affect the handling of a race car as much as any other suspension
component. However, shocks continue to be one of the least understood
and most overlooked aspects of chassis tuning. Consequently, most
racers have to depend on someone's recommendations when choosing
shocks for their race car. If the prescribed shocks are incorrect,
the racer ends up adjusting his chassis around the wrong shocks
while trying to correct the handling problem. The result, typically,
is mediocre performance.
if the chassis tuner understands how shocks work and how they affect
handling, he can use shocks to gain a performance edge over the
A shock is a
valved hydraulic device that resists motion. When its shaft, and
the piston assembly attached to the shaft, are moved, fluid inside
the shock is forced through a series of small orifices. Some of
these orifices are always open (permitting fluid to pass through
during any shock movement) while others are covered and permit fluid
to pass through only when the fluid reaches a certain pressure.
Since there is a volume of fluid on both sides of the piston, the
shock is able to resist the movement caused by suspension travel.
The size of
the orifices and the pressure levels at which the closed orifices
become open determine the stiffness of the shock at various piston
speeds. Generally speaking, the greater the force put onto the shock
the faster its piston attempts to travel. This increases the shock's
resistance to movement and slows down the movement of the suspension.
valving is necessary because the shock resistance required to control
the suspension when a tire goes over a severe bump (referred to
as the high speed control of the shock) is much greater than the
resistance needed to control body sway or suspension movement caused
by small bumps (referred to as the low and medium speed control).
For the best handling to occur, the resistance of the shocks at
low, medium and high piston speeds must be matched to the needs
of the race car. Since it is important to evaluate a shock's resistance
and low, medium & high piston speeds, you should know that whenever
you stroke a shock by hand you are forcing fluid only through the
valving orifices that are uncovered. Therefore, the resistance that
you feel is not an indication of how the shock will perform on a
race car when the shock moves much quicker.
control at low piston speeds affects how the race car handles through
the corners. Shock control at middle and high piston speeds affects
how the race car handles whenever it encounters bumps and ruts.
The speed of the piston, at which a shock develops a given amount
of control, should always be specified. (i.e. 250# of resistance
at 17" of shocktravel per second.)
control is a shock's resistance to extend. The amount of rebound
control developed by a shock will affect how quickly the tire is
unloaded during dynamic weight transfer and how quickly the suspension
"rebounds" or returns to its original position, after
the spring has been compressed.(more later)
or bump control, is a shock's resistance to compressing and is specified
at a given piston speed. Compression control will determine generally,
how quickly the tire is loaded during dynamic weight transfer and
how the suspension will react whenever a bump is initially contacted.
have equal rebound and compression controls are referred to as 50/50
shocks since rebound represents 50 percent of the total shock control
as does compression. Shocks with unequal rebound and compression
controls are referred to as "split valve" shocks. For
example, a shock that has 90 percent of its total stiffness in compression
control and 10 percent of its total stiffness in rebound control
is referred to as a '90/10" shock.
that the ratio number put on a shock does not indicate its stiffness.
However, to facilitate the shock selection process, most shock manufacturers
use a part numbering system that does indicate the stiffness differences
between rebound and compression controls.
Like shock stiffness,
the ratio between rebound control and compression control greatly
affects the handling of a race car
OVER BUMPS AND RUTS
We said earlier
that the resistances delivered by a shock at medium and high piston
speeds affect handling over bumps and ruts. When a fast moving race
car contacts a large bump the suspension must react smoothly and
with as little change in the attitude of the chassis as possible.
This allows the tire to maintain compliance with the track surface.
However, if the middle and/or high speed compression control of
the shock is too great, or if the rate of the spring is too stiff,
the race car will rise and upset the chassis set-up whenever a bump
is encountered. If the suspension is extremely stiff, the whole
car can actually bounce and allow the tire to lose contact with
the track surface. Remember that in "bump" the spring
is actually working with the shock to resist suspension deflection.
In "rebound" the spring works against the shock by trying
to extend the shock and deflect the suspension. Consequently, most
shocks, including shocks that are referred to as 50/50 shocks, will
have more rebound control than compression control at middle and
and high speed rebound controls are too stiff the shock does not
allow the spring (or suspension) to return to its original position
quickly enough after a bump is encountered. Consequently the tire
loses some of its compliance with the track surface. The shock can
literally hold the tire off the track surface for a period of time.
It will do the same if the tire runs through a rut.
If the race
car is shocked too stiffly the race car will tend to skate up the
race track whenever bumps and ruts are encountered. Many drivers
mistakingly describe this ill-handling as a "push" instead
of a "skate." Consequently, the wrong areas of the chassis
If the so-called
"push" only occurs over bumps and ruts, then the problem
is a "skate" and softer shocks are usually the fix (assuming
the springs are not too stiff).
shocks are too soft and bumps are encountered, a cycle referred
to as wheel hop or tire flutter can occur.
hop, the tire actually bounces on & off the track. The wheel
hop cycle begins when a bump causes the suspension to move upward
violently. This upward movement of the tire and suspension causes
the spring to compress excessively and store a large amount of energy.
If the rebound control of the shock is too soft to control the energy
stored by the spring, the tire is violently slammed onto the surface
of the race track. The tire bounces off the track and the spring
stores a slightly smaller (but still uncontrollable) amount of energy.
The cycle continues until the shock can control the energy level
of the spring. Wheel hop can be caused by any major deformity in
the racing surface or by violent rear axle wrap during acceleration
Wheel hop can
easily be felt by the driver and, if extreme, can be seen by those
watching the race car. During wheel hop, the tire bounces up and
down uncontrollably and causes the handling to be very unstable.
The fix, of course, is to install stiffer shocks. Keep in mind that
wheel hop to any degree, whether felt by the driver or not, reduces
chassis tuning in depth, a basic understanding of dynamic weight
transfer and its effect on tire loadings is necessary.
transfer is the transferring of weight from side to side during
cornering, from rear to front during deceleration and from front
to rear during acceleration. The distribution of weight that transfers
is affected by the rates of the springs used in the chassis. Basically,
if one of a pair of springs receiving weight is stiffer than the
other, the stiff spring receives proportionately more weight than
the soft spring.
The rate at
which a tire is loaded or unloaded during dynamic weight transfer
is affected by the low piston speed control of the associated shock.
In rebound, a stiff shock slows down and a soft shock speeds up
the unloading process (unless rebound control is extremely stiff).
In compression, a stiff shock slows down and a soft shock speeds
up the loading process(unless compression control is extremely stiff).
However, excessively soft or stiff shocks can produce effects opposite
to those started. Consequently, by changing the stiffness of the
shocks used on a race car, we are adjusting the loadings on the
tires at different points on the race track. If done correctly,
good handling will result.
THROUGH THE CORNERS
capability of a tire determines that tires influence on the race
car. Traction capability is greatly affected by the load put onto
of traction between the left side and right side tires determines
to a great extent how the car will handle while decelerating through
the corner. For example, a race car will tend to push (not turn)
whenever the left side tires do not have enough influence in stopping
the car (the right side tires are slowing the vehicle more than
the left so the vehicle tends to go to the right). By using stiffer
shocks (especially a stiffer extension control on the left rear,
and to a lesser degree, a stiffer extension control on the left
front), the unloading process of the inside tires (due to dynamic
weight transfer) to the outside tires slows. Consequently, the left
side tires remain loaded further into the corner which helps to
turn the chassis.
this adjustment, consider using the appropriate split valve shocks
so as to not increase the compression control of the left side shocks.
This change should allow the chassis to roll back onto the left
side tires more easily during corner exit.
Also, the opposite
of the above example holds true. Softening the extension of the
left side shocks, especially the left rear will cause the left side
tires to unload sooner during cornering. The balance of traction
between the left and right side tires moves toward the right tires
more quickly and the chassis becomes tighter on corner entry.
the balance of traction between the rear tires can be adjusted with
shocks also. A softer left rear shock (especially compression) will
quicken the weight transfer effect to the left rear tire during
acceleration. The result is a left rear tire that has added influence
initially in accelerating the race car off the corner. A race car
will tend to be tight off the corner whenever the balance of traction
between the rear tires favors the left.
can be enhanced by softening the extension control of the front
shocks. This enhances the front to rear weight transfer process
and helps to load the rear tires for improved forward traction.
Keep in mind that a softer left front shock (rebound) may tighten
corner entry handling also!
are a compromise like any other suspension component. Be careful
when using split valve shocks with soft rebound controls so that
handling over bumps and ruts does not suffer. Generally, side bite
(cornering ability) can be improved by softening the shocks (and/or
springs). This adjustment can stop the race car from skating up
the corners on slick, smooth tracks.
is no mystery to shock function and tuning. However, there are complexities
and qualities that need to be considered when choosing shocks for
a specific application. By keeping this basic information in mind
when troubleshooting handling problems, you should be able to install
the correct shocks for each situation. This should also enable you
to have the confidence to make shock changes with fairly good expectations
for the results.
Above all, remember
that chassis tuning is a compromise and shocks, though a very important
part of the set-up, are still only a part.