Reducing slop in the RC4WD AX2 transmission

[fyrstormer]

Several years after buying my Wraith and installing a RC4WD AX2 transmission in it, I finally decided to modify it to reduce the slop in the shifter assembly. Here's what I came up with:

I bought a piece of 4mm solid brass rod, cut some pieces off it, epoxied them into the slots on the sides of the gears, and then sanded the sides of the gears smooth so there would be no protruding edges for the shift puck to get stuck on and rip the brass inserts loose. (not that I don't trust the strength of JB Weld, but why tempt fate?) I could've gone further and filled-in the other side of each slot as well, but I would've needed to grind away some of the round profile of each brass insert to leave enough room for the shift puck to engage properly, and I just didn't care that much...this time around. ;-) Maybe next time I have to take this transmission apart, I'll finish the job.

Hope this helps someone.

 

[TheLetterJ]

Do you ever shift under load, or just from a standstill? My son has an AX2 and I notice the freeplay when changing direction, like it winds up then dumps the clutch so to speak, also when going from power to break and it rolls a little extra. I could see how reducing the freeplay would definitely reduce that shock load (and maybe prolong the life of the rest of the drivetrain) but is it noticeably more difficult for it to switch gears in motion?

 

[fyrstormer]

I will sometimes shift while lightly applying the throttle, but never above that. Shifting a dog-clutch transmission under load is generally a Very Bad Idea because of the enormous torque applied to the entire drivetrain when the motor and half of the transmission are forced to change speed by thousands of RPMs in a couple milliseconds while also transmitting significant power to the wheels. Does it matter for a vehicle as small as a RC truck? Not really, but I prefer to go easy on the transmission nonetheless. Besides, the purpose of having a 2-speed transmission in a truck like the Wraith is to avoid burning-out the motor when driving in difficult conditions, not to improve acceleration to top speed like in a road car, so there's really no reason to bother shifting while moving -- unless you simply forgot to shift while the truck was stopped. (and yes, that does happen to me on a regular basis, so I'm not judging.)

Anyway, what you describe when driving your son's truck is exactly what this modification is intended to reduce. There is no effect on shifting while the truck is moving, though if you don't have a servo-saver attached to the shifter servo, it's more likely that shifting while the truck is stopped will result in the servo straining until the motor turns the gears just enough for the dogs on the sides of the shift puck to drop into the slots on the sides of the gears. Ideally the slots would be only slightly larger than the dogs (no, I don't know why they're called "dogs"), and the edges would be beveled to help the parts self-align. Then shifts would happen without much resistance and also without lots of slop in the drivetrain, but...well, I really don't know why they made the slots so large. Maybe it cost less to have 3 wide slots per gear instead of 6 narrow slots.