Large Diameter Wheels and Low-Profile Tires: Why?

[Tim Maddux]

Subject says it all, really. Is there some technical reason (i.e. aside from the bling-bling factor) that people want/install the combination of larger-than-OEM-diameter wheels and lower-than-OEM-sidewall-height tires?

[Guillermo]

The sidewall being shorter is much stiffer so turning responce is improved. The stiffer sidewall reduces the amount the tire sidewall will flex, as the sidewall flexes the inside of the contact patch of the tire begins to lift reducing grip.

Also with a larger rim that is wider a wider tire is used.
With a wider tire there is more lateral grip because if the tire contact patch lifts 1 inch (from body roll and sidewall flex) a wider tire will maintain a higher percentage of contact patch in contact with the road compared to a narrower tire.

Disavantages of larger rims with lower sidewall height tires:
Decrease comfort,
increase noise
increase weight
huge increase in rotational inertia

Advantages:
More turning grip (assuming same tire model)
More precise steering feel
Bling Bling

[Guillermo]

I'm more important to use a much better tire then just getting larger rims and wider tires.
For example a top quality 14" ultra high performance tire will provide more performance then a wider 15" or 16" all season tire of the same outer diameter.

[Yves M.]

Disavantagem in a pothole, there is more chance to break the rim and/or the tire. As the sidewall is smaller, it takes less flex for the rim to hit the pavement.

[Holicow]

Yeah, you got it. For 95% it's just for looks.

[Guillermo]

Notice that I race with lightweight 13" rims and tires!

[Foxpaw]

I've heard that low profile tires supposedly give better performance on cornering because there less sidewall to flex. Personally, I would say that the benefit attainable through the use of low-profile tires is likely minimal, if present at all. If that were the case, logic would dictate that high performance racing cars would have low profile tires when in fact they have much "higher profile" tires. "Higher profile" tires are able to deform more, which allows you to get more traction, which is arguably a good thing. However, conversely, one might reason that reduced sidewall flex might mean less rolling resistance. So it might make sense from an efficiency standpoint.

[b1shmu63]

The amount of "flexing" will change depending on the presure, the surface area, and the rigidity of the sidewalls. In most cases the sidewalls are too flexible to add significantly to the equation. Tye problem with a low sidewall height is that the amount of "flex" takes place over a shorter distance. This will case heat buildup to increase at an exponential rate over a linearly decreased area. This means that a low profile tire will get much hotter due to "flex" and will waste significantly more energy. The ultimate expresion of this scenario occurs when you get a flat. Low profile tires do look cool however, and the larger the surface area in contact with the road and the larger the outside diameter of the tire , the less it will be affected by small potholes.

[Guillermo]

Originally Posted by Foxpaw

I've heard that low profile tires supposedly give better performance on cornering because there less sidewall to flex.

Actually it's because tires with short sidewalls are designed to have significantly stiffer and thicker sidewalls. It's not the fact that they are shorter but the fact that because they are shorter they HAVE to be stiffer to prevent rim damage etc...

logic would dictate that high performance racing cars would have low profile tires when in fact they have much "higher profile" tires.

You are talking about Formula cars and Nascar etc... their rules force them to use a specific rim diameter. If you look at powerful race cars based on street cars you will see wide low profile tires like WRC rally cars set up for tarmac stages, and british touring car racing on tracks.

"Higher profile" tires are able to deform more, which allows you to get more traction, which is arguably a good thing.

The best example of this is on drag racing tires with ultra soft sidewalls, but these tires are not designed for turning!
In a corner a soft sidewall will deform so much that the tire contact patch will begin to lift and the corner of the tire and sidewall will make contact with the road. That's why in an auto-x high tire pressures are required to prevent this. With normal all season tires 45 to 55 psi is required on the front tires to reduce sidewall flex. Race compound tires or low profile tires have very stiff sidewalls so 40 to 45 psi is all that's required on the front tires for maximum turning grip.

[Guillermo]

Originally Posted by b1shmu63

The amount of "flexing" will change depending on the presure, the surface area, and the rigidity of the sidewalls. In most cases the sidewalls are too flexible to add significantly to the equation.

My experience instructing at a skid pad has taught me that me that sidewall construction is a HUGE variable that determines how much a sidewall will flex. A common tire size that many cars have is 205/55R16. Tires in this size come in all flavours including : extreme condition soft winter tires, all season touring (comfort) tires, ultra high performance summer tires and Race compound tires. A skid pad is used to determine the minimum tire pressure required to prevent the tire from flexing onto it's sidewall in a maximum grip turn. The tires of the same size and profile but of different sidewall construction require a minimum pressure of anywhere between 35 psi to 55 psi to prevent flexing when driving the skid pad at about 50 km/hr.

The problem with a low sidewall height is that the amount of "flex" takes place over a shorter distance. This will case heat buildup to increase at an exponential rate over a linearly decreased area. This means that a low profile tire will get much hotter due to "flex" and will waste significantly more energy.

When one upgrades to a larger rim with a lower profile tire it has the same outer diameter but it is wider. The contact patch is the surface area of the tire that makes contact with the road. The size of the contact patch is only determined by the weight on the tire and the air pressure in the tire. Going to a wider tire does not increase the contact patch surface area, it only changes the shape of the contact patch. With a wider tire the contact patch is wider so there is less flex in the tire. For example think of tire with zero flex, it would be made of something like steel. The contact patch would be a line not a patch. With something soft like a balloon when you push it against the ground you will see that a flat contact patch appears and gets bigger as you push down harder on the balloon.

A tire that is narrow deflects like a balloom to have the contact patch of
a certain surface area. Now take a wider tire with the same air pressure, it will have the same sized contact patch but the shape is wider therefore the tire is not flexed as much when it has the same contact patch area.

Wider lower profile tires are meant for high performance so they have stiffer sidewalls not only for turning but to reduce flex to minimise the amount of heat they create. That is why low profile tires have much higher speed ratings then tall sidewall tires.
But for efficiency the wider tire is much less aerodynamic and that is a much larger factor then rolling resistance at higher speeds. Specially since you can reduce the contact patch therefore decrease the heat producing flex of a narrower tire simply by increasing the tire pressure.
That is why it's more efficient to use the skinny OEM Insight tires at 50 psi.
[/quote]