The colour of FS Drive Belt Tensioner springs that is!

This post is a bit overdue......from those halcyon days when pash put together a bearing renovation kit for knackered FS pivot pins, possibly for nutters like me who actually rode their Buell XBs for more than 2000 miles per year ....or who had the time to do those miles!

I may have been the first to have my pivot pin repaired. Around the same time we were discussing use of a softer spring for the FS unit. I don’t recall how it came to be, but in the event l tried a softer spring to the standard FS yellow item. I fitted a blue spring ‘IND4606217T’ rated as ‘medium’ strength. The yellow spring IND4603217T is rated as ‘extra heavy’ loading. I read somewhere that the blue has a 209N resistance compared to the yellow’s 269N.

In practice the blue spring feels much softer. Pushing the XB around it felt like no belt was fitted. The extra free movement was as much of a jump as comparing the yellow spring FS to the factory fixed idler wheel! You could flex the belt with only two fingers (or one stiff little finger! ).

All good it would seem, but after only one lengthy ride l checked the belt tension to find that all free play, or spring in the tensioner had disappeared. It transpired that the fatigue life of the blue spring had been used up in just one run!

I reinstalled the yellow spring, but when l removed this spring recently to clean the FS unit the yellow spring broke in two. This had also reached the end of the road, but it had endured some 10 years of abuse.

I decided to try the ‘red’ spring IND4601217T rated as ‘heavy’ strength, so somewhere between the blue and the yellow. Odd then that l read that the red spring is rated as 216N or not much higher than blue. Hopefully the red had just enough extra strength to avoid premature failure.
So today l put this to the test with a 73 mile ride around the Cotswolds. Pushing my XB by hand and flexing the upper belt by hand did seem to indicate that the red spring was a happy compromise between the blue and the yellow. On my return home the red spring still seemed to have plenty of life left in it. However this is early days and another longer day in the saddle is needed to show if the red spring is a stayer!

NB these springs were obtained from Cromwell industrial supplies, but there are a number of identical items out there which appear to be sourced from Indiamart and a factory in Mumbai (even Italian springs). I don’t know where FS obtained the original yellow spring, but could that be the source?
The original FS item is not stamped with a reference on the seating ends so l suspect that it is nothing very special.

I would have thought that some machine makers would have specified more durable springs for multiple cycles in more testing environments. Exhaust valves for instance and ‘oil tempered wire’ exhaust valve springs. I am looking for something to match the blue spring’s resistance like an inner valve damper spring of the same size. Small scrap motorcycle cylinder heads will be my first port of call.

I note that for such a corrosive environment as the FS tensioner that such tempered valve springs will need to be thoroughly cleaned and dipped in a protective coating before use......or regularly treated with SFC50!

Well that didn’t take long!

I was watching a video of Honda technicians stripping down a 250/297cc racing 6, but the valve springs looked much larger than expected. What hope for a tiny 15mm diameter valve spring?

Down in my cellar is my old Honda CB125K3 twin motor of 1971 vintage (based on the original Benly ). The valve springs are doubled meaning that there are slimmer inner damping springs. Hooking these out l see that these are only 16mm diameter. Damn close, but these are only 22mm long not the 32mm of the FS suitable springs. I could look at the springs inside the 50/70/90cc Cub series stepthroughs, but l suspect that Honda will probably have used the same sized springs as the Benly twins.

On the plus side these springs are even softer than the blue with a long travel and they still feel very ‘springy’ even after nearly 50 years. Two of these connected (spot welded?) will still compress comfortably to only 26mm which may give 4 to 5mm travel. These springs were run in an engine red lined at 11500, and which could go to 12 on a gradient or with a tail wind, so 6000 cycles per minute max. The FS on my Buell averaging 62mph should only see maybe 2000 compressions per minute......if my geriatric maths is correct! What larks lie ahead?

The spring seat machined into the FS unit is a bit too tight for these 16mm wide springs so l may make some kind of fitted stop for that end combined with an inner sleeve to keep the springs aligned (they are not rectangular in section and as self stabilising as the stock FS spring). Naturally l will have to try the springs in place on the Buell under belt tension before l undertake that much work or it’s back to the drawing board. Or hunting down alternative springs.

Maybe a midnight raid on Honda’s racing museum to measure the springs on their 50cc twins or 125-5?

As there seem to be a few looking in on this thread, maybe some of you could also look out for alternative springs?

I know a great bike breaker/salvage person l plan to visit who could name that spring in one!

FFS it's been staring me in the face: 50cc Malossi clutch springs are 15 x 32mm and you get 5 in a pack! I don't know what compressive strength these are, but might last 5 runs on my XB

There's and agent one block away from me.... might save me a trip to the breakers!

That’s Malossi ‘Reinforced’ Senda 50cc clutch springs. Fits Aprilia AX50 and Derbi 50. Only £8.65.

Worth a try.

similar power output i suppose

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Feros Ferio

Supposed to prevent the clutch slipping under the strain of 7 bhp and gives the boy racers added confidence and blagging rights down at the McDonald’s drive through.

Could be worth an extra gee gee from my XB though!

best get some for my commuter scooter vespa

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Feros Ferio

Funny but l was down Southampton way today and there were a few groups of Lambrettas heading west on the A27. Maybe a bank holiday rally somewhere, or on their way to cause some old school bovver in Bournemouth!?

A group of scooters had stopped at services on the 27. One of them was a sleek metallic black custom job with two big chrome spannies exiting under the rear side panels and a supercool see through clutch housing. The rider was wearing a full leather sports bike suit, but with a Parker coat stashed for later posing, so seemed to be taking it quite seriously. Shame l didn’t have time to stop and ask what motor was lurking inside.

Looks like my nearby Malossi agent is shut this week-end. Probably so that they can attend a scooter event down south!

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Feros Ferio

Now for some thoughts about drive belt tension.

Obviously you deviate from a manufacturers specifications or imitate me at your own risk, but as far as my own XB9 is concerned l am edging towards the belt being no tighter than it needs to be or as slack as l can get away with! Tubers had no tensioning wheel at all so the only way l would be sure the belt was at optimum tension was to set it when rear shock was compressed and axle, swingarm pivot and drive pulley were all in line. Then at any other point of suspension travel the belt would be slacker than optimum. Not that this appears to be a serious problem for owners?

With the XB Buell were a little bit clever (if they’d been a bit more clever they’d have included a spring!). The usual slack in the belt you would expect when the rear shock was fully unloaded is taken up by a fixed idler/tensioner wheel. FS claim that this set up creates excess tension when your XB does a bunny hop, or bounces over a sharp speed bump, but has anyone here actually studied the geometry of the pulleys to check this claim?
Just for argument sake as the rear wheel rises the belt effectively unwinds upwards from the idler wheel. If the swingarm pivot WAS concentric then the belt would get slacker, but because it is not concentric the rear pulley moves further away from the output shaft and so tightens the belt. Now if Buell got their geometry correct the idler slackens while the rear pulley tightens effectively creating constant belt tension. The only problem being that the belt is set too bloody tight to start with.

Not having studied the geometry l couldn’t say that Buell fixed the idler or tensioning wheel at or on this mythical optimum arc. Maybe the fixed position is only a compromise and an approximation of the ideal? Mathematicians or engineers who are better informed with a link are welcome to correct me here on this.

Whatever, l note that FS position the spring loaded idler wheel further forward than Buell’s fixed wheel. Is this because they see their geometry as superior to Buell, or because the spring they include in their design simply works more effectively at that point? Or was it simply because the Indiamart? springs they use prove to have an easier more extended life in that position? Come to think of it why didn’t FS use a red or blue spring to give a softer ride, or did the yellow ‘heavy duty’ spring prove to be the most durable?

This is where l came in with my quest for an alternative spring!

Having tried the blue and red Indiamart or Mumbai springs (do all springs come from India now?) l decided that l want less resistance, but more durability in a spring for my FS.

Then along comes Malossi with their reinforced clutch springs for the Derbi Senda 50, or tuned derivatives of. The part number is 2912018. This is the stiffest they do (said the bishop to...... ).
On Thursday we drove to Pedparts, which is on the Vale Trading Estate just off Hartcliffe Way south of Bedminster BS3 in Bristol. This is a scooter tuners heaven apparently. Some scooter dudes in the queue ahead of me were buying a basket load of goodies including a Polini big bore kit for some machine! Staff there seem to be efficient and knowledgeable.

The Malossi springs come 5 in a pack and are exactly 32 x 15 mm just like the FS spring, which seems too good to be true. We’ll see! The springs are more easily compressible in the hand than the IND blue spring, but once fitted to the Buell feel just as strong if not stronger in practice. The limiting factor on mine is the FS tensioner actually hitting the standard XB12 muffler. I don’t want to pack it out with spacers just to get more travel preferring full width support of the drive belt. I used a slightly thicker nut to the FS item because more threads means more support. I have yet to ride with the Malossi/FS tensioner so fingers crossed.

As there are 5 springs in the pack l can experiment with different coatings to protect the bare metal clutch springs from corrosion. I want to avoid heat treated plastic coating systems for fear of weakening the Malossi tempered springs. I was wondering if that process weakened the coloured springs from INDIAMART? Certainly their apparent stiffness seems to go hand in hand with their limited fatigue life. Hopefully l can give my XB a good thrash next weekend to see if l’m onto a winner with Malossi.