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Old 04-30-2014, 06:26 PM   #1
SloMo228 OP
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Twin shocks vs. monoshocks?

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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Old 04-30-2014, 07:06 PM   #2
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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.
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Old 04-30-2014, 08:04 PM   #3
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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.
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Old 04-30-2014, 09:25 PM   #4
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kind of an old idea

The rear wheel comes off like a car wheel. That is what I like.
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Old 04-30-2014, 10:19 PM   #5
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Sorry to go off topic, but is that the bike you spent 42 years on?? Good job man, I hope to have my XL half as long as that.
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Old 04-30-2014, 10:35 PM   #6
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The rear wheel comes off like a car wheel. That is what I like.
Nice one. You can't do that with trad twin shocks.
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Old 05-01-2014, 12:19 AM   #7
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Quote:
Originally Posted by SloMo228 View Post
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.
A monoshock has the significant advantage of allowing a longer rear suspension travel.

JohnCW screwed with this post 05-01-2014 at 12:33 AM
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Old 05-01-2014, 01:20 AM   #8
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A monoshock has the significant advantage of allowing a longer rear suspension travel.
Perhaps ...

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Old 05-01-2014, 03:06 AM   #9
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Some may find this article interesting

http://www.carbibles.com/suspension_bible_bikes.html


Look for
Motorbike suspension - back end.

about 1/3 down
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Old 05-01-2014, 06:01 AM   #10
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chain in oil bath

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Nice one. You can't do that with trad twin shocks.
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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Old 05-01-2014, 07:31 AM   #11
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Originally Posted by NJ-Brett View Post
Its hard to get 12 inches of travel out of the dual shock setup.
THIS ^^^^ [burp]
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Old 05-01-2014, 07:48 AM   #12
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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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]

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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Old 05-01-2014, 08:25 AM   #13
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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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Old 05-01-2014, 08:37 AM   #14
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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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Old 05-01-2014, 09:57 AM   #15
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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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