Does my bike have stock or aftermarket fork springs?

Hi - I pulled out the fork springs when I changed the fork oil in my 84' KZ700 last weekend. Bike was bought used with an unknown history. The springs had approx. 6 inches of tight coils at the top. The remaining coils were wider and evenly spaced.

Do you think these were stock or aftermarket fork springs? I thought stock springs were all evenly spaced coils. Not sure..what do you think?

Thanks!

AFAIK, stock fork springs are progressively wound, with the coils tighter on one end than the other. The springs on my 81 KZ650 are & I'm pretty sure the 82 750 Ltd springs are as well. You could measure yours to see if they're close to spec. I think Progressive brand springs are shorter than stock, so a a longer spacer is needed.

Ok, thanks for the reply. Sounds like my fork springs are OEM.

martin_csr wrote: AFAIK, stock fork springs are progressively wound, with the coils tighter on one end than the other. The springs on my 81 KZ650 are & I'm pretty sure the 82 750 Ltd springs are as well. You could measure yours to see if they're close to spec. I think Progressive brand springs are shorter than stock, so a a longer spacer is needed.

Bin those stockers and get some Sonic springs. They sell several different spring rates so you can select appropriately. The stock springs are weak because the forks used air assist (which leaks and is generally a pain in the butt.)

Nessism wrote: Bin those stockers and get some Sonic springs. They sell several different spring rates so you can select appropriately. The stock springs are weak because the forks used air assist (which leaks and is generally a pain in the butt.)

+1
Springs that old are done. Progressive, Sonic or Race Tech replacements will make a huge improvement.

TexasKZ wrote:
+1
Springs that old are done. Progressive, Sonic or Race Tech replacements will make a huge improvement.

I don't agree with that statement TexasKZ. As long as a spring has not been strained to the point of deformation it will continue to perform correctly. That's why the standard test is to measure the length and compare to a standard. Just because they're old is no reason for them to be changed out.

Springs are not designed independantly. They are selected to suit the fork damping, along with the mass being moved. Changing springs will change those relationships and could result in an unstable ride at certain speeds.

If riding conditions are different than those used during design there may be more suitable springs out there. It's hard to know what the design conditions were, but likely average rider on average roads. Someone wanting to ride more aggressively would definitely benefit from more suitable springs, but otherwise the original ones will be just fine as long as they have not been deformed.

Yes, they should be measured, but I am pretty sure that even just the static weight of the machine over the course of 40 years will cause them to fail the measurement test. Seems to be a pretty common problem with antique and classic automobiles. I suspect those companies I mentioned are well aware of the design parameters.

All of mine are within spec despite being as much as 45 years old. Same with my valve springs, which function the same way...under compression. I've heard of valve springs breaking but never a fork spring. Perhaps you have?

I doubt anyone knows the original design specs except the original engineers. I remember hearing during early testing that Z1's had a serious harmonic vibration (a tank slapper) around 85 mph. They corrected it somehow. I think they also changed the front tire to help stabilize the harmonic but they may have changed other things as well.

There is a mathematical relationship between the springiness, the mass and the dampening ability of a bike. Forces acting on the bike cause all three of those factors to change in ways that can create strong vibrations such as a front wobble. Moving your body forward and downwards while backing off the throttle is one way to stop a wobble. By moving your weight you change the center of gravity and this in turn changes the relationship with springiness and dampening and will end a wobble. A good example of how these factors can create vibrations under load is the Tacoma Narrows Bridge failure. There are videos online if you want to see a spectacular failure.

TexasKZ wrote: Yes, they should be measured, but I am pretty sure that even just the static weight of the machine over the course of 40 years will cause them to fail the measurement test. Seems to be a pretty common problem with antique and classic automobiles. I suspect those companies I mentioned are well aware of the design parameters.

If the bike is always on the centerstand when parked the weight on the forks when the bike is not being ridden is reduced quite a bit, and this would help reduce the stress on the springs. But if the bike has no centerstand or if it is normally parked using the side stand the front forks would carry the weight pretty much all the time. Measuring the springs is probably the best bet especially if the current owner isn't positive about the bike's history. Ed

The stock springs will work as long as you don't mind filling up the air assist on a regular basis. The air pressure will leak down over time (typically at different rates between the RH and LH fork) and create extra seal drag so the forks won't be quite as compliant as a non-air assist fork.

Personally, I hate filling up the air all the time and greatly prefer the aftermarket springs in my bike. Filling the air is touchy and a pain in the ass because even a quick jab with the compressor chuck spikes the pressure and then you need to bleed it down. There is a reason air assist forks are no longer in the marketplace.

Back in the day with my Harley I found the best way to fill the air fork was with a hand pump but even that was a pain. I usually had to go a bit higher in the pressure as I lost a little bit of pressure when I unscrewed the connector. Same for the rear air shocks. I used the Harley sourced hand pump with attached gauge. I was okay though and I still have it. A quick connect would have been much better. Looks good hanging on the old tool board in my garage.
Rick H.

I've seen old topics in which KZRiders recommended using a Harley hand pump to air up the forks. To make the job easier, my neighbor (R.I.P.) fabbed up an equalizer from copper tubing w a liquid-filled pressure gauge between the forks. It was kind of nifty.