M.L. Toys
M.L. Toys
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Need new motors? Grind a gearbox? Adding teeth to a pinion?

This is my first post, but I have benefited greatly from the posts on this forum. So thanks.

I have been tweaking and upgrading my son’s Dynacraft Tonka Truck, and have upgraded the battery to 18v by running a 12 and 6 in series without issue. I want for him to be able to drive on grass, so I replaced the stock 550 motors with Traxxas 10-turn 775’s. I had to modify the gearbox a bit, but I was able to mount it without issue.

The problem is that the wheels will only spin if the truck is in top gear when pressing the accelerator. If I switch to ‘low gear’ or reverse they do not respond at all. I have wired the motors with only one of the two black and red lead wires attached (2 come attached).

I cleaned out the gearbox and added new grease to decrease resistance.

I noticed that there is a 10 amp fuse in front of each motor, so I’m thinking that these motors might pull more power and require higher amperage fuses; however, I would think that top gear would be more likely to result in a tripped fuse.

Any suggestions would be greatly appreciated!

Please let me know if I can provide pictures or clarification.
Last edited by riesscar on Wed Dec 20, 2017 11:32 pm, edited 1 time in total.
"The problem is that the wheels will only spin if the truck is in top gear when pressing the accelerator. If I switch to ‘low gear’ or reverse they do not respond at all. I have wired the motors with only one of the two black and red lead wires attached (2 come attached)."

If you connect the wiring as it was prior to replacing the motors does it work properly?
Were you talking about the wires FROM the new traxxas motors or the original vehicle wiring TO the motors? After reading your responses I'm gonna go ahead and assume you are talking about the four leads coming FROM the new motors. You only need to connect to one red and one black. The other red/black are provisions in case you want to use them for another voltage tap.
If you replaced both motors then one is already running in reverse rotation from the other. Sounds like a failed hi/lo switch.
Thanks for the response. I was indeed talking about the 4 lead wires, and I wired them in the manner you described. I’m going to reconnect the stock motors and see if it is a failed hi lo switch or something to do with the motors.

I did replace both motors, but my thinking was that maybe dynacraft wires one of the stock motors with reverse orientation, such that the polarity is switched but red still goes to red and black to black. In this situation, wouldn’t one of the motors I’m installing need to be reversed as well? I’ll chech the orientation of the motors individually, and if they are both running in the same direction I’ll reconnect the stock motors and see if it’s a faulty switch.

One other thing I wondered is whether the voltage goes though a relay to each motor, thereby sending only 1/2 the voltage to each side. If this is the case, each would only be receiving ~9v since I’m running 18volt power source. Wouldn’t this create an issue given that Titan 775’s are rated at 16v?

Thanks for the advice!
I got it to work... I just basically went through the whole install again: repositioned the pinion gears, mounted motors to gearbox, and reinstalled gearbox according to manufacturer instructions. I suspect that the issue was either that I had one motor not connected, or that the pinion gear on one or both was positioned incorrectly, causing so much resistance that only the voltage supplied on ‘high’ setting could spin the wheels. It could also have been that I tightened the wheel but too much, leading to resistance. Anyway, it’s running well.

I have one major concern, though:

The gearbox on this ride-on had a plastic sleeve protruding from where the motor is mounted... in other words, the motor slid into this plastic tube pinion side down, and was then mounted with 3mm screws from the other side.

I assumed that this was simply a support for the motor, and because its narrow diameter prevented installation of the much larger 775’s I cut the sleeve off of the gearbox. I made a support of sorts and used blue thread lock when mounting the motors. I now realize that the protruding plastic that I removed was also a stabilizer for the gearbox: it slid into the body of the truck (along with the motor and wires that connect it) through a hole. When the truck accelerates, the gearbox shifts slightly, until that plastic part comes into contact with the edge of the plastic hole through which it sits. So, the gearbox is not screwed into the truck... it slides into the axle on one end, and inserts into a hole in the body of the truck in the other.

Now to my concern:

Without the plastic tube and with the 775’s being appr the same diameter of said tubes, it is now the motors themselves which come into contact with the body each time acceleration causes the gearbox to shift. My concern is that the motor might shift or that the metal end into which the mounting screws go will bend (which would in turn shift the motor).

My question: how strong are these motors with regard to durability? Will repeatedly agitating a few millimeters and then knocking into the edge of a mounting hole damage them. In my testing they seemed strong and did not demonstrate any shift in position or concerning behaviors. If there is a risk, has anyone ever come across this issue and is there a way to attach a larger sleeve to the gearbox (pvc piping or adapter with 2” diameter?).

I can take pictures if it would help to clarify, but I’m hoping that this is not an uncommon issue.

Thanks to all that have helped, and to anyone with advice on my current concern.

You need to find the directions about installing these and adding the spacer, either you can buy these, or drill out a piece of pvc pipe to make them. If I had the post right now, I'd attach it! There's a great post w/pics and all you need to know. They say you need those spacers to maintain proper spacing.
dlgg7- thanks for posting that. I think riesscar's concern is a bit different than the spacer inside the gearbox that I referenced in that post; he's talking about a support sleeve around the outside diameter of the motor's case (other end of the motor) -- presumably that connects somehow to the gearbox, and serves to limit gearbox movement within the body of the car.

I've never looked at one of the Duratrax cars but based on the description given would agree that using the motor case as the main limiter could possibly present a problem -- not because the motor case is too weak, but the mounting for the motor case to the gearbox may not be intended to support a large torque at the other end of the case.

Without a picture of the area around the gearbox (including how the old sleeve was connected), I can't really suggest much. If the old sleeve was actually part of the gearbox molding and nearly the same size as the 775 can, it would be pretty strong and would have a very stable connection (40mm-diameter circle of plastic) vs. the current setup, which is the face of the motor pressed against the gearbox and 2 M4 screws. However, if the gearbox is very rigid in that area it may still be sufficient.

I know if it was a Power Wheels gearbox I'd be hesitant to let the motors bang into things as the limiter, since it will flex the gearbox with a torque impact at the far end of the motor, temporarily affecting the pinion fit to the plastic gears, and it would likely result in damage to said plastic gear since that will happen at the time when it also has the most force on it (abrupt start/stop).

riesscar - do you have any pictures from the gearboxes, either before or after?

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