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SHOCK
ABSORBERS
Shock absorbers 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.
However, if the chassis tuner
understands how shocks work and how they affect handling, he can use shocks to
gain a performance edge over the competition. The following information,
gathered through testing on both dirt and asphalt race tracks should help you
better understand racing shocks.
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.
This staged 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. Typically, a shock's
resistance is checked at a minimum of three different piston speeds so that a
more precise determination can be made of how the shock will affect overall
handling. Since it is important to evaluate a shock's resistance and low, medium
and 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.
Basically, shock 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 shock travel per second.)
Rebound 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.
Compression, 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.
Shocks that 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.
Please note 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.
HANDLING 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 high speeds.
When middle 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 mistakenly describe this ill - handling as a
"push" instead of a "skate". Consequently, the wrong areas
of the chassis receive adjustments.
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).
However, when shocks are too soft and bumps are encountered, a cycle referred to
as wheel hop or tire flutter can occur. During wheel hop, the tire actually
bounces on and 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
or deceleration.
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 traction.
DYNAMIC
WEIGHT TRANSFER
When discussing chassis tuning in depth, a basic understanding of dynamic weight
transfer and its effect on tire loadings is necessary.
Dynamic weight 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.
HANDLING THROUGH THE CORNERS
The traction capability of a tire determines that tire's influence on the race
car. Traction capability is greatly affected by the load put onto the tire.
The balance 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.
When making 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.
During acceleration, 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.
Forward traction 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!
Remember, shocks 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.
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.
Do you have any questions?
Visit Stock
Car Products website, and you can e-mail questions to them.
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