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Color0's Micro RC Blog -- A technical brain dump from the mind of yours truly...
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MR03 Chassis Swap (or Why I Hate the 03 Sometimes)

Posted 12-30-2011 at 12:44 AM by color0
Hope everyone had a merry Xmas!

Just about 1.5 weeks ago I went to Inside Line Racing for their last points series race, and managed to come home 3rd in Expert Modified, not saying much though as there ended up only being 4 cars in the running! Anyways, my setup sucked -- traction roll and a complete lack of grip compared to the other cars, my fast lap was a full 0.6s behind everyone else! I suspect that the combination of low roll center from the double A-arm and stiff SP grey chassis on the 03 is combining low roll resistance with super-fast steering response, causing my traction roll issue. I've had enough of that, so we're going to revert back to my pre-RCX setup which actually worked well for most anything. Here's the old setup (goodbye!):

And we're going to swap to the stock black 03 chassis, and the PN reverse-kingpin setup. Softening the chassis will slow steering response and make the car more stable, while raising the roll center using the reverse-kingpin front is generally a good measure to stop traction rolling.

First off, prepping the chassis:

In RED is the spot that you need to cut into in order to make room for a 2x2 stack of FETs underneath the board. The MR03 comes with two 1x2 stacks, but I have a 1x2 above and a 2x2 below the board (AN0113, btw) so a little room needs to be made. To avoid heating and possibly warping the chassis, instead of a Dremel I use an Xacto knife to dig out the necessary plastic.

In BLUE are the two latches for the battery clips. I cut my battery clips in half to save weight, so I have no use for those two latch points and cut them off with side clippers.

In GREEN, finally, is the non-optional prep step, removing the plastic flashing on the main chassis. You must cut all the flashing away from the chassis tub or the top cover will not fit properly. I don't think the location I've circled is the only one, but it's the most common one that I've seen.

Next, plugging the switch hole:

If you haven't removed the power switch from your 03's electronics, don't do this step. If you have, however, filling in the hole with superglue is a good idea to prevent RCP dust from going straight up into the center tunnel.

Now to transplant electronics: The 03 is fairly easy in this regard. After taking apart the old chassis to expose the servo and PCB, you unscrew the battery terminals and just pry up the PCB with a pair of tweezers:

And then yank it out of the chassis by the servo case. Comes right out as a unit.

Plop the servo into the new chassis, but before you nudge the PCB back into its detents, one more modification. If you have soldered wires to your PCB instead of screwing them in, they will probably stick out through the board and if you tighten down the PCB like that, can tweak the chassis. So I carve out both screw holes with a knife to avoid all contact whatsoever.

Now for the (NOT) fun part: Reassembly. I like to use M2 threaded (machine screws, like the type that goes into aluminum motor mounts) screws in the chassis as they stay tighter, but the black chassis strips out way too easily! My solution for now is a tissue/rag.

Using the M2 screw itself as a tap, I cut 2mm of thread into each hole, pull it out, squeeze the screw threads (hard!) in the rag between my fingers, and with the other hand, unscrew the screw from the rag. This cleans out the threads of any Kyosho plastic stuck in there, and dramatically reduces the likelihood of stripping something. Then go back in for 2mm more until you've screwed it in and tightened it without feeling anywhere like you're close to stripping the threads. This takes time: For a 4mm screw you only have to do this once, but for 6mm it's twice, 8mm it's three times, etc. etc. and all together it took me 45 minutes to get the car back together without stripping a screw hole. I've already wasted three black chassis not being patient before, so I learned my lesson lol...

Once everything is painstakingly put back together, sit back and enjoy your handiwork:

I kept the grey top cover and battery clips because I need the extra strength in those two areas (that and my battery clips are cut already, too lazy to modify another pair). Now, since you completely replaced the base tub chassis it would be a good idea to re-tune the car's tweak and make sure the settings have stayed consistent through the chassis swap. I'll detail a quick and easy way of check that tweak, next week.
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