79 400B fork re-spring help needed for a lightened bike

hello all,
am looking for fork spring rate recommendations for my project bike, a 79 KZ400B. it came disassembled in 6 tubs, most parts were labeled and it's now running great. still need to paint the tank and seat, and fab a seat cushion. am dealing with a lumpy front wheel at the moment, but during test runs i noticed that the forks aren't moving up/down on the sliders at all. forks were fully disassembled for cleaning and filled with the correct level of full synthetic 20wt ( will be replacing w/10wt ). slider bushings were excellent, replaced the seals and dust shields. with the axle in place with no springs, the forks slide up and down freely by hand. no stiction noted.

i'm 155lbs and with 1/2 tank tank of fuel, the bike weighs in at 166 lbs front and 169 lbs rear, for a total weight on my calibrated scale of 335 lbs. period tests that i've found show a wet weight of 400 lbs so my bike is 65 lbs lighter than stock.

i've measured 150mm of exposed fork tube with the forks fully extended and 125mm with the front wheel resting on the floor and the bike vertical. when i get on the bike, it drops 0 mm and with a very little bit of shuffling around on the seat, can get it to drop 5 mm. if i apply the front brake and push hard on the front, i can get the suspension to drop another 50mm as measured using a cable tie.

a bike test i read recommended going to 23lb/inch springs and drilling out the internal orifices. knowing that the forks act as a parallelogram, i'm actually thinking about talking out one spring an going on a tentative, slow speed ride to see if the forks a little.

hoping i've provided all the info, any ideas for a spring rate for a little guy on a lightened bike?

Not being a suspension expert, I would contact Race Tech, Progressive, or Daugherty Motorsports, and ask one of their experts for advice. Since you will likely need new springs anyway, I reckon they will be happy to advise. They may also be helpful in finding a good solution for the rear as well.

noman wrote: hello all,
am looking for fork spring rate recommendations for my project bike, a 79 KZ400B. it came disassembled in 6 tubs, most parts were labeled and it's now running great. still need to paint the tank and seat, and fab a seat cushion. am dealing with a lumpy front wheel at the moment, but during test runs i noticed that the forks aren't moving up/down on the sliders at all. forks were fully disassembled for cleaning and filled with the correct level of full synthetic 20wt ( will be replacing w/10wt ). slider bushings were excellent, replaced the seals and dust shields. with the axle in place with no springs, the forks slide up and down freely by hand. no stiction noted.

i'm 155lbs and with 1/2 tank tank of fuel, the bike weighs in at 166 lbs front and 169 lbs rear, for a total weight on my calibrated scale of 335 lbs. period tests that i've found show a wet weight of 400 lbs so my bike is 65 lbs lighter than stock.

How have you achieved the weight savings?

Have you installed the front fender? Keep in mind that the fender is also a fork brace and will help to keep the front end in alignment. Without the brace, the forks are free to twist a bit, which could cause the condition you're seeing.

the front fender/fork brace is still present. i reassembled the bike keeping only that which makes it stop, go, or pass inspection. that's how i got the weight reduction. i believe i have two issues, one is using fork springs for a 400 lb (wet) bike while the bike weighs in at only 335 lbs (wet). the other is damping.

i utilized the clymer's manual that came with the bike when reassembling the forks. they say to remove the fork springs prior to adding the correct level of 10-20w. this was done, and i filled with full synthetic 20w thinking i would get smooth fork action.

when researching this fork problem i found a kawi service manual for my bike, and it says to keep the springs in when refilling the forks, and to use 5w-20 oil.

when i placed the springs back in after filling (according to clymer's) the springs displaced fork oil which rose up in the fork tubes. am certain the forks are overfull by some amount. i'll report back by how much, if interested.

the kawi service manual also gives the fork spring k-rate, at 1 kg/mm until the springs are compressed to 120mm, then the rate rises. these are progressive springs. the conversion gives a spring rate of 56 lbs-in. i can use this when speaking w/race-tech folks. thx for the replies.

Good looking bike.
I use 10W in most of my street bikes. 7W in the MX bikes. I am a tad heavier than you (ahem) but I think the stock forks are way too soft so maybe with the lighter oil and stiction/alignment problems resolved it will be acceptable. By alignment I mean make sure the forks are not binding. What I do is raise the front end, spin the wheel and apply the brake a few times. Then I tighten all the bolts.

+1 for racetech. I bought springs and cartridge emulators for my stock forks on a KZ750 and it completely transformed the front end. A lot of bang for the buck.

Off the top of my head, fork oil is added without springs AND forks completely compressed. If you had your forks extended, you surely have too much oil in them as well.

from my first post, first paragraph i don't think stiction/alignment is an issue. with the fender/fork brace in place, wheel off, springs out and axle installed i can easily raise and lower the forks as a pair. from compressed all the way to the triples, to fully extended there isn't resistance or sticking. everything just slides up and down freely. this was checked before, but i'll certainly recheck as the springs are out at the moment, and it's still snowing. no more checking stuff and test rides for me.

will be calling race-tech for sure. with the amount of cafe' bikes and street trackers being fab'd these days, am sure i'm not the only one ask.

according to the kawi service manual for my 79400B (attached) i am to refill to 16.4 inches inches with the springs installed.
see page 211

service manual