RE-Xtreme RC Drift Bible - Motor Choice

 What motor should I buy? One of the most common questions...



The answer is ... Whatever suits your purpose.

The purposes are...

Competition?
Tandem?
Scale Speed?
Surface Grip Level?
Track Size?
Matching Others?
Car Type?


These are repeating themes in RC Drift . So basically... as you get into the hobby more and more, you will buy a range of motors.  They will suit more purposes. As usual, talk to the guys you drift with, because matching your friends for tandem will bring the best results.

I am going to ignore brushed motors in this description as 99% of motors these days are brush-less.

the basics...

FAST = The Highest Grip Surface + the Softest Tyre.
to Slide on this surface with a soft tyre... you need the HIGHEST RPM to burst past the grip level and keep the wheels spinning.
 
A lower turn "T" motor allows more RPM. Another measurement is KV (kilowatts per Volt.)
The higher KV = More RPM





This KV number x Volts (7.4) with more at full charge can give you an approximate MAX RPM without load.You can check approximate values below.

7.5T = about 4600 KV x 8.4V = 38640RPM



So RPM can range with different motors  from 21.5T(10000 RPM) to about 4.5T(50000 RPM), which equates to a HUGE difference in how an RC car behaves. That's why people have many for different situations.

SLOW = The Lowest Grip Surface + the Hardest Tyre (within reason).
21.5T = about 1600 KV x 8.4V = 13440RPM

If you are getting into the hobby a 13.5T is a good middle of the range motor and gearing can change the behavior.

...but you can't talk motor with out the full picture.

So which is best....?

These machines have 7.5 through 21.5 and are used on high grip carpet at ludicrous speed and slippery polished concrete with differing tyres for different results.

Combine the motor with the Final Drive ratio and throttle response add to the equations among other concerns. Why do I have the smallest pinion and the largest spur here? Response is lower at 5.2:1

Response here is less than 3.8:1 giving you less control over the RPM... but in some situations this is exactly what you need. Instant power Spool Up and higher top end. Differential settings and other considerations also come into play.

You need to experiment and get a feel for what you like and what works.

Personally, I love free spinning high rpm motors, but I only need about 60% of the RPM on my surface.
So I use the ESC to de-tune the forward power setting. I end up with the low RPM of a Higher turn motor with the instant response of a high RPM motor. I also try to go for a lower FDR (12:1) to give me control. This is a real issue in RWD machines where a few RPM the wrong way can make a big difference.

I never use CURVE to confuse the linear settings and give predictable behavior. TURBO can be an advantage in certain conditions.

Sometimes it's this FEEL and CONTROL that contributes more to the way your RC drift chassis handles.

There are other reasons for motor choice too.

How about Scale Speed?

How do you want to drive this 800+HP 2.8L 6 cylinder turbo 2JZ Powered JZX100?

I can guarantee the answer is aggressively.Even in the real car, you don't need FULL 800hp to spin the tyres so you will find that throttle modulation becomes more important.

How do you want to drive this 86 with 180hp 4AGE 4Cylinder Motor.

You may want to mash the throttle most of the time, so it shouldn't really be the same end result as the MARK II. The real cars are not the same, so the RC result may not be the same..The 86 will run out of wheelspin ability. The Mark II never will.
 

FRD with 17.5T  Motor and with lower final drive gear ratio of  leaves max wheel RPM around 1000 only.

the below is a way of comparing wheel speeds. Don't forget your finger controls the RPM from ZERO to 100% so this is the best way to control RPM

USE YOUR FINGER. IF you are using only FULL THROTTLE or ZERO, you are never going to have control.

Throttle modulation is probably the most neglected skill in RC. 

ALSO remember as car speed rises you will need more RPM to overcome the loaded and geared wheels so you need some reserve RPM.

Sample calculations for wheel speed. If you build a spreadsheet you can calculate the Variables entered in the blue section.

			Motor Choice
	Motor		21.5	17.5	13.5	10.5	9.5	7.5	Formula
	RPM/V	KV	1628	2000	2593	3333	3684	4667
	Battery Volt	8.4	8.4	8.4	8.4	8.4	8.4	8.4	100% Charged
	Max RPM	MAX	13674	16800	21778	28000	30947	39200	KV*BatteryVolts
	ESC FWD	100	100	100	100	100	100	100
	EPA	100	100	100	100	100	100	100
	Adjust RPM	MAX	13674	16800	21778	28000	30947	39200	MAX RPM  - Adjusted %
	Spur	90	90	90	90	90	90	90
	Pinion	22	22	22	22	22	22	22
	Response		4.09	4.09	4.09	4.09	4.09	4.09	Spur / Pinion
	Shaft RPM		3343	4107	5323	6844	7565	9582	Adjusted RPM / Response
	Rear Pulley / Diff Crown	42	42	42	42	42	42	42
	Centre Pulley / Drive Pinion	13	13	13	13	13	13	13
	Rear Drive Ratio		3.23	3.23	3.23	3.23	3.23	3.23	Rear Pulley / Center Pulley
	Final Drive Ratio		13.22	13.22	13.22	13.22	13.22	13.22	Shaft RPM x Response
	Rear Wheel RPM	MAX	1035	1271	1648	2119	2342	2966	Rear Wheel RPM / Final Drive Ratio
	Tyre Grip will lose RPM
	LOAD LOSS %	20	828	1017	1318	1695	1873	2373	Rear Wheel RPM  - %
	WHEEL RPM by Throttle Percentage  AT THE FINGER!!!
	T	10	83	102	132	169	187	237	MIN RPM
	H	20	166	203	264	339	375	475
	R	30	248	305	395	508	562	712
	O	40	331	407	527	678	749	949
	T	50	414	508	659	847	937	1186
	T	60	497	610	791	1017	1124	1424
	L	70	579	712	923	1186	1311	1661
	E	80	662	814	1055	1356	1499	1898
		90	745	915	1186	1525	1686	2135
	%	100	828	1017	1318	1695	1873	2373	MAX RPM

Input values in blue


Here are two distinctly different setups achieving a similar result... but the feel and control are far from similar.

	Motor		21.5		Motor		7.5
	RPM/V	KV	1628		RPM/V	KV	4667
	Battery Volt	8.4	8.4	Full charge	Battery Volt	6.9	6.9	Low BAttery
	Max RPM	MAX	13674		Max RPM	MAX	32200
	ESC FWD	100	100		ESC FWD	100	100
	EPA	100	100		EPA	60	60	Set low
	Adjust RPM	MAX	13674		Adjust RPM	MAX	19320
	Spur	80	80		Spur	86	86
	Pinion	30	30		Pinion	22	22
	Response		2.67	high	Response		3.91	low
	Shaft RPM		5128		Shaft RPM		4942
	Rear Pulley	40	40		Rear Pulley	40	40
	Centre Pulley	17	17		Centre Pulley	17	17
	Rear Drive Ratio		2.35		Rear Drive Ratio		2.35
	Final Drive Ratio		6.27		Final Drive Ratio		9.20
	Rear Wheel RPM	MAX	2179		Rear Wheel RPM	MAX	2100
	Tyre Grip will lose RPM				Tyre Grip will lose RPM
	LOAD LOSS %	20	1743		LOAD LOSS %	20	1680
	WHEEL RPM by Throttle Percentage				WHEEL RPM by Throttle Percentage
	T	10	174		T	10	168
	H	20	349		H	20	336
	R	30	523		R	30	504
	O	40	697		O	40	672
	T	50	872		T	50	840
	T	60	1046		T	60	1008
	L	70	1220		L	70	1176
	E	80	1395		E	80	1344
		90	1569			90	1512
	%	100	1743		%	100	1680