Silvio V 1.0 on Air

Discussion in 'Road Series (S1.x, S2.x, S30)' started by super slim, Feb 23, 2014.

  1. super slim

    super slim Zen MBB Master

    I have installed a Firestone 2M2A air bag to replace the urethane rear spring, as my aging body wanted air bags at both ends to turn mountains into pebbles, and it does.

    I am running it at 25 psi to get a full 20 mm of movement instead of the 12 mm with the Urethane spring.
    the 1/8"npt to schrader valve sits 25 mm below the seat back, but at this stage I could only get a 34 mm air valve extender, so it is fitted to the shock valve so I can install the shock pump onto the air bag in the two stages.

    The beauty is that when I add 25 kg of rear luggage, I just increase the air pressure to maintain the same wheel travel.

    I have tried to attach a pdf file of my Autocad drawing, but the system does not like it.


    Super Slim




  2. Doug Burton

    Doug Burton Zen MBB Master

    Very interesting arrangement...

    I have one concern, and I can't quite evaluate it with your photos.

    You have increased the amount of flex on the titanium spring.

    In this case, it is critical that the spring is not presented with a sharp edge to bend over at either anchor point.

    The edges should be rounded on both the upper and lower mounting surfaces to relieve stresses at the bends. It looks like this may have already been done on the frame side surfaces, but it's hard to tell definitively from the photos.

    This is important; if repeatedly bent too sharply, the leaf spring will fail at the sharpest bend.

    Otherwise, an interesting approach, very innovative.


  3. Mark B

    Mark B Zen MBB Master

    Having just gone through the

    Having just gone through the ti spring breakage, Doug's thoughts mirrored my own.

  4. super slim

    super slim Zen MBB Master

    all surfaces have been

    all surfaces have been well rounded (10 mm radius) for deflections in both directions, after I broke a titanium spring 3 years ago, due to the flat top surface on the strut, creating a point contact when the spring deflected.

    Would 4 off 0.5 mm thick rubber sheets (i.e tyre tube cut up) on the top and bottom surfaces of the spring, assist the 10 mm radius surfaces even further?

    Super Slim

  5. Doug Burton

    Doug Burton Zen MBB Master

    The radiused interfaces should be fine..

    10mm is good. Rubber is useful, but probably not needed if you've radiused everything.

    A very clever modification.

    There's a data sheet out there on this air isolator. Looks like a good choice for this.


  6. super slim

    super slim Zen MBB Master

    Here is a pdf of the autocad

    Here is a pdf of the autocad drawing, that the system would not accept last night

    Super Slim
  7. georgec

    georgec Member


    Beautiful! I have often wanted to improve on the rear bumper and this looks like the ticket.

    Could you please cite a source of supply and cost?

    I'm having problems answering my questions from your drawing. Could you please write a few sentences describing the details, e.g.,

    Did you thread and tap the bracket or is there a nut involved?

    What is the centering pin made from?

    Any other wrinkles involved?
  8. georgec

    georgec Member

    ooh duh

    ooh duh, just discovered I need to be searching for W02-358-3008 instead of 2M2A.

    At ~$40 this has got to be the lowest cost bike suspension known to man... e.g., my son was just talking about the $800 shock he has on his mountain bike... my dad ran these air ride bogey axles on his truck fleet and these things are indestructible. woo woo!
  9. super slim

    super slim Zen MBB Master

    The titanium spring has

    The titanium spring has broken after some hard testing over bikeway road to footpath sharp transitions at 25 kph.

    I wanted to test the suspension to the limit, as I was going on a week a riding through the Victorian high country with 30 km 7% climbs and descents at 80+ kph.

    I think the titanium spring did not have enough strength, to resist high sideways forces from a slightly out of line air spring.
    An air spring is an active device

    The existing M8 tapped hole for the urethane spring location pin, was drilled out in 0.5 mm dia. increments using a hand drill, up to 13.5 mm then an intermediate and final 5/8" UNC hand taps used to create the thread.

    A pedestal drill would be better to keep the dill perfectly at 90 degrees to the existing frame face, so that there is NO side or end forces on the rear suspension strut and the titanium spring.

    I am now looking at two strong piano hinges, or a steel wire reinforced conveyor belt ground down from 12 mm thick to 3 mm thick in the clamp areas.

  10. Charles.Plager

    Charles.Plager Recumbent Quant

    Hi SS, 
    Thanks for the

    Hi SS,

    Thanks for the updates. Failures like this are expected. I'm following with keen interest to see how it looks at the end.

    cheers, Charles
  11. georgec

    georgec Member

    some semi-baked thoughts

    I got one of these things to play with. Way cool! Here are some semi-baked thoughts...

    I can see that it will give more travel (which is what I want) and will certainly flex the spring through a greater range. But this has got to happen when/if I achieve a softer suspension, which is what I think I would like (I only weight 150).

    Apparently the springs do break, so maybe your breakage is unrelated to the airbag?

    Mine came with an orange tag that says... WARNING DO NOT INFLATE ASSEMBLY UNLESS HEIGHT IS RESTRICTED.

    Installed as you have shown here, there is nothing really restricting the height, so I guess you Firestone warranty is out the window, EH?

    A snubber that restricts downward motion might be called for, particularly for heavier riders or loads that require higher inflation pressures.

    As far as upward motion, there is no doubt that the stock design (stock rubber bumper) snubs upward motion effectively. This air bag installed as you have shown it will also snub upward motion when it collapses, but this will be with more travel (eyeballing it with my bike ... maybe another 5/8") than with the current bumper.

    If this is really a problem for the titanium spring, perhaps a thinner spring would be the best way to go. With the air bag installed and the ability to adjust inflation pressure, we don't need the titanium leaf to provide much spring constant. A thinner leaf (say 1/2 to 2/3 the current thickness) will take much longer to fatigue and would probably be adequate to maintain lateral alignment.

  12. super slim

    super slim Zen MBB Master

    the lower carbon fibre

    the lower carbon fibre stay has approx. 6kg.f/10 mm, and the 1.6 mm thick titanium upper spring 40 kg.f / 10 mm deflection,
    so these keep the air spring from fully extending and at 20 psi limits the downward defection (from the neutral deflection of the upper titanium spring) to 12 mm.

    These load to deflection measurements were done with a digital bathroom scale under the rear axle, and checking loads for 5, 10, 15 mm deflection without the urethane spring installed.

    If i go to 1.2 mm then the load at 10 mm deflection, would reduce to 16 kg.f, and I would then need an extension limiting strap.
    I might also need to have the central 40 mm length wider, and folded up 10 mm on each side to limit the S curve bending of the Titanium spring for axial loads due to the rear brakes.
    The 10 mm high folds would also stiffen, and reduce folding of the spring to side loads.

  13. georgec

    georgec Member

    What you say sounds right.

    What you say sounds right.

    So the only issue with extension might be when you pump it up for a pack, at which point the extension when unloaded will be more. But if that is a small part of your total riding maybe it's not a problem to worry about.

    These fatigue failures are all about the number of cycles and strain in the extreme fiber, which is a maximum at the place where the leaf bends over the attachment point.

    How long have you had your bike? Maybe you titanium leaf's time was just up.

    So... if it were me, I would double check that the brackets are nicely relieved, put a stock replacement leaf in, and go riding ;)

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