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FF: Forks Modified Steering Stem


Chris Launders

Well Known and Active Website User
VOC Member
The damper could well be the problem, my twin with the 45s is usable with NO damper, the damper just being used to do it's proper job, just damping out excessive movement. The damper being used may be too stiff for such a light bike/rider giving the impression the springs are too stiff. Ask him to at least try it without, carefully of course.
Chris.
 

Bill Thomas

Well Known and Active Website User
VOC Member
Ron put my Air Suspension, Off an H.D. that I had on the back of my Special, "D" type, On the front of his "A" Twin Girders, Worked well. Cheers Bill.
 

timetraveller

Well Known and Active Website User
VOC Member
Chris, the chap in Italy is using a Vincent damper. I doubt that will be too stiff and he has tried it without the damper. I have now heard from the chap in Italy as to his progress. Here is what he has written;

I have several old Ducatis and while hunting around for a spare part the other day I came across a pair of fork springs that looked vaguely similar to the Vincent ones. Presumably from a Marzocchi, they have an od of 1 inch and are made of 4mm steel just like your springs. The gap between coils is a bit smaller and decreases a little half way down, so they are definitely softer than the 30lb/inch springs, and about 10 cms longer. Anyway, I decide to try cutting one down bit by bit, starting at the 'soft' end, and when that showed interesting results, I started on the other one from the 'hard' end. So, to cut a long story short (!), I ended up with a 40cm spring on one side and a 43cm spring on the other, producing a more or less horizontal lower link when I sit on the bike, no topping out (with the long lower eyebolts now fitted) and a spring rate that feels pretty good. It's certainly softer than with the two full length 30lb/inch springs fitted or with the one shortened one, but with no danger of bottoming out. On the other hand, the springing is a bit more than the Vincent damper can cope with, so for now I've fitted my Armstrong, which has better rebound damping.

You will see that he is starting off with multi rates springs so that is not useful for the rest of us. I have considered multi rate springs for the JE mod but am not sure the company making my springs can do that.
 

Chris Launders

Well Known and Active Website User
VOC Member
Thanks for that, multi rate springs are only made by winding sections of the spring at different pitches, the closer together the softer. The firm I used in Sheffield copied some Progressive springs for me many years ago which originally varied almost continuously their length but they made them with three different pitch sections as that was far easier, they performed just as well.
Chris.
 

oexing

Well Known and Active Website User
VOC Member
These days I am on the track for pneumatic shockers, like from Fournales. What I like is the complete adjustment for loads via air pressure that you set with a pump. Second, they say that the air volume inside gives a progressive damping towards full bump so no hard bottoming to be expected. Another plus is the small diameter of 50 or 60 mm , so no clash with top link on the Girdraulic. The company that has its roots in aviation struts seems to be prepared for special types - as in Vincent ?? Not quite so nice is the price, but then there is a lot of sophistication inside and overhauls any day. My idea is to hang one of them onto a Brampton.

Vic
Link to PDF
Another PDF
Concept Fournales
 

Flo

Website User
VOC Member
Having Worked on a number of air pressure base suspension and having lookes at the Fournales strut some 20 years ago, I'd be vera interested to hear about your results.

Frankly I am not entirely convinced of the sealed cylinder concept of air suspension.
 

Attachments

oexing

Well Known and Active Website User
VOC Member
Hi Flo,
would you care to give some more details as to what you did not like on pneumatics ? These are standard for many decades in aviation but , allright, aircraft don´t do a lot of mileage on the ground. Even so, the Fournales are in production for long time so should be tested. Comments are , as far as I found, very comfortable on common roads, not so much for motocross.

Vic
 

Bill Thomas

Well Known and Active Website User
VOC Member
I just Googled, Air Suspension for Harley Davidson, And there is lots there.
I used just one side of an H.D. rear, On the rear of my "D" Special late 1970 s, I think it was S&S.
I was very pleased, It stopped the rear wheel Hopping off the tarmac, When Braking hard for the hairpin at the race track, I also used it on the road.
Cheers Bill.
 

Chris Launders

Well Known and Active Website User
VOC Member
I've have a pair on my Harley Softail that were on it when I got it, 20 years ago, I think they have a pair of bladders inside not relying on seals. There's some serious pressures involved, 375 lb in the compression side and 175 lb in the rebound.
 

greg brillus

Well Known and Active Website User
VOC Member
Not only friction but the geometry of it all. Like I have said before, I have never been able to install any of the springs without them being hugely "Topped out" this means that you have to shorten the springs, or at least one side. That last Comet I did, after i installed the 33 or 36 Lb springs with the front shocker left off, it would readily bottom out, well the outer spring case would hit the front of the lower fork lug. I did trim some off the inner and outer spring cases, the inners were like a knife edge at the top anyway. With anything but the softest action shock absorber on there, the front end felt heavy and stiff. It was a juggling act between springs and shock absorbers, ending with a pair of 45 Lb springs with about 25 mm chopped off both, and an original Armstrong shock absorber used. The Thornton I had (brand new) worked very well, but I would not part with it, the second hand Koni was probably a bit stiff (heavy) even on its weakest setting, and the AVO was way too stiff..............Anyone with an AVO on the front of their Comet is going to have a very stiff action front end no mater what springs they use, the bike simply does not have enough weight to overcome the set up of that shock absorber. I have used the stock Vincent shock abosrber, (I think this was one of the new ones available from the Voc spares co) at full extension it's action was very stiff, almost felt like it was locked up, until the suspension compressed with a rider on, and then it felt ok. It is possible with that design that at full extension the upper bushes in the shock internals are binding as the piston is trying to overcome the rod bushes friction, and this gets better if the shocker starts to move further down its travel. I would recommend he use another type of shocker. He needs to back off the spring pre-load, cut the springs down so the suspension is not topped out.......................This goes for all of you trying to install this setup........................... Done about 30 or so of these now...................
 

hadronuk

Well Known and Active Website User
VOC Member
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

Well Known and Active Website User
VOC Member
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

Well Known and Active Website User
VOC Member
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

Well Known and Active Website User
VOC Member
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

Well Known and Active Website User
VOC Member
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

Well Known and Active Website User
VOC Member
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

Active Website User
VOC Member
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

Well Known and Active Website User
VOC Member
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

Well Known and Active Website User
VOC Member
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

Active Website User
VOC Member
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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