'77 GS 6 front Suspension

[wfopete]

I'm sorting out the front end of a buddy's 250 GS 6, 1977 model.  I ran this bike at the ISDTRR and found that it blew through about half it's travel which resulted in a harsh ride from the front end.

I'm not familiar with this bike & I need to know the correct fork oil quantity/height.  Right now I've measured the height at 10 inches (springs out, fully compressed) from the top of the tubes.  I drained about 8 ounces of oil out of each tube.  I expected to have an oil height of closer to 6 inches from the top of the tube.  The forks have very little preload also, less than 1/2 inch.  Typically I would expect more like 1 inch to 1.5 inches of preload.

Pete Petrick
175 Jackpiner
Slow but Good

[slvrbrdfxr]

Pete,
My records show the 220mm travel Marzocchi forks on that bike should take 240cc of 10wt oil if fully disassembled or 220cc of oil for a drain and refill. I cannot remember the measurement from the tops of the tubes but like you feel it should be closer to 6" than 10". Maybe someone else will chime in with more info. Hope this helps.
Dave McCullough

[Big Mac]

5"-6" from the top of tubes, ATF works for oil, or a bit thinner fork oil, a couple of 1" long pvc pipe spacers (3/4" dia) work to add some pre-load but heavier springs work better, or about 3-4lbs of air if you have air caps.

[wfopete]

My Ratio Rite sez I got about 8.5 oz of fluid out of each leg which converts to roughly 250cc's (at least using the online calculator).

Those numbers give me more than what Dave's info recommends and I was still at 10" from the top tube. I'm sure I had those forks compressed but now I'm tempted to refill and re-measure.

Pete Petrick
175 Jackpiner
Slow but Good

[brian kirby]

You need stiffer springs.

Brian

[wfopete]

quote:Originally posted by brian kirby

You need stiffer springs.

Brian

Eithier that or lose 50 lbs!

Pete Petrick
175 Jackpiner
Slow but Good

[wfopete]

Ok, I went back and put the drained oil back in (250cc) and re-measured.  I came away with an oil height of 10" from the top of the tube.  So what should I go by? Oil quantity or oil height?

Tell 'ya what; I'm just going to experiment with both and see which works best.


Pete Petrick
175 Jackpiner
Slow but Good

[slvrbrdfxr]

Pete,
The bike does have 35mm forks right ?? The only other thing I can think of is that maybe the bike has later model forks ?? The reason I ask is because the late 77 MC5 and some of the 78 model Marzocchi forks are still 35mm tubes but use more oil in the range of 265-280cc.
Dave McC

[Big Mac]

Go with height. Can't go wrong. Air compresses, oil doesn't. At full extend, you'll have about 8" + compressed fork air space of 6" or so, total stroke 14" to 6" less displacement of the spring. If you start with 10" with compressed fork, you'll have 18" to 10" range. Huge difference...that will only work if you run air pressure in forks to compensate for excess air chamber size.

[wfopete]

Well Dave you may be on to something.  The forks are 35mm with 9" of travel.  But yesterday I went with a height of 6" from the top of the tube then after reading your post this morning; I drained and measured the oil quantity.

Guess what? My Ratio Rite sez: 300cc.  That's a 50cc difference from what I measured on the forks originally.

So for now I'll go with that height/quantity and 1.5" PVC preload spacers (sorry no air caps).

But I suspect I only have a partial solution.  As Mr. Kirby pointed out, I think stiffer springs will be in order.


Pete Petrick
175 Jackpiner
Slow but Good