Rear Shock positioning

PT, the "fulcrum" is the swingarm pivot. The swingarm is the "lever". The rear axle pushes up on the lever, the shock pushes down on the lever. Since the shock is no longer perpendicular to the swingarm, the spring in the shock can only push down on the swingarm with .707 times the spring's actual force. The swingarm gets .707 times the spring's force pushing down instead of 1.0 times the spring's force. Since the force is reduced by a fraction of it's original, the leverage against the shock has increased, or the leverage the shock places on the swingarm has effectively decreased. (I was saying 1.414 to see if anyone would catch on that it's 2 times the square root of 2 since we are dealing with a 45-degree triangle.) But this is only for a static case, as I said, the rates vary as the swingarm angle changes.

I apologize, but I still don't see what gets twisted like a torsion bar. The shock can only push on it's pivot points, and a pivot point can't apply torque in the plane of motion. Also, if you twist a shock, the rod turns inside the slider so it can't produce a torque along it's axis.

I see where you're at - and where we're missing each other:

I'm looking exclusively at loads on the shock body itself, and you're looking at the shock/swingarm unit.

True, the *shock* can only "push" at the angle it's mounted between the pivots, but those are *not* the only loads being placed upon its structure - there is a *VERTICAL* component to applied force on the structure of the shock body - "twisting" was a poor choice of words on my part - "bending" - a torsional load is more accurate. Glad you pointed that out.

Your take is the more accurate if a shock is mounted in a free-swinging pivot.

If, however, like most shocks I've ever dealt with - they're cinched down pretty tight at either end - hence restricting that pivot action - the essentially *vertical* action of the swingarm is going to want to *bend* the shock in the middle.

Diggin' this!

Just for example's sake, here's an example of what I'm referring to:

The photo is a of shock piston from a touring bike that's been not only warped, but *cracked* by what Monroe refers to as "side loading".



Clearly, if compression loads were the only forces acting upon the piston/cylinder, this kind of damage wouldn't happen...

Thanks for these great responces. I am getting some really good ideas now. (Now I just need to put them into pratice. )

KB02 wrote:

scumbag wrote:

Lou you never cease to amaze me with your math and science...haha...

1.41 eh....I woulda said one and a half...haha but I am just an english major...math is like russian to me..

Hell, man, I've got a BFA. :woohoo: I'll take the math lesson.

Wireman: Would you know where I might be able to find a picture of one of those old Yamaha's? I got a bunch of great pics from Damon of different shock configs. They gave me some great ideas.

this one sits up like a 4x4 truck but its just to give ya a general idea;)

i got one of those yamaha swingarms and shock sitting at school. i will snap a pic tomorrow and post it for discussion. i have been eye balling it for a kz for awhile, but have come to the conclusion of no-go in the stock yamaha configuration. the shock is HUGE! the thing is like 2 feet long, seriously. plus on the YZ the shock went up the backbone of the frame and was bolted just a few inches behind the neck. if you want to run it you would have to transplant the YZ frame member onto your bike. i think the YZ i had was like an 82ish yz250, a ebay deal i got for .99 cents.

PT, soon as you said "bend", i got ya. Yeah I agree, if the pivots are not free, the pushrod in the shock will bend. I think the first indication (if it fails over time) would probably be oil leaking out.

Cool photo of that piston... not pretty!

My ZX900 has the top mount shifted forward to under the junction of the seat support. About an two inches forward. I definately makes the rear end sit up further but the reduced angle can allow bottoming out; especially two up or over a severe bump.
There has to be an amount of torsional twist in the shocks due to loading and unloading of the swingarm under acceleration and decel either via braking or torque. Monoshock and more rigid swingarms contribute to reducing this.
I like the comments from Lou. Great to read.

Not so much seat height as it is "looks" that I am after.

Well, then, look at the possibility of shortening your stock shocks. Shocks that are two inches shorter will still work well enough, but full-length re-angled shocks will not.

And at that angle, they will look foolish.

"E" Pete at Sons of Thunder Metric Cycles at www.sotmc.com and check him out. Cut, polished, and return-shipped, under $75.

Post edited by: DanVVArider, at: 2006/11/25 12:52

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JMKZHI wrote:

Dan, the link doesn't work.

JMK.

For some reason it has "kzrider.com" as part of the link address. Just remove it and you'll get to the site.

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