BSA Owners' Club Forum

General => The Star and Garter => Topic started by: Mike Farmer on 16 January, 2020, 12:22:08

Title: Off Piste
Post by: Mike Farmer on 16 January, 2020, 12:22:08
 :) :) :)

I know this ain't beesa but thought it might be of general interest.

I have just been told of a Sopwith ABC from about 1920 Evidently 400cc horizontally opposed across the frame as Douglas and Beemer. Is this a Douglas Dragonfly ancestor.

I was also asked-- what is/was so special about the Featherbed frame.

So if possible please enlighten me.

Mike 8) 8) 8)
Title: Re: Off Piste
Post by: DAVE BRADY on 16 January, 2020, 14:39:47
Mike,

Found an image of a Sopwith ABC.  Could easily be a forerunner of a Douglas.

I think that the Featherbed frame got its reputation for being the best handling frame at the time from use by Norton for racing.  At the time it seems that most frames were bolted together resulting in quite a degree of flexing.  Another plus was the size of the frame loop meant that almost any other engine could be installed from Triumphs, Vincents, Honda 750/4, Hillman Imp and many others.
I suspect that the BSA frame developed for the A10/7, B31/33 and Gold Stars was just as good being welded(?) and brazed.  Also the short and robust swinging arm of the BSA frame would have been a help. Usually the Roadholder forks on the Featherbed helped a lot as they were considered the best of that period.

Dave.
 
Title: Re: Off Piste
Post by: Mike Farmer on 16 January, 2020, 16:08:27
 :) :) :)

Thanks for that. Evidently designed by Mr Sopwith himself. He of WW1 aircraft such as Sopwith Camel fame.

Mike 8) 8) 8)
Title: Re: Off Piste
Post by: DEAN SOUTHALL on 16 January, 2020, 17:25:39
The Featherbed also has a wide looping top section, made up of two tubes rather than a single. I am sure this adds to the ridgidiy of the frame.
Title: Re: Off Piste
Post by: DAVE BRADY on 16 January, 2020, 17:44:42
The Sopwith Pup had 9 cylinder rotary engine - definitely would not fit in the Featherbed frame.  Designing a 2 cylinder engine must have been a piece of cake for Sopwith.  I wonder why they did not become more successful bearing in mind that BSA started off by sticking bits of tube on bits of wood as opposed to sheets of canvas on bits of wood.

The two tubes of the Norton frame were, I suppose, meant to give more rigidity but the single large and well braced tube of the BSA with its downward angle had a more direct connection to the swinging arm.  Many racing frames and more modern frames seem to have a geometry to get a direct line between the headstock and the swinging arm pivot.   

Any racers out there to comment?

Dave.
Title: Re: Off Piste
Post by: DEAN SOUTHALL on 16 January, 2020, 19:33:07
It was originally designed by the McCandless brothers after being commissioned by Norton to develop a frame specifically for racing. Obviously modern material and construction techniques result in superior frames but note how many racebikes and superbikes still have twin upper frame rails. I am no expert but I reckon ther McCandless lads got something right :)
Title: Re: Off Piste
Post by: royblackburn1@btinternet. on 16 January, 2020, 19:39:18
Mike I believe a road tester rode the Norton with the new frame and said it was like riding on a feather bed  Roy
Title: Re: Off Piste
Post by: DAVE BRADY on 16 January, 2020, 19:55:25
Dean,

Yes, and didn't McCandless develop the BSA swinging arm adaptation for racing Gold Stars using a plunger frame.  I do not know if this was a bolt on item as the back half of a plunger frame is or if it was welded/brazed on.  It follows that the McC brothers had involvement in the swinging arm A10 etc. frame so would they have designed or influenced an inferior frame?
The modern use of twin frame rails I believe is mainly prompted by the wish to connect the head stock to the swinging arm pivot.  If a DOHC, and therefore tall engine, is used the rails have to go around the engine.  Look at some of the frames on modern bikes of all sizes.  I believe that some, like Ducati, actually use the engine as the connection with everything hanging off it. 
I remember my first motorbike. It was a Honda 50 step through.  The pressed steel back end was connected to the headstock by a reasonable hefty tube and as you will all know the horizontal engine hung underneath.  The frame was quite rigid and the low engine gave a low centre of gravity.  This arrangement was only let down by non-existant damping. On smooth surfaced corners it was almost unbeatable by bike that should have been better.
Sorry for rambling.

Dave.
 
Title: Re: Off Piste
Post by: MGI on 16 January, 2020, 20:27:47
The Sopwiths originally employed a French Clerget 9 cylinder rotary engine, but towards the end of the conflict the W O Bentley (of subsequent Le Mans fame) designed a superior although similar engine which was then utilised, it was known as the BR1. Later, an advanced and larger engine was available, the BR2. In his early days, Bentley and several of his brothers all rode various motorcycles and covered astonishing long distance rides on them! He also raced in the Isle of Man races in 1909 and 1910. I believe they had Indians, Rex's, Quadrants and Triumphs. During World War 2, he used a Francis Barnet. He also raced at the Isle of Man post 1st world war with 2 other Bentleys gaining the team prize (on the same course as the bikes)

I had better get my coat!
Title: Re: Off Piste
Post by: DAVE BRADY on 16 January, 2020, 20:48:51
Going back to frames.

If the Featherbed was so good why was the Commando frame so different in design?  I know the engine was rubber mounted but the frames are so different.
Interestingly the Commando frame bears more resemblance to an A10/A65 frame than a Featherbed with the later A65 frame building on the idea of a big tube connecting the headstock and swinging arm.

Dave.
Title: Re: Off Piste
Post by: DAVE BRADY on 16 January, 2020, 20:50:39
This one should have been there as well instead of two red ones.



Dave