I have no idea why the drive belt tensioner on the XR1200 causes so much anxiety, but in my humble opinion Harley did a great job designing this gizmo.  Yes, Harleys have survived for years without a belt tensioner, but using that as logic we should still all be riding hardtail Panheads.  So I am not going to address whether the XR1200 needs a drive belt tensioner, simply whether it works as intended...

 
The XR1200's "Idler Wheel" (or drive belt tensioner)
 

 
The XR1200 uses a static roller as a drive belt tensioner or idler wheel.  The belt tensioner is mounted under the XR's frame in line with the pivot point of the swingarm.  As the swingarm moves downward (shocks extending), the top of the belt lengthens and the bottom of the belt shortens as it bends around the tensioner, or "idler" pulley.  On a stock rear end, the net result is no change in the length of the belt's path.  Alternately, as the swingarm moves upward (shocks compressing) the top path of the belt lengthens, effectively tightening the belt, while at the same time the bottom of the belt moves upward off the tensioner pulley loosening it.  Again, the end result is no net, or minimal, change in the length of the path of the belt.  With the XR1200's roller tensioner, on a stock rear end the drive belt stays under constant tension as the  swingarm moves through its arc. With the tensioner, if one sets the drive belt deflection at 3/8" (within the HD spec for belt tension), it will stay almost exactly the same as the swingarm moves.  Without  the belt tensioner, the drive belt would get significantly tighter as the swingarm moves up, and looser as the swingarm moves downward.

 
For a XR1200 with longer shocks and/or riser blocks, the idler pulley still works surprisingly well, drastically reducing the change in belt tension as the swingarm moves up and down - and by setting the belt tension slightly looser than spec, the tensioner is still highly effective.  Because the tension does vary slightly as the swingarm is moved out of HD's spec'd range for the tensioner, I prefer to set my belt tension at 5/8", which while a touch loose (by HD spec) at full shock extension, puts the tension in HD spec for most of the swingarm's movement.
 
Here is how I looked at belt tension and the idler pulley...
 

 
The XR1200's drive belt with its "Idler Wheel"
 

Belt-withtensioner.JPG
XR 1200 Drive Belt - With Tensioner

 
To give you an idea of what happens to the drive belt with the tensioner pulley in place, I measured the tension of the drive belt through a wide range of swingarm movement.  In order to do this, I measured the height of the rear end and length of the shocks as they are at rest on my bike, and then removed the shocks and raised and lowered the bike on a lift.  (Click here to see how I measured the drive belt tension/deflection.)  But just to get a quick feel for how much movement of the rear axle is needed to change the tension about 3/4 of a turn of the adjuster screw was enough to change the drive belt's deflection by 1/8".   And, tightening the adjuster screw 0.15", changed the belt deflection from almost too loose to measure, to 3/8 of an inch.
 
The following is a table of my tension measurements...
 
 

Drive Belt Tension

Shock Length

(to Stock Top Mount)

Shock Length

(to Riser Shock Mount)

Shock Spring Length

(for Ohlins Shocks mounted on Risers)

1/4"

28 mm

23 mm

 

1/4"

30 mm

25 mm

 

1/4"

32 mm

27 mm

11 mm - Ohlins HD144, shocks fully compressed

1/4"

34 mm

29 mm

 

1/4"

36 mm

31 mm

 

3/8"

38 mm

33 mm

 

7/16"

40 mm

35 mm

 

1/2"

41 mm

36 mm

Resting position -

unloaded bike, on side stand

5/8"

42 mm

37 mm

21 mm - Ohlins HD144, shocks fully extended

3/4"

44 mm

39 mm

 

7/8"

45 mm

40 mm

 
       

 
From my measurements I have to say that the tensioner pulley is a simple an highly effective way to keep a constant drive belt tension on a stock XR1200.  In the above table the shock length values in blue are in the normal operating range for a shock mounted on the stock upper shock mount, and for shocks mounted to a Free Spirits riser block.  Although I probably should have gone 2 cm shorter to be absolutely sure, it certainly looks like the idler pulley keeps the belt tension almost exactly constant through the full range of swingarm motion on a stock bike.
 
On a raised rear end, with Ohlins HD144 shocks and Free Spirits riser blocks, the pulley still does a great job.  The belt deflection does increase by about 3/8" of an inch as the shocks unload and extend, but this is excellent in comparison to running the bike without the pulley where the belt deflection varies so much that it can not easily be measured.
 
Belt Tension does not increase as the shocks extend - on a stock bike, drive belt tension remained constant.  And on a raised rear end, drive belt tension actually decreased slightly as the rear end unloads.  As can be seen in the table of measurements, if the belt tension is set slightly slack at full shock extension on a bike with risers, it will be within the normal belt tension spec for quite a bit of the travel of the swingarm.
 
I can't imagine why anyone would want to remove the tensioner pulley, nor can I see a reason to replace it with a spring loaded tensioner.  Harley did a great job on using simple geometry to effect a great improvement in drive belt tension on the XR1200.
 

 
 

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