I think that it is worthwhile just recalling how we got here. Originally Chris, a heavy rider with a fully equipped road bike, tried to fit 'D' long springs. That was not reported as they would not fit into the space available. The next step was just the outer 'C' springs, 56 lbs/inch, which fitted but gave limited travel. Next came the 36 lbs/inch springs which, in combination with the AVO damper seem to work perfectly. Then the Oilite bushes were changed to needle roller bearings , but only where the eccentrics go originally, and that has freed up the movement to the extent that there now seems to be not enough resistance. Note that all that has changed is the amount of friction. When it comes to measuring the amount of movement at the front end as various spring strengths are use I urge adopting the method of using a cable tie around the inner spring box. Position it where the base of the top spring box is with the bike either unloaded or with only the rider seated and then go out and find some rough roads. When you get back you will be able to see exactly what movement has taken place on the spring boxes. If it is bottoming out it will be clear and we will know that stronger springs might be needed. You are looking for a movement of about half an inch between the bike on its own and with the rider seated and about three inches total over rough roads. However, we still have to explain why a change in frictional resistance seems to indicate a need for stronger springs and to find out whether stronger damping would be the answer. Greg is absolutely correct, change one thing at a time and then measure.
FF: Forks Modified Steering Stem
- Thread starter greg brillus
- Start date
I'm hoping to do this mod, just waiting for confirmation that I can get the steering head etc but I have to admit to be confused about the range of springs being discussed. Would it be possible for someone to summarise the standard springs on series C and D in terms of length and rate along with the same for the various replacements discussed in this thread?
Thanks Mac
Thanks Mac
For those who don't know me I weigh 20 stone (nearly 130 kg) so almost anyone will be lighter than me.
At present I'm having difficulty getting much mileage in due to weather, work and all sorts of other stuff but as Gregg says change one thing at a time so I need to get more miles in playing with the damper before deciding where to go next.
So far I've only taken travel measurements with the damper turned up 1/2 way.
The cut down C inner springs is just an idea I had as so far I the suspension is excellent in normal use but is going down too far under heavy braking and if the damper couldn't control the movement then the extra springing in the second half of the travel MAY but without compromising the normal area used.
Chris.
At present I'm having difficulty getting much mileage in due to weather, work and all sorts of other stuff but as Gregg says change one thing at a time so I need to get more miles in playing with the damper before deciding where to go next.
So far I've only taken travel measurements with the damper turned up 1/2 way.
The cut down C inner springs is just an idea I had as so far I the suspension is excellent in normal use but is going down too far under heavy braking and if the damper couldn't control the movement then the extra springing in the second half of the travel MAY but without compromising the normal area used.
Chris.
I can only tell you the ratings etc for the standard springs I measured and what I have fitted.
Standard C springs were 15" long 56lb/in with inner springs (on twins only) 15" long 10lb/in
D springs were the 16.5" long but the same 56lb/in rating.
These are all around 14.5" fitted.
I am at present using two 16.5" 36lb/in springs with the modified steering stem which means they are 13.5" long fitted (around 1" less than standard)
At this the suspension sits down 1/4" on it's wheels and a further 1/4" with me on board, I tried one 30lb/in spring and one 36lb/in spring and they increased to 1/2" and 1" respectively, this being 1/3 of the travel available I decided not to try that on the road.
Chris.
Standard C springs were 15" long 56lb/in with inner springs (on twins only) 15" long 10lb/in
D springs were the 16.5" long but the same 56lb/in rating.
These are all around 14.5" fitted.
I am at present using two 16.5" 36lb/in springs with the modified steering stem which means they are 13.5" long fitted (around 1" less than standard)
At this the suspension sits down 1/4" on it's wheels and a further 1/4" with me on board, I tried one 30lb/in spring and one 36lb/in spring and they increased to 1/2" and 1" respectively, this being 1/3 of the travel available I decided not to try that on the road.
Chris.
As I understand it the Dunfey springs are shorter, stiffer springs to keep the standard forks in the best (most stable) operating range so they are fitted with no or very little pre-load and restrict movement by virtue of their stiffness.
With the modified steering stem providing the stability it is now possible to use more movement hence the softer springs. that is as I understand it, however someone will correct me if I'm wrong no doubt.
Chris.
With the modified steering stem providing the stability it is now possible to use more movement hence the softer springs. that is as I understand it, however someone will correct me if I'm wrong no doubt.
Chris.
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Perhaps I can help with this. Chris has given you all the details that you need for standard Vincent springs and the two 36 lbs/inch springs whish I specified. In the same context as those 36 lb springs there has also been mentioned 30 lbs/inch springs, once again with the same length, pre-load etc. as the 36 lb springs to try out with the new steering heads. These springs were used on a twin race bike used last year at Goodwood and other places and seemed to work correctly. I was hoping that they would also prove suitable for use on road going singles as they are lighter, but just in case I have had some 33 lbs/inch springs made, with the same length etc. to be used on road going singles. These have yet to be tried out.
So now you know about standard Vincent springs and the ones which are being trialled with the new steering heads. You now need information on yet another batch of springs and these were the brain child of David Dunfey. David had already realised that there was a problem with the standard front geometry and some years ago came up with a solution. This required the use of shorter, but stiffer, front springs. David specified 75 lbs/inch for Comets, 95 (from memory) for twins and 130 (from memory) for sidecar outfits. [Check those values by reading earlier MPHs]. It turned out that the 75s were too weak for road going singles and various combinations of the three spring strengths were used and tested by various people until a satisfactory strength was found. Eventually it transpired that the ones intended for twins were about right for Singles and a combination of Comet and sidecar springs were about right for road going twins. That combination resulted in a combined strength of 102.5 lbs/inch springs was about right and in the end I had some 102 lbs/inch springs made and sent to people. The idea behind these shorter but stiffer springs was to tilt the lower link up at the front so that the wheel only ever moved upwards and backwards rather than upwards, forwards and then backwards which is what happens with the standard geometry. This would result in less movement on the front forks but the wheel spindle path would now be what was wanted. John Emmanuel had also realised that the front geometry needed to be changed and he came up with a different but more expensive solution. It should also be mentioned that John Renwick had also changed the front geometry for his race prepared bikes and he did this by drilling another hole in the girdraulic blades above the original one for the lower link. Most people are not going to want to do that. So that is whey you read of several different springs. In the context of the new steering heads you should be able to forget about the original Vincent ones and David's short but stiff ones. Testing is taking place as quickly as it can be and people will have to wait until there is a consensus as to what works best for most people. I had thought that we had cracked it with the new AVOs and the 36 lbs springs, and possible we have, but the reduction in friction with Greg's modification has to be worked through and in the mean time we can get on with making all the other parts other than the springs.
So now you know about standard Vincent springs and the ones which are being trialled with the new steering heads. You now need information on yet another batch of springs and these were the brain child of David Dunfey. David had already realised that there was a problem with the standard front geometry and some years ago came up with a solution. This required the use of shorter, but stiffer, front springs. David specified 75 lbs/inch for Comets, 95 (from memory) for twins and 130 (from memory) for sidecar outfits. [Check those values by reading earlier MPHs]. It turned out that the 75s were too weak for road going singles and various combinations of the three spring strengths were used and tested by various people until a satisfactory strength was found. Eventually it transpired that the ones intended for twins were about right for Singles and a combination of Comet and sidecar springs were about right for road going twins. That combination resulted in a combined strength of 102.5 lbs/inch springs was about right and in the end I had some 102 lbs/inch springs made and sent to people. The idea behind these shorter but stiffer springs was to tilt the lower link up at the front so that the wheel only ever moved upwards and backwards rather than upwards, forwards and then backwards which is what happens with the standard geometry. This would result in less movement on the front forks but the wheel spindle path would now be what was wanted. John Emmanuel had also realised that the front geometry needed to be changed and he came up with a different but more expensive solution. It should also be mentioned that John Renwick had also changed the front geometry for his race prepared bikes and he did this by drilling another hole in the girdraulic blades above the original one for the lower link. Most people are not going to want to do that. So that is whey you read of several different springs. In the context of the new steering heads you should be able to forget about the original Vincent ones and David's short but stiff ones. Testing is taking place as quickly as it can be and people will have to wait until there is a consensus as to what works best for most people. I had thought that we had cracked it with the new AVOs and the 36 lbs springs, and possible we have, but the reduction in friction with Greg's modification has to be worked through and in the mean time we can get on with making all the other parts other than the springs.
Thanks
Mac
I just wanted to mention that the Works Performance dampers are adjustable, although they must be taken apart, which is quite easy to do and expected. I have taken them apart many times to change the oil. Changing the oil viscosity is probably the first step in the tuning process. The valve discs can also be changed.
Works is happy to do so, but for those far away I believe they will send you what you need.
My crude understanding of damping is that the springing needs to correct prior to changing the damping. The damper only controls the rate of change of the compression and rebound, it does not directly control the forces. The springs do that work.
David
Works is happy to do so, but for those far away I believe they will send you what you need.
My crude understanding of damping is that the springing needs to correct prior to changing the damping. The damper only controls the rate of change of the compression and rebound, it does not directly control the forces. The springs do that work.
David
Some thoughts on Vincent Front Suspension. PART 1
Observations:
· A Thornton Front shock Absorber has a total travel of 3 inches without the bump stop in place. With the bump stop the travel is reduced to 2 ¾ inches
· Spring length, installed in spring boxes at maximum extension is 14 inches
· Spring maximum compression (travel) inside the spring box is 3 inches
· Thus spring length at full compression inside a spring box is 11 inches.
· At all times it is essential that there is some pressure applied by the springs inside the spring boxes. I.e. at full extension there MUST be some pre-load, just 2 or 3 Lb is sufficient, so that there is zero free play.
· Static sag (front) should be in the range of 28 to 32% of total available suspension travel for normal street use.
· Original front outer springs have a free length of 15 inches, C series and 16 ½ inches for the D series bikes. Both series springs have the same spring constant of 65 Lbs/inch. Plus the same springs were used in all models – Comets, Rapide, Shadow. The inner front springs (when used) are the same across both series with a free length of 15 inches and a spring constant of 9 ½ Lbs/inch. (Thanks to the VOC Spares co and David Dunfey for this information)
Let’s take a side trip so we can have some understanding of what we are dealing with.
When studying springs and elasticity, the 17ᵗʰ century physicist Robert Hooke noticed that the stress vs strain curve for many materials has a linear region. Within certain limits, the force required to stretch an elastic object such as a metal spring is directly proportional to the extension of the spring. This is known as Hooke's law and commonly written:
F=−kx
Where F is the force, x is the length of extension/compression and k is a constant of proportionality known as the spring constant which is usually given in Newtons per Metre (N/m) or in imperial measurement, Pounds per Inch (Lb/In) .
Though we have not explicitly established the direction of the force here, the negative sign is customarily added. This is to signify that the restoring force due to the spring is in the opposite direction to the force which caused the displacement. Pressing down on a spring will cause a compression of the spring , which will in turn result in an upward force due to the spring.
If you really want to know more about the physics of springs, this site is a good place to start: https://www.khanacademy.org/science/physics/work-and-energy/hookes-law/v/intro-to-springs-and-hooke-s-law
OK – lets get back on track.
It is my understanding that the role of a spring and that of a damper are totally different and should not be confused. The Spring is used to control the movement by the absorption and release of kinetic energy within the spring itself whereas the Shock Absorber controls the rate (or speed) of movement by the conversion of kinetic energy (movement) into heat.
In terms of our Girdraulic suspensions, if we were to remove the spring boxes, leaving just a front shock absorber in place, the shock absorber would slowly compress to its shortest length and that’s where it would stay. Just how quickly the shock absorber compresses is a function of its stiffness (the rate at which it can convert kinetic energy to heat); The stiffer the shock absorber, the slower it will be in settling. As the shock absorber settled absorbing the kinetic energy imparted to it by the weight of the bike, some heat would be generated within the fluids in the shock absorber. That’s it folks – you can then push down as much as you like – but there will be no further suspension movement as the shock absorber will have already reached the mechanical limit of its travel. Of course, you can also pull up on the bars, but without the springs in place you are attempting to lift the ‘dead’ mass of the bike. Given super human strength you could lift the bike but this time the suspension travel will be limited, again by the shock absorber reaching the other end of its available mechanical travel.
If we leave the spring boxes in place but this time removing the shock absorber, what happens is that the weight of the bike starts to compress the springs to the point where the upward force from the springs equals to downward force being applied. It is a point of equilibrium where the kinetic energy from the mass of the bike balances the kinetic energy stored in the springs. Now if we push sharply down (or pull sharply up) on the front of the bike the equilibrium is disturbed the suspension will now start to move up and down and will continue to do with the amplitude decreasing with each oscillation, eventually returning to the point (height) of equilibrium or as some think of it, the ‘sag point’. As an aside, this is where the Shock Absorber then comes into play, soaking up excessive kinetic energy and damping out the oscillations. Remember – ANY friction in the suspension system will also act as a shock absorber, with that friction converting the kinetic energy of movement into heat at the points of friction.
Key Point One. It is the springs alone that determine the ‘sag point’ of the front suspension.
Key Point Two. It is the static sag point with the bike stationary AND the rider (plus normal luggage) on board that counts.
Key Point Three. It is the travel range provided within the shock absorber that sets the total amount of available suspension travel.
Key Point Four. There should be zero (impossible!) friction within the suspension system. Realistically, there should be minimum friction.
Key point 5. It is ONLY the sprung weight (that’s the weight supported by the springs) that we need to consider when looking at front spring calculation around % of static sag. In this discussion I have assumed an unsprung weight of 50 Lb covering the front wheel assembly with tyre,
Remember what we are looking for is a static sag where the weight of the bike and rider supported by the spring happens at the 30% of suspension travel point. OK, now you realize you need to get the weight on the front wheel measured WHILE you are astride the bike less 50 Lb (being the assumed unsprung weight) to help you determine the equilibrium point for static sag.
Observations:
· A Thornton Front shock Absorber has a total travel of 3 inches without the bump stop in place. With the bump stop the travel is reduced to 2 ¾ inches
· Spring length, installed in spring boxes at maximum extension is 14 inches
· Spring maximum compression (travel) inside the spring box is 3 inches
· Thus spring length at full compression inside a spring box is 11 inches.
· At all times it is essential that there is some pressure applied by the springs inside the spring boxes. I.e. at full extension there MUST be some pre-load, just 2 or 3 Lb is sufficient, so that there is zero free play.
· Static sag (front) should be in the range of 28 to 32% of total available suspension travel for normal street use.
· Original front outer springs have a free length of 15 inches, C series and 16 ½ inches for the D series bikes. Both series springs have the same spring constant of 65 Lbs/inch. Plus the same springs were used in all models – Comets, Rapide, Shadow. The inner front springs (when used) are the same across both series with a free length of 15 inches and a spring constant of 9 ½ Lbs/inch. (Thanks to the VOC Spares co and David Dunfey for this information)
Let’s take a side trip so we can have some understanding of what we are dealing with.
When studying springs and elasticity, the 17ᵗʰ century physicist Robert Hooke noticed that the stress vs strain curve for many materials has a linear region. Within certain limits, the force required to stretch an elastic object such as a metal spring is directly proportional to the extension of the spring. This is known as Hooke's law and commonly written:
F=−kx
Where F is the force, x is the length of extension/compression and k is a constant of proportionality known as the spring constant which is usually given in Newtons per Metre (N/m) or in imperial measurement, Pounds per Inch (Lb/In) .
Though we have not explicitly established the direction of the force here, the negative sign is customarily added. This is to signify that the restoring force due to the spring is in the opposite direction to the force which caused the displacement. Pressing down on a spring will cause a compression of the spring , which will in turn result in an upward force due to the spring.
If you really want to know more about the physics of springs, this site is a good place to start: https://www.khanacademy.org/science/physics/work-and-energy/hookes-law/v/intro-to-springs-and-hooke-s-law
OK – lets get back on track.
It is my understanding that the role of a spring and that of a damper are totally different and should not be confused. The Spring is used to control the movement by the absorption and release of kinetic energy within the spring itself whereas the Shock Absorber controls the rate (or speed) of movement by the conversion of kinetic energy (movement) into heat.
In terms of our Girdraulic suspensions, if we were to remove the spring boxes, leaving just a front shock absorber in place, the shock absorber would slowly compress to its shortest length and that’s where it would stay. Just how quickly the shock absorber compresses is a function of its stiffness (the rate at which it can convert kinetic energy to heat); The stiffer the shock absorber, the slower it will be in settling. As the shock absorber settled absorbing the kinetic energy imparted to it by the weight of the bike, some heat would be generated within the fluids in the shock absorber. That’s it folks – you can then push down as much as you like – but there will be no further suspension movement as the shock absorber will have already reached the mechanical limit of its travel. Of course, you can also pull up on the bars, but without the springs in place you are attempting to lift the ‘dead’ mass of the bike. Given super human strength you could lift the bike but this time the suspension travel will be limited, again by the shock absorber reaching the other end of its available mechanical travel.
If we leave the spring boxes in place but this time removing the shock absorber, what happens is that the weight of the bike starts to compress the springs to the point where the upward force from the springs equals to downward force being applied. It is a point of equilibrium where the kinetic energy from the mass of the bike balances the kinetic energy stored in the springs. Now if we push sharply down (or pull sharply up) on the front of the bike the equilibrium is disturbed the suspension will now start to move up and down and will continue to do with the amplitude decreasing with each oscillation, eventually returning to the point (height) of equilibrium or as some think of it, the ‘sag point’. As an aside, this is where the Shock Absorber then comes into play, soaking up excessive kinetic energy and damping out the oscillations. Remember – ANY friction in the suspension system will also act as a shock absorber, with that friction converting the kinetic energy of movement into heat at the points of friction.
Key Point One. It is the springs alone that determine the ‘sag point’ of the front suspension.
Key Point Two. It is the static sag point with the bike stationary AND the rider (plus normal luggage) on board that counts.
Key Point Three. It is the travel range provided within the shock absorber that sets the total amount of available suspension travel.
Key Point Four. There should be zero (impossible!) friction within the suspension system. Realistically, there should be minimum friction.
Key point 5. It is ONLY the sprung weight (that’s the weight supported by the springs) that we need to consider when looking at front spring calculation around % of static sag. In this discussion I have assumed an unsprung weight of 50 Lb covering the front wheel assembly with tyre,
Remember what we are looking for is a static sag where the weight of the bike and rider supported by the spring happens at the 30% of suspension travel point. OK, now you realize you need to get the weight on the front wheel measured WHILE you are astride the bike less 50 Lb (being the assumed unsprung weight) to help you determine the equilibrium point for static sag.