RE-Xtreme RC Drift Bible - Mathematics and Ratios

Do you remember high school!

Sometimes those lessons are useful.


I've decided to give you guys some ideas of why the numbers should be crunched.

I'll look at the following for common 2 belt CS setups

1 Motor RPM
2 Pinon Spur Combo
3 Rear wheel rotation 
4 Front wheel rotation
2.47 Samples


If you haven't considered changing all of these as an advanced runner, maybe it's time you took a look.

I'll use my chassis as a baseline and look at some tuning options and the ultimate effect.

Sometimes switching a motor is easier than changing a gearbox or belt. It should not be discounted.


1 Motor RPM


Accvance rate their 7.5Turn Brushless motors at 5,220KV  this rating = (RPM/V) which will of course vary as voltage drops.

the 6.4 (empty lipo) ~ 8.4V (full Lipo) range will generate from 37056RPM to 48636RPM

I'll use the high point for my calculations because, If I were to enter a comp, I'd want a fully charged battery for each run.

Why? So I can predict and perfect my setup. Ever wondered why your car may handle differently at differenet charge? This could be the reason. Its a flow on effect from RPM that can effect handling.

2 Pinon Spur Combo

Lets look at some basic pinion/spur combos which will ultimately control the RPM of the centre shaft.

Depending on the surface and grip available you will tune your chassis accordingly.

I run an 88spur (max my chassis will allow) to one of the smallest pinions which allows me a little more finger control over the wild RPM range as the pinion spins faster to one turn of the spur. I'm effectively slowing the spur and therefore the speed that the central shaft. 4.40 : 1. this results in the centre shaft rotating at max rpm of 11053RPM

i want to run 5 : 1 which would give me 100 : 20  which gives me about 9727rpm. quite a bit slower but with more control over the speed range.

With a stock 13.5T @ around 13.3T @ 23184 RPM
80: 30 which results in a 8694rpm. Which still generates a fast car. Only 1000rpm less than my monster power car.

The control over the RPM range is less. however 2.66 : 1 that's a whole 1.8 times less responsive to throttle input than my setup.

Now the shaft and wheels are spinning lets look at wheel rotation and introduce the CS ratios.

Stock 50:50 cars also run 2.66:1 for front and rear wheel ratios.
15t to 40t gears usually bring this magic number.

3 Rear wheel rotation

Yokomo FCD kits are designed to overspeed the rear wheels. A stock setup runs at 3260 rpm for rear axle rotation and a FCD 2.0x kit 20T rear axle of course doubles this to 6520rpm.

now any monkey will know that simply by doing this, the rear will be able to break traction easily because the rear wheels are spinning quickly. A lot like a rear wheel drive car... DRIFT!

Now... why do all the Japanese Pro's run a FCD 1.3 rear spool?

Simple. because they don't want soo much RPM on the rear wheels. It's a bit of a secret that their cars are not 1.3cs. Many are 1.6~1.8 which I will explain the next part.

But have you noticed a theme yet? I'm not talking about CS ratios I'm talking about Wheel RPM and Shaft RPM and Engine RPM. because ultimately its the tyre rotation (including size) that determines how much grip each wheel receives.

Why are the CS 2.0 kits bad? Well, they aren't. on a high grip surface... asphalt car park where most beginners run, the easiest way to get drift is to add rear wheel RPM and the more speed on the rear wheels, the easier it is to break traction and initiate drift. It's just like dropping the clutch and doing a burnout.

Anyway, My DRB's real setup is with a 1.5FCD rear spool with an 15T centre pulley to rear 27T pulley resulting in rear wheel rotation at 6140RPM
you will notice this is similar to a stock motor and FCD 2.0 @ 6250  very similar.

But I am using the power for control

If I used my motor with a FCD 2.0 set I'd be running 8250RPM on the rear wheels. 5000rpm more than a 50:50 stocker! thats gonna schred tyres like a mad bastard.

Getting the idea yet. I haven't listed where I used my car yet. but I should have...

On some tracks with more grip I run 18/27 which moves more to7369 rpm
I could have used a 1.8FCD for the similar effect, but there are other reasons to use different gear selections.

 
4 Front wheel rotation

Ok, so we still want to have the low speed on the rear but an even lower speed on the front for the RWD effect and that little bit of control assist for drift.

This is CS drift or Countersteer of Full Counter Drift (FCD) or whatever.

So, My car is running stock front gearing 15T centre to 40T front spool (one way) which results  in

front 4145RPM : Rear 6140RPM

1.5CS. People will say, you need more CS, but if you tune your chassis to reduce traction in other ways and change your driving style you can get all the impact of counter steer sliding with even these low CS numbers.

thats another chapter.

anyway... lets look at whats possible.
simply by changing the front pulley and belt I can reduce RPM by a massive amount.

40T/15T = 2.66 remember 4145 RPM

40T/9T = 4.44:1 which lowers the front RPM to 2487 RPM
So I now have REAR 6140RPM / FRONT 2487 this is CS2.47


2.47 Samples 

R15T/27 / F40T/9T   = CS2.47
R18T/20T / F40T/15T  = CS2.40

Why would I choose these ratios. (with a 7.5T motor) The answer is: Because wheel speed.

R15T/27 / F40T/9T -  = CS2.47
6140RPM / 2487RPM - sounds pretty low doesn't it.

R18T/20T / F40T/15T  = CS2.40
9948RPM / 4145RPM - I think you'll agree these are pretty high sounding.

If I change the Motor from 7.5T to 21.5T look at the difference.

R15T/27 / F40T/9T -  = CS2.47
1856RPM / 750RPM - SLOW. and sometimes that's enough for your surface.

R18T/20T / F40T/15T  = CS2.40
3006RPM / 1252RPM - I think you'll agree these are very different.

Conclusion

Depending on the grip your surface generates against the tyres, you should be tuning your wheel rpm to break traction at your desired point in your rev range. It's all trial and error in conjunction with soooo many other variables.

If you need more RPM, then do that, change your ratios or motor or pinon or spur.

But I can't answer your specifics.

Sometimes you may find your self saying, It handles well at full power, but as the voltage comes down, it's terrible.You can't maintain traction loss, there are many variables and too many scenarios to cover any particular one in detail.

But these are the mathematics.

So I am currently running on polished concrete. No GRIP at all, so I SHOULD change my motor to ripple down the line to wheel speed, but I can also just plug in my Keyence TAO and limit forward motor speed on my ESC and get a very similar result.

If you have read this to the end, you might be interested in your future Drift Car Development. Stay tuned.