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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: Front Suspension

Posted 06-10-2011 at 03:24 AM by color0
Updated 06-10-2011 at 03:27 AM by color0
As promised, this week we will be taking a look at front end setup on the MR03 -- but before I can even begin we have a problem: unfortunately (or fortunately, depending on how you look at it), the MR03 chassis has a LOT of front suspension options available. As hobbyists and weekend racers it can be difficult to choose between all them. To date, I know there are at least 5 possible configurations:
  1. Stock-style
  2. Long kingpin/spring-over
  3. Reversed kingpin
  4. Dynamic strut/spring-under
  5. Double A-arm
The 1) stock style suspension has been superceded by 3) reversed kingpin suspensions, given that the springs and geometry don't change, just the orientation of the kingpin (and it makes a world of difference for smoothness!). For a more radical departure from stock, the 2) long kingpin style and 4) dynamic strut styles, marketed by Reflex Racing, provide very linear spring action that 1) and 3) do not. And lastly, the 5) double A-arm that PN Racing produces does away with stock parts entirely to produce optimized custom geometry and spring rates at the cost of additional weight.

So which one does one choose? I would argue that it's a matter of personal taste and constraints, with the exception of the stock configuration which is just junk. The long kingpin suspension is immensely tunable and easy to live with, however the knuckle-on-kingpin sliding means you have to clean it more often to keep it smooth. The reversed kingpin and dynamic strut styles are nearly maintenance-free once you put them together, but the former suffers from progressive spring action and the latter has geometry constraints (can't lower the chassis too much, limited spring selection). The double A-arm is extremely optimized to produce high cornering grip, but only within a narrow range that it was designed to operate within; if your personal preference lies outside of that range, it can be difficult to make the double A-arm perform for you.

With that in mind, I ended up deciding to run with the reversed kingpin setup. I like being lazy, so frequent maintenance of the front suspension is not on my favorites list; I like running super low front ride heights, ruling out the dynamic strut configuration; and I run my tires at a variety of overall diameters while wanting to keep ride height consistent, and this range was too much for the double A-arm to maintain a reasonable geometry at all times. Thus, I purchased the parts for a PN reversed kingpin setup and proceeded to customize it to exactly how I want it. Take a look! Here's a semi-exploded view (missing the lower arm) to show you guys all the shims and tricks that I used to get the right heights and handling feel I want out of my front end.

Top and center is the PN MR03W (wide) tower bar, and attached to it the PN MR03 1deg camber suspension arms. These are mounted to the tower bar with two M2 0.2mm thick steel shims in the rear, creating about 2deg caster. This has been the optimal setting for me at most tracks, I keep finding that 4deg caster (Reflex tower bar) doesn't give me the turn-in I want and 6deg (turning the PN tower bar around) is entirely excessive. So no matter how much I tune the springs and geometry I keep the same basics: 1deg camber, 2deg caster. Considering my tire wear patterns I would have liked a 0.5deg suspension arm but unfortunately that is not available from PN. At RCX, I bumped ideas with Ryan Sagisi, who used 0deg arms instead and shimmed his front end appropriately to get some more camber out of them. Clearly this decision didn't let him down, as he smoked the entire field (including me in 2nd place!) on his way to the RCX win on Sunday.

Below the tower bar as well as above the knuckles you'll find several pairs of PN M2 0.2mm steel shims -- these serve to raise the entire tower bar/suspension arm assembly 0.4mm, keeping my kingpins from dragging on the track (I didn't want to grind them due to heat/distortion concerns). It also raises the roll center a tiny bit and reduces slop a tiny bit (same joint movement, less leverage = less slop) so I am not complaining. The shims underneath the knuckle are a Kyosho MR03 stock shim (~0.6mm), a PN M2 0.2mm steel shim, and a PN M2 0.2mm anodized aluminum shim (orange). I used these because I was in a hurry, however it's worth noting that the PN steel and anodized aluminum shims have different properties. The steel shims can be stacked in different ways to get different heights, you can change about 0.1mm worth by stacking differently. It's worth playing with. If you look very closely back at the picture, the shims under the tower bar are both cone-in and the shims above the knuckles are one cone-in, one cone-out. This is how I stack them in my actual build, under the tower the cones both face down, and above the knuckles the cones face opposite directions (outwards, both). The anodized aluminum shims, on the other hand, are 100% flat when new, so you know they're always precise and I make ride height/preload/droop adjustments with these whenever possible. Obviously, I ran out underneath my knuckles but I'll be getting a new set.

Now the front clip: because my front end is shimmed up so high I didn't want to buy a ton of spring shims to compensate. Instead I put two Ti-screws through the front clip, secured them with 1.6mm thick steel nuts, and put two PN M2 0.5mm anodized aluminum spacers (blue) on them as my preload "stack". This preloads the springs just enough such that with batteries and body on, the car sits with zero preload and zero droop. Just touching my front end lightly will depress the front suspension (that's how you know there's no preload), but letting go reveals there's no downtravel left when the suspension rebounds (that's how you know there's no droop). Note that this is my low-roll-center setting, for low-grip tracks such as RCX's. For high grip tracks, I do one of two things: 1) if the track is bumpy, the orange PN shim moves from below the knuckles to below the springs: as you might imagine, this has the effect of angling the suspension arms upwards, raising the roll center, and raises the ride height as well, letting me use smaller tires to avoid traction roll. Because one shim was moved from inside to outside, the preload and droop settings don't change. Or, 2) if the track is smooth, I simply switch to the blue PN springs. I get a tiny bit of preload, but the initial steering reaction is quicker and I have an extra safety factor against traction rolling, even if I decide to use softer front tires.

All assembled, my front end looks like this!

For smoothness, the upper and lower balls are all PN Teflon balls, lubricated with Kyosho Fluorine oil: this provides that silky-smooth frictionless feeling that you get with the PN A-arm suspension, and it lasts a pretty good while too before you have to reapply it. I also Fluorine'd the metal-metal hinge for the suspension arms... as metal on metal joints wear rather quickly I wanted to do everything possible (short of Teflon sleeving the hinge pin... that'd be ideal, but too much work!) to make everything smooth and reduce wear as much as possible. A quick note, unfortunately the Teflon balls are VERY soft, and attempting to fit them into the stock Kyosho arms or lower arm will result in destroyed balls. For any plastic arms you'll want to use Delrin balls, while they're not quite as smooth, they're much tougher than Teflon and you can actually snap Delrin balls into the pivots.

Take note of the suspension arm angles: if you want perfectly even handling left-right you'd better make sure these angles are equal, and that nothing else is messing up the front suspension geometry that you need to compensate for. Check the arms occasionally too, if you bend one and don't notice, your setup is going to be negatively affected. I myself bought the PN MR03 tower bar and lower arm because I didn't like how the Kyosho ones could tweak randomly and reversibly, so I switched them out for rigid versions and now my suspension geometry woes are all gone.

As a final word, some people like to use damping greases in the front end of the 03 to smooth out the overall drive. If you feel like you might need this, the damping grease should go into the lower ball joint (for reversed kingpin/dynamic strut), or the knuckle (stock style/long kingpin), or the spring collar itself (double A-arm). Personally, I feel that the dampened steering response is not worth the smoother overall drive, since the 03 is all about quickness and agility it doesn't make sense to me to take that away from the car, as long as the front end is not chattering under steady-state cornering. To date I have not had to use damping greases in my front end yet, the minimal friction provided by the snug-fit Teflon ball joints seems to be plenty sufficient. The Reflex crew tells me though that 3k and 5k diff gear greases can be useful for calming the front end of the car down, so you may find this information useful.

And I think that about covers it! To summarize, here's what I try to address when I build a new front end or tune my current one:
  • Front suspension style (see list at top)
  • Spring linearity/progressiveness
  • Camber
  • Caster
  • Toe (I almost always default to 0)
  • Shims under tower bar
  • Shims above knuckle
  • Shims below knuckle
  • Preload shims/stack
  • Knuckles (ride height tuning)
  • Arm geometry symmetry
  • Ball joint fit
  • Ball material
  • Ball/hinge joint lubrication
  • Damping

Since I've already covered my body and aerodynamics developments several times before, next week will be the wrap-up for batteries, electronics, ICS and transmitter settings. Cheers!
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