Fox RP23 tuning

[TЯΞB]

Hi All,

 

I got an '08 RP23 "medium compression tune" and "medium rebound tune" with my '08 Yeti 575. I have been blissfully riding it at 300psi (the maximum allowed pressure to carry all of my 115kg's) and using the "firm (3)" Propedal setting on the climps. Rebound is set at 2 clicks below max (out of 10).

 

This weekend I started experimenting a bit with my fork pressures ('09 Revelation) and found that by increasing the pressure in the fork from 150 to 190 psi the front end feel improved drastically (much more precise steering).

 

The rear then felt positively wallowy by comparison, so I tried riding it with Propedal on 1 (softest setting) permanently. What a difference! Feels like a different bike, really planted. Compromised a bit on the small bump performance, but that's a small price to pay. What bothers me a bit more is that I have now effectively given up the Propedal feature when standing in the pedals, on PP 1 the bike still bobs quite a bit.

 

Now for the question: Can my shock be modified to firm / hard compression & rebound. I think this would be far more appropriate given my weight and I should (in theory) still be able to use PP 3. In the US there are companies like PUSH and TFTuned that will do this, is there a local company like that? The Fox dealers themselves?

 

Alternatively I thought about going for a DHX Air.

 

Would it be a reasonable request to ask the dealer that sold the bike to me to exchange the shock considering they knew I was no lightweight? I was not aware of the "tuning options" until now.

 

Any help or suggestions are welcome.

 

Chrz,

 

Treb

 

P

[TЯΞB]

Just found this thread: http://forums.mtbr.com/showthread.php?t=495782

 

Seems it's definitely a Fox '08 RP23 problem, it just rockets through the mid stroke

 

The only remedy seems to be to have it PUSHed, $150 + shipping

 

Please tell me there's someone local!

[Johan Bornman]

Air-sprung shocks are not ideal, especially as rear suspension elements. Air is not a hookean material. In other words, the spring is not linear. Steel springs will move the same distance for each unit of a given weight put on it. Air springs initially move a lot and as it gets to the end of its travel, its travel decreases exponentially.

As the spring vessel gets smaller and smaller, like on a rear shock, the problem is even worse. Front shox have more space to use gadgets such as positive and negative air to compensate for air's unsatisfactory behaviour, but rear shox no space. A small vessel accentuates the effect.

 

Some shock manufacturers compensate by fiddling with the damping but at the end of the day, a good old steel spring is what you need.

 

 

[_im_from_earth]

^^ Confirm, there is however a mechanism built into the RP23 which compensates for this to a certain extent by having a "negative" air pressure preventing a drastic "ramp up" in the shock towards the end portion if its stroke.

 

Cutting to  the chase, a DHX is not going to solve your problem, the DHX uses a pro-pedal adjuster yes, but it will not be firm enough to give you te efficient and firm "On" you are looking for when on the climbs.

 

I do not believe that the firm compression tune will make too much of a difference either, but it certainly will help, give Omnico WC a call and chat to Russell, he is the Fox product manager and is very clued up with teir suspension and will certainly be able to rebuild the shock with the firm tune.

 

As for the "PUSH" rebuild... do a little more research on tese guys before sending anything off to them, all I can say is all is not as it appears.

 

[TЯΞB]

Thanks for the replies guys.

 

John, I totally agree with what you're saying, but I also think that the suspension design in the 575 is such that the "dogbone" exponentially reduces shock stroke the deeper you go into the travel, directly counteracting the non-linear nature of an air spring.

 

I love the plush feel of the 575 with the air shock, it's just that it's a bit too soft for me and ProPedal 1 is too hard.

 

Btw, from Yeti's website' FAQ's:

 

Can I put a coil shock on my 575?<?: prefix = o ns = "urn:schemas-microsoft-com:office:office" />

 

Answer:

 

No. The 575 can only fit air shocks. The Fox DHX air is an option for the 575 and is a nice mix between traditional air shock and a coil shock.

 

I have sent a lengthy email to Yeti, will post here if they respond. I am thinking 2010 RP23 right now (has the Boost Valve), or perhaps a 2009 which is said to have much improved mid-stroke compression damping and a wider rebound range. Will probable flog off my '08 model here on the hub soon.

 

MWKid, also agree, the Fox "Float" system also counteracts the nonlinear air spring. AFAIK most modern air shocks only ramp up right at the end of the stroke. I spoke to Russel, he was really helpful and can revalve to firm / firm if I want. I'm just not convinced that this will produce the desired result and will wait to hear from Yeti first.

 

Before spending any money on tuning however, I would rather just get a 2010 RP23.

 

Will keep you guys posted.

 

Chrz,

 

Treb

[Johan Bornman]

Thanks for the replies guys.

 

John' date=' I totally agree with what you're saying, but I also think that the suspension design in the 575 is such that the "dogbone" exponentially reduces shock stroke the deeper you go into the travel, directly counteracting the non-linear nature of an air spring.

 

[/quote']

 

That's one way around the problem and probably the best way of the lot.

 

I can't picture the frame but it sounds like the dogbone sweeps through an arc and at the end of its stroke it uses the cosine error to counteract the increasing compression force.

 

 

I love the plush feel of the 575 with the air shock' date=' it's just that it's a bit too soft for me and ProPedal 1 is too hard.

 

Btw, from Yeti's website' FAQ's:

 

Can I put a coil shock on my 575?<?: prefix = o ns = "urn:schemas-microsoft-com:office:office" />

 

Answer:

 

No. The 575 can only fit air shocks. The Fox DHX air is an option for the 575 and is a nice mix between traditional air shock and a coil shock.

 

 

[/quote']

 

It makes sense that you can't retrofit a spring shock in a frame designed to counteract the air shock's shortcomings. Should you do this, you'll have a bike that reacts in the opposite way of what it is you wanted in the first place.

 

 

[Wyatt Earp]

 

It makes sense that you can't retrofit a spring shock in a frame designed to counteract the air shock's shortcomings. Should you do this' date=' you'll have a bike that reacts in the opposite way of what it is you wanted in the first place.

 

?

 

?

[/quote']

 

JB and followers, this is not an attack, but please explain the above to me.

 

Not that I would want to fit a coil shock in any case.

[Johan Bornman]

It makes sense that you can't retrofit a spring shock in a frame designed to counteract the air shock's shortcomings. Should you do this' date=' you'll have a bike that reacts in the opposite way of what it is you wanted in the first place.

 

 

[/quote']
JB and followers, this is not an attack, but please explain the above to me.
Not that I would want to fit a coil shock in any case.

 

OK. Some background pointers first. I'm sure you're familiar with them all but for the benefit of anyone also scratching their heads:

 

1) Air springs are not linear. The first kilogram applied directly to the eye of the shock will make it move, say, 10mm. The second kg will make it move only 9 mm more and the 3rd kg applied will make it only move 6mm more. By the 10th kilo applied to the eye of the fork, it will move only 1mm more.

 

2) Coil springs are almost perfectly linear (called Hookean springs after the physicist    These springs react differently. For the first kilo they'll move say 10mm and for every kilo thereafter they'll move another 10mm until they've reached the end of their travel.

 

3) Fork/shock companies refer to coil springs as "smoother". They're not really smoother (that's a function of the seals and bushings), but they are linear and predictable.

 

4) Air springs don't act predictably towards the end of their travel. They tend to suddenly ramp up and become  very hard.

 

5) If a suspension dogbone/wishbone/swing member/whatever, was designed for an air shock, it would typically make use of the cosine error of a pendulum. Imagine a grandfather clock with a long pendulum. As the pendulum sweeps through its arc, it moves very little vertically and almost perfectly horizontally. That same clock with a very short pendulum, moving the same distance, will see the pendulum move upwards and sideways and the latter will be much more pronounced than that of the long pendulum.

 

This is applied to airshock frames so that it counteracts the sudden ramp-up of the air shock towards the end of its travel. The wishbone would be designed so that it moves forward (compressing the shock) a lot in the first 3/4s of its stroke. Past that, it would start moving upwards more than forwards.  In other words, on an airshock frame, for one unit of movement of the rear wheel at the beginning of the compression stroke, the shock would compress by one unit. For the second movement, it would compress the shock by 0.9 of a unit and for the third, oly 0.6 of a movement. This would exactly mimick the opposite reaction by the shock.

 

If you now go and fit an coil spring shock to that same frame (if indeed the frame was designed as was suggested), then the coil spring would become non-linear and towards the end of its stroke, provide too little force or, rush through its compression at the end of its travel.

 

Positive/Negative chambers in air spring forks is an attempt to balance this phenomena. The negative chamber doesn't really make it more sensitive to small hits, it rather just makes the fork a bit more linear.

 

I don't have a compression curve for ideal gases. Perhaps someone that works in that field could show us a compression and expansion curve and overlay the two to see if the movement in a balanced positive/negative fork is in fact linear.

[Johan Bornman]

Forgot to add. If you find the cosine error effect difficult to comprehend, visualise a set of V-brakes on a MTB. The pads move towards the rim as well as downwards. If the pads are new and there is little clearance between the pad and rim, then the movement is for all practical purposes, perfectly perpendicular to the rim. As the pads wear, cable gets longer etc, the pads move towards the rim and dive towards the spokes.  The shorter the lever, the more pronounced the problem. That's why we've moved from cantilever brakes to V-brakes.  Canti's suffered big-time from spoke dive.

[Wyatt Earp]

I still fail to see how the bike would act in the OPPOSITE way.

 

Coil shocks use a form of damping nowadays and have become a lot more linear than before and also have a rebound damper, so they are no longer a pogo stick, such as bikes found at major chain stores.

[Johan Bornman]

I still fail to see how the bike would act in the OPPOSITE way.
Coil shocks use a form of damping nowadays and have become a lot more linear than before and also have a rebound damper' date=' so they are no longer a pogo stick, such as bikes found at major chain stores.[/quote']

 

Many people confuse damping with compression.

 

Damping dictates how quickly a given force moves a spring.  Spring rate (spring strength), dictates how much the spring moves under a given force.

 

For example, if you dial compression damping to the full, a 1kg force will move the fork say 10mm. With compression damping completely off, that same 1kg force will still move the fork 10mm. The difference is just the speed at which it reaches the 10mm.

 

The damping technique does not change for coil or air springs. Standard damping is perfectly linear. Non-linear damping involves stage valves and is not applicable to this discussion.

 

A good example of the cosine error trick is a hatchback car's rear lid. When you click the lock and pick up the lid, it is relatively easy to move it a certain distance. However, when you want to close the lid from its open position, you require a bit more force to move it that initial distance. As it gets closer to the shut position, it over very easily. The lid uses as gas spring.

 

If you were to fit a steel spring in there it would become too slack towards the end of its travel (towards shut) and slam too hard.

 

 

 

[_im_from_earth]

however Johan damping is dynamic whereas a spring is static. the above assumes that the entire system is a static one.

 

[Johan Bornman]

Explain?

[Wyatt Earp]

however Johan damping is dynamic whereas a spring is static. the above assumes that the entire system is a static one.

 

 

 

.............and the compression rate can be changed by using different coils.

 

No I din't confuse compression with damping either.

 

I still fail to understand how the suspension will act in an ................................... sorry hit the button too quick.Summit Cycles2009-07-29 03:44:59

[Wyatt Earp]

however Johan damping is dynamic whereas a spring is static. the above assumes that the entire system is a static one.

 

 

 

.............and the compression rate can be changed by using different coils.

 

No I didn't confuse compression with damping either.

 

I still fail to understand how the suspension will act in an oppositemanner when a coil shock gets employed and let's not forget coil shocks that use a piggy back system.

[Johan Bornman]

however Johan damping is dynamic whereas a spring is static. the above assumes that the entire system is a static one.

.............and the compression rate can be changed by using different coils.
No I didn't confuse compression with damping either.
I still fail to understand how the suspension will act in an oppositemanner when a coil shock gets employed and let's not forget coil shocks that use a piggy back system.

 

Five smirking monkeys sitting on a branch clapping hands doesn't answer my question and doesn't add to the debate either.

 

Nevertheless' date=' compression rate is a meaningless term in suspension. Although we could invent a meaning for it....(the rate (speed) at which a spring compresses), could be one definition, it still doesn't help.

 

Spring rate on the other hand is a term with a well-defined meaning. We change the spring rate of a suspension component in order for it to carry more or less load. In English, we made the spring stiffer or shlapper. That is spring rate. Yes, by chaning the coil (thinner or thicker steel), we can change the SPRING RATE.

 

Back to the mythical COMPRESSION RATE. Yes, you can alter the speed at which the spring compresses. You can do this increasing compression damping, you could do this by slowly increasing the force applied to a spring (such as steering your bike into an inward-spiralling circle with a nice berm etc etc.

 

But we don't concern ourselves with the compression rate of a given suspension system since there are external factors influcencing it. Instead, we focus on damping. That can be altered by chaning the size of the oil channels or as you say (by external knobs we turn) or, by using thicker oil.

 

Let me explain my use of OPPOSITE MANNER.

 

You can compensate for an air spring's sudden ramp-up by designing the frame so that it's direct, inline movement towards compressing the shock is less, the closer it gets to bottoming out. 

 

You don't need to do that with a coil spring.

 

Therefore, if you have a suspension system designed for an air spring and you fit a coil spring, the suspension will "ramp down" (go softer) towards the end of the shock's stroke. 

 

Ramp up vs Ramp down = OPPOSITE MANNER.

 

Piggyback system: This has nothing to do with the spring. It is an oil reservoir for the damper system. It has several functions, amongst others it gives the total system more oil, which keeps the oil cooler, distributes the workload and shear forces amongst more oil so that oil lasts longer, it gives an opportunity for a larger gas cavity above the oil so that bottom-out events are less damaging to the damper and, increases the allowable margin of error of oil volumes in the shock. A piggyback system may also be necessary to create an extra gas volume above the floating piston, should the compression rod be large and displace large amounts of oil as it enters the oil chamber in its compression stroke.

 

Piggy-back systems could be used on air or coil springs since it has nothing to do with the spring and everything with the damping system.

 

I said "many people confuse...." That's because many people do.

 

In my hands I have a magazine with an article on suspension systems.  I quote: " .....With these [Air Sprung'] forks, compression damping can be increased or decreased using a shock pump..."

 

That is fundamentally flawed. A shock pump puts more air into an air spring which changes the spring rate, not the compression damping. Compression damping is done on the other leg by either chaning the oil or turning the dial.

 

All of this is not to be confused with the air or nitrogen charge inside the damper chamber. Here the gas compresses a piston floating on top of the oil reservoir. The piston's job is to prevent the oil from foaming. In the recent video that Morewood Kid posted of a glass shock on a dynamo, you can clearly see the floating piston. By adding or removing pressure from this chamber, you'll have no material effect on damping other than to invite or retard foaming and cavitation.

 

Interesting to note was that no matter where the damper piston is in the stroke, the floating piston did not move (or moved very little). I would have expected it to move more to allow for the increased volume of the oil chamber with the piston at full extension.

 

 

 

 

 

 

 

 

 

 

Johan Bornman2009-07-29 11:11:32