Trev aka Store from the BM Riders Club has been building The Mongrel. This BM started life as a humble mild mannered R80/7. Trev can tell us what he has been doing to it on full moon nights....
Style ; Anonymous 70's twin shock road bike.
Priorities : Anti rot parts for easy cleaning, riding all seasons. Ridability/drivability, using the best aftermarket components and the best mix of genuine parts to achieve excellent fuel economy, braking and handling. Top speed is not a priority but the ability to happily cruise at 90mph is required.
This started out as an 80/7 owned by a friend of mine whom I used to help set up his bikes and advising which parts to use. I'm the workshop and stores manger at Motorworks BMW you see.
It was originally built for pulling sidecars as efficiently as possible (which it never ended up doing). Anyway, he sold up to buy a boat to live on and knowing this bike and man well got first refusal on it. It was very scruffy but ran brilliantly.
All I've really done to it since is clean it up and set it to ride solo with an 81 on light flywheel/gearbox internals conversion. The rest is really cosmetic but after owning it a year realised I could possibly jet it down further. You jet down and retard the ignition with twin plugging, we used 80/7 jetting in '300' series carbs. On a steady run we both could 58mpg and on a very fast cruise or just town work would see it drop to 45mpg as it stood..
So the parts :
Originally a 77-78 80/7 converted to a twin plug 1000cc engine.
Because its twin plugged you have to have electronic ignition, in this case its a 'Boyer' and use PVL 356 1 00 dual output coils that you can use on the 81 on bikes too. They're cheap and reliable.
Barrels are 81 on nikasil type but you have to machine the o-ring lip off to fit into these cases. They're lightweight and last for ever.Pistons are 76-85 high compression type.
Heads are 88 on R100 mono type for smaller valves unleaded seats and smaller 32m carb inlet tract. The inlet tracts are cast as a more efficient shape too on these. These are very efficient heads. They are of course drilled and tapped for twin plugging and have a speedy rocker cover removal conversion on them too developed for sidecar racers who like to adjust their valves on the grid. Valve gear is converted to needle roller thrust type for less friction, again developed from the sidecar racing mob.
Gearbox has been rebuilt along with the bevelbox (resplined) by Roger Bennet to post 83 internals for better selection, aided by the nonadjustable 'direct' lever, and the 78-81 'wasted' swing arm and cushdrive driveshaft. The 81 on light flywheel also aids pick-up and drivability.
Suspension and brakes : Progessive fork springs in the 81 on Brembo fork/brake combination, far superior to the ATE forks and brakes and cheaper brake pads too! Koni adjustables keep the rear sorted. Also it has the cop 81 on 2 rib front wheel to match the Brembo forks properly. The spoked wheels are around 3-3.5 Kg lighter than cast apiece and look nicer.
The rear has a /5 stainless cover on it to neaten up the appearance and help it keep cleaner. You have to machine down the later hub and use the smaller /5 seal carrier to make it fit so there's no weight penalty. It has an 81 on subframe too which is marginally stiffer than the earlier bikes.
It has an 85 on handle bar clamp/master cylinder as it takes pre 81 twin pull throttle cables which make it easier to keep in carburettor balance with the much better and cheaper 15mm post 81 master cylinder. Pipes are by Goodrich to and from an 81 on under tank splitter.
Other mods include an easy-clean satin chrome top yoke plate from a 90S with bare alloy handlebar clamps but still hidden by the rubber crash pad. Various wiring mods to help stop the damp getting in and make switch testing easier.
Stainless nuts, bolts, battery cage, indicator bar, brake pivots front and rear, exhaust and brake line brackets some of these replica parts you can't get off the shelf. Powder coated frame, body work by 'Dream machine'.
So, that's 'my' version of a BMW.