FF: Forks Modified Steering Stem

[Martyn Goodwin]

Hi Martyn
I disagree slightly with the sag point being at around 30%, this may apply to other vehicles but with the restricted travel available on a girdraulic fork this would leave less than 2" travel so I am using about 20%, this puts the links in the horizontal position as original.
Also short stiff springs and long soft springs can give the same sag point, but completely different suspension characteristics.

Chris.

Hi Chris,

The sag point you choose is a matter of personal choice. All it does is determine how much travel you have left in BOTH directions, from that static sag point. I selected 30% for myself as I thought it was a good compromise to avoid both topping out and bottoming out when riding, but it is a matter of personal choice.

You are totally correct with your observation about short and long springs. It is ONLY the Spring Constant (sometimes called the spring rate) that affects the feel of the spring in use.

For any given Spring Constant you can move the Static Sag only ONLY by changing the length of the spring, either by adding pre load spacers or cutting the spring to shorten it. If you think that your springs are too soft, adding pre load spacers will NOT change the feel - all that will do is move the static sag point and change the topping out and bottoming out forces.

 

[Martyn Goodwin]

Any folks wanting to get hold of the Front Spring Calculator can now download it from the Oz Vincent Review Website. It can be found in Maintance Aids within the OVR Archive.
Here is a link to the OVR web site https://goo.gl/Br0hEJ

Martyn

 

[davidd]

I am all for on road testing and that is what Martyn is reporting on. It will be good to get more information from others.

For my own testing, I concluded that adjustable preload was mandatory. I felt that I needed to adjust the ride height to properly tune the bikes I was riding. I did this in a rather crude way by making the springs short so they could be shimmed up to a specific ride height that I felt worked.

I made spring limiters to make adjustments where I felt the the static sag (that is with a rider) was too great:

I made the limiters from scrap around the shop primarily for testing purposes. It allows you to turn the small nub inside the tube that is threaded to set the preload on the spring. I then put it in the press upside down to screw the cap on the end. As the spring compresses the rod moves inside the tube. As it extends, it can only extend as far as the rod will allow as the nub hits the shoulder of the hole that the threaded rod runs through.

I have a 1/2" preload on the spring assembled above. This required a 1/2" spacer in the spring box to keep the spring in the spring boxes with no slack. This is a quick way of testing different preload settings. I never felt the need to use one other than for testing. If I were to use a permanent one I would make one to size and that worked more smoothly.

I think this could help with heavy loads where you want the same rate, but need a little more travel. It allows you to steal some travel out of the sag (assuming you have a lot of static sag) and put it in compression. Thus, it raises the laden ride height. Sometimes these small adjustments can make all the difference, although I have to admit, that the few who could have used these limiters were not interested. They were happy with the ride as it was with the bare springs.

David

 

[greg brillus]

That's the thing I like about Vincent owners ........Always thinking outside the hexagon....................I mean Square..........

 

[Martyn Goodwin]

Before we all get bogged down in theory can we wait for feedback from users? I know that the 33lb springs and 36 lb spring are now with Greg, who will have to send them on to other people. If two 33s do not do the job on a Comet then two 36s should do. Two 36s with 3" of preload are exerting a force of 206 lbs. Another half inch of movement moves that up to 252 lbs. A further inch of compression increases that to 324 lbs. Use the whole 3" of travel and one is up to a force of 432 lbs, just about the total weight of the bike without a rider. Now start off with 90 lbs/inch springs. In order to get the initial required force of 206 lbs there would have to be a compression of 1.14". (Remember there are two springs). Compress a further one inch and the force is now up to 360 lbs and with a further one inch the total force is up to 540 lbs. That seems like a pretty stiff front end to me but let us see what users find. If that is what is needed than all the springs cost about the same to be made so there is no cost implication, only a comfort one.

I do not understand why you say there is a need for an initial force of 206 Lbs. or at what point in the suspension travel you are measuring it.

For my Comet, fitted with the (I must say fantastic) new headstem, the front of the bike is weighs 184 Lbs measured by putting scales UNDER the front tyre. To get to the base unsprung weight it necessary to remove the weight of those parts not supported by the front springs being the complete front wheel assembly including brakes, the Girduralic fork blades, the headlamp assembly and the speedo. I have NOT weighed these bits YET and for now have estimated them at 50 Lbs in all, giving an unsprung weight sans rider of 134 Lbs.

I weigh 190 Lbs and assuming 40% weight bias to the front then I will be adding 76 Lb to the front wheel unsprung weight, giving a total unsprung weight on the front end with me aboard of 210 lbs. Yes this is a bit different to the figure I used in my earlier post.

 

[Martyn Goodwin]

Any folks wanting to get hold of the Front Spring Calculator can now download it from the Oz Vincent Review Website. It can be found in Maintance Aids within the OVR Archive.
Here is a link to the OVR web site https://goo.gl/Br0hEJ

Martyn

I know there will be some folks who will want to know the effect of having different spring lengths, constants or pre load spacers in each spring box - and I am one of them! This is now resolved. I have developed a front spring calculator that is just for that purpose. Like the Front Spring calculator that I listed earlier, this one - that I have called the Differential Front Spring Calculator (err cause it compares different springs in use at once) is also available from the OVR web site, again under Maintenance Aids in the OVR Archive.

Martyn

 

[BigEd]

I do not understand why you say there is a need for an initial force of 206 Lbs. or at what point in the suspension travel you are measuring it. ........
I weigh 190 Lbs and assuming 40% weight bias to the front then I will be adding 76 Lb to the front wheel unsprung weight, giving a total unsprung weight on the front end with me aboard of 210 lbs. Yes this is a bit different to the figure I used in my earlier post.

I'm not much of a mathematician so your Excel sheets are useful.
Question: Isn't the riders weight sprung rather than unsprung?

 

[greg brillus]

Ok getting back to the real world of testing..............Today I installed a pair of the 33 Lb springs that Norman sent me, as a bit of a trial to see if a slightly weaker spring could be used as opposed to the 36 lb springs that Chris has been using for a while (remember that I am around 72 kg's). Now the springs I removed were the ones I had made earlier, and rated at 40 lb's, though because I had chopped 3 inches off them, the actual spring rate was 50 Lb's. I know these spring rates because I am using a spare spring box assembly as a test bed using a set of scales, and I have drawn a line on the lower spring box with one inch markers on it. Ok so I installed the springs, sat the bike back on the ground, and the shocker unit was showing about 20% of travel used, so quite close to being topped out. Took the bike for a good test ride, lots of good bumps and so on, the suspension felt very good, and probably as light as I feel I would want to go. It did not bottom out over bumps, but if I gave the front brake a really good squeeze, I would say it is up against the bump stop on the Thornton shocker. I would say after these trials so far, that the same set up on a Comet is definitely a lot softer and smoother than a twin, and I can only say that it must be just the extra weight of the machine. I feel that I will either try the 36 lb springs or even go back to the original ones I had in there, which have a higher spring rate, but are shorter with about 2 inches of pre -load. So this is near concluding testing from my point of view, other than when installing the springs it seems best if when the bike is resting on its front wheel again, that the front end is at or very near topped out, as this will settle and give you as much travel as is available. In conclusion I would have to say that the 33 lb springs are probably near spot on for a single and the 36 lb springs for the twins, or a combination there of. Cheers........Greg.

 

[timetraveller]

MartynG asks where the figure of 206 lbs comes from. The answer is that it was determined empirically. We had no idea what springs to start off with so a pair of 'C' outers were used. These have a rate of 56 lbs/inch. Chris then measured the installed length of the springs when the bike was vertical, but without a rider, and then again when he sat on the bike. Knowing the original length of the springs and the length that they had compressed to in order to take the weight of the both Chris and the bike allowed me to work backwards to find out exactly what force was being exerted. Then Chris did road testing over a variety of road surfaces, including some very bad ones and 'sleeping policemen', aka traffic calming bumps. The total movement of the spring boxes was measured after these runs, and that is when I started to work backwards. We knew what force was needed to keep the lower link at the correct angle and what spring rate would allow the whole of the potential travel to be used. That is where the spring rate of 36 lbs/inch came from and the pre-load.
What is not being discussed in some of the above postings is that the new geometry affects several things and that they have to be taken into account when discussing these matters. It is not just spring rate. First we now know that for optimum safety the lower link should point slightly upwards at the front when the rider is seated. We also know that without the rider and just the weight of the bike the lower link needs to point very slightly downwards at the front. This, approximately half an inch of movement, is affected by both the spring rate and the pre-load. The angle of this link when the bike is unloaded is governed by both the damper length and what eyebolts are used. The available travel on the damper limits the total potential movement. The springs are now not just fitted into a shorter length but are also more vertical when compared with the standard set up. All these things have to be accounted for in any final design, not just the spring rates. If stronger springs are required than that is not a problem but we know from Chris' early tests that 56 lbs/inch springs are too strong.
I have no idea if it is of interest to people but I do have a design to allow the spring force to be changed in moments by making a different fitting to the base of the inner spring box. This would consist of a M12 thread with a fork at the bottom to match the original. A new stainless steel inner spring box would be made from some thin walled tube, shrunk and spot welded onto a small block with an internal M12 thread. The upper part of this block would be cylindrical to fit inside the tube and the bottom of it would have a hexagon. An M12 thin lock nut, around the thread of the lower fork, would ensure that nothing rotated once it had been adjusted. It might be thought that a small amount of movement at this position would have little effect but suppose that 36 lbs/inch springs were used. Then moving the base of the springs and the inner spring box upwards by one inch would apply an extra 72 lbs of force, just at the front end. That is probably more than enough to compensate for the difference between a 100 lb rider and a 200 lb rider plus a pillion passenger. I could draw this up and post it here, (although I might need some help to get it to appear here). I could even get some made if there was enough interest but there is only one of me and I have other things in my life apart from Vincents. I already have to lay out over £6k for a new batch of steering heads and these are all being provided at a not for profit price. So be gentle with me.

 

[macvette]

Ok getting back to the real world of testing..............Today I installed a pair of the 33 Lb springs that Norman sent me, as a bit of a trial to see if a slightly weaker spring could be used as opposed to the 36 lb springs that Chris has been using for a while (remember that I am around 72 kg's). Now the springs I removed were the ones I had made earlier, and rated at 40 lb's, though because I had chopped 3 inches off them, the actual spring rate was 50 Lb's. I know these spring rates because I am using a spare spring box assembly as a test bed using a set of scales, and I have drawn a line on the lower spring box with one inch markers on it. Ok so I installed the springs, sat the bike back on the ground, and the shocker unit was showing about 20% of travel used, so quite close to being topped out. Took the bike for a good test ride, lots of good bumps and so on, the suspension felt very good, and probably as light as I feel I would want to go. It did not bottom out over bumps, but if I gave the front brake a really good squeeze, I would say it is up against the bump stop on the Thornton shocker. I would say after these trials so far, that the same set up on a Comet is definitely a lot softer and smoother than a twin, and I can only say that it must be just the extra weight of the machine. I feel that I will either try the 36 lb springs or even go back to the original ones I had in there, which have a higher spring rate, but are shorter with about 2 inches of pre -load. So this is near concluding testing from my point of view, other than when installing the springs it seems best if when the bike is resting on its front wheel again, that the front end is at or very near topped out, as this will settle and give you as much travel as is available. In conclusion I would have to say that the 33 lb springs are probably near spot on for a single and the 36 lb springs for the twins, or a combination there of. Cheers........Greg.

Hi,
Whilst the major contributors to this thread have a thorough understanding of spring mechanics, there will be followers of the thread who may not understand why Greg says in this post that shortening an existing spring increases its spring rate. This is because the material in coil springs is subject to torsional stress as they are compressed.
Shortening the spring reduces the amount of material available to resist the force so the spring rate is increased. To experience practically, clamp a piece of flat bar in a vice ( say 1/8"x 1/2"x 12"), grip the free end firmly with mole grips and twist it (putting it in torsion).. Now move about 1/2 way to vice and try again. You will find much harder to produce the same deflection. This is because in this latter case you have reduced the amount of material available to absorb the applied force without changing the cross section of the material. This is the same effect which happens when a coil spring is shortened .
Mac