Author Topic: Douglas Mk.3 Sports Special resurrected  (Read 5130 times)

0 Members and 1 Guest are viewing this topic.

Offline Dave

  • Administrator
  • ****
  • Join Date: Mar 2004
  • Posts: 4241
  • Location: Australia
Douglas Mk.3 Sports Special resurrected
« on: 19 Dec 2013 at 06:58 »
Many thanks to Doug Cross for sending in this great article and set of photos.

Douglas Mk.3 Sports Special resurrected

May 2013

What do you do when you realise that the newly restored bike that you brought home from the showroom is actually a hybrid, something that doesn’t have a straightforward CV from factory to garage? The Mk.1 that I bought back in 1994 was beautifully restored - but a little ‘odd’. The frame had too many rear sub-frame mounting lugs and a strange number, the engine was a genuine T35 unit - but the log-book showed first registration in 1955. I’d had a post-war Douggie since 1956, but an excellent workshop only since 1990 - so I decided to forgo originality and start playing with my new toy. The results have appeared occasionally in the Club magazine, most recently when I warned against letting old fibreglass tanks come into contact with modern petrol - it’s taken me a year to get over that calamity!

After the tank disaster I decided to go the whole hog and really uprate the bike for comfortable riding. So I swapped the engine for a modified Mk.5 unit, bored out to 62mm. It’s fitted with lightweight Suzuki GS650 pistons, with a 9 to 1 compression ratio, and No.3 camshafts. The gear change had to be moved back, to make room for the carbs, which are 28mm Amal Mk.2 Concentrics - yes, really - with 150 main jets, and real attitude. Result - startling acceleration, easy Plus 80 performance, and 100 miles to the gallon!

To calm the rear end down on bumpy bends I replaced the stock sub-frame with a more robust custom-made stainless steel tubing frame, and added a Vincent-style bridge to the swinging arm. This drives a fully adjustable hydraulic rear suspension unit off a Yamaha R1. I cut the spring off, to leave the old torsion bars to do their work as the rear suspension, and the Yamaha addition serves to tame the rear end and make the ride more comfortable. The hydraulic cylinder is anchored at the front to a heavy-duty bracket welded to the main frame, and hidden - well, almost - under the tank.

At the sharp end, the front brake is a modified Plus 90 unit. It had been severely cut about for use on a Triumph-engined grass track sidecar racing outfit when I swapped it for a standard front brake unit (yes, really!), so I converted it to twin leading shoe operation. The steering is calmed by a standard Yamaha R1 damper, also almost invisibly mounted under the tank. The rear brake unit is standard - why try to beef it up, when I can lock the rear wheel easily anyway? And the rev counter is driven by a Honda 2:1 geared drive, mounted on the timing cover and driven from the magneto shaft.

To get rid of the dreaded rust problem, I’ve gone stainless. Everything that could be remade in this excellent material has been - wheels, spokes, mudguards, rear frame parts, battery box, chain guard, engine protection bars, and all nuts and bolts are all rust-free for life. The new tank is alloy (£85 on eBay again), and most of the remaining original parts in mild steel are nickel plated and polished. The rear tail enclosure is aluminium, made from a second-hand hostess trolley to replace the former one made from an old fridge door - you get the picture!

The converted Lucas dynamo provides 12 volt 120 watt lighting, and I’ve used power LEDs for the turn indicators and lighting tell-tale lights. This reduces power drain from the 14 amp hour lead-acid gel battery, and allows plenty of spare power for the 55/50 watt halogen headlamp bulb. The electrics are in a closed box under the seat, with additional multi-point outlets for hitching up lights on a trailer for camping trips, for an external lamp, and for connecting the battery trickle charger in the garage if necessary

The seat was a real challenge, because I had to devise an automatic clip fixture to lock it in place that can be quickly released to get at the tools in the rear enclosure. The seat itself uses a high-density particle foam base covered with closed-cell 10mm self-adhesive neoprene foam. These materials can be shaped very precisely by very careful use of a fine sanding disc, and then finished with a 3mm layer of the same closed cell foam, stretched over the entire surface. The cover is in vinyl, knocked up on my old sewing machine, and fitted with judicious use of the hot air gum and aerosol spray contact adhesive.

And the decals on the tank? Well, this was a real find - I bought some waterslide ink jet decal paper from eBay, then printed out my design and lettering from my computer. So literally anything is possible - if you need an ‘unobtainable’ classic bike transfer, just take a digital photograph of one from a finished bike, or from an illustration, and use that to make your own. Once the printed image is dry (a hair dryer helps if you’re in a hurry), spray it with clear acrylic lacquer, dry again, and cut out the design. Then soak the transfer in warm water until it curls then straightens out again, slide it into place, smooth it gently and let it dry for a couple of days. Then spray with at least two coats of clear lacquer.

This upgrading lark is, of course, a never-ending job - Her Indoors keeps asking when I’ll be finished fiddling about with that dratted bike, and the answer is, of course, never. Douglas made a good start at producing a bike for real riders - but it was in competition with other manufacturers, and strictly at a price that sold. So we always expected to ‘adjust’ our new bikes as soon as we got them, and I am just continuing a long and honourable tradition. I’m not interested in preserving something for the sake of historical accuracy - bikes are meant to the be ridden, and threatened changes in bike laws in Europe really do challenge the continued use of low specification machines in the far more demanding conditions on modern overcrowded roads.

So my ‘Special’ is special to me - I’m perfectly happy to let the museums remind me of just how primitive things used to be, back in ‘the Good Old Days’ when I first started to ride a ‘modern’ Douggie!


Doug also attached some additional notes.

Quote from: Doug Cross
Since writing the above, I've also converted the magneto to run a Boyer Bransden MIcro-Power Electronic ignition unit. I used the TriumphBSA unit twin 12volt kit, and the BB special HT coil (UK£138 in all).  So for anyone who's got the nerve (and workshop) to experiment, I can assure them that it really is possible to do this without totally wrecking the appearance of a classic Douggie. And the transformation is amazing.

The twin-magnet rotor plate is mounted on the end of the mag spindle on a slightly raised spare contact breaker base,  and the stator unit mounting board is cut down to the absolute minimum diameter possible, so that it just fits inside a sleeve to allow the timing pickup pins (filed down slightly on the outside - this is a REAL tight fit!) to rotate a few degrees each way in the original slipring housing. The provides fine adjustment of the timing for Ignition at 35 degrees Before TDC. Total cost was UK£139 - and worth every penny!
So I've got the most amazing sparks - one per rev on each side - and really easy starting. Boyer Bransden workshop apparently hasn't heard of another mod quite like this. The great thing is that when it's in the bike the modified mag looks almost original, and the old contact breaker points can be put back again if I ever want to do that strange thing!
I'm also now running a BB Power Box voltage regulator (around UK£75) so the 12volt dynamo now gives 120 watts easily, keeping  the 14 AmpHour acid gel battery fully charged even when riding with all lights bvlazing. I did fit ( and then remove)  a 55 watt  Zenon High Intensity DIscharge lamp unit (UK£16, ex Hong Kong supplier), just for the heck of it - fabulous lighting, but it caused too much dazzle for oncoming traffic behind an old standard headlamp glass. But again - it CAN be done, if anyone wants to try it!
Doug Cross

« Last Edit: 20 Dec 2013 at 06:09 by Dave »

Offline Philmein

  • Member
  • *
  • Join Date: Nov 2022
  • Posts: 11
  • Location: Hassocks UK
Re: Douglas Mk.3 Sports Special resurrected
« Reply #1 on: 14 Aug 2023 at 20:23 »
Lovely Job!!