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Not sure where this topic belongs so I thought I would cast the widest net.

How do you choose a motor by RPM? For example, Slot.it offer a motor in each of these RPMs so how does one choose? Is it as simple as choosing the high rev motor for long tracks with lots of straights and 21,500 RPM motor for a tight winding circuit?

 

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How does the car feel on the track you race on regularly?
If its already a bit of a handful then more power is unlikely to be needed for a bigger track.
If it feels a bit underpowered then it's well worth trying one step up on motor revs for a bigger track.

That's just a staring point, there's no substitute for testing on a track to find the best set up for that track.
 

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Lee Green
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What makes more of a difference is the gearing (trackwise) i personally think it depends on grip , if you have the grip go for the higher revving motor
 

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I've played with a few fast rev motors, had a 46k that simply launched the car off the track even in a straight line (although that motor is now destined for a drag car)
On a plastic track, I have found even with the lowest accelerating gear setup a 29k is a handful, even with amazing tyres. I'm going to look at the 25k next to see if it gives me some control back.

A already stated its all about experimentation, don't forget if you increase the the motor speed you will need better than stock tyres, you may also want to look at gearing.

Good luck and keep us posted
 

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Be careful here. Motors might say 21,000 @ 12v, but they should say 21,000 +/- 10%.
I test all my motors I buy, after running in. I have a new Pioneer 21,000 motor here ready for a chassis. At 12v it is putting out 18,964 RPM.
The most reliable are NSR, but even they are about 5% down on what is stamped on the can. My advice, if it's worth anything. If you want 25,000, buy 30,000

Wraith, on that 46K motor, did you try different gearing.
eg. 46K with a wheel dia of 20mm and a ratio of 4:1 (P8, G23)will give you 12.04 M/s
go to the other end. 46K wheel dia 20mm, ratio 3.17 (P12, G38) that's 15.21M/s
go the other way. 1.92 (P12, G23) = 25.13M/s
4.72 (P8, G38) = 10.14M/s
So for a std. track, a ratio of 4.72 would be good, Correct me if I'm wrong here guys. BTTWICI
 

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Yes and no, depends on what you realy want. Quick get away, low top end. Lets face it, your never going to get a full 46K on the stright. It would need to be VERY long.
It also has a lot to do with the g/CM. Take the NSR 3016L - KING MOTOR 46 K . I would be surprised if it did 46K, more like 41K, which is still a lot. That 46K motor clames to have 310 g/CM. that's a far wack. So gearing it right could be very rewarding. But to my mind, you would be better off with Slot.it Boxer/2 motor, 21K5 @ 12V, 340 g/CM.
If the track has a very long stright. You first need to know. Will that motor peek at just the right moment before you break. And playing with the gears should hlep you find that.
Just wish we had rev counters on our controles, just to see what the revs are at any given moment on the track.
 

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Tore
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In general the more RPM the motor have the more torque it also has, so you will come to a limit where the motor overpowers the handling limit of the car.

Boxer motors can be torque monsters that can shoot your car out of the slot in a straight line, while S-can motors have a milder acceleration. But IMO the fastest usable motors for no-mag plastic cars have around 22.000-24.000 RPM and torque between 200 - 300 g*cm.

You can read a lot about motors here: http://slotcarnews.blogspot.com/2007/02/sl...motor-list.html

Besides RPM, Torque and AMP draw which can be measured the most important feature is how the motor behaves, how it brakes and how it delivers the power. A motor that deliver the power linear and smooth over the whole RPM range is much easier to control than a motor that dumps all the power at a certain voltage (throttle). In general, motors with more than 150 g*cm torque can often feel like an on/off switch with a standard controller, but if you have a good fully adjustable controller you are more likely to enjoy the more powerful motors as you can alter how they deliver the power, and smooth out the power-band to give you better control.

How you will use the motor may affect the choice too. If you plan to use stock gears then RPM is usually more important than torque unless you use strong magnets or have a very heavy no-mag car. If you plan to change gears then a combination of torque and RPM is more useful to look at, and as SN Motor list (link above) informs you can calculate RPM and Torque into Watt which tells how much power the motor have at a certain voltage.

( (Max RPM / 2) X (Stall Torque / 2) ) / 100000 = Watt

Hint! NSR motors are well known for being nice and smooth, and IMO the cheaper Scaleauto "blue" ball bearing motors are hidden gems


Tore
 

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Another thing to take in consideration in the magnetic downforce.

If you take two motors with the same specs : for example, a Cartrix TX6 and a Scaleauto SC025, they have almost the same specs :
- around 21500 [email protected] v and around 320 mg torque.

They should give almost the same performance.

BUT !! The Scaleauto SC025 (and the MSC "Thunder 1" and the Spirit long cans) have HUGE magnetic downforce.

So the car is sticked to the track (if not braided wood of course)...and goes faster.

But the TX6 is incredibely smooth with a totally regular power band. It's the perfect motor for no-mag class.
 

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Rich Dumas
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You should not choose a motor based on the RPM rating alone. The amount of torque that a motor makes is also important and the RPM and torque specs can be use to calculate a motor's equivalent of horsepower. Look here for more information: http://slotcarnews.blogspot.com/2007/02/sl...motor-list.html . It is also not always true that the RPMs and torque increase or decrease proportionally. There are high PRM/low torque motors and low RPM/high torque motors for example. The torque that is available will be a big factor choosing the correct gears. If a motor has little torque trying to gear for more top speed will be more likely to just get you sluggish acceleration and poor brakes with little or no increase in top speed. In the end you do have to experiment a little with gear ratios. There is one final thing that can only be found by actually trying a motor. Especially if you run without magnets it is nice to have a motor with smooth, proportional power delivery. I have found that some motors tend to be sluggish at low RPMs, then the power comes on with a bang as the speed increases.
 

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QUOTE (356speedster @ 27 Oct 2011, 12:20) <{POST_SNAPBACK}>A motor that deliver the power linear and smooth over the whole RPM range is much easier to control than a motor that dumps all the power at a certain voltage (throttle).
QUOTE (RichD @ 27 Oct 2011, 15:56) <{POST_SNAPBACK}>I have found that some motors tend to be sluggish at low RPMs, then the power comes on with a bang as the speed increases.
Important point there - there's more to motor characteristics than just maximum rpm and torque.
On most tracks there is a lot of lap time in cornering consistently quickly and a niece smooth torque curve can be a big help.

The calculations that have been mentioned can be of academic interest, but as has already been said, practical experience and testing are much more important to making a slot car faster round a lap.

The minority who take an academic interest in these things could take another look at how the formula for maximum power was worked out
( (Max RPM / 2) X (Stall Torque (gm cm) / 2) ) / 100000 = power in watts
If the motor has exactly liner torque characteristics, this is reasonably accurate (within about 2.7 %)
As 356speedster and RichD have pointed out, the torque curve in some slot car motors feels far from linear.
That means the formula won't give reliable answers for those sort of slot car motors.
Exactly how far out could be worked out from more detailed measurements of the motor characteristics. Anybody up for measuring that?
 

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QUOTE (Saviour @ 28 Oct 2011, 10:56) <{POST_SNAPBACK}>Just up my street. Could be, but would need more info.
It's all about what torque the motor produces at different speeds at a fixed voltage.
Measuring the speed and voltage with a decent degree of accuracy isn't difficult.
Measuring the torque of a slot car motor when its running with a decent degree of accuracy is more challenging. It's all too easy to put unintended loads on the motor that mask the results.
I've seen graphs and assertions from people who have simply assumed the answer must be a straight line and haven't actually done any measurements to discover to discover if it is.
I've seen some attempts at that sort of measurement reported, all a long way short of decent accuracy.
I've seen suggestions on how to measure this by people who think its easy and have failed to grasp the issues involved in achieving reasonable accuracy.
Of course a full set of rigorously accurate tests could already be out there somewhere.

That sort of measurement of torque against speed has been done on larger low speed electric motors in laboratory conditions, and the results come pretty close to what that formula assumes (a straight line from maximum torque at stall to zero torque at maximum rpm.)
The question is how these slot car motors that feel far from linear and those that appear to be fairly linear compare with that. Obviously you'd need samples of the different slot car motors to do the test.

Yeh it does sound a bit daunting - if it was easy I'd have got round to doing it years ago!
 

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Rich Dumas
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When you race with a club the tendency is to stick with motors that have worked well in the past, so there is a chance that you can get in a rut. We host a great many proxy races, so we get to try out cars from all over the world. I have noticed that motors with very long armatures are often reluctant to spin up, so at lower speeds they do not always perform as their published specs would indicate. I am not a big fan of bench testing, at the end of the day it is how a motor performs on the track that really counts. Published specifications are usefull in that they allow you to narrow the number of motors that you might try. Most of the time when a proxy series is run there is a published list of the entries with all of the technical details listed and also there are pictures of the guts of the cars as well. As the series progresses you get to see picture and videos of the tracks as well and soon a pattern will emerge showing what works well and what doesn't. It seems to me that it would be a good idea to see what motors the top running cars are using.
 

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Tore
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To quote myself from another baord:

QUOTE Did a funny test on my small track with a NSR 997 (no-mag).

First I turned the voltage way low, and then increased it slightly lap for lap until I felt there was not much more top speed to gain on the straight without having to drive like an idiot. Then measured the track voltage to 9.2 volt.

Then I did the same test, but this time I focused on acceleration (torque), and increased the voltage slightly until the car accelerated so hard that it became unpredictable. The result was 10.3 volts

At 12 volt the NSR motor has 21400 RPM and 322 g*cm torque and a 2.214 gear ratio so...

21400 / 12 * 9.2 / 2.214 * 3 = 22232
322 / 12 * 10.3 * 2.214 / 3 = 204

... it looks as a motor with about 22.000 RPM and 200 g*cm torque at 12 volt should suit my track well (no-mag, sticky tires and standard 3:1 gear ratio).

The Slot.it Orange Endbell is in that territory

Not so sure how good or bad this test method really is, but the amount of torque a slotcar can handle before it becomes un-efficient (overpowered) is related to the grip level on the track, so by using a variable voltage PSU you can dial in a car pretty good, and then calculate how much torque and RPM you would need to replicate that power level at a set voltage. But again this assume that the RPM and torque is close to linear.

Another fun motor is the SCX RX-4H, lot's of RPM (26-27000) but with relatively low torque (around 150 g*cm) it's very controllable even on shorter tracks.
 

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Nobby Berkshire
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I'm pretty sceptical as to whether the silly high g/cm torgue claimed by some longcan motors actually is real! For example, 300g/cm longcan motors have nothing like twice as much pulling power as 160g/cm hi-tech Mabuchi type motors.

rpm just means how fast the motor goes in a straight line and with enough straight track to let it get to that speed IF you have the right controller.

Beyond that it means almost nothing.

What you need is torque all the way through acceleration and brakes at the end when you need to slow down. That's down to individual motor design and gearing ratio.

rpm alone is not that important, but it is good to have a high rpm motor and then narrow the gear ratio so you maximise acceleration and brakes. This reduces outright rpm but makes your car incredibly responsive. But ONLY if you have the right low ohm controller, or a diode variable controller.

The best all-round performance motors are NSR, but unless you have a 20+ foot straight section then anything over 22000rpm is not going to make a lot of difference.
 

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QUOTE (356speedster @ 28 Oct 2011, 13:52) <{POST_SNAPBACK}>To quote myself from another baord:

<Tried different voltages etc, see post #16>

Not so sure how good or bad this test method really is, but the amount of torque a slotcar can handle before it becomes un-efficient (overpowered) is related to the grip level on the track, so by using a variable voltage PSU you can dial in a car pretty good, and then calculate how much torque and RPM you would need to replicate that power level at a set voltage. But again this assume that the RPM and torque is close to linear.
Turning down the volts is a good way of finding out how the car would go with a less powerful motor on full volts.
You can try different levels at the turn of a knob - much easier and cheaper than buying lots of different spec motors and fitting them in turn.

If reduced voltage produces the effect you want, is there any point in changing the motor? Just run the existing one on reduced volts.
Two ways of doing that.
1 Turn down the volts on an adjustable power supply
2 Run full track volts, and have some switched diodes in your controller power lead. There was a thread explaining how to do this recently. It's an easy add on you can do yourself and should cost less than one extra motor.
 
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