Neil Roberts Article on Shock Tuning
Dampers 201, we looked inside a shock absorber to find out
how it works. This time, we will discuss the damper adjustments which
can be used to change the behavior of the car in each phase of a
corner. We will begin by defining each phase of a corner and apply some
sensible simplifications, then list the relative damper adjustments
available to produce the desired handling change.
For the purposes of this discussion, we will assume the racing surface
to be perfectly flat, smooth, and uniform. So, all damper velocities
will be relatively low, assuming a smooth driving technique. We will
assume a simple road course setup, with no asymmetric adjustments.
The damper velocities and travel directions resulting from each
cornering phase affect the distribution of load among the four tires.
This change in load distribution changes the cornering balance. We will
focus primarily on the effects of diagonal weight transfer due to
damper forces and the resultant change in cornering balance.
CORNERING PHASE DEFINITIONS
PHASE 1: Increasing braking + increasing steering
This phase is the first part of a fast decreasing radius turn. This
phase will not occur at all if maximum braking is achieved before
turning in. Since weight is being transferred both forward and outboard,
the outside front damper moves in the bump direction. Also, the inside
rear damper moves in rebound. The other two dampers do not move as much
or as rapidly, so their effects are minimal. For our purposes, we will
consider the inside front and outside rear dampers to have a fixed
position during phase 1.
PHASE 2: Decreasing braking + increasing steering
This is the turn in phase of a slow corner. This phase may or may not
occur depending on the type of turn and driving technique. Weight is
being transferred outboard and aft, so the outboard rear damper moves
in bump and the inside front damper moves in rebound. The other two
dampers are considered to be stationary.
PHASE 3A: Increasing steering at constant throttle
This phase can be a course correction, a slalom turn in, or a turn
entry taken at full throttle. Weight is being transferred outboard, so
both outside dampers travel in bump and both inside dampers travel in rebound.
PHASE 3B: Decreasing steering at constant throttle
This is the opposite of phase 3A. During a slalom, this phase occurs
while the steering is changing away from the current cornering
direction. As soon as the lateral acceleration passes through zero, the
car reverts to phase 3A. This is part of why so many spins occur in slaloms.
PHASE 4: Decreasing steering + increasing throttle (or decreasing braking)
This is the apex-to-exit phase. Weight is being transferred inboard and
aft, so the outside front moves in rebound and the inside rear moves in
bump. The other two dampers are considered stationary.
EFFECTS OF DAMPER INDUCED WEIGHT TRANSFER
At all times, cornering balance is affected by the distribution of load
between the two front tires. Because the efficiency of a tire decreases
with increasing load, a larger difference in load between the two front
tires increases understeer. Also, a smaller difference decreases
understeer. The same concept applies to the two rear tires.
To illustrate the effect of damper adjustments, consider a phase 3A
flat out turn entry. If the front dampers are adjusted to increase
either bump or rebound damping, more weight will be transferred across
the front tires during entry. The same result occurs if the rear
dampers are adjusted to decrease either bump or rebound damping. This
increased front load transfer increases understeer during turn in, just
as a larger anti-roll bar increases understeer in steady statecornering.
The same increase in understeer results from diagonal weight transfer
from the inside rear to the outside front. The fact that the two
diagonally opposite dampers move in opposite directions allows us to
modify cornering balance with damper adjustments.
Note that we are only considering longitudinal weight transfer if
accompanied by steering change. Longitudinal weight transfer without
steering change moves both front and both rear dampers in the same
direction at the same speed, so damper adjustments cannot change the
diagonal weight distribution. Obviously, longitudinal weight transfer
affects cornering balance. But, since the dampers cannot affect balance
unless accompanied by roll, we will ignore this effect for damper tuning.
The following table presents the damper adjustments available to modify
the cornering balance in each phase. Each entry lists the phase, the
damper travel directions, the desired change, and the damper
adjustments available to produce that change. "+" indicates stiffer damping
"-" indicates softer damping. IF is inside front, OF is outside front.
DIRECTIONS UNDERSTEER OVERSTEER
Phase 1 entry OF bump
F bump +
F bump -
OR rebound R rebound -
R rebound +
Phase 2 entry IF rebound
F rebound +
F rebound -
OR bump R bump -
Phase 3A entry OF&OR bump F bump
+ F bump -
IF&IR rebound F rebound
+ F rebound -
R bump - R bump +
R rebound - R rebound +
Phase 3B exit OF&OR rebound
F bump -
F bump +
IF&IR bump F rebound -
F rebound +
R bump + R bump -
R rebound + R rebound -
Phase 4 exit
OF rebound F rebound -
F rebound +
IR bump R bump +
As you can see, none of the available adjustments affect only one
cornering phase. This is where the balancing act begins. Notice that
the same adjustments that increase phase 2 entry understeer also
increase phase 4 exit oversteer. Compromise is necessary even in the
case of a constant speed slalom.
It is worthwhile to spend some time
studying the table to figure out
how to fix more than one balance problem at the same time. For example,
consider a car that has phase 1 oversteer, phase 2 understeer, and
phase 4 understeer. Can this combination of problems be corrected by
damper adjustments alone?
Careful, focused analysis of the behavior
of the car during each phase
is necessary to begin real damper tuning. Then, the correct decisions
must be made concerning which phase(s) are most important and require
damper adjustments to improve cornering balance. The adjustments made
will alter performance in other phases, so the magnitudes of damper
adjustments must be selected accordingly.
As you can imagine, it is rather difficult
to accurately remember the
balance of the car in each cornering phase for each corner of the
track. This process can be assisted considerably by an in-car video
camera and/or a data acquisition system.
A truly optimum damper setup is only
possible with highly developed
active dampers. The optimum compromise with conventional racing dampers
is difficult to determine. This should not deter us from trying.
The restriction to symmetric damper
adjustments is the source of many
of the required compromises. If you have followed the discussion to
this point, you can work out for yourself the amazing cornucopia of
damper adjustments available to oval track tuners. If a particular road
course features all or most of the important corners in the same
direction, asymmetric adjustments can be used to fine tune the setup to
Neil Roberts firstname.lastname@example.org