81 GPZ1100 Road Racer

The intent is to make the shock perform in a more progressive manner, AC sanctuary do this mod to all their custom bikes, I'll drop a link here with their explanation, its in Japanese but I have google chrome set up to translate the page, if it shows in Japanese try right clicking over text and pressing translate.... I have google translate set up on my computer

www.ac-sanctuary.co.jp/factory/chassis-laydown/

I saved this part of the Yoshi PDF for someone on another forum a while back, it states, "Upper
shock absorber mounts are relocated to
provide 54.5' of shock laydown angle, and
11mm o.d. tubes are welded in behind the
gusseted new upper shock mounts and the
rear section top tube of the frame"

If you add 35.5 to the 54.5 you get 90 degrees, so depending which side you measure from either measurement works....


I found this in one of my files as well..

"When motorcycles first developed suspension, the rear shocks were mounted at or very near the back axle. It was easy to manufacture and had few moving parts, but they had very limited wheel travel. As the need for greater travel became more important, racers began to “Lay Down” the shocks. The top shock mounts were moved forward towards the front of the seat and tank and the lower mounts were moved towards the swing arm pivot. By “laying down" the shocks, the engineers created a progressive leverage ratio between the shock and the rear axle. This allowed for longer wheel travel without the need for excessively long shocks. How ever the shock and spring now needed to be made stiffer. This was a result of the loss of mechanical advantage of the shock and spring over the rear wheel. When the shock was mounted straight up over the rear wheel, for each millimeter of wheel travel, the shock also traveled 1 mm. When the shock was laid forward the shock moved less than 1mm for each millimeter of wheel travel. This is the leverage ratio of axle to damper movement. What engineers discovered was that laying down the shock creates a fairly Linear Leverage Ratio Curve. In other words if the shock moves .5mm for 1mm of wheel travel at the top of the stroke, it also moved close .5mm at the very end of the stroke."

Just ran across your thread on building a road racer out of a '81 GPZ1100. As far as having to brace the frame on the '81. The '81 J model frame and the '81 GPz1100 frame were designed using everything Kawasaki learned from the '79/'80 Superbike program. The Kz1000S1 their AMA Superbike you could buy uses basically the same frame as those two bikes. Only major difference is about 1.5~2 degrees of rack, and one little brace on the back top lefthand motor mount. I don't even believe you have to relocate the top shock mounts either when you use your frame. Now if you tried to use a '73 to '80 Kz900/1000 frame, you would have to do MAJOR frame work!!!!

The kick ass Vintage Superbike in this country is the one that is based on a GPz1100. Check out the bike of Dale Quarterly and see what he runs. That bike is the quickest and fastest Vintage Superbike in this country. Having Dale in the saddle helps a lot too!!!! He uses a real Kz1000S1 swing arm on that bike. Here is one photo of that bike.

First thing I would do is get a rulebook. Read it cover to cover a few times to see what you can do and what you can't do. Just remember if it don't say you can't, that means you can!!!!

I would get rid of those rubber mounted motor mounts and replace them with some aluminum ones first thing. Then some oversize ream-fit motor mount bolts. That way you make the motor part of your frame.

Dr Gamma, do you have any more pics of that bike ? , I'm interested to see how he's mounted that steering damper to the frame.
As a matter of interest, about 4 weeks back I watched a guy take about 3 degrees out of his GPZ1100 B2 frame, I think they are about 28 or 29 degrees stock, quite lazy geometry for the track....

I can't find many close up photos of his bike. I just found this one. But I ain't a close up.

The ELR had a slightly different rake. It was only a degree or two different than the J model and the GPz1100

Hi ZED-SLED, I am also in South Australia. Any chance you can let me know where that zr7 is being wrecked?
I am not after the front end, just some engine bits.
Cheers

Dr. Gamma wrote: I can't find many close up photos of his bike. I just found this one. But I ain't a close up.

The ELR had a slightly different rake. It was only a degree or two different than the J model and the GPz1100

I just found this one...Thanks for the heads up..

I am way out of my league here and showing my ignorance by even poking my head up out of the weeds to contribute. I remember reading a post about rear shock angles. I think it was posted by Les Holt of PDM, I know he is only a street bike guy but he did extensive mock up and testing to find the optimum angle. I cant find the post and hope I'm remembering correctly.

DOHC wrote: For the Brembo P108, has anyone tried to use these with a 78-80 KZ1000 aluminum cast wheel? I'm curious if the back of the caliper fits between the rotor and wheel spokes without contact.

I did a dual P108 setup on a stock KZ1000 spoked wheel, stock forks and stock disks. Needed 3mm spacers behind the disk for enough offset. I also milled about 10mm off the fork bosses so i could fit the caliper bracket on the inside but this was purely for aesthetics. you could easily do it on the outside and not mill the fork lowers

750 R1 wrote: This allowed for longer wheel travel without the need for excessively long shocks.

Based on my math, this is what laying down the shocks actually accomplishes. I don't see any progressive or rising rate in the wheel travel.

750 R1 wrote: When the shock was mounted straight up over the rear wheel, for each millimeter of wheel travel, the shock also traveled 1 mm.

What engineers discovered was that laying down the shock creates a fairly Linear Leverage Ratio Curve. In other words if the shock moves .5mm for 1mm of wheel travel at the top of the stroke, it also moved close .5mm at the very end of the stroke."

I read this as saying that the wheel rate is linear in both cases. But it says it in a roundabout way.

Here is my graph. I have lost my notes, so I can't give all the context about how I got here. I took measurements from my frame and shocks, and the spring rates from the FSM for the 1978 Z1R. I came up with 74 degrees for the stock shocks, and 55 for the laydown (shock to swingarm angle when the arm is horizontal). The laydown case has a different spring rate and preload, which I'm guessing came from me playing with numbers to get a bit more down angle of the arm at rest with rider (-2.6 deg. vs +6 for stock).



The wheel motion vs. ground force is basically linear through the entire range in both cases. But the wheel travel definitely increases. The FMS says the rear travel is 80mm. I came up with 92.4mm stock, and 108mm with the laydown.

Again, it's entirely possible that my math is messed up. If someone has a fancy CAD program that can just do this math for us, it would probably be a lot more believable.

Yes, they both may be linear but, you get more travel with the shock laid down, look at the way shock angle changed in the 1970's on motocross bikes before they went to monoshock, the later twin shock bikes had their shocks laid down to give the suspension more travel and tunability. Also when trying to work out rear suspension you have to include sag, and swingarm angle or anti squat, the swingarm shouldn't be parallel to the ground, I think, from memory, it should be set at somewhere around 10 to 14 degrees downward. There are a lot of things to consider apart from shock angle.. I'm moving the rear shock mounts on my bike , I won't be buying shocks until I work out the swingarm angle , that's when I'll know what length shock I'll need, actually, I need to work out ride height at the rear before repositioning the top shock mounts, lucky I have a friend that's very experienced with suspension and he has a frame jig, so the frame will be in the jig while all this is done. I'm not doing it to race, just building a race replica {Moriwaki}, my OCD is what pushes me to get things correct. I'm still trying to work out how to get the rake where I want it too, I am using a 17 inch wheel front end with 40mm offset so I'll need somewhere close to 24 -25 degrees of rake, {i have 28 degrees stock}, I can adjust the rake slightly by either raising the rear slightly, or dropping the forks a touch. Lots and lots of calculations, I'm a classic over thinker....!!

750 R1 wrote: I'm a classic over thinker....!!

I am right there with you, brother.

Fun isn't it.....:blink:

I hope we haven't chased away the OP...!!