The fork length numbers aren't particularily helpful because the don't factor the difference between the top of the fork and the top of the headstock. Drop and thickness of the top clamp have an effect. You could have a set of forks that are 800mm with a 30mm thick flat top clamp and end up at 770mm total or you could have a 770mm forks with a 30mm thick clamp with 30mm drop and end up at 770 total. There is only something like 10mm difference between stock KZ forks and ZRX forks but the stock triple has 30mm drop

Assuming a 25.3" front wheel diameter that Sanctuary bike is at 97mm of trail. That is probably at the lower limit of what is good and I wouldn't be surprised if it was a little twitchy coming out of turns as many race bikes are. That's fine for racing where the ragged edge is expected but most people, myself included, want a little more stability for highway cruising. Remember you are building a street bike.

I think if you massage what you have you can end up with a bike that handles well, even if it isn't optimized to perfection. I would tackle the things that will be the hardest to change once the bike is complete first (shock mounts) This way if you need to make changes to dial it in its not a huge pain later on. I ran the stock swingarm and stock ZRX triples on my 1000 for a year before designing and building the custom suspension. Part of the fun in building bikes (for me)  is slowly refining the bike, I just went threw a slew of smaller changes to my KZ1000 and it just keeps getting better and better.

These ZRX 1200 dims are all I have. I believe the 1100 has a slightly shorter wheelbase and some other small changes. These numbers are what I based my target on. I rode a friends ZRX1200 and was impressed with how it handled. I figured that was a good goal to aim for since its overall design and construction is similar to the KZ.

 

Thanks for that, some really good info there.

wrote:

don't guess, do some research, you'll find the information you're looking for...
 

It's less guessing and more trying to get the most out of what turned out to be less than ideal advice. Not really bad advice, just less informed than I assumed - and we all know what assumption is the mother of I should have posted more questions about the chassis here as I did with the engine 

More numbers:
I went back to the garage and adjusted the rear axle as far forward as it would go - 520mm pivot to axle.
Offset is 40mm
Forks are 770 (according to the internet, haven't actually checked)

My forks will end up sitting at a point about flush with the top yoke on the Sanctuary bike you posted - giving me about the same height as a 790mm fork on that bike (plus a bit more due to shorter offset).
I lifted the front about 14-15mm off the ground today to simulate lengthening the forks. Put me at about 24.5° (rough measurement, will do some trig later). Swingarm was at 11° and the shocks would sit about where the silver cross is in the picture below (pretty close to the sanctuary bike and where pretty much everyone else seems to put them). This would put the shocks at about 28° to vertical, as apposed to 18° with the stock mounts.
Lengthening the forks by 15mm and moving the shocks should sort things out. Wheelbase will be pretty close to the sanctuary numbers you posted (10mm longer swingarm, 5mm less offset), albeit with both axles about 5-10mm closer to the rear of the bike. Rake will be at 24-25° and trail will be a few mm longer due to the shorter offset. Should be a pretty good starting point though (still have room to move with fork height in the triples and adjustable shock length).

@DoctoRot: do you have any numbers for the ZRX rear suspension/geometry? The shocks are for a ZRX so I'd like to base things off that, couldn't find any numbers though.

 

wrote:

I think you should try a combination of several things
1. internally extend forks - 12- 20 mm I'm not sure but i think you can extend them a little without needing the top caps.
2. Mill bottom of top triple clamp. 15-20mm That sucker is way over engineered. You could cut it in half and it would be OK
3. Lower rearend till swingarm is at 10 degrees. 

Other things to consider:
Shorter swingarm? New swingarm is much longer than stock which means to achieve desired angle you need the rear end to be relatively higher.
Low profile rear tire
High profile front tire.

I sorted out point 2 today - it was more of a question of MOT paperwork. Milling out 15 mm leaving about 15 mm around the top bolt. Not going right across the bottom, just a 60mm circle around the bearing cap and nut. Will then cut a M28 x 1 thread in the stem and use a nut instead of the top bolt - will be easier than shortening the stem. This change will put me at around 23°/95mm rake/trail.

Point 1 is now on the backburner until I get the forks apart and see what is required. I'm pretty sure I can't just put a spacer at the bottom and use a longer damper bolt as the compression settings will get messed up (I think...). I think I'll look at adding about 15mm to the top of the damper rod [EDIT: that won't work ]. As you said, there should still be enough overlap between the uppers and lowers when fully extended.

I can shorten the shocks a bit (gets the swingarm to about 10.5°) but I probably won't bother moving the shock mounts to bring it up any more. May consider the mounts that scrirocco posted a while back just to lay them down a bit.

The wheelbase is at 151cm so may offset the steep front end - or it may just make things worse
Basically, once I've had the top yoke milled and decided what to do with the forks I think I'll just get the bike together and see how it rides (before I pull it apart again and paint it). If it's dodgy once I get it on the road I'll look at moving the shock mounts and/or putting a Zephyr swingarm on it. 

Thanks for the suggestions! I'm really hoping the changes at the front will get me where I need to be.

wrote:

Spent a while measuring things today and realised I made some mistakes on the calculations yesterday. Here's what I tried today:

Standard setup (forks flush with top yoke, rear axle in the middle of the chain adjument range): 

• Rake: 22.5°
• Trail: 89mm
• Swingarm angle: 11.3°

Forks dropped (forks sunk 10mm into top yoke, rear axle in the middle of the chain adjument range): 

• Rake: 23°
• Trail: 92mm
• Swingarm angle: 11.6°

Lowered back end (forks flush with top yoke, rear wheel raised 35mm towards frame): 

• Rake: 24.5°
• Trail: 100mm
• Swingarm angle: 8°

 

I think you should try a combination of several things
1. internally extend forks - 12- 20 mm I'm not sure but i think you can extend them a little without needing the top caps.
2. Mill bottom of top triple clamp. 15-20mm That sucker is way over engineered. You could cut it in half and it would be OK
3. Lower rearend till swingarm is at 10 degrees. 

Other things to consider:
Shorter swingarm? New swingarm is much longer than stock which means to achieve desired angle you need the rear end to be relatively higher.
Low profile rear tire
High profile front tire.

Spent a while measuring things today and realised I made some mistakes on the calculations yesterday. Here's what I tried today:

Standard setup (forks flush with top yoke, rear axle in the middle of the chain adjument range): 

• Rake: 22.5°
• Trail: 89mm
• Swingarm angle: 11.3°

Forks dropped (forks sunk 10mm into top yoke, rear axle in the middle of the chain adjument range): 

• Rake: 23°
• Trail: 92mm
• Swingarm angle: 11.6°

Lowered back end (forks flush with top yoke, rear wheel raised 35mm towards frame): 

• Rake: 24.5°
• Trail: 100mm
• Swingarm angle: 8°

All this means that I can't lower the back end or my swingarm will be too flat - makes it easier to deal with the shocks, don't need to move the mounts anymore. My only option is to raise the front end. Here's what I've though of so far, suggestions are more than welcome:

• Put a spacer in between the fork damper and the fork lower to lengthen the fork. Not sure how much I could gain with this before I start to lose stability in the forks, maybe 15-20mm and chuck a fork stabilisor on?
• Find some 43mm fork uppers that are longer than the ZRX's. No idea where to look though...
• Drop the forks in the top yoke - can get about 9mm out of this before going past the top clamp bolt. Would need some covers for the tops of the forks though - it looks pretty sh**. 
• Get new triples with a 30mm drop - really don't want to do this  
• I can probably get about 5-10mm if I play around with static sag once the bike is assembled.

Any other ideas?


Forks dropped in the top yoke:
 

Rear end lowered so the rake/trail is good - swingarm is too flat:
 

Probably not worded too well - seems to be a recurring thing with me
By raise the rear I actually meant raise the swingarm with respect to the frame. I'm thinking of where to put the axles to get the geometry right instead of thinking about the position of the frame. So yeah, I meant lower the frame (or raise the rear tire when the bike is on the stands).
I'm aiming for about 100-110mm trail. Everything else is just getting set to what I need to get about that value (the adjustment in the shocks will be enough to change things up if needed. I have a 40mm offset but stuck with 18" wheels - that gives me about 107mm trail @ 25°.
I waited until the wheels arrived before taking any measurements so I could be sure of the numbers. I am just going to set eveything up with the suspension unloaded - if I set the static sag correctly the trail won't change with the engine etc. in the frame.

EDIT: The plan tomorrow is to raise the front end on the stand until I have the numbers I want, then to see how far off the ground the front tire is (the rear is just touching). Then I can see where the shock mounts need to be to get the rear tire level with the front.

He has not logged on to KZR since January 2019.

Myke?

wrote:

My eyes keep going back to the Suzuki!  I had an '83 GS750E that I put almost 60k miles on.  Can you spill some info (without going too far off the topic).

Sure skidmark,
   1983 GS11es with some "period correct" upgrades. Astralite wheels, Fox Shox, Sandy Kosman rotors, Spondon calipers, Metmachex swingarm, 35mm Keihin CR Special carbs, Motogpwerks exhaust and RaceTech GSXR11 forks recently added

 

wrote:

wrote:

Hi Ed, I know from reading previous posts of yours you don't think they are needed, yes the 650 is very stable, I have fitted dampers to every bike I've owned that didn't come stock with them. Have you ever had a tank slapper ?   I have, one bad enough to dent my tank with my thumb, broke the steering lock tab on a 750 Honda back in the early 1980's at speed, frightened the poop out of me, have used them ever since, Think of them as an insurance policy...

  Besides this is my sons first bike, just being over cautious...




 

I asked because some folks believe they are a cure for speed wobbles even though the speed wobbles may be an indication of a serious problem that needs to be addressed.  My 1977 KZ650 developed bad speed wobbles several years back.  The problem ended up being bad Michelin Macadam tires.  I found the problem after replacing the swing arm bushings, steering stem bearings, shocks, etc., etc. Replacing all those items slightly dampened the problem, but replacing the tires made the problem absolutely go away at any speed.   It appears that the bike in question does not have any wobble so the damper is not being installed to try to correct an existing problem; that's good.  That's why I asked.  Ed

     I get where you're coming from Ed. The 650 has new stem bearings, wheel bearings, brakes, it uses my 750R1 forks that were completely rebuilt just before I bought it, {pulled them down to check them and found everything perfect}, which was a pleasant surprise, new rear brakes and all set up almost right, I need a little more preload in the front forks but apart from that, it rides and stops very well, actually I'm impressed just how well these 650's handle. I'm definitely not trying to hide any handling gremlins....