Custom Fork Spring Install.

[memphisninja]

I used a pair of 1982 KZ1100 fork springs from a Spectre shaft drive. They where 2 7/8" longer than the overall stock length of the stock ninja 250 shocks (including preload spacer), but they where the same 26mm diameter, so I cut them down to proper length and put them in.

According to Kawasaki the KZ1100 Spectre has 0.62Kg/mm spring rate, which for a 130-150lb guy is perfect (I'm 140). I also raised my fork oil level to 115mm from 225mm stock.

Much better handling up front. The bumps are smoother, the bike feels more stable in the corners (especially high speed), and the bike doesn't nose dive everytime I hit the brakes! Overall I am very pleased! The sag is right at inch when I sit on it which is perfect. The bike handles like a totally different machine.. I am running a newgen rear shock. Pirelli MT75 tires and it handles like a dream! It's so much smoother and responsive than it's factory suspension. I highly recommend this mod. To anyone wondering any of the KZ550,650,750,900,1000,1100 fork springs should work and those bikes where made from the mid 70's to the early 80's. The GPZ550 and 750 would likely have stiffer spring rates for heavier people but should be the same 26mm spring diameter. All I did was cut it down with a cut off wheel on a grinder and got it within 0.5mm of the other spring. Super easy. The job can be done in an hour or less.

Enjoy! I hope this info helps some people! Ebay is the place to go for used springs and sometimes just buying the whole fork set is cheaper!

First get it up in the air... A couple of jackstands under the exhaust headers and a jack under the oil filter works perfect.. An extra hand helps you not drop it


As you can see, quite a bit longer than the stock springs... Yes I did try it like this and it was WAY too stiff


First time I have done this, and I thought this was the better way, come to find out, doing it on the bike was a little easier. And a LOT easier once I cut the springs down


This is the easy way to do it... Get a 6mm allen wrench and take the handlebars off and hang them to the side. With the bike up in the air, get a large #3 or #4 phillips and push in the middle of the fork cap and have a pick in your other hand and pop the clip ring out. Its sometimes a good idea to have someone stabilize the bike while it is up in the air and you are pushing down the fork cap.


I took 2 7/8" off or 73mm off of the KZ1100 shocks. Other models and years may be different, so check with the service manual for whatever year you get. Most of them will all be longer than the stock ninja 250 spring, so you should be good, just the spring rate is the important thing, you can cut what you need off once you get it. I got these springs for $20 on Ebay.

[DannoXYZ]

Great DIY, thanks!

Just a note that spring-rate is determined by wire-diameter and number of coils. When you cut coils from a spring, this increases spring-rate. So your KZ1100 Spectre 0.62Kg/mm springs are stiffer than that now. You can calculate resultant spring-rate based on ratio of before & after lengths.

Also be careful with pre-load amount. It certainly changes bike's behavior independently from spring-rate.

[memphisninja]

Originally Posted by DannoXYZ

Great DIY, thanks!

Just a note that spring-rate is determined by wire-diameter and number of coils. When you cut coils from a spring, this increases spring-rate. So your KZ1100 Spectre 0.62Kg/mm springs are stiffer than that now. You can calculate resultant spring-rate based on ratio of before & after lengths.
Also be careful with pre-load amount. It certainly changes bike's behavior independently from spring-rate.

Excellent point.. do you know where I could find that formula?

[DannoXYZ]

Originally Posted by memphisninja

Excellent point.. do you know where I could find that formula?

Simple ratio:

SpringRate1*Length1=SpringRate2*Length2

SpringRate2 = (SpringRate1*Length1) / Length2
SpringRate2 = SpringRate1 * (Length1/Length2) <-- there's ratio


Example we can do in our heads is if you chop spring in 1/2, it will double its rate. So let's plug into equation with example spring of 100mm length with rate of 1.0kg/mm.

1.0kg/mm * 100mm = S2 * 50mm
S2 = (1.0 * 100) / 50
S2 = 100/50
S2 = 2.0kg/mm or double original with 1/2 length.

Reason this works is load-in-series equation, like a chain. Each coil faces exact same load and deflects exact same amount (except for bound ends). More coils you have, more you have of each little deflection and more "give" or total compression overall. Fewer coils you have, less total deflection and less compression or higher rate overall.

Other thing to consider is how much compression (travel) you have before spring coil-binds and becomes solid. Those KZ springs don't have as much travel as EX250, but as long as it doesn't bind before fork bottoms, it should be fine. Is there some sort of bumper for bottoming on these forks?

[memphisninja]

Originally Posted by DannoXYZ

Simple ratio:

SpringRate1*Length1=SpringRate2*Length2

SpringRate2 = (SpringRate1*Length1) / Length2
SpringRate2 = SpringRate1 * (Length1/Length2) <-- there's ratio


Example we can do in our heads is if you chop spring in 1/2, it will double its rate. So let's plug into equation with example spring of 100mm length with rate of 1.0kg/mm.

1.0kg/mm * 100mm = S2 * 50mm
S2 = (1.0 * 100) / 50
S2 = 100/50
S2 = 2.0kg/mm or double original with 1/2 length.

Reason this works is load-in-series equation, like a chain. Each coil faces exact same load and deflects exact same amount (except for bound ends). More coils you have, more you have of each little deflection and more "give" or total compression overall. Fewer coils you have, less total deflection and less compression or higher rate overall.

Other thing to consider is how much compression (travel) you have before spring coil-binds and becomes solid. Those KZ springs don't have as much travel as EX250, but as long as it doesn't bind before fork bottoms, it should be fine. Is there some sort of bumper for bottoming on these forks?

The specs say that the KZ1100 has a front travel of 6.4 inches or 165mm and the ninja 250 says 5.5 inches or 140mm... Does this mean the 1100 spring will have more travel?

The stock spring for the KZ1100 is 584mm long, I cut off 2 7/8" or 73mm down to, 511mm overall length, which is stock. I decided to use no preload spacer because the spring felt stiff enough without it for me.

The stock spring of the Ninja 250 is 460mm long (Spacer is 51mm or exactly 2") So overall 511mm overall length


So according to that formula, 0.62 Kg/mm x 584mm / 511 mm = 0.708Kg/mm spring rate with the modified spring that I have and 0 preload. So it's perfect for me at 140 lbs according to Race Techs calculator!