Brush Cutting

[MJ]

Where can I find more information on the finer points of slotting, serrating, drilling and generally deforming motor brushes to get better performance? What are the advantages and disadvantages of different techniques and brush materials?

Thanks!

[jeepinator]

I am no expert, so nobody laugh at me ;)
Your question is pretty darn loaded. If anyone really knew the answers, you could open a biz !
I believe that there is a lot of "voodoo" going on out there. There may be advantages to some designs, but there are just as many that could be argued all day as to whether they make a difference at all.
Serrated brushes help the brush face "seat" more quickly to the exact arc of the comm. This is a proven design. There are no performance advantages, only convenience.
The rest of the designs suggest removing parts of the contact area in order to gain something. It could be to gain RPM, it could be to gain efficiency, and there may be other reasons I am not aware of. I have read that the center of a brush face passes the least current, so if you remove that area drag is reduced, but not current carying capacity.
One old timer trick was to trim the trailing edge of the brush. They were (and maybe still are) called "timed brushes". Doing this in effect advanced the timing of the motor a few degrees. I have persoanlly used this trick and seen more RPM gain. On the dyno, this trick does not seem to help (I have a Tekin dyno), but on the track it can make a difference on the straightaways. Remember that, on any motor, advancing the timing increases power output overall but reduces torque and drastically reduces efficiency.
A top secret trick I used a long time ago was to drill a hole all the way through a brush and stuff the hole with cotton. Just before a race I would saturate the cotton with comm drops. I am not sure exactly how big a difference this made, but the motor produced more power on the dyno for about 30-45 seconds (presumably until the comm drops boiled off).
'H' cut brushes seemed to be the rage a little while back. Not sure why.
In general, I would say don't mess with your brushes. If you are good enough to "feel" the difference any brush mod would make then you would not be asking this question. I did not say that to be mean, it is just kinda true.
I say, "Full face or nothing baby !"

[Railman]

Nice post Jeep!
I have my own ideas about what different brush cuts do for performance on STOCK motors. I think it depends on your driving style, track layout & size, battery voltage, etc as to what brush will work best. I like to run serrated high silver brushes with v notch's or a full slot in my XXX. This gives me smoother cornering power with plenty of acceleration & good straightaway top end kick. The reduced brush surface also gives longer run time...important on big tracks & so so batts. Another reason I like to remove a little from the center of the brush to allow more pressure on the leading edge of the brush. Thats where the biggest current draw takes place just when the brush starts to pass the comm slot. This leading edge of the com is where they always go bad 1st. The other reason for a slot is to reduce friction. Thats where the top end kick comes from. You need to gear a little lower for this to work though. The serrated brushes effect can really be seen at handout motor races. There everyone starts out with the same motor it's pretty easy to see what works & don't work. Serrated brushes greatly reduce friction & still seem to transfer enough current to make good power. I think part of the current passes to the com through the hollow ares of the serrations, while not creating any friction at that area. A lot of guys like to go the full brushrout & overgear to take advantage of the increased torque. The reason I don't like to do this is it tends to make a motor run hot, which is how you lose your magnets.

There has been a lot of work done on the new MVP motors on brush cuts to reduce the brush overlap so as to reduce the oversaturation of the field. The end result is a motor with more torque that runs cooler while still making very good power. Thes brushes get cut down on the leading edge so that the brush face is square. From all that I've read this is a must for the MVP to run cool, as they tend to smoke when run out of the box, unless you find the perfect gearing. This is all of course just my $.02.
Grizzbob next? :rolleyes: