Fork oil ?

[Josh]

I have a 96 Sport 1100 with Marz 41.7 mm forks. I'm going to replace my leaking fork seals this weekend and I'm having a hard time deciding what weight fork oil to use and what air gap would work best.

The Guzzi manual says 10wt with air gap of 170mm. Hagon and Wilbers says 5wt with an air gap of 140mm. I contacted Marzocchi and they don't seem to have a clue! I know the heavier weight oil will increase damping and the smaller air gap can have an affect on sag. Will the smaller air gap also increase damping? Would the lighter oil with less air gap be more likely to bottom out? Any suggestions would be appreciated.

[Guest Phil_P]

As you have worked out, the weight of the oil will directly affect the amount of damping (heavier oil, more damping). As long as the piston/valve assemblies are submerged in oil then the quantity of oil in the fork should have no affect on damping (bar the effect of a smaller volume of oil will get warmer quicker and therefore tend to reduce damping). The smaller the air volume above the air, the greater the rising rate effect of the springing provided by the air, so a larger air gap will give you softer springing. I would hope that the fork springs themselves should be fairly resistant to bottoming out even without the air gap coming into play. With experimentation, if you found you WERE running out of travel then you could up the oil level, or even think of moving to stiffer springs.

[mike wilson]

Wait for it........

 

 

If you search the forum, you will find a wealth of information (much more than you could possibly want) on how to dial in your suspension.

 

I strongly recommend you do that and _then_ start asking questions.

[Tom M]

As Phil said, oil weight will only effect damping rate and oil height will have no effect on sag. How satisfied are you with the damping in the fork currently, and what are your present compression and rebound settings? If you're set at near max on either compression or rebound you may want to go with a heavier oil, and if you're near the minimum settings a lighter oil may work better for you.

[dlaing]

I have a 96 Sport 1100 with Marz 41.7 mm forks. I'm going to replace my leaking fork seals this weekend and I'm having a hard time deciding what weight fork oil to use and what air gap would work best.

The Guzzi manual says 10wt with air gap of 170mm. Hagon and Wilbers says 5wt with an air gap of 140mm. I contacted Marzocchi and they don't seem to have a clue! I know the heavier weight oil will increase damping and the smaller air gap can have an affect on sag. Will the smaller air gap also increase damping? Would the lighter oil with less air gap be more likely to bottom out? Any suggestions would be appreciated.

The effect on bike only sag will be pretty darn close to nil.

The effect on sag with rider, will be more than bike only, but probably negligible.

"Will the smaller air gap also increase damping?" FANTASTIC Question!!!

I have never thought about that. The higher the air pressure, theoretically the greater the compression damping and the lesser the rebound damping, but how much of a difference would it make? It could be next to nil???

Less air gap will make you less likely to bottom out metal to metal, if that is even posssible, but you might lose effective travel and bottom out more frequently on the air cushion.

Someone who knows those forks well might be able to tell you at what level you would lose travel.

I suspect a 140mm air gap will reduce effective travel a little bit, but provide good protection.

Are you bottoming now?

What is your current air gap? You should measure before changing.

Replacing springs could make a big difference!

[emry]

Air gap affects only affects the shock during about its last 1/4 of travel where it produces a rising rate curve that adds to the effective spring rate. This is normally adjusted when there is a bottoming problem but the rider is happy with the current spring rate. i.e. the front travel is correct in most turns at a track but on one or two bumps the forks will bottom. More air creates a softer longer curve while less creates a sharper more abrupt curve.

[Guest ratchethack]

"Will the smaller air gap also increase damping?" FANTASTIC Question!!!

I have never thought about that. The higher the air pressure, theoretically the greater the compression damping and the lesser the rebound damping...

There's not a whole lot to think about here. As Phil accurately noted above, as long as there's enough oil in the forks to keep the damping orifices in the cartridges completely submerged in oil (there is with any reasonable air gap), differences in air gap have NO EFFECT on damping whatsoever -- in practice, as well as in theory. As Phil noted (Part II), the air gap functions only as an "air spring", adding a rising-rate enhancement to the fork springs only at the end of fork compression. Damping is likewise not affected whatsoever by changing spring rates.

 

EDIT: I see Emry beat me to the post.

[docc]

Josh,

 

I thought your Sport already had different springs, no?

[Josh]

I've only ever bottomed once, it was a pretty severe pothole. I do have aftermarket .95kg springs installed.

I did the seal replacement today and went with the 10 weight oil and 170mm air gap.

Thanks for all of the responses

[docc]

Sounds like the springs would be comparable for a fellow of your weight with a V11. Comparable numbers for the Marzocchi on the V11 are 5 wt oil with 100mm air gap. I know your shock s White Power; what are the forks?

 

You must give us a ride report!

[dlaing]

For what words might have failed to describe...

 

 

From an Ohlins Fork Manual

[Josh]

Sounds like the springs would be comparable for a fellow of your weight with a V11. Comparable numbers for the Marzocchi on the V11 are 5 wt oil with 100mm air gap. I know your shock s White Power; what are the forks?

 

You must give us a ride report!

Marzocchi.

[docc]

Seems like the later 1100 Sport i had upside down forks, but yours have the male sliders on top, yes?

[dlaing]

I did not know they made an 1100 Sport with right side up forks!!!

http://daytonasite.free.fr/french/daytona_website.htm

[Josh]

 

 

I think they used the Marz "conventional" forks on all of the early Daytonas and Sports up to 1996. In 1997 they switched to White Power "upside down" forks.