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Brushless Motor in 870C

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rcnj
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Brushless Motor in 870C

Post by rcnj » Thu Aug 03, 2017 2:29 am

Hi everyone -

I've got an 870C that is finally almost fully restored! Here is a photo of the current motor, which came with my YZ-10 kit:

Image

As I learn more about brushed motors from back in the day, it seems they require an absurd amount of laborious regular maintenance... I simply cannot imagine replacing brushes after every 10 runs or so... That is just insane. I never touched the 540 silver can that came with my Hornet during the entire time that I used it as a kid!

So even though I love the look and nostalgia of my original motor and ESC, I am considering an upgrade to brushless.

Here is my question... If I get a quality brushless motor with approximately the same RPM as my kit brushed motor, can I just drop it in without changing any gearing?

My brushed motor is a Twister / Yokomo 1002 17x2 Terminator, which it says on the package is 34,000 RPM. I assume that's at 7.2V, which I am running.

For example, I could get a 3,600 kV 10.5T brushless, which would run at 25,920 RPM on my setup. Another possibility would be a 4,300 kV 8.5T, running at 30,960 RPM.

Would these be a good choice because their speed is so close to the Twister? Would they run the buggy slower than the Twister because of the lower RPM?

Does anyone with experience from the brushed days have any idea what kind of RPM the 34,000 Twister would degrade to after running a few dozen packs without doing any maintenance on the motor?

Thanks!

Brian
Last edited by rcnj on Sat Aug 05, 2017 8:21 pm, edited 3 times in total.

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Re: Brushless Motor in 870C

Post by rccars4sal » Thu Aug 03, 2017 4:56 am

I ran a 10.5 turn setup in my 870c, and it was ballistic fast. I dont remember the gearing, but it was about ideal, not over revving, and not over loaded. Heres the thing though,, the rear diff will quite literally melt. I would strongly recommend running a slipper clutch, or use a crappy old rear diff because it will be toast after a short while.

My car was run on the street with some sticky oval racing tires, so stress on the diff was pretty severe. However, I only ran it a short while and it was obvious very soon that the rear diff was getting damaged.

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Re: Brushless Motor in 870C

Post by rcnj » Thu Aug 03, 2017 2:16 pm

Thank you for this great information!

I'm having trouble understanding how a 25,920 RPM brushless motor would be harder on the drivetrain than the 34,000 RPM brushed motor. Can anyone elaborate on that?

If the motors I mentioned are too fast, I could use a 13.5T at 21,960 RPM, or even a 17.5T at 15,840 RPM. I would prefer not to change the gearing, though. Also, while I don't want it to be crazy fast, I don't want to to be slow, either. Seems like ideal performance would something comparable to a freshly serviced Twister.

I wonder what is the current RPM of my Twister, as it currently needs new brushes.

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Re: Brushless Motor in 870C

Post by GoMachV » Thu Aug 03, 2017 3:04 pm

The torque of a brushless motor is much higher and comes in at a lot lower RPM. Rpm really isn't going to hurt anything, the torque is what takes stuff out.
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Re: Brushless Motor in 870C

Post by Coelacanth » Thu Aug 03, 2017 4:47 pm

You can address the torque issue with modern ESCs by reducing the "Punch" setting, making it accelerate slower. I'm running 11.5T and 12T in some of my cars and dialed down Punch to about a third or a half of the max.

Gearing actually shouldn't make any difference because all the other factors that play a role in gearing haven't changed; tire height, internal gear ratio and spur-to-pinion gear ratio. No matter what motor you install, it's still the same diameter, and rotates the same distance on one rotation of the comm.

If you put in a stupid-fast motor, for example, if you gear down, the torque will hit that much harder; if you gear up to reduce the torque, you run the risk of overheating the motor. Protecting your drivetrain is much more about having a good slipper gear and/or adjusting Punch to compensate for the extra motor power.
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Re: Brushless Motor in 870C

Post by eco8gator » Fri Aug 04, 2017 2:22 am

I run a 10.5 in my 94 YZ10 and my diff is holding up but I have a slipper installed.

I have to be a little mindful with the throttle to not kill the slipper but overall I drive the car pretty hard.

My biggest issue was belts but I use are on poly belt brand belts for the rear belts and haven't had issues.

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Re: Brushless Motor in 870C

Post by rccars4sal » Fri Aug 04, 2017 4:12 pm

Those tires in your pic are the exact ones I ran on my 870c,, dominator tires I think... The rear tires and rear diff were shredding in noticeable amounts on every run. The 870 and mr4bc are probably the most efficient 4wd platforms I have ever run. They really translate power to speed very well. They make a draggy lazer or top force look slow in comparison.

Oh by the way, thats a nice clean 870 you have!

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Re: Brushless Motor in 870C

Post by Mr. ED » Fri Aug 04, 2017 6:07 pm

I see 7.2v ni-mh or ni-cad cells in your picture. That will help a little taking the edge of the more brutal brushless motor. Lipo's put out higher amps and maintain their initial voltage longer. So your car is exposed harder and longer increasing the risk of overheating the balldiff if it slipps even in the slightest.

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Re: Brushless Motor in 870C

Post by rcnj » Sat Aug 05, 2017 8:36 pm

Coelacanth wrote:
Thu Aug 03, 2017 4:47 pm
You can address the torque issue with modern ESCs by reducing the "Punch" setting
Thank you, I will check this out!

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Re: Brushless Motor in 870C

Post by rcnj » Sat Aug 05, 2017 8:45 pm

rccars4sal wrote:
Fri Aug 04, 2017 4:12 pm
Those tires in your pic are the exact ones I ran on my 870c,, dominator tires I think... The rear tires and rear diff were shredding in noticeable amounts on every run.
The tires are Duratrax, but I'm not sure what they're called... They seem to work great. I've been running the car only on the carpet in my basement so far, and I don't notice any extreme wear, but the street might be different.
rccars4sal wrote:
Fri Aug 04, 2017 4:12 pm
Oh by the way, thats a nice clean 870 you have!
Thank you! It's taken seven months. I will post more photos when it's done.

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Re: Brushless Motor in 870C

Post by rcnj » Sat Aug 05, 2017 8:54 pm

Mr. ED wrote:
Fri Aug 04, 2017 6:07 pm
I see 7.2v ni-mh or ni-cad cells in your picture.
I'm running a Ni-Cd pack I just made from fresh cells... runs great!

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Re: Brushless Motor in 870C

Post by rccars4sal » Mon Aug 07, 2017 3:51 am

The tires my car had were the genuine dominator dirt oval tires, very soft and sticky. Excellent info that yours are duratrax. I will search for some. The dominator tires were a bit too soft IMO, but might have been good for turf or carpet.

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