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rear frame alignment

In others experience of m20 rigid rear sections, is it common for there to be slight miss alignments? The tubing seems quite flexible.

Checking some of the alignments on my rear section reveals the drive side rear lower frame rail is not quite parallel with the other side.
also when the axle is inserted and tightened on the drive side, it points slightly upward (see exagerrated ilustrations)
Once the wheel is fully inserted and tightened, the parts all become force aligned (based on a visual inspection)
I am wondering weather it is worth attempting to bend the thing straighter, or just live with it.
Also I have no brake anchor, I suppose this coud be contributing


(pictures are exagerrated)

email (option): hutman80@hotmail.com

Re: rear frame alignment

Misaligned frames due to damage are fairly common.

The rear section on its own is "floppy" and often gets bent in storage.

The lack of parallelism of lower tubes (chain stays)indicated in your pic indicates incorrect spacing at bottom studs.

Lack of parallelism of lower tubes (chain stays) aft of the saddle post is not guaranteed, nor relative to correct alignment.

There are dimensioned BSA drawings of the frame in the BSA Service Sheets, copies of which which are available on Henk's excellent CD-ROM in For Sale section of this site.

A properly aligned and assembled BSA is fairly rigid structure and a joy to ride.

The missing brake stay is part of the structure and so will be definitely contributing.

The 15-5816 rear brake stay is 7/32" thick, to check your frame, suitable packing can be temporarily substituted.

The 15-5816 should be installed during initial frame assembly (when everything is "floppy"), or difficulty may be encountered and a damaged stud may result.

Check you also have the correct length left and right spacers fitted at this 3/8" stud point.

66-4601 Chainstay bridge distance collar, 3/8” clearance bore x 1.595” oal, waisted, right side, (shortest of the 3 x 3/8” bore frame spacers) common to all 1937-1960 500cc & 600cc rigid frame singles 1 per machine, black enamel for civilian.

66-4602 Chainstay bridge distance collar, 3/8” clearance bore x 1.763” oal, waisted, left side, brake stay is fitted between this spacer and left side frame lug, combined extra length of both makes up for offset of rear frame which accommodates chain & brake etc, common to all 1937-1960 500cc & 600cc rigid frame singles. 1 per machine, black enamel.

15-6240 Chainstay bridge distance collar, 3/8” clearance bore x 2.500”, waisted, fits between rear engine/gearbox plates, common to all 1937-1960 500cc & 600cc rigid frame singles 1 per machine, black enamel for civilian.

Also check length of right side 1/2" bore spacer (at bottom of saddle tube)

66-4581 Chainstay bridge distance piece, 0.5” clearance bore x 1.56” oal, not waisted, 1940-1960 inc WD 500cc & 600cc rigid frame singles 1 per machine, black enamel for civilian.

66-4600 Chainstay bridge distance collar, 0.5” clearance bore x 1.1” oal, right side OHV 1937-1939 is waisted and shorter than 1940-1960, 1 per machine, black enamel for civilian.

Install gearbox and check alignment of rear chain sprockets.

If general alignment or twist at rear persists after checking/correcting all above, have your frame re-aligned by a competent person, preferrably with a pair of crankcases, a gearbox with sprocket firmly bolted in-situ, and a correctly laced and true rear wheel supplied loose. (so both competent person and you know that everything is fixed)

Sort this out now or you may have subsequent issues with rear mudguard clearance and premature chain and sprocket wear, plus the bike will be a pig to ride (may even fail MOT), none of which you will enjoy.

"First, do no harm"

Neale

email (option): n.gentner@bigpond.com

Re: rear frame alignment

Neale
Thanks!! I was about to post a question about all those damn spacers as I have been having trouble getting the gearbox and rear wheel sprocket to line up and am not sure where the problem lies due to being a prat and not noting which spacer goes where when I took it all apart. Your info is a great help!
cheers
Pete

email (option): petercomley@web.de

Re: rear frame alignment

BSA M20 frame

email (option): ahum@quicknet.nl

Re: rear frame alignment

Thank You for the detailed and informative responses. and I will go and take all of those measurements now. That is great information

email (option): hutman80@hotmail.com

Re: rear frame alignment

Oh what fun and games, will I ever progress with this bike?

I'll have to have a word with Robb Nortier, I think he has his left and right mixed up. Not a problem as I don't know mine either!

Assuming some kind of symmetry, the Rear brake anchor plate strap. 15-5816 is 7/32" (0.2187") thick. (6mm S275 will be about 5.8mm or 0.228" once cleaned)
If the left hand side spacer 66-4801 is 1.6" wide and the right hand spacer 66-4602 is 1.8" the numbers add up better?

Lets change the numbers? 1.590" and 1.810" that gives us 0.220" for the strap.

Would this make sense, or have I missed something?

BTW, does anyone have the hole centers for the Rear brake anchor plate strap and it's width?

email (option): pes.sales@btconnect.com

Re: rear frame alignment

Please ignore the above post, I've been reading to many contradictory posts :(

email (option): pes.sales@btconnect.com

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