Belt/chain tension

Hi Adam,

Can you explain why you think the chain / belt is tighter or looser depending on where the crank is ?

Either the crankshaft is bent, the chain is wornunevenlyor something else I cannot even think of.

Just put the bike in gear and use the back wheel to put a small amount of tension on the top run and check on the bottom run (or visa versa).

If you do find it varies with crank position I suggest you chain is overdue forreplacement. The same with the magneto chain.

As far as I am aware the belt need to be fairly loose, about 30mm up or down movement, another way to measure is to twist the belt, you should be able to twist itthrough90 degrees mid run.

Someone who actually has a belt drive might be able to correct me on this, I am still using chains.

Don't forget when you are adjusting the gearbox that the pull on the final drive chain is greater than the pull on the primary so you should adjust the belt tight and then use the adjuster to loosen it. This means that the pull of the final drive will not move it once you apply power.

Regards

Tony

I tried running an RGM belt drive on my 19s, this set up used the old Dommie/Atlas clutch internals and a pulley for the singles, RGM did not offer alternative offsets for engine pulleys so the engine pulley and the clutch basket never did line up. RGM were not very helpful and seemed to feel that a belt engagement of only 15mm was adequate for the singles, I felt differently about that and after quite a bit of fiddling and tinkering and putting up with a balky clutch that persisted in dragging no matter what I tried. I reverted back to the tried and true chain drive with the proper clutch.

Regards

Albert

P.S. Whatever you do, do not over tighten the primary chain or belt, it puts a real strain on the bearings in the gearbox and premature wear is a likely result. If you do have tension fluctuations like you described, a new primary chain or belt is something to consider.

Hi Adam. I can see what you are asking. The amount of free movement is when you load the chain and take the measurement on the other run, the slack side.

In practice for the primary chain, I push down on the kickstart lightly which loads the bottom run, the measurement is taken of the top run.

For the rear chain, I select any gear, and sit on the bike (or get a helper) to get the bike loaded correctly and then wheel the bike backwards, the slack is then all on the bottom chain run. (Easy to get at)

For the magneto (fit the cutaway timing cover first) and turnby handthe magneto sprocket in the opposite direction it will be driven (therefore clockwise) the slack is the movement on the top of thechain.

However for the camshaft chain,you will need to fit a spanner on the cam nut and turn it anticlockwise to put the slack on the top run as the chain tensioner is on the bottom run. You will need a spanner, as depending on the camshaft position it could be loaded quite strongly by the valve spring pressure ina clockwise rotation and will resist finger turning pressure. The other way is to turn the enginebackwards (Plugs out is best) by turning the oil pump nut with a spanner anticlockwiseso it just starts to move the camshaft or loads it. Once again this will give you the correct working chain slack at the top.

IMPORTANT: For every type of chain you must check the chain slack obtained in numerous positions, to find the tightest position and use this position for the adjustment. Rememberall chain drives on engines tighten up slightly when the engine heats up due to the crankcases expanding slightly.

Les

OOPS; just noticedAdam'spost is for singles. Please disregard, any part not applicable (camshaft chain). However big twin owners might find this part is of some interest and value.

Les

Hi All,

Les is right about checking the chain tension in various places: even a new, out of the box chain can have a tight spot in it.

I haven't read any comments pro or con about "O" ring chain here so here goes; I've never run O-ring chains on my British bikes and had no reason to do so, I always ran the breather hose onto the chain where it runs over the gearbox sprocket, the light oil misting seemed to keep the chain moist and I rarely actually put chain lube on it.

On my Jappy bikes I tried O-ring chain and found that the spray on lubes just didn't get past the O-rings and into the important bits, no matter how much I diligently applied the lube before or after a ride. Before long, I could see the rust just oozing out of the side plates on the chain. I gave up on O-ring chain and went to normal chain where the lube actually worked and found much better service. My SR 500 now has the breather hose aimed at the chain as it passes over the gearbox sprocket and it too, gets adequate lubing from the light oil mist. If going on a long, hard ride I have sprayed lube on its chain just because, in most cases it doesn't seem to need much help keeping the chain moist. I'm also running the largest drive sprocket that will fit, a whopping 17 teeth WOW! (My 19s has 21 teeth)

Depending upon where the engine stops in its cycle, this in turnwill give different apparent tensions in camshaft, magnetoand rear chains. ie. an engine stopping just after a compression stroke will put a tension on the top run of the primary chain.

Theoretically, this should make little difference to the total measured play in these chains if a moderate up and down pull is applied (to the centre of the chain) as this will overcome any engine or camshaft forces.

On the heavy twins it is most important that a 'cutaway' timing cover is used before tensioning the chains in this area. This prevents a loose intermediate gear spindle from giving falseup/down freeplaymeasurements in the timing chains.

Also, as has been mentioned, it is also important to checkwith theengine in several differentpositions as chains do wear unevenly and shafts can get bent.

One important point to consider when comparing beltto chain primary drives is the 'heat' factor. When a chain drive gets hot the sprockets expand as doesthe chain and there is consequentlylittlerelative change in the chain up/down freeplay.

Not so in a belt drive. The alloy engine and clutch pulleys will grow significantly when hot. If the belt is overtensioned, thenthe teethmay no longerdrop intorespectivepulley slots and rapidly start to get guillotined.

Andy Dunn showed me his American madeCommando belt drive back in the 90s. The correct tension was achieved when the belt could just be pushed inwards, across the top edge of the clutch pulley - using only the tip of a little finger. This equated to above 30mm up/down freeplay in the mid-centre of the belt.

Of further interest, many commercially available Norton beltdrives now suggest that the free play is a minimum of 40mm. This helps to prevent the belt becoming overtight but can lead to rippling or whipping of the top run when the engine is suddenly shut down. if the alternator housing is a close fit to the bet then contact may occur.

Last bit of trivia. Due to poor product quality control quite a few clutch pulleys, at the end of the 90s,were manufactured with slighly oval drums. Now this does makechecking the tension an interesting experiece.

Just to be clear about the statement form Mr Hannam: "One important point to consider when comparing beltto chain primary drives is the 'heat' factor. When a chain drive gets hot the sprockets expand as doesthe chain and there is consequentlylittlerelative change in the chain up/down free play".

This may be mostly correct if one is considering the primary chain as the separation of the sprocket centres of the engine and gearbox is held apart by the steel engine plates. If we then consider just any warming of the steel chain and sprockets then there will be little difference in tension.

In my answer, Ispecifically said "engines" where the spindle mountings areheld on the same face or plane of the alloy "wallâof the crankcases.The Norton engine as we knowis made of an alloy of aluminium which has nearly double the coefficient of expansion of steel. The different coefficients will cause the separation distance of the sprocket centres to increase at nearly twice the rate of the expansion of the chain length assuming that the chain is heated to the same temperature as the crankcases. It matters not that there are two runs (top and bottom) of any chain run. It also matters not that there are two semi circles of chain run around the sprockets (one at each end) as the sprockets are also made out of very similar steel and consequently expand radially and circumferentially to absorb any linear increase for these sections. The only part of the chain that needs to be taken into account are the sections that run between the first points of contact on each sprocket which is a diametric straight line across themandexactly the same linear separation distance as the spindle centres. It is clear to see then that as an alloy engine heats up, a steel chain drive will become tighter. QED.

Furthermore, I do not concede that pushing and pulling on the cam chain with a finger to either move the entire crankshaft piston assembly and/or move the camshaft against valve spring pressures is easier and will give sufficient accuracy. The effect observed by doing the check in this way is to see a type of flexingor springiness in the chain and so is thereforedifficult to know how hard to push down and then have to pull up with similar and equal amount ofpressure to try to find the total âgiveâ when the chain is constantly under load by the camshaft. It is far easier to carry out the check as I have described with a spanner. This method gives the TRUE slop or slack of the chain without any need to judge the flexing and finger pressure. A ruler can then easily be held against the completely free chain movement to enable a simple and accurate adjustment to be made.

Les

Previously wrote:

Just to be clear about the statement form Mr Hannam: "One important point to consider when comparing beltto chain primary drives is the 'heat' factor. When a chain drive gets hot the sprockets expand as doesthe chain and there is consequentlylittlerelative change in the chain up/down free play".

This may be mostly correct if one is considering the primary chain as the separation of the sprocket centres of the engine and gearbox is held apart by the steel engine plates. If we then consider just any warming of the steel chain and sprockets then there will be little difference in tension.

In my answer, Ispecifically said "engines" where the spindle mountings areheld on the same face or plane of the alloy "wallâof the crankcases.The Norton engine as we knowis made of an alloy of aluminium which has nearly double the coefficient of expansion of steel. The different coefficients will cause the separation distance of the sprocket centres to increase at nearly twice the rate of the expansion of the chain length assuming that the chain is heated to the same temperature as the crankcases. It matters not that there are two runs (top and bottom) of any chain run. It also matters not that there are two semi circles of chain run around the sprockets (one at each end) as the sprockets are also made out of very similar steel and consequently expand radially and circumferentially to absorb any linear increase for these sections. The only part of the chain that needs to be taken into account are the sections that run between the first points of contact on each sprocket which is a diametric straight line across themandexactly the same linear separation distance as the spindle centres. It is clear to see then that as an alloy engine heats up, a steel chain drive will become tighter. QED.

Furthermore, I do not concede that pushing and pulling on the cam chain with a finger to either move the entire crankshaft piston assembly and/or move the camshaft against valve spring pressures is easier and will give sufficient accuracy. The effect observed by doing the check in this way is to see a type of flexingor springiness in the chain and so is thereforedifficult to know how hard to push down and then have to pull up with similar and equal amount ofpressure to try to find the total âgiveâ when the chain is constantly under load by the camshaft. It is far easier to carry out the check as I have described with a spanner. This method gives the TRUE slop or slack of the chain without any need to judge the flexing and finger pressure. A ruler can then easily be held against the completely free chain movement to enable a simple and accurate adjustment to be made.

Les

Hello Les,

How hot do think the Norton twin crankcase's gets then? I think it's not much more than blood heat, and you must have 3/16 to 1/4 slack on the primary chain and all so the rear drive chain, distortion of the rear engine plates as the engine resonates though the power band, which pulls the chain slightly out of line ?

Hello All,

The comments about heat expansion got me to wondering, just how much will a primary drive tighten up as the engine/gearbox approaches operating temperature? It can't be really all that much if everything is expanding at the same time, obviously the coefficient of expansion for the different metals will vary greatly, I just can't see it being that much of a factor. The loose, flexible nature of non-unit drive lines could be a bigger factor here than metallic growth through heating.

My copy of the "Maintenance Manual and Instruction Book" Norton Publication # P106/P calls for, a slack of 1/2" to 3/4" for the primary chain at the tightest portion of the chain, and a slack of 3/4" to 1" for the rear chain, again at the tightest portion of the chain. Sorry AJD, you need to loosen up your primary chain a bit.

Hi Albert. As I already described, the primary chain on a motorcycle with separate engine and gearbox held apart with steel engine plates does not have a problem with chain tightening to any degree as the component materials are all very similar ie; steel apart from the fact that this method tends to avoid direct heating and stays relatively cool.However engines thathave a UNIT construction, ie: engine/gearbox sharing the same alloy castings, such as late BSA Triumphs and even the Jubilee/Navigator are affected by this phenomena. The RoyalEnfield engines have the separate gearbox bolted directly to the rear of the crankcase with no intermediate engine plates and therefore these will have theprimary chain tighten slightlywhen the whole assembly warms. POI: the Enfields use a slipper mechanism for chain tension adjustment. it is adjusted by a bolt arrangement, and I have wondered why a spring could not have been substituted to take the slack up and keep the tension automatically.

Les

Hello Adam,

May I presume that your Easy 2 with the Hemmings belt drive uses a commando clutch? RGM had claimed that their "Dommie" belt drive using the old style clutch (Late 650 SS/Atlas type) was a simple installation I.E. take off the old, put on the new. My inquiry about fitting this to my 19s using their single cylinder pulley, got me the assurance that it too, was a doddle to install. Putting it on was easy enough, it was the lining things up part that proved awkward. My panicky faxes brought assurances that if I removed material from the side of the pulley closest to the engine that the remaining 26 mm width would be adequate for the 19s engine's power. Eventually I got it to work after a fashion, however, the belt was very close to rubbing against the inner primary case and sometimes did; after a time I grew weary of the fiddling about when I wanted to be riding. I would have gladly paid to have a special pulley made up with about 10 mm outwards offset to increase the engagement of belt and pulleys, and move the belt away from the inner primary case. RGM were of no help to me and I've noted that they did not have this clutch set up in their more recent catalogs. The pity of it is, the idea worked, the execution needed a bit more finishing up.

Lest it be said that I am displeased with RGM, it was this one item that let me down, I've generally gotten satisfaction from RGM's offerings. I've read the postings griping about prices and what not, maybe that complaint is valid with the commando bits. Riding a Norton single means that parts can be thin on the ground and the market isn't enough to justify making up a batch of new cylinder barrels or heads, I have to take what I can get.

Regards,

Albert