79 gs750e fork oil volume

[Northern Mike]

Doing the fork oil on my gs750e for the first time. Manual says 180cc is the volume per leg. Seams low or does this sound correct?

[Nessism]

180cc is correct

[Rich82GS750TZ]

As a double check, your manual may also give you a measurement, with no springs installed, from the top of your fork tube to the top of the oil level. You'll want to work the fork a few times to make sure the oil gets in all the places it needs to go and then compress the tube all the way down before measuring.

[Northern Mike]

Thank you guys for the conformation. Sounded a bit low so wanted to double check.
Dust caps were hard and cracked so went woth some batters I had on the shelf.20190325_214252.jpg

[Steve]

If you want to check another way, just to make sure, check the level, rather than the volume.

With the fork tube off the bike, spring out, fully compressed and held vertical, the level is likely in the 6 to 6.5" range from the top of the tube.

Some manuals give this measurement, others don't, but it's a pretty decent average.

Did you by chance change your springs while you had the forks apart? It's a VERY worthwhile upgrade.
Sonic Springs are a favorite here.

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[rustynissan]

I have a 1979 GS750LN (figured out based on engine and frame numbers). The FSM says 180cc for each leg, but doesn't specify bike model. My Clymer manual says 280cc per leg, and the fork oil should be 202mm from the top of the tube. I put 280cc in each leg and ended up with 229mm dead on for each leg, which is what is specified for the ET, LT, or LX models (but the capacity of those models is supposedly 237cc!). Spring lengths are consistent with an "L” model at 18.5" and 6.5".

I doubt I should add more oil to the forks, but thought I'd see if anyone knows something about this situation.

[Steve]

202mm is right at 8 inches. Set the level there and go for it.

Or, for a little more progression in the air portion of the spring, set the level at about 175mm, which is 7 inches.

Either way, make sure the fork is compressed, vertical and no spring in it.

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[rustynissan]

@Steve, thanks for the reply. 202mm, huh? Sounds like I would need another 43cc to get there. Weird that our manuals have conflicting info! Fork was compressed with no springs for the measurement.

[RichDesmond]

I agree with Steve, somewhere in the 7"-8" inch range is fine.
Too little oil and the damper rod might not be fully submersed in oil at full extension, too much and you'll get way too much progression in the overall rate and very high air pressure at full compression. As long as you stay away from those extremes it's not a terribly critical measurement for normal street riding.

Very important though, it is the air gap that you're measuring, so more oil = lower number. Lots of people get confused on this.

[Steve]

Weird that our manuals have conflicting info!

While I have consulted many manuals for fork oil measurements, I did not do so this time.

Just going by recommendations from suspension manufacturers for installation and set-up of their products.

As Rich Desmond mentions, too-little oil might not keep the dampers covered all the time. I have experienced this, and it's not fun. Too much oil leaves a very small volume above the oil, which will compress quicker.

Think of this: once your forks are filled with the proper amount of oil and installed, let's say there is 12 inches of air space. Let's also assume that your forks have 6 inches of available travel. At full compression, you will have half the volume, so will have twice the air pressure. Let's add a bit of oil so you only have 9 inches of air. Compress that 6 inches, you will have one third of the volume, so three times the pressure. Let's continue. Add more oil so you have just 6 inches of air. When you compress THAT 6 inches, ... let's just say that things will get rather interesting before it goes that far.

Setting your oil level to something in the 7-8 inch range compressed and without springs, will leave plenty of room when the forks are extended and the springs installed.

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[rustynissan]

Let's also assume that your forks have 6 inches of available travel.

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Judging by the upper spring being 6.5" and its position pre/post fork extension, I'd say I have somewhere between 5.5" and 6" travel (the full amount I hope to never see!). When filling, I just need to compress it by hand, and leave it there right? I let the tube fall all the way down and pressed on it lightly, then let it sit on my bench while filling. Is there another spring in there that needs to be compressed when filling?

And apologies for the continued questions in advance. I'm an engineer :-/

[Steve]

When the forks are off the bike and the cap is off, there will be no choice in the matter, they WILL compress.

And apologies for the continued questions in advance. I'm an engineer :-/

No problem, I'm an engineering tech, I deal with engineers' mistakes every day.

I have long had the philosophy that engineers should have to spend a year working on the stuff they design.
They would very quickly learn to do it right.

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[rustynissan]

I have long had the philosophy that engineers should have to spend a year working on the stuff they design.
They would very quickly learn to do it right.
.

Amen to that! I have worked with many a textbook engineer... :facepalm:

[rustynissan]

Some rough calculations, assuming 5.5” compression:

14.5" to 9" - 1.61 compression ratio
13.5" to 8" - 1.69 compression ratio
12.5" to 7" - 1.79 compression ratio

Not a huge difference. I think I'm satisfied that playing around with the oil level has a very minor effect, aside from keeping things lubricated.

[rustynissan]

Similarly, at 3.5" compression (mid range?), you get 1.26, 1.29, 1.32 compression ratios.