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FF: Forks New Coilover for Girdraulics



vibrac

Well Known and Active Website User
VOC Member
#22
Well I took the picture and the USA is where I got it from it could be made by you I seem to remember being told it was the last one
 

hadronuk

Well Known and Active Website User
VOC Member
#23
At this point, I would greatly appreciate all views on spring rates for the a Girdraulic coilover and also suitable preload.
CAD design work is now in progress, and it seems the greatest problem is fitting the spring in the available space.
It seems certain the spring will have to be specially made and will need to be high quality.
A spring with a fitted length of only 6" that will be compressed by 3" + preload is going to be very highly stressed.
So I would really like to get the specification as near right as possible first time, as several iterations will be prohibitively expensive for what is likely to be low volume sales. My initial suggestion is 150 lbs/inch, but there is time to change this.
Above there are several discussions of spring rates without it being clear whether this applicable to the Girdraulic or Brampton forks.
I am happy to be corrected, but with the differing geometries, I question how applicable Brampton rates are to Girdraulics?
Thoughts appreciated.
 

vibrac

Well Known and Active Website User
VOC Member
#24
No, I dont think the 180lb Brampton strength is much good to you
We know the range of spring strengths on the existing girdraulics so would some clever vector diagrams be able to transfer those characteristics to a central position? (dont ask me)
 

timetraveller

Well Known and Active Website User
VOC Member
#25
Here are some thought on the subject. For the JE modification I did lots of tests before coming up with the final specification for springs etc. The test riding of Chris Launders was a great help with this. At one stage I had a distance scale on an empty AVO damper and another on one of the spring boxes to determine whether there was a ratio in their movements. If there is one then it was too small for me to determine and my opinion is that the there is a one to one ratio between spring box compression and the damper compression.
Chris weighs 20 stone, 280 lbs, 127 kgs and with 45 lbs/inch springs with 2" of preload required about half an inch of extra packing on the springs. I think that give 225 lbs of preload for that rider. We also had to go for stiffer dampers than the standard dampers as soon as we replaced the Oilite bushes with bearings (needle rollers in Chris' case). I have been supplying the 45 lbs/inch springs to all twin riders and it has become clear that for lighter riders they are too strong and it is slowly becoming clear that for normal use on European roads, even two up with luggage, a twin is better off with the 36 lbs/inch springs with 3" of preload and a standard AVO damper. I make that 216 lbs of pre-load but with an easier movement for tackling bumps. Note that Greg in Australia is finding that they like 45 lbs/inch springs even on Comets and I have no explanation as to why this is. Riders over here certainly find that too stiff. For Comets I supply 33 lbs/inch springs with 3" of pre-load (198 lbs pre-load) but some people are also wanting to try 30 lbs/inch springs on Comets, 180 lbs of pre-load. Note that there are two things here, One is the pre-load to try and get the starting angle of the lower link correct and then the ease at which the front end can compress to handle bumps.
I did design a method for building pre-load adjustment into the front spring boxes. A drawing for that is included with every set of fitting instructions for the JE mod. So far no one has chosen to try that option.
I do not pretend to be an expert on suspension but from what I understand from the above the damper has to work harder when there is less friction and more spring force. It seems not to be just a matter of altering the pre-load but also the spring rate and in conjunction with that the damping rate. So for the proposed front coil overs the adjustable damper and pre-load adjustment is going to be vital but the spring rate will not be variable and, extrapolating from experience with the JE modification, it is going to require a choice of spring rates to suit different weights of bike and rider. What seems not to affect any of this is the weight of any pillion passenger and/or luggage. My guess is that they are so far back that their weight is taken by the rear suspension.
 

greg brillus

Well Known and Active Website User
VOC Member
#26
As great the idea of making a coilover available for Vincent owners, I feel the number sold will be minimal. If you look at the number of JE mod stem kits produced the numbers are not high, and the number of users with this mod in service is pretty small from what I can see, especially over your way. The reason I found the spring rates could be higher even on a Comet, is that some of the shock absorbers are too stiff in their action, especially the new Avo's, these on a light bike like a Comet are way to stiff. If riders took the simple action of removing their front shocker and ride the bike a short distance, you will soon see that the softer springs at 33 or 36 Lb's are too soft and will readily bottom out.............This is not the job of the shocker to control that. A Comet with two of the 45 Lb springs cut down about 20 mm and a soft action shocker like an original Armstrong or similar gives a fabulous and comfortable ride, far superior to Brampton's. I have had the ability to experiment around with many variations of parts to make this work. I think the spring I used on the front of the racer was 170 Lb's but that bike is quite light, more so than a standard twin. The clash between the spring and the upper link is the only real issue..........The coilover I used had open centers at 270 mm apart, and this was used with the very first JE mod stem kit that I installed. That with the fully sprung rear seat is probably the best handling and ride comfort Vincent I have ever ridden, well for a twin anyway.
 

davidd

Well Known and Active Website User
VOC Member
#27
I am unsure of the answers on preload. I have been reverse engineering based on what I know about the Works Performance springs.

Spring Rate 150 Lbs.
1.75 ID
6.75 Free Length
0.283" Wire
8.6 Total Coils ( I believe they subtract the inactive coils)
4.27" Total Deflection

6.0" seems to be a safe number with the Works spring perch to clear the upper link. So, I would be somewhere around 0.80" deflection, which Is under 150 lbs. with the 150 lbs/in rate. Based on Carleton's comments on his use of 150 lbs. with a stock D Shadow, I am guessing that a 280 racer is fine using a 120 lbs. spring. This spring has about the same free length at 6.81". The bike is not ready to ride, so I cannot offer any comments except I don't think these springs are special. I believe they are standard piano wire and the 100 lbs. spring can be as short as 5" free length. Works did make longer springs that could be used with a higher preload, but they go up at fairly regular lengths like, 6.7", 7.6" and 8.3". If you needed 1.6" of preload a 7.6" spring could be used.

I think Carleton would say that 180 lbs. would be much better for two up than the 150 he is using, so the Brampton springs do seem to be worth a comparison. He seems to prefer taking his chances using the lighter springs for most of his solo riding and being a little more restrained when two up. What is odd is that the spring boxes would normally require (short) springs to be somewhere around 220 lbs. to work on a twin, yet Carleton is finding 150 on the front coil over preferable to 220 in the spring boxes. That seems like a big difference.

David
 

hadronuk

Well Known and Active Website User
VOC Member
#29
Thank you all for your detailed comments, which I am still re-reading.

A few replies:

I wish I could better relate the existing spring rates to the coilover rate, as Vibrac suggests.
I spent a long time trying to accurately calculate the spring force to axle force ratio, as I think that might have told us a lot about Girdraulics behaviour and would be very useful now.

I did however succeed in accurately calculating and also measuring the movement ration of the springs vs the axle. It’s around 1.8: 1 with the forks fully extended, dropping to around 1.2: 1 with the forks fully compressed. It’s actually quite easy to see why this occurs and is a consequence of the axle following a 5.6” radius circular path. As the forks start to move from fully extended, although the lower spring box mounting is moving upwards towards the upper spring box mounting, it is also moving forward, away from the top mounting. So a lot of axle movement only produces a little spring movement. Approaching full compression, it’s the opposite, the lower mount is moving both upwards and backwards, so the movement ratio gets nearer to 1:1. As the JE forks don’t have this initial forward motion, it seems very likely they largely avoid this effect so need less preload. I also briefly looked at the damper vs axle movement ratio. There is some variation, but it is much less than for the springs.

I initially thought if I knew these movement ratios, I would know the spring/axle force ratio. But I realised it’s a lot more complex than that. The angle of the spring boxes varies quite significantly, changing the vertical force component. In addition, as the springs push the blades forward, the resultant tension & compression forces in the links produce vertical components. When I tried to add these calculations to my already large geometry calculating spreadsheet the complexity became too confusing. I couldn’t face starting from scratch with more modern methods.

A point about the amount of spring travel required. The original Vincent damper had a full 3” of travel because the hydraulic bump stops were very short and stiff. The dampers that came later all have much less useable travel, because they all have large rubber bump stops. I am hoping it is possible to come up with a damper that has a closed length of, say, 3/8” less than the Vincent damper but the same open length. If we then fit a suitable bump stop, the forks will be brought to a halt at the same point as the Vincent damper. This should be the best of both worlds, the greater travel of the Vincent damper but also the much gentler bump stop of more modern dampers.

But this increases the spring travel requirement.

Regarding potential sales, I was concerned about this and I hope I have made sure they don’t have unrealistic expectations. Also, IF this works, there might be a few extra sales in a version for the JE mod and also a replacement for the Monroe shock used for the Brampton coilover conversion.
 

timetraveller

Well Known and Active Website User
VOC Member
#31
Just one comment; with the JE mod set up properly there is no forward movement of the wheel spindle. I did publish a diagram some time ago and if it would help then I can retrieve it from the PC and publish it again here. Also, thank you for all the work you put in.
 

greg brillus

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VOC Member
#32
That's, actually not quite correct, the axle does in fact come forward from the lowest point but only a couple of mm at the most, or until the lower and upper links are parallel with the ground. I did some early pictures of the path itself when I first built the racer. The travel was about 85 mm from memory and moved aft about 15 to 20 mm. The other point with the JE mod is the upper attachment for the upper spring box is further forward given the eccentrics are gone in place of the bearings/concentrics. So this actually makes the effective spring rates work higher than a standard set up. They are only small changes, but they do make a difference. Sorry if it sounds like I'm nit picking, It is worth noting the very small working area if a coilover is to be used. It's a bit like trying to get maximum valve travel in the cylinder head when you cant see whats going on once the rocker bearing is in place..............:).
 

timetraveller

Well Known and Active Website User
VOC Member
#33
Greg above is correct but according to my measurements one should never get into that part of the movement regime. I will try and copy the curve I have shown before below. Note that this was made by having a pencil fastened into the front wheel spindle hole and moving the wheel up and down. The pencil itself was held up against a piece of imperial graph paper where the larger squares are one inch, and the smaller ones, one tenth of an inch (2.5 mm). The relevant part of the curve is at the lower end of the right hand curve. The line with a zero against it is when the lower link is horizontal and in this case the wheel was allowed to drop below the zero position by about one inch. If one starts off in that position then is it true that the wheel will move forwards by about 3 to 4 mm before is gets to what is intended to be its original starting position. It can be seen that if the spindle starts off about 5 mm below the zero point, the circle on the line, then there will be about zero to one mm of forwards movement before the lower link is horizontal. I have always assumed that this could be ignored and therefor I advise people to try to get the lower link horizontal with them sitting on the bike but always state that up to half an inch below that position will still be acceptable. If you have the option then it is worth while enlarging this graph and studying the details. I found it quite difficult to obtain these curves and it would be interesting to see Greg's measurements and compare his curves with mine.
1543418042801.png
 

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