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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Modified Steering Stem
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<blockquote data-quote="timetraveller" data-source="post: 78070" data-attributes="member: 456"><p>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.</p><p>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.</p><p>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.</p></blockquote><p></p>
[QUOTE="timetraveller, post: 78070, member: 456"] 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. [/QUOTE]
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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Modified Steering Stem
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