Wheel size discussion

[AG_]

Wanted to have a discussion about tire sizes - which is more suited, and for what.
Our 10s (Keizer rim, hooiser 18') weigh 13lb each. An older set of 13's weighed 18lb each. So, thats a 20lb difference, plus lets say 10 more for upright size (probably is less) - A arm would be about the same. So, around 5% weight gain with driver inside.
Looking at the TTC, 13s make much more peak grip - around 25%. So why go for 10s? Doesn't seem like a very advantageous choice. I joined the team after the choice was already made, and unfortunately didn't look into it till recently. The reasons I was told were "weight saving, spinning inertia" (on a high 400s steel frame car), "grip is more usable", and "heats up faster".
I havent dived into TTC in depth yet, relying on the findings of an alumni - who advocated for 13s, yet 10s were chosen.
Curious to see some of your reasoning in tire choice, as honestly I cant see the benefit in 10s. Or at least, not a one that warrants the switch.

[Claude Rouelle]

Consider
- Rim + tire weight
- Rim + tire rotation yy inertia. Think braking and acceleration.
- Rim + tire rotation xx inertia. Think roll and lateral weigh transfer in transient.
- Rim + tire rotation zz inertia. Think steering effort in transient
- Tire sizes effect on aerodynamics drag and lift. And cooling.
- Rim stiffness especially camber stiffness. A classical 13" 3 pieces aluminium rim has easily a camber stifness of 0.7 degrees per G. If you have 1.5 G lateral that is about 1 degree of camber compliance just for the rim
- Effect of non-suspended masses in yaw inertia. Huge effect. Four 10 Kg non suspended masses (not easy to be that light even more difficult with 13" rim/tires) have about 50 times more yaw inertia that a 4 cylinder traversal engine. Just make a simple calculation.
- Speed at which the compound operational temperature (and therefore grip) is reached.

When you compare the grip of a 10 and 13 in tire make sure the data are comparable. Look for example at the flat track belt temperature

[MCoach]

Fortunately, there are teams very close to you that have a variety of tires used.

Oakland University runs the Continental 13" tires.
Kettering University and UM runs the 10" LC0 tires.
Michigan State runs the 10" R25B tires (last I checked...).
Wayne State and CMU run the 13" R25B tires, 6" wide fronts, 7.5" rears.
Correct me if I'm wrong.

We've organized a few get together events for testing before and even done car swaps.
You can feel a lot of the minute effects that you can see from the tire data (and data log them!!!), and some of the terms may be more critical than first impression.
Aligning torque, range of peak grip vs slip, and combined lateral/longitudinal forces come to mind first.
Another thing that comes to mind is the average peak slip angle throughout a corner and how that plays into your aero package's ability to produce downforce and sideforce.


When looking at the tire data make sure to compare the same load cases. The 10" wheeled tires I think were only tested to 250lbs of normal load due to de-beading where the 13" wheeled tires were tested to the scheduled 350lbs.
Correct me if I'm wrong here again.


There are some benefits to differences in the tire construction towards 10" tires. I don't want to downplay the importance in Cluade's response, but unsprung inertia may be overhyped in FSAE due to how light these cars already are, compliance and component weight/stiffness efficiency seem to be bigger issues.

Have you driven your car? It would be eye opening for you to set up the car for the 13" tires you are advocating for (my guess is the 6" or 7.5" wide R25B on 7 or 8" x 13" wheel) and the current tires to be tested back to back.
This could be as simple as wheel centers for the 13" wheels to match your center lug pattern and new push rods (make them out of steel, ain't nobody got time for carbon test pieces).

[AG_]

Claude -
I will definitely look into it in the way you suggested - I am not fully sold on all of your points, but there is no use in me countering with no concrete data to back me up (yet). The weights I mentioned are for wheel and tire w/air.
What the alumni told me is that above a certain normal load (car mass, distributed between F/R, disregard aero - will not be added this year) 13ins are far superior in terms of grip, for the 5% extra mass I mentioned. Will have to look into that more, but thats the argument I want to examine first thing.

MCoach - Really, car swap? thats quite a bit of trust haha. I have driven the car, although it wasnt really an optimal track (tiny dusty parking lot), and it had a good 1.5 sec of throttle lag - so not a good objective test. But, The lug pattern matches, could probably steal 13s off of an older car at our shop and try it out. Uprights will take it for sure - they are like 9lb each, for A arms will have to make sure.
Yeah unsprung inertia is theoretically super important, but in the end we are still traction limited right? (maybe not as much the single cyl cars) so for cornering I guess it could play a role, but it all depends on when the tires reach their limit - I will have to do the calculations I guess, with TTC data.

[Claude Rouelle]

AG, MCoach,

I disagree with the idea that the contribution of non-suspended masses to total yaw inertia is not important because the car is already that light.
Just make a simple calculation of the percentage of yaw inertia due to non suspended masses on let's say a passenger car and a FSAE in 4 cases: 1 cylinder or 4 cylinders and with or without wings.

Worth for you to make a few simple step steer and frequency response simulation and decide from there if Yaw Inertia is important or not.

This thread initiated by Bill Cobb could be very useful: http://www.fsae.com/forums/showthrea...g-Testing-1-2-...

Look at what yaw inertia will do for a given step or ramp steer on Lateral Acceleration Response Time (LART), Yaw Velocity Damping (YVD), Lateral Acceleration, Cg slip angle, and Yaw velocity Overshoot, Lateral Acceleration, Cg slip angle, and Yaw velocity settling time and look at a Bode Diagram with amplitude and phase shift for different yaw inertia (and by the way different tire - 10 and 13 " relaxation lengths) and you will tell me if yaw inertia is important or not.

If you are non convinced by the simulation and/or as a useful addition to your simulation, put a ballast at the CG (let's say 10 to 12 kgs), probably close to the driver's butt to keep the same a and b (same weight distribution - that is very important because 0.25 % weight distribution variation makes a significant handling difference - the ratio yaw moment derivative vs the yaw velocity derivative is a^2 and b^2 sensitive - if you don't you will change too many parameters at the time) and the same CG height.
Ask your driver to make a few laps and/or, even better, a simple slalom with 6 to 10 cones.

Then split your ballast in the same proportion as your weight distribution in 2 parts as far away as possible, keeping the same weight distribution and CG height (a part your ballast somewhere in the nose, the other part near the differential)
Ask your driver to drive the same course and ask him if he feels the difference. Look at your gyro signal and the derivative of your gyro signal vs your steering input.
Compare the change of yaw inertia due to your centered or split ballast with the yaw inertia of let's say 10 kg and 12 kg of non suspended mass per corner.

Let us know.

Just remember everything is transient (except - maybe - to a certain extent - skid pad), especially on the type of circuits run in Formula Students competition.

[Z]

AG_,

As you may have noticed, old people often have strange growths of hair. Long whiskers growing out of eyebrows, or tufts of such growing out of ears or nostrils.

These are, in fact, finely-tuned BS-detectors. They start twitching whenever something seems not quite right.

Mine went ballistic when I read this.

Looking at the TTC, 13s make much more peak grip - around 25%.

Same track-surface, compound, temperature, Fz, width..., and ONLY a change in rim-diameter??? No, no, no... Not right.

Z

[Claude Rouelle]

25% more grip with 13" than with 10 " rim? Wow... Imagine what you would get with 15" or 17 "?

[BillCobb]

As my old friend Marcellus once said to Horatio: "Somethings rotten in the state of Denmark". We need a second opinion on this 10 vs. 13 grip comparison, eh ?

Maybe take this to the TTC forum and come back with the check sum.

[acedeuce802]

If that data if from the TTC (even if it's processed elsewhere) you'll want to take that picture down from here. This discussion is better suited for the TTC forums, you are not allowed to share data publicly, or to those who do not have the TTC membership.

[AG_]

Wasnt aware of that, makes sense though. Thanks for letting me know! Deleted it.
As for the rest of the discussion - I realize I'm somewhat misinformed on tire matters, will do as suggested and thoroughly go through TTC