Belt Drive Conversion - part 1

davidrayner

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There needs to be at least one lip or flange on both sides of the belt. Assuming there is only one each side, they can be both on one pulley or the other, or one on opposite sides of each pulley. The bike in my original post is like that. The trans pulley will have a flange on the outside (not fitted when the pic was taken) and the wheel pulley has one on the inside. The two others have a trans pulley with flanges on both sides and none on the rear pulley.

The pulleys must be perfectly aligned or the belt will wander off. Once you have the belt on and tensioned reasonably tight, you keep tensioning while turning the rear wheel by hand a few revolutions each time you adjust the tensioners on one side or the other. Do a little at a time and by the time the belt is tensioned correctly, you should have the belt running true and not wanting to wander off or rub hard against the flange on either side. If the belt ends up too tight by the time you get it running true, loosen the adjusters, push the wheel forward a little, and start again. All my bikes have custom swingarms which incorporate adjusters that can move the axle both backwards and forwards making this job much easier. Nice to have but you don’t need to go to that trouble.

Not only do belts need perfect alignment and tensioning whereas a chain can get by with ‘close enough’, but one thing they will not tolerate is debris getting between the belt and pulley. So, you must make a cover for the top side of the lower run to stop crap thrown up by the tyre landing on the toothed side of the belt to then be fed into the pulley but the top run needs no cover at all (see pics of belt cover on bike with panniers – they make it hard to get a good shot but best I could do).

I’ve sent this info to two other people (both in the US) who’ve done it and as I requested, they sent pics of their bikes to me. The first is the red Special. The owner was apprehensive about using a 21mm belt and so used a 36mm belt which he cut down to 30mm, necessitating a notch to be cut out of the swingarm to accommodate the pulley and filled with a piece of right-angle steel welded in (see pic of swingarm with notch cut out but not yet filled). The second is the blue chopper that’s owner is in love with white-wall tyres (see before and after pics). He used a 21mm belt. Both bikes have flanges on both sides of both pulleys. Is a 21mm belt wide enough? My first one used a 36mm wide Poly Chain belt but I made a new swingarm to suit (see pic of bike with skulls & flames paint job – getting a new exhaust). That was at least 15 years ago and since then, Gates has released their Poly Chain Carbon GT belt (they have blue teeth – see pic of belt hanging on handlebar) and a 21mm Carbon GT has a greater tensile strength than a regular 36mm Poly Chain. My other bikes have 21mm belts. There is also a fellow here in Australia who has fitted a belt but with little or no help from me. It’s the red street-tracker and it has a Yamaha Radian swingarm.

As for my four bikes, three have had Ivan Hoey’s high-ratio primary gears fitted which means I need a larger rear pulley to compensate so they have a 40:90 ratio and the next longer belt. That gives an overall ratio a little taller than stock which is like using an 18t sprocket on the trans or a 32t on the wheel. When I regeared the first one (the one with the 36mm belt) I bought 21mm items. The second and third had 21s from the start and the forth bike is yet to be done. It’s a chopper so I may make the swingarm longer and use an OD 5th gear in it which means I’ll need an even bigger rear pulley and an even longer belt. We’ll see. To save some money on the third bike, I used both 36mm pulleys from the first bike, cut down to 21mm as it’s the only one with stock primary gearing and there’s a reason for that that’s not relevant here and because the gearing remains stock, I’ll probably put an OD 5th into it. All four also have cush-drives but that’s another story entirely although I’ve included a video if you’re interested. Most people wouldn’t bother, but I’m not most people. No, there’s something seriously wrong with me.

I won’t lie or sugar coat it, this is not an easy thing to do. You must have at least some mechanical and engineering talent to attempt it. If you have that, I’ve given you enough to go on and if you don’t, nothing more I could say or do will help you. As for me, I’m neither a mechanic nor an engineer. I was a painter back in the ‘70s (I’ve painted every bike I’ve owned) and later became a train driver (railroad engineer if you live in Nth America) but I enjoy playing with machinery and wondering how I can make it better.

BTW, the 40t, 21mm rear pulley in the pic of me holding it fresh off the mill is now surplus to requirements and is for sale. I’m asking the cost of a new pulley from Gates and you get the machining and gold-zinc plating (as shown in pic of it next to the unmachined 90t pulley) for free. If you want it, email me davidrayner23t@gmail.com I have no idea how to insert pics here so email me and I'll send them to you. Ask questions about any of this if you like and as with the red Special and blue chopper, please send pics if you go through with it.
 
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H-Ds have had belts since the early ‘80s and several others have had them for a few years too so the pros and cons of belts v chains should be known by everyone who has been around bikes for more than five minutes, have already been discussed, and don’t need discussing any further. This is for people who think they might like to fit a belt to their XS-650 (or any other bike they own that has a chain). I fitted a belt drive to my ‘77 XS-650 more than 15 years ago. If I’ve adjusted it in that time, it’d be only once and needless to say, I’ve never had to clean any kind of lube from around the rear of the bike.

The belt and pulleys are made by Gates and are for industrial machinery. As such, the pulleys don’t fit any bike and must be machined. Are these belts suitable for a bike? The pic on the cover of the Gates catalogue is of a bike with a belt. If you can do the machining yourself, great but if you can’t and must pay a machine shop that’ll add to the expense and the pulleys themselves are not cheap to begin with. The trans pulley is part # 8M-40S-21 SH (8mm pitch, 40 teeth, 21mm wide). You take the stock sprocket and heat the teeth and in about halfway to the spline until they’re red, and let them cool naturally. This softens them so they can be ground off and the sprocket becomes round and you can drill and tap six 6mm holes. The reason you don’t heat the entire sprocket is you want the splines to remain hard. You can either tap threads into the sprocket and put the screws in from the outside, or countersink the back side of the sprocket, put countersink screws in from the back and secure with nuts on the outside. I’ve done it both ways and have no real preference. If you put screws in from the outside, they can only go one or two mm beyond the sprocket. Any further, and they’ll rub on the crankcase. If you countersink the backside, you must go deep enough so the screws are fully sunk.

You then take the pulley and machine a well into one side into which the sprocket will fit. You don’t have to make the well deep enough to sink the sprocket fully, but enough to locate the two concentrically will do for now. The deeper you make the well, the closer to the centre-line of the bike the pulley will be. Remember that, if you want to run a wider rear tyre. Drill six holes and screw the pulley and sprocket together. Depending on how deep you machined the pulley to go onto the sprocket, you may have to take a small amount of metal out of the crankcase but this can be done with a file and you don’t even have to take the motor out of the frame to do it, although it’s quite awkward and would be much easier with the motor out (see pic – this was hurriedly done to a junk crankcase for this story but yours can be much nicer). Fit the pulley to the trans shaft and using a straight edge like a length of drill rod, see where the inner and outer edges of the rear pulley will be. Will the inner edge of the belt clear the tyre? Will the outer edge of the belt clear the frame and the shock? Will the outer edge of the pulley clear the swingarm? Deepen the well in the trans pulley if the whole things needs to move inward. Make sure the screws you use to attach the two don’t go more than a mm or two past the inner edge of the sprocket.

As for the belt and rear pulley, it’s a matter of sizing the pulley and belt dependant on the distance between them and the gearing you want. If the distance between the trans and rear axle is stock or very close to it and you want stock gearing, you use pulley 8M-80S-21 and belt 8MGT-1760-21. That’s a pulley with 80 teeth which gives the stock 2:1 ratio and the belt is 1,760mm long. You may not want that ratio but pulleys and belts don’t come in increments of one tooth. Sometimes there’s quite a jump from one size to the next so it’s a matter of taking what’s closest to what you want from what’s available. As mentioned, if the distance between pulleys and the desired ratio is stock, you’re in luck and if you want to lower the revs on the highway you could always fit an OD 5th gear.

If the distance is not stock or you want a different ratio, or both, you select pulleys for the desired ratio and then see if there’s a belt that’ll fit. How do you know how long a belt you need? You could buy the size pulley you need, machine it to fit, fit it to the wheel and run a tape measure around the two only to find there’s no belt anywhere near the length you need. Don’t do that. Rather, you choose the pulleys you need (I always use a 40 on the trans) add the teeth of both pulleys, divide by two to pretend both are the same size, multiply by eight which is the pitch (distance between teeth) in mm, and keep that number. Then, measure the distance from trans to axle in mm, multiply by two, and keep that number. Then, add both those numbers which gives the belt length in mm.

For example, say you have a stock frame and swingarm (‘74 and up, earlier ones were shorter) and you want stock gearing. Gearing was 17:34t or 2:1 and will now be 40:80t. 40+80=120. 120÷2=60. 60x8=480. Keep that number. Trans to axle measures 630mm at roughly the mid-point of the adjustment range. 630+630=1,260. Keep that number. Now, add the two numbers and you get 1,740mm. Closest belt to that length is 1,760mm which is close enough for the axle adjustment to take up.

Use the same formula for whatever gearing you want and trans – axle measurement you have. You may not get exactly what you want and so may have to change one or more of the parameters. If you’re using a different swingarm, modifying a stock unit, or building a rigid frame then choose a belt and pulleys and then set the distance between pulleys but know that a 40t item is the largest that’ll fit on the trans and the belt wrapped around it will clear the clutch pushrod and gear-change shaft by a whisker.

The rear pulley needs a lot of machining to take it from a lump of iron (and it is cast iron) that Arnold couldn’t lift to something that’ll fit your rear wheel. I machine the centre section down to the thickness of the original sprocket and how much to take off each side will be obvious once you have the trans pulley in place and checked everything I mentioned earlier. Cut holes and/or slots to your liking for looks or lightness.
 
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Here are the pics. Can't load video so no can see cush drive or belt in action.
 

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The pros are clean running, relatively noiseless, long lasting
The cons are mostly from running them too tight, they're not rubber bands, they effectively do not stretch and if run too tight they'll destroy pulley bearings, the worst possible one being the transmission output shaft bearing.
HD uses a cush drive so the rear wheel pulley also rides on a bearing. I've seen over zealous owners destroy one or both on their Harley's from running them too tight

Nice work by the way
 
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The pros are clean running, relatively noiseless, long lasting
The cons are mostly from running them too tight, they're not rubber bands, they effectively do not stretch and if run too tight they'll destroy pulley bearings, the worst possible one being the transmission output shaft bearing.
HD uses a cush drive so the rear wheel pulley also rides on a bearing. I've seen over zealous owners destroy one or both on their Harley's from running them too tight
Running the belt (or chain for that matter) guarantees the front sprocket nut comes loose too. Doesn't matter how tight the nut is tightened or if there's locking washers etc.... Running too tight guarantees the nut comes loose, if it's not attended to, the spline wears to destruction.
 
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