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Thin or fat tires?


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9 replies to this topic

#1 gcleeton

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Posted 10 April 2007 - 07:40 AM

I had a surprise the other day. We live in a lake effect snow belt in Buffalo and have occasional skids which have written off two of my three Legacies (not me driving of course!).
Thinking of fat racing tires, I said to our local garage mechanic I thought I needed the widest possible tires fitted, but to my surprise he said no the NARROWEST tires!
I thought about this and wonder if he means to get maximum pressure on the snow (Pressure = Force divided by Area).
Is he correct please?
Buffalo Gil

#2 Manarius

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Posted 10 April 2007 - 08:18 AM

Wide tires do have increased rolling resistance. In snow deep snow, that can be a major issue if all you're doing is spinning because of the wide tread. I would say that a balance would be the best. I'm running 195's and haven't had any major issues getting traction in snow. I think that the way one drives in snow is a lot more important than the tires (although a good set of snow tires helps a good driver).

#3 ferret

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Posted 10 April 2007 - 08:20 AM

Snow tires are usually thinner for more pressure per square area. Wide tires 'float' on snow.

Lookup any snows on the web at various tire sites, they are usually not wider series.

Wide tires are more contact patch, but at a lower pressure per square area. These provide more 'skid patch area' when the surface is solid and provides some surface adhesion, like cement or asphalt.

#4 OB99W

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Posted 10 April 2007 - 08:21 AM

Your mechanic is mostly correct, for the reason you proposed; the tire lugs will bite into the snow better. However, I said "mostly" because there are other aspects of handling and load-bearing that limit just how narrow it's prudent to go.

#5 RallyKeith

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Posted 10 April 2007 - 10:35 AM

100% correct. Take a look at the Subaru World Rally team if you want more proof.

#6 porcupine73

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Posted 10 April 2007 - 11:15 AM

Hello Buffalo Gil! I've heard the same thing. The only anecdotal evidence I have is that I have a '96 w/14" narrowish tires and a '00 w/15" 225 wide tires. I think both are Michelin X Ice. The '96 handles much better in the snow; not sure if it's the tire width?

Hey can we get NY to stop using this vehicle devouring salt!?! I wonder if there has ever been a class action suit about it....

#7 ivantruckman

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Posted 10 April 2007 - 12:20 PM

I use to plow snow for a living, and i always used narrow tires, they would bite down to the pavement, helping me get traction to push snow. i had wide off road tires on it before, and they were worse. however if you need to go on dirt roads or trails it helps you float better and avoid getting stuck in ruts, my experience is having the right tire for the terrain. i dont think their is a universal tire.

#8 jamal

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Posted 10 April 2007 - 12:23 PM

Given the same tire pressure the contact patch will have the same area (well, nearly), for any width.

On pavement the reason for wider tires isn't for a bigger contact patch, it's for a wider one, which will develop lower slip angles while cornering (which means they flex less and make maximum grip more quickly than a narrow tire).

A narrower tire will grip better on loose surfaces, but it has less to do with the size of the contact patch as it does with the shape. The point is so the tires dig down through the top surface to something better. With a rally tire on gravel, it digs through the loose material to the hard packed dirt below. On snow, you might be having more tire contact with the pavement or hard pack when there's deeper, fresh snow.

#9 Snowman

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Posted 10 April 2007 - 12:39 PM

100% correct. Take a look at the Subaru World Rally team if you want more proof.


Precicely. In the Swedish rally, it looks like they're running on donut spares!

More ground pressure equals more traction.

#10 Commuter

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Posted 11 April 2007 - 11:56 AM

Given the same tire pressure the contact patch will have the same area (well, nearly), for any width.

On pavement the reason for wider tires isn't for a bigger contact patch, it's for a wider one, which will develop lower slip angles while cornering (which means they flex less and make maximum grip more quickly than a narrow tire).

A narrower tire will grip better on loose surfaces, but it has less to do with the size of the contact patch as it does with the shape. The point is so the tires dig down through the top surface to something better. With a rally tire on gravel, it digs through the loose material to the hard packed dirt below. On snow, you might be having more tire contact with the pavement or hard pack when there's deeper, fresh snow.

jamal has it right.

jcleeton is also right on Pressure = Force per unit area. The force (weight of car on the wheel) is for all intensive purposes a constant. (I'm not including dynamic forces here.) Tire pressure also is essentially a constant. (I know, it varies depending upon temperature, due diligence, etc.) Therefore, the area of the contact patch is also a 'constant'. Say there is 750 lbs load on the wheel. Tire pressure is 30 psi. Therefore, the area is 25 square inches. This is necessary for the balance of forces (ground supporting the car thru the contact patch of the tire). Tire profile does not change the total area, but will change the shape of the area (contact patch).

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