User Manual
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Motor Gearing: The important thing is to keep the motor in its optimal RPM range as much as possible around the entire track. This will
depend on the straight-away length and the size of the infi eld turns. The chart below is a guide to give you a starting point. You may want
to try gearing up (larger pinion or smaller spur) or down (smaller pinion or larger spur), one size at a time, noting the straight-away speed
and acceleration through the infi eld.
*NOTE: OVER GEARING (TOO LARGE OF A PINION OR TOO SMALL OF A SPUR) CAN CAUSE DAMAGE TO BOTH YOUR
ELECTRONICS AND MOTOR. USE CAUTION WHEN SELECTING YOUR GEARING.
Rollout: The overall gear ratio, including the tire diameter, is compared in terms of a distance traveled (usually in mm) per revolution of the
motor, called rollout. In foam tire racing, the diameter of the tire is left as an option to the racer (within a given range, set by the sanctioning
body) to adjust the way the car handles. When changing the diameter of the tire, the overall gear ratio is effected. The formula below should
be used to calculate the rollout of the car for a given tire diameter (d), internal drivetrain ratio (i), spur gear size (s), and pinion gear size
(p):
For example:
This example gives a Rollout of 20.6mm, meaning that this car will travel 20.6mm per one revolution of the motor. Opposite of the Spur/Pin-
ion ratio, the higher the Rollout value, the higher the gearing of the car. See Table 3 (above) for suggested starting Rollout values.
Tuning the Front End of the JRX-S
Shock Location: The JRX-S has three mounting locations on the front shock tower. Leaning the shocks in (moving them closer to the center
of the tower) will give a smoother transition as the car enters into the corner and improve consistency but will yield less total steering and a
slower reaction. This can be useful on high bite surfaces. Standing the shocks upright (moving them further out from the center of the tower)
will increase responsiveness and generate more total steering. This will also increase forward traction and on-power steering. This can be
helpful on tight, technical tracks where steering is vital.
Camber Location: The JRX-S has three different vertical locations for the front camber tierod. In general, the lower the inside position is,
relative to the outside, the more camber gain (total camber change through the total throw of the suspension) is present. Running the camber
tierod in the lower hole (more camber gain) will increase both off and on-power steering, however you will lose some consistency. If the in-
ner camber tierod location is raised, the car will lose some steering but gain consistency. We have found that running less camber gain in the
front of the car best suits the balance of the JRX-S.
Static Camber: This refers to the angle of the wheels/tires relative to the track surface (viewed from either the front or back). Negative
camber means that the top of the tire leans in toward the chassis. Positive camber means the top of the tire leans out, away from the chassis.
Camber can be precisely measured with after market camber gauges, sold at a local hobby shop. It can be measured (roughly) using any
square (to the ground) object by checking the gap between the square edge and the top of the tire. Testing has shown that 1 degree of nega-
tive camber is best for most track conditions. Increasing negative camber (in the range of 1-2 degrees) will increase steering for both foam
and rubber tire racing. Decreasing negative camber (in the range of 0-1 degree) will decrease steering and the car will feel easier to drive as
a result. This is, most often, a very critical adjustment in tuning your car that can be made track-side!
Toe-In/Out: This is the parallel relationship of the front tires to one another. Toe-in/out adjustments are made by changing the overall length
of the steering tierods. Toe-in (the front of the tires point inward, to a point in front of the diff) will make the car react a little slower, but have
more steering from the middle of the turn, out. The opposite is true with toe-out (the front of the tires point outward, coming to a point behind
the front diff), the car will turn into the corner better but with a decrease in steering from the middle of the turn, out. Toe-in will help the car
to “track” better on long straights, where as toe-out has a tendency to make the car wander.
Bump-In/Out: Bump-out (front of the front tires toe-outward under suspension compression) will result in more off-power steering. This
effect is obtained by adding washers under the steering spindle ball stud. Bump-In (front of the front tires toe-inward under suspension
compression) will result in less off-power steering and running too much bump-in can make the steering feel very inconsistent. This effect is
obtained by installing the steering draglink on the bottom of the steering bellcranks, with the ball stud pointing upward. Testing has shown
that running zero bump steer (kit setup) in the JRX-S offers the best overall setup.
( )
3.1416
Diameter of Tire ( )
where #
of Teeth on Spur Gear
# of Teeth on Pinion Gear
Internal
Drive Ratio (JRX-S=1.83)
d m
m
d
Rollout
s
s
i
p
p
i
π
π
≈
=
×
= =
×
=
=
( )
( )
( )
3.1416
57
3.1416 57 179.07
If 128 Then 20.6
128
8.67
1.83
27
27
1.83
d m
m
mm mm
s Rollout mm
p
i
π
≈
=
×
= = = =
×
=
=
Motor Manufacturer, Make/Model Spur Pinion
Suggested Rollout
Stock Motor
EPIC Based Monster 128 32-33 25.0mm-27.0mm
EPIC Based Binary (Two Magnet)
128 32-33 25.0mm-27.0mm
EPIC Based Binary (Four Magnet) 128 30-31 23.5mm-25.5mm
EPIC Based P2K/P2K2 128 37-38 28.5mm-30.5mm
TOP Based (Standard Brush)
128 36-37 28.0mm-30.0mm
TOP Based (V2)
128 36-37 28.0mm-30.0mm
Yokomo Based
128 36-37 28.0mm-30.0mm
All 19 Turn
128 37-40 27.0mm-29.0mm
Modifi ed Motor
7 Turn 128 25-26 18.5mm-19.5mm
8 Turn
128 26-27 19.5mm-20.5mm
9 Turn
128 27-28 20.5mm-22.0mm
10 Turn
128 28-30 22.0mm-23.0mm
11 Turn
128 30-31 23.0mm-24.0mm
12 Turn
128 30-32 24.0mm-25.0mm
Table 3: Suggested gearing for the JRX-S Expert Racing Sedan
SETUP GUIDE