student mechanic" posting this topic or someone else, I tend to agree with him.

I had a virtual A/B comparision in terms of low aspect ratio tires.

I had a 1981 Plymouth TC3 ("sporty" extended hatchback Horizon) with standard tires.

I also had a 1984 Dodge Rampage (mini-pickup based on the Horizon). Same front wheel drive, approximate same weight, same engines (2.2 liter). The treads on the two sets of tires were similar, but the Rampage had the low aspect ratio tires.

The Rampage was TERRIBLE in snow. The car spun out on me at slow speeds many times. I finally totalled it when it slid down a snow covered hill into a rock outcropping while doing less that 20 MPH.

The Plymouth TC3 was GREAT in snow....

Another similar experience happened about 14 years ago, when I pulled a 4 wheel drive Jeep off of a snow drift with my 1937 Hudson Terraplane (no kidding - yes, I'm an antique car collector). That car had the old bias ply 6.00 x 16" tires - tall and skinny. Of course, it did weigh something like 4,500 lbs.

Now my 1929 Model A doesn't do too well in snow but it has 4.50 x 21" tires. That's carrying tall and skinny too far....sort of like balloon bicycle tires.

My only theory is that "tall and skinny" tires put more load per sq. inch on the road, biting better into snow.

That's just my opinion, I could be wrong....

Doug

What tire pressure do you run in your Hudson and Model A?

Matt

Actually, for the same vehicle weight, short (small diameter) and skinny gives the highest load per square inch - if that's the only thing that matters (hint: it's not).

Bill Putney (to reply by e-mail, replace the last letter of the alphabet in my address with "x")

I've not thought about that recently. My recollection is that the Hudson's 6.00 x 16 tires run at "modern" pressures of around 32 lbs.

The Model A tires are higher pressure at around 50 - 60 lbs.

Doug

I never implied that "tall and skinny" was the only factor. There are probably dozens, including diameter, tread design, inflation pressure, rubber compounds, etc, etc..

As for tall and skinny versus short and skinny, the only "short and skinny" tires, in the sense that I meant it might be on go-karts (or

:-)

Doug

Higher pressures allow more pressure to be applied to the road surface. The area of the contact patch varies with the air pressure in the tire. Not linearly, but to a decent approximation, especially if you aren't at extremes of pressure, either high or low.

Matt

You made pounds of force per contact patch area the only comparison criterea by your statement ("'tall and skinny' tires put more load per sq. inch on the road). By that statement, I thought you were saying that (everything else being equal) tall and skinny gave more pounds of force per square inch of contact patch area than any other combination (i.e., more than tall and wide, short and skinny, or short and wide). My point was that, of the four general combinations, short and skinny would give the greatest force per contact patch area.

Bill Putney (to reply by e-mail, replace the last letter of the alphabet in my address with "x")

Just so there's no confusion - higher pressure (psi of air) in the tire allows more pressure, as in pounds of force per square inch of contact area, against the pavement, but not total pressure, as in total force counteracting the weight of the car - that stays constant for a given car (unloaded) at rest on level ground.

Bill Putney (to reply by e-mail, replace the last letter of the alphabet in my address with "x")

Total pressure does not equal force. Pressure and force aren't synonymous. Pressure*area=force. Higher air pressure in the tire yields higher pressure on the road surface. The force applied is the same as the area of contact will be reduced commensurately. I think that is what you are saying, but using the term "total pressure" as being synonymous with force is misleading at best.

Matt

I think I followed your point Matt. I might be naive on this but I would have thought that the air pressure in the tyres times the contact area, summed for all four wheels would equal the weight of the car.

So a lower pressure lets the tyre flex to add contact area.

I realise that there is some (fairly slight I thought) strength in the tyre sidewall (as an unmounted tyre doesn't collapse in a puddle of rubber), but I'd have thought that would be quite small.

My expectation would have been that the tyre profile is irrelevant when thinking of contact area, but that many people like the look of low profile, and to get the rolling radius that would match the original equipment, get a wider wheel and tyre combination. Now it makes sense to me that wider tyres could have greater difficulty in dispersing standing water and because wider (and same pressure) would equal smaller contact area, they would have more difficulty "cutting through" mud and snow.

-- Rickety

Yes, that is essentially the concept and it works over a reasonable range of air pressures.

Depends greatly on the tire. Some have sidewalls that barely hold up the uninflated tire and rim. Some are so stiff you can sit on an uninflated tire and it will hold up its own weight as well as yours.

I believe that is true for the most part. Obviously, tire design is fairly complex with a lot of variables involved, but some basic principles hold true.

Matt

Technically you are 100% correct. Notice that in my previous post I did not say that you were wrong. I said that I wanted to **clarify** - basically for those who may be thinking in terms of the looser (i.e., more ambiguous) meaning of the word "pressure" as used in everyday English.

The two sides of the coin (the "coin" being the word "pressure") are this: (1) As I just said, you are technically correct as far as the precise engineering term "pressure". There is a term "pressure force" in engineering that is the pressure gradient integrated over a surface which is the *net force* pushing against a surface due to the varying pressure (as you have pointed out - force per unit area). For example, a typical textbook problem in a statics or fluids course might ask you what the pressure force is on a gate of certain dimensions at a certain angle at a certain depth in water. That would be a force (due to the pressure per area times the area). In an engineering sense, and in agreement with your point, to ask what the pressure on that gate was would be meaningless, as it varies depending on where you are on the plate.

(2) In everyday language, the term "pressure" is ambiguous - this from Webster's New World Dictionary: "in physics, force exerted against an opposing body, the thrust distributed over a surface: expressed in weight per unit of area" (notice also the sloppy use of the word "weight" there). Technically, the first half of that is incorrect, but it is nevertheless how the "layman" thinks when he hears the word "pressure", as in "I applied pressure to the table with my thumb" - he would think to be another, more natural way of saying "I applied a force to the table with my thumb".

Again - that's why I though it needed clarification. But make no mistake - you are correct. You would also be correct to point out that "total pressure" would have no technical meaning, but, in the loose sense, if it did have a meaning, it would be the net force created by a pressure distributed over a surface (in precise engineering terminology, the "pressure force").

Bill Putney (to reply by e-mail, replace the last letter of the alphabet in my address with "x")