At about Xmas 2021/New Year 2022 Chris Launders discovered that his fully sprung seat was hitting the rear mudguard with sufficient force to dent the mudguard. We had a discussion and decided to try to compose a spreadsheet to help others to decide what rear springs strengths would be suitable for others using an AVO coil over rear damper and a fully sprung rear seat. The big advantage of the AVO is that both the damping and preload of the spring are readily adjustable and a range of spring strengths is available. The calculations only refer to twins. All the original measurements were made by Chris on his own bike with his wife providing the pillion loading.
There was one problem that occurred at once and that is that neither of us knew what the ‘G’ force would be when going over a bump. For those unsure about this the ‘G’ force is the increased amount that the rear suspension loading might experience when the bike goes over bumps. It is no good doing calculations for a static loading and then going over one of the ‘third world potholes’ that have become a regular feature of English roads. In the end we discovered that at least one motorcycle manufacturer uses G=3 for their calculations so that is what was used.
We did appeal through this forum for extra input from others, with either Comets or twins, to enable us to improve the accuracy and range of the final figures but the response was disappointing with only one other person giving us input. The table as presented should be regarded as a first attempt and it should be possible to improve it if we can get more input. The figures that are required are twin or single with a fully sprung rear seat, weight of rider, weight of pillion if applicable, length of spring between top and bottom retainers, all with the bike on its wheels or on the rear stand. The bike on a centre stand will not give useful data.
The first three columns are for the riders weight in Kg, Lbs or stones. Move down the column for increasing weight. The next column gives the force on the spring with the standard Vincent geometry and a fully sprung rear seat. The next column refer to the amount the various springs available from, AVO will be compressed with those forces. Springs with rates of 275 ( the standard one issued), 300, 350 and 400lbs/inch are considered. For each spring strength we have also calculated the force if a bump in the road imparts a 3G extra loading on the suspension.. Note that the AVO springs allow a compression of about three inches before they become coil bound. Therefore any value on the table above three should be regarded as a 'no go area'.
There was one problem that occurred at once and that is that neither of us knew what the ‘G’ force would be when going over a bump. For those unsure about this the ‘G’ force is the increased amount that the rear suspension loading might experience when the bike goes over bumps. It is no good doing calculations for a static loading and then going over one of the ‘third world potholes’ that have become a regular feature of English roads. In the end we discovered that at least one motorcycle manufacturer uses G=3 for their calculations so that is what was used.
We did appeal through this forum for extra input from others, with either Comets or twins, to enable us to improve the accuracy and range of the final figures but the response was disappointing with only one other person giving us input. The table as presented should be regarded as a first attempt and it should be possible to improve it if we can get more input. The figures that are required are twin or single with a fully sprung rear seat, weight of rider, weight of pillion if applicable, length of spring between top and bottom retainers, all with the bike on its wheels or on the rear stand. The bike on a centre stand will not give useful data.
The first three columns are for the riders weight in Kg, Lbs or stones. Move down the column for increasing weight. The next column gives the force on the spring with the standard Vincent geometry and a fully sprung rear seat. The next column refer to the amount the various springs available from, AVO will be compressed with those forces. Springs with rates of 275 ( the standard one issued), 300, 350 and 400lbs/inch are considered. For each spring strength we have also calculated the force if a bump in the road imparts a 3G extra loading on the suspension.. Note that the AVO springs allow a compression of about three inches before they become coil bound. Therefore any value on the table above three should be regarded as a 'no go area'.
