Now onto the forks

[dlaing]

Some more random thoughts.

No matter how hard you try,you wont bend an axle Unless you hit something.

Disclaimer: this graph ain't from a Marzocchi, but some Ohlins R&T and note they are talking oil level not air gap.

Perhaps flexing is a better word.

Now I would rather bottom on air than on metal, so a high oil level could be a good thing.

ASSUMING that one fork was set fourteen millimeter off of another, looking at the graph, that might be a difference in force of anywhere from 200 to 300 Newtons which I guess is not as much as I feared:

200N = ~50 pound force

300N = ~67 pound force

So, if I understand a pound force correctly, and assuming that chart is per fork and not per two forks, it might be like taking one fork clamping an axle in it and then hanging a 50-67# bag of cement from near the end of the axle and seeing if the axle flexes.

I am pretty sure it will flex, but now that I know the force, I am not sure how consequential such flexing will be. I was expecting hundreds of pound of force.

But keep in mind those are Ohlins recommended numbers, and an incorrectly set Marzocchi could be a problem.

For example if you added 400cc into each fork, but had not gotten enough out of one fork as the other, you might end up with an air gap of a hundred in one, and fifty in the other. This might have a much more drastic impact on the axle when you bottom.

In the case given here, the air gap enough that the 14mm may not have been a problem, but I suspect that combined with the force of the higher air gap being on the compression side, the forks are further out of balance and more stress is put on the axle. Does it result in substantial flexing of the axle? I don't know, but I would not set it up that way for fear that it MIGHT cause a problem.

[FuelCooler]

In the case given here, the air gap enough that the 14mm may not have been a problem, but I suspect that combined with the force of the higher air gap being on the compression side, the forks are further out of balance and more stress is put on the axle. Does it result in substantial flexing of the axle? I don't know, but I would not set it up that way for fear that it MIGHT cause a problem.

 

For what it is worth, gentlemen, I did not set the forks up like that (14mm differential oil height) and I would not set up forks like that!

But this is making for some good reading.

 

I did reset my air gap to 108mm (both legs of course). My 500# shock spring will be here Friday so I hope to have a test ride report by this Sunday. I am doing a bunch of maintenece right now (new stem bearings, new fuel line / filter, etc....

Thanks for the help!

Steve

[big J]

Well said Big J. Your post was neither patronising or smartarse. Rat hopes you will accept BS, mixed with ridicule and obscurity, in lieu of clear and lucid explanation. Seems to be enough for his sheeple to accept.

 

Rj

 

Yeah,I know.I just wanted a straightforward answer.It's not as if I made a pie from his cat.

[dlaing]

For what it is worth, gentlemen, I did not set the forks up like that (14mm differential oil height) and I would not set up forks like that!

But this is making for some good reading.

Yah, we know you did not do such a thing.

One of the points I was trying to make was that the difference between how you found your forks set, and how you set them should make a positive difference, especially when approaching the lower end of travel.

But others seem to disagree that the difference between right and left air gaps will make a difference. I am certainly open to the possibility that the difference is not noticeable and may not effect how it wears. But until proven otherwise, I'll stick to trying to balance the forks.

I recall reading that it is common practice to mix springs to get a given rate. For example to get the equivalent of a pair of 1.05 springs, the experts use a 1.00 and a 1.10 spring.

For my Ohlins forks, I would feel a little uneasy about that because I would be concerned that fork life might be shortened.

For the Marzocchi forks with right and left rebound and compression, I would happilly put the 1.10 spring on the rebound side and the 1.00 spring on the compression side.

But that is just my opinion based on very little experience, and based more on common sense and guzzichondria.

[Guest ratchethack]

Disclaimer: this graph ain't from a Marzocchi, but some Ohlins R&T and note they are talking oil level not air gap.

Dave, this graph is interesting.

 

If you notice in the legend on the graph, they clarifly their definition of oil level. It IS the air gap (oil level = air spring height).

 

The force rises with stroke on a curve with a decrease in the air gap, as shown.

 

This is the principle that's illustrated as simply as can be presented on Verdone's graph by the Ideal Gas Law:

 

P V = n R T, where P is the pressure, V is the volume, n is the number of mols of gas, T is the absolute temperature, and R is the Universal Gas Constant.

 

With a 150 mm air gap, at 1000 Newtons on the graph, there's 225 pound-force, and rising very rapidly.

 

Again with a 150 mm air gap, but at 1200 Newtons on the graph, there's 270 pound-force, rising even more rapidly up to the end of stroke.

 

At the limit of fork travel, a 20 mm difference in air gap between 170 mm and 150 mm = 450N, or 101 pounds-force. The way I think about this, even half of this difference right-to-left would be very significant WRT potential fork flex and binding.

 

Keep in mind the graph above looks at force curves at 10 mm and 20 mm above and below a 170 mm air gap for the Ohlins fork. If we looked at force curves at 10 mm and 20 mm above and below the recommended starting point of 100 mm air gap for the Marz 040 fork, the percentage changes (and relative differences in force curves) would be significantly greater based on relative air gap volumes alone (may we assume similar "piston swept volume" to overall air volume ratios for both forks?), and the curves would be even more dramatic for the Marz.

 

All these numbers are certainly helpful (at least when considering air gap settings on an Ohlins fork), but they can get a mite tedious, n'est-ce pas?

 

But of course, if the principles illustrated on Verdone's graph (taken from a Honda HRC Setup Guide as referenced, which presumably is a rough representation of a "typical fork" or a composite of many kinds of forks) are incomprehensible, it would follow that all the numbers here would appear to be "gibberish" as well. . .

 

Again, I want mine as close to balanced as I can get, measured by the air gap method, but that's just me.

 

NOTE: This is turning into an interesting study alright (not to mention the, er, rather spectacular ego dreckfest, which I find to be a bit of a revealing "study" in itself ) but here's an observation and question: Per above, it appears that these Ohlins forks run both a significantly greater air gap and a greater overall air volume than the Marz. If we assume from this that the Ohlins also run lower air spring pressures than other forks as well, could this be a design objective dictated by pressure limitations of the Ohlins ultra low-stiction fork seals?

 

Enquiring minds wonder, virtually dazzled now with curiosity on oh so many fronts . . .

[FuelCooler]

:!: Update on forks. :!:

 

Well, I am doing 'shake down' runs on my suspension mods. It is much, much better than before (thanks for everyone suggestions).

My findings so far (YMMV):

1. My shock oil change worked! (Bel Ray HVI 10wt)

2. When you adjust the rebound clicker on the shock, it increases / decreases rebound AND low speed compression somewhat. (Sachs).

2. The 500# shock spring is a huge imrovement. Thanks guys. (Eibach 6 inch x 2.25 x 500#)

3. The compression side fork has very little compression damping. Off the bike, the rebound leg has more 'slow shaft speed' compression damping than the compression fork does

And to cause several pages of contraversy, at the last minute before assembling the front end, I dumped the HVI 5wt out of the compression side fork only and repaced it with HVI 10wt. Yup, 5wt rebound, 10 wt compression and I am living to tell about it. And I have the compression adjuster screwed all the way in. It is by no means too harsh. Not even close.

4. A sensation I was getting (but haven't brought up yet) of the front end feeling like it was going to 'tuck' under lower road speed / rapid direction change was at least partially being caused by a bad (stock) steering damper. I should have known better, my GS500 did the same thing (but worse!). Too easy to check first?

Probably contributing significantly to the vagueness as well.

5. A 180/55 on a 5.5 inch later model V11 wheel works for me. It ain't slow steering the way I have my bike set up! It adds about 3mm rear ride height compared to a 160/60 on a 4.5. (I actually have both rims set up with Pilot Powers, and I measured them) I am going to experiment with raising (to slow the steering and add ground clearance) the front ride height (I am 10mm thru the clamp now).

 

I have only ridden 20 miles with this set up and I am really impressed with the rear shock. 20 feet out of my driveway I was grinnin'.

[dlaing]

Coool!

Kinda surprising about the compression side having less compression, but it seems like you took the right action and got great results

Knowing that your forks had firmer springs than many others, I guess I should not be surprised.

I wonder if someone before you had resprung and revalved the forks

[Guest ratchethack]

Roger that, Dave. Glad you're pleased with the progress, Steve. Just by way of recap, I've got the same forks as you, Steve, Marz 040 USD. But in my case, I know for double-dog dead-nuts certain that they hadn't been meddled with previously.

 

Both rebound and compression side on my forks provide some amount of low-speed compression damping at ZERO setting. How much this translates to on each side on the road is unknown, and I make no assumptions based on what it does off the bike on the bench with springs out and relatively tiny fractions of force applied. Since I don't have access to a shock dyno, my observations and analysis are limited to adjustment and effects of adjustment to damping characteristics on the road.

 

Regardless of fluid (again, I've run dino 10 wt., dino 7.5 wt., and synth. 125/150) I've never found a setting I like better for most circumstances than either twice the rebound as compression setting, or low rebound setting and ZERO setting on compression, (regardless of which side the comp damping comes from). In my case, if I went with asymmetric fluids, I'd be doing the opposite as Steve to gain more range of adjustability -- I'd go to lower viscosity on the comp side, which might allow me to run the same (or nearly the same) settings on both comp and rebound.

 

So I have to agree, it appears to me that Steve's may have been revalved.

[Dan M]

:!: Update on forks. :!:

 

3. The compression side fork has very little compression damping. Off the bike, the rebound leg has more 'slow shaft speed' compression damping than the compression fork does

 

 

 

This is odd. Is it possible your caps are swapped? How does the rebound damping compare off the bike? If your rebound leg has more damping in both directions maybe there is a restriction in the valve. Mine are clearly different (opposite) off the bike.

[Guest ratchethack]

Good point, Dan. Please don't ask me how I know how easy it can be to get the caps back in opposite sides.

 

For the record, compression goes on the left, rebound on the right.

 

Here's a more disturbing thought along the same lines: Assuming complete disassembly, isn't it equally possible to get the cartridge rod assemblies themsleves back in opposite sides?!

 

From memory, I b'lieve they're interchangeable ALSO!

 

Again, who knows wot the PO and/or unknown service dep't. may've done??

[FuelCooler]

This is odd. Is it possible your caps are swapped? How does the rebound damping compare off the bike? If your rebound leg has more damping in both directions maybe there is a restriction in the valve. Mine are clearly different (opposite) off the bike.

 

Hi Dan M,

My rebound leg had strong rebound and some low speed compression (checked with forks full assembled). The compression leg had zero rebound (pogo stick) and slighly less low speed compression until the last 1.5 to 2 inches of travel, where combined with the air gap and bottoming cap, it was getting stronger.

 

Now granted, as Ratchethack says, I can't mimic the forces or the speed the fork will see, but with all my weight I could move the legs pretty quick. Scientific? No. But I realized (from doing a half dozen different forks in the past) that more low speed compression was going to be needed. I didn't dissassemble the forks completely, so there may be something perhaps holding the compression shim(s?) open. Regardless, it felt much closer with the 10wt. I went for it! Do I like the idea of it? Not really. But the effect has been positive.

So far so good, but I am not done yet!

I may just rip that fork apart for a look.

[Dan M]

Hi Dan M,

My rebound leg had strong rebound and some low speed compression (checked with forks full assembled). The compression leg had zero rebound (pogo stick) and slighly less low speed compression until the last 1.5 to 2 inches of travel, where combined with the air gap and bottoming cap, it was getting stronger.

 

Now granted, as Ratchethack says, I can't mimic the forces or the speed the fork will see, but with all my weight I could move the legs pretty quick. Scientific? No. But I realized (from doing a half dozen different forks in the past) that more low speed compression was going to be needed. I didn't dissassemble the forks completely, so there may be something perhaps holding the compression shim(s?) open. Regardless, it felt much closer with the 10wt. I went for it! Do I like the idea of it? Not really. But the effect has been positive.

So far so good, but I am not done yet!

I may just rip that fork apart for a look.

 

Before you take it to pieces, try this. With the fork caps off, springs out and the forks blocked up, grasp the threaded rod and feel how much damping action there is in each direction. You will see the center actuator rod "float" as you move the assembly. Then, while moving (I'm trying my best not to tell you to stroke your rod) the rod, apply pressure to the center actuator rod that acts on the damping valve. When you hold that actuator in (down) there is substantially more damping. You can compare the action from each side, obviously it only makes a change on the push stroke on the compression side and on the pull stroke on the rebound side but the change is quite easy to feel. If you get increased damping while holding the center rod down then perhaps the adjuster in the cap is not acting on the rod (too much gap between the adjuster and the rod). If there is no change then you may indeed have a faulty valve.

[Tom M]

If I ran my compression adjuster all the way in I doubt that my forks would move at all! I think there's something wrong in there Steve. That said, if you ride the bike and it feels good I'd probably wait until a seal goes or it's time for a fork oil change before going back in there again.

 

FWIW I found something interesting when I was changing preload spacers the other day; uneven fork oil height. I was very careful in setting the height a few weeks ago when I installed new springs. I had the forks vertical in a vise and used vernier calipers to set it at 100mm after pumping all of the air out of both cartridges (so I thought). The rebound side was down about 7mm.

 

 

Off topic: Hey Dan, have you run your bike with the new MR pistons yet? If so please let us know how they're working out, maybe in another thread so we don't muck up this fork thread.

[Guest ratchethack]

FWIW I found something interesting when I was changing preload spacers the other day; uneven fork oil height. I was very careful in setting the height a few weeks ago when I installed new springs. I had the forks vertical in a vise and used vernier calipers to set it at 100mm after pumping all of the air out of both cartridges (so I thought). The rebound side was down about 7mm.

. . .

I had this happen one time after a complete fork strip down to the cartridges. Pumped out the air very carefully while re-filling (or so I thought). Rode the bike for a week, then opened them up to change preload and it was low on one side by 4-5 mm.

 

The next time I had the forks completely dismantled down to the cartridges, I went thru the same "de-gassing" pumping routine on re-assembly. Remembering well the last experience of discovering a low level on one side, after a few lengthy rides, I went back in to check the level and it was dead-nuts on the mark, both sides.

 

As with brakes, its a situation where air in the wrong place is NOT y'er friend, and it can be pretty elusive when you're on the hunt. . . .

 

IMHO it's worth an after-road-test inspection check after meddling with it.

[Dan M]

 

Off topic: Hey Dan, have you run your bike with the new MR pistons yet? If so please let us know how they're working out, maybe in another thread so we don't muck up this fork thread.

 

 

Not yet Tom, the motor is together but I'm waiting for my plastic parts to be painted. Hopefully (and I've been saying this for weeks) I'll have the stuff back next week.