Clutch

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Currently, the bike has a highly modified Barnett Scorpion clutch assembly installed. It uses the clutch hub and carbon fiber clutch pack from the Scorpion with a custom pressure plate. The pressure plate adapts the hydraulic clutch actuation from a Buell 1125. The pneumatic diaphragm from the 1125 slipper setup is also integrated and modified to provide supplemental force to the clutch pack when the turbocharger builds boost pressure.

XB1125 clutch 1
Barnett Scorpion 14Barnett Scorpion 1 Barnett Scorpion 2 Barnett Scorpion 4Barnett Scorpion 6Barnett Scorpion 5Barnett Scorpion 7Barnett Scorpion 10Barnett Scorpion 8Barnett Scorpion 11Barnett Scorpion 13Barnett Scorpion 12

Buell XB clutch options:

The factory XB clutch works very well for most XBs on the road, but it won’t live long if you try to put more than 100 ft-lbs of torque through it. There are few aftermarket upgrades available that will increase its torque capacity. If you need a more capable clutch, a good first upgrade is aftermarket clutch plates. I first installed a set of Barnett carbon fiber friction plates and they held up to about 120 ft-lbs with the stock diaphragm spring and maintained a very smooth engagement. I later upgraded to a Rivera/Primo Competition diaphragm spring (the stiffest available) that held over 140ft-lbs of torque. I did not run with the Barnett plate/Rivera spring with >140 ft-lbs for too long, so I can’t say for sure how long it would have lasted at that power level.

Here are some comparison shots of my old XB clutch assembly with the Rivera/Primo Competition diaphragm spring and Barnett carbon fiber plates next to the OEM 2007 Sportster Clutch assembly:


The XB basket is a “sprung” assembly, which has 6 coil springs that help to absorb drivetrain shock between the primary chain and the transmission mainshaft. The Sportster basket is solid or “unsprung.” While both work well in their respective motorcycles, the XB basket is a bit heavier due to the added weight of the springs.

The XB and Sportster also use different clutch hub support bearings. The XB basket uses a slim roller bearing that allows the hub to be removed from the basket by hand. The Sportster basket holds a press-fit ball bearing, which requires that the hub be pressed in and out of the basket.

The XB and Sportster basket share the same 57 tooth 3-row primary sprocket and 78 tooth starter ring gear.

The clutch hubs and packs are a bit different too. The XB hub is designed to use a conical spring that sits inside a unique(thinner) inner-most friction plate. The Sportster has a uniform friction plate size and incorporates a riveted dampening plate in the middle of the stack. It is common for Sportster clutch upgrades to eliminate the damper plate and replace it with additional friction and steel plates.

With the sprung basket, the XB clutch assemblies use a spacing washer between the transmission mainshaft bearing and the clutch hub. This spacer is required for the roller bearing to properly support the basket, to “shim” the basket preventing it from shifting side to side on the hub, and to ensure proper clearance and alignment of the primary sprocket and ring gear. The Sportster basket relies on the press fit of the hub in the support bearing along with circlips on either side to prevent shifting, and does not use a spacing washer between the mainshaft bearing and clutch hub.

What this means for an XB: ALL 2004-on Sportster upgrade options become a reality for XB models and the swap is simple. A 2004-2011 Sportster clutch will work in an XB primary if you remove the stock XB clutch spacing washer.

Here is the Evolution Industries 2004- Sportster Stage 1 Clutch Hub and plates. They sell two different versions. The Series D uses a diaphragm spring similar to the OEM spring and the Stage 1 uses 6 coil springs. You can also see the difference between the stock XB friction plate and the Evolution Industries’ plate. They share the same outer diameter and finger count but the Evolution Industries has a smaller inside diameter. I was unable to get the Evolution Industries clutch to hold more than ~140 ft-lbs. It could hold more torque with a stiffer spring setup but the pressure plate design only allows for a maximum of about 380 lbs of spring without coil bind. This is why I changed to the Barnett Scorpion clutch.

The Evolution Industries clutch uses a Kevlar material and they recommend ATF fluid or any non-synthetic primary fluid.