15T Motor - Running too hot

RC_Noob

Active Member
Messages
36
Hi Guys,

My ECX AMP hasn't been running for a while. I installed 15T motor and on the first run I stripped the spur gear. I replaced that with a metal spur and pinion and on the second run the drive shaft broke. The motor was also running really hot. During lock down I struggled to get a replacement part and then just forgot about it.

I finally got around to replacing the drive shaft, I loosened the slipper clutch a little to ensure it didn't happen again and the car is up and running.

However, the 15T motor is giving a faint burning smell and braking doesn't seem as good. When I replaced the spur / pinion I couldn't get like for like ratio's.

I believe the stock was 18T pinion and 87T spur, a 4.83 ratio.
When I ordered the HR replacement ones I had to go for a 20T pinion and 83T spur, a 4.15 ratio.

I already have a heat sink / cooling fan fitted so the motor isn't getting as hot as it did on those first runs but I'm guessing the smell isn't a good sign. Does a ~15% reduction in gear ratio cause that much of a difference to running temperatures ? I do have a 16T pinion I could try that would raise but I would need to modify the car otherwise I can't get them to mesh.

Just throwing this out for ideas / thoughts as to what to do

Thanks
 

Lephturn

Well-Known Member
Messages
206
How often do you clean that motor? How hot did you get it?

If you take the pinion off or slide the motor down to disengage the pinion, does the drive train spin freely? You could have bad bearings.

How do you set your gear mesh?
 

Music411

Well-Known Member
Messages
141
You most likely have toasted the motor.

A 15T motor runs faster but has less torque than the stock 20T, so with that said by decreasing your gear ratio dramatically you effectively put more load, on a weaker motor - torque wise.

Adding methods of cooling won't/wouldn't have made a difference, the motor just doesn't have enough strength to move the car with the reduced mechanical advantage of decreasing the gear ratio. So it wouldn't have survived regardless.

To make it work and see some performance increases you would have just needed to leave your current gear ratio alone.

Another thing, adding a metal spur gear adds a decent amount of weight and drag on the motor aswell. So taking that into account you probably would have even had to slightly increase your gear ratio by bumping up your pinion a tooth or two.
 

Spanky

Active Member
Messages
44
Location
Loomis Ca
Yes you would need to go back to the stock spur gear and drop the pinion at least 2 teeth which in turn creates the lost torque. Make sure your gear mesh is right, don't over tighten your slipper its always best to run it a little loose.
 

RC_Noob

Active Member
Messages
36
Thanks guys - I've ordered a 87T spur which I'll run with the 16T pinion.

The motor is well and truly toasted, it crapped out today so i need to replace it. Would a 550 motor be better - pretty sure I've seen folks mention they can be fitted in a Amp MT and that would compensate for some of the lost torque?
 

Music411

Well-Known Member
Messages
141
If you want a better motor and want to stay brushed the 12T Titan from Traxxas is your best best. They come stock in the Slash 2WD (base model obviously).

It's 12T which is plenty fast. Equivalent to roughly 3300kv (if I remember correctly, the conversion between turns and kv's on brushless motors is iffy at best). But it's very fast for a brushed - and it is a 550 can motor. So even though it's rather low Turns, the can size compensates very well and gives it plenty of punch. I remember a few years ago being impressed when my buddy got a stock Traxxas Slash with that motor, and the Slash is on the heavier side of 1/10th scale vehicles.. roughly 5lbs I believe. Whereas the AMP is only 3.5lbs ? And I think the Torment is a touch over 4lbs... I might be mistaken or off a bit, it's been awhile. He obviously upgraded his Slash but for a brushed motor in a heavier truck it definitely didn't slouch around.

As far as getting the 12T in the vehicle, yes you will have to get creative and might have to start shaving some parts and plastic down to get it in there. But im pretty confident it's possible. I'm almost certain after my bud upgraded his Slash I put his 12T motor in my AMP, but that was a few years ago and I upgraded pretty quickly. But regardless, I'm sure it's doable.

The Motor itself is only about $35 CAD, so if your American it would be cheaper. But you can't run it off of your stock ESC. It's not rated for a 12T motor and it will melt. Quickly.

So you can run another 15T if you like, or it might just be worth looking to a cheaper brushless setup (motor/esc). But... to get the most out of that you will need to switch over to Lipo batteries.. and get a new charger...and the metal transmission gears.. aswell as some better driveshafts (the stock ones are very weak). So it's not a cheap or pain free transition. But it is worth it.

Now remember, a 12T or 15T motor is much different than the stock 20T motor. So be careful with your gearing. With either motor, i wouldn't go any lower than your stock gearing, and even then, depending on ambient temperature and surface your driving on you will have to watch it until you figure out what works.
 
Last edited:

RC_Noob

Active Member
Messages
36
Thanks - already running LIPO batteries and have upgraded the ESC though I'm not sure it is rated for 12T, would have to check.

My local store stocks a 16T 550 motor, perhaps I'll give that a go Monday to see how that works out.

Even though the 15T motor burnt out the cars fun factor was increased significantly whilst it was running :).
 

RC_Noob

Active Member
Messages
36
OK Amp back up and running with 16T 550 motor and stock pinion / spur, I finally managed to get a replacement ECX 87T spur. Performance is on par with stock so will look for a 12T motor later in the month once the metal 87T spur is delivered from HK and I can play with the gearing.

At least for now there's no motor over heating.
 

Lephturn

Well-Known Member
Messages
206
A 550 motor is exactly the same diameter, it is just 10 mil longer. It will absolutely bolt on with no modifications required.

With brushed motors, fewer turns = less torque and more RPM for the same voltage. That means that a motor with fewer turns will need a smaller pinion gear or they will overheat trying to love that truck faster with less torque. This does depend on the terrain, on pavement or something smooth with little drag you can run a lower turn motor on the stock pinion likely without too much of a heat issue. Get a cheap infrared heat gun thermometer and check your motor temps every 10 minutes when you make a change. If it’s below 100f all the time you can go up a tooth or two on the pinion. If it runs 150-170 you should probably drop a tooth. If it gets above 170 f you need to stop and let it cool down And you should drop two teeth on the pinion so you don’t burn it up. You can run a much larger pinion on smooth flat terrain than bumpy off-road or the motor-killer.... grass. Grass is a high drag surface especially with a fairly low 2WD vehicle. Don’t mistake grass as “easy”, it’s usually the terrain that generates the highest temps for me.

If you are running LiPo and have replaced the stock ESC you should just go brushless. What ESC are you running? The ESC and3,300 kv (540 size) motor in my Amp MT cost me maybe $40 total.
 

RC_Noob

Active Member
Messages
36
ESC is the Dynamite WP 60A. I don't believe I can run brushless with it though.
 

RC_Noob

Active Member
Messages
36
ARRMA 12T fitted today with stock gearing.
That motor brought the AMP to LIFE.

The motor was getting hot but not too hot to touch when running around wet grass in my back yard. Will look to get a cheap IR heat gun so I can play with the gearing safely.

The only issue was batteries drained after about 10 minutes - 2S LIPO, 4000mah. Not sure how much impact the fan is having on that and if it is even that effective at cooling the motor. Another thing to test with the heat gun.
 

Lephturn

Well-Known Member
Messages
206
Wet grass is brutal on an Amp. Very high drag terrain and worse when wet. Unless you drive on grass a lot I wouldn’t tune your hearing for grass.
 

RoyRoy

Member
Messages
22
Location
Rochester MN
A 550 motor is exactly the same diameter, it is just 10 mil longer. It will absolutely bolt on with no modifications required.

With brushed motors, fewer turns = less torque and more RPM for the same voltage. That means that a motor with fewer turns will need a smaller pinion gear or they will overheat trying to love that truck faster with less torque. This does depend on the terrain, on pavement or something smooth with little drag you can run a lower turn motor on the stock pinion likely without too much of a heat issue. Get a cheap infrared heat gun thermometer and check your motor temps every 10 minutes when you make a change. If it’s below 100f all the time you can go up a tooth or two on the pinion. If it runs 150-170 you should probably drop a tooth. If it gets above 170 f you need to stop and let it cool down And you should drop two teeth on the pinion so you don’t burn it up. You can run a much larger pinion on smooth flat terrain than bumpy off-road or the motor-killer.... grass. Grass is a high drag surface especially with a fairly low 2WD vehicle. Don’t mistake grass as “easy”, it’s usually the terrain that generates the highest temps for me.

If you are running LiPo and have replaced the stock ESC you should just go brushless. What ESC are you running? The ESC and3,300 kv (540 size) motor in my Amp MT cost me maybe $40 total.
Thanks for this! Since purchasing my MT a few weeks ago, it gets a lot of run-time in the grass. As I think about upgrading it, or just upgrading to a new RC, I’ve wondered if running in grass is something I should factor in. I’m not making any decisions yet, but This helps. Thank you!
 

Lephturn

Well-Known Member
Messages
206
Thanks for this! Since purchasing my MT a few weeks ago, it gets a lot of run-time in the grass. As I think about upgrading it, or just upgrading to a new RC, I’ve wondered if running in grass is something I should factor in. I’m not making any decisions yet, but This helps. Thank you!
If you run on grass it absolutely is important to factor that in. Monitor motor temperatures amd expect to choose a lower gear than you normally would. Fewer teeth on your pinion. If you upgrade to brushless you should also choose a lower kv rating and a longer motor. Go with a 550 sized brushed motor with more turns or for brushless 3660 and I’d stay down near 3,000 kv. Lower kv means lower rpm at the same voltage but more torque which is what you want. A longer motor is more torque. You can do just fine with a 540 sized or 3650 motor too, just gear it a bit lower.
 

RoyRoy

Member
Messages
22
Location
Rochester MN
If you run on grass it absolutely is important to factor that in. Monitor motor temperatures amd expect to choose a lower gear than you normally would. Fewer teeth on your pinion. If you upgrade to brushless you should also choose a lower kv rating and a longer motor. Go with a 550 sized brushed motor with more turns or for brushless 3660 and I’d stay down near 3,000 kv. Lower kv means lower rpm at the same voltage but more torque which is what you want. A longer motor is more torque. You can do just fine with a 540 sized or 3650 motor too, just gear it a bit low
If you run on grass it absolutely is important to factor that in. Monitor motor temperatures amd expect to choose a lower gear than you normally would. Fewer teeth on your pinion. If you upgrade to brushless you should also choose a lower kv rating and a longer motor. Go with a 550 sized brushed motor with more turns or for brushless 3660 and I’d stay down near 3,000 kv. Lower kv means lower rpm at the same voltage but more torque which is what you want. A longer motor is more torque. You can do just fine with a 540 sized or 3650 motor too, just gear it a bit lower.
Thanks!
If you run on grass it absolutely is important to factor that in. Monitor motor temperatures amd expect to choose a lower gear than you normally would. Fewer teeth on your pinion. If you upgrade to brushless you should also choose a lower kv rating and a longer motor. Go with a 550 sized brushed motor with more turns or for brushless 3660 and I’d stay down near 3,000 kv. Lower kv means lower rpm at the same voltage but more torque which is what you want. A longer motor is more torque. You can do just fine with a 540 sized or 3650 motor too, just gear it a bit lower.
Thanks! The last couple of runs in the grass, I noticed the batteries were really hot (1800 nihms that came with the box), I wonder if that’s why? 🤔. The grass hasn’t grown much in several week (MN change-of-seasons) but it’s not putting-green short either. Maybe a 15T brushed isn’t such a good idea if I don’t hear down? -but then I’m not really gaining anything on speed anyway. (Kids aren’t ready for brushless yet). You’ve been very honest and helpful, thank you!
 

Lephturn

Well-Known Member
Messages
206
The best upgrade I did was 2S LiPo. Bone stock and I ran a cheap voltage alarm to avoid discharging it below a safe level. 40 minute run times on a charge and slightly more acceleration but only slightly. It’s 7.4v LiPo compared to 7.2 NiMH. I still run those 2S batteries now with brushless although it rips with 3S. To be fair the 7,000 mAh 55c LiPo is not far off of a 5,000 mAh 3S. It runs coolest on 2S and my kids can handle it fine but they are 10 and 8.

My son drives my Notorious 6S better than I do...
 

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