Front and rear tire choice and weight distribution

[Claude Rouelle]

In any case I was told it could be difficult to find any Hoosier tires for FSAE / FS competition next year. I heard the company has been bought by Yokohama and they have merged together to create a new company with a new name....

[JulianH]

Still.... That tire does exist and to my knowledge (and I attend at least 6 or 7 FS competitions each yea) it has never have been used in any FSAE / FS competition. One team was close to the answer and then gave up a few years ago. Just think about the type of car and the type of race duration that are similar to FS in the racing world. The search is not that difficult. Well it could depends on how many languages you know. Think outside the box. It is there.

Languages in AMZ:
German (including the famous Swiss, Austrian and Liechtenstein dialects)
English
French
Italian
Spanish
Luxembourgish
Croatian
Romanian
Serbian
Greek
Russian
Portugues
Dutch
Afrikaans
Chinese
Japanese

Still, we were not able to find it.

"The team that gave up" could only be Berkeley with the American Racer tire, but this one did not show any performance out there...

So we are back at "nope, there is none".

[Silente]

hillclimb tires?

Anyway, regarding the front weight distribution, i guess one thing which still deserve to be mention is the differential.

In my experience, it could change dramatically how a car handle.

What i have seen, for example, is that big/heavy GT cars with pretty good power and very high CG requires something completely different than, say, a small aero single seater with not too much power.

It turns out that they also use completely different TLLTD to Weight distribution settings, still having different front and rear tires.

[Claude Rouelle]

Julian H,

Finding the "right" tire.... well....how do you know if it is the "right" or "better" tire if you do not have any information of, for example, measurements of forces and moments tested (lab and ideally also track) in similar test conditions than the "reference" testing conditions you are used to?

You also need to design and tune your car around those specific tires. If you have something like 5 times the critical damping on your rear dampers because that is the only remedy (I mean patch on patch) you found to cope with your rear axle longitudinal compliance and your chain jumping out of the sprocket then you won't get a lot of grip and consistency and driver confidence for that tire whether it is a bad, good or magic tire.

Claude

[JulianH]

Julian H,

Finding the "right" tire.... well....how do you know if it is the "right" or "better" tire if you do not have any information of, for example, measurements of forces and moments tested (lab and ideally also track) in similar test conditions than the "reference" testing conditions you are used to?

Claude

Luckily, we have the possibility to test all tires that we get our hands on with a tire test truck on real tarmac, not on unrealistic ground like Calspan. Sure on the car it is sometimes difficult if e.g. a tire needs more camber than the Hoosier (which determines the design of the suspension).

So far, there was no tire out there that showed any promisse compared to the known ones (Hoosier, Goodyear (sad to see them go) and Conti).

But if you tell the world which option you see out there, we will of course try them out...

[JT A.]

Yeah the American Racer tires for midget oval cars were the closest thing I could find when I was digging around. Then Berkeley tried them on their ultra-light car and they apparently had no grip. So the search continues...

[Claude Rouelle]

Yes, you are and your team is lucky Julian . In fact not "lucky" just smart, curious and resourceful. You use you R&I (Resources and Imagination)

We have done test on a trailer too as well as with WFT (Wheel Force Transducers) on a vehicle. These testing methods have their limitation too.

No simulation or simulator are perfect but some are useful.

At the end it depends more of the experience and skills of the user / operator / test program definition and data analyst. I think the important skills of such person(s) are non emotional objectivity, open mind and curiosity, more than just education.

Understand the sensor and making sure you have reliability is important too.. We have a rule of minimum 7 ideally 10 test sessions of the same tire in the same conditions to have enough data base to build useful, applicable, reliable trends and conclusions that will allow you to use that tire.

Unfortunately and ideally you need a lot of tires to do that job: you cannot restart a test with a tire that has a different temperature and went through a different number and type of heat cycles.

[Claude Rouelle]

Silente,

You have 12 +1 causes for the yaw moment: 4 tires Fy, 4 tires Fx, 4 tires Mz and the aerodynamic Mz Vs Beta (and if you are picky the roll and steering angle) of the car. In some conditions (part of the corner, brake and throttle application) the tire Fx plays a big role in the Grip, Balance, Stability and Control. That is what ABS, Traction control and mainly ESP is about.

You do not need to have an heavy car to make the tuning of the differential an important part of the car performance. The reality is that most FS/ FSAE teams (much lighter car than a GT) do not have a differential model, do not take the time to test different differential settings

***

As far as weight distribution and roll stiffness distribution of, for example, an LMP2 (I think you play in that area) you will have about something like 46 % weight front and 70 % antiroll stiffness front (K&C or 7 post rig measurements, not Excel spreadsheet). There are are reasons for that... The tires, the aero, the compliance and the transient response have to be taken into account. An LMP2 is rigid - surely compared to a FSAE - but "more rigid" front than rear: that is why a small change of FARB stiffness will have more effect than the same % change of RARB stiffness: a stiff spring on a soft spring is still a soft spring.

[maxay1]

"2. If we exclude the effect of camber and pressure...." ???????

And what are students to do if they are unable to procure this mythical tire that only Claude knows about? Will they have to actually 'engineer' their car at this point? There are probably a couple of racing series in which the tire is specified, and the front to rear mass
distribution is not suited to the cornering stiffness of the tire. And certainly most mass-produced vehicles available commercially have the same tire front to rear, are we to believe that only the designers of the SMART car have the requisite knowledge to
properly design a vehicle?
Some foods for thought.....

[Claude Rouelle]

MaxAy1,

It is all about perspective and the way the reader(s) want to interpret my posts .....or the productive debate (or controvesry) that we want to create.

1. The "no camber no pressure" is a simplification of the question to Tim. No need to remind me or the audience of the importance of these factors

2. Of course you need to "cope" with the tire(s) you have and sometimes with the weight distribution you have. In racing and in passenger car. A Nascar weight distribution will change close to 1.5 % from full to empty fuel tank. Same (but lesser) issue with a Porsche GT3 wit their front fuel tank (compared to a central fuel tank of a Ferrari 458 for example). On a passenger car .....with or without passengers and luggage? And yet, some engineers and students will do a better job than others. What is your point?

3 I never said that the Smart was a smart car. Just pointing that there is some limitation in Tim approach; that car was initially designed with the same tires front and rear and it made sense on the economic point of view. The "Smart" guys tried everything to get that car work; kinematics, suspension stiffness and damping....you name it. Until they put larger rear tire they could not find a good and even a safe balance. GM went through the same issue on a other car a bit more than a decade ago.

4. I am not in favor of students getting performance with "just" a better tire. FSAE / FS is an engineering competition not an apply the recipe challenge. I am in favor of students finding a better tire, AND knowing WHY it is a better tire and AND designing a better car around that tire. And when possible working the other way around by designing also a tire around the car. That is what Delft is doing and doing pretty well.

By the way it is not about the Max Ay, neither the Max Ax. It is about having the best possible acceleration in the appropriated combined X, Y, Z direction at any time. It is called the Optimum Acceleration: the OptimumG.
I could not help!


Claude