Springs
Springs are the quickest, easiest, and most effective way to change weight transfer on your chassis.
Springs have only one job; to control ride height. Softer springs allow more weight to transfer, while
stiffer springs allow less weight transfer. The more weight transfer you have to a particular corner of the
chassis, the more traction you can gain on that corner’s tire. In general, running softer springs will
produce more traction until you get so soft that the chassis rolls over to the point where there is no
suspension left and it starts to slide the tires. Also, running springs that are too soft can result in
overloading of the tire, which can also result in a break of traction. A softer front spring will usually
produce more steering while a firmer spring will tend to decrease steering response. A softer rear spring
will usually produce more rear traction, but on higher bite tracks, you will need to increase your spring
rate to keep the car sitting on the rear end, causing a push. Most chassis can achieve maximum balance
with stiffer springs up front than in the rear. Rare situations may call for softer front springs than rear
springs, usually when cars have extreme aerodynamic conditions such as EDM style bodies with high
amounts of downforce and plenty of side panel for stability.
Shock Mount Positions
Shock Mount Positions on the suspension arms and the shock tower can change the effective rate of the
shock/spring package. Moving the location of the shock mount changes the amount of leverage the
shock has on the suspension as it compresses. The more a shock is laid down, the softer that shock
becomes, due to decreased leverage against compression of the suspension. The more a shock is stood
up, the stiffer that shock becomes, due to increased leverage against compression of the suspension.
Laydown front ends typically have a more aggressive feel for the driver, due to decreased leverage of
the shock, and increased weight transfer to the right front. Stand Up front ends typically have a more
“positive” feel for the driver. This means the car feels more into the track, and more consistent,
although possibly more of a “tight” feel. Stand Up and Laydown front ends need very different spring
setups due to the varying amounts of leverage.
Shock Dampening
There are three key factors in shock dampening: Pistons, Oil, and Bladders. Shock Pistons move through
the Shock Oil inside the shock body during compression and rebound, and the amount of resistance
caused by this movement is known as dampening.
Shock Pistons
Most shocks have multiple shock pistons available for fine tuning of dampening. Shock pistons are
mounted on the shock shaft and have holes in them that control the flow of the shock fluid as it moves
up and down with the suspension movement. There are various options available for each brand of
shock, usually varying in number of holes and the diameter of the holes in the piston. Using pistons with
smaller and/or less holes provides stiffer damping, slower weight transfer, and slower response. Pistons
with larger and/or more holes provides softer damping, increased traction, quicker weight transfer and
response.
Shock Oil
Shock Oil is graded by its viscosity or thickness. On most dirt oval chassis, racers use between 30wt and
60wt Shock Fluid. Lighter shock fluid will give more overall traction and allows quicker response to
suspension movement but also has a tendency to allow too much chassis roll in the corners. Heavier
fluids have less overall traction and react slower but on high bite and smooth tracks, heavier fluid tends
to be easier to drive. Heat makes shock fluids lighter and cold makes them heavier. You may need to
adjust shock fluid when there is a drastic temperature change (20-25 degrees) to maintain the same
dampening. If it gets cold you will need to go to a lighter weight shock fluid. If it gets hotter you will
need to go to heavier weight shock fluid. In extreme temperatures, you can change 5wt and get very
close to the feel of your dampening settings from typical temperatures.
Bladders
Not all shocks are designed to use a bladder; however I recommend bladder shocks for all racers looking
for consistent handling. The shock bladder acts as a silicone membrane that keeps the shock oil and the
air in the shock separated from one another. This prevents air bubbles for getting into the shock fluid,
causing inconsistent dampening as the pistons moves past the fluid and into air pockets. Be sure to
check your bladders for punctures any time you open your shocks up. I suggest that racers replace their
bladders every time they replace their shock seals (which should be a somewhat regular maintenance
procedure).
Chassis Height
Chassis Height is a very important measurement and setting that many racers neglect or simply do not
understand. Chassis height is set by the amount of pressure applied to the springs by the spring collar on
the shock. Adding Pre-Load to your shock collars DOES NOT do anything to the shock besides change
chassis height. You cannot stiffen an 8 pound spring with more pre-load; instead preload simply jacks up
the chassis height on that corner of the car. The proper way to stiffen up the chassis and maintain
balance is to go to a stiffer spring, and reset your chassis height. Typically, we treat chassis height as a
setting, not an adjustment. We use a particular chassis height setting that we have found to work well
on a particular track surface, and no matter what spring changes we need to do to control weight
transfer, we readjust our chassis to that chassis height before each run.
A 1mm change in chassis height can be a very significant in handling and weight transfer. The higher the
chassis height is set, the higher your center of gravity will be. Higher center of gravity creates more
chassis roll and weight transfer when cornering. This will also cause the car to transfer more weight
when accelerating or decelerating. The higher you set your chassis height, the more weight transfer you
are promoting. Likewise, the lower you set the chassis height, the lower you move the center of gravity.
Lower center of gravity promotes maximum corner rotation and cornering speed, along with keeping
the chassis flatter and resisting traction roll. If your chassis height is set too low, and your chassis is
bottoming out, you will experience inconsistent handling. Just like any other chassis adjustment, when
it comes to adjusting Chassis Height, you need to find balance.
Adjusting Balance and Setting Your Chassis with Chassis Height
A typical Foam Tire Chassis Height is 12mm all around measured to bottom of chassis plate. A typical
Buggy Tire Chassis Height is 17.5mm up front and 19.5mm in the rear of the chassis. Chassis Height is
measured on the flat portion of the chassis just behind the front wheels, and just ahead of the rear
wheels. We typically measure Sprint Cars race ready, with body and cage and measure Late Models and
EDM’s race ready minus body.
Tuning Balance with Chassis Height and Springs
Now let’s assume that we have a chassis that is setup with the right tires, and setup with the “standard
setup” and set to 12mm chassis height. Now we go out and practice, and the car is a little too aggressive
getting into the corner, so we end up putting two turns into the right front spring to counter the less
than desirable handling situation. Now the car is feeling more drivable. What adjustment do we make
next? First thing you should do, is remember what changes you made, and since we put turns into the
RF, we should replace the RF spring with the next rate stiffer spring, and then reset your chassis height
to 12mm. Now we go back out for more practice and find that the track is losing some grip on corner
exit, so we end up cranking a turn into the LR and taking a turn out of the RR. This helps us get the feel
we are looking for. So what do we need to change after the run? We need to go up one rate on the LR
spring and down one rate on the RR spring, and then reset our chassis height to 12mm all around. By
changing springs, we are controlling the amount of weight transfer to find the balance we need.
Likewise, by maintaining the 12mm chassis height, we maintain a consistent Center of Gravity, keeping a
consistent and comfortable feel, and we always have a reference point, just in case we go too far or get
lost. For best results, take plenty of notes, and work on writing down detailed descriptions of how the
car felt in each of the 4 segments of the track (Straights, Corner Entry, Mid Corner, and Corner Exit).
Basic Spring Adjustment Cheat Sheet
Loose In: Stiffer RF Spring
Tight In: Softer RF Spring
Loose Off: Stiffer LR Spring, Softer RR Spring
Tight Off: Softer LR Spring, Stiffer RR Spring
(Stiffer RF Spring will tend to tighten up the car all around the track)
Chassis Tuning 101
Matt Murphy’s Dirt Oval Chassis Tuning Guide