FF: Forks Cadwell Landsdowne video + Suspension Setup

[BigEd]

The reason you all feel the 45 lb springs are too stiff is because you are installing them at their full length.........This makes the front end "Top out"........Every one that I do I cut around 15 to 20 odd mm off each spring.........From my findings almost every shock absorber made to suit a Vincent is way too stiff.........

Greg has highlighted the conundrum of getting the spring rate, pre-load and damping combination acceptable for the variations in machine use. e.g. Solo, pillion, luggage, touring, racing, etc.

Compared to suspension systems on modern machines, you are working with very little suspension travel in the first place. Our aim is to make the best use of what we have, maybe 3 inches at the front and 4 - 5 inches at the back depending on RFM length and spring unit.
Initially, I experienced topping out with full length 45/lb springs so started experimenting with different rate springs and spring lengths.
With only a very basic understanding of suspension setup here are a few things I learned and took into account when experimenting. This was regarding Girdraulics fitted with the concentric fork mod and ball journal bearings but applies to the rear suspension as well:-

If you shorten a spring the actual spring rate increases so you have a stiffer spring. You will also have less pre-load unless you fit packers.
If your damper is adjustable you may be able to reduce topping out by increasing the rebound damping. This may also make the suspension stiffer on compression so less comfortable but It might also help to reduce the chance of bottoming out.
Most dampers you can fit to a Vincent have more rebound damping than compression. Some dampers have a separate adjustment for rebound and compression. I don't know if there are any like this to fit a Vincent.
A lighter spring rate gives a softer ride but then you have to take into account that there is more chance of the suspension bottoming out. You need to check how much travel you are using. (A tie-wrap around the lower spring box is an easy to fit indicator and it is a good idea to at least have a bit of a bump stop rubber fitted to the damper shaft.)
You will become quite adept at removing and refitting the front spring boxes when changing springs/pre-load packers. (Do one side at a time and be careful as we are dealing with quite a lot of stored energy in a compressed spring.)
You may end up with different rate/length springs and packers in each spring box to get the best results.
Sometimes you may have to compromise to get the best balance of performance and comfort that is acceptable to you.
My Vincent is one of the most comfortable bikes I ride so take your time and experiment a bit.

 

[timetraveller]

Let’s talk about springs. When I designed the springs for the JE steering head stem I tried to ensure that the maximum travel that was possible was available. This was following a journey I made as ballast in Dick Sherwin’s sidecar attached to his Comet. The rally was In France and Belgium and I had little to do for several days but watch the front suspension. The example which stands out in my mind is when we crossed the rail way tracks at a level crossing and the spring boxes moved about three quarters of an inch. It seemed to me that the front tyre was providing much of the suspension.

The maximum travel that is possible with the JE system is about three inches and is controlled by the damper travel. The length of the damper and the length of the eye bolts used at the base of the damper control the position of the lower link and I spent a lot of time with a set of forks set up so that I could play around with various settings in order to ensure that the JE geometry was being made best use of. The first iteration was 36 lbs/inch springs with three inches of pre-load. This gives an upwards force of 216 lbs. This figure came from a road equipped twin weighing about 450 lbs and having roughly half of the weight in each wheel. To this must be added some of the weight of the rider and the weight of the front forks, wheel, headlamp etc. must be taken off the total as these are all unsprung. Chris Launders did all the early test riding and all seemed well. Then Greg came up with the idea of using a ball or roller bearing in place of the Oilite bushes at the rear of the lower link and when Chris tried this the reduction in frictions was so dramatic that at first we thought that the damper had failed. The solution was to go to 45lbs/inch springs and a stiffer damper. Chris’ weight required this extra force. The 45 lbs/inch springs were installed with two inches of pre-load which gives 180 lbs of upwards force but eventually Chris had to add about half an inch of packing at each side in order to give two and a half inches of pre-load which gives 225 lbs of upwards force, similar to the first figure given by the 36 lbs/inch springs with three inches of pre-load.

Now, going back to the JE steering geometry; to get the best out of the JE conversion it is important that when the forks are at their lowest position with the bike on its wheels and the rider seated that the lower link is just about horizontal, or at a maximum no more than about one quarter of an inch down at the front. By removing about three quarters of an inch off each spring Greg is reducing the upwards force by about 67 lbs although a slight increase in spring rate will reduce that value slightly. I would have thought that this would be enough to make the lower link up at the front, rather than horizontal. This reduces the total travel possible. There is something else involved in this which further muddies the waters. I suggest that people shorten their front spring boxes by one inch. Without this, the outer spring box can hit the front fork blades at the lowest part of their travel. Greg does not find the need to do this which suggests to me that he is getting less travel than is possible.

My intention was to get the maximum travel and comfort out of the front end together with the added safety of the JE geometry. It is possible that this can be improved upon and if anyone can get a softer modern damper then I would be happy to get springs of any strength made to allow experimentation. One thing that worries me is that in the seventy years since Vincent designed their dampers, which give equal resistance on both compression and extension, it is now realised that it is better to have the extension damping several times that of the compression damping. This allows the bike to ride more smoothly over bumps but, if correctly specified, does not cause ‘ratchetting’ of the springs.

 

[greg brillus]

Unfortunately we have hijacked Tim's thread........But still relevant to his race bike.........There is too much emphasis on the shock absorber.........the load is taken by the springs.........So soft springs, yes you will get a lovely soft ride, but the front end will dive badly on heavy braking, just as is the case on the race bike above. a shock absorber will not cure this unless the one you use has a lot of compression damping built in. The most simple test to check the springs and their ability to function without bottoming easily is to simply ride the bike with no front shock absorber in place..........hit the brakes and see what happens.......... I did this on a Comet some time ago with the soft 33 lb springs and the front end bottomed out very easily, then the 36 lb springs which was a little better, then the 45's and these worked the best.........But if you use a stiff shock absorber then the front end will feel ridged and terrible. One point I should make here........Of the many bikes I have done, the ones where I assembled the kit with the link bearing mod and steering damper kit, then the owner finished off the assembly themselves.........I have found they somehow got lost in the verbal instructions and did things their own way........Once I caught up with them at a rally or bike show somewhere, Upon inspection I found that the front end of their bikes was terrible.......generally topped out, because they had failed to trim the spring length down........another trap is the spring cases need to move absolutely free within one another, any slight drag will really inhibit the freedom of the front forks........Often on customers bikes I do in fact trim the spring cases down about 15 mm just for a bit of extra travel........generally only necessary if the rider is a bit on the heavy side. On assembly of the spring cases, I can nearly install them by hand.......The cases only have about 30 to 40 mm of pre load to fit the top bolt.

 

[Chris Launders]

I have, as been said earlier 45lb springs with packers as I weigh 300lb, but have my damper on the lowest setting, in fact I once rode 60 miles with no damper at all and experienced no real problems with excessive diving or topping out, obviously I avoided speed humps and emergency stops but it was mostly back roads.

I feel Greg is right in that the springs should be almost stiff enough to use without a damper, the damper being there just to help with the extremes of operation.

 

[Chris Launders]

I wonder if setting up the springing initially WITHOUT the damper would be an avenue to pursue, an empty standard one could be used to prevent too much travel.

 

[Chris Launders]

I have to say what I learned doing the testing for Norman, helped on other bikes as well, my Brough Superior used to pogo unless the friction damper was screwed down tight but then wouldn't react to small irregularities, I realised after working on the Vincent the rebound springs were far too strong and ended up shortening them from 9" to under 7" so they virtually only come into play with over extension and I need hardly any tension on the friction damper and the ride is infinitely better.

 

[greg brillus]

Yes as Chris has pointed out, the ability to try many changes is what gives the best answer........This is where I have been able to carry out many variations of springs and shockers........This even before any springs were made by Norman for these kits.......The first ones I had in my Rapide were the softer "red" springs as per David Dunfey's method of using shorter springs.........From memory these are rated at 75 Lbs and these did work, but felt quite stiff given the lesser travel due to the lower stem motion block being lower down. Of the 2 scenarios whereby you go with longer soft springs with lots of pre load, verses shorter stronger springs with less pre load, the latter is the one to go for. Chris you mention about using the 45 Lb springs with packers installed.......... I have never managed to do this ever.........Or if I do, the front forks sit "Topped out" quite a lot.........To the point where I can not compress the forks at all with all my body wight pushing down on the bars.........I am around 74 Kg's if that means anything...........I can tell by the lack of likes/dislikes from readers that they are lost in this topic........that alone shows how general folk have no idea about suspension changes......... And that's ok, we cant all be interested in these things.........There are still riders out there having tank slappers on these bikes.........I'm helping a chap who lives not far from me to repair his bike after one that tossed him off at speed only a couple of months ago........Cheers.

 

[Chris Launders]

Hi Greg, I have 11mm packers, but I'm 133 kg at present, which will make a considerable difference, and I CAN bounce mine up and down.

I can't remember the exact figure for sag but seem to recall it was about 1/2 with me sat on the bike.

The bike is parked on the side stand at the moment so almost all the weight will be off, but the cable tie is 3" down from the upper springbox so I reckon I'm using full travel as I reset it occasionally.

 

[timetraveller]

Greg has mentioned the David Dunfey short, stiff spring mod above. When DD first published this I did get springs to his specifications made to sell to those who wanted to try it out. I don't now remember the numbers of people who tried them but it was tens of. The 75 lbs/inch springs were much too weak. Even on a Comet without a rider the front end was way down. From memory those springs were probably right for a very light weight racing single but no use for a road going bike. In the end most people ended up using one of the medium strength springs and one meant for sidecar use. A local friend still has this set up on his Comet and I do not find that it gives a comfortable ride.
The different experiences between Chris and Greg when it comes to the combination of weight and flexibility is instructive. Chris is getting the full range of movement, which is what I designed for, while Greg finds that, in general, it is not necessary to shorten the spring boxes by much. It would not be a lot of effort to try Greg's ideas here if we had access to a very weak damper. I still have original Vincent and Armstrong dampers here but they both have the same resistance to both compression and extension and therefore would not provide a meaningful test.
Greg mentions speed wobbles. Well I am in the process of making 20 hydraulic steering damper kits to fit standard front ends and two days ago I collected the stainless steel brackets from the company who does the laser/water jetting for me. I have spent a few hours on each of the last two days polishing stainless and by the end of today I should have the first ten of these ready for shipping. I will be posting that information in the correct thread later.

 

[Bill Thomas]

I still run Your/Davidd short strong springs with standard front end,
But with Vibrac's Broom handle !, Or was it the Wife's .
About 2. 1/2 " !, On all springs, I think, I have a red = soft and a blue in the Comet ,
And a Blue and a white = Strong in the L/ning,
I run a standard rebuilt front damper on the Comet,
But the L/ning still runs a SPAX, I know I know, But I think it was a fine damper ,
Took off the market because of one man in AUS' got hurt.
It's adjustable and I run it on softest, I have raced and used it on the road for a long time,
I have also snapped a standard damper.

I think a lot of our trouble came from Owners fitting the long "D" springs,
Too long I think.

I am pleased with the L/ning,
But the Comet is still not as good as I would like, Not bad but you know !!, Christ there 70 years old .
I think it would need the bearings on the Eccentrics to work well,
Davidd did tell me of a way but it was too much for me.
Cheers Bill.