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monoshock, backwards step?

Discussion in 'XJ Technical Chat' started by bensalf, Sep 12, 2014.

  1. bensalf

    bensalf Well-Known Member

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    well I know that 25 years ago , the Yamaha monoshock innovation was in its infancy, and was seen to be a real step forward in handling, and frame / suspension design.
    but the more I get into renovating my 1991 xj600, the more (i think) in this instance, it was a backwards step ,from the twin shock set up.
    here is the setup on my xj600.

    [​IMG]

    the system comprises of , the swinging arm, coupled to a rear relay arm (in two halves) coupled to a front relay arm, pivoting on the frame, the 2 relay arms pivot together on the bottom shock mount.
    then the top shock mount pivots on the frame.

    to me the system is just over complicated.
    it has no less than 14 grease/dust seals ,yes FOURTEEN seals just for the rear suspension. most of which I have now replaced, the others which are not available I have made new seals from shaved down "o"rings
    here are the two relay arms
    [​IMG]

    and all these spindles ,bushes, seals etc are slung under the bike ,in a direct central position where all the road shit from both wheels is thrown right at them.
    not to mention a replacement shock (which has to be bike specific) is £200 around $300?
    don't get me wrong I love this bike and am enjoying every moment of renovating it.
    it just all seems a backwards step in these early developments?.
    cheers
    stu
     
  2. KDub

    KDub Member

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    What you have there is a rising rate linkage, typical of just about every modern mono-shock system. The benefits in these systems are numerous: they are cheaper to produce; allow for the use of a shorter shock absorber which reduces weight; move said weight closer to the centre of the motorcycle which aids in mass centralisation and handling; allow for fine tuning the suspension characteristics through changing the length of the linkages without the need to redesign the shock absorber; allow for changing the geometry and handling of the motorcycle, again without changing the shock absorber.

    Quickly changing those same characteristics on a twin-shock motorcycle is impossible. The only ways to effectively alter ride height are to swap the shock absorbers for longer ones, or to change their position across the frame or swinging arm. Either alteration is expensive and time consuming. To eliminate flexing or to increase the torsional rigidity of the chassis, they require stronger subframes which adds weight away from the centre of gravity and makes the motorcycle harder to handle.

    The downsides to the mono-shock, as you point out, are increased complexity of the swinging arm. Though generally the bearings used are cheaper than a shock absorber, and the entire system can be completely serviced in a couple of hours; less if they only need cleaning and lubing. Inadequate protection or lackadaisical servicing can greatly shorten the life span of the system.

    £200 for a mono-shock is cheap. A mid range one from the likes of Hagon will set you back more like £350 but they are completely rebuild-able. WP, Ohlins, etc will set you back £500 and up towards around £5k for top flight race-spec components. Again, all rebuild-able and they can be set up just about any way you like.

    To the end user, maybe those factors are irrelevant. All that is really important is that the motorcycle is fun to ride and nice to look at, in which case either system is fine. I get as much enjoyment from my slow and steady XJ as I do from my faster sports bikes. But from a performance perspective, mono-shock wins every time.

    Those early systems paved the way for the modern systems we have today. A huge improvement over the basic twin-shock. Hardly a step back.
     
  3. bensalf

    bensalf Well-Known Member

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    I agree with much of what you say, but most modern monoshocks are slung much higher, ie up under the seat and tank, this one sticks down below the swing arm, where all the crap is, im not saying modern monoshocks are a step backwards- just this one, I doubt you would get a rebuildable shock to fit this bike, this one certainly isn't, and only has 7 mechanical settings, "no air/hydraulic settings
    [​IMG]

    but I suppose this paved the way for more modern systems
    stu
     
  4. bigfitz52

    bigfitz52 Well-Known Member Premium Member

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    And that is pretty much "it."

    It was an early, evolutionary phase. Without which, more advanced designs would have taken longer.

    Which is not to say the execution wasn't a tad overdone.
     
  5. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

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    Every engineering decision is a compromise. Trading one thing for another to gain a benefit for a specific purpose.

    Take as an example the humble single-cylinder steam engine, and compare it to any multi-cylinder gasoline engine. The steam engine offers greater flexibility when considering a fuel source, greater long-term reliability (less down time over any given time period), lower maintaianace requirements, and fewer moving parts, but it's heavy and needs a lengthy warm up time before being able to do any work.

    The gasoline engine offers greater power per unit of displacement (typically), near instant warm up time when compared to a steam engine, lower weight, a much smaller physical size for a given power rating, and greater flexibility when considering applications (steam powered airplanes won't fly), but has limited choices when fuel is considered, and has a multitude of moving parts which must all work within a comparitavely small range of adjustment.

    Compromise. There is no way to work around having to do it.


    Also consider that 90% of vehicle sales are based on customer impressions and not logical, measurable performance. An "ugly" bike that outperforms a "pretty" bike just doesn't sell as well. Monoshocks were the "pretty" bikes in that era (monoshocks with single-sided swingarms even moreso).
     
  6. hogfiddles

    hogfiddles XJ-Wizard, Host-Central NY Carb Clinic Moderator Premium Member

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    My 1999 Intruder VL1500LC has a single shock.
     
  7. TIMEtoRIDE

    TIMEtoRIDE Active Member

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    I never could figure out why ANYONE would want to build a single-sided swingarm outside of the "wow" factor.

    It would HAVE to be heavier, and NEED a stronger (heavier) attach point to the frame. It would only be an advantage to an endurance race team.

    As for the mono-shock, the smartest place is horizontal, under the motor (IMHO).
     
  8. hogfiddles

    hogfiddles XJ-Wizard, Host-Central NY Carb Clinic Moderator Premium Member

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    I think diagonal, and behind the engine is best. Diagonal will be in the line of compression, and behind the engine makes most sense to me. Maybe because that's how mine is already, and I know how it works.

    I don't think horizontal is the best....you'd have to have a riser of some sort to mount the shock to, and then still have to account for compression and shaft rise/arc. If the shock were under the engine, the the shock would lift into the bottom of the engine, unless you used a shock that dampens on extension rather than compression.

    Dave
     
  9. Polock

    Polock Well-Known Member

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    my BMW has a one sided swingarm, it weighs about half as much as a xj swingarm, the bearings in the frame are twice the size. i guess it's a mono shock but it's in the conventional twin shock position.
    i think they made it a light one sided swingarm to have room on the other side for the two hundred pound muffler that rattles
     
  10. hogfiddles

    hogfiddles XJ-Wizard, Host-Central NY Carb Clinic Moderator Premium Member

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    lol
     
  11. KDub

    KDub Member

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    Single sided swinging arms were popular in the late eighties / nineties as a result of popularity the TTF1 class and endurance racing. Specifically the Honda VFR750R RC30 springs to mind, and the later RVF750R RC45. The single sided 'arms in addition to left side exit exhaust systems allowed for much faster wheel changes which negated the additional weight required to make the 'arm acceptably rigid. Ducati used a similar system through the nineties with the 916, 748 and onward. Beyond the wheel changes and aesthetics, there are no benefits in having a single sided 'arm.

    Current endurance racers employ the traditional dual sided 'arms with more sophisticated quick release systems.

    As for positioning of mono-shocks, the only bikes I can think of that had their shock-absorbers under the tanks are the early cantilever Yamaha TZ race bikes, and the road going RD LCs that followed. Honda RS125 GP bikes also use the system now that I think about it, though most follow the same pattern set by the early motocross bikes; mounted through the swinging arm. Most bikes are equipped with either a rear hugger or some kind of mud flap to protect the shock absorber.

    As a side note, the cantilever systems are not considered to be rising rate systems, and require fairly long shock absorbers to accommodate the necessary wheel travel. Primitive, but still a step forward from twin-shocks.
     
  12. bensalf

    bensalf Well-Known Member

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    -------------------------------------------------------------------------------
    If the shock were under the engine, the the shock would lift into the bottom of the engine, unless you used a shock that dampens on extension rather than compression.

    Dave

    ---------------------------------------------------------------------------------------
    I think the Buells fit into that category,
    if my memory serves me right ,they had rods the full length of the shock, which pulled on the forward end, thus compressing it. but at least it was protected by the enclosed fairing
    stu
     
  13. Polock

    Polock Well-Known Member

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  14. hogfiddles

    hogfiddles XJ-Wizard, Host-Central NY Carb Clinic Moderator Premium Member

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    Interesting. That sure is one UGLY bike, though........

    dave
     
  15. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

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    Single sided swingarms are practical (or rather were at one time).
    Henkel and Vespa both utilized single sided swing arms to facilitate easy tire changes, just like on a car. A very handy feature for the 1950's commuter. Instead of patching a tube, you just slap on the spare.
     
  16. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

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    It gets real purdy when you ride it.
     
  17. markie

    markie Member

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    Bensalf - I did exactly what you did, making my own seals. I had the (fortune?) of finding a "Betor" (Spanish) monoshock for only £130 but that was 4 years ago.
    On my relay arms, there was some corrosion on the relay arm pivots that locate in the plastic bushes. This meant that there was play even with new bushes - but fortunately I "Found" some oilite bushes at work from a packaging machine that were a slightly better fit.

    The rising rate is probably better than the mor basic system on the diversions!
     
  18. bensalf

    bensalf Well-Known Member

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    markie"-- yes I had the same problem, (mainly because the relay arms cop all the shit).
    my rear relay arm ends were completely shot, where they pivot in the swingarm
    I managed to get some new plastic bushes, and luckily my brother is an engineer, and has a couple of lathes.
    he cut off the relay arm ends, made some new bobbins and bored out the ends of the relay arms, and welded the new bobins in.
    then just a little grinding up and filling, and there as good as new

    [​IMG]

    stu;)
     
  19. Polock

    Polock Well-Known Member

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    think you could find needle bearings that fit ?
     
  20. bensalf

    bensalf Well-Known Member

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    ---------------------------------------------------------------------------------------------------------------------------
    think you could find needle bearings that fit ?
    ------------------------------------------------------------------------------------------------------------------------------
    yeh, that could be a possibility. but they still need a smooth surface to roll on, the ends of my arms, the shafts were all rusted and pitted ,
    hence the new bobbins, pictured above a little "blue" with the welding heat
    stu
     

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