FF: Forks Modified Steering Stem

hadronuk

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A suggestion for anyone who finds the AVO damper is too stiff.
Check it has not been bent!!!!
Try rotating the damper rod to see if there is eccentricity.
If you think about it, there are 8 difference ways the bottom eyes can be fitted. So any inaccuracies can accumulate.
Heights of my eyes were about equal, but both had some offset, plus the holes in the bottom link were offset and oversize.
Inevitably, during many tests I unknowingly managed to get all the offsets going in the same direction and bent the damper rod.
My eyes are now stamped so they go back in the same place the same way round. They are also sleeved to centralise them in the holes in the bottom link. The offset in the eyes is actually useful to correct the offset in the bottom link holes!

PS. Please remember that although the AVO front dampers that I and TT specified are externally identical, they do NOT have the same damping characteristics. My understanding is that mine is softer.
 
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hadronuk

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At my request, AVO dyno tested one of the new IKON Vincent look alike dampers.
It was a lot stiffer than the AVO I specced for the front. In fact it was about as stiff as the AVO rear damper, though these are of course also adjustable.
 

vibrac

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I think the stiffness of the AVO damper was at the root of my problems with the comet I have a selection of dampers here standard vin modified vin my AVO (made some years ago from a car one used on our comet racer) and new AVO what I lack is some way of getting relative measurements off bike in compression and extension I could build a little rig with weights and a stop watch I suppose at the moment it's all a bit subjective
 

greg brillus

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You have the best test rig you need, straight outside your property. Put the softest shock absorber on you have and try it. If you like your Brampton's then you will be in for a pleasant surprise.
 

timetraveller

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Tim, the first thing I recommend to anyone who thinks the springs are too strong is to remove the damper and just try pumping the forks up and down by hand. Remember, if you do find that the springs are too strong then I can get some made which are even weaker. The problem will be to find out just how much weaker.
As more food for thought, one of our USA based friends is just setting up his 'D' twin with one of the new steering heads. He found that he could not move the forks with 36 lbs/inch springs and has removed half an inch from each spring and now the front end moves. That is giving 36 lbs less upwards force and that has made the difference. Although shortening springs means that their rate goes up, removing this small amount is making negligible difference to the rate but is reducing the spring pre-load. All those who bought the kit from me will find that there is the design for an adjustment in the inner spring box to allow the preload to be modified. Up to now, no one has expressed any interest in this and so I have not had any made. The design is there for anyone who likes to experiment.
 

vibrac

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That is my intention after I have thoroughly inspsected forks for sticktion, head races for damage, and wheel bearings for slop. All thats for the spring I have 3 jobs ahead of me first :confused: I also have a great piece of paper from Paul Ennis detailing all his mods on a Vincent damper which exceed those I have carried out before. I will do that anyway whatever I find in the experiments as I need a new front damper for the Flash and it needs to look standard
 

Gary Gittleson

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I am the "USA-based friend". The springs were actually over-sized when I got them. According to Mr. Timetraveller, they were supposed to be 16" but were actually 1/2 inch longer. That caused the forks to simply lock, at least with my 165 lb weight and no fuel tank or seat on the bike. I cut the springs down to 16" and all seems well. It's amazing how much difference that 1/2 inch reduction in preload made. The bike is fitted with a Thornton shock with the long eye bolts. The Thornton has some 7,000 miles on it and can easily be compressed by hand. As I mentioned to Norman, there's no chance I'll be trying the bike for a while. The weather hereabouts is too awful to discuss in polite society. Once I test it, I'll report the findings.
 

hadronuk

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As I have mentioned before, there is a DIY way to shorten a spring without cutting it.
As TT points out, chopping coils off will cause a presumably unwanted increase in spring rate, plus the cut end will not be correctly formed to sit square under load, even if ground accurately flat.

Method: Thread the spring onto some stout studding, then clamp it up solid with nuts and washers.
Heat a small area of the spring to the highest temperature you can achieve with a hot air gun.
Allow to completely cool (perhaps leave overnight), then unclamp and measure. If more shortening is required, repeat heating a different part of the spring. Only do a bit at a time, as over shortening is difficult to reverse.
Before doing this, it is highly recommended to count coils, calculate the solid length, then check the spring cannot become coil bound at full bump.
This may be a particular problem if you have replacement spring cases that have a much shorter spring space than they should have.
From memory, the original spring cases with an original Vincent damper fully extended will have a spring space (spring fitted length) of over 14 inches. Replacement cases can easily be 1/2" less than this.
 
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timetraveller

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I agree with everything Hadronuk has written above. My understanding is that the increase in spring rate is a linear function of shortening. For example, if you shorten a spring by 10% the rate will go up by 10%. If that is correct then taking a 36 lbs/inch spring down in length from 16" to 15" will result in a spring rate of
38.4 lbs/inch. As I have been providing springs of up to 45 lbs/inch for twins with heavier riders this figure of 38.4 is well within the range of spring strength experimented with. When trying to set up the forks with the JE modification there are two things one is trying to get correct. One is the pre-load and the other the spring rate. It is encouraging that Gary, by reducing the pre-load by 36 lbs, has managed to improve the angle of the lower link to where it should be and my hope is that the increased spring rate will not be noticable.
 

Gary Gittleson

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Since my springs were originally not 16 but 16 1/2 inches, should we conclude that the reverse calculation would be true? That is, were my springs weaker than the nominal 36 lbs due their excess length? If so, wouldn't they now be correct at 36 lbs? That is, assuming all other spring dimensions and materials are the same. I suppose I could rig something up to measure their rate, but they're on the bike now, so I'll test the pudding in the eating -- er -- riding.
 
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