Why does your team run 10s?

[Menisk]

I'm currently in the process of looking over TTC data and debating whether or not we should look at changing from 13" down to 10" wheels for our next car. I'm comparing the Hoosier 20.5x7-13 R25B to the 18x6-10 R25B and LC0 using the data from round 5 (I've ignored the 18x7.5-10s for now due to lack of data and the fact they're really not that much lighter than a 13). From my analysis of the data the 13s generate more force, have more cornering stiffness and the self aligning moment looks to be virtually the same as the 10s. In a steady state situation my understanding is that 13s should provide greater performance than the 10s. The only advantage I see in the 10s is transient stuff - yaw inertia, relaxation length and a reduction in unsprung mass.

I'd put a guess at being able to drop 2kg a corner in wheel and tyre which is definitely significant and makes a decent difference to your yaw inertia. Relaxation length on the 10s also appears to be shorter. However when I look at data from our car and overlay the steering trace and the lateral G's the delay between peak wheel angle and peak lateral force is usually 50-100ms in the absolute worst cases. I'm not convinced a drop in yaw inertia is going to make us faster either because I highly doubt our drivers can even feel the effects of yaw inertia and relaxation if the delay is usually around 50ms.

From a steady state perspective it looks like the only way to drive 10s - especially the LC0s - is sideways as hell. Surely the delay in generating all that slip angle as you throw the car side to side through slaloms offsets any gain you've made in yaw inertia and relaxation length?

My last final grasp at understanding why they're so popular is a large reduction in unsprung mass and shorter relaxation length means that if you know what you're doing with high speed damping you can get them to maintain a more consistent level of grip than the 13s on bumpy ground and overall end up faster. My understanding of this area is pretty fuzzy and when the majority of teams spring their cars ultra stiff to give them a responsive feel they might as well be entirely unsprung mass. Our team runs a soft twist suspension system and will likely run soft twist again on our next car. With wheel loads varying less than a regular car over uneven ground this may be even less relevant to us. The only difference I see is the inertia of the wheel and tyre when being deflected in the twist mode may be a consideration for the change to 10s.

Despite the fact the 13s look better on paper to me there are plenty of very successful teams winning competitions on the LC0s. My only conclusions at the moment are that they could be faster on 13s or I'm missing something big in my understanding.

So now I ask, for those of you running 10s, why did you choose them? Do you have access to the TTC data and was this an influence to the decision?

[mech5496]

When we started using 10's there were no data available for the R25B or LC0 Hoosiers. The reasons we did was the decrease in unsprung weight (more than you think) and subsequently yaw inertia. Structurewise, a smaller wheel means a smaller component in bending and a smaller moment arm.

[BillCobb]

Just a quick observation based on comparison of the dimensions of two tires for handling performance regardless of the TTC data should tell you this:

1) Even though the tires have about a 3" OD difference, the rim size (10 vs 13) produces an identical section height. (4").
2) Yes there is a tread width difference and we're not sure what rim width was used on each. This is a big player in relaxation and side bite metrics.
3) The mass difference (9# vs 13#) x 4 is only a 16# difference to the whole car.
4) Yes the OD has a theoretical effect on the relaxation length properties if you have only the tires circumference fraction theory to go on. However, test data for other types of tires indicates this is only a very weak relationship. The materials and 'parts' of the tire at the tread to shoulder interface are the big players in relaxation metrics.
5) A good guide to picking tires for performance can be made from a metric calculated from the ratio of tire section width to corner mass. You'll have to figure out for yourself what actual numbers are best for your race car, but there is a startling relationship for this metric especially when you compare cars and trucks you love vs ones that suck.
6) Yaw dynamics and response times are easy to measure with a simple yawrate gyro and a strip chart recorder of some sort, but experiments and comparisons of cars with vastly different yawrate response time are not discernible even to expurts because they are just too short for humans to sense. (some may call this 'too fast'). Sideslip response time on the other hand are very well sensed and can really identify a slow or sluggish responding car. For that matter, the front and rear axle mass (total weight) is the key ingredient. However, the 12 lbs difference could also be removed via a short, leaner and better conditioned driver. BTW, FSAE car sideslip response times are quite disappointing becase even though the stiffness are there, the stiffness to mass ratio (cornering compliance and linear range understeer is just not there.
7) Still, the inertia to mass ratio (radius of gyration) effect shows up in the handling equations and an optimum ratio can be determined for a chosen speed range once the tire cornering stiffnesses are designated by a design choice (go find a tire) or accepted (go produce a mass distribution for a tire you pick).

That's the way its done in the production world, with constant fights over powertrain mounting and component and accessory positions, and even glass and fuel tank size and positioning (you know, the heavy stuff). Trying to squeeze tire property changes from demands on handling is commonly done because tire produces are easy to bully.

Happy Holidays ! (Make some measurements). BTW:
Brake sizes and steering dementia are unfortunately also players in tire and wheel size picks.

[MCoach]

Menisk,

We used to tout ourselves as the lightest car on 13s (363 lbs). Of course, that didn't turn as many heads as we thought it should and when I came into this tire game, started to cause me problems. There is something around here where the guys from Stuttgart were saying that if your car is lighter than ~400lbs then you should be on 10s and I agree with that because one thing I've had to explore in all of this is how much tire mass can the car cope with? As in, how much tire is too much tire? Most people don't run 13 x 9s because of this thinking because they won't "come up to temp" at all. Similar happens when you end up too light, and sprung softly for the smallest option on the 13s for these types of cars. You have to find something with less tire mass. I think this goes into Bill's 5th point.

When we moved to 10s we calculated that we could save about 10lbs per corner between wheel bearings, uprights, tires, wheels, and associated control arm, stick things that hold the car together.
Ultimately, we didn't, opting for a safe design, and only dropping 30lbs on the whole car. We'll be back, as the saying goes. ;)


We made the change for the 2014 season, with the benefit of having had the tire data to look at gave us the same conclusions. I have only had the joy of testing 10" R25Bs second hand, so most of my experience and correlations exist with the LC0. I don't want to get into details on the data because there is another forum for that but I'll talk about general dynamics and handling characteristics. When putting our drivers in our new car for the first time, they tend to fail at the limit bits, not pushing the car hard enough, so I have a few things I tell them before their next drive:

1) Sideways is fast. Sideways is life. (only applicable to LC0s and drifting)
explanation) Usually if you feel like your at the edge of drifting, your not yet, the car is just getting started. The slip angle at which peak force is found is so far out there there the car can literally be at 10 degrees of body slip angle before yaw settles out.

2) If you want to be a hero, you have to drive like one. The tires will do the rest.
explanation) Fortunately, the peak is not really a peak, more of a plateau and so the transition from the transitional range to the frictional range of tire grip is about the same force output. For drivers that need a helping hand and don't care that your tires will be worn out by the end of endurance, this helps keep you on the track.

3) The tires will come up to temp before you do, your job is to make sure they stay there.
explanation) Because of the decreased tire mass, you may have to wait one lap...-ish for the tires to come up to operating temp. Unless you're semi-pro or above, drivers will need some time to get situated to the car.


Yes, steady state, the 13" R25B seems to dominate, but there is more to tires than what's putting out the most cornering force and highest stiffness and lowest mass. That'd be like rating an engine based on those numbers alone, but of course those engines rated on power alone are known as "dyno queens", only good for making those numbers. :P Similarly, when is FSAE events ever steady state? There is skid-pad and acceleration. Take a look at the FSAE results and see what their results are compared to your simulation. Do they seem reasonable?

Yaw inertia at FSAE level is almost negligible unless you're doing something wrong. I do enjoy how soft the sidewall is (leading to the above characteristics) because some of the other handling characteristics we were trying to achieve on the 13s was only doable at low (~6psi) pressure, and useless by that point, but settled nicely into the TTC test range for pressures for 10s.

Brake size hasn't played a huge problem for us, but we've recorded temps of 950C. Steering and kinematics was more of a pain. There's a lot less space in there than it looks.

Oh, taking a look at pressure effects with respect to load sensitivity may reveal some fun stuff.

[Menisk]

Interestingly we've had the exact opposite problem when it comes to 13s and heat this year. Our old car was 240kg had a 340mm CG and spent most of its life on two wheels so there was heaps of normal force and hence plenty of lateral load. The tyres came in with extremely even wear and it normally took about a minute and a half of hard driving to get the temperature into to them. This year we've been between 210-220kg (Bandaids for comp), with a CG of 265mm and we've been seeing graining and smearing of the tread to the point we've had tread wear markers filled in by smearing of the surrounding rubber. You could turn your hands black just by pressing them on the tyre and there was a nice bead of rubber ready to marble off on the inside of the tread. By the end of the first lap the tyres are already nice and tacky from rolling out of the pits stone cold.

I've had a look at load sensitivity of the 10s and their relative lack of pressure sensitivity, but it's hard to get a good picture due to the lack of data for the higher load case where you can expect an aero car to be operating. Also there's always a linear section of the load sensitivity where the drop in COF isn't so bad and then there's a point where you start falling off the cliff. But that's a discussion for the other forum.

[MCoach]

I think this year we were at 236kg with driver, with a CG of 220mm, and on LC0s. Bit heavier than we wanted, but we "chopped it, slammed it" with an overall height of 912mm.

Overall, I think it was a good move for us due to our low mass, but I still know cars like the Chicago "Beefcake" cars can be very fast on 13s. Because of our relatively low load transfer all around, I didn't really need to bother with the muddled upper range of the 10s data, so I'm not much help there. We've had a good time with these tires but I'm doing some exploration on trying to cope with peculiar low sidewall stiffness problem where the car seems stable and then "kicks" back the other direction rather violently, particarly in slaloms, and seemingly from displacing the wheel about 100mm (not exaggerating) laterally through the sidewall. This never happened with our 13s so it's not like there is a magical solution out there. Even the 10s have their own quirks.

[Claude Rouelle]

Good to take into account the outside and inside tire deflection influence (as MCoach mentioned quite big on soft side wall tires), estimate where the "real" contact patch surface center "point" is and from there what the real track and the real weigh transfer is.

Also and I need to insist on that many students do not take into account the weight transfer (or jacking forces if you want) due to
a) the tires Mx,
b) the difference in altitude between the non-suspended mass CG
c) the different inside and outside grip

On a FSAE / FS it is NOT negligible