[rsheidler]

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?

 

[rsheidler]

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!

 

[rsheidler]

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.

 

[rsheidler]

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.

 

[rsheidler]

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.

 

[rsheidler]

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.