Britbike forum Forums British motorcycles in general The competition forum 1966 BSA A65/A50 LSR Bike

HI all,

Figured I'd start a new thread on my next race bike, an A50. So this will built from parts I've had lying around for years with the addition of some new ones as well. The chassis is a 1966 A65S that I never got around to collecting all the correct bits for a full restoration so this will be my chassis to use along with a 1968 A65LB cases and bottom end with a A50 barrel and Wasp head, rotating assembly balanced as needed. The front forks and aluminum triple trees are off a 1975 Can Am TNT 175 which will be mated to early Suzuki Katana wheels (front and rear)with proper Goodyear LSR type tyres. The plan is no front brake and a rear disc operated by handlebar lever and possibly extending the swingarm some too. Need to figure out a seat/ side panels as well as a gas tank. Was able to get the rear sets figure out using parts from an old Hyde T140 Rear set setup.

Hope to get the empty cases set into the chassis this weekend for further mockup as well as the rear wheel swapped out. The current LTA record for the M/PG 500/4 class is 124.169 in the mile and 122.98 in the mile in half. Not sure if A50 will achieve this but we will see as my plan is its debut is next summers 2023 LTA Maine Event.

Can you get away with no front brake?
In UK we have to have brakes on both wheels when riding on tarmac/concrete.
On my Rickman build I have fitted a "line lock" valve for when riding on Pendine sands. I can use the front brake normally to hold the bike when on the starting rollers, then before a run move the valve to the closed position so I can't grab a handful of front brake at high speed and plant my face in the sand.

Andy, front brake is not required I recall on motorcycles unless your over 175mph.

Be nice to have a BSA grab those records.

It will be fun to follow this thread! I am currently tinkering with a 744 build. PRT

Upgraded motor for the MPS bike??

Tough row to hoe Chris! As you are probably aware, I have achieved 125 mph on my highly tuned B50 motor at the Salt Flats (only 117mph at Loring back in 2011), but only as an "A" class specially designed bike with very low frontal area.

I have actually written a book, never published as it never seems to be finished, on "How to make a slow bike FAST", which resides only on my computer.

I'd be glad to offer you a look at it, but it would be a large file. Maybe there is a platform (dropbox?) that I could post it to if I knew how to do it as I don't think I'll ever get to the point of actually publishing.

Tom

tom, im interested too, although my BSAs are all cuurently only street legal

Tom, Im sure it will definitely be a challenge but really curious as to what a built A50 can achieve. On the plus side, once I get the bike built, I can always upgrade the CC's. Its fun piloting behind a fairing but trying to achieve some good speed numbers all out in the open is a fun challenge. I would definitely be interested in your book as well. Dropbox or even One Drive may work well. PM me.

Thanks

I will be keenly following this thread and I hope you can get the bike set up to break the current record.

I don't know too much about LSR bikes, but imagine that streamlining and tuning to get max engine power are the key factors to look at.

The A50 is a long-stroke engine 65,5 x 74 mm, so potentially scope to make it short stroke to allow higher revving?

Anyway good luck!

That's what race guys wanted from BSA was a Lightning bore with short stroke. It did not prevent BSA running them over 9,000. But it was very peaky. It could be 80xsomething or 75-76xsomething. So shortening the stroke would not be a bad idea. But either way the combustion chamber needs filling with something to get compression. It sounds fun whichever way you go.

Good question Chris. The 650 has not had a good chance to prove itself yet but if the 744 looks good it may find itself into a bike someday. Plan to use the street bike for testing. PRT

Gunner, streamlining and tuning are definitely important but I believe having a good wide power band over a peaky tune is better. I haven't really looked into the engine internal just yet to see if shortening the stroke and enlarging the pistons is an affordable option but this would allow for higher revs. For some reason or another I like using the underdog marque trying to bump a record, it will surely be interesting to see what I can achieve on an A50 setup.

Mark, Definitely will be filling the combustion chamber/ shaving the cylinder deck height to increase compression. I would think with the internals built properly, it should be able to handle 8k revs with the stock stroke. Will be an interesting build for sure.

Hi All,
I thought you All might like to have a look at this A50 racer that's coming up for auction in UK
It has some interesting features
https://www.bonhams.com/auction/27434/lot/602

John

They wasted so much money on those 500's it was just to win one race.
They had a good 650 that needed sorting and some redesign to make it really great
but all that development dough went to those slugs. The group was winning with
the triumph why waste money on a 500 beezer?

A7 pre-unit BSA 500 had 72.6mm stroke. Will these cranks fit the A50 motor and are they strong enough? Remember, horsepower is torque times rpm so higher rpm can produce more hp if the torque does not drop off faster than the rpm gain.

Tom

You may be better off with a norton 88 crank.

Chris Vincent won the TT using an A7 powered outfit so it was possible
to get them to go BUT he worked at the factory so we'll never know how
standard his crank was.

If you are 75x56.6 = 499. 80x49.7 = 499.68. The factory ran an A50 75mm bore short stroke to 9,250 with 57hp.

That would be fun to see what's possible now. It's easy enough to weld a head and tub it in. 30ci 150cfm race spec predicts 77hp 11,500, if that's possible. I wonder what it could be made to make at a bit lower rpm? 75 or 76mm bore would just use a shortened block. And short stroke crank. A T140 piston might suit tubbing the chamber a flat top would lose too much compression. Retard a std Lightning cam.

In my opinion, any reduction in stroke will be a bonus in land speed racing. Ed Valiket made a couple of long stroke B25 cranks by having them welded up and re-machined up from 70mm to 75mm, which when combined with my 92mm (bored) B50 resulted in a killer B50 motor with which I hold several records. You might check with Ed as well as old speed shops to see if anyone is still doing that. It's not uncommon to weld up crankshafts for refinishing them.

However, a reduction of 5 mm in the stroke in a A50 still leaves you way undersquare. But once you've done this, you could make a killer A65 motor!

Tom

thefairing mounts on this are interesting. the rear mounts are at the cylinder studs. while the fron mount is to the steering damper location rather than the headstock.





i cant remember the mounts for the factory machine.but ill look

added: look at that plate under the coils. heat sink for something? does this repro-racer have an alternator?

here it is







the factory mounts for the fairing were more conventionally affixed to the frame, both at the rear motor mounts and to the headstock.

if out and out speed is what youre after, th fairing is likely to be as important as the motor. or more, once youre up above 100 mph.

i run naked in order to be aesthetically simple, but i will never be as fast as the fibreglas guys.

Worst clutch cable route imaginable.

All good comments and yes, a short stoke would be great. If using an A65 top end @75mm bore, the stroke would need to be 56.5mm to achieve 500cc, would this be even possible? I did have the crank welded on my A65 racer to A10 specs achieve 750cc displacement, this resulted (with combination of cam) in a great torque band. If the torque numbers are high enough and dont start to taper off until around 6500 rpms but HP is still workable, is excessively revving the engine really needed? I've always thought that (with pushrod engines) if your not making the power by 6-7k, increasing the rpm's for a bit more isnt worth the risk of the internals escaping. I could be totally wrong as well.

Kevin, You have achieved some amazing speeds with your naked bike.....I had to add a similar type brace on my fairing at the top yoke after my first event with the faired bike just to keep the top side of the body from moving around with the air pressure.