D series AVO rear shock absorber: Info needed please!


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In the past, D owners have bought a standard AVO unit and adapted it to fit. AVO are in the process of looking at an original Armstrong D rear suspension unit to see if they can draw up a spec for a unit that is a good physical match, will fit straight on and have the optimum characteristics.

So I need the following information please:

1. The Armstrong unit we have is quite good, but it is bare, and we need the spec for the mountings. I assume the top and bottom eyes originally had metalastic bushes pressed into them?

2. What are the diameters of D suspension pins FT276 & FT277? Are they a sliding fit in the inner sleeves of the metalastic bushes?

3. What does rear damper bush FT189 do?

4. How wide are the metalastic bushes and how wide are the slots in the frame they fit into?

5. Are the standard units open and closed lengths optimum? It seems likely the new unit could have a longer stroke. Would this be useful, and how could this extra travel best be used?

6. Is the D suspension lever ratio about 1.5:1 like the C series? If so, we can use similar spring and damper stiffness settings.

7. AVO will dyno the Armstrong unit, but on first impression it seems much too soft. What is the general opinion?

8. Anyone who has an AVO unit fitted, was it a PA140/095, is the damping right and are the open and closed lengths right?

9. What spring rates are preferred? I would have thought with adequate damping, much softer springs could be used.

10. Are we best to try to replicate the Armstrong lengths and fitting arrangements, or could they be improved upon?

11. Anything else we need to know?



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There is a .5" metalastic bushing in both end and the RFM bushing is necked down for a 3/8" pin while the top must use the .5" diameter.


The stock assembled free length is 13.75."

This is the Works Performance Series D shock. It has three springs and the middle spring can be engaged with three positions the fourth being no movement. I don't believe I have the numbers anymore.

D Rear Shocks 2016 4.jpg

Sorry that I did not orient them the same. the top of the Works is on the right.

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Bill Thomas

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My "D" Spax is 14" eye to eye, the spring is 2.77" outside, I remember I had to have a new spring with it because the old one would not go over the Spax body, The old spring was a 400lb and felt better to me, ie I think the Spax one was stronger ?.
The top pin is a sliding fit, The bottom of mine was a Bodge in a "C" frame after much Grinding !!.
Funny you talk about the bushes, The top metal part has walked halfway out the rubber, But it was a Standard "D" top tube ?? , Maybe the Spax metal bush was not as wide as it should have been ?.
Shame on me for not spotting it. Cheers Bill.


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I will measure my armstrong on the bike and put the results up here. when on the centre stand the rear wheel is about 1" off the ground. It requires to be raised a little to remove the pins which are a sliding fit. I will also measure the sag in terms of spring length change with and without rider but that will have to wait a few days until I can get a mate to help. if you don't need this info let me know. How do you determine the lever ratio?


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The tag on is says $395, but that was a while ago. I had one for my Knight, but I did not get to try it. Pat Manning has one on his Shadow and so does Carleton Palmer. They were pretty popular here, but I am not sure that Laney or Dave Malloy stock them anymore. I don't know for sure, but I think someone in the UK complained that it was no longer available a few years ago.

With no offense to Spax users, when Penske had the spax on the shock dyno, the operator would not give out the numbers. He said they were so far off that it would not qualify as a damper. I suspect that is a little harsh. but he tested them all as a favor. He said the Koni was the closest to ideal, being only 300% off in rebound damping. I do have a new bare Series D Spax damper here and as Bill points out it is longer in its extended position than the Armstrong.

The Works is 13 5/8" in its installed position. It is a lovely piece of work, not only that it reflects its custom machined look of all the Works products, but that it out-performed all other dampers in the test by a large margin.


It was late yesterday when I posted, so I did not have time to do many measurements, but I have the units downstairs and can provide dimensions. I don't have access to a machine at present.


Bill Thomas

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When I was breaking hard for the hairpin at Cadwell, Norton front disc, People said it hopped at the back and then slid a lot, After fitting an air unit off an H.D. it was fine. That gave a Super ride, But was at the end of it's safe max' pressure, The H.D. used 2 !!. Cheers Bill.

Jim Richardson

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I live in Aylesbury, not a million miles from AVO, I could lend them my rear unit for a few days if required, complete with original gaitor.


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Power cut deleted my post, will have to start again!

Thank you everyone for your input.

From David's picture of an original D unit, it seems the unit I have left with AVO is not an original one. Strange, it is nearly right, but the eyes seem too big.
So Jim, I would greatly appreciate the loan of a unit. Meet you part way perhaps, or at AVO? (NN4 7PW)
That should ensure we get the best fit.

What about the rest of the spec gentlemen?
There may be the opportunity to improve on the original if appropriate.
I think there are 3 things to decide:

1) Spring rate.

There seems to be a bit of a consensus for about 400 lb/inch, but from C experience, I would be very inclined to test a 300 lb/inch first.
Easy to change the spring if I'm wrong or the D-owning test pilot has a preference for a firmer ride.
I think there is a general issue here:
If an owner replaces a worn and/or original Vincent suspension unit with almost any modern unit, it will be a vast improvement, so people are satisfied. But it is still not necessarily as good as it can be, that won't be evident unless you try something even better. Which is why I am keen for a soft spring to be tested first! Seems a shame not to use this opportunity to do some back to back testing.

2) Open and closed length.
This needs to be right first time, as it will not be possible to modify the prototype in this respect.
It seems likely the AVO unit will have perhaps 1" more travel than the original. Is this good and where might it be best used? 1" shorter closed length than standard would allow a lower seat height IF the mudguard does not hit the seat or something else. There is also the chain to consider. Perhaps someone with a D could experiment with the correct original damper fitted but no spring? See if extra upwards wheel travel looks possible. But remember the bump stop could compress quite a bit further sometimes.
An extra 1" on the extended length seems unlikely to be useful, as I gather the seat is already high, plus the wheel would droop more when the bike is on the stand, perhaps making the bike unstable.
If both are a liability, a restrictor could easily be included in order to match the originals open and closed lengths.

3) Damping.
This may be easy to decide, as both bump and rebound can be adjusted separately and as far as I know the standard off-the shelf AVO unit was well liked.

Macvette. Thanks for the info.
Re the lever ratio, in the past I have assumed this is simply the distance between the RFM pivot point and the axle centre compared to the distance between the same RFM pivot point and the suspension unit bottom eye centre.
(This is a bit of a simplification, as it assumes the suspension unit is aligned to be exactly tangential RFM movement, but it is near enough for our purposes.)
For the C series it is 1.5: 1, so 1.5 inches of wheel movement produces 1 inch of movement at the damper.
If the D series is reasonably close to this, then spring and damper settings that work for the C should work for the D, so it would be useful to know the ratio.

Bill Thomas

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Hello Rob, Just my thoughts, I don't think you want a longer unit, With a "D" you have to allow the chain to be more slack than a "C", A lot of people do the chain too tight, Not thinking it would alter that much. Cheers Bill.