30 October 2005 20:33
Dampers (Shock Absorbers)
Most Formula Student cars use Mountain Bike Shock absorbers similar to the Fox Vanilla RC Shock shown here.
When choosing dampers, we must first understand exactly what the dampers do. They are designed to work in concert with the spring to keep the tyre contact patch on the racing surface. In bump, the damper compresses to help control the wheel travel and prevent "overshoot", and in rebound, the damper helps absorb the energy stored in the spring.
Good damper control is the most significant contributor to the "mechanical grip" we hear so much about. Mechanical grip is all about keeping the tyre patch in contact with the racing surface with as little excitation as possible. It is the task of the damper to dissipate that excitement.
Bike dampers are generally pretty suitable for the mass and forces imposed in a FS car, and when the suspension travel is matched to the damper shaft travel by use of properly calculated motion ratios, they can give pretty good results as long as the damping rates can be matched to the application and the spring rates used.
Newer bike dampers have very high initial compression damping to prevent the power of the riders pedal pressure being dissipated in the spring, causing the bike to bob up and down as the bike is pedalled. This is not a good thing as can be seen from the table below. If this feature can not be removed from the damper or otherwise tuned out, this type of damper should be avoided.
Mountain bike dampers are also designed to allow a bike and rider to survive a leap from a cliff, and as such really are "Shock Absorbers" in combination with the springs.
On a FS car, there should never be massive impact loads on the shocks, and so they need to be adjusted to better match their new application. This adjustment may be available from the bump and rebound knobs on the damper body, but it is very unlikely the optimum high and low speed damping can be achieved without a modification to the internal valving, or by a change of fluid or gas pressure at least.
Many makers of Mountain Bike shocks know that their product is used on FS cars, and so can supply them with custom damping curves to suit the application. Teams should investigate this opportunity when buying dampers. If dealing directly with the suppliers, dampers should be specified with spherical bearings in both eyes, rather than the simple bushes usually fitted. Invariably, rocker arm motion and mount and chassis flex will hinder the straight-line operation of the dampers, thereby degrading their performance. If the dampers are supplied with plain bushings, these should be replaced with spherical bearings.
Usually, mountain bike shocks have a shaft travel of less than 3 inches (75mm) and FS cars are required to have a minimum of 1 inch (25mm) wheel travel in both bump and droop. Designers should adopt motion ratios that make the best use of the available damping travel without allowing the damper shaft to bottom in the body.
Although the damper is usually used as the suspension droop limiter, this should be avoided in normal driving. A damper that "tops out" repeatedly will rapidly fail. Motion ratios can be calculated to impart a raising (or falling) rate to the suspension but such a linkage should be very carefully matched with the hydraulic damping rate. Dampers may have a progressive or digressive rate, and that rate may differ with shaft speed. The judges will want to see tables of wheel travel to damper shaft travel, and will ask the suspension designer to justify his solutions. Most dampers use bump rubbers on the shaft to prevent damaging "bottoming" by significantly increasing the spring rate in the last 20mm or 30mm of shaft travel.
Modern dampers invariably use a compressed gas (usually nitrogen) to prevent aeration of the damping fluid. The gas pressure keeps the damping fluid from frothing up and becoming ineffective. Unless the team are lucky enough to have some of the brand new "through shaft" dampers, this compressed gas adds a degree of spring to the damper. In the illustrated Fox, this high-pressure gas is in the end of the piggyback canister farthest from the damper, and separated from the damping fluid by a floating piston. As the damper is compressed, the increasing volume of the shaft displaces the fluid against the floating piston. The compression of the gas affects the spring rate and the performance of the aerated fluid.
Gas pressure regulation is an important adjustment. Care should be taken to keep a hard working damper cool, or the increase in gas pressure will alter the performance.
Usually dampers are described as having both high speed and low speed compression and rebound rates. The "speed" mentioned is the speed of the piston shaft inside the body. Low speed damping is sometimes called "bleed". High and low speed damping affect different areas of the on track performance, however there is a considerable overlap in the "speed" phases that can complicate analysis and adjustment.
High-speed damping adjustments affect low speed performance and vice versa.
The following simple table should help a team sort out their damping problems, or at least identify if there is sufficient damping adjustment available with the current hardware. I always suggest tuning dampers from "Soft" to "Hard" rather than the other way around, therefore this table deals with the effects of adding more rather than less damping adjustments.
Setting up dampers at the track can be a lengthy and frustrating task, however the benefits are well worthwhile. One frustration to watch for when testing dampers is a reduction in tyre performance as the test goes on. Chasing a good setup on deteriorating tyres is not only frustrating.
