Fork Oil Change

[txrider]

Thanks, Gio.  Since I'm in the middle of cutting spacers right now, this is helpful for comparison purposes as I dial things in. 

 

Using your figures with the 120 mm spacer, your laden sag at 23 mm translates to 18.4%.  Your unladen sag at 10 mm translates to 8%. 

 

My target sag figures with my new springs are 33% (38 mm) laden, and 18% (21 mm) unladen.  This is a "target" that if I can get close, I figure will yield more ideal fork performance, only achievable with considerably stiffer springs than stock for my weight (175 lb. without leathers).  Of course as always, YMMV.

 

My "First slice" at shortening spacers wasn't short enough.  Off to the workshop now for another round of PVC mayhem.

 

Before I make my next pile of plastic sawdust, any comments?  Gio?  Brian?  Dave?  Anybody?

64049[/snapback]

 

Ratchet, I've been following all threads concerning setting static suspension sag with considerable interest and most describe bike unladen (rider off)sag numbers as well as bike laden (rider on)numbers.

As a matter of interest why is unladen sag even mentioned? My impression is that total suspension travel and sag measured with rider/load aboard were the only numbers critical in setting sag so a % of total travel could be calculated like your 33% mentioned.

[Guest ratchethack]

As a matter of interest why is unladen sag even mentioned?  My impression is that total suspension travel and sag measured with rider/load aboard were the only numbers critical in setting sag so a % of total travel could be calculated like your 33% mentioned.

Great question, txrider, let's see if I can do something like a halfway decent job answering this. It's actually the relationship between laden and unladen sag that yields the best fork action for a given rider's weight (total load). It's the same for the rear shock. I can understand it in terms of the physics involved, but expressing it is another thing altogether.

 

OK, think of it this way. Extreme examples always seem to help. I got myself through Newtonian Physics with A's and B's by recalling visual images of the Roadrunner and Coyote...where lots of springs, ACME rockets, and humongous Archimedian levers were all standard stuff. No drugs involved - honest.

 

For argument's sake, if you had a 300 lb rider on a all-stock Guzzi, you could put enough preload on the suspension to get the laden sag figures at 30%. No problem. However, the unladen sag would be out of range on the low side. This means that the rider would be riding around near the limit of the spring's compressibility on a prohibitively limited fraction of available suspension travel, on a very rough ride that would tend to overwhelm the stock compression and rebound damping.

 

On the other hand, if you had a 100 lb rider on a stock Guzzi, you could back off the preload to get the laden sag figures at 30%, yet the unladen sag would be out of range on the high side. This would also yield a rough ride, also on a prohibitively limited fraction of available suspension travel, but this time the rider would be riding around on a fraction of available suspension travel at the start of the spring's compressibility, and low settings on compression and rebound damping adjustments would be relatively harsh.

 

As Brian pointed out earlier in this thread, the general rule-of-thumb (target) is: Laden sag = 30-35%, and unladen sag = 15-20%. To state it as simply as I know how to state it, a proper match of laden sag to unladen sag is achieved only by the quality of the match between spring rate and load.

 

In other words, if you can't adjust preload to get within the rule-of-thumb ranges on both laden and unladen sag, your spring rate is incorrect for the load. In each of the above cases, the suspension is not properly set up due to an improper match of spring rate to load, and a harsh ride is the result.

 

I figure if I botch this badly enough maybe it'll conjure up somebody who knows how to state it better and/or set me straight.

[dlaing]

Knowing free(unladen) and static(laden) sag helps to determine if you have the correct spring rate.

Static sag is usually set to 25-33 percent of travel.

If static sag is 30% and free sag is close to zero, your spring is too soft. EDIT (as it would be for John's example of a 300# rider)

If static sag is 30% and free sag is close to 30%, your spring is too hard. EDIT (as it would be for John's example of a 100# rider)

I imagine one could develop a chart to determine ideal numbers, but it would vary by ratio of bike to rider weight.

But because we all have roughly the same weight bike, we can get a pretty good idea of the correct spring rate, from the numbers.

I think about 10% free sag and 30% static sag is about right, YMMV EDIT, I changed my mind and now think more than 10% free sag is necessary to prevent bottoming.

Probably a better way to determine if the spring is correct, is to set the static to 30%, go for a ride on your typical road with zip ties on your forks and shock and see if they don't bottom and that they are using up atleast 75% of available stroke.

If you are not using 75% of stroke, your spring is probably too hard.

If you are bottoming, your spring is too soft.

Now keep in mind, I am making this stuff up from common sense, so others may disagree and be right.

My guestimates of the 30% 10%(edit, not sure about that 10%, maybe fine for rear, but not front.) stuff that is digested from experts....who are all over the place on what "correct" numbers are.

I guess you have to try it, and I don't have much experience there, yet.

EDIT Note that on a thousand pound bike, 29% free and 30% static might be acceptible, and on a professional 125cc dirt bike, with a heavy rider, 1% free sag and 30% static sag might be acceptible. For our weight bike somewhere around 5-25% free sag and 25-33% static sag is probably fine. Perhaps with tighter numbers in the rear. Is that vague enough...

[Gio]

Thanks, Gio.  Since I'm in the middle of cutting spacers right now, this is helpful for comparison purposes as I dial things in. 

 

Before I make my next pile of plastic sawdust, any comments?  Gio?  Brian?  Dave?  Anybody?

 

Hi again,

 

I also have additional measurements where I sat on the bike with gear and bounce up and down a few times in an attempt to simulate a "dynamic" situation (NB this is not dynamic sag as measured with a zip tie after a test run - can't find these notes) :

 

Stock (111mm) spacers measured at 124-75 = 49mm (~40%)

Longer (120mm) spacers measured at 125-100 = 25mm (~20%)

 

Again this is for stock springs, 10w oil and ~180lb plus gear.

 

Gio

[dlaing]

My target sag figures with my new springs are 33% (38 mm) laden, and 18% (21 mm) unladen.

Before I make my next pile of plastic sawdust, any comments?  Gio?  Brian?  Dave?  Anybody?

64049[/snapback]

The progressive springs make it tougher to predict the results.

Generally, I suspect that a 33% laden sag number is a good target for progressive spring, and that smaller sag numbers should be used for straight rate springs.

I really don't have too much of an idea on the unladen number, I would have said 18% is too much, but I just looked at my notes and my front sag is set 33%static and 21%free, and I don't think the spring is too stiff.

So, I now revise the 30%/10% target, and confess that I really do not know what is ideal, but for the front I suspect 30%/20% is about right, and for a progressive spring, more static sag...like 33%

I'd have to go back and look at my notes for the rear....

[Guest ratchethack]

Dave, you're right about progressive springs being a little harder to predict. I just cut the spacers down again and my laden sag is now 33% (38 mm). The unladen sag now reads 21% (26 mm). I'm not at all displeased with this (what're the chances it'd match your figures exactly with about a 50 lb. weight diffential between us and entirely different kinds of fork springs!?!), though I missed my target unladen sag by a tad. Will give 'er a seat-o'-the-pants "chassis dyno" on the rides tomorrow from EuroMotoFest-West. I expect Couser Canyon, maybe Mt. Palomar? See you then, my friend!

[BrianG]

Dave, you're right about progressive springs being a little harder to predict.  I just cut the spacers down again and my laden sag is now 33% (38 mm).  The unladen sag now reads 21% (26 mm).  I'm not at all displeased with this (what're the chances it'd match your figures exactly with about a 50 lb. weight diffential between us and entirely different kinds of fork springs!?!), though I missed my target unladen sag by a tad.  Will give 'er a seat-o'-the-pants "chassis dyno" on the rides tomorrow from EuroMotoFest-West.  I expect Couser Canyon, maybe Mt. Palomar?  See you then, my friend!

64088[/snapback]

 

 

I'd ride it like that... Sounds pretty darned close to ideal. A little MORE sag means you'll have a cushier rided........ less sag = harsher. The Guzzi is a heavy machine so I would maybe aim for a bit less initial and loaded sag with a fixed-rate spring. You're so darned close the ideal street numbers that I'd go give her a go!

 

Cangrats on hitting the nail!

[BrianG]

As a matter of interest why is unladen sag even mentioned? 

64055[/snapback]

 

This question arises very often, and it's because it people don't generally think about the bi-directionality of suspension action. Springs especially, are mostly thought of in terms of their compressibility.

 

If you think about the other end of the suspension action... (the extension part) and the bike encountering a pothole, as opposed to a "bump" you will see that other aspect. If free sag were "0", when the wheel encountered the void of a pot hole, the spring would fire the forks to the extension-stops, and top out hard. That would be as undesirable as bottoming the forks over a bump.

[dlaing]

snip

That would be as undesirable as bottoming the forks over a bump.

64291[/snapback]

Excellent point.

I think we will see a trend in future bikes for longer travel, not just to give more travel between bottoming and the point of loaded sag, but also to allow for more sag and extension.

The first signs of it are the SuperMoto bikes and the Buell Ulysses.

I believe the trick to setting up the long travel suspension bikes is to give them more than 30% sag.

As an example:

The Ulysses is basically the less short wheel based Lightning, but with longer travel suspension.

6.51 in vs. 4.72 in for the front forks

To set both Buells up properly, the Lightning would probably have 25-30% or lets say 1.22" sag and the Ulysses ~40% or lets say 3.01" sag. This would give the both bikes the same ride height, but more importantly, by having lots of sag, it could comply with potholes very nicely.

And sure, if the center of gravity is not too bad, give the Ulysses a little less sag, so it can have more effective protection from bottoming, but the point is don't go overboard, and don't use a 25% sag number for a long travel suspension that is going to be ridden on crotch rocket ridden canyon roads.

[BrianG]

I think we will see a trend in future bikes for longer travel, not just to give more travel between bottoming and the point of loaded sag, but also to allow for more sag and extension.

The first signs of it are the SuperMoto bikes and the Buell Ulysses.

I believe the trick to setting up the long travel suspension bikes is to give them more than 30% sag.

64301[/snapback]

 

I would suggest that there's a "law of diminishing returns" with regard to fork travel and sport bikes. Certainly, suspension travel is desirable, but the opposing issue is ride height. On the one hand, you don't want the high center of gravity as seen on modern dirt bikes with their long-travel suspension systems. The higher the center of gravity, the less "flickable" the machine feels. As well, you have to be cautious of the amount of squat travel available because ground clearance in cornering is of concern.

 

An adventure tourer like the Ulysses is, to my mind, a compromise that sacrifices maneuverability for rough terrain ground clearance.

 

Of course all facets of suspension are inter-related and advances in damping control technology will affect the need for, and allowable maximum of, suspension travel.

 

Suspension design is an interesting study, but I'd suggest it's only advanced to a 50/50 ratio of science vs black-art in it's present state!

[Guest ratchethack]

Of course all facets of suspension are inter-related...Suspension design is an interesting study, but I'd suggest it's only advanced to a 50/50 ratio of science vs black-art in it's present state!

Amen, Brian! Cartridge forks being relatively new to me, I've found that "mastery" of something as apparently simple at first blush as springs with damping control is a surprisingly complex and elusive thing. Thankfully, at this point, at last I seem to have a chunk of the "mystery" on the run! An incremental improvement approach has paid big dividends in terms of cost-to-value. Without any question, the Marz fork (and therefore the handling of the entire machine) is a whole bunch better today than the relatively sad "un-meddled-with" state in which it arrived from Marz/Guzzi, (despite exhaustive and continuing setup efforts from day 1!) but I also understand that I've got a ways to go - in fact, I may never "get there". Y'see, as I think you've hinted at, Brian, I'm starting to figure out that "achieving optimum" here is a bit of a fallacy.

 

Good thing I enjoy fiddling with it. Making each change individually and observing the results has increased my understanding along with the handling. Yesterday's rides at EuroMotoFest West were the best handling rides I've had since I've owned my Guzzi. Wow, what an improvement getting the stiffer springs dialed in has been - it ain't no small thing. Next, instead of raising the fork tubes in pursuit of a little quicker handling, I'm gonna experiment with lowering the oil volume in the forks by another 10 mm to increase the air gap. With the progressive Wilbers, I've already discovered that I can increase usable fork travel this way (another 5 mm last adjustment!) without getting too close to bottoming-out.

[BrianG]

Good thing I enjoy fiddling with it. 

64350[/snapback]

 

Well, let's really make your day!

 

If you start messing with the rear suspension as well, you'll quickly discover that the front suspension and the back suspension are connected.... Who'd have guessed!

 

If you take those same "sag" parameters and set the rear end to them (which will mean re-springing this end of a V-11 as well) you'll see "order of magnitude" improvement in the way the bike goes around corners! But getting the sags right is just step one!

 

Step 2 is setting the damping. I'm a big fan of the Penske shock, so I'm not going to play with the original Boge unit. For both front and back, set the compression damping to ZERO and set up the rebound damping. When that is right you dial in a tad of compression damping, and you'll swear it's somebody else's bike!

[Guest MikeC]

Well guys you ran me a bit scared with some of the posts of getting the job done right and having the correct tools for cartridge fork work. I don't have them.

 

I spoke to Traxxion and I have sent them my forks to do the spring swap, replace bushings and basically give the forks a over-haul. Seemed like money well spent.

 

I spoke with Lee and they seem like good people, turn around a week or less.

 

Thanks for everyones help.

 

Mike

[Guest ratchethack]

Well, let's really make your day!

 

If you start messing with the rear suspension as well, you'll quickly discover that the front suspension and the back suspension are connected....   Who'd have guessed! 

 

If you take those same "sag" parameters and set the rear end to them (which will mean re-springing this end of a V-11 as well) you'll see "order of magnitude" improvement in the way the bike goes around corners!  But getting the sags right is just step one!

 

Step 2 is setting the damping.  I'm a big fan of the Penske shock, so I'm not going to play with the original Boge unit.  For both front and back, set the compression damping to ZERO and set up the rebound damping.  When that is right you dial in a tad of compression damping, and you'll swear it's somebody else's bike!

64455[/snapback]

Brian, I'm afraid I'm [ahem] a ways beyond "starting" to mess with it... Yep, one of the "benefits" of making one end better is the discovery that now the other end is way out o' whack - I'm familiar with this particular leap-frog. By my experience, the process is (usually) a progressively positive one. Kinda fun, tho, innit? I note your comment on the Penske shock. Thanks.

[Guest ratchethack]

Question - I b'lieve this fits this thread well enough.

 

Has anyone taken measurements from the top of the Marz fork tubes to establish a reference point for setting the air gap? I've come to the conclusion that somehow I may not have drained all the old oil out last pass, therefore my level may be higher than the recommended 400 ml per side - but how much higher? At this point it's unknown.