smallest usable comm diameter
 

How for can the comm be cut, as in, its smallest usable diameter before it is considered no good anymore ??
Thanks

I'd say under 7 mm it's near crap.

Hi Brad,

It all depends on the motor that you are using. The Reedy quad-magnet motor requires a larger com - whereas some of the Top motors can be cut as small as 7mm and still offer some good performance.

Keep in mind, as your com-size decrease, the rpm of the motor increases - equalling an adjustment needing to be made to your gearing.

Hope that helps.

realy

First I've heard that one. I thought it increased the RPM's, guess I was wrong.

well what dose your motor do when the com gets small
dose your motor get faster or slower
theres a sweat spot where the motor is fastest then when you make it smaller the performance go's down.
depending on how you cut you brush too.
but its still easier than a damm nitro motor. :lol:

You are correct... the RPM DOES increase.... typo on my part - I fixed it.

:o

I found that as long as you geared properly the Epic based motors get faster as the com decreases to about 7.2 mm - then they just overheat and run strong for 2-3 min... after that, they die off quickly in a race.

Small Comm = More RPM, Less Effenciency, Less Torque

Bigger Comm = Less RPM, More Effenciency, More Torque.

Small Track = Bigger Comm

Large Track = Smaller Comm

.270 on the small comms is really the safe point, though I have seen people run them smaller.......yer playing with fire, certainly something I would not do in the A main at a big race....

.310 on the large comm (reedy stuff, like quad mag 19t)....

We have never really messed with cutting the stuff to some magical diameter like other companies have claimed at big races....we cut tills its round and run it......

Just remember, as it gets smaller.....you may need to adjust your gearing some....not always, but sometimes.

Later EddieO

You can go to 6.8mm but after that you're probably screwed :)

It will still be fast until 7.0 or maybe 6.9. This is from my experience from Orion armatures during the last few years.

From my experience .270 is pretty much the cutoff point. If the comm doesn't start to flake at the grooves it's still not worth risking damage to your lathe to cut it smaller.

I have seen a few motors peel up the copper off the comms at .275 though.

For us living in the metric world:

.270 = 6.86 mm
.310 = 7.88 mm

Personally - when I cut a comm to around 7mm that motor turns into a practice motor. I've had too many die-off during a race for my liking at this size.

yep at that point you have gone past the sweat spot.
and the arm is junk.
the trick is to fined what a given motor likes
magnet
com size
brush
and rollout
all motor's are different and to find what a given motor wants is an art form
and you duckman have that little extra touch to find it prity good

.275

common be brave .264!!! :D

I stop at .270 or ealier if the comm starts to chip.

If ya can't peel the copper segemnt up with your finger nail then there another run in her.... :lol:

or... keep grind'n till ya see daylight... :lol: :lol:

thas correct@!!lol

I used to true them till the comm peeled off. :nod:

Checkpoint 19T
 

we have run a Checkpoint 19T motor at 6.86mm with Infinity brushes for the last 3 weeks - very quick.

regards

Don

Thanks Everyone,
Thats a lot of good info.

That is the same experience I've had with the common Trinity motors. However, you can get around this by shaving both the leading and trailing edges of the brushes. As the comm gets smaller, the edges of the brush wrap around more comm surface area. This causes more overlap of the brush between the comm segments as the comm rotates. Another simplified way of looking at it is that more overlap usually means more heat with the increased RPM.

Shaving the edges of the brushes will reduce the comm overlap back to a level where the efficiency picks back up and the motor doesn't overheat. But you have to keep in mind that you also need to increase the spring tension some as the comm gets smaller. All of these things can be quite hard to master for consistency though.

donwhit is correct about the CP motors. I can run the comms AND brushes quite small without losing power. I'm not sure, but I think the CP motors don't heat up with the increased overlap since the amount of brush edge touching each segment changes as the comm rotates (round face versus rectangle face with long flat edge). However, I do have to spend some time with each motor on the CE dyno to find out how much to change the spring tension as everything gets smaller.

I have tried shaving the leading/trailing edges as well... however, still see a noticeable drop within the last couple minutes in TC. In 1/12 - not so much - leading me to think that the voltage running through the motor has something to do with it as well. As the comm does get smaller - I have also adjusted spring tension by up to 2-points on the Trinity spring-thing and did notice RPM increase. I have also tried reducing spring tension and found that with a small comm, cut brushes, the motor is more reactive to spring changes.

I think that CP motors don't heat-up as much since the design is much more effecient (round brush)... but I also think that the brush tubes are far superior to the convensional design for minimizing brush movement - reducing arching on the comm = less heat.

i agree :nod: