Rebirth of a Lightweight Panther. 4. The Forks

     The forks for the 1947 Model 70 are lightweight Dowty Oleomatic forks. These use air as the suspension medium with oil damping. Each leg contains a moving cup seal mounted on a piston, three square section static seals and a rubber buffer. The piston slides up and down a guide tube compressing the air in the upper tube. The two legs are connected by a copper tube so that pressure in both is equal. These forks are alleged to give a very smooth and comfortable ride but are prone to air leakage especially when of considerable age. Any damage to the internal surface of the upper tube where the seal bears means that air retention is impossible and the only answer is to fit springs. P&M did this for the Australian market and had so many warranty claims that they were only fitted for 1947 and 1948.

       Photo left. Lightweight Dowty fork internals. Left; inner sliding tube with axle cap, centre; filler plug with static seal and outer tube, centre right, top to bottom; internal fitting and tube with static seal and balance pipe, circlip, seal retaining ring, seal, piston, buffer, washer, Simmonds nut, right; scraper locating ring, scraper ring, tapered shroud. Not shown, the inflation valve.

       The lightweight forks differ from the heavyweight Dowtys, apart from having smaller diameter tubes, in that the lightweight piston forms the upper bearing. On the heavyweight forks a resin-bonded fibre split bush is fitted to the piston as the upper bearing.

          All the parts for the Heavyweight forks are shown in my Cycle Parts Manual.

         Below. Seal set, left to right, filler cap static seal, top fitting static seal, piston seal, piston static seal, rubber cushion, scraper locating ring, scraper ring.

      I have two sets of lightweight Panther Dowtys, one that came with the Game Changer, and the other I bought via an ad on the POC forum. Both sets had damaged inflation valves so I cut off the outer part from the brass hex and silver soldered in a valve body from on old inner tube. Blowing the forks up revealed both were leaking badly. I disassembled both sets, fitted new seals and found both were too worn to retain air so I decided to go for springs, first stripping the remains of the paint and repainting. Dismantling requires a special tool, home made in this case, in order to unscrew the piston from the lower tube. The two sets of forks differ. In one the piston has slots in the top, in the other a pair of holes for a pin spanner.   

Double ended fork tool for slotted or pin type pistons.

          P&M produced a spring set for these forks comprising two upper bobbins which screw onto the tubes on the top fitting, a pair of bobbins for the bottom of the springs, and a pair of springs. The POC now produce a similar set made to original P&M drawings. 

Spring bobbins, left; upper, right; lower

          The POC also have seal sets for these forks if you want to us them on air as designed. Springs from the late Panther fork can be used; they need to be cut down and are a snug fit inside the bottom tubes on the lightweight fork. Often when springs are fitted the forks are extended too far. The ride height under air is shown by small dots on each of the lower tubes; these should align with the bottom of the tapered shroud with the bike off the stand and the rider seated. Achieving this when springs are fitted is a bit tricky and cannot be checked when, as in this case, you have no wheels.

      I tested a spring from a heavyweight Panther fork using the trusty bathroom scales and found it to have a rate of about 50 lbs per inch. Sitting on my rigid Model 75 with the front wheel on the scales gave an indicated 200 lbs so the forks needed to be set unloaded with the dot two inches below the bottom of the shroud. 2 inches by 50 by two legs = 200lbs. These springs will give a pretty stiff ride but I have them in the Model 75 and they seem fine. I found that with the springs cut down to 16 inches overall length and the upper bobbins at the top of the thread I achieved close to the 2 inch target. Whether this works in practise only time will tell.

      The forks also need the Oleo bit to provide some damping. The specification suggests filling the forks completely with 30 grade oil with the bottom of the shroud one inch above the axle fitting, then allowing the fork to close completely (with the air valve removed). Whereupon excess oil will spill from the valve orifice. You cannot of course do this with springs in place of air. I filled one fork leg in the recommended manner using diesel as it is easier to pour out than oil. I measured the volume at 230cc. To allow for the volume of the spring I did it again with the spring in place, filling to the same level as before giving 160cc. 

     I fitted new races to the headstock of the frame and the top and bottom yokes then assembled the steering using 3/16 inch ball bearings. After fitting the fork top fitting and balance pipe the legs were offered up in turn and the pistons screwed into the top of the lower tubes. Finally the outer tubes were passed though the bottom yoke and up over the static seal on the top fitting. This can be tight so was aided with a sash cramp between the bottom of the outer tube (shroud removed) and the filler plug. Previously I had drilled and tapped the top fitting on each leg for a small screw which prevents the outer tube sliding down when the clamping action of the bottom yoke is inadequate. This was a problem on my Model 75. This drilling must not penetrate right through if air is to be retained. Finally I put 160cc of 30 grade oil through the top fitting and into each leg. This all took much longer than it takes to write.

Completed Dowty forks.