twin shock motorcycle suspension

[Quarantine]

I came across the twin shock suspension most motorcycles in the 70's used and would like to get to know the reasons why it was designed the way it is. One end connects to the rear hub the other end vertically connects to the sub-frame, usually under the seat.

The shock does not have to be vertical 90 deg exactly from the pictures I have seen, but how does the angle of the shock affect the bike's performance? I've seen some almost 90 deg vertical some 80 deg and others mounted close to 45 deg. Why is that? is it damping related? only aesthetics? Are mounting points to the sub-frame crucial?

I know raising the height of the back shock can change handling characteristics making them sharp or less. But I don't know what maintaining the same sub-frame height but changing the angle or rake (if you can call rear shock that) does

[Duccout]

I'm no expert, but I believe that the angle of the shocks affects the wheel travel, or the ratio of wheel travel to shock travel. Completely upright shocks give a 1to1 ratio, ie 1" of shock movement gives 1" of wheel movement, then if the angle of the shocks is increased, the wheel travels further for a given amount of shock travel.

[Quarantine]

Thanks for the feedback.
But what are the pros and cons for both 1 to 1 ratio shock/wheel travel and the other setup? stability? road feedback? comfort?

[Duccout]

I think that it depends on the road surface. A bike ridden on smooth roads needs little wheel travel, but over a bumpy road, more travel = better comfort and better contact with the road. Look at twin-shock motocross bikes and see how much travel they have to try to soak up the bumpiness of the tracks.

[Quarantine]

Thanks for the info.
I also think that you can keep the shock vertical and still affect road travel
by moving the shock farther away from the wheel hub closer to the main frame

[wdietz186]

Shock location, angle,travel etc. are all variables dictated by the design of the chassis,swing arm location,length,desired angle[pro squat or pro lift] and intended use.Also in the equation are the size and desired damping capacity of the shocks themselves.If you look at the bikes from the late '40 and '50s when rear suspension was in its infancy you will see most all of the shocks were vertical and the swing arms fairly short because the travel was only a few inches.Later designs added travel as the engineers figured how to improve the damping and chassis integrity to keep things controllable.

[Quarantine]

wdietz186 thanks for the valuable input. You are right too many variables go into suspension engineering and design. Thinking of buying a bike and converting it into a cafe racer and one of the first things I want to play with is the rear shock. But I going to have to learn a lot more before getting my hands dirty.

[st2lemans]

It also changes the effective spring rate (and damping). Basic mechanics (leverage). If you angle the shocks, you get more wheel travel (for the same shock travel), but you also soften the spring rate. It's pretty easy to calculate. Some bikes have multiple mount points, so that you can simple move the shock around rather than change springs or shocks.

Tom

[Quarantine]

Thanks Tom,
Now I think I grasp the idea behind twin shock suspension.
A single shock will also give more wheel travel. I understand that need for
off road use, but do modern road bikes benefit from the single shock when it come to wheel travel? Or would it be just as good with twin shock

[Duccout]

There are so many variables here that a straight answer is difficult, but the fact that a single shock has much more scope for mounting, including the use of rising rate linkages, then I think that a single shock is better.