I was incorrect in saying that the FS tensioner hits the muffler. It was actually brushing against the crescent shaped bracket under the motor which the muffler is clamped to. This is with a 1.5mm washer on each stud inboard of the tensioner.

The lower inner frame of the FS has a nice shiny strip where the exhaust mount bracket has rubbed away the anodising. This is handy as it shows where to shave off more metal if you have a mind to. This part of the tensioner is in compression and should be able to stand the grinding off of a little more alloy to finish what the exhaust bracket started. A minute or two later l had filed and polished away a smidgeon more alloy to the inner edge of where it had previously been clipping. There is now lots of free movement of the tensioner, which is surprising given how little metal l removed.

Had a brief run this afternoon, and while not a fail the Malossi spring is showing some limitations.

The spring needs winding up to about a third on the adjuster just to overcome the inertia of the drive train, but once there it seems to be at a sweet spot. There is free movement without bottoming out. Slackening the adjuster just makes the tensioner hit the exhaust mount again, which is annoying. I need to use a mini mirror to see just where it’s fouling again!

I will do a longer run at the weekend to test durability. I guess that you can’t defy physics no matter how good a springs pedigree. To get the power with the springiness l may just have to use a much bigger cross section of coil. There is room in the FS to use a wider spring if l machine out the spring seat at the front (l have a spare unit which l may sacrifice) or just make a small seating flange for whatever spring l eventually find. 18mm or 3/4 inch diameter coils may be possible. At the adjuster end it is quite easy to insert a stop washer. I am using a 16mm brass stop washer now between the adjuster and the spring. Still never give up (never surrender! ) The Malossi might go the distance yet.

Stop Press: If anyone has the need ‘The Range’ are doing those plastic coloured number plate bolt sets on their motor bits stand and they have the extra long ones which hold better. I needed longer ones for my modified rear plate holder.

Regardless of how my current Malossi spring stands up to a longer run, l have designed a revised spring set up for in my quest for FS spring compliance over power! Although l may not actually fit this one until after my XBs pending MOT.

One annoying aspect of looking for springs online is that there are many sellers of stronger racing clutch springs for various bikes, but few specify actual dimensions! I still have to visit a bike breakers with my electronic micrometer to measure a few with.

There is a 2mm gap above the spring and the FS frame. This is why l estimate that an 18mm OD spring is possible. Towards the front of the FS the frame curves down towards the spring socket, which presents a snag, but also a potential solution. By grinding off the end of a larger springs tapered end coil you can create a natural recess which the curved FS frame will slot into. My plan for a larger spring was always to get an end stop with a 15mm stub one side and a 12 or 13mm stub opposite turned on a lathe which will clip into the FS spring socket and retain a larger spring without having to modify the FS.

Not having obtained an 18mm spring yet, l hit on the idea of using one of my Honda 125 valve springs as a ‘test mule’. These springs are 27mm long by 20mm OD. Too wide, but possible by grinding a flat on one side of the spring so that it will fit in the FS. Then l thought about using this as well as the Malossi by resting it piggy back on the smaller spring. I have a 20mm stop washer to hold both springs at the adjuster end. At the front end l ground off the end of the Honda springs last coil so that it slots in above the Malossi due to it’s shorter effective length. This dual spring combo is significantly more powerful than the previous single, but strangely more easily compressible than any of my Indiamart colour springs. I can’t wait to try this out, but there’s no time to install it before my next ride out.

Ok so how did the Malossi Derbi Senda 50 Reinforced Clutch Spring do?

After my 136 mile jolly to the Welsh border the spring looked and felt much the same as before. The belt was held in what seemed to be adequate tension to wrap itself around all three pulleys and the FS tensioner could be compressed down then released so that it returned to its previous position. When pushing my XB around it nearly felt like ‘no belt was attached’, like the blue spring, but not quite as taught as the red spring l had tried before.

Typically l forgot to actually measure the Malossi spring length before my Sunday run.
And when you come to measure something, it’s never quite what you expect! After my ride out the 32mm spring has been compressed by the adjustment nut to a mere 18.27mm, or 10.5mm approx of the way alongside the adjuster thread. When l pull the belt upwards the spring can compress to a mere 16.78mm.

So the actual maximum spring compression movement as set is only 1.5mm. But the actual belt at mid point was displaced upwards by 14mm in the above exercise due to the effective length of the FS lever arm. The eye can deceive as the spring seems to be moving far more than only 1.5mm. Whatever, it seems to work.

Most FS equipped XB owners would swear that my belt is far too slack, but is it? I felt no ill effects on Sunday. Although being a tiny bit too lose around the pulley teeth where maximum load is applied may result in increased wear? I will have to read up on this. Although on my last ride in cooler late summer air at 1020mb, the effect of my exhaust Venturi when winding the throttle on at 3800rpm felt like a supercharger, and things can go a bit ballistic for the next 1500 rpm. At one point the front end went very light, next the rear broke traction crossing a line on overtake, and it seemed to go from 50 to 90 more like an XB11 than a 9! Or maybe l just get excited more easily in my advancing years. I doubt if it would drive so well if the belt was dangerously slack!

Next stop the double spring experiment!

Some more thoughts about belt tension.

There are lots of papers on belts and tension on the web. Some get very technical to the point my brain just switches off!

The main concern about toothed or synchronised belts being too slack is what happens during load reversal when the slack side of a drive switches from the lower to the upper belt run in the case of motorcycles. Experts recommend that idler wheels should be equal or greater in diameter than the smallest drive pulley. Observant Buell historians may have noticed that the XB’s idler was enlarged early in the life of the CB9 Firebolt.

Spring loaded tensioner are also not recommended as they can be overpowered by load reversal unless they employ a stiff spring to avoid the risk of ‘ratcheting’. I assume that is when the slack in the belt switches to the upper belt run and the teeth ride over the upper pulley teeth? I may be wrong, but even with my softer spring l doubt if that’s going to happen as there seems to be too little play in the belt. Maybe when a belt gets older and you slam the throttle shut at high speed then ratcheting is a risk. Maybe my XB has done this without me even noticing! FS may also have moved the position of the idler closer to the drive sprocket to reduce the impact on the spring from load reversal. I would envisage that had the tensioner been fixed just in front of the rear pulley then load reversal would hit the tensioning wheel at a more acute angle. Another beneficial factor is that Buells have relatively high crank mass and so less engine braking than say an old 900 Monster, which slows very rapidy when you shut off.

Load reversal may also be a good reason to have the tensioning idler wheel fully under the belt, but then there is a risk of lack of play before hitting the exhaust mounting. Which would be more damaging in the case of an actual belt ratcheting event is hard to say. Maybe the FS tensioner needs modifying so that both the above factors can be dealt with. I also read that strong tensioning springs put excess load on shaft bearings, but no more than a fixed idler wheel that is too tight to start with?

As l suggested before, if the fixed idler can be perfectly positioned to counter the effect of a non concentric swing arm mount, then load reversal wouldn’t even be an issue. But the XB is what it is so l’ll go with a spring for now. Given the risk of ratcheting from my softer spring being overloaded l will go ahead with my twin spring set up. I think what l am trying to find is a progressive spring which is soft enough to reduce loads, but is still strong enough to return quickly and damp out fluctuations. I think that was the thinking behind Honda using double valve springs on their high revving small engines. A double spring may have the same benefits in my FS tensioner.

CB9??? A Honda/Buell hybrid!

I gave in to temptation and fitted twin springs to my FS tensioner today. My original thoughts about a 20mm OD spring being too large were misplaced. The Honda 125 valve spring IS the spring I needed.

This allows 16mm stop washers for the 15mm Malossi spring to fit comfortably inside the 20mm OD coils to push the smaller spring forward. So in fitting I used the original FS adjuster nut wound to the idler end of the threaded bar, then a 20mm washer, then a 16mm brass washer, then the 20mm OD x 27mm long Honda 125K3 exhaust valve spring with the 15 OD x 32mm long Malossi spring inside.

The Honda spring can be pushed inside the FS frame just using my fingers, but the longer Malossi spring needed a small screw driver to get it to seat in the FS forward spring socket. I need not have worried about the bigger spring grinding on the FS frame as it fits with room to spare. I did however cut or grind off the last few mm of the tapered end of the Honda spring so that it slots in easier at the front above the Malossi.......and it all looks perfectly at home as if was meant to be there. Even the 20mm spring doesn't look too big, just more purposeful than the original.

After winding up the rear wheel spindle I wound the adjuster nut a couple of turns to make sure it is fully threaded, but there seems to be enough power in the twin spring set up. I gave the new (+1 old) spring a liberal dose of ACF50 (a bit like SFC50, but less of a Laverda ). It now looks the dogs dougnuts! Just have to see how she runs on the way to my MOT. NB copyists fit the components as described above at their own risk of course!

Looking for yet more spring alternatives l remembered a CB175K4 head with valve springs intact in a box in my father’s attic. A trophy from an old blown motor after foolishly spending too much time overrevving to 11000 rpm. The old 175 motor was much more prevalent than the 125.

Everyone else’s uncle must have had the single carb CD version dumped at the back of a shed somewhere!? Before quads these were sometimes seen on farm yards without mudguards and fitted with nobblies for animal herding! There must be hundreds of old CD valve springs languishing in boxes somewhere.

Got the 175 head home today.....and the valves appear to be the same OD as the 125, despite the bigger motor having significantly bigger pistons and valves ( 52mm pistons up from the 125’s 44mm ).

I can’t see the spring length as l think you need an M7 bolt to pull the rocker shafts. If l recall correctly, according to Haynes there was a fine pitched M7 bolt secreted about the engine for this purpose, but that’s gone now.

So much for Honda scaling components to the optimum? Maybe one size fitted all in the old CB range of 125/150/160/175 and last of that line CB200 Dredd!

Still ll appear to have plenty of spare springs for my FS.

Doh! In first line; 175 ‘springs’ are same OD as 125!