Tires – Sidewalls vs Air pressure

The Vredestein tires I’m running have very soft sidewalls (carcass) making for a smooth ride. Unfortunately, handling and stability has suffered and is something I miss quite a bit. So why not just inflate the tires with more air pressure to counter the compliant tire construction?

First, some background

Unknown to most, the tire is the most crucial part of a cars suspension system and every good suspension is designed to optimize a specific tire. In previous posts, I have shown the great lengths engineers have gone through to learn the characteristics of how a tire performs so they can design a better suspension.

In this regard, race car engineers have it easy over us folks with production cars. Professional race series usually have a mandated tire, so the engineer only needs to design the suspension to use that specific tire. In contrast, our street cars have an endless supply of tires from the aftermarket, and our cars may even come from the factory with many different wheel and tire size options. So, our suspensions, by their nature, are less optimized but will work with many different tires.

In general, tires made for maximum handling performance have very stiff sidewalls. Stiff sidewalls allows better control of the tire contact patch when cornering and can create more stability by reducing overall body roll. Unfortunately, stiff tires transmit more road imperfections to the chassis which can be uncomfortable; thus, non-performance tires have very soft sidewalls.

One of the most difficult parts of manufacturing a good tire is to achieve a blend of performance and comfort. This seems to be particularly the case in the high-performance summer tire segment where all the major brands tend to bring out new tires every few years. The goal is to make a tire that provides fantastic handling while still being comfortable enough for the street.

What happens when you increase the tire pressure?

It’s no secret that adding more air pressure can improve the overall handling (to a point!!! it will actually decrease traction as well); but, adding more pressure is not the same as having a tire that has stiff sidewall built for cornering. Here are some of the negatives of adding air to counter sloppy sidewalls:

  • Air is a spring – As you add more and more air to the tire, you may notice that the suspension may start to skip over road imperfections. When this happens, the tire is no longer in contact with the road and therefore has no traction. This is because air is a natural spring, and increasing the pressure in your tire also increases this spring rate.
  • Air wants to be in a round container – I’m sure you have noticed that high-pressure vessels are typically round in shape with some being perfect spheres. The pressure of the air acts evenly on all surfaces, so as it increases, the tread of the tire begin to bulge out decreasing the contact patch. Traction is significantly reduced with too much pressure.
  • High pressure does not provide a lot of lateral support – While adding more air is easy to notice as far as comfort is concerned,  the sidewalls seems to flex disproportionately to the amount of pressure added. Cornering ability does increase, but ride quality is reduced at a greater rate. Handling generally gets sharper with higher pressure, but lateral traction will start to taper off with increased pressure as the contact patch decreases.

How do stiff sidewalls prevent these problems?

  • Sidewalls provide some damping. While the sidewall has its own spring rate like air, it also provides some isolation. Simply put, a tire with stiff sidewalls won’t be as bouncy as a tire with high air pressure. This allows stiff tires without the shock absorber punishing high-frequency vibrations that you get with air.
  • Stiff sidewalls control the tire tread much better than air does. Tires will maintain their profile and…
  • Stiff sidewalls don’t roll over as much as soft ones when cornering.  Adding a lot of air to prevent this roll over also deceases the contact patch; while a properly engineered sidewall prevents this.

It is possible to counter soft sidewalls by adding more camber to the suspension, but most street cars have very limited camber adjustment if any at all. The only real option we really have to tighten things up is to add more air or get different tires. Unfortunately, getting new tires can be onerous so we will have to make do with just adding air…

Can sidewalls be too stiff?

As with everything else with a car there is a balance. If the sidewall is too stiff, the longitudinal contact patch will suffer and the spring rate of the tire might be hard to damper and/or provide too harsh of a ride.

Some other notes

Actually, adding a lot of air doesn’t bulge the tread as much as it use to. This is more of a problem with old bias-ply tires because the ply’s in the tire are not independent of each other and the tread area is not as supported. Where in a radial tire, there are different ply’s for the sidewalls and the tread. Because of this, it is easy to spot a bias-ply tire as they have a concaved tread area when they are deflated.

You might notice that most cars recommend the tires to be inflated to about 35psi, this seems to be close to the optimal pressure for virtually all cars that weigh 3200-3400 lbs and have normal sized street tires. This pressure is also a good target for a hot tire at a track day. I recently drove at C7 at the Autobahn Country club which had a cold pressure of 28psi (recommend by GM for a track day), but after a few laps, I noticed that the pressures increased to about 35psi. The goal is to have as little pressure as possible without causing damage to the tires (Don’t drive on the sidewalls) as the contact patch will always be larger with less pressure.

The weight of a vehicle has a large impact on tire pressure required. For instance, an Ariel Atom is about 18psi front and 20psi rear; and if I recall correctly, my schools FSAE car ran 10-15psi.

biasPly

This tire shows the typical shape of a bias-ply tire when deflated. The tread area dose not have the supporting radial plies that are found under the tread in modern radial tire. Because of this, bias-ply tires are more sensitive to pressure changes. 

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1 Comment to "Tires – Sidewalls vs Air pressure"

  1. Wilson's Gravatar Wilson
    September 4, 2013 - 12:46 am | Permalink

    I just read about the new Tweel design being experimented by Michelin.
    http://www.scienceonline09.com/technology/tweel-airless-tires/
    It’s really interesting as it’s based on the concept of airless tires.

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