<<Shaping the Angle Brackets>>

 

The boom, wishbones, seat and rear fork are joined to the chassis by means of eight light-alloy brackets. This page covers the transformation of the angle brackets from the form shown on the left, into something like the shot on the right.

 


The brackets are available from us pre-drilled and countersunk, but if you want to, you could drill your own, starting from standard light-alloy sections which you can probably get locally. The front wishbone brackets are 25x25x5mm; the rear are 50x25x6mm (the last dimension in each case refers to the thickness; the imperial equivalents 1x1x3/16" and 2x1x1/4" would be OK). You need a countersink bit 15 or 16mm dia (centre pic), and a decent drill press helps as it is easy to get 'chatter' on a small machine. The shot on the left shows a good countersunk hole; on the far right is a poor-quality countersink (in steel, harder to machine than light alloy), which would make a bad wishbone pivot. A bit with a hole in it, if you can find one, usually leaves a good finish, in my experience.

The drawing on the left shows the layout of the front wishbone brackets. The top shows the face with the wishbone pivots; the lower part is the face of the angles in contact with the chassis. The overall length is 6"; the separation of the wishbone pivots is 5"; the chassis mounting holes are 4.25 to 4.5" apart.

The plan for the boom-bracket drillings is shown on the right. The brackets are 170mm long. The top chassis bolt is 6mm or 8mm diameter; the lower one is 6mm. The holes for attaching the boom are 8mm diameter, to engage the 8mm studding described in build boom.htm. The lower holes are 8mm further forward than the top ones, to lift the pedal bracket by about one inch compared with my own trike which is mostly shown on this site. See build put together.htm for a discussion of this.


Given brackets which you have either drilled yourself, or obtained predrilled, you need to shape them, to improve the weight, aerodynamics and appearance of the finished trike. The easiest way to do this is with an electric jigsaw. With an appropriate blade, you will be able to cut a gentle curve easily. However you do it, aluminium tends to stick to cutting tools; some kind of lubricant will help.

The front wishbone brackets are the simplest to shape, because all four are similar. The rear brackets are mirror images of each other, with two countersunk holes at the top (for the seat and rear wishbone bearings) and one hole (for the fork pivot) at the bottom. The front boom brackets are dissimilar, as the righthand bracket is partly cut away to clear the chain.

Before drawing the profiles, have a good look at the pictures on this page and scattered around the site. It is better to err on the side of caution than cut away too much metal! Generally speaking, you want to curve the lines you cut so you don't concentrate stress in one particular place. The sketch on the right is wrong: the highest stress is at the weakest point just where the hole is. The line of the cut must come well below the mounting hole in this picture. Remember that aluminium is liable to fatigue, and surface finish is important because any minor blemish or notch can concentrate stress, eventually leading to cracking. (Hence, fancy light-alloy cycle components come highly polished, or anodized.)

The Wishbone Brackets

There are four wishbone brackets, two for each front wheel. They are all similar, and interchangable, except that the bottom of the front brackets are secured with an 8mm screw, while the other fixings are 6mm. This is because of the pull of the front suspension (right), so you may want to leave a bit more metal around this pivot than the others..

A good way to get a nice sweeping curve is to make a template from a piece of cardboard. You want to make it symmetrical, perhaps by cutting the card when it is folded in half. Mark the profile on the aluminium with a pen that makes a clear line. A CD pen should work well. It is probably easiest to cut pairs of these brackets while they are still joined together, as they are easier to hold this way in a vice .

When you drill out the pilot holes, bear in mind there are six of them if you bought pre-drilled brackets; the two end holes are for screwing the brackets to the chassis, so try not to drill these out. You will probably get swarf around the holes (left), which can be removed with a countersink bit, or the point of a drillbit larger than the hole. Be careful when drilling out these holes: it is easy to injure your fingers if you copy the method on the right.

You will need to trim the corners with a hacksaw and file. You should also file the edges you have just jigsawed, as they will be rough. A deburring tool (right) is a handy gadget like a potato peeler that works on metal. You can use one for internal (hole) deburring, and along the filed edges. You won't find one exactly like this in your toolshop, as I made the handle myself. Don't worry if you cannot find one at all; a file does the job almost as well.

The Rear Brackets

The rear brackets are made from aluminium angle 25x50x6mm in section. When you mark these for cutting, leave a decent amount of metal around the bottom hinge (above), as there is more stress here than at the top, and the distance between the pivot and the attachment point to the chassis is greater than elsewhere. (The hole in the top right corner above provides a low rear mounting point for carrying luggage when touring on the trike, and is not predrilled.) The shot on the right shows roughly the cuts you want. Again, you want to ensure that the two sides have a similar shape. With a piece of card cut to the required shape, you can mark the same profile onto both aluminium pieces. (This is a better idea than using the first bracket to mark the line on the second, as you are unlikely to get them similar if you do this.)

As before, you also need to drill out the pilot holes, circled in green in the picture (left). The bottom-most hole on each section is a sawing guide; the next hole up is the bolt-hole that attaches the bracket to the chassis, and the rest of the holes are pilots to be drilled out, up to the top hole, which again is for attachment. Note the sizes for the attachment screws in the picture: 8mm at the bottom and 6mm at the top.

The reason for the saw cuts is to create the clearance (shown left) required for the rear of the trike to fold properly. If you have the finshed rear fork, and a pair of 8mm countersunk screws, you can check that the fork will be able to move through a sufficient angle. You need an angle of 95-100 degrees for the rear wheel to be able to rest on the floor in the folded trike. This is necessary, else an unacceptable amount of force would be exerted on the hinge of the folded trike.You will probably need to do a bit more filing to achieve this angle, but you should leave as much metal as possible. If you have a rear wheel, you should mount it in the fork set up in its folded position on the chassis, to check it can fold sufficiently.

One way to ensure the pair of angles match is to bolt them back-to-back through the pivot holes, and then file both profiles together, as in the picture (left). You can drill out metal from the faces of the brackets that are in contact with the chassis, as on the right. Note that you do not want these small holes to go right through to the front face, so you need an accurate stop on your bench drill. Another thing to bear in mind if you lighten the brackets in this way is that since they transmit forces between the pivot holes and the nearby mounting holes on the chassis, you should avoid drilling the area between these two points.

UPDATE - JULY 2003: A batch of rear brackets have been drilled with a second 8mm hole at the bottom, countersunk on the inside. This does not substantially alter the information above. The hole is intended for luggage mounting: a bracket to fit may be available later.

 

The Boom Brackets

The boom brackets are made from aluminium angle 25x50x6mm. The two pieces will probably come joined. Each piece has three 8mm holes in the face that attaches to the boom clamps.The two pairs of holes at the bottom ends need to be joined to form closed slots. At the upper end, the holes need to be made into open-ended slots, to allow the boom on the finished trike to be lifted up and then folded downwards.

One way to cut the slots is to use a sawing wire in a hacksaw frame. These useful gadgets, sold in the UK as 'Abrafiles' are about as thick as bicycle spokes, but have teeth all around their length. You use them in a hacksaw frame with a pair of special links; an exercise in mindfulness, since they break if your attention wanders. Fortunately they come in packets of three. Be happy if you need only one packet. To saw a straight line, you can clamp a piece of scrap steel along it as a guide. An alternative way to cut these slots is to chain-drill small holes and then clean up with a narrow file. Use an 8mm bolt to check the slots are wide enough; it needs to slide freely in the slots.

When you have cut the slots, the next thing is to shape these brackets. The leftside bracket is simpler as the other is partly cut away to clear the chain. One way to get this cutout is first to chain-drill and then file a slot wide enough to get a hacksaw blade in, as shown on the right, and then saw down.You can run through the pilot holes in the chassis faces with a half-inch drill, except for the three that are adjacent to the chain cutaway, which should be left untouched - as in the righthand picture of the red chassis above.

Leave a healthy distance under the open upper slots when you draw your cutting line - remember that when you are pedalling, the force of your legs is trying to pull these slots open.

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