Air Gap

[golftooter]

We were having a discussion about the effect the air gap has on the motor. Does a large air gap (non shimmed magnets) result in a car with less torque but high RPM, or a car with more torque and lower RPM?

[palm_dog]

slick 7 has a wonderful website with way too much good info on magnet and magnet theory. tips and air gaps and timing. I think. been to long since I've read it. go check it out

[hesketh]

Increased airgap increases RPM and decreases torque. It's a nonlinear relationship as well, as the airgap increases, the torque decreases faster.

[D.Z.]

Also the bigger air gaps let's the motor run cooler.

[hesketh]

As long as you are running more gearing, since you are losing torque the motor will run hot if your gearing is too tall. Same relationship as timing.

[gotboostedvr6]

Its more complicated then that.

dont forget to consider unloaded and loaded rpm, brush overlap, magnet tip design ect

[hesketh]

True but all other things being equal an increase in airgap will decrease torque and increase rpm, whether it's cooler or not becomes subject to a lot of other variables.

[Marty]

Logic test. Take the magnets out of the motor. How much torque or rpm will it have? None! Visualize the magnets installed but a foot apart. Now how much? Again, none. How about half a foot, three inches, inch and a half and so on. At some point the motor will start to turn. It will be hot and lazy. As the gap becomes tighter and tighter BOTH rpm AND torque will increase. As the gap gets tighter it will reach a point where rpm no longer increases but torque does. All along this ever tightening gap the motors efficiency continues to climb and the motor runs cooler and cooler. Continue to tighten it more and rpm will start to fall as torque continues to climb. Peak motor efficiency will remain about the same but average efficiency will increase. Tighten it some more and both rpm and torque dive. Both peak and average efficiency starts to fall and the motor once again starts to heat up. You've just made the journey from too little to Goldie Locks perfect to too much gap flux.

The most efficient gap will vary and is influenced by the can, the magnets gauss AND mass on the field side of the gap and the wind, the iron and, for road race motors, track voltage, average throttle position (current draw) for a lap on the armature side of the gap. It's a balancing act between the magnets flux contribution to the total air gap flux which is pretty much stable in a running motor and the armatures amp/turns which vary with throttle position and choke settings. Both the field and the arm contribute to the gap flux.

This line in the first paragraph, " Continue to tighten it more and rpm will start to fall as torque continues to climb". Seems to be the area bench racing revolves around but that spot is exceedingly narrow and often not obtainable with ceramic magnets especially in road race cans. You have already observed this comparing acceleration on long straight sections of track between C-12/G-27 motors verses their X-12/G-20 counter parts. Try it at the same car weight and the outcome is still the same.

[phlirv]

MARTY,

That answers, to me, why some arms work in some cans & mags and not in others. I have 4 can and mag setups that I test my 12 arms in cause I did find years ago that sometimes changing can & mag setups can hurt or help performance. If you have an arm that is slower. Change setups. It can make a difference.

OLPHRT
PHIL I.

[Marty]

Absolutely Phil. Road motors, which are not my normal play pen, have a whole different list of requirements from drag motors which have little to do with the motors physics and a great deal more to do with the drivers preferences, track, controller and abilities or reflexes.

A road motor, which spends a good deal of it's time throttled and choked so the magnets "see" the armature, on average, as a much milder wind (lower average amp/turns) and the more technical the track the milder it looks to them. You could say the opposite would be true as well in that the magnets would "look" stronger to the arm when throttled.

The opening question to this tread concentrated on a very narrow area of this relationship which is taken as gospel by the racing public by and large and not the whole picture and often wrong. Problem is, it's right enough of the time to perpetuate the myth that it always is.

[phlirv]

What I did find is that on a flat road course with many turns. MOST C can motors like a fairly close gap... .527 with a .513 arm and the more punch bowl style. A .535/.540 with a .513 arm. It seems to give me smoother, higher power, easier to drive curve coming out of a slow corner. I have not had a reason to work with the large dia. arms yet.....

OLPHRT
PHIL I.

[Marty]

There ya go. Tuning the motor for control instead of ultimate power or efficiency. Perfect example.

[D.Z.]

I know my motors tend to like bigger air gaps lately. It seems like every track I've been going to my big air gap motors are just plain faster. Most of the time. It all depends. I believe the fact that we are running such lighter cars now that you can get away with bigger air gaps beacuae they brake so much easier being lighter cars. Also, controllers have better brakes now then say 5yrs ago. It at least works out that way for me. So many, many, many options. Pick your poison.

[golftooter]

Thank you for the surprisingly clear discussion of this. Wish I had been interested in electric motors when I was in college - i might have taken a class in the physics of them.

[phlirv]

Thank you for the surprisingly clear discussion of this. Wish I had been interested in electric motors when I was in college - i might have taken a class in the physics of them.

SLOTCAR 101....Took about 8 years........Trouble is.....I'm still learning....Started in 64

OLPHRT
PHIL I.