Friday, March 3, 2017

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









4 comments:

  1. Awesome article as always!
    What FDR would you suggest for a 10.5T motor with MST silver or gold tyres on polished concrete?

    ReplyDelete
  2. It's so hard to recommend without seeing the feel and how fast everyone is drifting on track.

    We use HDPE carpet tyres on polished concrete for the slowest speed possible.

    All concrete's behave a little different also. And the length of the track also determines speed and gearing.

    I try to go as high as possible for response over speed.

    ReplyDelete
    Replies
    1. Thanks a lot, I'll start from 8 and will will increase up to 10 or more to find the best for me

      Delete
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    ReplyDelete

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