FF: Forks John Emmanuel Fork Shock Absorber Length

Vincent Brake

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Hi Norman and others.
I finally found time to do me a revision on a girdraulic and did the alu pivot in with conical bearings. Bottom one with a brass "nylos" ring 1,5 extra mm.

Anyway i am wondering what the streched length on the damper would be.??

As per picture it looks an inch to short.
This is with Works P. At lenght of
254 mm ( with botom pivots FF57 at 27mm) i know the supplied damper is at 260 mm.

Btw. Hail to mr Graham S. Whom made this possible just by an Android P.
Btw2 maybe its in the pool old thread. But thers 1600 posts to look up....
IMG_20191129_165856.jpg
 
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timetraveller

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Hi Vincent, I've just been out to the shed and measured a new AVO damper, top hole centre to bottom hole centre and I make that 252.3 mm, fully extended. There are two continuing mysteries with this conversion. The first is that on some bikes the front wheel can move far enough rearwards to hit the front exhaust pipe. This only happens if the handlebars are moved to the left and a very large bump is hit. On other bikes this does not happen. The second mystery is that on some bikes the AVO damper fouls the inside rear of the top link and the link has to be relived. On my own bike the maximum depth removed was 5 to 6 mm. On other bikes this does not happen. I cannot believe that either top or bottom links can differ in their for and aft centre distance to cause this and so far no one has found an explanation. Neither stops everything working well once the system is installed.
From your photograph and the text it seems that you have put a 1.5 mm spacer in under the bottom bearing. but you also show a Works damper. I don't know their centre to centre distance but unless it is about the same as the AVO, 252 mm, there might be some problems. Although John Emmanuel designed the new system I spent a lot of time with a Vincent front end off a bike and held on a work bench to work out the wheel spindle movement and over what range of total movement the system would work properly. With the original length over the steering head lug it is important that the lower link is about horizontal when the bike is fully loaded with the rider etc. If the front of the lower link is up to about 3 -4 mm lower then that will not be a problem. If it is, say, 8 - 10 mm lower, or more, than the rear then the wheel will move forwards at the start of its upwards movement, rather than upwards and rearwards, and so defeat what John was trying to achieve. If you start off with the front of the lower spindle too high then there will be no problem with the geometry but it will cut down the total movement available.

The extent of the downwards movement and position of the lower link front is controlled by the combination of damper length of eyebolt length. You can check this before the springs are fitted.

I designed the combination of spring strengths, damper lengths etc to try to give maximum movement and thus maximum comfort at the front end. You will have to experiment with the system to discover whether a 1.5 mm spacer makes any difference.
Also, just to clarify, the system is designed to work with the long lower eyebolts, as used with the Armstrong dampers, not the earlier short ones which work with the original Vincent dampers.
 
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Vincent Brake

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The wheel hasnt been fitted yet and forks is at fully bottom down. Ie. Fully extended.

I can think you see it as if it was on wheels.

So a empty tavel of 20 mm ( only with motor weight on wheels)
I need to lengthen the bottom pivots about 15-20 mm longer.
What do you think Norm??


Peter indeed its for grease nipples.
And use of needle bearings.

IMG_20191102_140056.jpg
 

timetraveller

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*Timetraveller has noticed some errors in this post. He has corrected them in post #13. Ed
You need to be aiming for a total travel of the damper of about 75 mm. This is the same on the spring boxes. The problem with the AVO dampers is that they have a bump stop inside and it is impossible to get the full travel by hand as the bump stop is so strong. I had to use a sash clamp to convince myself that the full 75 mm was possible. So starting with the lower link horizontal, or just a bit low on the front, no springs, wheels or anything else get the right combination of damper length and eyebolt length. That is the starting point for the 'at rest' position. From your picture it looks as though you have it about right. That is although the picture is taken from an angle the bottom link looks to be about the right angle.
Interesting modification to use needle rollers rather than the ball races. Are they going to run on the stainless steel concentrics or is there an inner bearing surface to go on the recesses machined into the concentrics?
 
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Chris Launders

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Norman. If you remember I have needle rollers in mine, I used an inner race to go with them and made a sleeve for the bolt and new side plates. I can't remember the exact dimensions at the moment, I would have to take it apart to see.
 

oexing

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I can see a standard inner race for the needle bearing. Myself not concerned with girdraulics I would not go for ball or needle bearings at this place as there is no point aiming for nil friction here. You can get problems with rust by water entering the bearings . So definitely my choice is permaglide or IGUS plastic bushes for Bramptons and same for girdraulics. Needle bearings are a strong type and to be preferred over the tiny twin ball bearings but even so it is no easy job to get a perfect fit . A bit of friction in there does not matter as you want to have damping in the hydraulic damper anyway, so a small add-on from the bushes is no factor.

Vic
 

timetraveller

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Yes Chris, I do remember very well your use of needle roller bearings, My memory is that they were two millimetres wider than the eyes in the rear of the lower link, that is about 16 mm wide whereas normally I supply two 7 mm wide ball races for each side, a modification developed by Greg Brillus in Australia. What intrigued me about Vincent's mod was that, if I understand the photograph properly, he has machined the diameter of the concentrics but left the part which fits into the steering head lower motion block the original diameter. I was wondering how he was going to fit it all together but Vincent is a proper engineer so I am sure it will be well thought out.
Regarding Vic's comments; I do provide sealed for life ball races so the wearing surfaces are unlikely to go rusty. In addition when I fit these I put thick grease at each side of the bearings in order to further prevent water entry. Vic's comment regarding friction were not born out by the tests you did. When you changed from Oilite bushes to needle rollers the friction reduced so much that the spring strengths had to be increased from 36 lbs/inch to 45 lbs/inch and the AVO damper, even on its maximum setting, was not stiff enough and I had to get AVO to produce specially stiff dampers
 
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Vincent Brake

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Norm, Vic, et all,
When i looked up the static load on the thinnsection bearings.
I got scared, and figured me a way out of it.
I have seen a bump or two in my life. Not only the ones of the wife.....

By ina 33 x 20 x wide16 or so i got about 10 times the load factor, and more over if they damage. You know yes, over those bumps, i still can carry on as where a skinnier ones would brake.
Side plates are skf ones thrusting one hardend stainless.
As are all spindles.
1600HV.
By chrome diffusing in process.
We will see the life span.
My old forks went up to 50T km.
And had steel hardend spindles.

Btw.
I made a new bottom part for the works P damper a bit longer so now the lower front pivot is about 17 mm lower as rear.
At fully extended damper.
Hoping it to be with me on the bump ride to be level.

Cheers
 

macvette

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Non-VOC Member
The wheel hasnt been fitted yet and forks is at fully bottom down. Ie. Fully extended.

I can think you see it as if it was on wheels.

So a empty tavel of 20 mm ( only with motor weight on wheels)
I need to lengthen the bottom pivots about 15-20 mm longer.
What do you think Norm??


Peter indeed its for grease nipples.
And use of needle bearings.

View attachment 30703
Vincent damper, long eyebolts, front wheel off ground so damper fully extended. Centre to centre of damper pivots 270 mm.
Mac
 
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