Hey guys, i want to do some upgrades to my stock forks, could i cut a piece of PVC for each side and stiffen them up that way?

i am not trying to be a squid but i am also short on money at the moment is there a cheaper way to stiffen it up a bit without new springs? if not, are people just using progressives or going with racetech?

Thanks for any opinions

bearshawk wrote:


If not already done, would first try a higher viscosity fork oil.
That would of course be after draining and flushing out the old oil.
And assuring correct volume of new oil per FSM specs.

Good Fortune!

Bear, In theory, a constant rate spring compresses at the same rate thru its applied travel...Progresive springs thru the virtues of different coil distance & spring wire diameters increase resistance (what else) progressivly as compressed. adding spacers (or Pre-loading) on constant rate springs will give you a taller ride height initially, but compression rate (in theory)will remain the same....HOWEVER, since compression on a constant rate spring is dependant upon its length (number of coils/distance compressed) you can cheat....... cutting a constant rate spring reduces the amount of "compressable" media, thus "stiffening" the springs travel rate. a rough "rule of thumb" is that you should have (with 1 inch pre-load) @ 1 inch sag static (mounted) and use @ 80% of travel in "spirited riding". Fork travel is GOOD as long as it's within its perameters, it not only provides creature comfort, but maximum adhesion. END PART 1. Dampening Is simple on these old beasts, its the same for compression & rebound. It's nothing more than the rate at which fork oil travels from the inside of the fork tube to the outside. changing fork oil weights will vary this rate. As will playing with oil amounts. go play, luck G

That makes alot of sense, thank you! i have changed the fork oil to 15wt and it helped a little bit but i am a big guy, 245# or so and its a cafe racer at the moment so im putting a bit of weight on the front end...i also have the factory amount of oil in there right now, i assume that ADDING some oil will help with dampening?

also if im understanding correctly, i could cut the stock constant rate springs lets say 1 inch and compensate with a 1 inch spacer thus stiffening up the front end a tad?

honestly i have a good amount of "bandit bucks" stocked up i might just get some progressives if i cant find a deal on race tech springs

Don't add more oil than is called for; that won't stiffen things up. If the problem is that the forks compress to low when you are sitting on the bike while it is not moving you will need to adjust the springs. If the problem is that the forks compress too quickly when you hit the front brakes or a bump you should try using heavier fork oil (Bel-Ray 20w comes to mind). If you do use Bel-Ray fork oil you can experiment using a combination of 15w, 20w,or 30w until you get the viscosity that works best for you. Ed

www.belray.com/bel-ray-high-performance-fork-oil

The FSM should provide specs for fork spring length and service limit.

For example, Z1 standard length is 495.5 mm with a service length of 485 mm, which is a difference of about ⁷/₁₆" .

Don't see anything in the FSM about feasibility of adding spacers to help compensate for reduced spring length due to wear.

Many owners have long since upgraded to progressive springs from "Progressive Suspension" (brand name).

Good Fortune!

Could also look at RaceTech's much stiffer than stock single rate springs.

www.racetech.com

bearshawk wrote:

As Ed mentioned, adding oil won't really help with dampening. However having a slightly higher oil level may make the spring rate more progressive. Actually, it makes the compressive force of the air inside the fork more progressive due to the lower volume of air to begin with. This requires the forks are air tight. The affect is that the fork's rate is more progressive. The only problem is, in my experience, the amount of oil you can safely add does not usually provide nearly the amount of stiffening you might need.

bearshawk wrote:
Yes. As a matter of fact I just did that, but I cut off 7.5 inches! That's because I wanted a pretty big jump in spring rate. You can cut a significant amount of the spring and still not reduce travel because the KZ's seem to have a pretty long travel compared to modern bikes. (At least the 550 forks do.) I went from 57.6 cm to 38.4 cm on the springs. It worked out almost exactly how I planned. (I measured the spring rate and calculated accordingly, it was not just some random guess.)

On mine, I had to cut-off the bottom end since that end of the spring in question was already cut from a previous test, years earlier. This means I lost the end that was dressed for riding up and down inside the fork.

You have to heat the end of the spring to a dull red and reduce the outside diameter of the spring slighltly so the cut-off section won't scrape the walls of the fork. At the same time, while it's hot, you have to alter the bend on the last coil or two so they stack correctly. (Notice the end of a spring is not just cut, it's comressed and formed so the last coil ends up close to prependicular to the axis of the spring. Then when it's ground flat, to make it perpendicular, there is a nearly contiguous circle of spring material to contact whatever the spring rests on. Just simply cutting and grinding won't create this. It has to be formed by bending the wire.

The one drawback to cutting a lot off of the spring, like I did, is that the spring's remaining life will be greatly shortened because each individual coil of the spring has to move a lot more for every compression the fork undergoes. But I have a bunch of old springs so it's worth it for me.

If you increase the fork's ride height too much (and then compensate by raising the forks in the triple clamps), you run the risk of shortening the life of forks seals and bushings. This is because you reduce the amount of fork tube in the slider, and this increases the leverage the fork has on the bushings for wear, and the amount of slop the seals have to take up to prevent leakage. I shoot for a close-to-stock ride height, but stiffer rate. (After many years, the ride height lowers quite a bit, so there is some to be made up, of course.)

Before, cutting, I made sure the internal space of the fork was adequate so that the fork travel was not reduced. There is a surprising amount of space to accomodate the spring, so the 20 cm preload I used leaves plenty of room before it would start to affect fork travel.

There are many good ideas posted, and you must of course try to find the solution that you think will suit your needs. Also, there are plenty of stuff on various sites from makers of suspension bits that will provide insight.

Thicker oil slows the forks action, but doesn't change spring rate in any way. I personally find compression damping generally to be too harsh on old machines - which is also why Race Tech calls for drilling out the compression damper holes to get it close to nil, then compensates with their blow-off valve. I don't mind the quick compression under hard braking, but I do mind a harsh ride. So my personal trick is to drill out the compression holes in the damper rods with a 1/4 in drill bit and use heavier oil, around 25 weight. This keeps some compression damping despite the bigger holes while fixing another issue typical for old machines; lack of rebound damping.

Adding preload will, as mentioned, only raise the bike without altering the spring rate. But sometimes, raising the bike to get the desired total sag is enough to satisfy the rider - it depends on your needs and preferences.

On my Z400, I first upped the oil level by 40 mm (1.5 in) which made the spring rate far too stiff for my 215 lb frame (plus 20 lbs in riding gear) but with 1 inch / 25 mm increased oil level over stock the front performed great for my wants. Together with longer Koni shocks lifted from a Z1000 handling was excellent and cornering clearance all that the tyres could take.

Racing bikes use straight rate springs, usually, whereas progressive springs suit most street riders better due to a softer ride combined with a high resistance to bottoming. Upping the oil level, together with the more or less straight stock springs, will give such a progressive action.

Total sag with you plus bike plus riding gear should be between 1/3 (for comfort) and 1/4 (for hard riding) of total suspension travel. But that's not all. A a light touring bike will often have little sag with the bike resting alone, with the majority of sag coming from your weight. Why? Because they use soft springs with lots of preload. A sportbike will drop as much as 3/4 of the total sag from the bike weight alone, the little bit remaining coming from your weight. A harsher ride, but much greater resistance towards bottoming.

One other downside to cutting springs is that you may not get them cut exactly equal thus having two slightly different rates (one on each side of the fork). I don't know how much this would affect the ride quality, though.

personally , i would not recommend a progressive spring, ,at your weight, u will compress the soft part of the spring, so u will likely have a lower ride height. Then to correct the height, u would add a 2” sleeve above the spring, this would compress the spring even more. These old bikes have a habit of diving when u apply the brakes. Stronger springs will help, not progressive springs.
The best solution for a ride that is too soft, is a stiffer spring (higher spring rate).
Call Racetech and order the proper spring rate , this is based on the total weight of the rider and bike. Springs cost about $124.
the racetech spring calculator calls for a 1.048 kg/mm spring rate

racetech.com/ProductSearch/2/Kawasaki/KZ1000/1977-80

thanks mark, thats what i was going to do i think...i just replaced my fork springs in my vtr1000 and it made a world of difference. I would rather do it right and have the peace of mind that i wont bottom out just got my kerker ceramic coated

oh wouldnt thats suck the big one. its hard to find a pipe that dosnt have a flat spot on the bottom. Take good care ofr it.