fork oil level question Progressive Springs

Overhauled the forks today. New washers, bushes, oil seals, dust wipers, and Progressive springs. The 1988 forks are quite simple with no air assistance and no anti-dive. Big thank you to Chacal for the parts, and everyone on this site for posting tips. I made my own tool to hold the rebound damper by welding a 22mm, across the flats, nut onto a long piece of 20mm RHS. Forks are much firmer now. The old oil was black, and very smelly - like old gearbox oil. Put in new 10W Castrol fork oil.

My Haines says the oil level should be 168mm from the top of the fork tube with the spring out and the fork tube fully pushed into the slider. With the new thicker Progressive springs it should be 31cc less oil. I took a guess and set the oil level at 200mm below the top of the fork tube.

Does this sound right?

 

If the inside diameter of your upper fork tube is 35.68mm then you did it correctly. I think your forks are probably a smaller diameter than that.

To calculate the decrease in fluid level divide 40743.7 by the inside diameter in mm squared.

Your inside diameter is probably closer to 30mm. 30 squared is 900. 40743.7/900 = 45.27mm

168mm+45.27mm = 213.27mm fluid height.

**EDITED to correct math**

 

Actually, since the fluid level is measured from the top of the fork down to the fork oil, I believe you'd SUBTRACT 45.27mm from the original 168mm, making the new measurement 122.73mm. If i am wrong, please correct me.

 

Ummm.........I don't think the fluid HEIGHT should change at all.......you change the fluid VOLUME (amount of oil) in order to obtain the same fluid height when changing to the Progressive springs....

 

That makes even more sense. Progressive should state any difference in the literature or website.

 

The quote from the Haynes manual said to measure the oil without springs.

Aside from the damping provided by the oil, the air pocket above it is also a spring. That's why we need to be fairly precise in filling them (don't want different spring rates right and left. Progressive may be specifying a lower level (farther from the top CNCguy) so there is a lower spring rate to the air pocket.

 

Thanks for your responses everyone. The forks are 34mm i.d, so, going off CNC's calculation to use 31cc less oil, gives an oil height of 203mm, which should set the oil at the same height as would be with 168mm of oil with the old stock springs.

While I was dismantling the forks, I found that the PO had put a 25mm long spacer, made of copper pipe in one fork leg, and a 20mm spacer made out of iron water pipe in the other fork leg. These have now gone on my rapidly growing scrap metal pile.

 

Sorry, I should have said going with MiCarl's calculations. And Chacals fork springs are excellent. My forks are nice and firm now.

 

Well, we did measurments of the amount of fluid displacement of a stock spring versus a progressive spring, and the difference is about 31cc of fluid.....the progressive springs, which use a thicker wire and have more coils, displace 31cc more fluid than a stock spring does (spring fully immersed in fluid). Therefore, to achieve the same fluid LEVEL, one would think that----ignoring the small air pocket----that you would need 31cc less fluid than the stock fluid settings (realizing that the fluid level is measured without a fork spring in place, but-------you're going to add a fork spring in there eventually, and the difference in fluid displacement of stock vs. progressive spring should directly and pretty much impact the fluid level in a 1-to-1 relationship).

Tell me where/how I'm looking at this incorrectly? It sure would have been nice if Yamaha had released fluid level specifications for all of their fork systems, rather than just oil volumes..........

 

I'm doing springs on mine and have been doing some info hunting, th nearest I found was here.....For th fork oil level measurement, I take it we are talking about th forks being on th bike and measuring at th middle of th fork not front or back and not off th bike sitting vertically?
Or are any of those differences not worth worrying about??
And on chacals comment about 31ml difference from stock to progressive...different models have different length springs, so that must be specific to a particular model.

 

We're talking off the bike, measured vertically.

On the bike the front will read low and the rear will read high. The average (middle) should be the same as a vertical reading.

 

Maybe I'm missing something, but if the measurement is WITHOUT springs, then the specified volume would always give a certain level, and vice versa, right?

So the question becomes, what is the oil level at the specified volume? Rough calculations say about 12 inches from the bottom...around 7 inches from the top. Since they are BOTH measured without the spring, then it becomes a meaningful measurement. And the difference between progressive and stock would be for the purpose of:
a) ensuring you didn't put too much in...so you just need to make sure it is less than 5.5 inches
b) altering the level to suit your preferences...so you need to experiment to find the right level....you just need a decent starting point.

So to find a good starting point...how many inches of oil are lost by removing 31 ml of oil? I can't be too precise because of the different forks we're talking about, but it's about 35 mm, or an 1 1/4 inches.
So we should start our new Progressive springs about 7 inches from the top, or about 31 ml less than stock...which SHOULD be the same thing.

***In case you didn't catch it, the numbers are based loosely on an 82 xj650. I didn't work too hard on the math, because I don't feel like going outside right now with a micrometer, and also because we all have different bikes. I just wanted to ask the questions and show what the answers would look like.

When I actually do mine, I'll do the math for real.