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Color0's Micro RC Blog -- A technical brain dump from the mind of yours truly...
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RCX 2011 Setup: Wheels and Tires

Posted 05-17-2011 at 02:30 AM by color0
I've wanted to do this for quite some time, but had to wait till I had some race results to back it up lol. With this next series of articles I'll be doing a full documentation of my setup and setup theories as applied to a Stock-class Mini-Z Racer. At the same time I'll be commenting a bit on how Stock and Mod-class racers differ, and how might you approach the particular points of the setup differently. It's gonna be long and comprehensive, so stick around! At the end of each section I'm going to try something new, and sum up all the points that I addressed or brushed over, that I think are important and pertinent to having a winning setup.

Let's start by saying that my RCX setup was specifically geared towards the PN 80t Stock motor, and for a VERY slippery, brand new RCP track. I'm only going to cover the setup that I finalized on Sunday, as that was definitively the faster (and possibly the fastest at RCX!) setup I wound up with. And to be honest, most of this setup was in the wheels and tires -- not only are picking the right tires and offsets 90% or more of a good chassis setup, tire management throughout the race day has proven to become increasingly important with one- or two-run tires like we have today.

So, as we've been through before, for 2WD cars the rear tires are the all-important tire. You have to have sufficient rear traction to handle all the power you have, yet not too much that the rear end can't rotate off-power. So for RCX, the tire of the weekend was the super-gooey PN 6 radial:

This was the only tire anybody found that weekend that had enough grip for the new RCP, and so honestly we didn't really have a tire choice! (Sometimes, in Stock racing you don't need the max-grip rear tire, and instead can go one or two steps harder to reduce scrub and increase corner speed and rotation.) Interestingly, in the recent batches of the Radial 6's, some of them were molded super-wide and super-fat (13mm wide mounted, WTF??), and if you happened to get one of these, you had to either A) make it fit, B) trim it so it fit the rim, or C) screw it and deal with the funky looks. The compound on the super-wide 6's was particularly soft and particularly fast-wearing, which would have led to overheating and traction rolling problems for sure if I was driving Mod. So I picked through several packs of PN 6's and got the ones I thought were the least "fat" and the least soft. For maximum grip we found that these tires should NEVER be trued: when I tried, the rubber compound turned to silly putty, warm silly putty in fact, and picked up so much track dust they became unusable. So the tires you see here are untrued, and the radial pattern has been worn down by racing action.

For their part, these 6's were mounted on Atomic S6 20mm rear Delrin rims, +1.5W offset. I then spaced the rims out on each side with three PN M3 0.2mm shims for a total offset of +2.1W in the rear. These wheels are plenty strong (only have one small dent, thankfully they did NOT crack like their black plastic counterparts) and are some of the lightest rims available. I choose spoked rims over dish rims whenever possible for flex reasons: all the dish rims I've ever tried are too stiff in the hub area, which makes them strong, yes, but at the same time they don't flex enough to give you maximum traction. So, especially for Stock 80t where every bit counts, and this brand-new slick track, I chose the Atomic S6 rims for more flex. The PN spoked rims would have also sufficed, and are roughly the same weight, but I made a style decision here.

I use a bearing in the rear left as well as rear right wheel: the rear right wheel bearing is a Reflex high-grade 3x6x2.5mm until I can get another ceramic, and the rear left wheel bearing is a rusted-over Reflex high-grade 2x6x2.5mm. I have no idea why it rusted over (I cleaned and oiled it every 2 months or so) but since it did, it serves its final purpose in my rear wheel. I tend to use bearings here instead of a plastic bushing for precision, and I pick bearings over the Qteq nut because the Qteq nut's centering mechanism could easily crack a rim hub if you crank the nut down too hard and subsequently get hit. Hence, metal bearing. Purely a precision and durability decision on this point.

Meanwhile, the front tires have to balance out and complement the traction afforded by the rear tires. Too much grip will overwhelm the rears, too little will cause understeer, and too much scrub will just make you slow. Thankfuly, at RCX the PN X-Pattern 15 tires were not finicky at all, I used up completely only one pair and the second pair, as you see here, still has some tread left.

I really have no complaints whatsoever about this tire, it's very versatile and can be adapted (read: trued or prepped) for many different track conditions. I true this tire down to 23mm (mounted on 20mm rims) when I run at Kenon, then it warms up nicely and provides lots of grip without traction-rolling or overwhelming the rear. At RCX though, not even the trued tires would hook up, so Grant's idea (and he was right!) was to run untrued, full-height X15's on 19mm rims. The X-pattern tread reduces the amount of rubber in actual contact with the track, so what did contact the RCP heated up better, providing more grip. This was one of the primary factor in transforming my car from a mid A-Main car to a definitely-the-fastest car. The full-height X15's had a bit of scrub, actually, but also had so much steering! The scrub allowed me to brake using the front tires, meaning that I never actually had to get off the throttle completely, and the steering advantage I had over everyone else was small, but very evident.

Now a short discussion on front wheels and front wheel bearings. To get the maximum amount of steering with very little setup time, I decided to mount the X15's on Atomic S6 19mm front Delrin wheels, +0N offset. The spoked design, as mentioned already, increases flex over dish designs and improves grip; the zero offset reduces scrub, but increases steering responsiveness so I can chuck the car into turns faster and turn in inside other cars. Yes, it did make the car difficult to drive when the rear tires started to fade, but given fresh rear tires I had wonderful amounts and balance of steering and traction, and that's what gave me the ability to keep that TQ even after Ryan Sagisi got his binding problems fixed. In a discussion I had with Ryan after the race, we both agreed that a little bit more front offset than +0N made the MR03 a little more manageable on slippery surfaces, but alas I didn't have +0.5N wheels or enough extra threads on my front axles to space out the front wheels using shims. (Fairly certain that if I had spaced out the wheels I might've lost a wheel nut mid-race, and DNF'ing is much worse than 2nd place!)

As for the front bearings, you may have observed that one of them is a Reflex, the other not a Reflex. For whatever reason, two AVID bearings did not have enough material on their inner races to avoid binding up when I cranked down the wheel nut on the entire bearing stack! So, feeling adventurous I dug out some old Reflex bearings, cleaned them, put them in the outside hub face, cranked everything down and -- it works! Everything spun more freely than I had seen in a very long time. So that's the setup I'm putting in these particular wheels for the time being.

As for the race performance: I've mentioned before that the PN Radial 6's are a one-run tire, and this became painfully evident over RCX weekend. I got to experience for the first time how fast they really degrade! I ran the first qualifier on new rear 6's, setting a very good TQ pace. I kept this pair of tires for the next 2 qualifiers and steadily watched my performance get slower and slower, despite my nerves stabilizing I could feel that the tires were slowly but surely reaching the point where I actually wasn't getting enough traction to even lay down the meager power of a PN 80t handout motor! So for the fourth and final qualifier I went to fresh tires again, and immediately reset the TQ pace by 6 seconds to achieve a perfect TQ record, taking all four rounds.

Here I made a mistake: I didn't change tires before the race. Lo and behold, on the very first lap I already found myself at a traction disadvantage! All I had done between Q4 and the main was run a couple laps of practice, but even this had done enough to the tires that my race was already lost to Ryan. Hard lesson well learnt, I will certainly be prepping more tires in advance from now on such that I can go into a main with optimal traction and full confidence that I can push the car and not lose it.

So to sum up this first section, I care about the following points in the wheels and tires:

- Tire compound/absolute grip
- Tire fit on rim
- Tire sidewall stiffness
- Tires' temperature during race
- Wheel weight
- Wheel flex
- Wheel strength
- Wheel offset
- Wheel-bearing fit, precision
- Bearings' freeness
- Nuts' thread engagement
- Obsessive tire management!

Get all these figured out and you will at least have a solid platform on which to work the rest of your setup and make it through your race day. Next time, a discussion on diff building and gearing for the 2WD Mini-Z's!
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