Fork Rebuild - Wrist Twisters
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post #1 of 33 Old 11-26-2017, 05:46 PM Thread Starter
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Fork Rebuild

OK, just completed the fork rebuild, and the one thing the Honda manual I had neglected to mention was the adjustable fork story. So I kind of winged it, since the fork nut in the pictures didn't reference what to do when the fork was adjustable. That being said, I kind of winged it, reversing the deconstruction since the adjustable sleeve didn't need to be torqued as indicated in the manual. It did not exist in the manual, in fact. Therefore, i only ask that someone explain how to set the adjusters at the top of the stem. I assume cranking the main part of the adjuster affects spring, and the little inset, risen screw adjusts the damping? Insights, or corrections, no matter how brutal, will be appreciated. I believe my oil levels are right, and the forks respond as expected post operation, and I've set both at the same line on the adjuster post as I would expect them to work if I were the designing engineer. It's just the little screw at the top I'm confused about.

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post #2 of 33 Old 11-27-2017, 02:27 AM
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I think you're correct. What I did was started all over, meaning that I backed things off quite a bit, then started adjusting downward. The reason I mention this is that my fluid was filled with water and wasn't working. Starting back at the top, allowed me to find the ideal spot.

I think someone will offer some better info, but what I wanted to mention is setting the forks after you mount them.

This wasn't in the manual and I guess it's supposed to be "common knowledge", but I didn't know about it.


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post #3 of 33 Old 11-27-2017, 01:59 PM
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Quote:
Originally Posted by Devin4242 View Post
OK, just completed the fork rebuild, and the one thing the Honda manual I had neglected to mention was the adjustable fork story. So I kind of winged it, since the fork nut in the pictures didn't reference what to do when the fork was adjustable. That being said, I kind of winged it, reversing the deconstruction since the adjustable sleeve didn't need to be torqued as indicated in the manual. It did not exist in the manual, in fact. Therefore, i only ask that someone explain how to set the adjusters at the top of the stem. I assume cranking the main part of the adjuster affects spring, and the little inset, risen screw adjusts the damping? Insights, or corrections, no matter how brutal, will be appreciated. I believe my oil levels are right, and the forks respond as expected post operation, and I've set both at the same line on the adjuster post as I would expect them to work if I were the designing engineer. It's just the little screw at the top I'm confused about.

Cheers,
Devin
Yes, the "main part" relates to the spring, and the "risen part" relates to damping, low "speed" (velocity) rebound damping to be precise.
Given the poor instruction of the manual and your being forced to "wing it" in order to finish the reassembly, I strongly suggest you check to see how much range of movement you have in the damping adjustment screws.
Slowly turn in (clockwise) and feel for as gentle a bottom out as you can.
Then back out fully (counterclockwise) while counting the turns, until it reaches a gentle stop.
You should have at least 2.5 turns and perhaps 3 or so.
Assuming you have something like that, then move to making the initial adjustment.
Again, slowly turn in (clockwise) and feel for as gentle a bottom out as you can.
Then back it out 1.5 turns.
This will be A OK for you to start with.
Make very slight adjustments based on how the front end behaves.
Typical, is making changes that are firmer than the initial setting.
NEVER make adjustments any coarser than 1/8 turn when going firmer.
(A very picky setup for track day tuning would be in 1/16 turn increments.)
Spring and/or oil changes will affect the position of the ideal setting.
As will rider preference.

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post #4 of 33 Old 11-27-2017, 06:48 PM Thread Starter
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Hints of yes on both counts of the last two replies. On the one hand, starting at zero is a good idea for the spring gauge, to measure weight balance, and then circumspect riding to review damping. mcroomo44, the screws turn easily after the re-build, I will take your advice also, regardless. Before any of that trial can happen is the installation of the new 520 sprocket set, which is waiting in the theater wings. At this point, Renthal Mediums are in place (though I worry about the cable stretch), new sliders, which fit perfectly, fork rebuild, aka this thread, and finally the sprocket and chain set, the next thing on the list. With the weather holding strangely warm in Texas, I might have a test run before the end of December or early January. Ah, the lengths we go to in honor one of the most reliable machines out there....

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post #5 of 33 Old 11-27-2017, 07:27 PM
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Originally Posted by mcromo44 View Post
Yes, the "main part" relates to the spring, and the "risen part" relates to damping, low "speed" (velocity) rebound damping to be precise.
Given the poor instruction of the manual and your being forced to "wing it" in order to finish the reassembly, I strongly suggest you check to see how much range of movement you have in the damping adjustment screws.
Slowly turn in (clockwise) and feel for as gentle a bottom out as you can.
Then back out fully (counterclockwise) while counting the turns, until it reaches a gentle stop.
You should have at least 2.5 turns and perhaps 3 or so.
Assuming you have something like that, then move to making the initial adjustment.
Again, slowly turn in (clockwise) and feel for as gentle a bottom out as you can.
Then back it out 1.5 turns.
This will be A OK for you to start with.
Make very slight adjustments based on how the front end behaves.
Typical, is making changes that are firmer than the initial setting.
NEVER make adjustments any coarser than 1/8 turn when going firmer.
(A very picky setup for track day tuning would be in 1/16 turn increments.)
Spring and/or oil changes will affect the position of the ideal setting.
As will rider preference.
So what would you expect to happen if you dampen too much and too little? In other words, what effect does too little dampening have vs too much?

If you change the outer settings (spring rate) should you re-adjust the inner or are they independent?

The rebound dampening would be how fast the spring is allow to return the forks back to their height?

The outer adjustment make the spring tighter, so it springs back faster and doesn't spring down as far, right?

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post #6 of 33 Old 11-28-2017, 10:49 AM
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Quote:
Originally Posted by KarlJay View Post

1
So what would you expect to happen if you dampen too much and too little? In other words, what effect does too little dampening have vs too much?

2
If you change the outer settings (spring rate) should you re-adjust the inner or are they independent?

3
The rebound dampening would be how fast the spring is allow to return the forks back to their height?

4
The outer adjustment make the spring tighter, so it springs back faster and doesn't spring down as far, right?
1
Too little makes the front end bob.
Too much makes it "pack".
Too much is worse than too little.

2
If spring rate is changed, the Rebound setting should be checked.
Stiffer springs and/or lighter oil will need more turning in of the screw, but the change needed can be very small.
The opposite also holds true, the degree of change needed, included.

3
More or less.
It's a bit more complex than just a simple "how fast" though, as one needs a controlled state of motion that still also provides enough compliance throughout the stroke back to the normal point.
So in terms of concept, you had it nailed, but we need to remember the added detail, especially as an insight towards interpreting ride results from setting changes, and what direction(s) to try when doing further adjustments, be those adjustments towards needed improvement or hoped for betterment.

4
In a word, no.

In terms of 919 forks in the normal range of stroke, as in nowhere near full collapse or full extension in particular, the only thing that happens by screwing that adjuster in or out, is to change the elevation of the front end.
The stored spring energy remains exactly the same, as does the spring rate. (aside from a weeny net theoretical change in the trapped air spring rate by the minor changes in fork extension at the new equilibrium position)
This is why I call them Ride Height Adjusters, although strictly speaking, they are "Internal Spring Preload Adjusters".

On the other hand, sophisticated forks with very long top out springs, actually have their effective their overall spring rate altered by making adjustments to the so called "Preload Adjusters", which are more accurately described as being "top out spring installed height adjusters".

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post #7 of 33 Old 11-28-2017, 12:31 PM
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You failed to mention that even the most linear coil spring will become progressive as it nears the end of the stroke so the rate isn't really constant. Additionally most OEM springs are not that linear to begin with so you get a lot more of the progressive effect than you would think.

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post #8 of 33 Old 11-28-2017, 08:05 PM
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I feel like too many people neglect to change their fork oil. It made such a huge difference on my bike. Was nice to get rid of the grey muck that came out after 25k miles

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post #9 of 33 Old 11-29-2017, 02:21 AM
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I feel like too many people neglect to change their fork oil. It made such a huge difference on my bike. Was nice to get rid of the grey muck that came out after 25k miles
It should be a yearly thing, it's all of what $20? Oil, cleaner, 1/2 day...

Too bad they didn't make the rear shock just as easy to rework.

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post #10 of 33 Old 11-29-2017, 08:21 AM
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You failed to mention that even the most linear coil spring will become progressive as it nears the end of the stroke so the rate isn't really constant. Additionally most OEM springs are not that linear to begin with so you get a lot more of the progressive effect than you would think.
I've been wondering about that, too. It basically means that increasing pre-load actually puts the spring further towards its higher-rate progression, thus making it overall stiffer, correct?

Which somewhat contradicts the theoretical view that pre-load doesn't affect the stiffness of the spring/ride because it merely changes the starting point, which, if the spring rate would actually be constant would not affect how soft or stiff the ride is. But real-world experience is actually different and I think the reason is exactly what LDH stated.

On a different note, I've got a new pair of Sonic .90 springs (cost was around 100 bucks including spacer material and shipping) and two bottles of Honda SS-8 w10 fork oil so'll do the thing as soon as it starts raining here.

I read all threads here regarding the topic but honestly I'm still slightly confused about the rebound adjuster and there seems to be a 'needle' somewhere (?). I guess I'll dig in and see what I find..

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post #11 of 33 Old 11-29-2017, 01:52 PM
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Quote:
Originally Posted by LDH View Post
You failed to mention that even the most linear coil spring will become progressive as it nears the end of the stroke so the rate isn't really constant. Additionally most OEM springs are not that linear to begin with so you get a lot more of the progressive effect than you would think.
You are right, I didn't even think of it, plus you took it even further.
I assumed standard linear springs, seeing as that's what I have been using for so long I forgot about the OEM springs.
The OEM springs by my eye were dual rate with a transition winding zone in between.
It sure would be interesting to explore for coil bind towards and at the end of bottoming stroke.
(oh to have access to a fork dyno!)

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post #12 of 33 Old 11-29-2017, 01:56 PM
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You would have to screw up the preload spacer lengths really bad to get coil bind on the normal 120mm stroke of the forks and the air spring would go super progressive long before that point as well with normal oil heights.

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post #13 of 33 Old 11-29-2017, 02:02 PM
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Originally Posted by druegeme View Post
I've been wondering about that, too. It basically means that increasing pre-load actually puts the spring further towards its higher-rate progression, thus making it overall stiffer, correct?

Which somewhat contradicts the theoretical view that pre-load doesn't affect the stiffness of the spring/ride because it merely changes the starting point, which, if the spring rate would actually be constant would not affect how soft or stiff the ride is. But real-world experience is actually different and I think the reason is exactly what LDH stated.

On a different note, I've got a new pair of Sonic .90 springs (cost was around 100 bucks including spacer material and shipping) and two bottles of Honda SS-8 w10 fork oil so'll do the thing as soon as it starts raining here.

I read all threads here regarding the topic but honestly I'm still slightly confused about the rebound adjuster and there seems to be a 'needle' somewhere (?). I guess I'll dig in and see what I find..
IF you have linear wound springs in 919 forks, and only make a ride height change within the range of the adjusters, in the centre stroke zone of the fork there will not be any spring coil bind, therefore the spring rate and stored energy will both be constants.

Progressively wound springs entirely changes the equation.
The short story being that if changes result in any coil bind, then the effective spring rate will go up, and the more coil bind there is, the greater the spring rate becomes.
Stock 919 springs are anything but linear.

Don't be fooled by no or small height changes on 919s when changing the adjuster positions.
So much static stiction in the 919 forks, being the primary reason.
(Keeping in mind that static friction always significantly exceeds dynamic friction)

The "needle" you referred to is at the end of a long shaft that ends with the slot you see top side.
The needle is inside the cartridge, and registers into a tapered seat.
(The rebound needle taper on F4i cartridges is different than 919s, and needs to be closed further in than a 919 needle to get the same low speed rebound damping effect. Likely the result of the F4i forks having been designed around much softer spring rates than the 919 has, let alone what many get retrofitted with.)

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post #14 of 33 Old 11-29-2017, 03:02 PM
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Quote:
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You would have to screw up the preload spacer lengths really bad to get coil bind on the normal 120mm stroke of the forks and the air spring would go super progressive long before that point as well with normal oil heights.
Piggybacking on the above.
OEM 919 forks have low oil levels, thus increasing the air springing effect.
High oil levels, such as 125 mm, reduces the air springing effect and makes the overall springing more reliant upon the mechanical spring effect.
Further, the air springing effect is not linear. (trusting I remember my Gas Laws correctly.....)

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post #15 of 33 Old 11-29-2017, 04:45 PM
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Quote:
Originally Posted by mcromo44 View Post
Piggybacking on the above.
OEM 919 forks have low oil levels, thus increasing the air springing effect.
High oil levels, such as 125 mm, reduces the air springing effect and makes the overall springing more reliant upon the mechanical spring effect.
Further, the air springing effect is not linear. (trusting I remember my Gas Laws correctly.....)
I'm hoping I am misunderstanding you as semantics, but I am not following you at all outside of the non-linear effect of the air-spring.

Now I have admittedly been drinking & Bombay Sapphire Martini's are not exactly brain friendly, but the lower the oil level the less effect the Air Spring has due to the larger air gap and the additional room to compress the gas.

The higher the oil level the less air there is and thus less room for compressing the gas hence the very progressive curve as it takes effect during the compression of the forks.


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post #16 of 33 Old 11-29-2017, 05:25 PM
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Originally Posted by LDH View Post
I'm hoping I am misunderstanding you as semantics, but I am not following you at all outside of the non-linear effect of the air-spring.

Now I have admittedly been drinking & Bombay Sapphire Martini's are not exactly brain friendly, but the lower the oil level the less effect the Air Spring has due to the larger air gap and the additional room to compress the gas.

The higher the oil level the less air there is and thus less room for compressing the gas hence the very progressive curve as it takes effect during the compression of the forks.

We are on the same page.
When I said low oil level, I meant low in the tube, not the numerical measurement down from the top when setting the level.
Your chart is exactly what I would expect to see.

By the way, I can't show your post to my dear sweet wife of many a year.
She is continually tossing the "semantics" barb at me, she'd be in stitches instantly if she saw what you said.

Have another Bombay', be it the shaken or stirred type.

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post #17 of 33 Old 11-29-2017, 05:28 PM
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Good, I'm going to have another drink to celebrate
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post #18 of 33 Old 11-29-2017, 05:37 PM
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Good, I'm going to have another drink to celebrate
Not until we warn everyone not to set their oil level at full fork extension as the curve might erroneously intimate to some, and instead at full collapse.

Then I'll hae a nice single malt scotch at my end.

Deal?

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post #19 of 33 Old 11-29-2017, 05:45 PM
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Nikka Coffey Malt Whisky sir.

Find it, drink it, live it.

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post #20 of 33 Old 11-29-2017, 06:06 PM
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Regarding oil level.. My understanding is that lower oil level means more air means softer damping- not spring rate.

Counter-intuitively the oil level measurement per manual actually measures the distance of oil level to top of fork, means you actually measure air level. The higher the value the softer the damping. Semantics I know. But what oil level would you guys suggest? 140?

Having beefeater and tonic here

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post #21 of 33 Old 11-29-2017, 06:37 PM
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Quote:
Originally Posted by druegeme View Post
Regarding oil level.. My understanding is that lower oil level means more air means softer damping- not spring rate.

Counter-intuitively the oil level measurement per manual actually measures the distance of oil level to top of fork, means you actually measure air level. The higher the value the softer the damping. Semantics I know. But what oil level would you guys suggest? 140?

Having beefeater and tonic here
The air acts as a spring, with the reactive force being in the same directions as those of the main spring.
In other words, the spring forces summate. (unlike a top out spring with is a counter spring)
Oil level affects the air spring rate curve.
The air does not dampen, only the oil does, as it flows through the various circuits within the cartridge.

As for the measured level, 125 to 140 is the suggested range.
My understanding is that forks set up with reduced internal preload allow for a more shaped spring rate curve towards full bump, when a high oil level such as 125 is utilized.
For sure, it is a bonafide chassis tuning element.

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post #22 of 33 Old 11-29-2017, 06:49 PM
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Nikka Coffey Malt Whisky sir.

Find it, drink it, live it.
I know zero about Japanese malts, but the reviews on the above are good.
They have it next door in British Columbia.
Maybe I can find it here in town, I'd think so, as Albertans are heavy on Scotch - no doubt in part to by far having the lowest prices in Canada for it.

I'm an Islay guy through and through, with Lagavulin 16 being my favourite dram.
My collection is heavily weighted with Jura, Islay, Mull and Orkney, mostly Islay.
Love that peat!
Have a fair representation of the mainland makings.
No doubt the Nikka would be in line with a mainland offering.

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post #23 of 33 Old 11-29-2017, 06:51 PM
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Quote:
Originally Posted by mcromo44 View Post
The air acts as a spring, with the reactive force being in the same directions as those of the main spring.
In other words, the spring forces summate. (unlike a top out spring with is a counter spring)
Oil level affects the air spring rate curve.
The air does not dampen, only the oil does, as it flows through the various circuits within the cartridge.

As for the measured level, 125 to 140 is the suggested range.
My understanding is that forks set up with reduced internal preload allow for a more shaped spring rate curve towards full bump, when a high oil level such as 125 is utilized.
For sure, it is a bonafide chassis tuning element.
ok that all makes sense but here's another question, if you don't mind.

Along the lines of your side note of NOT filling the oil into extended shocks, oil by itself can hardly be compressed. Thus the only available travel is that of the air, which has as we saw a pronounced progressive compression rate. So, if one sets the oil level at say 140mm -or to be more accurate the total air gap at 140mm- then that would be the total available travel, actually less than that because the air can't be compressed to 0.

By the same token if someone would decrease the 'air level' to 125 then that would reduce the total fork travel to the same value.

Am I getting this right?

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post #24 of 33 Old 11-29-2017, 07:19 PM
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Originally Posted by druegeme View Post
I get your logic but the upper end of the spring touches the spacer which touches the preload adjuster. It's not that the air 'cushion' is inbetween the spring and the upper fork cap/preload adjuster. Therefore I think that the air does not affect the spring rate but the damping. I could be wrong of course..
Imagine no spring fitted.
Imagine the rebound adjuster fully backed out so there is the least effect.
Imagine extending the fork fully.
Then putting the cap on, which will then seal in the air.
Then imagine trying to push down on the fork tube.
Imagine the resistance increasing, as the trapped air is forced into an ever decreasing volume thus increasing its resistive pressure.

Such an imaginative exercise also explains why the chart that LDH posted looks the way it does, including the units of measure used.
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post #25 of 33 Old 11-29-2017, 07:26 PM
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Originally Posted by mcromo44 View Post
Imagine no spring fitted.
Imagine the rebound adjuster fully backed out so there is the least effect.
Imagine extending the fork fully.
Then putting the cap on, which will then seal in the air.
Then imagine trying to push down on the fork tube.
Imagine the resistance increasing, as the trapped air is forced into an ever decreasing volume thus increasing its resistive pressure.

Such an imaginative exercise also explains why the chart that LDH posted looks the way it does, including the units of measure used.
yep thanks I think I get it (sorry I edited my post after thinking about it some more!).

So the total available travel is the length of the air 'gap', or actually a little less because air has a very progressive curve.

Per 919 service manual: front axle travel: 109mm, recommended oil level: 155mm.

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post #26 of 33 Old 11-29-2017, 07:33 PM
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Originally Posted by druegeme View Post
ok that all makes sense but here's another question, if you don't mind.

Along the lines of your side note of NOT filling the oil into extended shocks, oil by itself can hardly be compressed. Thus the only available travel is that of the air, which has as we saw a pronounced progressive compression rate. So, if one sets the oil level at say 140mm -or to be more accurate the total air gap at 140mm- then that would be the total available travel, actually less than that because the air can't be compressed to 0.

By the same token if someone would decrease the 'air level' to 125 then that would reduce the total fork travel to the same value.

Am I getting this right?
You're on the way, and likely some semantics, context and jargon are unfairly conspiring against you.
Oil level does not dictate total fork stroke length.
The only way it could, is if there was no air gap.
But there is always an air gap, more correctly an air column, at the top of the fork tube.
When you set your oil level, you do it with the fork fully collapsed, all the way through the hydraulic snubber to the hard mechanical bottom out limit.
The oil is down from the top, thus creating the selected trapped air column height at full bump, by whatever level one selects.
Even after putting the springs back in, there will still be an air column at the top of the fork tube when the fork is fully compressed.
That air column acts as a air spring.
The change in trapped air volume between fully extended and fully compressed, is what dictates the trapped air pressure curve, ergo, the air spring rate - as changes constantly through the range of travel according to The Gas Laws.
Just like an air bag of old as fitted inside coil springs of old station wagons, or an air bladder fitted shock absorber instead of air bags inside the main coil springs of the car.
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post #27 of 33 Old 11-29-2017, 07:44 PM
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yep thanks I think I get it (sorry I edited my post after thinking about it some more!).

So the total available travel is the length of the air 'gap', or actually a little less because air has a very progressive curve.


Per 919 service manual: front axle travel: 109mm, recommended oil level: 155mm.

Yes, the total mechanical travel is 109 mm.
Because of the residual air column at full bump, there is no hydraulic limit to bump stroke so fork can reach it's mechanical limit of 109 mm of travel from it's fully extended length.

Yes, the air has a progressive spring rate curve, fairly flattish for the first part, then kicking up at the end, more so with an oil level of 125 as compared to 155, all as LDH's posted curve depicts.
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post #28 of 33 Old 11-29-2017, 08:43 PM
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Originally Posted by mcromo44 View Post
You're on the way, and likely some semantics, context and jargon are unfairly conspiring against you.
Oil level does not dictate total fork stroke length.
The only way it could, is if there was no air gap.
But there is always an air gap, more correctly an air column, at the top of the fork tube.
When you set your oil level, you do it with the fork fully collapsed, all the way through the hydraulic snubber to the hard mechanical bottom out limit.
The oil is down from the top, thus creating the selected trapped air column height at full bump, by whatever level one selects.
Even after putting the springs back in, there will still be an air column at the top of the fork tube when the fork is fully compressed.
That air column acts as a air spring.
The change in trapped air volume between fully extended and fully compressed, is what dictates the trapped air pressure curve, ergo, the air spring rate - as changes constantly through the range of travel according to The Gas Laws.
Just like an air bag of old as fitted inside coil springs of old station wagons, or an air bladder fitted shock absorber instead of air bags inside the main coil springs of the car.
Thanks. You are correct I was neglecting the fact that measuring the air column takes place at fully collapsed fork. Which means that by the time you are tightening the upper fork bolt and sealing the air column, the fork has extended (I assume to its max) and the total length of the air column is the initial measurement -say 155mm per manual- plus the total travel which is spec'd at 109mm.

As mentioned I've got .90 springs and 10w Honda fork oil ready and I'm thinking 140mm will be about right for my ~220 pounds curb weight.

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post #29 of 33 Old 11-29-2017, 10:11 PM
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Quote:
Originally Posted by druegeme View Post
Thanks. You are correct I was neglecting the fact that measuring the air column takes place at fully collapsed fork. Which means that by the time you are tightening the upper fork bolt and sealing the air column, the fork has extended (I assume to its max) and the total length of the air column is the initial measurement -say 155mm per manual- plus the total travel which is spec'd at 109mm.

As mentioned I've got .90 springs and 10w Honda fork oil ready and I'm thinking 140mm will be about right for my ~220 pounds curb weight.
Suggested is no lower than 140 down from the top when measuring.

Suggested is 15 mm of installed preload, with that determined on the basis of the adjusters being fully backed out so all of the rings show.
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post #30 of 33 Old 11-30-2017, 07:41 AM
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Quote:
Originally Posted by mcromo44 View Post
Not until we warn everyone not to set their oil level at full fork extension as the curve might erroneously intimate to some, and instead at full collapse.

Then I'll hae a nice single malt scotch at my end.

Deal?
Quoting this to save everyone from being dumb like me and spending half a year with rock hard front forks because I skimmed and didn't RTFM
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post #31 of 33 Old 11-30-2017, 08:23 AM
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Quote:
Originally Posted by mcromo44 View Post
Oil level does not dictate total fork stroke length.
The only way it could, is if there was no air gap.
But there is always an air gap,
You obviously underestimate the average rider that wants to work on his own suspension!

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post #32 of 33 Old 11-30-2017, 09:51 AM
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Originally Posted by LDH View Post
You obviously underestimate the average rider that wants to work on his own suspension!

That is indeed a picture of epic proportion, that is far better than any million of words could ever be!

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post #33 of 33 Old 11-30-2017, 11:18 AM
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Quote:
Originally Posted by LDH View Post

NOTES as added by McTavish McRomo
Remember that Oil Level is set at full fork collapse, not at full extension. Meanwhile, the curve below plots from fully collapsed to fully extended. Assumed is that the curve is predicated upon the fork atmospheric pressure being 0 Gauge at full extension before stroking to full collapse.

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