TRC

Brushless Radio Controlled Cars

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RC Handling & Tuning Guide

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Droop

Droop is determined by the amount that the chassis can lift from normal ride height until the wheels lift off the ground.

When the car turns a corner most of the weight of the car is transferred to the outside wheels. If we had a fixed axle front and rear, the inside wheels would lift. But because our cars have independent suspension we can compensate for this. By giving our cars droop we allow the springs to exert pressure onto the inside wheels give us more traction at the front, reducing under steer. And increasing traction at the rear, preventing spin out and diff-out.

The droop is adjusted at the front by adjusting the screws on the lower wishbone. Screw out the screw-more droop. Screw in the screw-less droop.

On the rear this is usually adjusted on the top wishbone, screw in the screw –less droop. Screw out the screw-less droop. Remember, too much of something can be as bad as too little.

To start with, 3-6mm front. 4-8mm rear shouldn’t be far away.

Torque RC .com 2011

The dampers fitted to your car are there to ensure that the tyre remains in contact with the track at all times. The springs are fitted to support the weight of the car and prevent the chassis from scraping the ground.

That’s the basics, now for a bit of fine-tuning.

I am talking about oil filled dampers here. No matter what spring rate the springs are, the oil in the shocks must be the right viscosity to counteract the spring oscillation but still allow the piston to travel through the oil. In other words it stops the car bouncing up the track but still allows the car to soak up the bumps.

The shock oil is graded in numbers, the lowest number being the thinnest oil and visa-versa. Different manufacturers have different number systems so pick a brand and stick with that one brand throughout so you have a common reference.

To be precise you must ensure that all the wishbones are free to move and none of the hinge pins are bent. Remove your shocks and wheels and put the chassis on a block to clear the surface. Disconnect any anti-roll bars. The wishbones should lift and drop with no tight spots or resistance. If there is any, solve the problem before testing the shocks. Now re-fit the shocks and wheels and put the car on the flat surface.

To get the car to its normal ride height you must push down and lift the chassis a couple of times finishing with a push on the chassis. Release the chassis and let it rise on its own. Now without pressing on the chassis, roll the car back and forth about a car length. This will ensure the tyres don’t have a grip on the worktop.

You must do this every time you test the shocks.

Now, to test the rear spring/oil balance, push the rear of the chassis down to the stop and release. Note how the chassis returns to its ride height. If the car springs back straight away then the oil is too thin. If the car starts to rise and stops then the oil is too thick.

What you want is the car to rise up to its normal ride height under the control of the dampers.

If you don’t know what oil is in the shocks to start with have a starting point. This depends on the spring rate, if it’s a soft spring start with thinner oil and the same with stiffer spring use thicker oil.

You may have to buy 2 or 3 bottles of oil for both front and rear to get the exact oil/spring balance but it is well worth a few pounds to get it right. And the oil you don’t use can be used if you change springs or do a bit of tweaking to suit track conditions which I will cover later.

If you want a bit of a reference try it on your full size car, push it down and release and you will see it slowly returns to its ride height.

Now we have covered the balance between the oil and springs I will just say a bit on the springs themselves.

In general the springs on the front should be stiffer than the springs on the rear.

This may confuse some people as a lot of cars come from the factory with stiffer springs on the front or the same on both. To simplify matters, the softer the spring the more grip. So to get the drive to the rear wheels we tend to put a softer spring on the rear, within reason. 1 or 2 spring ratings of a difference is enough. As long as you have the oil matching the spring you can balance the car with toe-in, camber etc.

Always remember, car set up is a compromise. What you gain at the front you loose at the rear and what you gain at the rear you loose at the front.

I have gone on a bit about spring/oil balance but it is essential that whatever springs you have the oil in the shocks should match them.

Just a quick note, if you have some fancy pistons fitted, the set up is the same; the oil still has to match the spring.