Twin shocks vs. monoshocks?

Discussion in 'The Perfect Line and Other Riding Myths' started by SloMo228, Apr 30, 2014.

  1. SloMo228

    SloMo228 World Class Cheapass

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    Maybe this is the wrong forum, but I'm interested in hearing people's thoughts on the differences between an old-school twin shock and a modern monoshock rear suspension. Assuming you could somehow have two otherwise identical chassis, one with a monoshock swingarm, and the other with twin shocks, and the shocks/springs themselves are of equal quality and appropriately set up for the rider, what are the inherent advantages/disadvantages of each?

    Are there any handling (or other) advantages to a twin shock swingarm, or are bikes which have them simply designed that way for aesthetic or perhaps economic purposes?

    I'd like to hear some people's ideas about this.
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  2. NJ-Brett

    NJ-Brett Brett

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    Dirt bikes or street bikes?
    Its hard to get 12 inches of travel out of the dual shock setup.
    On the street, twin shocks make it easy to adjust for added load, and the shocks keep bags out of the rear wheel, otherwise the single shock setup likely has some performance advantages but uses up space.
    #2
  3. Moronic

    Moronic Long timer

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    Consider how it would work if you replaced the monoshock on a "modern" rear end with twin shocks mounted in the same place.

    What difference would that make? 1: More parts. 2: Probably more static friction (stiction) from using two small seals on two small diameter shafts rather than one big seal on one big shaft.

    But probably, looking only at performance, not much difference at all.

    So, looking purely at the choice of one or two shocks, the main advantages of the single shock seem to be space efficiency, cost efficiency and serviceability.

    Okay, but twin shocks on a monoshock chassis wouldn't be an old-school twin-shock suspension. So let's look at the question from the opposite end.

    How would it work if you replaced the twin shocks on a traditional twin-shock bike with a bigger single shock just on one side?

    In fact plenty of bikes these days are built like this, and some makers (e.g. BMW) were doing it in the '80s. The advantages of the single-shock set-up will be exactly the same as in the former case: space efficiency, cost efficiency, serviceability.

    However ... in order to use the shock just on one side, the swingarm and its pivot on the frame have to be stiff enough to carry the significant torsional loads generated by having the load on the tyre resisted by a single component that is offset several inches to one side of the tyre.

    On a twin-shock bike, that torsional loading is balanced by the torsional loading from the shock on the opposite side. So ... one manufacturing advantage of the twin-shock set-up is that you can get away with a less robust swingarm.

    As suggested above, there are also design and serviceability advantages to running the shocks either side of the wheel (or on one side) rather than in front of the wheel: it frees up space between the tyre and the engine, and shocks placed there can be (much!) easier to remove and adjust.

    IMO that just about sums up the theory. What about the practice?

    In practice, the main advantages of the single shock chassis tend to be improved stiffness and a better shock (as it is cheaper to build one big shock than two little ones). The cost of upgrading the single shock will be lower also, to similar performance levels.

    A secondary advantage is that some chassis designs run the single shock through a progressive linkage, which allows a single-rate spring to act like a progressive-rate spring - at the cost of placing a bunch of small bearings in some of the grittiest spots on the bike. :1drink
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  4. scootrboi

    scootrboi Long timer

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    The rear wheel comes off like a car wheel. That is what I like.[​IMG]
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  5. RFVC600R

    RFVC600R SAND EATER

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    Sorry to go off topic, but is that the bike you spent 42 years on?? :ear Good job man, I hope to have my XL half as long as that.:clap
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  6. Moronic

    Moronic Long timer

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    Nice one. You can't do that with trad twin shocks. :lol3
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  7. JohnCW

    JohnCW Long timer

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    A monoshock has the significant advantage of allowing a longer rear suspension travel.
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  8. Moronic

    Moronic Long timer

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    Perhaps ...

    [​IMG]
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  9. Rucksta

    Rucksta SS Blowhard

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  10. scootrboi

    scootrboi Long timer

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    ]
    [​IMG] The chain drive monoshock swingarm design is from 1951 and was used on all Heinkel scooters and minicars. Changing wheels on the back takes a few minutes, and a spare is mounted under the luggage rack.
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  11. DAKEZ

    DAKEZ Long timer

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    THIS ^^^^ [burp]
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  12. Rgconner

    Rgconner Long timer

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    The Vespa put everything into the swingarm in '47. More unsprung weight, but also allows for a monoshock.

    Front is built like a nosegear, so it is also monoshock. (well, mono SPRING on the early models)
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  13. cccolin

    cccolin Long timer

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    this is relevant to my interests. would like to do a conversion to dual shocks for aesthetic reasons but hadn't considered the extra cost. hmm
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  14. SloMo228

    SloMo228 World Class Cheapass

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    What's been posted pretty much sums up what I expected, though I never thought there would be a difference in suspension travel. Makes sense, though. The single shock is usually mounted far forward of where dual shocks would be, so the shock doesn't require a full 12" stroke to get 12" of suspension travel due to the lever effect of the swingarm.

    One plus of dual shocks that I liked on the ZRX I owned was that not having the single shock allowed for a pretty big storage area under the seat. It was very handy when using the bike as a commuter.
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  15. NJ-Brett

    NJ-Brett Brett

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    Yes, there are also two setups for single shock, a small shock with a linkage in front of the rear wheel, and the early setup like my xt200 has, a giant shock under the seat and gas tank.
    That takes up all the space on a bike, the battery was put in a small box mounted behind the motor and in front of the rear wheel.

    Twin shocks gives lots of room under the seat.
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  16. markk53

    markk53 jack of all trades...

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    Didn't read it all, but key points for a single shock set up were:


    • less mechanical parts to make a rising rate suspension system.
    • hard to bend a single shock in the center of the bike when you crash on a log or such.
    • One shock to adjust.
    • If hung on the one side with a single side swing arm makes the rear wheel accessible quite easily for road bikes.
    As far as travel, there were 12" travel twin shock bikes in the late 70s, so that really isn't the reason.

    The thing that is hard for me to comprehend is why it is so much more expensive for a quality single shock over a similar quality twin shock replacement set for a road going bike.
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  17. scootrboi

    scootrboi Long timer

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    Twin shock designs are cheap to design, cheap to make. There is an element of cantilevering in a single springleg design that requires more robust parts, right down to the bearings.
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  18. markk53

    markk53 jack of all trades...

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    Definitely need a serious swingarm and all for single side.

    Of course twin shock may be easy, but they still bend if you fall over on something like a rock hard enough - not good.
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  19. dirt hokie

    dirt hokie Been here awhile

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    adjustability and balance and weight
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  20. markk53

    markk53 jack of all trades...

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    Which one?

    Seems a shock on each side is as balanced as one in the middle. Most single shocks are as heavy as two single shocks. And adjustability comes with either as well. All the major shock manufacturers make top shelf shocks in either design.

    Biggest reasons I see for single shocks would be having it out of harm's way, narrowing the rear of the bike, and one shock to adjust (although sometimes buried in the bike).
    #20