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Rear Suspension - Mono Shock vs. Dual Shock

67248 Views 31 Replies 22 Participants Last post by  pplassm
The point about available room is a good one, but a monoshock has one definite advantage: It's easier to tune.

Many monoshocks are fitted with variable linkages, so that effective spring rate and damping can be increased or decreased as the suspension moves off "neutral" position. As I type this, I can't think of any dual shock setups I've seen that incorporate variable linkage (OK here comes the onslaught of counterexamples - have at me, folks) unless you count the variable attachment point used by Velocette about 40 years ago.

Of course, variable rate springs and very fancy valving in the damper units could do everything a variable linkage accomplishes, but it would surely be more expensive to manufacture. Not only that, but a suspension tuner can surely (within limits) fab and install new rods and linkage parts to change the geometry with straightforward and readily available machine shop facilities. And the manufacturer can fine tune the linkage during final product development without upsetting the whole production operation.
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From my personal experiences I've come to feel that bikes I've ridden with monoshock are less subject to wallowing in hard turns. This could be more a factor of the improved frame designs that reduce flexing, though.

I'm certainly no expert, but it seems logical that a dual shock setup has the liability that one shock is going to compress differently than the other no matter how carefully you try to tune them. Monoshck eliminates any possibility of an imbalance between the two.

Another advantage would appear to be weight saving.

All this in addition to FrankS1's comments about the ability of monoshock systems to allow the design of variable rates.
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Its not so much a question of which is better, it all depends on how well the suspension is designed for the application and quality of the components.
A single shock would of course have less unsprung mass, so, possibly better performance. Of course it's all down to the design target. As for wallowing or not, I think that also falls into the design target. In other words, generally, single shock models are more "perfomance" targeted.
I can certainly see that a monoshock rear suspension would produce a more rigid feel as the wheel moves through its travel, but I seriously doubt (depending on the materials used) that it would weigh less than a two-shock system. Keep in mind that a monoshock has to do the work of two shocks, so it must be a beefier unit with a bigger spring (maybe a larger gas/oil reservoir too). The monoshock swingarm with all its linkage and adjustment pieces add to the weight of the rear suspension system.
True, a mono shock does have to be beefier, but don't forget about the differences in mount points and hence the impact of leverage on what becomes unsprung weight. In other words the monoshock being further away from the rear wheel has less impact on unsprung weight. The difference in leverage however means the monoshock has to control a stronger force (longer lever ie the swingarm) and hence may be heavier than twin shocks. Unsprung weight is however more significant for handling than overall weight.....

It seems that you really don't know what you are talking about when it comes to levers and springs. There aren't many monoshocks that are mounted far forward without linkages to decrease the force/amount of compression among other things. In addition, the only part of a shock that will contribute to unsprung weight is the lower half of the shock body. This is because the upper half is supported by the spring, and the spring itself is in some state of quasi self support. In other words, if I put the swingarm/wheel/tire/brake/etc. on a scale it will weigh the same for 2 shocks or 1.
It ain't how many shocks, but the quality of the ones you got. Early twin shocks were of low quality. Performance was inconsistent between a pair of shocks. Combined with spindly swing arms there were all kinds of handling problems. Girling sold 'matched' pairs for extra dollars.

Replacing low quality stock shocks with high quality aftermarket units was one of the first things a serious rider would do. The rumour was that japanese shocks of the 70s used fish oil for a damping medium. Any damping ability they had usually disappeared after 5000 miles or so.

It costs as much to build a single shock as it does to build two, all other things being equal, which is probably the biggest reason that single shocks are so popular with bike makers today.
The baseline problem with dual shocks is that there will always be variables between two shocks. Simple manufacturing tolerances assure that there will be differences, albeit slight, between two units. Over time these differences will increase. A monoshock set up eliminates the possibility of these differences. Does the difference really matter if you're not riding hard? Probably not. It's like single-sided swingarms. They're a great idea, but a dual sided is more stable.

Dual shocks with linkage set-ups

I remember a couple of designs that were available on dirt bikes in the '70s. One example was the last series of Ossa MAR/Plonker trials bikes that came out. I think the trade name of the linkage set-up was Bolger.

Sorry. Couldn't resist.
Anybody out there actually know anything?

For sport/racing bikes, I would think the biggest advantage of the mono-shock is aero-dynamics. Next biggest might be weight. Nobody so far really knows, right?
I agree that a spring/shock unit acting through linkage offers better tuning opportunities, and is certainly one reason for use on race bikes. Of course, for 99.9% of the bikes with mono shocks, nobody will ever even consider replacing the linkage.

I think the biggest reasons for chosing one or the other are:

1) Image -- do you want the bike to seem modern or retro? A mono shock on a Triumph Bonneville just wouldn't seen right, same for a Harley Sportster. Dual shocks on a new sportbike would seem equally out of place. Of course the Harley Softtail is also a Mono shock dictated by styling considerations more than performance.

2) Packaging, as mentioned, is a big factor. Where does the designer need the exhaust pipes to run? Will dual shocks get in the way of saddle bags, etc?
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Remember the Virago!!

Remember the first Virago models? Mono-shock rear suspension. Worked great! Looked like hell! However, Triumph has done a great job of making a bike look "classic" while retaining the monoshock configuration. There's no question: monoshock setups are easier to tune, weigh less, and streamline the looks of a bike. Dual-shock setups are easier for a non-technical rider to adjust for preload (usually with step-collars) and are more visceral, which is why they show up on most cruisers. However, many new "retro" sport bikes now have the dual-setup as well, especially the Kawasaki ZRX models. And that system works great!

In the end, mono is better. But better has never meant "domination" in the market. Just ask Bill Gates. Or Harley (although HD makes beautiful- if slow- bikes).

I remember when the running joke was that the only purpose for the shocks that most Japanese bikes came with was to keep the rear tire from rubbing against the rear fender while you push it around on the showroom floor -- sorta like they said about the original equipment tires -- that their purpose was to protect the rims during shipping and handling.

Back in those days, it seems that most riders, at least performance-oriented ones, routinely replaced the OEM tires and shocks almost immediately. I know I purchased several Yamahas in the 70s for which I replaced both at the first service after engine break-in. Things sure have changed!
Assuming that the swing arm has the level of torsional stiffness that modern handling standards demand, it should not matter if the two shocks are tuned alike -- hell, there are quite a few monoshocks that work just like dual shocks with one shock removed (obviously requiring a stronger spring/damper).

When I used to race my old SR500, with a welded on swingarm brace, I often deliberately used different springs left and right. I only had two different rates to chose from, and if I wanted something between these rates, using one of each allowed me to split the difference.

On older bikes, the swing arms had the torsional strength of overcooked noodles, hence the recommendations to be sure to set preload etc equally. On any modern bike, I cannot believe that any imbalance within the entire range of adjustability could effect handling (of course the resulting overall spring and/or damping rate would do so).

I think the wallowing you note is likely a function of the fact that more performance-oriented bikes tend to use monoshocks, thus they have more rigid chassis and generally more attention given to suspension tuning.
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When one talks about leverage, one must consider the total leverage, which, as you point out, is a function of not only the length of swingarm relative to the shock, but also of any linkages. What is relevant is to compare shock travel to wheel travel.

I don't have specific figures handy, but I know that most mono-shock units have much less travel, relative to a given amount of wheel travel, than is the case with typical dual shock setups.

Aside from packaging, and the ability to fine-tune the rate of progression in spring/damping rates, this is probably the big technical advantage. This is particularly important in dirt bikes, where getting 12+ inches of travel with conventional shocks was difficult, and controlling damping with a shock moving through such long distances (at such resulting high speeds) was problematic. Not a coincidence that modern monoshocks first appeared on dirt bikes, and only later were adapted for streetbikes.
Actually, the Softtail has two shocks, they're just both tucked underneith the engine between the frame rails.
You are, of course, correct. In this case, the choice of 2 rather than one shock is probably dictated by packaging -- probably hard to find a space for the larger single unit and still keep it out of sight.
Single Sided Swingarms

Clearly there is no performance advantage to single sided swingarms. It seems intuitively obvious that for a given amount of material (= weight), a double sided swingarm will have greater torsional stiffness. Note that Ducati have gone back to the double sided swingarm for the 999 for this reason.

Single sided swingarms have a couple of possible advantages that have led to their (limited) use:

1) Facilitates tire change -- important in endurance racing where quick pit stops are important, also probably of interest to touring bike riders, hence the use by BMW and Goldwings.

2) May facilitate exhaust routing -- this was a factor in the Ducati 916 series. Part of the reason for the funky muffler on the 999 is to compensate for the unequal pipe lengths dictated by the rew routing required.

3) it looks "trick" and high-tech.

For shaft drive bikes, I suspect that the shaft housing etc required anyway is already heavy enough that it is close to being stiff enough anyway, so there may not be much weight penalty, and in any case, the designers are not that worried about cutting every possible gram from the weight of such bikes.

For chain drive bikes, the system has an additional disadvantage in that chain adjustment requires an eccentric adjuster, that changes ride height and suspension geometry with a chain adjustment.

In my opinion, they are kind of like single sided forks that have been used in a few cases (I think mostly on scooters, and a few mountain bikes) -- they are a cool technical novelty -- it is neat to be able to do it, but why, other than to show off your technical prowess.

Still, I do think that the single sided swingarms on 916, VFR and Honda Hawks do look pretty sharp.
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Check out the duel rear shock set-up on the Japan and Euroup only CB 1300. VWW
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