forks: More oil or heavier oil?

So I've just changed oil the other day to a 15w and still think my forks are way too soft (manual says 10w and the springs are newish). I couldn't find any heavier oil locally so I'd have to order it online. I'd rather not use air pressure, so do I change the oil again or would it make any difference adding more oil? The factory manual allows for +/- quite a lot (can't remember how much atm.) so would it do anything if add some more, within limits?

Thanks!

Are you sure you put in the correct amount of fork oil? How did you measure it? Ed

Adding more oil increases the progressiveness of the compression force, but too much will obviously blow out seals etc. I prefer going to stiffer springs rather than change the oil level drastically or adding air. I might change the oil level by 10mm or so at the most.

I stiffened up the springs by cutting down the stock ones (I had a bunch of old ones laying around). Then just make up the difference with a PVC spacer. It costs almost nothing, lets you put the old springs to use, and feels really good. I usually take off about 25% to 30% of the spring. On a stock spring (on a kz550) it leaves plenty of travel so the spring does not bind up even with hard stops and bumps.

By TheForumTroll: the factory manual allows for +/- quite a lot (can't remember how much atm.)

You should double-check what the fork oil table says & also make sure the table you're using applies to the 700. In case there's more than one table, since the 750-4 manual covers quite a few models. Most factory manuals say to add "about" xx ounces & the amount depends upon whether you're doing a simple oil change or a rebuild. Also be sure to note if the fork oil measurement is taken with the springs in or springs out & the fork legs extended or compressed.

The quantity of oil has very little affect on spring rate. You need stiffer springs for that. Shortening the springs will work or get some Sonic springs or similar. Oil weight affects damping not stiffness.

Also-

The KZ700-A1 has air assisted front forks (see link below). How much air pressure was put into them? The air pressure is to be measured with the front wheel off the ground and the pressure should not exceed 36psi. Also, make sure air is not leaking out past the valve or anyplace else. No air will impact the ride. Ed

www.powersportswarehouse.com/pages/oempa...00-A1-1984/080199F-7

Thank you all for the input. I don't have the manual at me and cannot remember the numbers, but I did pick between different models in a table. I measured 335mm (I think?) according to the guide in the factory manual with one of those devices where you suck out any oil that is above whatever point you set it to (does that even make sense?). The guide does state "around so and so much oil" but does also gives a precise measure (without springs) from the top of the inner tubes. It was something like "335mm, +/- X mm". It was those X mm I thought might make a difference. I'm pretty sure I'm using the correct measurements in the manual as the amount I took out is the same as I put in.

I read in here that 20 to 25w oil was great even though the manual says 10w, especially if you weigh above average.

Does the trick with shortening the springs also apply to progressive springs?


Edit: Oh, and I did put in air according to the table in the manual but at the lowest setting. I have tried the high setting too but didn't think it did much. I'd rather be careful with the seals and stiffen up the springs than adding more air.

I guess it depends to some degree of the weight. I use Bel-Ray 15w fork oil in my KZ650-C1. It doesn't have air valves. I weigh 200lbs. and even with a 115lb. passenger the forks work fine.

I agree on not exceeding the 36 psi max air pressure. Ed

The fork oil doesn't affect spring rate, but does effect the overall rate by affecting the rate of compression of the air in the remaining space. Taken to an extreme , if the oil were filled all the way up, the overall rate would be infinite. At intermediate levels, the compression rate of the air varies with the remaining internal volume.

The effect of the air rate will also depend on the initial air pressure. Taken to an extreme, if there was no air at all, as in a perfect vacuum in the remaining volume, then the oil level would have zero effect on overall rate.

The fork compression rate is really the result of a gas spring in parallel with a mechanical spring. If you start with very little air pressure, then its contribution to the overall rate will be minimal. But the type of gas spring in question, where the fork displacement directly affects air volume, is relatively progressive compared to a typical factory metal spring, which tends to be constant rate.

But as nessism said, you will not feel much effect of oil level change at all unless you make pretty large changes, and are using higher air pressure to begin with.

Also, should mention...
Oil weight effects damping, and most damping on old forks is rebound damping.

My bike originally used I guess the early version of progressive springs. One on top of the other of different rates.

I installed true Progressive springs cataloged for my bike, with PVC on top cut to the specified length. I forget the weight, but went 1 up on fork oil viscosity, and mid to high on level range. I also, and crazy not to on these older skinny tubes, installed a fork brace. It gave me a significant improvement in low and high speed stability.

And if you have adjustable rear spring preload, finish off with adjusting race sag to your weight. Another big advantage you don't realize until you do it.