Topic: Need advice from anybody experienced with suspension... gurus?

[Thunder]

So i am hoping somebody here can help me as i can't wrap my head around it and i'm looking for some guidance...


I have been toying with modifying the stock suspension on a conventional setup (std normal twin shocks mounted close to the wheel axle ) to something like this




By converting the swing-arm into  a Y configuration and utilizing a single shock system (nothing has been done yet I'm just trying to understand the physics of it)

To my mind, this would utilize a stronger spring? If i were to use the original twin shock it would be too strong/weak???

I keep looking at the picture and changine my mind... engineer i am not.

If anybody has a clue, or even better can explain it to me like i'm 5 , please let me know, because short of building it (at the moment it is a mind exercise) i cannot figure out how to work out how much weaker, stronger a single spring would need to be in the y configuration.


I hope i got my meaning across ok?

Thanks as always
Thor

[greenjeans]

Most guys that try this route use the shock and swing arm from a donor bike.  The swingarm will have to be modified to fit into the Honda frame.
You will likely have to move/change your oil tank location and electrics as well as the airbox.  It certainly can be done, but it will take care patience and skills.

[calj737]

If I understand you correctly, you want to use dual shocks in a side-by-side installation, as a monoshock would be? The two shocks would be installed in front of the tire, reacting to the seat/frame and supported at the center of the swing arm. Do I understand you correctly?

If so, I will give you my "non-technical" non-guru understanding-

I recently asked about something quite similar to someone who does know a great deal more than me on bike suspension. His response was: "Why do it? It offers no advantage over a conventional monoshock in that location, and it defeats the purpose of having dual shocks where they should be mounted (at the leg of the swing arm)."

I took his explanation to mean that spring rates for mono shocks and dampers differ sufficiently (as do the shock body) to provide for the performance of a single shock. Travel, rebound rates, river weight, etc are all factored into the shock design/build.

Could you accomplish tuning dual shocks to perform as a mono would in that location? Sure, I guess. But then why do it? Is it a design aesthetic you seek? If so, then I think you would truly need to speak to a suspension company that offers custom built shocks to get the best/most accurate design guidance.

As for transplanting a different swing arm that uses a monoshock to a CB, lots have done that. And many of the shocks use a damper setup to displace the forces. So nothing is straight forward when it comes to customizing suspension.

Does any of that help? And I may be off quite a bit, but I think generally it's close.

[Thunder]

Thanks guys, well wouldn't you know, its been covered in the forums to a degree befor...
http://forums.sohc4.net/index.php?topic=41453.0

Thanks so much for the help.
It was more of a idea that i thought i would try get some answers on before looking deeper, as the electrics and oil placement would then need to be relocated etc...

[Muckinfuss]

There are a number of dynamics within the context of suspension shock performance.  Every aspect of suspension tuning/modification has an effect on the planes of movement and how the kinetic energy is either displaced or advantaged.

I'll just touch on a couple....first off, everything an engineer does is to create a performance 'leg up' and always results in some compromise somewhere.  Start with unsprung weight:  Less is better.  Heavy wheels consume HP during acceleration, hence aluminum rims and race tires that weigh half of a DOT approved tire.  Next, a lighter wheel benefits from a lighter swing arm....aluminum is the choice here as well.  Cut one leg off the swing arm to make it lighter yet and you can only use one shock....  Now to shocks....by design, a shock is how you keep the wheels on the road/track surface.  Incorrect dampening in either travel direction will give less time for the tire patch to be in use during the opposite extension or contraction.

A shock with two inches of piston travel installed vertically will give up to two inches of vertical wheel travel.  Canting a shock 'forward' allows a much lower travel shock to result in more vertical travel of the rear wheel....ie a two inch travel shock canted up to 45 degrees would give from more, to much more than two inches of vertical travel.  It may also result in a rear end ride height that is above ideal for the front end geometry if the spring rate is off.

In its simplest definition for a motorcycle application....spring rate basically determines ride height front and rear both at rest and in motion.  This affects front end trail as much as anything else, including rake in the steering head.  Trail is how caster is handled for straight line tracking stability.  Too much and turns can range from difficult to scary.  Too little and all sorts of scary things happen.

To the OP's questions....ultimately, you want the end result to yield the same ride height front and rear unless you are trying to change handling characteristics intentionally...which you haven't expressed.  If it's cosmetic ....  Then you are going to be involved in finding the good guesses to get what you're after. 

IN either case....yes you need an entirely different shock to do what you want to do both for spring rate and dampening characteristics.

[Thunder]

Mucking, thanks for that:)

I hope this will illustrate better what im asking


As the more horizontal one would have more rotational play at the shock mount than the near vertical one, but how does this effect the compression and damping, supposing both swingarmas an wheels were the same weight?

[Muckinfuss]

Your picture....the travel (compression stroke) is a function of spring rate, not shocking. You haven't offered enough information to answer the question as an exact yes or no. 

You are asking questions that require an M.Eng. on both sides of the conversation.  The Kinetic energy portion of suspension design is wrapped up in a soup made from Physics, Trigonometry, Hydraulics, Metallurgy, etc.    The only thing a shock delivers to your question is the stroke distance of the piston.  If it's not enough for the travel needed, then the bottoming out will be harsh and noticeable.  Your red line symbol would be articulated using trig (arc cosine) based on known angles (to figure the unknown angles), vertical motion measured at the axle, distance of the fulcrum.  Sprung and un-sprung weight motion and engergy dispersion is a function of the shock.  FYI, shocks that are designed to be 'upside down' are done this way to reduce a few ounces of the un-sprung weight, it isn't a cosmetic 'cool factor'.

If you have a university near you with an engineering department...go there.  Seek out a mechanical track undergrad that would take on your design issues and let them have at it. They will know more than you.  You will benefit.  It may benefit them as well to do a real world task.  They might be able to sell the idea as a paper to get credit, you may want to pay them....whatever you can work out.

Or.....copy what someone else has already done that fits your concept and accept the result.  Take the path of least resistance.

[Thunder]

 

Shot budd, i appreciate you taking the time.

It was a question born of curiosity, as you know when we build bikes in our heads, eliminating the problems (hopefully) in our minds eye before actual cutting.
Thanks for the explanation , much appreciated

Thor