Bicycles/Maintenance and Repair/Appendix/Moulton/Leading link

ApplicationEdit

The AM leading link fork is used on the AM series (1982-current), also ATB, APB, TSR, Moulton Speed and Moulton Jubilee. While there are detail differences, the basic design is common. A number of running changes have been made over the years.

Principle of OperationEdit

The leading link suspension is analogous in operation to an automotive MacPherson strut suspension but with the moving elements rotated 90 degrees in plan view.

Component IdentificationEdit

Starting at the bottom, the stirrup or spring_fork includes dropouts for the front axle, pressed in DU bearings (bushings) for the leading link pivots, attachment hole for caliper brake (or braze-ons for canti or V-brakes) and at the upper end a thread that carries the bump stop and height adjuster. In normal operation the stirrup carries most of the weight of the bicycle and rider.

Assembled on top of the stirrup is a threaded washer with bonded on rubber bumpstop, this will contact the bottom of the fork crown when the suspension is fully compressed. The bumpstop is threaded all the way down, for maximum suspension travel.

The threaded height adjuster is used to vary the standing height for different rider weight and riding position. Many models include a knurled/serrated ring or thin jam nut to lock the height adjustment, this is tightened against the bottom of the aluminum height adjuster. The thread is UNF(fine) 7/16" x 20 threads/inch (also, AN7). If a hex nut is used the wrench size is 5/8", smaller than the 11/16" wrench used for a standard 7/16" nut--if not available from the Moulton factory, another source for this nut is through aircraft suppliers of military spec AN hardware. On most models there is a rubber wiper seal around the top of the height adjuster, the ATB and some other models use a rubber gaiter (boot).

The upper end of the height adjuster is spherical and the suspension bearing (also referred to as plastic guide bushing and base plug) is snapped (or squeezed with a vise) onto the top of the height adjuster. Original suspension bearings were black plastic (Nylatron) cylinders and recent versions are bobbin shaped. Most are approximately 22 mm in diameter, but early APB models used a steer tube with smaller ID, these take a ~20 mm diameter bearing. Nylatron bearings have been known to swell in hot/humid climates and lock the suspension, the newer gray bobbin type are made from a different material that is believed to solve this problem.

The steel coil spring is above the suspension bearing. As with the plastic suspension bearings, these come in two coil diameters with the smaller for the same early APB models. Springs have been produced in different lengths, wire diameters and even progressive wound for variable rate (this is sometimes referred to as the "race spring"). Stiffer springs are recommended for heavier riders and also riders that use a forward position with more weight carried on the front wheel. A small amount of grease on the spring is recommended to prevent squeaks and premature wear.

Moving back to the lower end, stainless steel pressings (stampings, plates) are bolted together to make the leading links. There have been several styles, the original version (1982 to about 1987?) have a shallow depth and the two plates do not touch. The later style are deeper and the center sections butt together when assembled, making the link stiffer. Three loose bolts, spacers (the bearing journals), nuts and washers hold the early style together. More recently bolts, journals and studs have been bonded/pressed into place. All variants of the leading links are interchangeable. Plastic washers trapped between the links provide adjustable friction damping. The center bolt holds the links together and also carries the rebound stop (early type hard blue plastic, newer type red elastomer). The center bolt is slightly offset toward the front, closer to the stirrup.

The main fork legs (blades) also carry DU bearings for the rear end of the leading links. The legs are made from tapered tubing on early AMs. Constant diameter tubing in stainless (and 531 on later AM models) gives increased fork stiffness. Heavier gauge tapered tubing was introduced with the Esprit model to match the stiffness of the constant diameter legs. ATB/APB/TSR models use larger diameter tubing for the fork legs. Unlike a rigid fork, the main fork legs only carry a small portion of the weight of the rider in normal riding, but they do carry braking loads.

Fork legs are brazed to the steer tube (steerer) with a traditional investment cast crown or welded/brazed in unicrown style.

Original AM steerers (steer tube) were externally butted. Later models use constant wall thickness tubing. Part way up the steerer a pin is brazed (or pressed on APB) across the tube for the upper spring stop. Most steerers are threaded for English headsets. Esprit and other recent steerers use the Ahead Set system.

The small parts noted above are shown on the last page of the factory TSR Service Instruction Front Suspension (PDF). The factory service instructions are supplemented here by additional tips for DIY, collected by members of the Yahoo list.

ServiceEdit

DisassemblyEdit

The TSR Service Instruction Front Suspension (PDF) linked nearby includes instructions to disassemble the fork. As noted, it may be necessary to compress the suspension slightly to release pressure on the leading links--this can be done with a rope or strap looped around the stirrup and through the frame (or over the crown if fork removed from bike).

The Instructions also include removal techniques for a stuck Suspension Bearing. A long steel rod threaded down the steerer from the top could scratch the inside of the steerer. Brass welding rod (welding wire) or even a wooden dowel removes any chance of damage (but the wooden dowel may not be strong enough).

Repair or replace suspension bearingEdit

If a stuck suspension bearing (black Nylatron version) was removed without visible damage, it can be reused, although the newer bobbin shaped bearings are slightly nicer.

Rust inside the steerer can be smoothed out with a flap wheel (Instructions). An automotive brake cylinder hone with plenty of oil also works well, the type with three stones on a flex shaft, spun by a drill motor.

After the inside of the steerer is cleaned up and wiped dry, check the bearing for a smooth slip fit. If it is tight, careful sanding can reduce the diameter slightly. To avoid making the cylinder into a "barrel shape" (smaller diameter toward top and bottom ends), the sandpaper can be backed up with a flat surface (plate glass, machine table).

A slightly loose fit (range??) is normal, this can be felt by rocking the (assembled) bike fore and aft with the front brake locked. The small amount of play will not be felt when riding.

Care and feeding of the Friction WashersEdit

The blue plastic friction washers at the ends of the leading links are a common discussion topic on the Yahoo list. Experience seems to fall into two camps:

  • Primarily dry weather operation -- clean the washers with spirit (USA = paint thinner), clean out the contact surfaces on the leading links, fork blades and stirrup and assemble dry. The lack of oil or grease should minimize dirt pickup. The initial author of this section falls in this category and has very few problems with the washers.
  • Wet weather operation -- if assembled dry as above, the washers are prone to annoying squeaking. Before blaming the washers, it may be worth adding a few drops of oil down the steerer (remove stem) to make sure that the suspension spring isn't squeaking inside the steerer. To stop the squeaking, a variety of lubricants, often "Teflon" loaded have been tried with varying amounts of success. Users are invited to edit this page with their personal experience.

Per the manufacturer, the self-lubricating DU bearings work best dry. Lubricate the washers sparingly and try to avoid oil/grease in the pivots.

Damaged washers -- lightly scratched washers can be smoothed out by rubbing on fine (400 grit?) sandpaper backed by a flat surface (plate glass, machine table). Pairs of washers on each side of each leading link should be the same thickness to maintain uniform contact in operation. The metal bearing surfaces should be clean and smooth.

Starting in 2005, replacement washers from the factory are slightly thicker than the original spec, they should not be accidentally mixed with the thinner version. When in doubt, replace all eight washers at the same time.


ReassemblyEdit

The TSR Service Instruction Front Suspension (PDF) linked nearby includes instructions to reassemble the fork. It may be necessary to compress the suspension spring slightly to install the leading links. With a smoothly sliding suspension bearing (inside the steerer) and the height adjuster screwed all the way down, this can usually be done by hand. If necessary, a rope or strap may be looped around the stirrup and through the frame (or over the crown if fork removed from bike).

AdjustmentEdit

Setting Ride Height (standing height)Edit

Sweetening the DampingEdit

Dr Moulton's method for synchronizing (equalizing) the four sets of damper washers. He called this "sweetening the damping":

Start by slacking off all four damper nuts (but not the central nuts which hold the leading links together). Stroke the front suspension, the motion should be smooth and very bouncy (no friction from the plastic damper washers). To set a baseline, tighten each of the four damper nuts, one at a time, until they just start to squeeze the damper washers. Again, check for smooth suspension action. Next, tighten the four nuts equally in very small increments (1/12 turn or "half a flat") to add friction damping. Press down on the handlebars and then release the pressure slowly. The release (rebound) should be smooth, with only one "jump" at the top end of the stroke. A series of small jumps (multiple "stick-slips") indicates that the friction is uneven across the four sets of damper washers -- slacken them one at a time (stroking after each change) to find which one(s) are too tight or binding slightly.

It will be quickest if the rider strokes the suspension after each tiny adjustment, while a helper with a good "mechanics touch" makes the adjustments. The torque level is very low and may be dominated by the Nyloc self-locking nut, so a torque wrench is probably not helpful.

Once the four dampers have been synchronized then ride the bicycle to check for your preference of damping. Adjust to taste -- for a general guideline, a smooth spinning riding style usually allows very light damping (and the smoothest ride), while a pedal stomping style might work better with more damping to reduce front suspension motion.

If unable to achieve the desired single "jump" at the end of the rebound stroke, it may be time to service the friction washers in the leading links, see above.