All You Need To Know About – Shocks
The job of the four shocks fitted to any car is to keep the wheels and body in control, this helps ensure that the tyres remain in as much contact with the track as possible at all times. Without shocks the car’s body would bounce and oscillate uncontrollably making its handling completely unpredictable. The springs are fitted to support the weight of the car and prevent the chassis from bottoming out. Working in conjunction with the springs, the oil in the shocks must be the right viscosity and counteract the spring oscillation but still allow the piston to travel through the oil. The shocks must absorb the changes in the surfaces without inducing ‘bounce’ or affecting the handling adversely. Matching the correct spring rate and oil viscosity is an art in itself, but most manufacturers get a good all round compromise with their RTR and kits. All that will be required usually is some fine-tuning for your track. If in doubt, always ask to see what others are running at your local track and why.
First insure that all the components that the shocks have direct contact with are free to move unhindered. If wishbones stick at any point in their range of movement, then anything you do with the shocks will be in vain. Remove shocks and wheels and put the chassis on a block to lift it above the surface. Remove any anti-roll bars that may be fitted and check that the wishbones lift and drop under their own weight with no tight spots or resistance of any kind. Re-fit the shocks and wheels, then place car on a flat surface.
FIRST SET THE NORMAL RIDE HEIGHT
Push down and lift the chassis a couple of times, now release the chassis and let it rise on its own. Then roll the car back and forth about a car length or so without pressing the chassis down. This is to ensure that the tyres haven’t ‘gripped’ the surface you are working on.
TEST THE SPRING/OIL BALANCE
Push the rear of the chassis and release. Then take note of the way the car returns back to its original ride height…
- If the car springs back very quickly the oil viscosity is possibly too thin
- If the car starts to rise slowly and stops, then the oil viscosity is more likely to be to thick.
What your looking for is the car returning to its normal ride height, but under full control of the shocks. This means that on the track the car will soak up the bumps and absorb changes on the track surface without exaggerating them. Running to stiff/viscous a set-up will not allow the wheels to move quickly enough to conform to the bumps. Too soft a set-up will cause the suspension to over compensate for surface changes and exaggerate them. What your trying to find is the sweet spot where the bumps are soaked up and the shocks can rebound upper control, with all four wheels in contact with the ground as much as possible.
One quick and inexpensive tuning aid is swapping out the stock springs. Most manufacturers offer a range of different rates. This is to aid the rear generate grip, and within reason 1 or 2 ratings difference is enough. For a track car you want stiffer springs, this will limit the amount of body roll in corners keeping load transfer to a minimum allowing maximum cornering speeds, too stiff and the car won’t absorb any bumps or curbs becoming skittish. Don’t forget go-karts don’t have any springs/suspension so are ultimately stiff and they handle pretty well as long as they don’t hit any bumps! For an off road car the balance will be between being soft enough to allow the wheels to ride the bumps out when landing jumps.
Most kits and RTR cars these days come with optional shock pistons, allowing you to change the shocks’ action. Usually these just vary the amount and size of holes the pistons have and the testiest way of understanding what difference they make to a shock’s action is this…
- Less or smaller holes will require lower viscosity oil for the same damping or offer a slower reacting shock if using the stock oil viscosity. Smaller holes also increase the amount of pack.
- More or larger holes decrease the amount of pack, and will require higher viscosity oil for the same damping or offer a faster reacting shock if using the same shock oil viscosity.
SHOCK MOUNT OPTIONS
Most R/C cars and trucks offer the possibility to mount the top of the shock absorbers closer to the centre line of the chassis, or vice versa. On many off road vehicles, there are several rows of mounting holes on the shock towers, giving an infinite number of mounting options, and all have a purpose and an effect on handling. The lower end of the shock absorber is usually attached to a control arm, or wishbone, and this too may have several mounting points, enabling the damper to be located closer to the wheel hub or vice versa.
ANGLE IN OR OUT
So let’s get start by taking a look at the chassis centre line, or closer to the wheel. If we mount the shock closer in its outer mounting holes (top and bottom), i.e. closer to the wheel, then what we achieve is a more proportional relationship between wheel and tyre articulation, and the extent of the shock shaft travel. Therefore, if your tyre hits a bump on the track, pushing the wheel and tyre upwards, the amount of shock shaft travel is roughly the same. The net effect of this is that the shock action will feel slightly stiffer, even though you haven’t fitted a harder spring or swapped to thicker viscosity shock oil.
Not surprisingly, if we go the other way, mount both ends of the damper closer to the chassis centre, on the inner holes, we have now changed the relationship between actual wheel movement and shock travel. Now we get a lot of axle articulation, with relatively little compression of the damper. So that the effect will also be the opposite, and now you will experience a softer ride, but with more chassis roll.
STAND UP OR LAY DOWN?
Stand up configuration, is something that is more commonly used for on road conditions on high grip tracks. As the car turns into a corner or strikes a bump in the track, you will experience less damping and the car will feel as if you have gone to a stiffer spring or thicker oil. This is why the less inclined the angle, the more responsive the car will feel, making it ideal for flat, technical tracks with lots of direction change. As you would expect, the more inclined you angle the shocks, the effect is almost, but not quite the opposite. Now, you will find the handling more progressive, softer initially, but increasing as the damper is more compressed.
Terry Crew – Reality Racing