Double-Check my Math pls :D (Bandit Suspension Stuff)

[Devilman]

I've slowly been sourcing together parts for the rear end of my Mk1 bandit, But wanted a double-check of the math to make sure I'm not going to cock anything up  

Standard Mk1 B6 Shock is 300mm center-to-center with 55mm stroke.
Standard Mk1 B6 dogbones are 174mm center-to-center from what I've been reading, posted by kickstart and others in various threads.

I have a very nice rear shock with 293mm center-to-center and 60mm stroke.
I've a set of RF600 dogbones which as near as I can tell are 168mm center-to-center.

From doing some measuring, raising the rear axle 1inch shortens the length between the shock mounts by 12mm (effective shock length center-to-center).

Using that and comparing it to the apparent "rule of thumb" that 4mm change in dogbone length equates to a 1inch change at the axle (I assume this is what people are comparing to when measuring ride-height changes) it makes sense that a 4mm change in dogbone length will alter the distance between shock mounts by the same 12mm mentioned above.

So in theory (please correct me if I'm wrong)...

The (roughly) 6mm shorter shock should raise the axle by aprox 1/2" (effectively lowering ride height by 1/2")
While the 6mm shorter dogbones should try to lower the axle by roughly 1.5" (effectively raising ride height by 1.5")

Resulting in a net difference of 1" lower axle compared to standard, translating to a 1" increase in ride height? (or put another way, distance between a fixed point above the rear axle on the subframe and the rear axle itself should increase by a total of 1")

Does that all sound logical and reasonable to you guys? 

[TLRS]

My head is starting to spin.. probably easier to use photo's or make a drawing

A little extra rideheight doesn't have to hurt, might actually be welcome.

[bluedog59]

Quickly reading through it you're probably right about the starting point of your suspension movement , however, you will probably have changed the geometry of the rising rate as it moves through its travel.

To find out what affect this actually has you can either,

A. sit down with a pen, paper and calculator and plot the movement of the spindle in relation to the bottom shock mount .

B. Bolt it all together and bounce your arse on it.

[wraith]

 

29 minutes ago, bluedog59 said:

Quickly reading through it you're probably right about the starting point of your suspension movement , however, you will probably have changed the geometry of the rising rate as it moves through its travel.

To find out what affect this actually has you can either,

A. sit down with a pen, paper and calculator and plot the movement of the spindle in relation to the bottom shock mount .

B. Bolt it all together and bounce your arse on it.

I'd go for B.

Ever make dog bones to suit or get some adjustable ones 

[bluedog59]

Just to throw another curve ball into the equation. What is the 293mm shock off ?

Different spring and damping rates ?

[Devilman]

2 hours ago, bluedog59 said:

Just to throw another curve ball into the equation. What is the 293mm shock off ?

Different spring and damping rates ?

Oh it will definitely have different spring and damping rates, I'm kind of counting on that fact Bandits are pretty softly sprung to start with, it's not uncommon to upgrade B12's with a GSXR1000 rear shock (K7-K8 fit easiest from what @fatblokeonbandithas mentioned).

It's from an '07 R6

[Tony Nitrous]

I’m no expert but from personal experience I’ve found that changing shock lengths and changing dogbone lengths has got me the height I want…

…. but lowering or raising on the bones also seems to change the leverage on the shock and not always in a good way. 

[bluedog59]

10 hours ago, Tony Nitrous said:

I’m no expert but from personal experience I’ve found that changing shock lengths and changing dogbone lengths has got me the height I want…

…. but lowering or raising on the bones also seems to change the leverage on the shock and not always in a good way. 

If you alter the shock length then, for the majority of its movement, the linkage ratio will work as designed. If you alter the dogbones you change the angles/leverage ratio all the way through its movement.

[TLRS]

Depends on the design!

I think generally the knuckle has more influence on the progressive ratio because it’s pivot points are closer together. Describing a smaller arc, which causes a greater variation in movement vs compression.

Edited August 20, 2022 by TLRS

[coombehouse]

Have you considered modifying the top mount on the frame to lower the shock & put the original knuckle & dogbones in the correct position. 

Or make a height adjustable top mount?

[Digs]

This many variables makes it difficult to say what the outcome will be, it’s good practice just to make one change at a time. 

if you really want to dive into suspension you can make your life easier by simplifying your design… Keep the standard knuckle geometry, standard shock, but add an adjustable top mount and adjustable dogbones. Keep everything standard height and length to start with and make incremental changes. Only changing one thing at a time.

This, or set your design goals and get ready to measure and calculate/plot all the current geometry and start designing the linkage with every variable accounted for. In order not to make a pigs ear of this you really want a CAD model or a good maquette and a spreadsheet. 
 

You mentioned the bandit was undersprung? What about just setting the sag and measuring the Motion ratio as it is? Give yourself a baseline.

Here’s a bit of light reading; don’t be put off by the maths it’s still a great resource. 

https://www.researchgate.net/profile/Ettore-Pennestri/publication/276907029_Optima_design_and_dynamic_simulation_of_a_motorcycle_with_linkage_suspension/links/555b33ad08ae6943a8794913/Optima-design-and-dynamic-simulation-of-a-motorcycle-with-linkage-suspension.pdf

Edited August 22, 2022 by Digs

[Digs]

Section 5 here should help! 
 

https://racetech.com/page.aspx?id=100

 

and here’s my last one, really concise 

 

https://www.shimrestackor.com/Physics/Spring_Mass_Damper/Link-Ratio/link-ratio.htm