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Build Complete: MST CMX

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Build Complete: MST CMX

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Old 12-20-2016, 04:11 AM
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Have you ever had tires you liked so much you bought an RC to use them on? I have.



About a year and a half ago I built the Vaterra Slickrock shown above, and I scored some discontinued Pro-Line Chisel 1.9" tires to put on it. The tires are incredibly good for their size, but the Slickrock just couldn't keep up; the center of gravity was too high so the truck could clear obstacles without high-centering all the time, and it had terrible torque-twist. After finalizing the build, I only drove the truck a few times, and eventually it just sat on my shelf. Eventually I decided to sell the Slickrock, but I wanted to keep the tires and find another vehicle to use them on.

This is what I came up with:



This is a CMX crawler, made by Max Speed Technology. It's sized to fit Tamiya bodies intended for the CC-01 and CR-01, and in fact the Ford Bronco body on this truck was made by Tamiya and painted by MST, so all I had to do was attach the grille and wire-up the lights. Most of the mods I made to this truck were along the lines of replacing plastic parts with metal parts, and finding ways to protect the edges of the lovely Tamiya body.



The gearbox gears were all plastic, and honestly they probably would've been fine, but I prefer steel gears so I sprung for the steel gear kit. I had to take the entire drivetrain apart to replace the bushings with bearings anyway, so it wasn't out of my way to upgrade the gears while I was at it. The center driveshafts were also plastic, and I replaced them with the optional steel CVDs. The motor was a generic 26-turn disposable motor, which I replaced with a Holmes Hobbies TrailMaster Sport 27t motor, because motors just work better when they spin on ball-bearings.



The front axles are CVDs, but an interesting quirk about them is they're the old style that just has a cross-pin passing through a slot, with no barrel-pivot at all. I bought a spare set of CVDs, which do have the newer pin-and-barrel design, but the old style works fine as long as you shim the stub-axles so they don't slide around. I saw no reason to take the front axle apart a second time just to replace them, especially since you have to disassemble the diff case to remove the axles -- you can't just pull the axles out the sides of the diff like you can with an Axial or Vaterra crawler.



In keeping with the "replace plastic with metal" upgrade theme, I replaced the stock plastic links with aluminum links; I went with brown anodizing because it matched the motor-mount plate. The aluminum links also came with aluminum pivot-balls instead of the stock plastic pivot-balls, so it's a double-whammy for improving durability.

You can also see the side rails I added; they are Junfac side rails for the SCX10, mounted directly to the frame via an extra set of holes I drilled for the purpose:



The sides of the body have a bunch of surface scratches on the outside where it leaned against rocks during initial testing, so hopefully that won't happen anymore.



The stock front bumper was durable enough, but it was positioned too low and too far out, so it seriously interfered with the approach angle. The rear bumper, on the other hand...didn't exist. So I bought a spare stock bumper, and some RC4WD metal bumpers, and sliced-and-diced the stock bumpers to form mounts for the RC4WD bumpers.





Now the front has a much better approach angle, and the rear edge of the body is protected from damage.



I mentioned the aluminum suspension links I installed, but I didn't address the steering links before. There actually wasn't a steering link upgrade kit, so I made one myself; I chopped the ends off the stock plastic steering links, drilled holes for mounting screws, and attached them to links from a second suspension-link kit (the 267mm-wheelbase kit). I measured carefully before cutting the plastic links, but I didn't get it quite right, so the truck will have to make-do with having steering links about .1mm too long.

The aluminum pivot-balls from the second kit came in handy for upgrading the pivots on the main steering link, whereas the drag-link uses cylindrical bushings so it didn't need any upgrades. I added an O-ring to one of the pivots on the main steering link, so it wouldn't wobble on its pivot-ball. (I don't know why, things like that annoy me.) Because the drag-link is bound to smack into rocks on a regular basis, I decided to cover it with clear heatshrink to try to reduce scraping. No idea if it will work, but it's cheap insurance if it does.



The battery is held down by a couple of large O-rings that serve as rubber-bands, which stretch over hooks on either side of the battery tray. I got annoyed with the O-rings falling off when there wasn't a battery installed, so I drilled a couple holes just below one set of the hooks and drove in a pair of set-screws to capture the O-rings. Now they *can't* fall out.

In a couple previous photos, it can be seen that there are springs surrounding the front body posts. As with my SCX10 II build, these springs serve as electrical contacts to power the lights on the body, so I don't have to unplug anything when I remove the body. (I tried that approach before on my CC-01, and I damaged the wiring because I kept forgetting I had to unplug it when I'd remove the body.)



The lights on this truck are pretty simple, just two headlights and two taillights. To save wiring, I was able to power both taillights in-series with a single resistor to keep them from burning-out, because red LEDs have a very low voltage drop of 1.6V each, whereas the power supply from the ESC is 5V. The headlights had to be wired separately, because white LEDs have a voltage drop of 3.2V each, so two of those in-series would require a 6.4V power supply to drive them properly.

I like it. It works a lot better than the Slickrock did, with much less torque-twist, and the Pro-Line crawler tires may not be scale-accurate but they make it possible for this little truck to clear the same rock gardens my SCX10 II can.



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Last edited by fyrstormer; 11-24-2017 at 06:30 PM.
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Old 11-24-2017, 06:30 PM
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Fixed all pictures in this thread.
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Old 11-30-2017, 08:33 PM
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How do you shim the stub axle? I just split my cvd axle ball end in half and split the cvd steering cup in half. Also where did you find the new version of steering cvd with pivot inserts?
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Old 11-30-2017, 11:01 PM
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The new parts are listed on Max Speed Technology-MST. It may also be possible to buy them on eBay. As for shimming, you just get some 5mm x 0.1mm cylindrical shims and stack enough of them on the stub-axle that the wheel doesn't slide side-to-side inside the bearings when everything is tightened down. Don't overdo it though, or you'll pinch the bearings and they will feel notchy when rotating.
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Old 03-25-2018, 03:17 PM
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I converted my CMX into a CFX using MST's conversion kit.



The drive ratio with this kit is much lower than the CMX originally had, so I had to get creative to fit a small-enough spur gear to increase the drive ratio to match my CMX's original configuration.



The spur gear shown above is made by Kawada, and has been fitted with a center adapter from the Tamiya XV-01 parts tree (which the XV-01 never actually uses, strangely enough), drilled-out slightly to fit the CFX's input shaft. The screw holes in the gears are stuffed with teflon friction pegs from the Traxxas T-Maxx Classic to produce a usable slipper surface -- a trick that I also used on my XV-01 FF FWD rally car.

The CFX is a little more tippy with the motor mounted higher, but it performs about as well in practice because the truck is about 100g heavier overall, which helps anchor it to the ground a bit better. Anyway, it's definitely more interesting and realistic with the front-motor conversion.
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Old 03-28-2018, 03:13 AM
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Nice Bronco , so nice chassis that mst cmx , so nice...
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Old 03-31-2018, 08:32 PM
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To commemorate owning this truck for...however long I've owned it...I decided to finally paint the grille properly:



I painted the "spaces" between the "slats" flat-black, and I painted the FORD logo gloss-red. I kept the turn-signal portions of the original decal, though.

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Old 04-01-2018, 11:36 AM
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How do you like this truck?? What is it comparable to? Any performance differences between the CMX vs. CFX? Sorry for all the questions but I actually just bought a CFX kit last night.

Edit: I wanted the Bronco but settled for the Unimog... Love your Bronco body!
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Old 04-01-2018, 07:41 PM
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There are only two directly-comparable chassis that I know of, and those are the Tamiya CC-01 and the ECX Rampage. The CMX/CFX is better than both.

The CMX has a slightly better center-of-gravity, but the CFX has a more realistic layout, and also allows more space for a decorated interior if you're into that sort of thing.

I had a Unimog body on my CC-01. I much prefer the CMX/CFX chassis vs. the CC-01.
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Old 04-01-2018, 08:06 PM
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I'm not much into realism vs. That its gonna tackle the job. Will I notice less center of gravity on the CFX? I couldn't find any CMX kits for a good price that's why I went with the CFX... I just don't want to be tipping all the time with the not so great COG.
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Old 04-01-2018, 09:53 PM
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The CMX is only slightly better. Buy some lead tape for balancing tennis racquets, and wrap it around the wheels before installing the tires.
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Old 04-01-2018, 09:57 PM
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O ok. Will do! Is that just for weight? Any thing I should look out for when building it?
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Old 04-01-2018, 10:41 PM
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Mine was originally a RTR CMX, so I'm not sure anything I noticed will be especially helpful to you. I think the CFX comes with bearings stock; my CMX didn't. The CFX also comes with metal gears stock; my CMX came with plastic gears.

The stock spur gear is 0.6mod. If you want to use a different gear pitch, get a different spur gear before installing the transmission, because it's a huge pain to get in there afterwards.
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Old 04-01-2018, 10:44 PM
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Cool man! Thanks for all the helpful insight. Just getting into crawling si I hope this was a good choice!! I don't expect it to be up there with Axial rigs, just want it to hold it's own on the trail!! Haha
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Old 04-02-2018, 03:12 AM
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It will do just fine on trails and smallish rocks. It's a nice truck to take for a walk in the woods; it won't be boring in that environment like a 2.2" truck would be, but it will struggle in piles of dead leaves and twigs unlike a larger 1.9" truck would.

Personally I think you need all three sizes to get the full range of experience.
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