Clutch Control Ring

 XiU-rdi Clutch Components

Josep “Xiu” Serra operates XiU-rdi Engineering (a design consultancy in Spain).  As I understand it, Xiu had a hand in much of the OSSA's design.  Search “retrotrials.com the secret engineer.”

Xiu also sells aftermarket parts for several makes of trials bikes.  I use his Kevlar clutch plates and Clutch Control Ring and like both very well.  The Kevlar plates outlast OE by a factor of two or three.  The Clutch Control Ring provides three adjustable spring preload settings to compensate for friction plate wear: standard, +0.1mm, and +0.2mm.

The thickness of the OSSA clutch pack is critical.  A new clutch pack measures about 9.8mm.  By the time it measures 9.5mm, the clutch slips constantly.  Consistency of the thickness measurement around the pack is important.  It should not vary more than 0.03mm anywhere – a smaller variation is better.

When the pack gets to around 9.65mm in thickness, I select position #2 on the control ring.  This gives an additional 0.1mm of spring preload.

Although I have not yet needed it, when the pack gets to about 9.55mm, I will select position #3.  This would give an additional 0.2mm of spring preload.

I had some problems with Xiu's first Kevlar pack being too thick (about 9.9mm) and had to use one of my OE fiber plates in the pack.  In Xiu's system, one of the steel plates is 0.5mm thinner than OE.  This allows his friction plates to be thicker. 

The above was written specifically about my 2011 TR280i.  I am no longer certain it is fully applicable to the later bikes.  It is no wonder the OSSA clutch can be problematic - note the difference in step height (and resulting preload) for the three different control rings shown below.  What Xiu calls the Clutch Control Ring, OSSA calls the “CLUTCH SPRING SUPPORT PLATE.”  OSSA has two different part numbers for it: 2430020211 and 2430020214.  I think there must be some variation in the clutch baskets as well based on how different clutch pack thicknesses respond in my various bikes.

XiU-rdi Clutch Control Ring installed

Variations in OE CLUTCH SPRING SUPPORT PLATE dimensions from my 3 bikes

OE Clutch Spring Support Plate Identification

Measuring my three OE clutch spring support plates revealed the following: 

3.92mm thick – 2.69mm = 1.23mm step

3.46mm thick – 2.23mm = 1.23mm step

4.01mm thick – 2.41mm = 1.60mm step (so this was almost certainly from my 2011 bike)

 XiU-rdi Control Ring Measurements

While measuring other clutch components, I discovered the stated 0.1mm difference between control ring positions is not exactly right.  

By installing the clutch control ring without fibers, steels, spring,  or pressure plate I was able to insert pin gauges as shown in the adjacent photo.

The pin gauges are available in 0.001-inch increments.  In position #1, I could insert a 0.157" pin.  In position #2, it was 0.151", and in position #3 it was 0.145". 

This gives deltas of 0.006" (0.15mm).

0.157 - 0.151 = 0.006 and 0.151 - 0.145 = 0.006 

So maybe the 0.1mm specification is how much clutch pack wear each change can compensate?  Or maybe Xiu changed the specification but not the documentation?

Pin gauge being use to measure control ring

XiU-rdi Clutch Pack Preparation

The following information comes by way of XiU-rdi Engineering with each clutch pack.  New friction plates are not perfectly uniform in thickness.  Slight variations can make the clutch grabby, and increase initial wear. 

Assembly Process:


Webmaster's notes: To further elaborate, let's say one of your discs is perfect with no deviation in thickness.  This disk could have any orientation relative to the other two, which would have their “T” marks placed 180 degrees apart.

I always soak new friction plates in gearbox fluid at least overnight, preferably longer.