The Tuesday Tune Ep 14: Myth Busting

 It's because forks typically generate the majority of their compression damping over the base valve, with very little over the mid valve (ie the rebound piston, equivalent to the main piston in a rear shock). As a result, the pressure built up in front of the base valve is sufficient to force oil through the mid valve (which in most cases is functionally just a check valve with very little pressure drop across it) without needing pressurisation of the whole damper.

Also, last week's video was on fork dampers

 I'm curious about that, too. If there's nothing pressuring the oil in an open bath damper why doesn't it cavitate?

 To add to what @VorsprungSuspension said, it's not strictly true that fork cartridges with bladders or open bath don't have any pressurization. both the headspace that is compressed as you go through the stroke with an open bath, & the air outside the cartridge + the elasticity of the bladder itself are going to provide some pressure against the oil. When talking about pressurizing a damping circuit to prevent cavitation, any pressure beyond that needed to prevent cavitation at the maximum pressure differential is generally a negative with regards to tuning, as it's another air spring you have to account for when figuring out the right spring rate. The older FOX DHX shocks actually use the IFP resevoir as a tuning tool for bottom out & pedal platform, via changing the air pressure & reservoir size.

Also, some brands do use a pressurized cartridge, DT Swiss(formerly Pace) has been claiming a pressurized cartridge for years, & Suntour's most recent damping revamp uses an IFP.

Incidentally, Vorsprung, if you wanted to do an teardown & explanation of exactly how the IFP pressure & volume changes work the FOX DHX shocks, & how exactly the DT Swiss fork cartridges are constructed, I'd eagerly tune in for that.

 @groghunter: How would changing IFP pressure affect damping? The only way I can see that working is if the IFP is the only mechanism that allows for oil displacement by the damper shaft, like in the simplified single tube damper in this week's video.

I asked some experts that I know - one a professional engineer, the other a former student of hydraulic engineering - and they told me that the pressure of a liquid shouldn't affect the coefficient of damping (ie damping force) of a piston in said fluid.

 @WaterBear: IFP pressure doesn't affect damping unless it's too low to prevent cavitation(in that, gaseous oil has a different damping coefficient than liquid oil.) What if affects is spring rate. it's essentially another air spring you have to compress.

& yes, the IFP(or bladder) is essentially the only mechanism for dealing with the shaft displacement. that's actually their main purpose. There are a bunch of other ways to deal with that displacement(some kinda rare & very funky, never saw them before I started reading the motorcycle suspension bible.)

 @groghunter: largely correct, however to clarify, open bath or bladder-type cartridges have zero gage pressurisation at topout (in fact some Fit4 dampers are actually very, very slightly sub-zero gage pressure due to how they are assembled, and only begin to stretch the bladder at all quite late in the travel) and the pressure provided by those factors increases with travel. However, since the highest velocities occur quite early in the travel, this is not really sufficient to provide any resistance to cavitation, and the compression check valves essentially need to be able to function at atmospheric pressure.

 @VorsprungSuspension: is the increase in pressure inside the lower leg significant in bladder equipped damper to provide any help against cavitation as the fork compresses?

 @Happymtbfr: Everything helps, it's just a matter of proportion. The pressure increase inside say a 34's damper side between topout and bottom out is pretty tiny, only a few psi. By contrast, damping pressures can run into the hundreds of PSI.

 @Happymtbfr: to amplify what @VorsprungSuspension is saying, & to clarify some of my earlier points(after getting a little education on the effects of that bladder in the above reply,) it's also largely irrelevant. because of the design constraint of that low pressurization, the rebound piston is designed to not create a big enough pressure differential to cause a low pressure phase change(cavitation) when moving through the fluid.

A piston that did cause a large enough low pressure area to cause significant cavitation wouldn't(or at least shouldn't) make it past the design phase.

& furthermore, any more pressure on the compensator would be a detriment, in that it becomes another air spring you have to account for when doing setup(& one you can't tune, to boot.)

 @groghunter and @VorsprungSuspension thank you!

 Wish I could UPvote this ^^^ 100x
Many Thanks @VorsprungSuspension for providing these excellent technical tutorials!!

 The RC has a LSC adjuster (needle), just like the small small dial on top of the RCT3.
The RCT3 also has the pedal switch with does the following:
In open mode, it doesn't doe anything
In pedal mode it closes that needle port with a sleeve, and starts preloading the shim stack.
In lockout mode, it increases the shim stack preload, therefore increasing platform.

Another difference is that most RC versions of the Pike have a wayyyy stiffer shim stack. If you would replace it with the shim stack from an RCT3, it would feel identical to the "Open" mode of the RCT3 damper. There is a MTBR thread somewhere that discusses these tuning options.

 @two-one: Exactly this. most of the time, when people ask these questions, they're leading into "my Pike RC feels like shit in high speed chatter." that's because the high speed shim stack is intentionally crippled in order to provide a pedaling platform when the knob on the top is cranked closed. This leaves the low speed circuit to handle basically all events, which leads to to terrible feel over chatter, as it can not flow enough oil to allow the fork to move enough at those shaft speeds.

I'm actually on a softer stack now than the one in the RCT3, & I'm still not satisfied. This is partly because the air spring in the pike is tuned for far more sag than I'm trying to achieve. I like a stiff spring & minimal damping (at least on a big bike like my Spartan) & the pike doesn't really allow for that in it's tuning range, & the air spring doesn't work as well with less sag. I think it's partly that the Pike is trying to cover too many bases, the tuning needs of a bike with 130mm of travel is not the same as a bike with 160mm.

 @two-one @groghunter: Thank you both for the info. My reason for asking is that I have a 160 mm Pike RCT3 on my AM bike and got a 120 mm Pike RC for my hardtail a week ago. I'm just trying to figure out how the adjustments compare to achieve a similar outcome. This is helpful.

It sounds like a lighter compression shim stack in the RC would give a more usable range of LSC adjustments for descents.

 @DMal: Yup, at the cost of a lockout when you crank the knob. Which may be more important to you (120mm) than it was for me(160mm.)

When you lighten the shim stack, oil will flow past the HSC shims when the LSC is closed, it does not as configured stock.

 I too am interest

 Extremely unlikely.

 @VorsprungSuspension: care to elaborate? No worries if not, I expect you have better stuff to do than answer random pb queries!

 @VorsprungSuspension Everything I have read about the Millyard bike sounds like OLEO would be a great idea. Maybe not playful but certainly fast and efficient in terms of travel use.

 @alexhyland: several reasons:
1. Oleo struts are excellent for big single impacts and preventing really heavy things from damaging components by providing extremely strong, progressive bottoming resistance. They are used as elevator cushions, landing gear struts, and so forth. They are not a refined damper in the variable aperture, speed sensitive context that all other high performance dampers are.
2. They've had 10 years to make a dent since Millyard first debuted their bike and have made none beyond Steve Jones talking it up - nobody has even tried to copy them or adapt the idea in other ways. The closest thing really is the KTM PDS shock, which was universally lauded as inferior to everyone else's dampers at the time and even then that still had shimmed valving in parallel with the massive bottoming needle.
3. Second hand info has been... somewhat contradictory to the positive reviews published elsewhere.

 @VorsprungSuspension: Cheers. I think your 2nd point sums it up. Also MX is way ahead of MTB therefore if they aint used in MX the same follows for MTB.

 @fartymarty: generally agree with you there - MTB does have slightly different requirements (pedal platforms etc) but on the whole the high end tech is very similar.

 @VorsprungSuspension: Cheers!

 ...really?

 It's not an unreasonable way to get a rideable baseline before you hit the trails, but making adjustments on the trail is the best way. Check out our previous Tuesday Tune video, particularly the part about bracketing - www.pinkbike.com/news/the-tuesday-tune-ep-6-ls-and-hs-damping-2016.html

 @VorsprungSuspension: just wanted to say I've just fitted one of your corset air sleeves to my Fox ctd shock - AMAZING. Night and day difference, now feels like a coil shock. Thank you and keep up the great work.

 If she's happy with it, chasing theoretical ghosts is a good way to spend a lot of time making your life less enjoyable

 Come maintenance time, when the monarch needs a rebuild, send it to Vorsprung for the overhaul and a custom tune. We just did the same for my partner who is in the same boat at 125lbs or so. Steve sent it back with lighter compression (and rebound) valving to help her better use her travel.

I will commend Steve/Vorsprung for their help getting her bike sorted for the upcoming season! Super courteous in amswering all our questions, and very professional output in the end.

 @marshalllu Some bikes just are very hard to utilize full travel with. For instance, I have trouble doing so on a Rocky Mtn Thunderbolt, and also weigh the same and am able to use full travel on its Pike. It's a bike that makes tight turns fun, that gets easy thrills from merely letting go of the brakes in tight sections, not needing to go fast to have fun... I've made a switch to a SB5c (1st gen), since I believe the rear suspension is superior and the bike felt more in tune with me and how I like to trail ride (not so tight and twisty, with more high speed sections).

 @zooey: OT, but can't resist echoing your opinion on the sweet Thunderbolt. Most local trails are tight'n twisty and this rig is SO much fun!

 RS adjusters can be relabeled as beginning stroke = low speed, ending stroke = high speed. In reality though the RS adjusters are more like beginning stroke = all speeds, ending stroke = particularly high speeds.

 If it's imperceptible, why ask the question? We've only got so many minutes, days, weeks, months, and years on this roofless spaceship, spinning through infinity. Why waste time on something that would never even be felt?

 The spring can be considered partially unsprung weight, you are correct. However, its centre of mass only shifts by half the shock's displacement. The shock moves roughly 0.35x as far as the wheel, and the weight difference between Ti and steel is in the region of ~200g on a DH bike (and virtually nothing compared to SLS springs), meaning you have the equivalent of about 35g weight difference when measured at the wheel. That is smaller than the difference in weight between two Minion DHFs in different rubber compounds (everything else being the same). The effect is slightly bigger in forks, but then Ti fork springs don't have the greatest reliability record with any manufacturer. In the context of what most people are looking for as far as suspension performance goes, the difference really is not significant - it's in the vicinity of 1% reduction in actual unsprung weight (for the rear of the bike), which on a bike is already very low relative to rider/sprung mass. 1psi of tyre pressure has a far larger effect.

 @VorsprungSuspension: I'm sure I'm not the only one to think this, but I must admit that I'm impressed by your knowledge of physics fundamentals - is this something self taught after years of experience?? I think it is great that you are helping educating the wider mtb community. We could all use some help understanding the difference between marketing BS and the real deal.

I know it is likely complicated for you, but don't hide the fact that some mfrs are infusing marketing into something that should be strictly performance based. Call them out, we could all benefit by leveraging your knowledge on who is full of BS and who is saying it as it is.

 @VorsprungSuspension: Maybe you should do a Tuesday video on Hooks Law, and the math behind hydraulic and pneumatic principles. BTW, well done! I thoroughly enjoy Tuesday Tune videos.

 @rivercitycycles: Hookes*

 @Deadskittles: Thanks for correcting my spelling mistake...........I'm just a dumb engineer

 @rivercitycycles: one does not simply out oneself as an engineer in PB comments.

 @rivercitycycles: Didn't mean to be rude (Currently training to be an engineer)

 @rbelak: Not belittling @VorsprungSuspension , but these topics are standard mechanical engineering. However, very refreshing to see someone use engineering to explain the topic versus marketing.

 @rbelak: I know I'm speaking for him, but chances are good he has a degree in engineering. Probably mechanical. Typical engineering students undergo plenty of physics. Typically all the intro physics classes and then a really slow treatment of classical mechanics dividing it into statics and dynamics, then off to thermo, etc so on and so forth.

Chances are equally good that they can't (or shouldn't) give too honest an opinion of any particular manufacturer or product.

FWIW, a college student who has taken physics 1 is, more or less, in a position to understand suspension kinematics. It all boils down to various torque calculations, many of which are not that complicated. Leverage ratio is the result of assuming conservation of torque, for example.

 @rbelak: thanks for the kind words, though as @profro said, this is all standard mech engineering stuff, just applied directly to bikes. Within the realm of engineering, pretty well every concept you could dream of has been investigated to a completely insane degree by somebody somewhere on a theoretical level if not a practical level, and usually written down in a textbook somewhere. What separates that fundamental understanding of principles from the products we ride in the real world is a combination of multiple factors - cost, information accessibility, data integration and the ability of the engineers to recognise proper prioritisation & necessary compromise of design parameters. For example, you could go absolutely nuts trying to reduce unsprung mass on the bike, but if you do so by making the swingarm out of half a dozen drinking straws, obviously you're neglecting strength and stiffness as a priority and placing too much emphasis on that unsprung mass.

For sure the bike industry has a lot of marketing hype within it. I think in the suspension world, what often makes things difficult is that the people designing stuff are not necessarily the ones responsible for marketing it, and in order to be technically accurate in everything you say about a product, you have to understand how it works very thoroughly - and likewise, in order to actually connect with your customers, they have to be on the same wavelength as you.

 @ThomDawson: @Muckal: My intention wasn't to out myself as an engineer. I really enjoy Tuesday Tune. I've been working the night shift this week managing the installation of some processing equipment and got caught up in the moment of the video due to being tired...........But on the flipside, just like I rubbed you the wrong way people that publically correct someone spelling or grammar pisses me off. Sorry for coming across as a d*ck

 @rivercitycycles: I was just kidding btw, should have added a few ;-)
:P
:P
But srsly you wanna be careful with that engineer talk around here. I've seen things. Terrible things done to engineers. Filthy things.

 @ThomDawson: thank God i didn't mention that i am an...yeah well...forget it...

 @ThomDawson: *puts hand up for filthy things to be done to an engineer*

 Designing an entire fork or shock is one thing (one of our staff actually designed the first Elka MTB shock prototypes), but actually producing it is another. Off-the-shelf parts in bike suspension are nearly non-existent - pretty much just (some) seals and the occasional fastener. Nearly everything has to be purpose built in order to keep weight down and performance up. But on top of that, why would we? We aren't going to produce better fork chassis' than Fox/RS/DVO etc. They have enormous manufacturing capabilities dedicated to that, we have a small workshop with one (admittedly quite capable) CNC machine and a handful of staff. Rather than taking on the huge development cost, quality assurance measures & risk associated with creating structural parts that have essentially already been made as well as they're ever likely to (give or take - but we aren't going to beat them on that front by any measurable amount), we can deliver far better ride quality by isolating single points of weakness and improving otherwise good products relatively inexpensively. That's where our strengths lie, and that's what we focus on - not to say we don't plan to expand in the future but we'd rather absolutely nail one single component that makes a substantial improvement to your ride, than spread ourselves too thin trying to "think big".

 @VorsprungSuspension: Thanks for the answer. Was kind of wanting an idea of just how challenging it was for smaller players like Cane Creek, MRP (which acquired Elka), Avalanche, PUSH, Stendec, and others to get a suspension product out there to get a slice of the market, alongside a manufacturing and engineering powerhouse like Fox. Your answer reminds me of this vid: www.youtube.com/watch?v=ZMMUKUhxwdY

 OTT is just the amount of preload on the negative spring which is a coil on a dvo.

 @Happymtbfr: I know, but that preload (and correct me if i'm wrong) adjusts how much the OTT spring participates in the initial stroke.
With fully open OTT, more of the initial travel is due to the metal spring, while OTT closed means initial travel is mostly air sprung. So i'm wondering if those springs are independently dampened, and whether the damping reacts significantly different depending on the spring type.

 @foxinsocks: the ott spring works in the opposite direction of the air spring.
And yes, substantial changes in the spring curve will affect the damping somehow but I doubt that you will be able to use the full range of the ott for a given air pressure...

 @foxinsocks what you're referring to is just the spring system. The damper is a completely separate system that is not directly related to any adjustment you make to air pressure or OTT (except if you also adjust the damping because you changed the spring setup enough to require that). Everything to do with the LSC/HSC in the DVO forks is equally applicable anywhere within the travel.

 @VorsprungSuspension: Thanks. I assumed the damping is probably uniform throughout the travel. Rather, what i mean is the compression/rebound might act differently (faster/slower) depending on the OTT. For example, maximum OTT --> more plush initial stroke --> reaches the HS circuit faster --> effectively less affected by LSC (pretty much in relation to your earlier videos). Though, now when i think of it, the compression speed must be a continuous function anyway, so it's not like the damping will magically change when transitioning from "OTT spring" to "Air Spring".

 @foxinsocks: Ah I see what you're getting at. There is no defined transition between negative-dominated and positive-dominated aspects of the spring curve - negative force is declining at the same time positive force is increasing. A softer beginning stroke will sometimes mean you want more LSC to push against... unless you're running more pressure as well, which firms up the middle/end of the stroke, in which case you may want less. It's not a simple relationship unfortunately.

 What we were talking about with speed sensitive vs position sensitive damping was nothing to do with MRP's Ramp Control cartridge, which is actually both of those things. It is a simple device with a very complex output in that it simultaneously affects spring rate and damping rate, but in very nonlinear ways that require 3D mapping to describe even remotely accurately. They are also compatible with our Luftkappe in most travel configurations (150mm and less on the Pike, 160mm and less on the Lyrik/Yari) so they are not a direct competitor per se, they affect different things and are not mutually exclusive.

 @VorsprungSuspension: Much appreciated,thank you for clearing that up! Looking forward to the next Tues Tune!

 as I understand it foam is a concern unique to emulsion & open bath dampers, in that it is a product of oil/air mixture. most modern dampers are sealed, in that there is no interface between oil/air to allow foaming.

 That is a pretty large topic, though something we have considered.

Foam is a constant concern in open bath and emulsion dampers, but basically a service interval issue with sealed dampers. With regard to sealed dampers becoming aerated - fluid typically (this is not a comprehensive list) becomes aerated basically by air (or whatever other gas, eg nitrogen) making its way past seals into the damper fluid, or by a loss of damping fluid causing a lower static pressure at rest which draws dissolved air out of the oil. In most air shocks, air is constantly being ingested into the damper, but because it's pressurised (google Henry's law) it takes quite a while for the dissolved air to reach saturation point, at which point the oil can no longer dissolve the air. At that point it remains as bubbles in the oil. That makes damping response slower, less predictable, lower damped, highly dependent on fluid mixing and so forth.

 @VorsprungSuspension: So this dissolved gas problem is the main consideration for service intervals between the two styles (open and sealed) of dampers? I guess there are other performance benefits here with sealed damper and IFP systems. Higher static pressure being one advantage, meaning more dissolved gas possible. Seems also relevant that according to Henry's law the hotter a fluid becomes the more air will likely come out of solution. Could this lower static pressure causing dissolved air to come out of solution be the reason why some shocks will feel awful until cycled a few times? My old concept of service intervals was focused around contaminates that weren't air: things like dirt and water. So if I didn't ride in the muck, then the service interval could be a bit longer. Oops. Thanks.

 @jcklondon: Interested to hear what they say as well, but I believe the air contamination problem is really only an issue with sealed dampers: open bath & emulsion dampers should be able to offgas via the same oil/air interface that they ingest gas. Also, since they were designed to have air & oil co-mingling, they're built to tolerate air/mix in the damping circuits, to a certain extent, & the oil used in these type of dampers have additives to keep foaming to a minimum.

As for why some shocks feel crappy until they get a few cycles, I'd put that down 90% of the time to friction. seals swell at rest, lubricated surfaces dry up, & interfaces at rest adhere to each other. you've got to break that friction & get the lube oil coating the surfaces again when a damper has been sitting for a while.

If a sealed damper is legitimately not functioning until cycled, it's got air bubbles or broken parts inside, & needs service.

 @jcklondon: It is a part of it yes. In the case of fork dampers, open bath dampers run a ton of oil, which means you have a lot more oil to dilute any contaminants that get in there as opposed to the smaller amounts of lubricant used in the lowers in sealed cartridge forks, which vary from 5ml to 50ml. Air shocks do commonly have issues with aerated oil causing poor behaviour until it's mixed in with the oil a bit - this is usually noticeable as an airy squelching sound the first one or two bounces on it, that seem to disappear or reduce after a few bounces.

 Yes and no. Not in the sense of reducing friction as such, since there is no friction involved in bending (or twisting) the coil spring anyway, however in some cases they can help linearise the spring rate slightly near the end of the stroke (in the case of low coil count/high pitch springs like Ti shock springs) and somewhat reduce buckling of the spring. More useful for reducing noise than anything IMO.

 @VorsprungSuspension: Sweet, thanks!

 ..and a stiffer spring rate? like 25 in the rear and 15-20 on the front? full spacer and high spring rate on a rockshox fork does not work for me. at the moment i like 100-105 psi on my lyrik with 0 spacers and just a little lsc. but a note more hsc would be nice. my weight is 85 kilogram. on my x2 shock i like a bit less than 30% with full spacers, open hsc, lsc like u describe, slower lsr and a bit faster hsr. it should not bang u up on a huck, but faster hsr is nice for blowing trough stuff.

 Depends on the degree of rotation the mounts see as to how much effect they can have. Little rotation - use bushings, they're cheaper and lighter. Lots of rotation - bearings start to provide more of an advantage. That said, polymer bushings are sufficiently low friction that you do start splitting hairs here.

 @VorsprungSuspension: Thanks again -- Awesome series!

 Sure am