Wednesday, November 4, 2015

RE-Xtreme RC Drift Bible - Wheel Speed.

Wheel Speed. Is this what drift performance is all about? The grip threshold.

Real machines are getting more grip... Isn't drift about slide?

Is the above image RWD or Counter-steer? It doesn't really matter. the fact is, the car is doing what resembles RWD burnout and thats what we want isnt it? Well not really.

If we do a BURNOUT at FULL THROTTLE we don't actually go anywhere.
The car just sits there spinning the wheels.

So what do you do?

1. You can put on sticky/grippy tyres until the grip is enough to stop the wheels spinning and the grip moves the car forward somewhat.


2. You use less throttle until the tyres are slow enough that they grip.

The point at which the tyre loses traction is known as the grip threshold.

You only need just enough throttle / power / gearing to just break traction to achieve drift / slide.

But at what car speed do you want to break traction. When the car is already travelling at low speed you have enough reserve power to break traction again. At full car speed, this ability is reduced. Just like you can't do a burnout on the highway in 5th gear.  Car speed and wheel speed require balance.   

First up 4WD.

Counter-steer cars suffer an imbalance between front and rear grip that really becomes evident as the surface grip increases. The ability to slide the front and rear at high speed is a massive RC issue. If the RC speed is low, then this imbalance doesn't really matter, but as the rolling speed of the front and rear increases, A high counter-steer car around 3.5:1 cannot create enough front wheel-speed to overcome grip and a spin will ALWAYS result. The crude solution is a high RPM engine to force the wheel-speed higher and higher.

This is why on high speed tracks like Japan's Yatabe arena, CS ratio is rarely more than 1.5:1. You need the car speed and the fast front wheel speed balance on a large fast circuit. You never want the front wheels to generate a LOT of grip at high speed. 

Off throttle with a one way bearing, the car exhibits RWD charter anyway, so this can be ignored. 

Tight tracks like RC-Oahu, the speed is so low that even at full throttle the car momentum will never reach the front wheel speed. So you can always break traction.

In a similar way, a high grip surface or tyre will demand a lower cs ratio as the wheel speed required to overcome traction at a fast car speed is demanded.

Its kind of like applying the front brakes on the car after it reaches a certain speed.

I have tried Counter Steer up to 5:0:1  and this relates to about 15% front to 85% rear drive. As the car speed increases past the rotation that the front wheels are turning then they will start to exhibit grip and make the car uncontrollable.

It's the reason I always ran a high RPM motor in a high CS car with the smallest pinion and largest spur possible. You can use the throttle to control the wheel speed but you need to practice.  Painting a section of the front wheel in a visible colour will help you match the FRONT rolling speed.

So what about RWD?

Rear wheel drive will ALWAYS have the correct rolling front wheel speed. So the front wheels will ALWAYS be at the correct speed for the car. They are not driven.

So the above discussion about front to rear ratio can be ignored to an extent. But the amount of front grip at high speed is still in play.


Remember the burnout. We don't want this unless we have a front brake.

We want MAX accelleration without wheelspin until initation, then we want max accelleration / grip + just enough wheelspin to allow the car to slide to our desired angle. Any more REAR wheel speed will cause a spin.

Just like your real car. Keep you boot in on a wet road and you will spin. But control the throttle and you do still go forward and you can hang it out. But go to far and you are in trouble. Your hands on the steering wheel control the angle within may 2 degrees. The gyro takes care of the small corections.

You have NEVER seen a real car go from LOCK to LOCK in 0.8/s so don't let your RC do it either.

Apply real car drift techniques here. Got too much slide... Back off and get traction again.

With 4wd countersteer, you want to add more throttle to braeak the front traction also and help pull you out of the slide. RWD doesn't have this GT-R style ability, so your only choice is ... back off or spin out.

Paint the REAR wheel of you car to practice this technique. 

Matching the wheel speed to the gearing and motor selection is of course similar in all forms of RC but the RWD is especially dependant on this setting.

High grip tracks demand big power or fast wheel speed to overcome the grip levels. and that means the moment they let go... you may already be travelling too fast to catch the slide.

Scale speed drift requires real world physics.  


More front grip means that front wheels may never slide. If you are fast on the steering in the front then the reaction at the rear will be violent as the initiation will throw the rear around.  So you need rear grip to hold it.

If you have a little slide built into the front, then the impact to the rear will be reduced also.  Depending on your driving style and the grip level of the track you can tune your suspension to cope.

As my cars have evolved the search for grip vs slip is a constant battle. How much wheelspin do you need?

I've tried all kinds of settings, but the focus with 4wd is on reducing the front wheel speed to a "rolling speed". If you can do this to suit the track speed you are in business.

Rear Wheel Drive is a little easier to find the grip threshold. But it means you need to "drive" the car more to balance the grip available.

So I hope you can think about these ideas and balance your chassis.

1 comment:

  1. Thanks alot! Great read and alot of info to help me understand the why and the what and eventually the how :D


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