Tire Width and Rim Width Configurations

[Goody]

I am with the University of North Texas, as a second year team, we haven't been able to invest in Tire Data and are trying to consider the advantages and disadvantages of different tire and wheel configurations. I am hoping our limited experience racing is not coming across as a lack of enthusiasm or research but am hoping you all could help with a discussion of wheel configurations.

Our current car has 20.5x6x13 Hoosiers along with 13x6" wheels.

Looking at other teams, we have seen similar configurations, staggered configurations and also some ~8" wide wheels with 7" tires on very successful cars. We are looking to see what the best process for analyzing the differences in combinations to find which might suit our needs the best.

My initial thought regarding the 7" tire on the 8" rim would cause the tire to be much more load sensitive. it could maximize the section width, but remove the tires ability to generate a slip angle, negatively affecting drivability.

In the middle would be a 7" tire with a 7" wheel, we would increase our contact patch and presumably be able to put more power down and and carry more cornering speed to negate the weight increase of the components.

What methods, without Tire Data, could be used to help generate a decision for the best configuration for our car?

Thank you!
Mike Goodman

[JT A.]

You could always buy several sets of new tires and rims of various sizes and test them.

Or you could contact OptimumG and hire them to fit your hubs with 6 axis force/moment sensors and create your own tire models from driving data.

$500 for tire data doesn't seem so expensive anymore does it?

As a 2nd year team you can probably get away with just looking at what the fast cars are using, and focusing on getting the rest of your design solid. There's some "rules of thumb" I've heard of, such as choosing your front & rear tire width to be proportional to your front & rear weight distribution. That will probably get you a decent car, but not very far in the design event. When your team is at the point where you feel that tire choice & non-optimized kinematics are the biggest factors holding you back, you will need tire data to make any progress. The TTC data is by far the best $500 you can spend on the car, once you're ready to use it & design the suspension around it.

[Edward M. Kasprzak]

Mike,


We are looking to see what the best process for analyzing the differences in combinations to find which might suit our needs the best.

Honestly, the TTC data is probably your best bet. We specifically tested each tire using two different rim widths to give students some data on this variable.

The variety of tire and wheel choices by FSAE/Student teams shows there is no consensus on what's best. Students defend their choices based on the criteria they think are most important. Whatever you decide, be sure you have some firm engineering ground to stand on. A comparison of two tires, or of the same tire on two different rim widths, will be very interesting. There's no shortage of metrics to investigate, and tradeoffs to consider.


I am hoping our limited experience racing is not coming across as a lack of enthusiasm or research but am hoping you all could help with a discussion of wheel configurations.

UNT has gotten off to one of the better starts for a new team. I'm glad you're continuing to develop your program. Asking questions like this says you're thinking about good, relevant issues. No worries here.


My initial thought regarding the 7" tire on the 8" rim would cause the tire to be much more load sensitive. it could maximize the section width, but remove the tires ability to generate a slip angle, negatively affecting drivability.

In the middle would be a 7" tire with a 7" wheel, we would increase our contact patch and presumably be able to put more power down and and carry more cornering speed to negate the weight increase of the components.

Be careful. This is sounding like buzzword talk, a classic sign of speculative mental constructs without firm hypotheses or methods to prove/disprove them. You can get there from here, though. Just don't get stuck where you are now.

[BillCobb]

The Weight on the car, the weight distribution, the yaw inertia to mass ratio, the wheelbase and the front and rear tire properties are really the only parameters that make up the performance characteristics you have mentioned.

If you have performance goals and a foundation chassis and powertrain, you can synthesis the tire property requirements and tell a tire manufacturer what you need and beg or order them to produce the hardware. You will need performance specification for gain, frequency response, maxim limit and driveability to constrain the solution. If you don't have that luxury, you will have to try out all your options either in Math or in track testing to produce a 'winning' combination.

Its just part of the discipline of vehicle engineering.

[Claude Rouelle]

If a larger rim would increase the contact patch surface then why not using a 7.5" or a 8.0' or a 8.5" rim... What is the rational? For what load, what pressure, what speed, what camber, what slip angle, what slip ratio?

The advantage of a different rim size are not the same if you are looking for lateral or longitudinal grip, steady state or transient. Also major influence on the MX and My.

For a given size, the shape of the rim especially in the anchorage zone of the tire bead is very important. IT is called the hump of the rim. Google rim hump. Yeah I know....

Understanding rim compliance influence on car behavior are in my experience as important as 0.5 " rim width difference

It is not only about contact patch size or shape, it is also about contact patch vertical, lateral and longitudinal load distribution that you would like as homogeneous as possible. Carpet plot load distribution made with pressure sensitive paper (Fuji is one manufacturer) is quite useful although it is only working at rest. Kevin Hayward and his team from UWA in the mid 2000 made a pretty good first study about such contact. Not perfect (is there such a thing?) but very useful.

The dream of a tire manufacturer is to understand forces and moment distribution at speed in cornering, braking (or acceleration), or combined situations in the contact patch area. FEA sure helps. Unless you go that far (you would need a strong association with a tire manufacturer) just using and understanding the forces and moments from the tire data from the TTC will help to determine the ideal windows of efficient tire use (including the rim size effect) even if you do not understand what is happening at the macro and micro levels at the contact patch.

Buying the TTC data is a must. Not necessarily to make your car quick but to understand why. If you do so do not forget the Calspan sticker on your cars. It is part of the agreement.