The Aim Variable
Pressure Clutch provides variable pressure
clutch engagement that increases in clamping
force as the engine's rpm
increases. It consists of a set of
weighted throwout arms, and is installed on
top of the clutch diaphragm spring.
Initial spring rates are selectable at 150,
190, 240 or 320 lbs but the clamping force
increases dramatically as centrifugal force
spins out the throwout arms in the
mechanism. At idle and around town the
feel can be significantly lighter than stock,
while providing increased clamping force when
the torque builds at above 4,000 rpm.
VPC comes nicely packaged with
instructions, and a longer adjusting screw
and spacer for the Ball and Ramp assembly
for clutch release.
For comparison, the
AIM-04Sport VP-011-320 has the same feel and
nominally 320 lbs of clamping force at idle as
a stock Sportster diaphragm spring,
but over 40% more clamping force as the
rpm spins up! So, at idle the clutch
lever would feel like stock, but it is spec'd
to hold an engine with 140 ft-lbs
of TQ at 4,000 rpm.
Dropping down to the
VP-011-190, one could drop the clutch lever
force at idle by a stated 40%, while still
holding 98 ft-lbs of TQ at 4,000 rpm.
more information on the AIM VPC
clutch, click here - VP-Sport04.
NOTE: I determined
as I went through the VPC installation
process that the VPC is not compatible
with the XR1200. Because of the
position of the clutch cable entry point
in the XR1200, the VPC will not work
without major modifications to the
XR1200, including relocating the left
side driver's footpeg and repositioning
the shift lever. This
discussion shows some of the needed
modifications, but the VPC will
now work with an XR1200.
information discusses modifications
needed to fit the VPC in a Sportster
primary cover, and modifications needed
to mount the VPC to the XR1200 clutch
pack, but, as noted above, the VPC will
not work on an XR1200 without
For information, I am detailing the
steps that would be required to
install the VPC in an XR. Some
of this information transfers over to
installation on an XL Sportster, for
which this model of the VPC is
Sportster, the VPC's added parts require
internal modification of the primary cover,
derby cover, clutch release arm, and clutch
hub - but it is all doable in a home shop
with a Dremel tool.
is what the clutch pack looks like with the
|VPC on Clutch
Pack - Assembly Complete
The centrifugal weights on
the VPC swing outward as RPM increases,
providing leverage against the fingers of the
clutch diaphragm spring and increasing the
clutch clamping force.
A discussion of VPC
installation considerations follows...
Sportster Primary Cover for Fitment
In order to install the
VPC in a Sportster primary, there are a couple
of parts that need to be clearanced to let the
additional VPC parts fit.
- First, the primary
cover needs to be relieved around the
support bracket for the Ball and Ramp
mechanism to provide room for the VPC's
- Next, the Clutch cable
coupling for the Ball and Ramp Mechanism
needs to be made thinner to fit between the
Ramp body and the VPC.
- The center of the
Derby Cover also needs needs to be modified
to provide clearance for the longer clutch
Adjustment Screw which is used with the VPC.
case and modifications are common to
installation in all Sportsters.
XR1200-Specific Modifications for VPC
The XR1200 uses a
different clutch pack than the rest of the
XR1200 family. This different clutch
pack, and the reduced clearance between the
end of the clutch pack and the primary
cover, require to additional modifications
to allow the VPC to work in the XR1200.
- The Buell style clutch
of the XR1200 requires modification of
the clutch hub (not required in other
Sportsters) to allow the VPC to attach
to the clutch hub. The Buell style
clutch hub must have the fingers that
hold the diaphragm spring retaining
ring, clearanced for the centrifugal
counterweights, to let the retaining
ring snap into place during installation.
Click here for
additional detail on modifying the
- However, even if
all of the above is completed, the VPC
will still not fit in the XR1200
primary, because the clutch cable entry
point in the primary interferes with the
centrifugal weights of the VPC.
In addition to clearancing the primary
cover as shown below, in order to install
the VPC on a XR1200, the primary cover
would need to be spaced out about 3/16" -
14" to gain clearance so that the
centrifugal weights do not strike the end
of the clutch cable. This would
require either using a different primary
cover, modifying the cover, or spacing the
cover outward. This modification
would then cause a need to relocate the
left rider's footpeg and to move the
shifter outward. This
complication therefore, makes the VPC
unusable on the XR1200, unless one is
willing to do significant
additional modifications because of
the Primary Cover
In order to provide
clearance for the centrifugal weights of the
VPC, the primary cover of a Sportster must be
clearanced around the support for the Ball and
Ramp assembly. A notch needs to be cut
in the center support ribs. I used
a Dremel tool and cut the ribs with about
a 1" radius, as shown in the picture to the
(More detail is
For a better look at
the clearancing required in the primary cover,
I've shaded the machined surfaces of the
reinforcing webbing with black marker in the
picture to the left.
The next piece that
needs to be modified according to the AIM
instructions for Sportsters, is the clutch
cable coupling. AIM calls for grinding
the back of the coupling down for a total
thickness of 0.080" (2.0 mm).
I was a little skeptical about grinding the
back side of the cable attachment button down,
since it is a press fit. So, I stopped
short of grinding the full back portion of the
coupling, and I only ground the coulpling
thickness down to 0.100".
The unmodified clutch cable
coupling is to the left.
Coupling - Modified
the right is the clutch cable coupling,
modified for use with the VPC.
The AIM instructions
for a Sportster call for grinding down the
center boss in the Derby Cover, to provide
clearance for the clutch adjustment screw with
the VPC installed. In a Sportster,
this would be needed. An alternative for
XL Sportsters is to use a spacer ring to push
the Derby Cover outward.
But, since the XR has a different clutch
fitment than the XL's and I had already
determined that the VPC was not going to work
on my XR1200, I stopped short of performing
In order to install the
VPC on a Buell type clutch pack, the clutch
hub needs to be modified. The same
modification would be required for use on a
Buell, although I do not know whether other
fitment issues will prevent installation on a
you would like to see the modification to
the XR1200 clutch hub that is required to
install the VPC, click here.
with this clutch pack modification, the VPC
will still not work on the XR1200 because of
interference problems with the primary cover
and clutch cable entry point.
the Clutch in the Primary with the VPC
Sorry, as noted above,
this is not possible due to interference
problems with the primary case cover and
clutch cable. Additional modifications
to the primary case cover, changing its
appearance and relocating the rider foot
controls outward would be necessary. I
worked on this for a while, but decided there
were much simpler approaches for increased
clutch performance, e.g. higher clamping force
diaphragm springs. If possible, I will
add additional photos and information here
later showing the XR1200-specific fitment