AVO coil-over or damper for B series bikes with unslotted UFMs.

hadronuk

Well Known and Active Forum User
VOC Member
This is my rough sketch of one way of doing it.


Coil%20over%20for%20unslotted%20B%20Small_zpszamx84wz.jpg




AVO have said they would be willing to make this, subject to costing. It consists of the new (but well tested) coil-over, with the top mounting modified as shown. A thicker round plate to support the spring is welded to the damper rod. The spring (+collet) is fitted over this, then a sort of wide clevis is screwed to this plate with four countersunk screws.
The ends of the clevis fit in place of the original spring eyes.

Anybody see any problems?
Anybody suggest a better way?
Anybody want to order a prototype to test?!!!
Would B owners want to go straight to a coil-over, or would they mainly want to fit a hydraulic damper with original spring cases?

A damper could probably be fitted the same way, but there are several possible problems that don’t occur with the coil over.
Rob
 

Vince Farrell

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VOC Member
This is my rough sketch of one way of doing it.


Coil%20over%20for%20unslotted%20B%20Small_zpszamx84wz.jpg




AVO have said they would be willing to make this, subject to costing. It consists of the new (but well tested) coil-over, with the top mounting modified as shown. A thicker round plate to support the spring is welded to the damper rod. The spring (+collet) is fitted over this, then a sort of wide clevis is screwed to this plate with four countersunk screws.
The ends of the clevis fit in place of the original spring eyes.

Anybody see any problems?
Anybody suggest a better way?
Anybody want to order a prototype to test?!!!
Would B owners want to go straight to a coil-over, or would they mainly want to fit a hydraulic damper with original spring cases?

A damper could probably be fitted the same way, but there are several possible problems that don’t occur with the coil over.
Rob

Would this be the best way to make the coil-overs for slotted tubes as well? It puts the load hard up against the outside of the UFM in the same place as the original twin spring boxes, a spacer could fill the gap.
 

nkt267

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VOC Member
Surely the sleeve F57/2 (which should be the one piece sleeve) would be used..John
 

nkt267

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VOC Member
I just fitted a new sleeves to a B rap with a damper, the sleeve is a running fit in the UFM..
I must be misunderstanding you over where the gap is..john
 

hadronuk

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VOC Member
Would this be the best way to make the coil-overs for slotted tubes as well? It puts the load hard up against the outside of the UFM in the same place as the original twin spring boxes, a spacer could fill the gap.


Fair point. As you say, the UFM was designed for the spring loads to be right next to the strongest part of the UFM, which this would achieve. But doing it this way the load still goes into the centre of a beam. We can make the beam as thick as we like, but the thicker it is the higher the seat height. I did give some thought to the altered load distribution with the "standard" AVO coil over. The Thornton coil-overs (and others) have been in use for many years with apparently no problems. And remember that when the standard Vincent hydraulic damper bottoms out hard, that substantial shock load goes straight into the centre of the top spring bolt anyway. I took the precaution of replacing the bronze bush in the top eye with a steel one. Consequently the top spring bolt can be tightened much more firmly so that the spring bolt and the spacers together form a composite beam 9/16" in diameter. I found with the bronze bush fitted, firmly tightening the top bolt resulted in the ends of the spacer bushes deforming because they only overlap the steel part of the eye by 1/32”. This crushes the bronze bush onto the spring bolt making removal difficult. And if the top bolt is not tight, the sleeves add nothing to the beam stiffness and we effectively only have a circular beam of 3/8” diameter. This is a fair bit weaker than a 9/16” diameter composite beam because a solid circular beams stiffness depends on the fourth power of the diameter. Area Moment of Inertia, Ix = π d4/64
 

davidd

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VOC Member
I made the coil-over for the racer back before Works had made one. I had the same concerns that Rob had about weight distribution. It seemed reasonable to be careful about routing all the suspension forces through the damper mountings when they were not specifically designed to accommodate them. As a result, I made the top mount out of SS to give is some heft and the lower mount was aluminum, but it spread the load to the spring mounts.

100_1600.jpg


I cut off the tang of the shock and threaded the body to take the SS replacement which had the new tang and a cup for the spring.



100_1602.jpg


The lower mount simply screwed on and I locked it with a nut.

100_1255.jpg


There was plenty of clearance for the top of the RFM. It was not an adjustable spring platform, but the 200 Lbs. spring worked fine.

The center mounted shocks owners are running seem to have performed fine. My mods may seem too conservative now, but I think it helps to follow good design principles and I had always planned to do a similar mount on the next racer, but lighter.

I would note that sometimes you can turn the shock around and put the base up top (like the Works damper). It will depend upon what AVO says. However, if it is possible to do that you could design a more robust way to span the non-existent slot by starting the spring perch or spring perch threads a little further down the body which would give you a bigger and maybe slightly taller base to build from.

By the way, you cannot run the Works damper the other way around due to its design,

David
 
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