Low Profile Tires are DANGEROUS

Ever hear of the 6.70-15 tire? Its aspect ratio was about 92.

In the 70's, small cars typically used ratios of 80.

In the 80's, ratios of 70 were common.

Today, absurd ratios as low as 40 are being fitted to cars. Avoid at
all costs these killer tires.

The lower the ratio, the wider the tire has to be to support the
weight. All things being equal, this means hydroplanning occurring at
lower speeds; possibly 10 mph lower. This is dangerous.

Hydroplanning with normal tires can be expected at speeds of 55 to 60
mph. Low profile tires may let loose at 45 mph. Since these "high
performance" and very expensive tires are sold to car enthusiasts who
likely will assume better, not worse, specs, this is dangerous. Such
lead footed drivers will quickly get themselves into hydroplanning
situations and wreck.

 

Student Mechanic: Go the **** away.


On Sat, 4 Oct 2003, Anonymous via the Cypherpunks Tonga Remailer wrote:

<frass about low profile tires>

 

Yes and the fuckin sky is falling!!!!!!!!!!!!!!!

 

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

 

Ummmm.... no. Tread design plays as large, if not larger role than tire
width in determining how easily a tire hydroplanes. Of course, if you
have to rationalize the "4 for $99" whitewalls you just bought at Pep
Boys, be my guest.

nate

 

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

 

Anonymous via the Cypherpunks Tonga Remailer

[snip gibberish]

"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.

******************************
Got wood?
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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
1951 Crosley Hotshots....). I was just dealing with what was standard
equipment on those cars.



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.

 

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")