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Seat Failure

Martyn Goodwin

Well Known and Active Forum Website User
VOC Member
Had an interesting experience today at the end of a 170 mile run. During the run I managed to break the decompression cable (that is another story) and when I got home in preparation for the cable replacement I went to remove the seat and tank.

I was askance to find that during the run the seat frame had broken. The two vertical legs at the front that the seat pivots on had moved from their normal vertical position to a horizontal position - the metal in each leg had "torn" allowing the seat to move rearwards about 1 inch - just enough to put a not so nice crease dent in the rear guard.

Only recently I moved to a fully sprung seat by installing the luggage carrier described in KTB and I wonder if this change was the root cause of the failure.

Would be very interested to learn if any others have had this happen to their seats, especially anyone using the luggage carrier as described in KTB, figure 21

Martyn
 

clevtrev

Well Known and Active Forum Website User
VOC Member
Had an interesting experience today at the end of a 170 mile run. During the run I managed to break the decompression cable (that is another story) and when I got home in preparation for the cable replacement I went to remove the seat and tank.

I was askance to find that during the run the seat frame had broken. The two vertical legs at the front that the seat pivots on had moved from their normal vertical position to a horizontal position - the metal in each leg had "torn" allowing the seat to move rearwards about 1 inch - just enough to put a not so nice crease dent in the rear guard.

Only recently I moved to a fully sprung seat by installing the luggage carrier described in KTB and I wonder if this change was the root cause of the failure.

Would be very interested to learn if any others have had this happen to their seats, especially anyone using the luggage carrier as described in KTB, figure 21

Martyn
By moving to a fully sprung arrangement you need to design the set up accordingly. If you look at the side view, you will see that the lug that bent, is now trying to get itself into the correct geometry. Maybe you can overcome it by putting a rail on either side from the bottom of that lug to the rear down tube. What you have done is to introduce a bending moment about the front pivot bolt. That`s what you have to overcome.
 

cinquecento

Well Known and Active Forum Website User
Non-VOC Member
little knowledge and not much understanding: but could the cause be that the seat bolt F35 and pivot bearing F39 were locked up either by seizing or being over tightened and therefore the necessary movement caused flex and fatigue fracture in 'verticle legs'.

or,

conversly, the damper drump had insufficient preload/damping set,allowing for front of seat to pivot beyond normal 'arc' of travel.

just a thought.
 

timetraveller

Well Known and Active Forum Website User
VOC Member
This type of failure became common 20 to 30 years ago. Recall that Vincents were producing most of our bikes between 1947 and 1954, the post second world war years. It is instructive to look at old films of people on the beach in those years. Nearly everyone has a stomach curved inwards and their ribs show through their skin. Rationing and lack of availability of many foods types means that post war people (remember that we were not occupied so the situation must have been much worse on some countries) were considerably lighter weight than most of us today. The series ‘D’s had been produced with fully sprung rear ends for more comfort and so many of us fully sprung the rear of our seats and the failures began. Recall also that there are two lengths of these particular stays. The solution adopted by most of us was to weld in a triangular gusset to the rear of these brackets to spread the load over a larger area and to reduce point loadings. There is a limit as to how far rearwards these gussets can extend because of the sliders for the tool box. If one does not want to go too far with changing things then this is probably the easiest and cheapest way to get round this problem.
However, Ian Savage of Spares Company fame made a very nice and almost invisible mod years ago. I hope that Ian will not mind me mentioning that he is robustly built as is Jenny, the lady in his life, and so his modification had to take a serious amount of force. I do not have photographs of the mod but it consists of a frame, roughly the same size and shape as the rear of a B/C seat base. It can contain a larger tool box than standard and the seat pivots from one side. It takes all the tensional forces of the seat trying to pivot rearwards and relieves the brackets in question of the forces which lead to their failure. It is a very neat mod and possibly Ian could post either a photograph or drawing here.
Within the context of ‘D’ rear ends and also with Laplander seeking advice on what model of Vincent to purchase it is worth noting the following. The ‘D’s were developed after consultation with members of the VOC and other riders and it was Vincents attempt to make the bikes better. Both spark plugs on the same side of the bike and both carb also on the same side of the bike for easier maintenance, coil ignition for easier starting, a fully sprung rear end for more comfort, a larger tool box and oil tank sited for easier removal and cleaning, different springs at the front end for more comfort and so on. Lack of finances and difficulties with post war supplies of material bedevilled the ‘D’s reputation and yet after all this time quality control problems have been long overcome. I have no vested interest in this as I have both a ‘C’ and a ‘D’ but it is the attempts of owners to make the rear end of ‘B’s and ‘C’s as comfortable as ‘D’s which leads to the failure of these brackets.
 

Vincent Brake

Well Known and Active Forum Website User
VOC Member
mine did as well, not that it would pivot, i am just to heavy with my 74 kg,
\so i am sliming down/
now making them myself with thicker material, for one change, and bit new design, so i can hook up some cases on the saddle frame.
hell of a job though.I better do an engine up.

Vincent S
 

clevtrev

Well Known and Active Forum Website User
VOC Member
little knowledge and not much understanding: but could the cause be that the seat bolt F35 and pivot bearing F39 were locked up either by seizing or being over tightened and therefore the necessary movement caused flex and fatigue fracture in 'verticle legs'.

or,

conversly, the damper drump had insufficient preload/damping set,allowing for front of seat to pivot beyond normal 'arc' of travel.

just a thought.
When you go fully suspended, the whole frame needs to be solid. There is no pivoting involved.
 

A-BCD

Well Known and Active Forum Website User
VOC Member
The Eddie Stevens luggage rack seems to support the seat fully with no bending moments at all. I made up some brackets so the original appearance and geometry were maintained, didn't have a problem. Really depends how old the seat is !!
TEV1402005_zps72e58e85.jpg
 

Peter Holmes

Well Known and Active Forum Website User
VOC Member
By moving to a fully sprung arrangement you need to design the set up accordingly. If you look at the side view, you will see that the lug that bent, is now trying to get itself into the correct geometry. Maybe you can overcome it by putting a rail on either side from the bottom of that lug to the rear down tube. What you have done is to introduce a bending moment about the front pivot bolt. That`s what you have to overcome.

I do not comprehend what Trevor means when he says "trying to get itself into correct geometry" it seems to me that if you go fully sprung by mounting the seat rear frame brackets onto long tubes going down to the footrest plates, then the seat supports are at an acute angle and when you sit on the seat you will create a rearward force as the seat support, at the acute angle tries to collapse to an even more acute angle as the weight is being brought to bear onto the seat, in effect the force tries, and sometimes succeeds in tearing the welds or the metal on the seat front bracket, as Timetraveller points out, the heavier the weight carried the worse the problem, especially under fierce acceleration with a heavy pillion passenger.
However if you go fully sprung by using a custom made sledge, then the seat rear frame brackets should be fixed at 90 degrees vertical to a flat horizontal surface and no forces will be introduced, of course if the seat is badly manufactured or the plywood rotten then the seat could fail however it is mounted. A few folks have addressed this problem by making the entire seat base out of Duralumin and beefing up the seat frame.
 
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