Rear Shock positioning

Admitantly, I am not an engineer. However I do have a really cool idea for a rebuild of an old bike that I would like to do, but I have a question as to whether part of my idea is doable or not. For what I want to do, I need to move the rear shocks.

Now, the bike I am thinking of working on is a '74 (I believe) KZ400. It is currently sitting in my cousin's field, rotting away. I basically have a blank canvas for whatever I want to do. (I'm thinking Cafe racer/street fighter lookin' thing, yet small enough - it is a 400 after all - for a beginner to ride on). For the image I want, I need to reposition the rear shocks.

Basically, I want to lay the top end of the shock down toward the front of the bike. So intead of them sitting sitting at about an 85 degree angle, they'd be closer to 40 or 45. Would this be possible (pronouced, "Ridable") with the stock shocks? Or even at all?

Let me know what you all think.

The further the axis of a shock's piston action is angled away from vertical (90 degrees), the progressively less effective is becomes.

Angled at 45 degrees, it'd be acting more as a torsion bar than a shock absorber - more stress load being taken up in sheer than compression.

Taking a wild guess that the desire to angle the shocks is based in lowering the seat height, I'd let the height requirement determine the shock angle rather than the angle determine the height.

In any case, I'd try to keep the angle at 35 degrees or less....

The "ride-ability" issue is a toughie - frame geometry is an art as well as a science. By laying the shocks forward, once in a turn, torque would be applied to the frame structure at different places and values than it was designed for, and all bets are off. The frame might tend to flex in places it wasn't anticipated, and how that would interact with the front end geometry is a tough call.

The short answer is, "who knows?" It's certainly do-able - but I'd take my time exploring the outer limits of how the configuration would handle.

Post edited by: Pterosaur, at: 2006/11/07 08:31

Not so much seat height as it is "looks" that I am after. basically, I would love to get rid of the seat almost entirely. There will be a seat, but I'm not thinking about a long distance cruiser or anything.

maybe steal the swingarm off an early-mid 70s yamaha yz dirtbike.they had a single shock that laid almost flat the way the swingarm was layed out.

A shock only works along its axis of travel. Due to the pivot points, there won't be any torsion, but there will be more leverage against the shock.

Initially, the 45-degree shock will need to have 1.414 times the spring force that a 90-degree shock would have, assuming they use the same mounting point on the swingarm. That is, initially only, though, because the 90-degree shock has a falling-rate geometry, but the-45 degree shock has a rising-rate geometry (which is why shocks are usually tilted forward... to prevent a falling-rate). For the same amount of vertical wheel travel, the 45-degree shock will initially travel much less than the 90-degree shock.

Works Performance can probably make whatever you need. You just give them some frame measurements (like the position of your new shock mounts etc.) and they'll do the engineering for the spring rate, travel, length, pre-load, dampening etc. Just don't go too extreme, and it should work out.

Post edited by: loudhvx, at: 2006/11/08 03:11

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..

I've been watching this thread, and have a second, related question. How would you fabricate the upper shock mounts onto the frame? Any photos from someone who has done a similar mod?

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.

wireman wrote:

maybe steal the swingarm off an early-mid 70s yamaha yz dirtbike.they had a single shock that laid almost flat the way the swingarm was layed out.

my 78 yz had duals

Recently, I worked on a bike where it had a 6" over swingarm and struts. The normal "85 degree" orientation of the shocks was closed to 45 degrees due to the longer swingarm. Since the struts were tearing up things, I put some 14" shocks on the bike and while it was MUCH lower than I preferred, it was still higher than with the struts and the shocks seemed to work a some... It would probably have been better to weld new mounts but that is getting into engineering and liability to name a couple problems so I left the shocks laid over with the original mounts. If the bike had been mine, I would have tossed the extended swingarm and put a normal one on for height and handling.

i realize you want to move the top shock mount forward, but amother option might be to move the bottom shock mount backward:

cgi.ebay.com/ebaymotors/kawasaki-Z1-KZ90...QQrdZ1QQcmdZViewItem

www.jcwhitney.com/autoparts/Product/tf-B...11+600014338/c-10111

loudhvx wrote:

A shock only works along its axis of travel. Due to the pivot points, there won't be any torsion, but there will be more leverage against the shock.

One minor quibble: regardless of pivot points, as the angle of applied force varies from perpendicular (90 degrees - compression), total applied force would be a sum of compression and *twisting* about the longitudinal axis - torsion by definition.

So in effect, a laid-over shock would partially be acting as a torsion bar, but *because of the pivot points*, it just wouldn't be a very good one...

Since a shock is connected to the frame at both *ends*, with no fulcrum point in between. "leverage" doesn't really come into play.

Initially, the 45-degree shock will need to have 1.414 times the spring force that a 90-degree shock would have, assuming they use the same mounting point on the swingarm. That is, initially only, though, because the 90-degree shock has a falling-rate geometry, but the-45 degree shock has a rising-rate geometry (which is why shocks are usually tilted forward... to prevent a falling-rate). For the same amount of vertical wheel travel, the 45-degree shock will initially travel much less than the 90-degree shock.

Works Performance can probably make whatever you need. You just give them some frame measurements (like the position of your new shock mounts etc.) and they'll do the engineering for the spring rate, travel, length, pre-load, dampening etc. Just don't go too extreme, and it should work out.

Really well done explanation...

One other effect that comes to mind - because of the increased torsional (bending) force applied on the shock lengthwise as the angle increases, I'd expect the seals to wear out at an increased rate...