Oil height

[wence]

I am changing the oil in my 07 marzocchi forks and am trying to get it clear in my head what does what.
If the standard oil height is 90-100mm and I put in oil up to 120mm (ie less oil), this will give a larger air gap .
By having a larger air gap the suspension will sitting higher or lower in the stroke?
Will sag be effected?
What will characteristics be compared to standard.
Any help will be appreciated.
Cheers guys,
Mark

 

[Eric K]

The oil height has no significant effect on the sag.

The oil height has the biggest effect in the lower portion of the fork travel. As you raise oil height it reduces the space for the trapped air. Reducing the trapped air space increases the effective air pressure at the bottom of stroke. At top of stroke the air pressure is zero if you bleed you forks.

 

[wence]

Ok, so increasing air gap will make the forks feel harsher or plusher? When should this happen in terms of progression through the stroke.
Sorry if I am asking the same question in a different way.
I am having trouble with my forks in that they are way to soft and I can't seem to get them right. I am trying to decide whether to icrease air gap to 120 mm as a lot of others have on here or whether to leave it as 90-100mm as standard.
Cheers Mark

 

[DWreck]

If you go from 90-100 up to 120 on your oil level your forks will bottom easier. If you think that they are too soft now I would increase your compression. (adjuster on bottom of fork legs) I believe most consider the stock springs ok for a rider up to 180 pounds. If you are a lot heavier than that springs are probably your problem.

 

[wence]

If I go from 90mm from top of fork and then change it to120mm from top of the fork I am increasing air gap.
Just making sure we are both on the same page.
This is what you meant...yes?
Cheers Mark

 

[rpduc]

This gets a little confusing because the larger number actually means less oil, more air.

I'm no expert but as Erik already mentioned the effect of the air gap is mostly felt at the bottom of the stroke but more air/less oil (in a general sense) would be "plusher." If you go to 120mm air gap there will be less bottoming resistance, which usually becomes a problem when forks are "too soft."

I am having trouble with my forks in that they are way to soft and I can't seem to get them right.

Maybe describe what you mean by "way too soft."

Have you checked sag? This will help tell you if your springs are the correct rate for your weight. Any good suspension shop (or some of the folks on here) can tell you what springs you need for your weight. No amount of fiddling with oil height, viscosity and clickers can make up for the wrong springs.

 

[wence]

Thanks rpduc,
I just wanted someone to say that.
I thought that was the case but was unsure.
When I bought the bike it had a leaky seal and I did the old note trick.
I don't know how long it was leaking for though.
I guess by going to a heavier weight oil I will be adding damping to the fork as thicker oil through athe same size hole therefore one should counteract the other and I should be still slightly more rebound and bottoming resistance than standard even at 120 mm ???
Thanks Mark

 

[rpduc]

I guess by going to a heavier weight oil I will be adding damping to the fork as thicker oil through athe same size hole therefore one should counteract the other and I should be still slightly more rebound and bottoming resistance than standard even at 120 mm ???

Yeah, but the deal is if you have the wrong springs in there no matter how thick or how much oil you have in there the fork will be working in the wrong portion of it's stroke. Think of it like this. The springs hold the bike up and the oil and dampers control the speed of rebound and compression. If the springs are too soft it's like giving up several inches of travel at the get-go.

I had my MC suspension resprung and revalved by Powerband Racing here and it is a COMPLETELY different machine. I'm an old MXer (really old) and hadn't been on a dirt bike for many years. Before I had it redone I thought had completely forgotten how to ride. The thing was impossible to keep on line. Now she's sweet. IMO suspension will always be the FIRST investment I make in a bike.

 

[Eric K]

I suggest starting with the simple changes - like the oil level and damping adjustments.

Reducing the air gap (decrease in measured distance to top of oil level) will increase bottoming resistance without making a big difference in the plushness of the initial travel.

Increasing compression damping can help slow down how fast the fork compresses.

If those adjustments are inadequate, then you are looking at spring changes and/or revalving.

 

[wence]

Thanks to both of you guys Rpduc and EricK.
I think I will try as Eric said but if no happiness then I will definitely get the suspension done properly by a suspension shop.
I did get my Rmx suspension done by a suspension shop and was very impressed too, it was a completely different bike to ride.
At least now I have it worked out in my head what does what concerning oil, springs and oil height.
Thanks again guys , I'll let you know how it goes after oil change etc.
Cheers Mark

 

[wence]

Oil change done and this would be the first one on these forks looking at the state of the oil.
I did notice a black 60-70mm long plastic sleeve under the spring when I removed the spring .
Any idea what this is for?
I ended up using 7.5wt oil and filling it to 95mm from the fork top...
I guess we'll see how it goes but it feels a lot stiffer already and actually has some rebound now.
More than likely the oil that was in them was probably stuffed.
Keep you posted ,
Mark

 

[rpduc]

I did notice a black 60-70mm long plastic sleeve under the spring when I removed the spring .

If I understand you correctly that would be the preload spacer.

 

[wence]

No worries , thanks rpduc.
Just couldn't see it in the pics form the pdf.
Cheers Mark

 

[darren]

Did you strip them out completely? if so how did it go? Did you need any special tools?

 

[wence]

Hey Darren,
No I didn't strip them completely, just took the top off undid spring and removed and then removed old oil.
The oil was grey/black in colour.....very old probably what it was first sold with.
Replaced with motul 7.5wt to 95mm from fork top.
Tested in shed and feels a lot firmer on the same setting as before therefore I should be able to now adjust the forks and get them to actually do something.
Was going to strip the forks completely but cannot afford the seals etc right now.
Cheers Mark.

 

[rpduc]

FYI, in the last year I've done forks on 4 seriously neglected dirt bikes and on all of them there was a good amount of sludge caked up in the bottom that will not be extricated by just changing oil. I'd think it would pollute your new oil in no time. You might be able to wash it out of there with kerosene with the forks still assembled, bit if they were my forks I'd want to see that the sludge was outa there.

Ideally you'd replace seals, check bushings etc. as long as you had them apart but the forks will come apart without damaging the seals.

 

[wence]

Yeh , to be honest didn't really think about the sludge but I think I will probably get the forks and shock done professionally once I have some more cash so this was really a case of just trying to see if new oil and different weight made a difference.Time was also a constraint as I have 2 young boys.
Enough excuses though.
I will see how they go and then at least can tell the tuner a bit more maybe.
By the way, the shock oil is too hard to do at home as there is no way to get the gas back in, Haven't really looked at the shoock at all.
Cheers Mark