[tmcgee]

What effect can I expect by raising the oil level? Lowering the oil level?

 

[greywolf]

Air in the forks gets compressed by an upstroke. More oil and less air will effectively increase the spring rate, especially when deep in the stroke. Too much oil and the fork can't achieve a full stroke.

I had a Can-Am 175 TNT with Schrader valves added to the fork caps. I ran it with compressed air and no springs for quite a while. At other times, it ran with springs and vented caps. It really needed new, longer travel forks to work properly. There was a reason people switched from Betor to Marzocchi.

 

[jokermtb]

GW is right - increasing oil volume only affects the last bit of a fork's travel. It's a fine line you run by increasing oil volume in the fork - you do run the risk of reducing the total travel of your fork because, the fork will pretty much be unable to compress, due to the excessive air pressure, in the last inch or so of it's total travel [not great for oil seals either, when the pressures are sky high!]. Conversely, too little oil in the fork, you can bottom out harder than normal, since there's a reduced progressive build up of air in the fork as it compresses. That volume of air in the fork is pretty handy! There's a reason why fork oil height/volume is what it is - but you can go 5mm [or more] above the normal fill height without creating too much of an air spring 'bottom out lock' - experiment a little. You may need a little bottoming-out protection, courtesy of a bit more oil in the fork.

 

[richw]

Kibbitz

different springs use different amounts of oil

Oil volume hard to use as draining 100% very difficult

Spring supplier SHOULD have a recommended height and the should be rechecked after a short amount as trapped air may have distorted original measurement

 

[tmcgee]

Air in the forks gets compressed by an upstroke. More oil and less air will effectively increase the spring rate, especially when deep in the stroke.

That's what I'm hoping for. The forks WILL bottom under hard braking with the springs I want to run and the suspension is too harsh using any of the recommended spring rates from the online calculators. It's easy enough to increment oil level increases and test the results with the cable tie around the fork method.

I figure an increase in oil level should be enough to cushion that last bit of compression. And yeah, I realize there's a risk to the fork seals, but I already have the next set on hand.

FWIW, spring rates I've tried: OEM, .9, .95, 1.0, and 1.1 springs in this bike.

 

[tmcgee]

Spring supplier SHOULD have a recommended height and the should be rechecked after a short amount as trapped air may have distorted original measurement

BTDT. It's a question of fully understanding things when reading the directions didn't work. :mrgreen:

 

[PTRider]

Tom, are you correctly setting the sag, the combination of spacer length and preload adjuster, for your weight and each spring rate you run?
Suspension Adjustment

 

[V-Tom]

...FWIW, spring rates I've tried: OEM, .9, .95, 1.0, and 1.1 springs in this bike.

What spring rate are you going to use now?

I have only tried stock and .95. I have a set of .85 ready to install and hope to do that in the next week.

..Tom

 

[tmcgee]

Tom, are you correctly setting the sag, the combination of spacer length and preload adjuster, for your weight and each spring rate you run?
Suspension Adjustment

I figure spring rate comes first, then sag. I'm only using about half the available fork travel with the 1.0 springs in there now. This is clearly wrong, although it feels very good on smooth roads.

The .9 springs felt very good on the bumpy stuff when they were in there, but the bottoming on panic braking made me take them out.

I've given up on doing things by the official instructions because I'm not getting the desired results. There are also RaceTech emulators in there and I've been through three different oil weights (all the same brand).

 

[RichDesmond]

The trapped air volume acts as an additional spring. A spring with really weird characteristics. Essentially it's exponentially progressive. If you plot it's rate as a function of fork compression it will look somewhat like the right-hand side of a parabola.* Assuming you assembled the forks at atmospheric pressure, the air spring rate starts off at zero and then climbs as you compress the fork. Slowly at first, and then more steeply as each unit of fork compression results in a greater decrease, percentage-wise, in the remaining air volume. Raising the oil level puts you into that rapidly rising region more quickly, and makes the final rate at full compression higher.
At "normal" fork oil levels the air spring component adds about 10-20% to the total spring force in the fork at full compression.
Like anything else to do with motorcycle suspension, the trick is to find the right balance among competing and often contradictory priorities.

What setup are you running now?


*For you math/physics geeks out there, yes, I know a parabola isn't really the right model. But it's a decent approximation in the region under consideration, and it's easy to visualize. So chill...:mrgreen:

 

[Scott E. Bonds]

I am truely impressed with all the fuxxoring around with suspension you guys endure on a street bike. Apparently my tastes are not as sophisticated as I run mine stock. I am thrilled with the suspension. I have reworked several dirt bikes...but suspension is more critical in the woods.

Now, I also ride a Road King so suspension quality of the strom really impresses me when compared to my Milwaukee mini-bike.

 

[tmcgee]

Like anything else to do with motorcycle suspension, the trick is to find the right balance among competing and often contradictory priorities.

Very true, very true. To the horror of some, I prefer to do some things by trial and error.

And I may by wrong about this, but after trying so many springs, the spring rate seems to be the biggest variable. The ONLY thing I didn't like about the .9 spring was the bottoming on a panic stop. To me, based on what you and Pat have said, it sounds like an increase in oil level may be able to increase the effective spring rate for that last bit of suspension travel, prevent bottoming, and thus fix the part I don't like about the .9 spring.

Of course, I may also blow fork oil all over the bike too. It wouldn't be the first time a seal has let go on me.

As to the air spring having weird characteristics, I'll take weird over bottoming for that last little bit of suspension travel if it gives me a better ride on a bumpy road by using a lighter spring rate than I have in there now.

More later after I swap the springs and change the oil and the oil level.

 

[V-Tom]

I am truely impressed with all the fuxxoring around with suspension you guys endure on a street bike. Apparently my tastes are not as sophisticated as I run mine stock. I am thrilled with the suspension. I have reworked several dirt bikes...but suspension is more critical in the woods.

Now, I also ride a Road King so suspension quality of the strom really impresses me when compared to my Milwaukee mini-bike.

I rode my Strom's suspension stock for the first (roughly) 80,000 miles. Then I upgraded both the rear and the front. While there are things I don't like about my new setup, there is enough of an improvement in many areas to make the effort worthwhile for me. There are some here that say you *HAVE* to upgrade the suspension, and I would say no it isn't mandatory, but there are benefits of doing so.

..Tom

 

[RichDesmond]

Very true, very true. To the horror of some, I prefer to do some things by trial and error.

Nothing wrong with that, especially if it's backed up with a little understanding of the physics of the system.

And I may by wrong about this, but after trying so many springs, the spring rate seems to be the biggest variable.

That doesn't correlate with my experience. Especially when it comes to sensations of harshness or stiffness, that seems to be driven more by high speed compression damping or a spring so soft that it bottoms all the time. There are bikes like the Ninja 650 that have insanely stiff springs (in that case a progressive one that goes from 1.2 to 1.6kg/mm) and if you go on that forum you don't hear much about a stiff ride

The ONLY thing I didn't like about the .9 spring was the bottoming on a panic stop. To me, based on what you and Pat have said, it sounds like an increase in oil level may be able to increase the effective spring rate for that last bit of suspension travel, prevent bottoming, and thus fix the part I don't like about the .9 spring.

Of course, I may also blow fork oil all over the bike too. It wouldn't be the first time a seal has let go on me.

I think you'll be fine on that score as long as you don't go above 100mm.

Keep us posted.

 

[tmcgee]

Thanks, Rich. I realize that when you make some adjustments, other parameters are affected and whatever arbitrary reference points I'm using get shifted.

The problem most of us tinkerers have is that we are not suspension engineers and there are many variables we have to figure out. There's 'fun for the curious' in this less than scientific approach, but then again, the scientific approach requires that you test all your theories. You, and others, have provided a wealth of information on suspension and it does take time for the uninitiated to even understand the language.

I've been an electronic tech for 40+ years and enjoy getting things working right. I've never been on a bike that was properly set up for me, so it's a discovery and so far, I do not have that "optimized for my style of riding" reference.

I'm working on it.

 

[farmerlou]

Maybe a heavier weight fork oil would help you?

 

[tmcgee]

Maybe a heavier weight fork oil would help you?

Yes, it's part of what I plan to try if I don't get where I want to go with other planned changes.

 

[PTRider]

Now, I also ride a Road King so suspension quality of the strom really impresses me when compared to my Milwaukee mini-bike.

Ricor makes some suspension products that will tame the ride of your Milwaukee mini-bike. Many of use use the Ricor Intiminator damping valves in our 650s and like the action a lot. I haven't heard results of the shocks.
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