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Discussion Starter · #1 ·
I bought one of these recently and took it apart last night. The motor brush carbons have been pre-profiled to the commutator radius, but they are fixed to the brush arm at right angles to the comm. axis so that only two high spots are in contact. Weird!

Is this a new idea to reduce friction (and mess-up the timing), or carp design/quality control?

There is a rectangular lug on the top of the carbon which mates with a rectangular slot in the brush spring arm, so there is no possibility of assembly error at the factory - or of rectifying things. I just hope that the commutator doesn't wear out before the brushes have run in to the correct profile. This could take many days on the test rig. I am reluctant to attempt to give a start to the process by re-profiling with a file as the last time I tried it, I pinged the carbon off the brush arm, never to be seen again.
 

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I've always thought that this brush design/layout was to allow the brushes to bed in more quickly to the com dia. It would be quite difficult to get pre profiled brushes to align accurately enough, so this method is seen as the best compromise for a cheap motor. All of the can motors of this type I have dismantled have had this brush layout.

Mike T
 

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Discussion Starter · #3 ·
Thanks! I can see how that would work as the narrow part of the carbon will wear to the full profile quicker than a flat face, or a curved face in the correct plane, but not accurately centred. It's counter-intuitive but now makes sense. Damn clever these Chinese.

Now, if I can just find a way to kill the magnetic effect, I will be happy. I tried the previous suggestion of a steel shim under the car and it reduced the downforce by about 20% which is nowhere near enough. For the time being, I am trying the arm. and end-bell on an old NSR King can.
 

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I would have thought a bit of sheet with high iron content would have helped more.

Quick kitchen experiment here shows simple steel like that pop-out from a computer case even blocks that hefty harddrive magnet on the right.

EDIT - how big a sheet did you use? Nothing 'blocks' magnets, only reroutes the force into something it likes better. Maybe you tried too small?

Wood Purple Rectangle Tints and shades Hardwood
 

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Discussion Starter · #5 ·
how big a sheet did you use? Nothing 'blocks' magnets, only reroutes the force into something it likes better. Maybe you tried too small?
About 25 mm x 40 mm - just enough to cover the motor to the width of the pod so as not to interfere with the free movement of the pod. It appears from what your post that it was too small and too thin. I used a bit of steel sheet cut from a 5 litre oil can. I can see how the computer steel is better, but the thickness would reduce the ground clearance too much as a direct add on, so it will mean larger diameter rear tyres, and a possible change of gearing. As I love experimenting, this is something to get my teeth into - not a chore. Thanks again for the steer.
 

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Kevs Racing Bits
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Steel has iron and iron is what you need to attract the magnetic field. Try putting some on the sides of the motor and even the top, the magnetic effect will reduce even more. 0.4mm is the thickest shim I have, I had a neo mag motor with shim all round but ultimately I opened the motor and pushed the mags out and put a pair from an old SlotIt motor in...now that motor is back to minimum mag effect for our club rules.
 

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Kevs Racing Bits
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I bought one of these recently and took it apart last night. The motor brush carbons have been pre-profiled to the commutator radius, but they are fixed to the brush arm at right angles to the comm. axis so that only two high spots are in contact. Weird!

Is this a new idea to reduce friction (and mess-up the timing), or carp design/quality control?...
Do you mean the orientation of the brush? If yes then this is an old idea where brushed motors are concerned, before brushless motors took over in r/c cars there were two types:

Standup brushes - are your 'everyday brush' used in most standard motors. They produce the best low end torque, but don't give best RPM and power can be lacking if you don't choose the right compound.

Laydown brushes- are mostly used in stock racing motors and some mild-wind racing modified. They produce extra rpm and mid-range to top-end power due to overlap. But this benefit comes at the sacrifice of bottom end torque. They are also harder on the comm due to arcing due to this overlap.
 

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Discussion Starter · #8 ·
Do you mean the orientation of the brush? If yes then this is an old idea where brushed motors are concerned, before brushless motors took over in r/c cars there were two types:

Standup brushes - are your 'everyday brush' used in most standard motors. They produce the best low end torque, but don't give best RPM and power can be lacking if you don't choose the right compound.

Laydown brushes- are mostly used in stock racing motors and some mild-wind racing modified. They produce extra rpm and mid-range to top-end power due to overlap. But this benefit comes at the sacrifice of bottom end torque. They are also harder on the comm due to arcing due to this overlap.
Yes - the orientation of the radiused carbon tip is at 90 degrees to the way you would think it would be to match the commutator periphery.

As for standup and laydown brushes, this is all new to me and I don't understand your descriptions. I presume that this is r/c motor technology. Are these different compounds and/or different sizes (to get different overlaps)? Slot car motors all seem to use the same overlap as the carbons and comms. are pretty much the same sizes. I am not familiar with the Mabuchi 540 types - do they have a flat disc commutator? I am hoping to learn something here that will give me an advantage, and hoping that our other club members miss this thread!
 

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Kevs Racing Bits
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Slot car motors are just smaller versions of old r/c brushed motors. You could get different brush compounds, different orientations, different brush face cuts, different brush springs etc.

If you got a motor with the brushes the 'wrong way round' you just water dipped the motor to speed up the run-in process.
 

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Discussion Starter · #11 ·
Thanks for the explanation. Water dipping I am familiar with but I haven't seen differing brush compounds since my ECRA days.
 

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I haven't seen differing brush compounds since my ECRA days.

There are considerable variations in brush hardness between different types of motor. But for the sort of motors you are talking about, the makers rarely mention the different brush compounds that are used. These motors are normally used as sealed units, it is rare to swap or service parts of the motor. These sort of motors come in just a few standard sizes and a vast number of brands and specs. It is possible to swap bits between other same size motors, but it seems very rare for anybody to do that.

Of course faster slot motors are designed to have parts replaced or serviced, but these are quite different to the sort of motors you are talking about.
 

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The motor brush carbons have been pre-profiled to the commutator radius, but they are fixed to the brush arm at right angles to the comm. axis so that only two high spots are in contact!

There is a rectangular lug on the top of the carbon which mates with a rectangular slot in the brush spring arm, so there is no possibility of assembly error at the factory - or of rectifying things. I just hope that the commutator doesn't wear out before the brushes have run in to the correct profile.
I've always thought that this brush design/layout was to allow the brushes to bed in more quickly to the com dia. It would be quite difficult to get pre profiled brushes to align accurately enough.
I have no more motor engineering expertise than the next man here and, obviously, the big manufacturers know exactly what they're doing, but this seems wrong on so many levels:

1) Of course they can pre-profile the brushes to the correct radius. The little rectangular lug shows the precision to which these tiny chunks of carbon are manufactured.

2) Even if the pre-profiling is only 99% accurate... A brush which starts out 99% not contacting the commutator cannot bed-in more quickly than one that is already 99% correctly aligned. The mis-aligned brush has to wear down to the state that the correctly aligned brush already has, before it can start the same, final polishing process.

3) No manufacturer would want their motor to require lots of brush wear. The dust has to go somewhere and the likeliest home is the commutator slots, with disastrous consequences.
 

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I've always thought that this brush design/layout was to allow the brushes to bed in more quickly to the com dia. It would be quite difficult to get pre profiled brushes to align accurately enough, so this method is seen as the best compromise for a cheap motor. All of the can motors of this type I have dismantled have had this brush layout.

Mike T
Indeed nearly all the motors with that sort of brush gear do have the brushes aligned like that.

Agreed that sounds like a likely explanation for why. Also it'll give somewhere near the intended comm timing before it has bedded in.

On those sorts of motor the brushes are on a flexible arm so they swing through an arc as the brush wears down. That inevitably changes the brush alignment during the life of the motor.

If the end of the brush was curved to match the comm, production tolerances mean some motors would inevitably have only one end of the curve making contact with the comm till it beds in. That means quite a large error in comm timing till it beds in.

I have no more motor engineering expertise than the next man here and, obviously, the big manufacturers know exactly what they're doing, ........
These sort of motors are made in vast quantities by a small number of manufacturers. The slot racing market is a tiny proportion of their production. Many different slot car brands have their motors made in the same factory in China.

Yes the manufacturers do know what they are doing - what they are doing is producing vast numbers of motors at very low cost and they want and absolute minimum of rejects. That means they aim for something that works OK every time with no running in etc. You don't get a motor finely tuned for best slot racing performance at that price!

(Further up market you can buy motors finely tuned for best slot racing performance - but that's not the sort of motor the OP is talking about)
 

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There is a rectangular lug on the top of the carbon which mates with a rectangular slot in the brush spring arm, so there is no possibility of assembly error at the factory - or of rectifying things. I just hope that the commutator doesn't wear out before the brushes have run in to the correct profile. This could take many days on the test rig.
Have you done anything with it yet?

I'm just thinking, since it's a novel design, maybe one shouldn't presume the usual best-methods will apply as they have. I'm thinking test it on the dyno and the track before running in. Then again at each step to see what steps do. Make sense?
 

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Rich Dumas
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My experience has been that the motor brushes in 1/32nd cars often do not align perfectly with the commutator. When that is the case there would be no point in pre profileing the brushes.
 

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Discussion Starter · #19 ·
Re countering the magnetic effect, try some plastic tape under the motor, any metal will transmit the force, but plastic is a good insulator.
Its a good electrical insulator but is nearly transparent to a magnetic field.

I am sure that this is the true explanation.

If the end of the brush was curved to match the comm, production tolerances mean some motors would inevitably have only one end of the curve making contact with the comm till it beds in. That means quite a large error in comm timing till it beds in.

I'm thinking test it on the dyno and the track before running in. Then again at each step to see what steps do. Make sense?
Yes, except that I have just put it in a car and set it all up. I will try it and then do the tests

It is possible to swap bits between other same size motors, but it seems very rare for anybody to do that.
I do it.. One of my best motors has a Scaleauto arm in an NSR can. This means I stay within the no-mag club rule and the motor rotates the correct way to get the crown wheel to fit into the moulded blister in the rear of the car body (it has a really low rear deck). The original can had too much pesky mag downforce.

I digress, but am I the only racer that wants the cars to corner at realistic speeds? The slower the better as far as realism is concerned for me. Standard Cartrix on Scaley Sport track is a real test of skill and looks good too. I applaud the Scalex Sierra class which had the video posted too. It's all a bit like the real thing for me - it's not the speed which counts, but the closeness of the racing. Some of the best 1:1 races I have seen were for 850cc Mini's, Citroen 2CV's, 750's and Formula Junior, all low powered and very closely matched. At 80 mph, 0.1 sec is less than a car length, at 180 mph it is more than two car lengths, so extra speed makes the racing look less close. (New topic perhaps?)
 

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Kevs Racing Bits
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Re countering the magnetic effect, try some plastic tape under the motor, any metal will transmit the force, but plastic is a good insulator.
Plastic tape will do nowt to counter the magnetic effect. The only way is to use iron (found in steel shim, but much less so in stainless steel)
 
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