AIM-Tamachi VPC fof the XR1200
 
  
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.

The VPC comes nicely packaged with instructions, and a longer adjusting screw and spacer for the Ball and Ramp assembly for clutch release.


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VPC in Package
 
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.

For 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.


The VPC


The following 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 significant modification.  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 designed.

In a 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. 
 
This is what the clutch pack looks like with the VPC installed.

 

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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...
 
 
Modifications to 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 centrifugal weights. 
  • 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.
 
The case and modifications are common to installation in all Sportsters. 


Additional XR1200-Specific Modifications for VPC Fitme
nt

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 installationClick here for additional detail on modifying the clutch hub.
  • 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 special requirements.
 


Modifying the Primary Cover
 

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Primary Cover Modification

 
 
 
 
 
 
 
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 left..
 
 
 
 
 
 
 
 
 
 
 (More detail is provided below.)
 
 
 
 

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Primary Cover Marked

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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.
 
 
 
 
 
 
 
 
 
 
 

Clutch Cable Coupling
 
 
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". 
 

 

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Clutch Cable Coupling

 
 
 
 
 
 
 
 
     The unmodified clutch cable coupling is to the left.
 
 
 
 
 
 
 
 

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Clutch Cable Coupling - Modified

 
 
 
 
 
 
 
 
To the right is the clutch cable coupling, modified for use with the VPC.
 
 
 
 
 
 
 
 
 
 

Derby Cover
 
 
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 this modification.

 
 

Clutch Hub


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Modified Clutch Hub

 
 
 
 
 
 
 
 
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 Buell.
 
 
If you would like to see the modification to the XR1200 clutch hub that is required to install the VPC, click here.
 
 
Note:  Even 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.
 
 
 
 
 
 
 

 
 
Installing 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 problems.
 
 
Conclusion
 

The AIM VPC is a high quality piece of hardware.  In Big Twins and XL Sportsters it can provide a means to increase clutch clamping pressure, while maintaining a light clutch lever feel.  Unfortunately however, it will not work on the XR1200 without a modification to the primary cover that most riders will not want, or be able to perform.

 
To return to a discussion of the XR clutch and possible enhancements, click here.
 

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