I know very little about chassis design (I do like triangles though
), there are many people here who know a lot more than me, however if you want to be learned up, go here -
http://www.tonyfoale.com/The belt and pivot pointsOn an XB, the swinging arm pivot point is slightly higher than the sprocket in the horizontal plane:
So looking at an XB, you can see that with the suspension unloaded, the axle (centre of rear sprocket), swinging arm pivot point and the sprocket centre are not aligned.
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As the suspension compresses, the rear axle rotates in an arc with the centre at the pivot point. As the rear axle gets moves closer to being aligned, the distance between the centre of the sprockets increases, the maximum being when the points are aligned. A practical analogy is straightening your arm.
So what you say... Well, this gives problems with belt (or chain) adjustment, and also the effect of applying load (opening the throttle) to the back wheel on the suspension movement. So much so that Bimota created frame designs to ensure these points were always aligned:
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The BMW X450 is similar...
On an XB, we can see (as Nigel pointed out on his 1125R) that as the suspension compresses, the belt will become tighter. So to ensure that the belt does not overtighten and restrict suspension movement as the suspension compresses, it it will be very slack when the suspension is extended (bike has no weight on it).
Tightness of the beltThe tightness of the belt is important. Too slack and it will jump - when you open the throttle and load the belt up, it becomes tighter at the top and it stretches slightly, making the belt longer at the bottom. Being longer, the belt will need to accommodate its extra length and it does this by appearing to sag. As the belt doesn't like to bend, and there is no tension as it nears the rear sprocket, it has difficulty aligning the belt teeth with the sprocket teeth and the belt jumps.
Contrary to this, if the belt is tight, it applies a static load to the wheel bearings, gearbox output shaft bearings and the suspension movement. This means compromised suspension (equivalent of a higher spring rate) and bearing life.
Tightening the beltThe S1/X1/M2/S3 etc. have conventional "chain" adjusters, however as with the XBs (and 1125s) as the suspension compresses, the distance between the sprockets increases and the belt tightens, hence it is very important to set the belt tension for when the bike is loaded:
viewtopic.php?f=17&t=9&hilit=beltThis chap from over the pond aligns his axle, pivot point and sprocket to check belt tightness:
For the XBs and 1125Rs, there is no belt adjustment one can perform, and Erik has seen fit to add an idler pulley to the design to take up the slack when the suspension is unloaded. So as the swinging arm drops, where the belt would normally become slack, it becomes supported by the pulley wheel. The pulley wheel will never be in the right place to ensure the belt load remains constant, the position is a compromise. This is one reason why a sprung tensioner, like a Free Spirits or a Pink Willy makes sense as when the suspension on an XB is unloaded, the belt tension actually increases.
What is the right tension?As we said before, too tight and there is static load on the bearings. If it is too slack, it will jump.
In addition, if it is too slack, the shock loadings are higher. If you shut off the throttle, the bottom of the belt becomes tight as the rear wheel turns the engine, and the top is slack. When the crack open the throttle (to do a wheelie past your mates at the pub), the engine accelerates, the gearbox sprocket accelerates and is free to do so until loaded by the belt. A practical analogy is a bungee jump, where the load is applied as a shock and the bungee stretches bigtime.
Following this thread:
viewtopic.php?f=8&t=9080the failure document suggests that Buell belts fail through shock loadings or damage from stones...
I remember reading somewhere, maybe here or on BadWeb that when Erik was asked why he ran the belts so tight, he responded that it was on recommendation of the belt supplier, presumably to minimise shock loadings...
Why fit a sprung tensioner?Both the Free Spirits and the Pink Willy sprung tensioners reduce the load on the belt. With the load reduced, the load on the gearbox output shaft bearing and wheel bearings are reduced. In addition, there are those that believe that they prevent damage from stones finding their way into a belt, allowing some compliance to accommodate the stone between the belt and the sprocket. My experience is that you can still pick stones up and they can cut into the belt. Of course, if they are not sharp flints, they may have come and gone without my knowing...
With an extended swinging arm, such as Nigel has, the stock idler wheel position is no longer optimised, so a sprung tensioner is a fix.
Interesting to note that sprung tensioners are not at all popular on BadWeb, yet a very high percentage of people on UKBEG have them...
Do you need a tensioner on a tuber? Good question... You will be told that they don't need one and one of the reasons they don't need one is that the maximum length between the sprockets is halfway through the suspension travel (which is not such a good design for belt load / effect on suspension as the bike is much more prone to squat when you pass the mid travel point), so the slack is less that it would be on a non tensioned XB or 1125.
Design criteria for a sprung tensionerThis is by no means an exhaustive list (and I have probably missed the obvious
)
1. As big a wheel as possible to minimise the arc through which the belt is forced - this applies to a tensioner wheel if fixed also - note the picture at the top of the pre-production engine with a tiny wheel
2. Minimum tension to accommodate stones and other foreign objects (eg. castanets)
3. Minimum tension to increase bearing life
4. Minimum tension to minimise effects on suspension as the suspension is compressed
5. Sufficient tension to reduce shock loadings
6. Sufficient tension to minimise tooth jumping
7. Wheel positioned to minimise length of chord through which the belt passes to minimise stretching of toothed face
8. Wheel positioned as close to front sprocket as possible to minimise spring movement and save weight
9. Good torsional stiffness to minimise sideways belt run off (XB and 1125 rear sprockets do not have guides)
10. Clearance for rider and other parts of the bike
Points 2. to 6. and 7. and 8. mean that there is always going to be a compromise.
There is more than one way to skin a cat. The Pink Willy moves the rotational centre of the idler wheel further rearwards. This results in more movement. For the XBs, the Free Spirits tensioner more or less maintains the stock idler wheel rotational centre position. From Maz's description, I am guessing that this more or less maintains the standard rotational centre position.
Bring on the discussion (and criticism)
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..... with the shock disconnected the swingarm can be moved through its entire arc without change in belt tension 
Wouldn't the perfect tensioner be one that was controlled by a cam driven off the swing arm offering zero lift at the point when the axle was level with the sprocket and progressively more either side?
