when to change brushes?
 

Hi, was wondering when is it time to change the brushes?

is there a rule as to when you should change them? when there is xx mm left?

I normally reuse them by serrating after cutting the comm during club meets and practice

and replace them every 2 runs during big meets.

I usually do mine by performance , I have some motors that they get faster about 4 and 5th run

Others say to change the brushes right after each comm cut.

it's not others, it's every motor prep', this is a RULE, unless you can re-serrate your brushes (but then you won't expect the same performance as with perfectly brand new brushes).

Rules and guidelines :

- always change brushes after cutting a comm.
- look at your brushes, if the sides of the faces are discoloured then it's time to change them, even if it's only slightly and next to the face in contact with the comm, they're on their way out.
- you can change brushes without cutting the comm, but they won't last as long and the motor won't be as quick as with a cut.

Finally the lifetime of the brushes depends on a lot of things :

- what brush it is (long life, high silver content, 2/3 runs...),
- what motor (27T / 19T / Mod),
- how good is your gearing (overgeared you'll burn your brushes very quick, undergeared you can too if by a lot),
- how long you run for at a time : 5 mins only, or until you dump,
- your spring combination : too soft --> brush bounce --> arcing --> burning the brushes,
- Your drivetrain ! Is it free running ?
- External temperature, are you using a Fan blowing on the endbell ?

As an example, you can get 50 runs out of a set of CS long life brushes in Mod, and you can get only 2 to 5 runs of a set of 4499s in stock (that's with proper gearing, if overgeared you'll burn them in the first run).

i took second in the Series race here with a motor that had 30+ runs on the brushes without a comm cut as well

i usually rebuild when I notice a drop in performance

I don't always change brushes after cutting the comm. If you havent abused the brush (overheated, worn out) there is no need. Just keep them clean. The second worst thing you can do to a fresh comm is run break in new brushes on it (the worst is to run them without being broken in). If I need to change brushes, I cut the comm, break in brushes, recut the comm, run.

I replace brushes when they are worn, probably when 1/3 - 1/2 of the new brush length has been worn off, or if they are discolored due to overheating. You can see some rainbow colors when this happens.

Wow. Glad I'm only a part time elec guy. COME ON SPRING!!!!!! I wanna get my gassers out and foget about brushes.

Ok, that's done. I was actually wondering the other day about breaking in brushes. They come serrated, do you guys break them in till the serrations are worn off? Or change them when the serrations are gone? I'm just curious.

I use a fussy brush cutting tool on my brushes, much easier on your motor than breaking them in by running the motor. Usually throw a new set of brushes in cut them with the fussy brush until I can see that the entire brush face has been seated, then run the motor for 30 seconds or so to make sure its perfect. I usually use Reedy 766 brushes, mabye they aren't the most powerful, but they last a very long time, and since our track here isn't ultra competitive its easy to get away with. Running for only 5 minutes at the most I've found also really helps to increase brush life, running a whole pack (especially with the 3800+ cells now) is really hard on the motor. By the way I run 19t.

If you cut the comm you don't have to replace brushes. It's the other way around: if you replace the brushes you have to cut the comm.

My motors seem to get faster and punchier the more I run on a set of brushes. For this reason I use a long break-in time at very low voltage. The serrations are about 3/4 gone by the time I race. After breaking in brushes I skim cut the comm just to make sure that it's still round. Re-seat the brushes for about 30 seconds, and get ready to race.

If I burn the brushes, or discolor them, they go out. I re-stratigize my gearing and break in another set. I always break in using a fan (racer's edge). Heat is what distorts the comm and creates the major reason for having to cut.

I check my motors after every run or so to make sure they are good to go for the next round, or if I need to re-consider my gearing.

Wether my method is absolutely correct or not, I don't know. Some of the fast guys at my track cannot understand why I have so much rip, so I must be doing something right

Not that breaking in motors with a fan is bad, but it does put a load on the motor, so if you can have an external fan cooling it, that is a better option. Of course everyone has their own ways. What I do usually is:
Break in brushes initially with fan and equal springs on each side.
For the last 30 seconds or so, remove fan and put on the heavier springs you are going to run the motor with.

The good brushes I have appear to be 9.6mm long when new, that's the total length of the copper 'block' ignoring the shunt.
The brushes I've got that began to give problems are around 8.6-8.7mm long.

Skiddins