Frankin bike GPZ900r rear suspension issues

I need help from the forum. Some of you might be aware of my frankin (Frankenstein) bike, GPZ1100 81 model hybrid. The rear suspension has been changed to a monoshock from a GPZ900r or 750 (I think they were both the same). My question is what kind of ride should I expect from this rear end, subtle , hard, soft???.
Currently my setup is pounding my kidneys to death and I did everything from change the shock oil to ATF, played with the dampening etc. I just got the bike registered and hit the tar, when I got to a known bumpy section it felt like the rear was off the ground at 110kph.
I managed to get about 65/70mm of travel with around 25/30mm with rider sag.
All suspension parts are original Kawasaki.

Previously someone commented that my linkages were not in the correct position to there GPZ900R, so its possible my linkage set needs changing - any ideas would be appreciated but as stated what kind of ride should I expect, include rear swingarm travel etc.

Did you use all the GPZ linkages and mount them exactly the same..? If you've changed the length or position of those linkages, or the shock, then the suspension will have different geometry, small changes can have a big effect, Look up the GPZ900R and see how much travel the suspension is supposed to have, also take into account your 1100 is probably a lot heavier than the 900...

I bought the frame with the linkage setup, As you stated I'll have to get my abacus out and start to work out the travel etc, Maybe during our December Christmas run I'll find a GPZ with the same swingarm and linkages to compare.
This bike is actually missing a bit of weight compared to the standard, the Honda wheels are a lot lighter and other unnecessary parts removed. so it might be around the same weight as the GPZ900r.

Thankyou for your input.

750 R1 wrote: Did you use all the GPZ linkages and mount them exactly the same..? If you've changed the length or position of those linkages, or the shock, then the suspension will have different geometry, small changes can have a big effect, Look up the GPZ900R and see how much travel the suspension is supposed to have, also take into account your 1100 is probably a lot heavier than the 900...

100% agree with that!

I recently worked on a mono shock on a project that had been started but not really tested or finished complete…Unless it is EXACTLY set uo like it was from the bike it was taken from expect do play around A LOT with spring rates .
Is it a progressive rate linage or straight rate? it looks progressive but measuring is the only way to know if it is and how progressive it is.
You may need measured the distance of swing arm travel vrs shock travel in 10mm increments or less to determine this. This requires removing the spring and finding a way to move the swing arm and measure the shock travel vrs swing arm travel,
Also have you set the static sag and do you know how much shock travel you have? This will help determine where the problem is, to try to find out if the spring is to stiff or the linkage rate is to high or low if its progressive,

Custom mono set ups can be a challenge , I am still messing with that project, I only went through 5 springs to find on that would be close :blink:

Looks like a really cool bike, don't give up!!!

Joe, I did check the sag with rider - static at around 30mm the only problem I have is that the overall travel at present (bouncing on seat) is 65mm. Yesterday I tried to check a few things and found I had no air pressure in the shock, that would explain the bouncing over bumps. Funny thing is that the ride is acceptable around town with slower bumps (remember my back is sensitive so I'm a bit more picky).
You are correct in taking time to get things working, I nearly sold my FJ1200 for the same reason, eventually I added an RPM shock and emulators in front (with racetech springs) the ride is now as good as any on the road.

I have no intention of giving up on this bike, I got it this far with hard work and apart from the harsh suspension on rear it is a dream to ride.

I still would like to know what I should expect out of this rear suspension in terms of ride quality, I want a bench mark to work to.

Forgot to ask how much air should this shock have and do they came apart to change the internal spring?

I have no intention of giving up on this bike, I got it this far with hard work and apart from the harsh suspension on rear it is a dream to ride.

Don't give up...!!! The bikes a beauty...

I still would like to know what I should expect out of this rear suspension in terms of ride quality, I want a bench mark to work to.

See if you can find a half decent GPZ900 to ride, I had a friend that raced one when they were new, they handled very well.....Any chance of getting a weight of the 1100 ?

My brother had a 1986 GPZ900R in 1988 but I don't remember how it felt on bumps, yes it handled but then so does franki, its just that the suspension needs work.

I'll see if I can measure the weight next week when I ride it to work, stay tuned.

I had a GPZ900 it handled very well and was a fairly plush ride not harsh.
I don't think comparing the two will help much.
The linkages are on a completely different bike and even if they were set up exactly the same it still will have a different affect.
Weight bias front to rear, steering geometry although it won't affect bumps on that rear will have impact on handling.

What is the free sag of the bike, when you lift the rear end how far does it settle from the top of the suspension under its own weight?
You want it to be around 1/2 or so if I remember correctly
Also is the shock bad , because thats what it sounds like, sharp bumps make it go hard, the oil and the valves do go bad and the shock becomes unpredictable. Rear shock damping goes off when the oil gets old. Look on the web for how to rebuild or replace the oil, many times its just a matter of dumping out the old and adding new.
I have found in the past a lot of the old air shocks are blow because in there past they were over filled and blown out. GPZ900 shocks are notorious for being sagged out and in need of a rebuild.
Does it get progressively worse as you ride it? Is it compressing and not returning as it is ridden, you cannot replicate that by bouncing it as sits still. Sharp repeated bumps put a lot of very quick inputs if the shock is bad it cannot respond and finally you end up with no dampening, Is it just bottoming out eventually over harsh bumps.

This guy helped me a lot when I was working with a customized mono shock set up on my H2 750, he's sharp and his services are reasonable,
www.twfracing.com/contact.html

It got confusing really fast for me last winter when I was playing around with my custom set up!! I never had a shock apart so many times and measured so many different things. But it is much better! And went from being mostly unridable to being a stable comfortable and predictable ride (for a "raceish"the set up)

When you push down on the seat the suspension should compress then rebound slowly. Not too slow but not spring back up either. Too slow it will pack.

JOE MKII wrote: I had a GPZ900 it handled very well and was a fairly plush ride not harsh.
I don't think comparing the two will help much.
The linkages are on a completely different bike and even if they were set up exactly the same it still will have a different affect.
Weight bias front to rear, steering geometry although it won't affect bumps on that rear will have impact on handling.

What is the free sag of the bike, when you lift the rear end how far does it settle from the top of the suspension under its own weight?
You want it to be around 1/2 or so if I remember correctly
Also is the shock bad , because thats what it sounds like, sharp bumps make it go hard, the oil and the valves do go bad and the shock becomes unpredictable. Rear shock damping goes off when the oil gets old. Look on the web for how to rebuild or replace the oil, many times its just a matter of dumping out the old and adding new.
I have found in the past a lot of the old air shocks are blow because in there past they were over filled and blown out. GPZ900 shocks are notorious for being sagged out and in need of a rebuild.
Does it get progressively worse as you ride it? Is it compressing and not returning as it is ridden, you cannot replicate that by bouncing it as sits still. Sharp repeated bumps put a lot of very quick inputs if the shock is bad it cannot respond and finally you end up with no dampening, Is it just bottoming out eventually over harsh bumps.

This guy helped me a lot when I was working with a customized mono shock set up on my H2 750, he's sharp and his services are reasonable,
www.twfracing.com/contact.html

It got confusing really fast for me last winter when I was playing around with my custom set up!! I never had a shock apart so many times and measured so many different things. But it is much better! And went from being mostly unridable to being a stable comfortable and predictable ride (for a "raceish"the set up)

Joes right about the shock, its 30 years old, while they were quite good when new, most shocks of that era died a rather quick death, usually from overheating. They can be rebuilt but it may be better looking at a replacement. Still, the very first thing to do is make sure all the linkages are in the right place, I understand what Joe's saying but if those linkages are all in a factory position, that suspension will work, regardless of it being on a different bike, there's only 20 something pounds difference in dry weight of both bikes, yours is lighter than stock so they should be close in weight, yes. the fronts have different geometry and set up and there's nearly 5 inches different in wheelbase but, even though I don't think the 1100 will handle like the 900, its should still handle well and done properly, the 900 suspension should be an improvement...

Thankyou all, it looks like I have some work ahead of me. My steps would be-
1. Check that the rear shock oil is not overfilled (I changed it not long ago hoping it would be better.
2. check linkage spacing etc, and the sag verses full movement of rear.
3. Check that the damping on the shock does work by changing it while riding. Including air pressure changes.
4. Last resort I'll try a different quality shock,

The other thing I should mention is that the front end is Off a GPZ900r including the triple clamp which does give it a similar wheel base to the GPZ900R but the seating position is a bit further back so more load goes on the rear. The handling overall (barring the rough ride) is brilliant, I can't fault it (up to 200kph through fast bends errrr er on a race track off course)

.[/quote]..Joes right about the shock, its 30 years old, while they were quite good when new, most shocks of that era died a rather quick death, usually from overheating. They can be rebuilt but it may be better looking at a replacement. Still, the very first thing to do is make sure all the linkages are in the right place, I understand what Joe's saying but if those linkages are all in a factory position, that suspension will work, regardless of it being on a different bike, there's only 20 something pounds difference in dry weight of both bikes, yours is lighter than stock so they should be close in weight, yes. the fronts have different geometry and set up and there's nearly 5 inches different in wheelbase but, even though I don't think the 1100 will handle like the 900, its should still handle well and done properly, the 900 suspension should be an improvement...[/quote]

750R1 I just weighted Franki = 215Kg with around 5Ltr of fuel, the seat is light plastic base so it weights bugger all