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Talking Technical: Wheely Simple
However old and quaint your bike, it should have a pair of true-running wheels in exact alignment. If not, says Paul Friday, then you are riding a crab. Your tyres and chain will wear out sooner than they should and the gearbox and wheel bearings may soon follow. It's not hard to get everything lined up, it just takes time...
Gather the tools you will need during the horrible wet summer, ready for when you take the bike off the road for the mild and dry winter. You will need a long straight edge; long means as long as the bike. A plank is OK, providing it has a good straight edge - try it against the garage or kitchen floor and look for gaps. Failing that, try one of the big DIY chains for a length of alloy channel. While you are at the DIY shop buy a plumb bob to replace the one you can't find in the garage. Find at least two paint tins or similar of the same height, ideally about half the height of your wheel spindle above the ground - say six inches. You will also need a ball of thin string.
Setting Up The Bike
If your bike has a rearward weight bias -- it lifts the front wheel when parked on the centrestand -- then a ratchet tie-down strap will come in handy.
To prepare the bike, park it on a level garage floor on the centre stand. If you have no centrestand you will either have to use a box under the engine or tie the handlebars or frame up to the rafters. The box is safer, providing you can rest the bike on the lower frame rails rather than have it teetering on the oil drain pug. If you use two stacks of bricks, take care that they won't wobble and collapse.
Invest some effort in getting the bike vertical. If the handlebars aren't bent, then the ends ought to be the same height above the floor (with the steering straight ahead). If the bike is on its centre stand, you may need to use bits of packing under one leg.
The first job is to check the wheels for trueness. Hold a pencil against the fork leg, swinging arm or rear fork and spin the wheel. Bring the tip of the pencil in towards the wheel rim from the side until it just brushes it. If the wheel has big dings in the rim or is oval, get it trued first. Check that the front wheel is central between the forks, and the same with the rear. If either of them is offset, check that the bike wasn't built that way! Most bikes are symmetrical though, so if one of the wheels is offset, find out and cure the cause. It will probably be due to the wrong spacers, or the right spacers in the wrong order.
If the wheels are true, then you want to get the bike level fore and aft. This is easily done by putting packing -- bits of wood -- under the lowest wheel. It's at this point that anyone with rear suspension needs to think about the tight spot. There will be a point in the suspension movement where the chain is at its tightest. For swinging arms, this may be in the centre of the range where the engine sprocket, swinging arm bearing and wheel spindle are all in line.
Other quaint arrangements like plungers may be tightest at full compression or rebound. Unless the chain is new, it will also have developed some tight and loose spots. Spin the rear wheel to find the place where the chain is tightest. Break out the tie-down strap and use it to pull the wheel to the position where the suspension movement makes the chain tightest, in the position where the chain itself is at its tightest. If the bike rests on the front wheel you may be able to get away with removing the rear shocks and using a bit of rope. The alternative is to get the bike off its stand and persuade a lardy mate to sit on it and support it upright.
So: bike level, chain in tightest position, steering set full ahead. Now we begin.
Put one of your tins next to each of the wheels and balance your straight edge on them. Bring the edge up to the sides of the wheels. The centre stand will probably get in the way -- cut a lump out of your plank to clear it, or try to feed a thinner version through. You want the long straight edge to touch the rim or tyre of both wheels at two points -- which is why you stood it on tins rather than put it on the floor.
It's likely with most old bikes that the front and rear tyres will be close in size. If they are the same size, the straight edge should touch both tyres in two places. If the rear tyre is wider, then there will be a slight gap between the straight edge and the front tyre. Adjust the steering to make the front tyre touch the straight edge, or have an even gap.
Without touching the steering, transfer the straightedge to the other side of the bike. It should touch in four places, or have an identical even gap at the front. If so, preen. If not, tighten the chain adjuster at the rear wheel, that is on the side with the smallest gap. If the chain is bar-taut and you need to put some slack back into it, let the chain adjusters out by an equal number of full turns or flats. If you have to move the wheel forward, go back and check both sides with the straight edge again.
When the straight edge shows the same gap both sides, or four perfect points of contact, then your wheels are in line. If the gap is greater on one side of the front wheel than the other (the left side of the bike or the right), then the rear wheel may be out of line -- go to the next check.
If the wheels are in line, you must now ignore any markings on the chain adjusters, and only ever turn both adjusters by equal amounts. Undo the tie-down strap or put the rear shocks back on.
The engine and rear wheel sprockets should be in line, and the chain should run straight and free between them. You may have to take the chain guard off, but get down behind the bike and take a squint down the top run of the chain. MPS sell a gadget that clamps to the rear sprocket and indicates its alignment. Check if the rear sprocket shows greater signs of wear on one side or the other. As a last resort, take the chain off and pull a length of string tight across the faces of the sprockets.
If the rear wheel is offset to one side or the other, confirm it by measuring the gap between the rim and the forks or frame. If it is definitely offset in relation to the frame, suspect the spacers on the spindle. If the rear wheel appears central in the frame but the chain is out of line, suspect the engine plates and spacers. Go back and get these right, as it is crucial to the life of your gearbox, chain, sprockets and wheel bearings that the chain line is perfect. Once you have juggled the spacers to get the chain line, go back to the beginning and get the wheels in line again.
The final job is to check the chain tension is still correct (at the tightest position), then tighten the spindle and all the adjusters. Check the adjustment of the rear brake too, as they often vary when the rear wheel is moved. You could leave it there and ride the bike, or you can go on to check that the frame is straight too.
One sign of a bent frame is that you can't get both wheels and the chain to line up. If you find yourself kicking the cat and throwing tools about, use the ball of string to make some basic comparisons of dimensions. There should be equal distances on the left and right sides of the bike from the steering head to the swinging arm spindle, and from the steering head to the rear wheel spindle. It's a specialist job to straighten a frame, but most old bikes are not made of exotic materials so it's only really a form of careful blacksmithing.
However, if the frame has been twisted rather than bent, in the sense that it has a 'thread' rather than a banana curvature, it can be difficult to find. You have obviously left the bike up on its blocks, all nice and level. Wrap the plumb bob over the rear tyre at the highest point you can, and let it hang down. It should rest over the top side of the tyre and just graze the side of the lower portion.
If it doesn't, check the other side and fiddle the bike to get it vertical.
Without moving the steering, which should be straight ahead, try the plumb bob on the front tyre. The string will also tell you if the tips of the handlebars are the same height off the floor.
If both wheels are vertical at the same time, your frame is straight. If they are not, your bike will try to veer one way or the other when you ride it. It may be very subtle, but you will get backache from applying pressure to steer straight.
Check everything is tight and don't drop the bike getting it off the blocks. Providing you only turn the chain adjusters by the same amount, you will have a true-running bike -- until the next time you take the rear wheel out.
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