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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: Rear Suspension

Posted 06-02-2011 at 01:48 PM by color0
This week we take a look at the rear end, particularly the motor pod and rear suspension system. Obviously, the rear end in a 2WD is responsible for laying down the power, and thus you have to pay it some attention to make it perform for you. So let's start from the bottom, shall we? The rear pod I'm using as of this moment is the PN Reconfigurable Motor Mount:

There is of course the Reflex mount also out on the market but for Kenon, I need to have the super low-CG motor positioning that only the PN pod provides. (Both are light and very versatile compared to old-school style mounts.) For RCX you can see in the picture that I left it at a "standard" low-CG position for the 94mm wheelbase; the shorter wheelbase provides more traction and the motor positioning provided a little more roll and a little more corner "pivoting" action, both good things to have on the tight, slippery RCX track.

There's nothing special I did to the rear pod, though I did replace the full gamut of steel screws with titanium socket-head screws. The RCX handout 80t motor is completely unmodified, in fact I didn't even break it in other than running it HARD for as many laps as I could before qualifying started. I'm not denying it, I got very lucky with this handout motor and in a class with such little power, this particular motor provided a sizable advantage. Lots of torque -- gearing up to 3.94 made me equal to everyone in the infield but a relative rocket down the long center straight! That was pretty much my strategy for the race, hang on in the infield and catch up/blast ahead on the straight. I think it worked OK, hehe...

The motor pod is attached to the chassis via a PN G10 #5 T-plate. I like these T-plates a LOT, they've proven to be stiff enough that the pod isn't floppy and sloppy but they're soft enough that the vast majority of the spring and damping work is handled by the tri-shock. For RCX, I left out my grub screw center pivot -- the grub screw usually improves precision and responsiveness at the expense of total traction, and RCX was a case where traction was certainly the more important commodity in demand!

Make sure everything's square when you attach the pod to the chassis. If it's not square the car will not travel straight and it'll feel tweaked as well! Once again note the thorough use of titanium screws: on the MR03, replacing all the steel screws with titanium shaved me almost 2g off the total weight. That allows me to replace all that weight in the form of lead in the center tunnel of the chassis, lowering the CG and polar moment of inertia in all axes. I have neglected to take a picture of my lead weight, but it was about an inch long strip of lead, 1.5g total weight, mounted to the chassis center tunnel via electrical tape. I didn't press the tape all the way down into the center tunnel of course, just had the tape sealing off the bottom of the chassis. This kept the lead as low "in" the chassis as possible, and possibly had an aero benefit as well. I still think CG is king, and my setups and "hacks" are always going to reflect that.

The loads transmitted from the rear pod are handled mainly by the tri-shock:

I elected to use tri-shock instead of the disk damper because I think this is the future; the tubes are smoother and return to center more consistently than the disks, allow easy tweak adjustments and require less maintenance from race to race. The disk damper is easier to work with, true, but for ultimate performance I don't think I'm ever going back.

For my car I chose the PN tri-shock system: it is heavier than Reflex's but I prefer the metal shafts rather than plastic (less prep work!). The side springs are 3racing AWD Black springs, smaller and lighter than PN springs. By feel they are roughly in the stiffness range of PN's Blue or Orange springs, but I don't have a proper measuring tool to prove that. The spring collars were adjusted till there was zero tweak and minimal preload, then I increased the preload by two full turns to make sure the springs wouldn't pop out under normal suspension loading. You'd be surprised how little the suspension actually needs to move -- even with a soft setup it's only about 1mm each way on a smooth track! Damping grease used is Kyosho 15k grease, this seems to be the best-damping, longest-lasting grease in these little tube dampers and I seem to be able to use it no matter what the traction conditions are. I had brought some Associated 7k grease in case the car wasn't hooking up, but I never even had to use it. Note: RCX was a very smooth, very green track, so 7k or 15k probably both would have worked, providing more dynamic or more consistent handling respectively. On a bumpier track I would have dropped down to 7k on the top shock, otherwise the rear end can leave the ground on large bumps!

Please note the angle of the side shocks: to get the most linear feel you can, you want to mount these shocks perpendicularly to their direction of action. It's not always "perfectly flat" that produces linear motion. Mine are a little angled down, probably a little bit more than the setting that would create perfectly linear movement.

Furthermore, note that I'm using Delrin balls in the tri-shock to reduce weight. Don't overtighten these! Delrin is not the stiffest material and cranking down on the nuts will bind up the shocks. I just put some threadlock on the nut, tighten the nuts gently and leave the whole assembly alone.

Meanwhile, the top shock is also PN. I've been debating between using the Kyosho oil shock or the PN for some time, both perform great but the PN is easier to tune and the Kyosho is lighter. For RCX, not knowing what to expect, I went with the PN shock.

The main spring is a PN Red, to provide a little more on-power traction at throttle tip-in, and increase overall steering a little, versus my typical setup which involves a PN Blue. Meanwhile, my rebound spring is super hard! Because I needed even more rotation off-throttle but didn't want to screw too much with the main spring, I switched from a PN MR02 Yellow spring (fairly hard) to an Atomic MR02 RCP Yellow spring (ROCK hard). This way, as soon as I lift completely off the gas at speed the tail end is lifted upwards more quickly than with the PN Yellow spring, hence more weight lies over the front wheels and I get more off-throttle steering. The Atomic spring is a little short though so I spaced it out with a Kyosho AWD spring spacer (about 0.5mm thick) to get some clearance with the PN shock pieces. Once again, damping grease is Kyosho 15k for its excellent damping and longevity.

For 94mm and full-height rear tires it seems that I had to raise the entire tri-shock system to avoid suspension binding and tire rubbing. I did not disassemble this for pictures but underneath the main mounting plate (on each side) are two Atomic AWD Spring Shims, one 0.4mm (blue) and one 0.3mm (red), for a total of 0.7mm spacing upwards. Above the plate there is an additional 0.3mm (red) shim to help the PN anodized washers clamp down on the main mounting plate.

Take note of the differential axle height adjusters and the damper arm height on the PN reconfig mount, this was the lowest I could set the arm and still have sufficient clearance between the tires and the Ti-screws on the bottom of the carbon tri-shock arm. For 94mm you can put the damper arm right in the middle of its "travel" (but to get to the lower parts of that travel you need to do some manual grinding of the damper arm mount -- FYI). For 98mm I try to get the arm as low as possible, and then of course lower the main mounting plate all the way.

Finally, please note the top damper angle. The more horizontal the damper, the more linearly your bump stiffness will increase as you move across the suspension travel; angling down creates a positive progressive feel (stiffness increases exponentially) and angling up creates a negative progressive feel (stiffness increases less as you go through travel). Nobody I know angles the shock up on purpose, it doesn't make sense to have negatively progressive stiffness. I myself went in between linear (horizontal) and progressive (angled down); mine is a little bit angled down, and the spacer is a ground-down PN Delrin ball. Progressiveness was a nice thing to have at RCX, I was constantly pushing my car past its traction limits (how else can an untalented driver like myself TQ a big race? ) and having a progressive rather than sharp breakaway made it much easier than it could've been.

I think that about covers my rear end setup! A summary of the important points I'd be looking at when putting the rear end together:
- Wheelbase (shorter = traction, longer = corner speed)
- Rear pod CG (higher = pivoting, lower = smoother cornering)
- Rear pod weight
- Gearing strategy (infield vs outfield advantage)
- T-plate flex
- T-plate precision
- Grub screw pivot? (Yes for precision, No for traction)
- Overall chassis weight before ballast
- Ballast positioning!
- Tweak!
- Tri-shock vs. disk damper
- Shock weight
- Shock shaft smoothness
- Side shock springs
- Spring preload (don't let them fall out!)
- Top shock spring
- Rebound spring (PN shock only)
- Spring size, weight
- Damping grease
- Side shock angles
- Tri-shock clearance to body, wheels
- Ride height

The list is fairly extensive so I hope I have everything. It's a lot! The rear end's behavior is crucial to getting maximum performance of the car. Next week I'll cover the front end, and how to put one of those together for optimal results.
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