While reading another thread, it reminded me that I wanted to discuss this topic. The purpose of this thread is simply to make people aware of the risks you take when using a larger-than-OEM wheel with a lower aspect ratio tire. I'm not claiming that upsizing your wheels is sure to result in disaster, just that you subject your car to higher suspension loads when doing so.

There are two factors at work when you upsize your wheels. One is that you will most likely be using lower-profile tires, the other is that the mass of the wheel and tire will sometimes be increased. Both of these factors result in significantly increased suspension loads, especially during harsh road events like potholes, uneven pavement, dirt/gravel roads, speed bumps, etc. The physics behind the increased loads is common sense for the most part.

1) The lower the aspect ratio of the tire, the stiffer the tire side wall. A stiffer tire sidewall will transmit more energy to the suspension components. An example of this in everyday driving is driving on underinflated versus overinflated tires. Underinflated tires are much more compliant than overinflated tires, thus harsh road events are more readily "absorbed" by the tires. When you overinflate your tires, the harsh road events can really be felt in the passenger cabin.

2) Increasing the mass of the wheel and tire increases the unsprung mass of the suspension system. When the wheel and tire experience significant acceleration due to harsh road events, the force transmitted to the suspension increases proportionally with the mass of the wheel & tire.

Why should the increased suspension loads concern you? The manufacturer designs every car to handle the road loads it will experience with the OEM equipment. During the development of a vehicle, the early test mules are instrumented and run over durability courses. The road loads are measured during these tests, and subsequent design of suspension components is based upon these measurements. So as you might expect, if a car comes with 17" wheels from the factory, it was designed to handle road loads associated with that wheel and tire combination.

Sure, there is a factor of safety involved with the design of any component, which is why I'm not claiming that you will surely experience durability failures when using bigger wheels. However, designing a component to handle higher loads increases the cost of the component. So don't assume that manufacturers include a large safety factor during the design process. Designing suspension components to handle the loads associated with 19" or 20" wheels that many kids are sure to put on the car is not exactly a point of concern for the manufacturer. After all, you do realize that is very possible that your warranty on suspension components will be voided if you have a durability failure while riding on larger-than-OEM equipment, right? This is why the manual states you should use the OEM specs when getting replacement wheels and tires. Going smaller is not an issue since this will actually decrease suspension loads.

 

Good food-4-thought there RX8_Buckeye. A question that comes to my mind (and as you are in the car engineering business perhaps you have light to shed about this) is just how much 'saftey factor' IS typically built into modern cars to cope with upsizing of unsprung weight and lower aspect ratio tires? none, 10% 20%, or more? Manufacturers must assume some kinda fudge factor to include to keep the lawsuits from suspension failure down when big heavy aftermarket stuff is put on a car, yes?

But even more interesting to my mind (as I see no functional point to the 'bigger is best' philosophy seemingly driving tire/wheel diameter sizing), is that when DEcreasing unsprung weight and tire/wheel size from OEM, as many race car teams do, are there any negaitive suspension factors besides a somewhat rougher ride to consider and compensate for?

Do you need to soften the suspension at some point to compensate for less weight? And what about the very stiff spring rates (and stiffer dampers) that come with many lowering springs and coilovers, is that also considered more stress on the rest of the suspension components as road irregularities are put into the chassis more quickly and directly with less damping?

And lastly, does all this that we try to do to decrease unsprung weight really matter that much? For example, unsprung weight includes the moving suspension components and their weight, not just the wheel/tire, so the % decrease we make with light wheels/tires isn't really very much and is in the end, hard to actually determine short of taking apart and weighing the suspension too.

In the RX-8 if the OEM wheel/tire is 48lbs, and purely guessing the suspension components are another 48lbs, that's approx 100lbs - so a very low weight 15lb wheel saves maybe 10lbs at best over the OEM wheel. So 10/100 is a 10% unsprung weight reduction. Can that difference be felt / and how much gain in performance, if any, is really realized at the track?

In other words, is it really worth it?

 

Good questions Spin9k. I am not extremely knowledgeable when it comes to vehicle dynamics, so you'll have to rely on someone else to answer those questions. As far as the safety factor goes, it really varies from company to company and even from department to department within the same company. It is important to note that many of the suspension components affect the ride & handling characteristics of the vehicle as well as NVH. When designing these components, trade-offs must be made between these attributes as well as durability. It is often the case that what is good from a durability standpoint is not so good for NVH. Therefore, it's not as simple as implementing a large safety factor for durability.

Other suspension components are essentially rigid castings for the sole purpose of transmitting loads. The most important attributes of these components are durability, weight, packaging, and cost. Once again, trade-offs must be made among these attributes to arrive at the best design. Improving the durability of a component often involves adding or changing material, and/or adding complexity to the design. In an industry where saving $0.50 or 1 lb in a vehicle is substantial, you can see why a component might be designed to be "adequate" from a durability standpoint, rather than "more than adequate".

Another thing I should mention is that the peak loads that the manufacturers consider during the design process are intended to represent worst-case scenarios. These are some of the very worst road events you would experience in the real world, and the average driver typically won't come close to experiencing these loads on a daily or even monthly basis. This, in itself, is a bit of a "fudge factor".

In the rare cases in which a severe suspension durability failure occurs, it is most likely due to extreme driving conditions, heavy modification of the suspension, or a combination of both. In my opinion, anyone who changes out the OEM suspension components with little regard to what the manufacturer has to say about it does not have a leg to stand on in the courtroom. I think a judge (or jury in the case of a criminal lawsuit) would feel the same way once educated by someone from the manufacturer. It is simply impossible to design a vehicle that is robust to all the possible modifications that an owner can make, which is why warranty is usually voided when doing so.