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Techie question for the suspension guru's
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thisisnotaspoonFree Member
When an orrifice plate spikes is it;
a) transitioning form laminar to turbulent
b) reaching its sonic velocity
c) cavitatingFresh Goods Friday 696: The Middling Edition
Latest Singletrack VideosFresh Goods Friday 696: The Middlin...geetee1972Free MemberAre you referring to inertial (or transient) cavitation, or noninertial cavitation?
clubberFree Membersuspension guru's what?
orifice plate (one r)
it's b).
And the term is choking really
thisisnotaspoonFree Memberserious question, just thinking back to some research I did at uni
laminar flow is what you imagine the flow in a pipe to be (in distinct streams, the ones at the edge moving slowly, the one right at the edge is theoreticaly stationary)
turbulent flow is when the inertial forces are larger than viscous forces, so you get turbulence, the boundary layer at the edge thins and the velovity profile accross the pipe is largely flat.
Theres a step change in the flow when you go from one to the other, which would give a spike.
You can't make something flow faster than the speed of sound (well not very easily)
And you cant pull a vacumn sufficient to boil the liquid (without creating a big pressure drop and erroding away the restriction)
thisisnotaspoonFree MemberTa, the talk the other day about adding electronics to dampers got me thinking, and combining two bits of research I did at uni, recon I might have come up with something quite interesting……………….
coffeekingFree MemberDepends on exactly what you're asking about I would have thought, but I would have thought you were talking about choking as clubber suggests?
LoCoFree Memberhttp://en.wikipedia.org/wiki/Choked_flow
for the laymen in STW, quick scan read and it seems right.
jemimaFree MemberI don't think sonic choking is an issue in terms of dampers. Wouldn't worry about Reynolds numbers either.
I think the key is that an orifice is a quadratic damper. I.e the pressure drop over an orifice (and thus resistance to motion) is proportional to the square of the flow velocity. Therefore, in the context of a MTB shock I would say 'spiking' is when you get a really big hit thus giving very large shock displacement and velocity and in turn fluid velocity through the orifice. Therefore the resistance to motion goes right up and is felt as the shock stiffening (usually when you want it most compliant).
Can I be a suspension guru now? Do I pass?
coffeekingFree Memberjemima does point out the obvious very well (while we're all hunting for more complex explanations!).
jemimaFree MemberIts not obvious if you don't know! And just because an explanation is more complex doesn't mean its right! By way of further reasoning: I don't know what the sonic velocity in oil is but I'm guessing greater than 1000 m/s. Can't really see dampers porting shock oil at these velocities…
LoCoFree MemberJust seen this which refers to up to:
'a racecar that's running over harsh rumble strips tops out with shaft speeds of around 13 metres per second'
http://autospeed.com/cms/title_Damper-Dyno/A_108385/article.html
Push/Tf always said fox forks spike, and market their kits/tunes as solving that problem.
If your thinking of developing something do a bit of research in the motorcycle industry as most things have been tried at one point or another if they haven't there's probably a reason why.
However you may have something!thisisnotaspoonFree MemberI was guessing closer to 400m/s (i.e comparable to water/solids)
Assuming a 3mm circular orifice, and a 28mm internal diameter, you'd hit 400m/s at about 4.6m/s which sounds like the right ballpark for a suspension fork to be compressing?
LoCoFree MemberThis is annoying can't find any specifics in terms of logged data from motorcycles to give definite answer on shaft speed, obviously a MX will be travelling quicker than an MTB but would give a better comparison than a race car.
Anybody know a Motogp/superbike technician?jemimaFree Memberspeed of sound link
OK – quick bit of googling gives speed of sound in oils of >1400 m/s. I'd be very surprised if these shocks are designed to be porting fluid at those speeds.thisisnotaspoonFree MemberHmmm,
Its given me two thoughts.
1) My magura menja's desperately need a shim in the rebound damper
2) I recon its possible to build an electronic version of the specialized/fox brain/terralogic damperI predict some tinkering………….
thisisnotaspoonFree Member1400m/s through a 3mm port in a 28mm piston is a piston speed of 16m/s.
does kind of explain why you cant remove spiking with a thinner oil (similar sonic velocity, lower viscocity)
clubberFree MemberThe cannondale idea on this (Simon?) sounded like the right way to go about it – very basic (basically just a hole with a partial blockage electronically controlled – but I can't see why it shouldn't work. I guess that ideally you'd make the hole quite big and then use the solenoid to control the size so that at full open it'd never 'spike' though technically I guess exponentially more damping force related to speed is what we're really after otherwise you'd just use a simple spring.
thisisnotaspoonFree Memberhaha, we'll see, I recon it's possible, but maybe beyond my shed tinkering abilities.
Anyone at magura reading this want to loan me a load of internals for some magura menja's to play with?
coffeekingFree MemberIts not obvious if you don't know! And just because an explanation is more complex doesn't mean its right! By way of further reasoning: I don't know what the sonic velocity in oil is but I'm guessing greater than 1000 m/s. Can't really see dampers porting shock oil at these velocities…
Wasn't intending it as an insult, I was pointing out that (I specifically) had overlooked the basic properties of OP damping and jumped to more complex explanations when yours was clearly the better answer and obvious when you're not looking for more complex ones!
clubberFree Memberhttp://www.cannondale.com/none/none/simon
I reckon that in theory it's not that complex but you're right, it's probably beyond most people's shed tinkering.
The control software and power requirements are probably the big headaches.
thisisnotaspoonFree Membermost dampers are designed to give a falling damping rate (i.e. if you draw a line with speed Vs force you get a steep line (the harder/faster you push at low speeds the harder it pushes back) which transitions to a shallow line (at high speeds you want to shift as much oil as possible) unlike an orifice which gives a shallow line going to steep.
clubberFree MemberI've always wondered about that though – if you're taking a big hit then it's usually going to be at high speed so you want to use all the travel – while you don't want to spike the shock, you do want to stiffen it up to prevent bottoming at speed I reckon.
thisisnotaspoonFree Memberposition sensitive and speed sensitive damers are different.
Take manitou TPC+
It has a normal compression system, then another piston that doesnt kick in untill the end, so you get the performance you want/need 90% of the time in the first 2/3 of the travel, then a second damper kicks in to soften the bottoming out.
You could also use an air spring (air shocks) or controll the volume the oil is displacing into (fox DHX-5, manitou swinger, 5th etc) but thats fixed most of the time (new Van-R, vanilla RC/TC, float, old vanilla R)
jemimaFree Membercoffeeking – I didn't take it as an insult 🙂
Regarding damper (i.e. motion control/RP23 system) rates versus orifice plate damping rates you have to remember that the system damping rate is affected by numerous orifices and shim stacks (springs) which change the port area according to the velocity input. The actual damping is still imparted by oil flowing through holes which are governed by the orifice equation. The force-velocity curve shape and gradient is tunable in a damping system by varying various settings and geometries which is why people get their shocks pushed I guess.
So, I guess there are two ways to make a smart shock. One is to vary the actual orifice areas the other is to vary the force required to open extra orifices…
clubberFree Memberposition sensitive and speed sensitive damers are different.
Yeah, I understand that and I think I could have worded my post better as I wasn't talking about position related damping (since it's still relatively uncommon in bikes) and my point is that unless it seems logical to me to actually have a rising rate (relative to speed) right through the movement – maybe with a flatter curve at medium speeds to allow for better absorbtion of medium sized hits (achieved through porting via shims as detailed by jemima).
philjuniorFree MemberI'd suggest that getting the fork to react quick enough to avoid a spike would probably be accomplished far more easily with the use of more complicated blow off valves (maybe opening bigger and bigger orifices to avoid spiking – all that I've pulled apart have only gone from one orifice size to another at a given force).
It also seems more logical to me to control the opening of the valve with a preload, as this allows you to pick a force rather than having to have electronics react to something. Any decent fork should have something like this already, so your best option would probably be to first make sure this is working, and make sure spiking is your problem, then investigate making sure that the "blow off" orifice is big enough not to cause a problem on anything that's not going to bottom out the fork incredibly hard anyway… I can't really see why you'd want to vary the force required to open the extra orifice on the fly electronically, although if you did I'm sure it could be accomplished reasonably easily (if you're good at making oil-proof electronics).
clubberFree MemberI think you're thinking too much about adding electronics to an existing design of damper there phil. If you're going to add an electronically designed control then I reckon you'd want a damper that would do very little on it's own – eg a fairly simple piston with relatively large sized hole that would provide next to no damping on its own. You can then completely control how the damper works depending on how you set up your system control – eg you could have position dependent damping combined with speed dependent in any combination pretty much.
For example, if you're riding over a rutted trail (shallow ruts, smaller than the bike's travel), you don't really want any compression damping except maybe to damp out pedalling effects, so you run with the port wide open (at least on the compression stroke) and you get excellent suspension for that type of terrain.
A little further, the ruts become worse – bigger than the bike's travel – this means that you can't fully absorb the bumps so you can use compression damping to ensure that it's absorbed as smoothly as possible (eg hitting full travel at the top of the rut).
And of course if you really wanted to take it to the limit, you could get rid of the spring entirely (a la F1 active suspension c1992 IIRC) and control the whole suspension through oil flow alone though you'd need some sort of pressurised system for this to work. Imagine though – you head down a steep hill and the fork automatically adjusts to keep sag the same rather than diving. Same for braking and so on…
OK, all getting a bit tomorrow's world but…
philjuniorFree MemberYeah that sounds good, but I was thinking about what would be realistic to achieve "in the shed" – I can't imagine (having worked on fuel injectors a couple of years ago) getting anything like the speed and power needed without a fair old wedge of electronics (like at least getting on for bike bottle sized) plus a fair old battery.
I guess it's possible, but you'd have to be quite clever with how the valves actuated etc., and you'd probably still want a blow off for when the electronics thought you wanted compression damping but in fact you were spiking on something.
As for the active suspension, I remember reading something about using that on a WRC car – apparently there was a prototype, but it used about 80bhp… Still allowed the car to go faster down a special stage, but not really feasible on a push bike (even if you assume that as the weight is a lot less the power would be a lot less you're still looking at strapping a big lawnmower engine on to power your suspension!).
I sound way too negative. Give it a go and let us know what happens…
clubberFree MemberI think that most of this is going to be tricky in the shed (that's a proper shed, not one equipped with mills, lathes and other such proper manufacturing equipment…)
I guess it's possible, but you'd have to be quite clever with how the valves actuated etc., and you'd probably still want a blow off for when the electronics thought you wanted compression damping but in fact you were spiking on something.
Possibly but in theory if you can get the positional sampling (and any other telemetry you'd need) fast enough and the control similarly quick, then it shouldn't ever really be necessary.
thisisnotaspoonFree MemberHmmm,
few interesting thoughts there, I think I'm thinking about it in a much simpler way to most of you, there are no constantly variable orifices or anything like that in my head.
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