As I have discussed on the damper pages, the standard rear springs are very short and stiff because they were only designed for the friction damped “B” series, so they make very little use of the available suspension travel.
The well-known Petteford springs are good, but at 189 lb/in they are rather stiff and have little or no pre-load.
Softer springs with more preload can make better use of the available suspension travel without increasing bottoming out, so they are more comfortable.
Fortuitously, Girling rear springs are of a diameter that is a good fit in the Vincent spring cases. New spring platforms are needed to replace the claws.
It is important to put a chamfer on the face that abuts the inside of the spring boxes, as the inside of the spring box end is not flat. Make these custom “platforms” as thin as practical, as the springs are on the long side.
A spring compressor will probably be needed to compress the springs so they can be fitted to the bike.
Eddy Grew has devised an easily made device to do the job, details here: - Spring Compressor
Obviously, partly sprung bikes can use softer springs than fully sprung bikes and bikes mainly used solo can use softer springs than bikes used for two up touring.
I note that PEI said that the rear springs carry 66% of the rider weight and 100% of the passenger weight. (This is for the standard semi-sprung arrangement of course.)
My testing has all been with a fully sprung bike mainly used solo.
I was surprised how well 100 lb/in Girling springs worked, and they may be just right for a solo rider with a semi-sprung seat. I had expected them to be much too soft, but with the good damping they worked very well, giving a very comfortable ride. I used them for several months. I eventually decided that they bottomed out a little too much. I could have reduced or cured this by increasing the preload, but I like a low saddle height so I decided to use stiffer springs.
I now use the Girling 132 or 145 lb/inch springs, which are just right. For the 132lb springs, these are Girling part numbers 64539963 (black) or 64543764 (chrome), paint code red/orange, free length 8.47 inch. For the 145lb springs, these are Girling part numbers 64543818 (black) or 64544234 (chrome) paint code blue/yellow, free length 8.19 inches. Both are made from 0.282 inch diameter wire.
Most of the many varieties of Girling springs available are too long for our purposes. I found a simple way to shorten springs that are only slightly overlong. Close up fully with some stout studding, apply hot air gun to one end only. Leave to cool while stress creep takes effect. This works surprising well, so well that I overdid the first one. Reversing the process was trickier and best avoided, so proceed with caution.
Note that replica Girling units often use springs of slightly larger diameter than the genuine Girling springs, and are unsuitable as they are likely to jam in the cases, as I found out.
If you can’t find Girling springs at reasonable prices, custom made springs are likely to cost around £50 a pair. This could be greatly reduced if you can find four other people to share an order for ten springs.
David Kitrosser in Massachusetts sent us some feedback that you might find useful:
Thanks for the work on dampers. Some feedback:
I am now using the AVO dampers Front (TA1447) and Rear (TA1446 ) and one each Girling 64539963 (132 lb/in) and 905463(?) (126 lb/in) springs.
Fully sprung seat. Little bit higher than I'd like but tolerable. Springs only shortened by 1/8 inch or so. (If shortened more, the damper doesn't go to full extension without the springs rattling loose?) Springs fit without compressing first with damper at full extension.
No change to mudguard positioning. Seat rear links adjusted so seat just clears mudguard at full compression. With my wife (125 lb) on the rear, me on the front (160 lb), appears to just bottom out on good size bumps according to an observer (watching gap between seat and mudguard) following behind. Bottoming out not at all noticeable to riders.
A quick comment on the seat supplied by the VOC Spares Company, which is the most comfortable seat I have used.
I would strongly recommend these seats for comfort, but don’t buy the long version unless it is really needed!
All of the extra length is added to the passenger’s part, and when the seat is fully sprung, it often collides with the rear lifting handle.
If you do have this problem, it can be remedied by reversing the lifting handle (FT22AS) and fitting it behind the mudguard hinge, which is how it is fitted on a Comet.
This also moves the chainguard bracket to the other side, so a new bracket will need to be welded in place. If you don’t wish to modify an original part, a thin steel strip can be wrapped around and clamped by the chainguard bolt.
After I had got the rear suspension working well, it was more apparent that the front forks had a rather harsh action, so I embarked on what became a rather lengthy attempt to understand and improve them.
Summary here: LINK TO FOLLOW!
If you wish to contact me: - Click here to email Rob Staley
Rob Staley, 12 November 2015