Motor voltage not reaching 7.2V. Vintage setup. Car slow.

[Bakauata]

I've felt, from the first time I ran it, that the RC10 I put together last year (seemed very slow compared to videos I've seen of vintage cars running.
I've never had an electric before, so really didn't know what to realistic expect, and had nothing to compare it too either.

Last week, however, I got my hands on a Tamiya Hornet re-release (Expert Built, RTR), which has hardly been used, and today I gave it a quick run.
To my surprise (or maybe not) it ran incredibly fast (and smooth) compared to the RC10. So I did some tests:

I measured the voltage at the motor on full throttle on both cars. The Hornet measured the expected 7.2V, but the RC10 measured only about 6.7V.

Connected the Hornet's ESC to the RC10's motor and again it measured around 6.7V

Connected the RC10's ESC to the Hornet's Motor and it measured 7.2V.

So it's not the ESC or battery.

What could account for the voltage not reaching 7.2V at the motor? The motor itself? Resistance in the drive train? I haven't yet tried uncoupling the motor from the transmission and measuring again. However, I measured both cars with the wheels off the ground, so I figure the resistance has to be pretty low compared to having to move the car.

The Hornet seems so much smoother when I engage the throttle. Not sure if it's the motor or the transmission.

The RC10 has a six gear transmision, Trinity Green Machine 2, 27 turn stock motor, and a Futaba MC112B ESC.

Thanks for any pointers.

[radioactivity]

Bakauata

Maybe a couple of thoughts.
1. Are you using the same radio for both cars? Not sure of the radio you are using for the RC10, but could it be a throttle endpoint issue?
2. Binding in the transmission is easily diagnosed by removing the pinion and turning the rear wheels. Should spin easily.
3. A pinion set way too tight on the spur will cause binding.
4. Either 2 or 3 will probably cause the motor to heat up and even overheat quickly.
5. Pull the motor and disconnect wiring. Thoroughly inspect and clean brushes and commutator. Check solder connections, bearings and capacitors.


Chuck

[Bakauata]

Thanks Chuck.

When I did my tests I used the same transmitter on both cars. Basically put the two cars side-by-side and connected the motor from one to the ESC of the other. So, tried both cars with both radios. Hornet measured 7.2V with Futaba radio and ESC, and also with Tamiya Radio and ESC. And vice-versa: RC10 measured 6.7V with both Futaba and Tamiya radios and ESCs.

I'm going to disengage the motor form the transmission of the RC10 and see if the voltage remains low. I'm thinking maybe there's a short or leak in the capacitor or soldering.

The pinion and spur are not binding, but when I remove the pinion, the transmission in the RC10 isn't the smoothest. I'm going to have to re-check that I think. The transmission in the Hornet feels very free in comparison. The transmission in my nitro cars feels much freer also.

[coxbros1]

Looks like u got it narrowed to tranny or motor. Pull that hornet motor and see if u get 7.2 out if the rc10.....if not, u got sone 6 gear work comin your way

[TRX-1-3]

If it was a tranny / drivetrain issue, wouldn't you get 7.2V at the motor regardless? The drivetrain would be harder to turn and the motor temp would raise quicker and the battery would die sooner ( all other things being equal ). I would lean towards a frayed, badly corroded, or partially severed motor lead. Maybe under the insulation. Or a really "cold" solder joint.

Any suspicious looking bulges or floppy points in the RC10 motor leads? Or a really dull or chunky looking solder joint? Bad ground?

Just my thoughts.

Happy troubleshooting!

[Bakauata]

That's my thought too. I'm not the electricity expert though. But I figure if the voltage is low, there is something going on with the circuit itself. I have other motors I can try also.
I'll do some troubleshooting and get back.
Thanks guys.

[Timmahhh]

It sounds to me like the RC10 is putting an additional electrical load on the system, causing the voltage drop. This could be because of the motor itself (fewer turns, binding, brush/winding issue - try a different motor to test) or it could be mechanical (driveline load - remove the motor pinion to test). Batteries don't put out their full resting voltage when a load is put in them, and the voltage decreases as the load increases. I think internal resistance is the measurement on a battery that relates to that, but honestly I haven't really researched that much, so I could be wrong.

When a brushed ESC is at full throttle, it should basically be a direct pass through of battery voltage.

[jwscab]

so you have three basic things that can be happening to review:

the motor could be worn or dirty, or the advanced timing could all contribute to drawing more current than the stock silver can in the hornet

the 6 gear and spur/pinion combo can be creating excessive drag in some fashion, binding, friction, gear spacing, etc, all leading to excessive current draw. gear ratio setup can also be loading the motor down as well.

the wires leading to the motor can be undersized, meaning higher resistance. the above two items cause greater current draw, and with added resistance, the voltage will drop.

so rule out a dirty motor, run it directly from the battery and check the voltage

inspect the 6 gear and try and fix any drag issues, BTW, this could also include binding in the axles when the suspension is in full droop.

the undersize wire you can't really fix if they are soldered to the esc. probably have to live with that.

check you gearing. you may want to drop the pinion size down if you're ratio is too high.

[Bakauata]

Thanks for the tips.

I did some further tests based on your suggestions, and my vintage motors seem to behave quite differently to the Mabuchi 540SH silver can in the Hornet.

Regardless of the speed controler I hook up, the vintage motors have a larger drop in voltage than the Mabuchi.
I tried a Kyosho Mega Motor, a Trinity Green Machine, a Reedy Mr Outlaw, and a Reedy Mr R., paired with a Futaba MC112B, MC210CB, Novak 610 rc, and the Tamiya ESC in the Hornet.

Of the four, the Mr R had the least drop in voltage, just slightly more than the Mabuchi.
I ran the tests directly from the battery also. Same results.

So maybe the voltage drop values I’m seeing are normal.

Next I wanted to check how free the Hornet’s transmission was compared to the RC10. So I pulled the Mabuchi out and, wow, I was surprised at how freely the wheels turned; It felt like there were no gears in there at all.
So I’m going to be looking into RC10’s transmission.

But I did come across a surprise when I had the Mabuchi in my hand: It’s very easy to turn this motor with your fingers! There are no perceivable “detents” where (I imagine) the armature aligns with the magnets. The other motors I have (all of them) have a significant resistance you need to break in order to turn them.
I figured this would be related to the strength of the magnets, but I didn’t notice a significnat difference when using either motor to pick up a piece of metal.

Can anyone tell me what makes a motor much stiffer to turn than another? Does stiffer make for a “stronger” motor?

Next I’ll be trying the Mabuchi in the RC10 and the vintage motors in the Hornet, for comparison. See if I can draw any conclusions from that.

[Bakauata]

Pulled the transmission from the chassis and was happy to find that the gear box itself spins very freely. So I don't have to go in there.

Next problem is the axles. I'm pretty sure most of the drag is coming from here, and not only when the suspension is in full drop.

First thing is, I mounted the suspension mounts in the rearmost position (that is, I used the two holes which position the mount closest to the rear, and therefore extend the wheelbase).
I didn't do this for any other reason than that the chassis plate I'm using has a little extra material which does not allow for the A arm to move freely if the mounts were installed forward (See picture in next post).

This makes the dogbone axles always be at an angle from the gearbox to the hub. Should I file the chassis plate down so that I can fit the mounts forward?

I've checked a couple of manuals and none mention which position to mount at, as if there were only one set of holes in the mount.

[Bakauata]

Here’s the picture of that extra lip on the chassis plate which touches the A arm when the suspension mount is installed in the forward set of holes.

Bottom chassis in photo is the A-stamp I’m using, top is a B-stamp.

[jwscab]

yeah the A stamp chassis should be trimmed if you plan to use the forward mounting position. That was a running change from A to AB to B. the car should still be OK in the rear position, as it was run that way with the 6 gear as shown in the manual.

an important piece to the rear suspension that is often missed is to be sure to add the 5/8" silicone tube (or plastic spacer) inside the rear shocks. if you don't do this, the suspension will get bound up on droop.

I would very carefully check over the hubs/bearings/axles/wheel to figure out if you've got binding there somehow. It can happen, as I've had issues in the past there as well.

yes, motor fields can be drastically different in strength. old worn out motors that got hot and overheated will have w a weaker field. cobalt motors and rare earth magnets can generate a much great field also. It's a tradeoff between generating power with a strong field, to losing power due to an overly strong field compared the overall motor design.

[Bakauata]

I've narrowed down the source of friction to the rear axles.

I had hoped to get away with using the worn bushings and axles that came with this car, but the wheels wobble and bind rather than spin freely. So I'm already replacing the bushings with a set of bearings I had set aside for another build.

But I'm also getting some binding in the slots where the dog bones engage the axle. The slots are a little rough, so I'm going to file down the edges and make them as smooth as possible.

On a side note. Can anyone ID these hub carriers? They fit the A arm space perfectly and have all the holes in the right places, which makes me think they are intended for an RC10.
The bearing space in these is 10mm, which might make it easier for me to get bearings for them. I would need a new metric axle too; maybe for hex hub? Wheel clearance might be an issue.
Curiously, the hinge pin hole is smaller then the RC10 pin, so I would need to enlarge that too.

Not sure what some of the other parts are. Too short and too few holes to be servo or aerial mounts.

[GoMachV]

Those are all Ultima parts (not rere). The rectangles are servo mounts (kyosho didn’t use 4 screws per servo just two) and the shield shaped part is an access cover for the ultimas gearbox.

[Bakauata]

Thanks. There was a Kyosho Rampage gp 10 in the lot, but the hub mounts are different.
I’ll put them aside for now.