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A Little More Tire Science

Posted 04-20-2011 at 05:28 PM by color0
Sorry for the missing week, guys, I've been catching up with real life for a bit before sinking back into RC debauchery.

This week I'd like to share with everyone some of the little things I've with regards to tires, especially some of the things I picked up from RCX. If you've ever scoured the net before for RC info, you're most likely familiar with the notion that tires are 80-90% of your setup. And it's true! You get your tire choice wrong and you won't even be competitive in a race; choose the wrong spring and usually you can drive around it. With that in mind, you can see why it's important to get every bit you can out of your tires, not just slap 'em on and go. And here are some of the "bits" that I've learned or figured out as I myself have matured as a racer.



1) Tire temperature matters. Despite what you might think about these small cars not generating a whole lot of load on the tires, they do wear -- and that means they're being heated up at least a little bit. From the 1:1 racing school we know that warm, but not overwarm, tires are the grippiest ones, so to get the most out of your tires you have to warm them up somehow!

1a) Softer compounds heat up more easily. There's not much else I can say behind this, a softer blend of rubber in the tire allows the molecules to rub against each other more easily, which creates more friction inside the carcass which produces more heat. There's also heat produced by the rubber grabbing the track, but primarily it's the rubber working itself up. This heating effect is the reason why you don't use 6 degree tires on an extremely high-grip track: they'll generate lots of grip, yes, but they can and will overheat, leaving you with a faded tire and no grip at the end of a run. On the other hand, if the track is cold you definitely want to try out those 6 degree tires because probably nothing else is going to heat up. At RCX, the rear tire of choice was the PN 6 degree radial, having the softest compound mean the tires would heat up the most. And it paid off! Nobody NOT running the PN 6 radials was competitive that day.

1b) The amount of rubber you have affects how much it heats up. The load on the tire generates about the same amount of heat energy no matter what your tire size is; these cars are small, I'm going to approximate. However, a smaller tire (less rubber) has a smaller volume of rubber in it: if you try to distribute the same amount of heat energy into a smaller volume, physics dictates that the smaller volume is going to get warmer than the larger volume. Essentially, smaller tires get warmer. The difference can be pretty noticeable, too! I raced the same setup twice at Kenon, the first night with the front tires at 22.5mm diameter and the second night at 24mm. Just that 1.5mm difference made my car a LOT pushier the second night, the fronts just would not heat up! I lapped the field the first night, the second night I was only four seconds ahead. Keep that in mind when truing your tires, the diameter is (unfortunately, since it's wasting rubber) very important to generating maximum grip.

But truing isn't the whole story, because you can also decrease the amount of rubber that is immediately in contact with the track, i.e. add a tread pattern. This is the philosophy behind the notoriously grippy Reflex Racing SSG tires, as well as PN Racing's X-Pattern and Radial tires. At RCX I chose to use 15 X-Patterns in front and 6 radials in the rear, and the combination of soft rubber and aggressive tread pattern allowed the smaller amount of rubber hitting the track to heat up more, which generated more grip as a result. While there were other front tires in attendance at RCX that also worked (Kyosho 30 slicks/radials trued down aggressively also generated the right front grip), the un-trued, full-tread PN 15 X-Pattern was by far the tire that provided the most steering that day.

1c) Since the rear tires are normally a softer compound than the fronts, I suggest NOT truing them down to a set diameter. They can overheat if you have any decent traction or power (both factors load the rear tires), and if that happens you're pretty screwed.

2) Sidewall height matters. As the 1/12 pan car boys picked up a long time ago, a taller sidewall has more deflection, so the car is more forgiving, while a shorter sidewall has less deflection, so the car is more precise and responsive.

2a) A side benefit of truing down the front tires is that you get faster steering response. I don't see a downside, in a sport where speed and reflexes are everything I just don't see how faster turn-in can be a bad thing.

2b) In the rear, however, you might want to avoid truing down the tires. Responsiveness is good, but when you're laying down the power you might want to give yourself a small margin of error, of progressive breakaway characteristics so that even if you do slide, you can catch it and enter a four-wheel drift instead of spinning right out. Besides, the softer rear compound most racers use will heat up just fine even without truer assistance -- you risk overheating if you run the rear tires down too low.

3) If you're traction rolling even with properly mounted tires, it's most likely because your sidewalls are folding over themselves and unsettling the car before finally flipping it on its lid. A trick I've learned is to smear superglue over the outside sidewall, and let it dry. You may even want to heat up the superglue (our friend EMU uses a lighter!) to really secure that bond. The superglue does two things, firstly it provides a low-friction surface so that if your tire begins to roll over itself, it slides instead of continuing to grip the track, and secondly it significantly stiffens the sidewall to prevent folding over in the first place. This is immensely helpful for high-CG cars such as the MR-01, MR-015, and MA-010.

4) Watch the inherent sidewall stiffness of different brands of tires! I've noticed that Kyosho tires have soft sidewalls, Atomic silicone tires have VERY soft sidewalls, and PN tires generally have hard sidewalls. This gives Kyosho and Atomic tires better grip/a more forgiving drive on cold or slippery surfaces, as their sidewalls deflect more, and heat up a little more. On the other hand, PN tires with their stiffer carcass respond faster and more precisely on high-traction surfaces, and the stiffer sidewall prevents traction rolling even without using the superglue trick.

5) If you want to soften up your tire a little bit without changing compounds or using traction compound (ILLEGAL on RCP btw, it kills the surface), drop the rim diameter. A tire stretched over a 20mm diameter rim is going to be slightly harder than a rim stretched over a 19mm rim. So, if you run the same thickness of tire, you'll notice slightly more front traction, but at the expense of a slightly delayed steering response. You have to have some pretty fine-tuned hands to feel this difference, but I assure you it helps when you're looking for that last extra bit of steering on a cold track and there is no softer compound you can get.
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