These too will need slots to allow the cables to be put on and off. The holes will need to be able to carry an appropriate cable adjuster each (again motorcycle is an ideal source). A third hole, parallel to the others is then needed in the middle and a cap-head bolt of appropriate size into the original adjuster thread on the clutch lever mount hold it all together. Place the modified rod where the original motorcycle clutch cable nipple was intended to go. You may have to run a drill through the lever to get it to fit as sizes vary and some levers have a blind hole. Adjust the brakes so the outfit wheel is slightly softer than the front wheel. Trial and error here. Again, the difference is decidedly more than the sum of its parts.
cheers, Peter Watson

Making an Earles-Type Leading-Link Fork
By Peter Watson
An equivalent of the top triple-clamp will also need to be made. A piece of 4-5mm plate with a hole for the steering stem and appropriate bend must be manufactured. An angle-grinder with a fine blade does just fine. The plate needs to be basically a triangle with the other two points being the bolt holes to attach to the tops of the 20mm girder tubes mentioned before (see drawing). An easy way to provide attachment at the top is to weld a piece of 6mm thick angle iron across the top of the tubes and trim it neatly to fit afterwards. Drill straight, centred holes with a 7mm bit and tap and 8mm threads for high tensile cap-head bolts (ie Allen key bolts). You will need a bearing adjustment nut under the plate and one with a lock nut to hold it in place. Be sure the forks you choose to make this out of have a long enough threaded steering stem to enable this to be done. The triangulated addition will stop the forks bending up under load at the bottom of the steering stem.

After the basic girder is complete, set up the swinging arm pivots and the top shock absorber mounts. Both these need to be pretty precise. There are a number of ways to tackle the problems. One is to drill the holes in some shaped 3mm plate, attach them to the shock absorber, (or a spacer of the same width) with the correct sized bolt, then weld them onto the front of the fork supported by a simple jig and clamps. Make sure both are in line, fore and aft, laterally and equidistant from a central datum point. Use your ruler accurately and devise a method of clamping them in place while you weld. You can then remove the 'spacers'. Another way is to accurately drill the plates after welding them in place, again with a temporary spacer clamped between them when welding.

The next trick is adding the rear-facing swinging-arm pivot mounts. This will need a simple jig and clamps. And will need to be as accurate as you can get it. I used a piece of RHS and a pair of gussets, one either side. You could just as effectively use tubing of the same diameter as the forks and a pair of gussets. They need to be parallel in all planes. Again a piece of 8-10mm plate at the end, accurately drilled and taped after attachment will carry the rose joints. Use 10mm ones. The thread on the rose joints provides a handy point of adjustment. If you move one side, you can adjust the track of the front wheel. If you move them equally in or out together you can adjust the caster by about 10-20mm. Another real advantage of rose joints is that they are, to a degree, self-centering, and can thus tolerate a small degree of imprecise welding and drilling.

The swinging arm needs its redundant steering stem removed. Again get out the hacksaw or cut-off saw. I always use forks that have good brake mounting posts already on them in exactly the right positions, assuming you get a set designed for the wheel size you intend to use. Put the brakes on the underside. This is not only better for the brakes, but allows the appropriate placement of the pivot mounts. Said mounts are essentially the same as the shock absorber mounts and can be done using the same tricks. These need to be as accurate as you can get them, though the rose joints do allow a little bit of play if necessary. Likewise do the bottom shock absorber mounts. And that is basically it. Clean off the burs and paint it. Ohh, and you may need slightly softer springs than the standard ones found on second hand mountain bike rear ends, they are, after all, intended to be used as a single unit supporting your weight, not as a pair. There is always the option of using a pair of suspension struts saved from an old motor-scooter or very small bike.

The second approach:
You can of course go the other way and actually start with all new materials. This will doubtless give a much better finish, but does require a good workshop and a high level of trade skills. In this instance many of the design and construction parameters are no longer governed by what is available, but only by what is possible. There are clearly numerous elegant ways to achieve the same geometric imperatives if this route is taken.

In the end, the real object of the exercise is to shift the contact patch forward about 80mm (as in the case of the illustrated 24" wheeled design) leaving a caster of between 40 and 50mm. Caster is the distance between the projected steering axis, where it meets the ground, and the point directly below the axle (ie 90 degrees to the ground), being the contact patch of the tyre. It is this caster length which is important. You would not believe the improvement in handling over the original single track front end geometry. The front suspension is also a real bonus with the extra weight involved.
The basic geometry of leading-link forks was developed for motorcycles and was most prominently used by BMW, among numerous others in the 1950s. Several firms still manufacture Earles-type forks specifically for motorcycle outfits. With a bicycle there are a few other considerations, though the design dilemma is essentially the same. I am assuming anyone attempting to build their own will spend the time to do some homework and drawings based on the real parts, tools and skills they have available. They should thus gain an understanding of what their own project actually needs. There is also the overall geometry of an asymmetric vehicle to consider. Issues such as toe in, lean out, placement of the third axle, track width, wheel base and projected weight distribution are all critical and should play a part in the design.

There are a couple of ways to tackle a fork project.

What I have done is to follow the simplest route with basic tools. The KISS principal. The guiding principal is not to try to reinvent the wheel, or more accurately, why start at it from scratch when someone else has done most of it for you already? In the instance illustrated, we are talking about a 24" kids mountain bike with a steering head angle of 24-25 degrees, as a starting point.

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The component forks themselves are readily available as junk from a tip recycling shop. They come with all the correct thread sizes and bearing mounts, are straight (or should be) and have brake mounts and the like already done. They are however not strong enough. This is because most bicycle steering heads are not of the motorcycle triple-clamp variety, and are unsupported over the length of the steering axis. They are also simply too light for the planned application. I'm sure the strength issue will have already been discovered by many aspirants. The solution is to convert them into a girder, as per 1930s motorcycle design, with a triangulated brace down the front. 15-20mm bicycle tube is exemplary in this application. A hacksaw and a couple of dead bike frames will provide all you need here, or you can always buy new stuff. I simply cut the correct radius at the ends with the appropriate hole saw in a drill press, though very similar results are achievable with two accurate hacksaw or cut-off saw cuts. The bends should be smooth and accurate, so use a pipe bender. To achieve this try a friendly engineering or exhaust pipe shop.

Cut the forks to length first.

An equivalent of the top triple-clamp will also need to be made. A piece of 4-5mm plate with a hole for the steering stem and appropriate bend must be manufactured.
An angle-grinder with a fine blade does just fine. The plate needs to be basically a triangle with the other two points being the bolt holes to attach to the tops of the 20mm girder tubes mentioned before (see drawing). An easy way to provide attachment at the top is to weld a piece of 6mm thick angle iron across the top of the tubes and trim it neatly to fit afterwards. Drill straight, centred holes with a 7mm bit and tap and 8mm threads for high tensile cap-head bolts (ie Allen key bolts). You will need a bearing adjustment nut under the plate and one with a lock nut to hold it in place. Be sure the forks you choose to make this out of have a long enough threaded steering stem to enable this to be done. The triangulated addition will stop the forks bending up under load at the bottom of the steering stem.

In the note attached to the previous pictures, I mentioned using coupled brakes. I have always done this with motorcycles, and would never do otherwise now. It is very easy to achieve. Start with a motorcycle after-market replacement clutch lever and its handlebar mount. Cut a length of 3/8" or 10mm rod to about 40-45mm. Drill two parallel holes, one at each end, about 10-12 mm from the actual ends, square across the rod, on the same plane. With a hacksaw or very fine angle grinder blade, cut a slot from the end to the holes, again on the same plane as the holes. With a round file, the same size as the end nipple on a brake cable, cut a rounded trench across the original holes to retain the same said cable end nipples from sliding out of their slots. Replace the original cable adjuster on the handlebar mount with a piece of angle or RHS with a hole at each end in line with holes in the piece of 10mm rod.
These too will need slots to allow the cables to be put on and off. The holes will need to be able to carry an appropriate cable adjuster each (again motorcycle is an ideal source). A third hole, parallel to the others is then needed in the middle and a cap-head bolt of appropriate size into the original adjuster thread on the clutch lever mount hold it all together. Place the modified rod where the original motorcycle clutch cable nipple was intended to go. You may have to run a drill through the lever to get it to fit as sizes vary and some levers have a blind hole. Adjust the brakes so the outfit wheel is slightly softer than the front wheel. Trial and error here. Again, the difference is decidedly more than the sum of its parts.
cheers, Peter Watson

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