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Color0's Micro RC Blog -- A technical brain dump from the mind of yours truly...
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Lot of Pivots -- MR03 Double A-Arm & T-Plate Hack

Posted 02-13-2011 at 07:54 PM by color0
Updated 03-08-2011 at 09:30 AM by color0
Hey everyone, this week I finally built up my MR03's new PN double A-arm front end properly, as well as implemented a clever T-plate hack taught to me by EMU on the Mini-ZRacer forums. I haven't hit the track with these changes yet, but I do have some impressions to give, especially pertaining to the new front end by PN Racing.



1. Horizontal play is VERY minimal. That's wonderful, steering precision should be improved over my old front end (reverse kingpin, worn down though).

2. Close to zero bump steer. The stock-type and reverse kingpin front ends have a little bit of bump steer, you can see if if you take out the springs and slam the car to the ground. On the double A-arm this is reduced. Not eliminated, but reduced. Every little bit counts.

3. Just like the original MR02 double A-arm system, suspension geometry looks right and on-the-fly camber adjustments are awesome. But the best thing about the transition from 02 to 03 is that the upper and lower arms can be shimmed up or down respectively -- potentially useful for fine-tuning roll centers.

4. Need shorter springs. With the ride heights that I like to run AND 20mm rims, I'm on the lookout for shorter springs that will actually droop a little bit under load, unlike the stock springs in the kit which are a good stiffness but way too long, thus getting preloaded like a rock. Maybe AWD springs will be a good candidate?

5. Massive toe out. The "0" degree tierod with the double A-arm kit gave me a good 1 or 2 degrees of toe out, which is NOT what I like to run: in 70t classes it's all about efficiency and so I run the least amount of toe out possible, if not 0 toe exactly. I put in the "1" degree tierod after the initial install and now the toe is OK, less than half a degree of toe out.

6. Shim the Delrin balls. If you look really closely at the first picture you'll notice blue washers under the E-clips, those are Philip's M2x0.5mm thick machined aluminum washers. There is a lot of vertical play in this front end if you don't shim these balls. In my case I sanded down the ball's flange about 0.1mm, put the washer in, E-clipped it back together, and now I've finally gotten rid of the play.

7. Break in all the hinges. When I first threw mine together, every single hinge in the front end was tight. Spend the time to work each hinge free (not to the point of slop, just make it free) so that you don't have to deal with it later.

8. Threadlock screws or tension them somehow. I dropped two setscrews out of the aluminum when not using threadlock -- definitely use it!

9. Once the front end is broken in, lubricate joints for maximum performance and better longevity. Metal-on-metal joints are NOT ideal for lasting a long time, so if you want to avoid replacing parts on this front end, you'd better oil the hinges, ball cups and spring perches.

10. While you're at it, you should probably grease the ride height adjusting screw, it will give you front damping since the spring perch slides over it. I haven't put any grease on yet, as I haven't even properly broken in the front end yet, but once I myself get to step 7 I will be greasing it up, probably 3000 or 5000 CST oil to soak bumps and avoid numbing my steering response.



In the rear end of the car I got taught a trick by EMU: introducing pivot point set screw.



Look closely and you'll see the head of a set screw poking against the G10 T-plate. This is an easy way to get the "pivot screw" functionality of the Reflex T-plate mount, although of course the Reflex mount provides many more options as to where you can put this pivot point (this method would give you two locations max, and only one if using the PN T-plate mount like I do). The set screw pivot is a little time-consuming to set, it's all trial-and-error. You basically try to get the screw even with the T-plate mount such that the T-plate never leaves contact with the pivot screw (important!) but the set screw also doesn't excessively preload the T-plate (that'll break the T-plate eventually). By supplying an explicit pivot point to a soft T-plate that normally might flop all over the place, it makes the suspension feel much more solid and I imagine I should register an improvement in precision. My set screw is so far back though that bump stiffness does get raised a little bit. I have to back off my top shock preload a tiny bit to compensate for the increased stiffness. We'll see the actual handling effect once I hit the track again.


Weight with the double A-arm is about 2g heavier than the stock front end, but considering the number of moving parts that is not bad at all.



The chassis is 92.1g, the body (not shown: still the heavily modified 2008 GT-R GT500) is 34.6g, and the batteries (TRP 900) are 47.3g for a grand total of 174g flat. Somehow, I'm still under weight for PNWC competition! So no worries about weight, per se, the double A-arm is fine to use unless your body has not been race-prepped at all.
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