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Rear frame loop out of shape

Submitted by stephen_whybrow on Sun, 16/02/2020
Forums
Commando

Hi there. 

I have just joined the NOC after the purchase of my 1971 750 Commando Fastback.

I have a question regarding how to fix a slightly out of shape rear frame loop.

When looking at the bike from the rear the numberplate and tail light were slightly off centre towards the right hand side. I thought I might just have been the fastback fitting or the wheel not put in correctly. After taking all the back end apart and taking some measurements the rear loop is off to the right by about 1cm. there seems to be sign of a crash The past owner who has had the bike for 20years said he did not crash it. There are no signs of any mental out of shape. could this have been a manufacture fault with the welding of the frame or maybe a fall off the kickstand, as this has been removed. 

 

Has anybody had this problem with their frame. If so how did you go about fixing the bend. I have a TIG and MIG welder so cutting it off putting and installing a slug back in the tubes for strength should not pose much of a problem. 

 

Cheers Steve

 

Submitted by katherine_scott on Mon, 17/02/2020

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Hi Steve 

Cutting and fitting slugs would seem a little over the top. I have straightened a couple of  rear loops that have drooped, they bend back without to much effort . I would suggest that you run your tape measure around a few frames  before you chop.  If you are convinced  you need to make adjustments maybe bolt or weld a jig to you frame that you can jack from or pull against.

Best regards 

Katherine Scott

Submitted by stephen_whybrow on Tue, 18/02/2020

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Thanks Katherine.

I have cut it off already. If the loop was drooped it would be a easy fix the bend back. Because it is a sideways bend it much easier to the cut off and replace. I did not want to put stress on the frame trying to pull it back. 

Will let you know the outcome.

 

Cheers

Submitted by philip_hannam on Wed, 03/06/2020

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For a short period of time the early 70s Commando frames were supplied from Italy. Probably on the grounds of cost. This was not the best of moves as the metal used in the manufacture of them turned out to be a little too soft and drooping rear ends became a well-known feature of these bikes.  

I do believe that for small deflections a cold bending back into position will suffice. For anything  larger I would be heating the metal to up to cherry red and then flexing my muscles.

Submitted by john_holmes on Wed, 03/06/2020

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The 850's at some point had gussets added to try to stop the droop, you can get the pieces from Andover Norton or just fabricate from steel sheet, they are just extended inner shock mounts.

Submitted by stan_thomas on Wed, 03/06/2020

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They bend when you crash. But normal wear 'n tear is just a droop.

You can buy a new rear loop for £55 (+VAT) : https://andover-norton.co.uk/en/shop-drawing/459/commando-850mk3-frame-repair-items

Or just weld the bit you cut off back on straight. 1" 16g ERW if you need to stick in a bit extra. The frame dimensions in the shop manual don't tell you the overall length but a saw cut or two isn't going to make any noticeable difference.

If you're not too concerned about originality, the late 850 suspension mounts are a useful reinforcement.

one I prepared earlier

 


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