ADJUSTING THE BUMP DAMPING CONTROL
Bump damping controls the unsprung weight of the vehicle (wheels, axles, etc.). It controls the upward movement of the suspension as when hitting a bump in the track. It should not be used to control the downward movement of the vehicle when it encounters dips. Also, it should not be used to control roll or bottoming.

Depending on the vehicle, the ideal bump setting can occur at any point within the adjustment range. This setting will be reached when “side-hop” or “walking” in a bumpy turn is minimal and the ride is not uncomfortably harsh. At any point other than this ideal setting, the “side-hopping” condition will be more pronounced and the ride may be too harsh.

STEP 1: Set all four dampers on minimum bump and minimum rebound settings.

STEP 2: Drive one or two laps to get the feel of the car. NOTE: When driving the car during the bump adjustment phase, disregard body lean or roll and concentrate solely on how the car feels over bumps. Also, try to notice if the car “walks” or “side-hops” on a rough turn.
STEP 3: Increase bump adjustment clockwise 3 clicks on all four dampers. Drive the car one or two laps. Repeat Step 3 until a point is reached where the car starts to feel hard over bumpy surfaces.

STEP 4: Back off the bump adjustment two clicks. The bump control is now set. NOTE: The back off point will probably be reached sooner on one end of the vehicle than the other. If this occurs, keep increasing the bump on the soft end until it, too, feels hard. Then back it off 2 clicks. The bump control is now set.

ADJUSTING THE REBOUND DAMPING CONTROL

Once you have found what you feel to be the best bump setting on all four wheels, you are now ready to proceed with adjusting the rebound. The rebound damping controls the transitional roll (lean) as when entering a turn. It does not limit the total amount of roll; it does limit how fast this total roll angle is achieved. How much the vehicle actually leans is deter-mined by other things such as spring rate, sway bars, roll center heights, etc.

It should be noted that too much rebound on either end of the vehicle will cause an initial loss of lateral acceleration (cornering power) at that end which will cause the vehicle to oversteer or understeer excessively when entering a turn. Too much rebound control in relation to spring rate will cause a condition known as “jacking down.” This is a condition where, after hitting a bump and compressing the spring, the damper does not allow the spring to return to a neutral position before the next bump is encountered. This repeats with each subsequent bump until the car is actually lowered onto the bump stops. Contact with the bump stops causes a drastic increase in roll stiffness. If this condition occurs on the front, the car will understeer; if it occurs on the rear, the car will oversteer.

STEP 1: With rebound set on full soft and the bump control set from your testing, drive the car one or two laps, paying attention to how the car rolls when entering a turn.

STEP 2: Increase rebound damping three sweeps on all four dampers and drive the car one or two laps. Repeat Step 2 until the car enters the turns smoothly (no drastic attitude changes) and without leaning excessively. Any increase in the rebound stiffness beyond this point is unnecessary and may in fact be detrimental.

EXCEPTION: It may be desirable to have a car that assumes an over-steering or understeering attitude when entering a turn.
This preference, of course, will vary from one driver to another depending on individual driving style.

Road Racing Setup

How to determine the required damper lengths.

2812:

A. Prepare the car for making measurements: put it on a flat and level surface, support it on jacks or such to lift the wheels of the ground. Remove the wheels, springs and dampers. Disconnect the anti-roll bars if fitted. Note: If your car features adjustable droop-limiters, set them to maximum droop.

B. Check if the standard A top eye (see here) clears its attachment point on the car under all normal operating conditions.

C. If you run into clearance problems, check if top eye #2 or #3 will fit. This should only be done as a last resort, because using a longer top eye will give you less stroke for a given damper length than a shorter one will.

D. The suspension should now be set at its maximum droop position. Take careful note of which suspension component is limiting the suspension from traveling any further.

E. Lift the suspension just enough to prevent that component from binding.

F. Measure the center to center distance between the upper and lower damper attachment-points. This is your open length or Lmax.

G. Look up here for the column that corresponds with the top eye you are going to use.

H. In these columns, search for the Lmax you measured. If no exact match can be found, and your car has no droop limiters, decrease Lmax to the next available fit. If your car does have droop limiters, increase Lmax to the next available fit.

I. Make a note of the corresponding closed length or Lmin.

J. Raise the suspension to the point where the chassis would hit the ground, or a suspension compo- nent uses up all its available travel.

K. Now again measure the distance between the damper mounting-points.

L. Check that this figure is greater than the Lmin found at point D1.

M. If this is not the case, decide if you need all the available droop-travel. If not, decrease Lmax to the next available fit and go back to step C5.

You have now found a suitable stroke/length combination. Check the table on the right to find the correct Design code Together with the type code as found in the first column of the stroke/length table here, this defines the required damper. Only the valving is now to be decided, look it up here.

2817:

For the following paragraph, it is assumed that the car is already equipped with dampers.

A. Put the car on a flat and level surface. Mark the position of the lower spring seat.

B. Jack the car up to its maximum desired ride height. Be sure that the wheels can be removed! Measure the distance between the upper and lower spring seats.

C. Support the chassis on jacks. Remove the wheel and springs. Reset the lower spring seats to their original position found at A! For convenience, disconnect the anti-roll bars if possible.

D. Now raise the suspension, to the point where either the chassis would hit the ground, or a suspension component uses up all its available travel. Again measure the distance between the upper and lower spring seats.

E. Subtracting the value found at D with the value found at B gives the required stroke.

F. Find a 2817 here that has this required stroke. Note its Lmin.

G. Check that this Lmin fits within the limit found at D.

H. If the Lmin of the 2817 is too long, see if the next shorter length would do the job. Note its corresponding Lmax and see if this is still sufficient.

I. Remember that the Lmax can always be limited by choosing another built-in droop limiter.

You have now found a suitable stroke/length combination. Please check here for the stroke table to find the correct Design code. Now only the valving needs to be decided.

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How to determine the required valvings.

On this page the currently available standard valvings are documented in the form of Force vs. Velocity graphs. Only the Minimum and Maximum adjustment curves are drawn in. A typical, complete adjustment diagram is also shown on this page, in this case a 233.

If you want us to help to pick a suitable valving, we need additional information about your car's suspension:

the spring rates
the "motion ratios"
Motion ratio is the term used to indicate the ratio between wheel movement and damper movement. This ratio is a very important factor when the required valving is chosen, because it determines the piston speeds the damper will "see".

This ratio is easily measured: assuming the car is without its wheels, springs and anti-roll bars:

1. Lower the suspension to its maximum droop position.
2. Measure the distance between the damper mounting points.
3. Raise the suspension to the minimum ride height position as found earlier and repeat step 2.
4. The mean motion ratio can now be calculated using the formula stated above.
5. Mention both the spring rates and motion ratios when ordering.