F: Frame Spring Strengths to be used with Fully Sprung Rear Seats and AVO Coil Over Dampers

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Nigel Spaxman

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I finally measured my spring. My seat is fully sprung. I have the standard AVO spring it measures 160mm when on the stand. (it has a lot of preload) The same when sitting on the ground. 155mm with me on the seat 220 lbs. I have it adjusted this way because most often I do have Allyson as passenger. 133mm with me and Allyson on the the seat. Total weight about 390 lbs. The ride was comfortable but sometimes on really big bumps especially with luggage the shock would bottom On the last long trip I broke a few spokes so before our next trip in a few weeks I am going to install the stronger spring.

Once I do that I will measure it again. I expect it is going to be better for two up. Hopefully it will also be OK for solo. I might even be able to run with slightly more sag and have less chance of bottoming, that's what I hope. Probably it won't be as good for solo but really it is not used that often that way and it matters less then anyway.

My bike has the B RFM, I don't know if that makes much difference. I have the AVO on the front as well and I am very pleased with both. Before that I had a rear Koni and a modified Vincent front shock with the original seat stays.

Nigel
 
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timetraveller

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OK Nigel, thanks for the input. Going through your figures and comparing them with the above table I see that for a 220 lbs rider we found that a compression of the 275 lbs/inch spring would 1.9". Converting your measurement of 160 mm back to inches for the benefit of the table gives 6.3" which when removed from the 8" length of the spring means that there is 1.7" of compression. If our figure of 1.9" is right you could give it a bit more. We did find that about 90% of the weight of the pillion passenger is passed onto the rear spring. 90% of Allyson's weight is about 150 lbs and if we add that onto your 220 lbs it just takes us off the bottom of the table but near enough to show that you woiuld need about 2.4" of preload but a 3G bump would certainly bottom out. Chris has found that with his weight, about 300 lbs, he is not getting much movement on normal road surfaces with the 350 lbs/inch spring. He is a very ingenious person. Currently he has created an indicator using a speedo cable that will show the movement of the top of the RFM along the crash bar. That way he can actually see how much damper travel is occurring. In the mean time I am going to try to produce a graph that will show what would happen if one used a double spring, say a 275 plus a 400, i.e. two equal length springs. I suspect that the 275 would be coil bound much of the time.
 
ClassicBiker

ClassicBiker

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I'm interested in the AVO dampers but on AVO's website there is a lack of information about lengths and purpose. Using AVO's nomenclature there is a Type C Standard Length, a Type C Short Length, a Type C Short Coil Over, a Type D Short Coil Over, and a Type D Standard Length Coil Over. I don't think I'm too far out on a limb in thinking that as Type C and Type D are listed separate because they are not interchangeable. But am I correct in thinking that the reason there are short versions is because folks want to lower their seat height? Am I also correct in thinking that the Type C coil over is intended to lower the seat of a fully sprung Series C as a standard length coil over for the C would be to tall for most?
The standard AVO dampers are less expensive than Ikon versions that can be purchased from Ikon USA, which makes the coil over rear version all that more appealing. Any clarification would be greatly appreciated.
Steven
 
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timetraveller

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I will try to answer these questions. AVO would be the best people to answer but failing that try my answers. When I am in any doubt I will mention that in the text.

'D' Normal. This is intended to replace the original Armstrong units provided on the 'D's. You should note that the front UFM fixing for these dampers is totally different from the 'B's and 'C's so the dampers are longer and not suitable for earlier bikes.
'D' Short. They are intended only for 'D's but aim to reduce the seat height.
'B'/'C' coil over. Intended to provide springing and damping to the rear of 'B's and 'C's. Both damping and spring preload are adjustable. The limited length available forces compromises on the length of springs available and that is what we are trying to understand.
Now this is where others might be better informed than me. As far as I remember AVO originally made a damper for the rear of 'B' and 'C's. This was intended to be a direct replacement of the original dampers. The total length available caused some problems and so it was decided that if the lower pivot on the damper was displaced from the centre line it would be possible to find a more effective design. On these dampers the tube which acts as the lower pivot is welded to the side of the lower boss giving at least half an inch of extra travel. If you can find pictures then the difference is obvious in that on some the lower tube is as the very bottom of the damper while on the others it is at the side of the lower boss. Any corrections from those better informed than me would be useful and welcome.
 
ClassicBiker

ClassicBiker

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TT,
Thanks for those explanations. The AVO website only has generic photos for the standard B/C rear so I could not compare to the short version. Realizing that the C front would probably look similar to the standard C rear I had a quick look and I see what you mean by where the pivot is welded. I will contact AVO about ordering as the website doesn't have any country to ship to outside of the UK. Even with the added cost of the coil over and the need to order the spring separately it is still on par with a set of Ikons, which are not compression or rebound adjustable.
Steven
 
N

Nigel Spaxman

Well Known and Active Forum User
VOC Member
I find the standard
timetraveller said:
OK Nigel, thanks for the input. Going through your figures and comparing them with the above table I see that for a 220 lbs rider we found that a compression of the 275 lbs/inch spring would 1.9". Converting your measurement of 160 mm back to inches for the benefit of the table gives 6.3" which when removed from the 8" length of the spring means that there is 1.7" of compression. If our figure of 1.9" is right you could give it a bit more. We did find that about 90% of the weight of the pillion passenger is passed onto the rear spring. 90% of Allyson's weight is about 150 lbs and if we add that onto your 220 lbs it just takes us off the bottom of the table but near enough to show that you woiuld need about 2.4" of preload but a 3G bump would certainly bottom out. Chris has found that with his weight, about 300 lbs, he is not getting much movement on normal road surfaces with the 350 lbs/inch spring. He is a very ingenious person. Currently he has created an indicator using a speedo cable that will show the movement of the top of the RFM along the crash bar. That way he can actually see how much damper travel is occurring. In the mean time I am going to try to produce a graph that will show what would happen if one used a double spring, say a 275 plus a 400, i.e. two equal length springs. I suspect that the 275 would be coil bound much of the time.
Click to expand...
When I looked at your chart I noticed that for the 220 lbs rider (me) the compression of the spring is shown as 1.89" (275 lbs spring) That means the spring is compressed to 6.11" which is exactly 155mm the same as what I measured. I think you didn't understand what I had written. The 160mm measurement was with the bike on the centerstand and remained the same with the bike on it's wheels but without anyone on the seat.
 
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