Does tire size really affect speedometer?

[Compaq888]

Originally Posted by MetroMPG

shouldn't you guys be going to narrower tires for mpg? less frontal area & aero resistance.

unless of course you're using the improved grip to corner at higher speeds, thereby braking less and conserving more momentum.

I'd never put narrower tires on my car. My car is 3000lbs so I need all the contact patch I can get.

[Matt Timion]

Originally Posted by MetroMPG

shouldn't you guys be going to narrower tires for mpg? less frontal area & aero resistance.

unless of course you're using the improved grip to corner at higher speeds, thereby braking less and conserving more momentum.

Yes, we should. I went from 175/70R13 to 185/70R13 because the 185s came on the rims when I bought them. I sort of wanted to get rims with tires. Now I kind of wish I'd just found some rims and swapped. Whatever.

I guess the good thing about this is that I have another set of rubber in my garage when my current tires go out. Considering how often I go through tires that is a good thing.

[Sludgy]

It's easy to correct odometer and mileage when you change tire size. If anyone wants to email me sludgy@aol.com, I'll send an Excel spreadsheet that calculates an odometer correction.

[CosmicMC]

By putting wider tires on your car, you have spread out the 3000 pounds over a greater surface area. Your rain, snow, and ice performance will probably be negatively affected by the change. What problem were you trying to solve, or did you just want tires that looked wider? You said your gas mileage didn't go up or down with the new tires. Why did you change sizes?

[SVOboy]

By putting wider tires on your car, you have spread out the 3000 pounds over a greater surface area. Your rain, snow, and ice performance will probably be negatively affected by the change.

'

The spreading out part is true. But negatively affected is not true. By the magic of physics, we know that 3000 pounds on 1 square inch of tire will experience the same frictional force in total as 3000 pounds on 100 square inches.

[Matt Timion]

Originally Posted by CosmicMC

By putting wider tires on your car, you have spread out the 3000 pounds over a greater surface area. Your rain, snow, and ice performance will probably be negatively affected by the change. What problem were you trying to solve, or did you just want tires that looked wider? You said your gas mileage didn't go up or down with the new tires. Why did you change sizes?

As I mentioned in a previous post, these tires came with the rims I bought. I purchased these rims because they were lighter. The tires came with them so I saw no need in taking them off.

[Capcom]

Originally Posted by SVOboy

'

The spreading out part is true. But negatively affected is not true. By the magic of physics, we know that 3000 pounds on 1 square inch of tire will experience the same frictional force in total as 3000 pounds on 100 square inches.

Unfortunately this is not true.

Friction is thought by teachers as not being dependant on the area for educational purposes but in fact it is affected.

Also there are 2 types of friction:
1) Sliding Friction
2) Rolling Friction.
Rolling friction is always greater then sliding friction.

For best MPG, narrover and smaller tyres should be used in order to decrease contact area, and wider and bigger tyres have more contact area therefore increases grip at dry surfaces and decreases braking distance (with or without ABS).

Also: Losing grip at snow/rain is highly dependant on the geometry and shape of grooves on outer surface of the tyre.

[SVOboy]

There are indeed two types of friction, static and kinetic. But if you look at the friction equation you will get Friction = uForceNormal where u is the coefficient of friction and Force Normal is the force that the ground exists on the tire (in this case). The force normal equation is FN = mass * gravity. So therefore, you get your frictional forces without any consideration off size, only mass.

[Capcom]

Originally Posted by SVOboy

There are indeed two types of friction, static and kinetic. But if you look at the friction equation you will get Friction = uForceNormal where u is the coefficient of friction and Force Normal is the force that the ground exists on the tire (in this case). The force normal equation is FN = mass * gravity. So therefore, you get your frictional forces without any consideration off size, only mass.

I know.
I was referring to this formula that being thought on priliminary physics.

Unfortunately "u" (thecoefficient of friction) increases as the area increases. In fact you should never compare different areas of same materials under same load.

And also why do you think wider tyres decrease braking distance (with or without ABS) as compared to narrover tyres ?

[SVOboy]

Aha, I see now that cylindrical surfaces have friction proportional to length, but I dunno if the tire is considered a cylinder or a square/rectangular surface, as I don't think cylindrical implies give in the substance. I dunno.