Monday, 16 February 2015

Counter Steer Ratio and Final Drive Ratio

Counter Steer (CS)


My previous technical posts have all been quite similar; all based around suspension and geometry. This is because it plays a big part in RC drift. What I haven't talked about is drive mechanism...

(Image source: Broadtech)

The drive mechanism consists of any component that takes power from the motor to the wheels. For example: the motor itself and the pinion fitted to it, the spur gear, the pulley(s) or gear(s) that are connected to the spur gear, belts (pictured above) or a shaft (pictures below) and finally the front and rear differentials.

(Image source: Broadtech)

These components are like any other and can be swapped out for varying results. The first one; counter steer.

Counter steer, or CS as it is referred to, is the difference in speed between the front and rear wheels. There are many apps and websites dedicated to assisting you with your counter steer setup (links at the bottom of the page) but I am a numbers guy and so we are going to go through it step by step.

All you really have to remember is - driven÷driver
This is the key to the equation regarding both CS and FDR for example, on my triple belt R31-16FM the CS pulleys are as follows:

Diagram ref
Pulley
Teeth
A
Front belt front pulley (one way diff)
39
B
Front belt rear pulley
16
C
Center belt front pulley
18
D
Center belt rear pulley
18
E
Rear belt front pulley
23
F
Rear belt rear pulley
32


Pulley B and C is where the drive comes in from the spur gear so these are our driver pulleys. B drives A making it a driven pulley and C drives D also making it a driven pulley. Pulleys D and E are connected together on a shaft and so E is the driver and F is the driven pulley. Bear with me, here comes the maths...

Like I said, there are plenty of online calculators and apps available but I want to enlighten you on what CS is and how it's calculated.

Going by driven÷driver, the first set of pulleys are A - 39 and B - 16. As B is driving and A is driven, we take 39÷16 = 2.44. That is the ratio for the front wheels.

There are 2 belt for the rear to is becomes driven÷driver x driven÷driver.
D÷C x F÷E = 18÷18 x 32÷23
That gives 1 x 1.39 which gives us 1.39. That is the ratio for the rear wheels.
Confused yet?

We then take the front wheel ratio and divide it by the rear wheel ratio:
2.44÷1.39 = 1.76 and that is my CS ratio. The full calculation looks like this:

(A÷B)÷((D÷C)x(F÷E))

WTF?!

If you have a 2 belt chassis, you just take out the E and F pulleys.

That is a lot to take in but that is the calculation for CS. As I previously mentioned in another post, the CS ratio that you decide to use is dependent on the track. Put simply, the larger the track, the lower the CS. Having the rear overdriven or the front underdriven, the rear of the car will "want" to slip out. On a large track with large corners, this will have an adverse effect. Similarly, having a low CS on a small or tight-cornered track will cause the car to not slide as much and will end up being difficult to get the car drifting realistically.

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Final Drive Ratio (FDR)


Once you've got your head around CS and are quite comfortable with changing it on how the chassis feels around the track, you may notice that you're either faster or slower than the other cars going around. This could do with your technique or you may have messed with the FDR when you changed your CS.

(Image source: HobbyKing)


Final drive ratio is the difference between motor rotation and wheel rotation. Let's say that you have a 10.5T motor (like the one above) that has a kV rating of 3250, with a fully charged battery at 8.4V, this motor will have a maximum rotation (speed) of 27300 rpm. At 6V, you'll be getting 19500 rpm.

The rpm of any motor is worked out by multiplying the battery voltage with the kV of the motor, most brands will have kV ratings for their motors readily available. Most people will measure battery voltage at either 6V, 7.2V or 8.4V depending on the projected outcome. 8.4V is a battery fully charged but it doesn't stay there long. Measuring at 6V will give you additional speed while always giving you your desired speed.


This graph is a visual aid to the point I made; let's forget about motor speed for the time being and concentrate on wheel speed. The red line shows a steady incline in speed, this happens naturally as you apply more throttle using your trained trigger finger, the blue line is wheel speed. The difference between the 2 lines is 7.

Random number 7? Where did you come from?

7 is the FDR that I run, Yokomo recommend that for a 10.5T motor you should have a final drive ratio of 6.5. Another reference to Mizunaga San from Japan, again relayed to us by Mitto at Soul RC, you should set the FDR to what track surface you use. An ideal FDR is one where you can move off from a stopped position and up to running speed with no wheel spin.

I run a FDR of 7 because I feel that it's a good number for my local track. Paired with my 7.5T motor, I get great results allowing me to run close to the other cars without pulling away or falling behind. Similar to CS, for larger tracks you'll want a lower FDR, smaller tracks higher but you final decision should depend on surface and wheel spin.

FDR 2

FDR 3

FDR 10

Now that you understand FDR, how it works and it's purpose, let's take a look at  the numbers. Here is the table from before but I've included the pinion and spur sizes as they will make a difference; remember FDR is the difference of rotation between the motor and the wheels.

Diagram ref
Pulley
Teeth
A
Front belt front pulley (one way diff)
39
B
Front belt rear pulley
16
C
Center belt front pulley
18
D
Center belt rear pulley
18
E
Rear belt front pulley
23
F
Rear belt rear pulley
32




Pinion
24

Spur
120

Some may disagree but I always measure at the rear wheels. The reason for this is because I have a one way differential fitted in the front and along with practising the drift technique of "Korogashi", the front wheels only ever roll at ground speed and are barely powered.

Ready for the numbers? The motor rotation goes through the pulleys in this order:

1. Motor
2. Pinion
3. Spur
4. Center belt front pulley (cbfp)
5. Center belt rear pulley (cbrp)
6. Rear belt front pulley (rbfp)
7. Rear belt rear pulley (rbrp)
8. Wheels

Remember the key to it all? Driven÷driver. This works here too...

(Spur ÷ pinion) x (cbrp ÷ cbfp) x ( rbrp ÷ rbfp)
(120 ÷ 24) x (18 ÷ 18) x (32 ÷ 23)
5 x 1 x 1.39

= 6.95
= 7 FDR

I take that 7 FDR, run it with my 7.5T 5135kV motor and get a top speed of 43,134 rpm at the motor and just over 6100 rpm at the wheels with the battery fully charged. As the battery gets used, the top speeds drop to 30,810 rpm at the motor and 4400 rpm at the wheels; these are the numbers I use when making the calculations. 

But of course this is drift and I'm very unlikely to be at top rpm for long if ever but "chance and fortune favour the prepared".

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So you see, it's not that difficult to work it out your self with a bit of paper, a pen and a calculator but it does make a big difference in driftability (I literally just made that up! Maybe I should patent it?).

It is very simple and if you're still struggling with the numbers, here are some useful links.


Drift Mission App - Includes CS, FDR and more




Countersteer Calculator for Android (use only for CS, these guys have calculated FDR using all wheels)

Right, hope you learnt from this and as always, keep drifting fun!