Automotive theory: tire sizes (how is this decided?)

[lizpat]

How do automotive engineers/designers decide what tire size to give their cars?

cars = any automotive vehicle, including electric cars. SUVs, trucks, crossovers, etc. are all included
tires = the total size of the tire including the rolling diameter and the width (we will ignore wheel sizes)

First a few observations:

1. LOOKS: Bigger cars have bigger tires (this fills out the wheel well and gives it a "proper" look)
2. POWER: more powerful cars have bigger tires (bigger tires can properly put down the horsepower)
3. SUSPENSION: taller cars (Crossovers, SUVs) have bigger tires to properly clear road imperfections and hurdles (a smaller tire may get swallowed by a pothole, whereas a bigger tire will roll over it)

What do you think order of importance here is, and give it a estimate weight percentage.

Here's an example:

I would say it's mostly about looks (50%) then it's about power (30%) and finally suspension (20%)

 

[JohnnyG]

I think you left out handling. Did you ever notice on the RWD Mercedes model, the rear tires are often larger than the fronts.
Larger tires on the rear (wider) will increase understeer.

So, no doubt, there's more to the size decision than we realize.

 

[BMWTurboDzl]

From what I've read in interviews with automotive designers:

Vehicle design governs rim size.
Vehicle performance characteristics govern tire size.

 

[Skippy722]

I’d say brake size is first. Can’t fit anything smaller than a 17” wheel on my truck for example.

 

[Snagglefoot]

I’d also include marketing. The sales people have noticed many buyers gravitate to the vehicles with larger rims.

 

[dogememe]

I’d also include marketing. The sales people have noticed many buyers gravitate to the vehicles with larger rims.

Maybe I’m boring but I’m the opposite LOL

 

[DriveHard]

I think you left out handling. Did you ever notice on the RWD Mercedes model, the rear tires are often larger than the fronts.
Larger tires on the rear (wider) will increase understeer.

So, no doubt, there's more to the size decision than we realize.

I can say that larger rear tires DO NOT increase understeer...

My C5 has much wider tires on the rear...during AutoX, understeer is RARELY a problem.

 

[justalubetech]

Maybe I’m boring but I’m the opposite LOL

My Geo Metro had 12" rims from the factory lol, I had to "upgrade" to the massive 13" wheels to find any tires

 

[earlyre]

Maybe I’m boring but I’m the opposite LOL

I pine for the days of 15" rims and bigger sidewalls.

 

[FowVay]

I think it's the bling factor.

 

[Jimmy_Russells]

I can say that larger rear tires DO NOT increase understeer...

My C5 has much wider tires on the rear...during AutoX, understeer is RARELY a problem.

They absolutely do. Any time you increase grip in the rear, you are effectively decreasing comparative grip in the front, and it will increase understeer.

That doesn't mean the layout of the car, alignment etc can't compensate for that, but if increased tire size in the rear is the only change, it will increase understeer.

 

[ATex7239]

Here's an example:

I would say it's mostly about looks (50%) then it's about power (30%) and finally suspension (20%)

Clearly you didn’t poll any rednecks on this one…100% looks. The end.

 

[DriveHard]

They absolutely do. Any time you increase grip in the rear, you are effectively decreasing comparative grip in the front, and it will increase understeer.

That doesn't mean the layout of the car, alignment etc can't compensate for that, but if increased tire size in the rear is the only change, it will increase understeer.

Hummm...I am not an expert...but...my understanding of traction is the normal force multiplied by the coefficient of friction to get the amount of grip. Wider tires of the same rubber compound have benefits, like they increase the amount of loading it can handle before overheating, increasing contact patch which reduces the effect of abnormalities in the road, and more importantly, they allow a softer rubber compound to be used without reducing life as much...but does just increasing width of a tire without changing other parameters (rubber durometer, inflation pressure, etc.) actually increase grip?

Questions I am not qualified to answer.

 

[SC Maintenance]

I would say its 99% appearance - for most utilitarian cars.

Look at any modern family car. The low trim gets the 16 inch steel wheel with 70 series sidewall, and the high trim gets the 19 with 50 series (filling removing) sidewall. Neither the suspension nor powertrain change.

So it must be for looks.

Enthusiast cars excepted. In those cases there may be more to it.

 

[y_p_w]

It's all getting pretty silly these days. I remember the Acura Integra Type R. It handled exceptionally well on 195/55R-15 tires. The original NSX had 16 inch wheels.

The current penchant for larger wheels and lower profile tires is not a good thing. Not only do they cost more, but they make for a poor ride and damage easily from potholes and other road hazards. Here's a guy who got better performance from downsizing. Apparently the tire/wheel set was lighter and much cheaper to replace the tires. Probably wore better too.

The Only Good Thing About Low-Profile Tires Is When You Get To Replace Them

Low-profile tires are ruining modern cars, but that doesn’t mean you have to live with these monstrosities.

jalopnik.com

 

[Jimmy_Russells]

It's all getting pretty silly these days. I remember the Acura Integra Type R. It handled exceptionally well on 195/55R-15 tires. The original NSX had 16 inch wheels.

The current penchant for larger wheels and lower profile tires is not a good thing. Not only do they cost more, but they make for a poor ride and damage easily from potholes and other road hazards. Here's a guy who got better performance from downsizing. Apparently the tire/wheel set was lighter and much cheaper to replace the tires. Probably wore better too.

The Only Good Thing About Low-Profile Tires Is When You Get To Replace Them

Low-profile tires are ruining modern cars, but that doesn’t mean you have to live with these monstrosities.

jalopnik.com

The cars are heavier now and have much larger brakes. You can fit 18" on the new Type R but the brakes are way larger than the ITR.

 

[userfriendly]

Clearly you didn’t poll any rednecks on this one…100% looks. The end.

View attachment 138235

That's the modern equivalent to the Marlboro Man.

Only one thing missing; orange www.bullsballs.com on the trailer hitch.

 

[Pew]

Hummm...I am not an expert...but...my understanding of traction is the normal force multiplied by the coefficient of friction to get the amount of grip. Wider tires of the same rubber compound have benefits, like they increase the amount of loading it can handle before overheating, increasing contact patch which reduces the effect of abnormalities in the road, and more importantly, they allow a softer rubber compound to be used without reducing life as much...[/b]but does just increasing width of a tire without changing other parameters (rubber durometer, inflation pressure, etc.) actually increase grip?[/b]

Yes, increased surface area allows for more friction. You'll want as much tire as you can fit on an autoX course where speeds are normally limited to first and second gear with maybe a touch of third on a straight. Tire and brake temps don't really get push to their limits on an AutoX course so cold performance can be more important. On a road course, there's a limit of "too much" where the added weight and rolling mass of a too-wide-of-a-tire will negatively impact your times including impact brake and tire temps.

 

[ATex7239]

That's the modern equivalent to the Marlboro Man.

Only one thing missing; orange www.bullsballs.com on the trailer hitch.

Don’t for the “Stroking” sticker on the back window.

 

[barryh]

Wider tyres than necessary result in worse fuel economy and larger diameter rims than necessary result in a more uncomfortable ride plus increased susceptibility to wheel/suspension damage. When the above is being applied to ordinary everyday cars rather than sports cars then it has to 95% marketing.

You can say the same about suspension settings. I've been a passenger in a car with suspension so hard it defied belief. It was so bad the driver actually had to slow down which rather defeated the point of a car with a sporty image. Have buyers asked for cars this bad or is it the designers producing cars they think we should have ?