Topic: How do I figure out where to place my shocks?

[fastbroshi]

Hey folks,

  So I've decided to try a swingarm on my 550 racer that's longer than the stocker by about 2.5 inches.  This was a boxed steel monoshock arm from a CBR600 F3.  I want to just add mounts for new shocks at the best location on this arm, so obviously the stockers are too short and I wouldn't want to use them anyway.  Problem is, I have no idea the geometry or formulas I'd need to use to find the best angle to lay them down at. 
  It seems Tony Foale has a suspension program for sale on his website for a reasonable price that can do this, but I'd rather not have to do that, there's got to be a simpler way.  Any thoughts?

[bwaller]

Have a shock mount with three or four placement options.

[dusterdude]

broshi,when you get them mounted i would love to see some pics.i couldnt figure out how to mount shocks with the same arm on my 750.they would look like dog doo.

[fastbroshi]

Hmm that's a good idea Brent.  Yeah I'll post some pics no problem.  Still don't know where I'm going to put the flux capacitor though.

[Rick4004]

I'll give a quick, if incomplete answer. There isn't one place that is "correct". A shock that is mounted vertically will work, but it will give a regressive action as it is compressed. (less force will be required to compress the shock the more the shock is compressed).

Moving the top mount forward will start to give the shock a progressive action, ( more force is required to compress the shock the more the shock is compressed) up until the shock is layed down at approx. 45 degrees. If layed down further than approx. 45 degrees, the action will move back to a regressive action.

It is a bit of a chicken/egg situation. Are you going to buy shocks with off the shelf springs? Then you need to mount the shocks to use the existing spring rate. Or, you can decide on the layout you want, and then calculate the proper spring rate from there.

Moving the mounting locations can have a dramatic effect on the spring rate. Calculating the force that needs to be balanced by the springs is fairly easy to do if you have an Engineering background, or Tony's software will do it for you.

Rick

[fastbroshi]

I'm tracking you Rick.  No way I'd have to buy off the shelf, I want springs for weight and they'll be revalved as well.

[Hannibal Smith]

I used to do this stuff 10+ years ago professionally-
I would do this with "Mono" shocks normally, and design and manufacture bellcranks to get the desired effect.

Anyway, as stated above, the shock anchor point setup is a lot more complicated but yet quite simple too.

Shade tree style, I would get a yardstick etc. and pivot the stick/rod from the swingarm pivot from the top (frame) allocation and swing it down, this gives you the swingarm arc from mount to swingarm. Mark the swingarm. This is your "neutral" mount to mount arc.

But.......your rear shocks travel on a linear plane, so this arc is just you engineering basis.

The trick is to exaggerate the action in your mind.

Let's say we have a 10 foot swingarm, one with the shock mounted 1 foot back, and one with the shock mounted 10 feet back (long swingarm!)  Lift the rear wheel 6 inches................the shock mounted 10 feet back will lift 6 inches, but the one 1 foot back compresses much less for the same 6 inch travel at the rear, and the arc will have a correspondingly smaller radius.

Once you do the yardstick bit, determine the outermost point of the arc (farthest away from swingarm pivot), this is the "hump" that must be accounted for. Since you already have a set shock anchor (frame) this point is set already, you just need to find it.

The kicker is the arc, if you run the system so that the shock is always running to compression, and not going "over the hump" in the arc, you have a pretty basic setup that is not out of line- lay the rear shock almost parallel with the swingarm and anchor it, it will extend in action and not compress if the swingarm angle at sag is not engineered correctly.



This is where it gets a little complicated, you can engineer the angle of attack so to speak, to get your shock to run the spring rate up (progressive) at just about any rate- coming on fast, or relatively slow, but your spring rate must match the whole shebang.

As long as you are in the ballpark, you should be fine.

I had a bike come in once (water cooled GSXR 1100 in an 750 frame) and the hotdog rider was having rear suspension/handling issues. I tore the back end down, and found out that the swingarm had been frozen up solid. This guy raced like an animal for many races that season with no rear suspension- It's critical, but this tale just keeps things in perspective.
Anyway, this is just 1/10 of 1% of the stuff involved, but you get the idea.
 

[Hannibal Smith]

Forgot to add- If the swingarm is angled "down" like 99.9999% of all bikes, then you will always run into the "hump", but it's effects are minimized and accounted for, with a monoshock and a bellcrank, it can be engineered to run basically however you want it too with little trouble- Hence the modern monoshock.

Just get the swingarm mount so that you always have the shock in a compressive (throughout the entire travel you want/need) state for starters, then tweak from there.

[fastbroshi]

  Thanks for the response Hannibal, I was thinking of just what you spoke of but couldn't really figure out which way was best to figure it out.  I was thinking of maybe making a cardboard template to lay on the swingarm and use a pen attached with string to the swingarm pivot to mark this arc to show me the "hump".
  And then depending on where I assumed a swingarm mount point for the bottom of the shock, scribe the different arcs.  Sound about right?

[Hannibal Smith]

My 550 is in parts, but if it was together with the stock shocks, I could get an idea where Honda ran the mounting allocations in relation to the travel arc.

You could just mimic the factory mounting locations, and just run a correspondingly shorter shock (in relation to the arc) to get the same ride height as stock, or just run stock length and calculate how much your backend gets jacked up- 2 inch longer swingarm won't jack it up 2 inches (it will be less), but it can easily be calculated.

I might slap on my swingarm tomorrow and I can get all the numbers crunched.

If it was my bike, I would replicate the factory setup, and determine if I could live with a slightly jacked rearend, or just run shorter shocks, I will also need your swingarm box section dimensions, as the mount will be "higher" on the swingarm, just a little, but it adds up!

[fastbroshi]

  Sure I can get those tonight, probably real late.  I've been working this late shift for a bit and I've been staying up til like 5AM.  Got some forks to disassemble anyway.

[Hannibal Smith]

Once the swingarm goes into the upward arc (hump) the ground clearance becomes less if the swingarm is longer.

Just did some math-

My stock swingarm is 18.5 (center of adjustment range) so I calculated the difference with a 2" longer swingarm.

If the swingarm is "swung" 10 degrees upwards under compression, we get the following figures-

18.5 swingarm...............distance is 18.2189" and height from 0 degrees is 3.2125"
20.5 swingarm...............distance is 20.1886" and height from 0 degrees is 3.5598"

Look at the height figures, distance is a straight line application that only applies when getting pyscho with shock engineering.

So, height differential in regards to ride height is tiny, relatively speaking. You will also note that the longer swingarm has deflected the shock just a teensy tiny bit less (relatively) to the same wheel input. These figures would have me duplicating the stock mounting locations and not sweating it.

.3473" is the ride height differential at 10 degrees (swingarm deflection) in a nutshell when we run a 2" longer swingarm than an 18.5" one. You can easily determine racing and max deflection in degrees, and run the figures. I would strongly suggest that you mimic the stock shock locations.

Edit to add- Now, spring weights might have to be played with a touch, as the longer swingarm now has more leverage for compression, but this is splitting hairs in your application.
I gotta stop, now your CG is changed....................nothing to worry about, but this is what you asked about!!!

[fastbroshi]

   Just to clarify, my swinger is 2.5" longer, not 2".  So assuming I had a 2" longer swinger, we get over 1/3" or 7-9mm difference in travel between the two swingarms when going through a 10 degree arc correct?  So .5" longer would probably add a couple mm or so.  I think I understand this;  to move the same amount of degrees it moves a shorter distance because it's radius of travel is larger.
   So we have a larger lever needing less force to move a shorter distance with the stock swingarm shock angle.  What's the advantage to moving the shock mounts farther rearward?  Judging by one of your posts, the force needed to compress the shock becomes more progressive maybe? 

These figures would have me duplicating the stock mounting locations and not sweating it.

Did you mean on the CBR swingarm or the 550 swinger?

This is all really fascinating.

[Hannibal Smith]

You already have a hard point on your frame, unless you start from scratch, this point is predetermined.

If you run the mounting point on the swingarm to the same spot as on the stock CB550, getting the issue solved is 100% easier and without any surprises. You can run a shorter/longer shock and/or adjust preload which is easy, very easy.

If you move the location on the arm 1" back let's say-relative to stock, you will have changed your action quite a bit......................if it sucks, you are stuck with it.

This is all based on the fact that you already have a mounting point on the frame, so 90% of your work is done for you.

All you need is to sit on the raceweight bike, and take the swingarm angle from zero (roadsurface) then get another at max deflection. We can then build a graph that will show the progressive action of your shock. I can tell you exactly how it will act (math wise) real application is another thing, but we need a mathematical base for starters. You don't need a fancy computer, just some poster board, a ruler, and some string can get all the same figures.

[HB-1fan]

Just a thouht!!  Roadracing or dragracing? Why would you want it longer if roadracing??

HB

[fastbroshi]

Just a thouht!!  Roadracing or dragracing? Why would you want it longer if roadracing??

HB

Well because as everyone knows these little 550s are little balls of fire and they'll melt these little pizza cutters we call tires at a moments notice. JK.  It's a much cheaper means to building a full custom swinger, plus it's gonna look really cool.

Hannibal,
  I hear what you're saying.  Keep in mind my bike is a bare frame right now, so I'm not sure how to proceed after your last comment.   I can mockup the CBR600 arm and get the radii for different assumed lower shock mounts if that would help.

[turboguzzi]

brent's idea is good, but just like others are asking, why a longer swingarm?

is that for the road racer you are building of totally unrealted?

just wondering if this is hi-po or just for kicks. not even sure it's "gonna look real cool" like your saying......

TG

[fastbroshi]

brent's idea is good, but just like others are asking, why a longer swingarm?

is that for the road racer you are building of totally unrealted?

just wondering if this is hi-po or just for kicks. not even sure it's "gonna look real cool" like your saying......

TG

  Yes this is CBR arm is potentially for the racer.  I've yet to weigh the CBR arm but it's definitely heavier than the stock one.  I'm going to check this shortly.   I'm getting a stock swingarm ready to just in case this ends up being too long and drawn out.  If I end up getting it sorted and don't like it I'll just transfer it to my street 550.

[Rick4004]

There could be additional benefits to the longer swingarm for roadracing,,, longer arm puts more weight on the front, raises the rear-end, potentially allowing you to lower the front without loosing ground clearance, tighten up the rake and shorten the trail. It is possible to quicken the steering in spite of the longer wheelbase.

Rick

[fastbroshi]

   I thought about that too but didn't want to add it to the thread for fear of mucking things up.  A Yamaha R6 front end is in the works.

[Retro Rocket]

I don't think you will have too much trouble with mounting shocks on a longer swingarm. I am using a longer swingarm on my bike and have been wondering about the different rake of the shocks as well but after checking out early Kawasaki's gsx suzuki's and a pile of other bikes, none of them had the shocks in the same position, they were all mounted in varying degrees of rake yet the same shocks were being used.
These are old bikes and suspension theory is still not a perfect science and i have seen a lot of early race bikes with altered rake both front and back so i suppose what i am trying to say is do what i am going to do and put a 3 position rear shock mount incorporating the standard rear shock position as one of your settings and go from there. The front shock mount on my swingarm will be the standard position..

Mick

[turboguzzi]

There could be additional benefits to the longer swingarm for roadracing,,, longer arm puts more weight on the front, raises the rear-end, potentially allowing you to lower the front without loosing ground clearance, tighten up the rake and shorten the trail. It is possible to quicken the steering in spite of the longer wheelbase.

Rick

I race a 500 and a longer swingarm is not something you want. does your experience differ? some VERY fast guys here actually go quicker by fitting shorter arms.
True, modern supersports have relatively long s'arms that are supposed to reduce chain torque reaction and give better traction but they can have them only due to the stacked transmission that makes the engine shorter. I say supposed because the ducatis have very long engines and still win races with a relatively short arm.

so Broshi, unless you go dragracing, keep that CBR swingarm for your street project where it will surely generate more ooohs and aaahs, if youd see how fast some people go with the stock unbraced swingarm you'd understand.

TG

[fastbroshi]

  Thanks for the input TG, I'll take that to heart.  The more I think about it the more I'm leaning towards putting it on the backburner as just using the stocker would save me a lot of time getting this thing setup. 

   Now that the CBR swinger's out of the way, what would you recommend for shocks using the stock swingarm?  Any particular ones you'd stay away from?

[turboguzzi]

I found a very simple way to mount any kind of shock and not just honda clevis type to the stock s'arm that will also let you regulate the length.

will post  a pic when I am back home

I am using 7160 Koni's  picked up in a flea market and overahuled. they work pretty well but these are the only ones I tried, should do more homework/testing really. Our dutch friends are running YSS shocks. Works Performance have very high end shocks legal for classic racing but you might want to wait till you are at the sharp end of things before moving on to them. People claim that the Works shocks will shave a second from your lap time out of the box, not really relevant if you are still 5-10 seconds per lap away from the class record. but hey, who knows, you might be the next Rossi....

If anything, now that I am running closer lap times to the front runners, i find the front end needing attention rather than the rear, am starting to get chatter, have to find a solution. Gonna try a different front tire next race.

TG

[fastbroshi]

From Hannibal:

All you need is to sit on the raceweight bike, and take the swingarm angle from zero (roadsurface) then get another at max deflection. We can then build a graph that will show the progressive action of your shock. I can tell you exactly how it will act (math wise) real application is another thing, but we need a mathematical base for starters. You don't need a fancy computer, just some poster board, a ruler, and some string can get all the same figures.

  I was playing around with paint and got the circle approximately where it needs to be.  I rotated the pic to get the frame approximately horizontal.  Anyone know a better tool than paint to do this?   
  I didn't find the angle yet, need to go to the hardware store and get an angle finder.