|How the Polar Junior
by the guy who created it
By Stephen Payne
In the 10 years the Polar Junior
clutch system has been in use, it has produced excellent results
in reliability, consistency, and performance. While the stock
setup works well for most cars, the optional tuning parts
can really improve its performance.
Here's how it works: The drive clutch
uses centrifugal* force to move a roller down a ramp and shift
the clutch. (See Fig.1).The driven clutch uses a combination
of springs and cam angles to control how quickly it will shift,
which depends on the load needed to turn the wheels. This
type of driven clutch is called "load sensitive."
THE DRIVE CLUTCH
The drive clutch has three pivoting arms bolted to the sheave(the
part of the clutch that moves), with rollers on the ends of
these arms. Once the engine starts, centrifugal force pushes
the rollers along the ramp. As the amount of weight attached
to the rollers is increased, so is the shifting force available
to load the engine. The opposite is also true: when there
is less weight, less force is available to move the rollers
down the ramps.
The standard ramp has a shift angle
of 28 degrees, changing to 22 degrees, but additional ramps
angled up to 32 degrees are available. If the ramp angle is
increased by any degree, the weighted roller will move faster
down the ramp, it's really just like riding a bicycle: pedaling
down a hill is easier then pedaling on flat ground. This will
cause the clutch to shift faster and load down the engine
The drive clutch is designed to use
the amount of weight on the arms to raise or lower the rpm
of the engine when it is shifting on the way down the track,
not for adjusting stall speed. Available for each clutch are
weights ranging from 3 to 9 grams. If you find that you are
using the heaviest on your car, you should change the ramp
angle. The object of the game is to use the middle-range weights
so, as the weather changes, you can compensate for the loss
or gain of engine power.
The roller first touches a small
section on the wide end of the ramp. The angle of this section
determines stall speed with a given spring. Flatter angles
and stiffer springs increase stall speed; steeper angles and
softer springs decrease stall speed. If you want a high stall
speed, it is better to change to stiffer springs before changing
The stock clutch stalls at about
4800 rpm with an 18 degree stall angle and 7 gram weights.
This can be reduced to about 4400 rpm using a 22 degree stall
angle on the ramp. The green spring will increase stall speed
up to about 5200, and the heavy black and yellow spring will
increase it to about 5600 rpm.
The Polar Junior drive clutch is
very tunable, but remembers this: more weight or steeper ramp
angles slow down the engine. Less weight or shallower angles
speed up the engine. No clutch is perfect for every car. Tunability
will allow you to make these adjustments for the consistency
The driven clutch takes the power from the drive clutch and
turns the wheels on the car by way of the chain. The best
system for mounting the driven clutch is on a jackshaft. This
allows the best alignment and reduces flex, which helps the
clutch belt last longest. The driven clutch uses a spring
that is wound up by the cam to control how quickly it opens.
The amount of load it takes to move the car also determines
how quickly the driven clutch will shift. Tightening or loosening
the spring has a small effect on how quickly the driver will
shift. If the engine is mostly stock, the spring can be placed
in the fourth whole in the cam. If the engine is modified
(or you're not sure), use the fifth hole.
Steeper cam angles shift more quickly
and increase efficiency. Shallower cam angles make shifting
slower. Steeper angles are better for lighter cars that require
less power to move, while shallower cams help heavier cars
move better. The stock cam is 34 degrees, which will work
in any car, but if your car is light, a 38 or 42 degree cam
might produce a quicker e.t. The 46 degree cam is for light
cars. Steeper cams can be used on heavier cars, but will need
more spring tension.
What's the advantage of changing
the cam? If the track is slippery, a shallow cam will not
spin as much as a steeper cam. As a rule, a good-hooking track
will produce lower ET's with a slightly steeper one. Increasing
the spring pressure to the 6th hole will help the car be more
consistent but at a slight sacrifice in ET.
Alignment dots on the Polar Junior
clutch ensure that the cover is placed properly in relation
to the movable sheave. The cover should be removed by using
two bolts, each turned a half turn at a time, in the threaded
holes near the retaining bolt.
The Polar Junior clutch uses bushing material, which should
last a long time. Due to manufacturing tolerances, the clutch
may not engage and shift smoothly when hot. Use a feeler gauge
to check the clearance on all bushings (square or round).
It should be .007 to .010 inch. If the clearance is tighter,
increase it by gently scraping the inside of the old bushing
with a sharp knife. With the springs out, all parts should
slide on their own weight easily, and the proper clearance
will help the clutch provide even better consistency.
ON THE TRACK
Following is a typical tuning situation
to help you know what to expect the first time out with Polar
Junior. If you've already been running it for a while, keep
reading. The following information may help you work out a
First, let's presume you have installed
a Polar Junior clutch on your Jr. Dragster. You made or bought
a jackshaft motor mount and everything bolted up fine. The
next step is to set center-to-center distance and offset.
Use the special tool available from Polar or a piece of flat
bar and a tape measure.
The centre-to-centre distance should be about 8 15/16 inches.
If it's right, the belt will deflect only about ½ to
¾ inch in the middle. To set the belt tension, hold
the driven clutch from moving with your right hand. With your
left hand, push the belt backward around the driven clutch.
If the belt is too tight, you won't be able to push it. If
it is too loose, it will go very easily. When the belt tension
is right, it will just go backward. At idle, the car will
roll ahead slightly by dragging on the belt. A new belt might
squeal a bit, but after it's broken in, it won't squeal unless
it's too tight. Proper belt tension makes for easier staging.
For the belt to be in perfect alignment between the drive
clutch and the driven clutch, the two parts must be offset
by 5/16 inch. Offset is measured from the back of the moveable
sheave on the driven clutch (with the belt off) to the backside
of the fixed face on the drive clutch. Proper offset makes
the belt last longer and helps the clutch perform at its best.
If the centre-to-centre distance
and offset are correct, putting the belt back on should be
easy. With the belt already on the drive clutch, just push
and turn slightly clockwise on the cam of the driven clutch
so that you can slide the belt right onto the driven. (Tip:
Removing the belt when towing will also help it last longer.)
I prefer to set up the driven clutch with +/- 1/16" float
from aligned. This helps let the belt optimize alignment during
The clutch comes with a 18 degree stall and a 28 degree changing
to 22 degree shift ramp, with seven gram weights on the arm.
Weights ranging from three to nine grams are recommended for
tuning a Jr. Dragster because no two race cars are the same.
Polar Motorsport International does not recommend using weights
heavier than nine grams.
A tachometer that records the run will help when dialing in
the clutch. Note that the clutch stalls at about 4800 rpm.
If the belt tension is correct, the driver will feel a small
tug when the car moves.
REVIEWING THE DATA
Let's say the engine in your Jr. Dragster should make the
most power at about 7500 rpm. The car ran an eighth mile e.t.
of 10.19 with the old clutch system, but the time slip on
the first pass with the Polar showed an e.t. of 10.15. The
tach showed that the engine was running at 8500 rpm most of
the way down the track. This is past the power band (remember?
7500 rpm). You know that adding weight to the clutch arms
will reduce engine speed, so for the next pass, you replaced
the four gram weights with six gram weights. With the heavier
weights, the ET was much better: 9.95. The engine was in its
power band at 7600 rpm, and driver felt the car pull all the
With the old clutch, the gear ratio was changed to help load
down the engine. With a 54 inch circumference tire, a 16 tooth
top gear, and a 75 tooth bottom gear, that speed is 81.8 mph.
Here's the formula, where TG equals Top Gear, BG equals Bottom
Gear, and TC equals Tire Circumference:
MPH = [RPM x TG}
[ BG ] TC
1056 (constant no.)
Changing to the Polar clutch increased
the car's speed to 58 mph. Switching to the six gram weights
brought it up to 59.5 mph. What happens when you trade the
stock 16 tooth top gear for a 12 tooth? The car runs a 9.93
at 59.4 mph, with the engine rpm at 7650.
Let's say the last problem left to solve is that the engine
over revs to 8000 rpm up to the 60 foot mark, but runs fairly
constant after that. Changing the ramp angle slightly for
the first shift will solve the problem. You change to a combination
of 18/30/22: 18 degree stall (same as stock), 30 degree first
shift angle (loads the motor more), and 22 degrees for the
last part of the ramp (same as stock). On the next pass, rpm
remain constant at 7600 rpm. The ET is 9.91 at 60.0 mph. The
next several time slips show consistent runs. Gearing the
car for more MPH than it can go, gives a slower ET and highest
MPH. The shorter gear ratio for least MPH will usually give
the best ET. For consistency in bracket racing, a slightly
taller gear ratio will give the best results.
That's it - a typical tuning situation
with a new Polar Junior clutch. You may go through a similar
process when tuning your own. Just remember that a good understanding
of any component will always help you get maximum performance