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FF: Forks Modified Steering Stem



greg brillus

Well Known and Active Website User
VOC Member
The easiest way to sort out between springs and shock absorber is once you have it all back together, just remove the front shocker and ride the bike at slowish speeds and hit the front brakes, and see how the front behaves. With the weaker springs you will bottom out quite easily.......Remembering that I have cut most all the springs I have installed, as without doing so the front end is topped out. This seemed more obvious to me when carrying out the entire mod on the Singles as they are much lighter than the twins........The action of the forks seemed too stiff for a bike 30 Kg's different in weight.
 

bmetcalf

Well Known and Active Website User
VOC Member
OK Thanks Bruce. I looked through my records but you must have had your springs quite some time as you are not in the latest spread sheet. If the springs were shiny then they are stainless steel and are the 36 lbs/inch ones. If dark steel then probably 45s which some people are finding too stiff. Let us have the feedback when you have had chance to test it out.
Our last PM was in April 2017 and the springs are the heavier ones, but I weigh 240#, so maybe OK.
 

greg brillus

Well Known and Active Website User
VOC Member
What springs are you using Chris.....?......I have never had to use packers on any springs, in fact I have always had to cut them down to stop the front end "Topping out"..........Cheers........Greg.
 

chrislaun

Well Known and Active Website User
VOC Member
Hi Greg. I'm using 45lb springs, but as I said I weigh 280lb, I've not noticed it topping out but with the roads round here I need as much travel as I can manage, If I took the packers out I'd lose nearly 1-1/4" of travel, possibly more now I've done a few thousand miles and that would be nearly half.
I'm still getting down to touching the bump stop.
 

greg brillus

Well Known and Active Website User
VOC Member
Chris you might find the 45's might still be a bit light on. I'm sure on a couple of bikes I've done I used one of the Red davidd springs on one side and a weaker one on the other. My findings are that you need not too much pre-load, say about one inch per side using springs that suit the weight combination of the bike and rider. The twins will handle most all of the shock absorbers, but the singles need a very soft shocker or else it will feel too stiff in the front even with weak springs installed, as the shocker "masks" the springs.
 

chrislaun

Well Known and Active Website User
VOC Member
Hi Greg.
I had occasion to ride it 60 miles the other week without any damper, the ride was not noticeably different, I will admit it was a bit livelier and was possibly going a little too far but nothing over the top, so I'm happy with it the way it is, I normally have the bearing conversion AVO damper fitted, set at minimum.
I've forgotten how many spring combinations I used when arriving at the 45s but Norman will tell you it was a considerable number.
 

timetraveller

Well Known and Active Website User
VOC Member
Here we are again. I've just received an order for another 20 of these items, some steel and some aluminium. If you want to get in on the back of this order then now is a good time to contact me, enw07@btinternet.com . To recap, the original production was 10, followed by 15, then 10, then 30, then 50 and most recently another 10. That makes 125 out there somewhere. There are options for using either the original Oilite bushes in the lower link or using the Greg Brillus mod of fitting ball races as replacement for the bushes. If you want to learn more then either read some or all of this link or email me for the fitting instructions and they will be sent to you as a PDF. Thank you.
 

macvette

Well Known and Active Website User
VOC Member
Here we are again. I've just received an order for another 20 of these items, some steel and some aluminium. If you want to get in on the back of this order then now is a good time to contact me, enw07@btinternet.com . To recap, the original production was 10, followed by 15, then 10, then 30, then 50 and most recently another 10. That makes 125 out there somewhere. There are options for using either the original Oilite bushes in the lower link or using the Greg Brillus mod of fitting ball races as replacement for the bushes. If you want to learn more then either read some or all of this link or email me for the fitting instructions and they will be sent to you as a PDF. Thank you.
I was reading a book on Broughs and came across this snippet so it seems that brakes locking Girdraulics when applied is not unique. In a chapter 1927-29 20190109_181854.jpg 20190109_181808.jpg
 

everiman

Active Website User
VOC Member
Back to the 40 Lb springs.......We decided to use these and chop them down in increments and see what happens, first we cut 25 mm off and tried this, but it felt too heavy, so we chopped another 25 mm again, and this felt quite good. .
If you are happy, we are all happy for you, but cutting down springs does not make softer, quite the opposite, it makes them stiffer.
 

timetraveller

Well Known and Active Website User
VOC Member
As an example of this I am in the middle of communicating with a rider preparing a Comet for racing. It is very light. I supplied him with 30 lbs/inch springs, the lightest I have had made up to now, and these are still too stiff. As part of the experimenting he left one spring as standard and then progressively chopped 20 mm lengths off the other spring, four times. If I have done the sums correctly that is now a 37 lbs/inch spring and of course it is still too stiff. Alternatives being considered are either shorter 30 lbs/inch springs to give less preload or 27lbs/inch springs, the standard length so that if required they can be shortened and still not exceed the 30 lbs/inch rate. I will report back on what we find and no doubt Vibrac will be interested as he found the 30 lbs/inch springs too strong on his single. Note that at £30 a pair these springs are the lowest cost part of this modification.
 

chrislaun

Well Known and Active Website User
VOC Member
I know it's not best practice Norman but given how strong the Girdraulics are couldn't he experiment with only one spring, starting with a standard D spring then a C spring then a 45lb spring etc also possibly a C inner to add more variations.
Chris.
 

vibrac

Well Known and Active Website User
VOC Member
Yes I am still interested .it's not often I am defeated with a Vincent but the lack of movement and the almost imperceptible wander at low speeds compared to the standard Flash on the same test road countered by the glowing reports of other successful implementations meant that I put my comet 2020 project on hold till warmer weather I intend to go through the fork to check for friction before experimentation on the road one thing is pretty clear that AVO unit is too stiff which in itself is a puzzle
 

timetraveller

Well Known and Active Website User
VOC Member
First a reply to Chris. One of the first things I suggested was to remove one spring and see what happened with one 30 lbs/inch spring. I was worried about damper and fork bearing friction. See below.

Greg; my first thought was that either the damper or the fork spindles were not set up properly so I suggested that he removed the wheel, damper and spring boxes to make sure that the forks dropped readily under just gravity. They did and he sent me a video to show the tests. He then sent me some more measurements of damper and spring box movement and I became suspicious that he was not using the long eyebolts. He was not and he then replace those with the long ones. He is using a standard Vincent damper, which has a slightly different length to an AVO but only by a few millimetres and at that stage I was trying to ensure that the angle of the lower link was correct, not just the spring rate. Perhaps I should say that the chap is in Italy so I cannot just pop round and have a look for myself. At the moment he is back to trying something else and will let me know what happens.

One thing that occurred to me during our correspondence when he was weighing the bike's front end was that it is possible that one should remove the value of the unsprung weight before doing the sums for the springs. For example the bike weighs 73 kg at the front and 105 with him on board. Later today I am going to weigh a wheel and stripped down front forks to see exactly what they weigh but is going to be a significant fraction of those amounts. So the springs are having to support the weight of the bike plus rider minus the unsprung weight. The damper is having to control the inertia of all of that.

Vibrac also had problems with the 30 lbs/inch springs on his latest creation for road use, hence my comment that he should find the results of the Italian tests useful.
Thanks for all ideas and feedback.
 

timetraveller

Well Known and Active Website User
VOC Member
Ok. I have now weighed the forks. To make it clear these are lightened Grey Flash blades, upper link, a wheel, tyre and two disc brakes so no callipers etc. The weight is 19 kg, lets call that 20 and if there was a mudguard, stays, headlamp etc that would probably go up to 21-22. Removing 20 from the figures given above for the owner in Italy gives 53 kg bike only and 85 kg with him on it. 85 kg is 187 lbs so as there are two springs then 30 lbs/inch springs would need just over three inches of preload to compensate for that. That is proving too much for both Vibrac and our friend in Italy so what am I missing?
 

vibrac

Well Known and Active Website User
VOC Member
Got to be friction thats where I shall start
A stray thought occurred to me I am slowly (very slowly) rebuilding a Difazio hub center steering BMW and that had two pnematic suspension units on the front swinging arm (they are completely gone) how about two pnematic struts inside (or forming) the spring boxes and a valve at the top pump up to the required pressure adjust to taste
Not one for me I have too much on the list already ;)
 


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