Hello everyone,
I have an interrogation for the suspension neerds among you. While this isn't MX specific, suspension tech is suspension tech regardless and the knowledge in the moto world is much greater than in the Mtb world (specific piston is off a enduro/DH shock, Formula MOD). I also ride (soft)enduro which is why I am frequently here.
So the base valve piston on my rear shock happens to have a very strange profile. When I took it apart I noticed that rather than having the ports covered by the shims, all the ports are connected in what is essentially a circular groove, which is covered by the shim stack. Since a picture is worth a thousand words here is what it looks like:
So this leads me to think that the damping profile should be very digressive by nature. Now Formula provides 3 base valves that can be swaped easily and advertise very different damping curves for each piston. Port sizes varies but they all end-up in that groove so I am unsure of what the point is in the change of port size and number. Stacks are very different so I am wondering if the port arrangement is just to show something to bikers that don't have a clue about stacks. The difference between the 3 pistons is definitely noticeable so it isn't just a gimmick either.
One questions that I was wondering about is, what would be the effect of flipping this piston ? The back face is flat and the design of the part is symmetrical so it is not a problem to flip it around. For a given stack would it actually change the damping profile of not at all ?
It is something I plan on testing if nobody can give me a prediction of what it would achieve but if I can have an answer without having to spend hours testing it out of my ride time that would be better.
A variation of the size and number of holes in a piston (valve) will result in different behavior at higher flow rates of oil. People often use the term "velocity squared". As the flow of oil reaches the speed of sound in the throat of a port, the flow becomes "choked". The speed of sound (mach number = 1.0) in a port for incompressible flows can basically be considered a speed limit. Oil is considered an incompressible flow here. The speed of sound I'm referencing is in your particular oil at the pressures your shock is operating with, not the speed of sound (in air) you'd find if you search google.
Shims are pressure sensitive damping elements. They allow flow when pressure acting on a portion of their area causes them to bend. Variation in the size and position of that groove under the shims will change how the shim initially lifts off the piston. The large groove will allow the oil pressure to act on a larger area of the shim when its closed, resulting in dramatically greater force against the shim at low lift, corresponding to opening easier at low pressure / shaft speed. If you flip that valve so the small holes act against the shim, you'll theoretically increase the shim's resistance to initially opening.
In this particular shock, is there another path for oil to return on the rebound stroke?
Is this a dual adjuster shock or a twin tube shock or some other arrangement?
A lot depends on the layout of the damper system and how it flows oil.
i was aware of the "orifice damping" effect yes, although I thought it was only dependent on overall port surface rather than how that surface is split in terms of number of ports and their diameter. I need to experiment more with Restackor when I have more free time to see how different port arrangement can be at constant surface. I already tried to make the "circular" port but it is not possible in the software, which makes sens as it is probably the only shock to have such port shape.
Your take on groove vs port is interesting. While I was aware of the effect a greater surface area can have, I would also consider the influence of the number of ports. For instance you need a thicker stack for a dual port arrangement vs a 3 or more ports piston because it is easier to bend a shim in half than it is in 3, 4 or 5 bends. Because of this I thought the groove would basically behave as an infinite number of ports, hence creating more damping. Now maybe we are both right ? Could it be that the increase in surface area helps with initial lift which would lower the initial LsC, while the mechanical advantage of the shim being bent everywhere creates more HsC damping ?
The piston is attached to the shaft in the picture making a whole assembly with a LsC bleeder adjuster. There is no check valve on this piston so it is a one way design, the shock head is big enough so I expect a return circuit to be somewhere in there, I haven't opened it further so far (this design allow for reshiming in seconds without having ot open the shock, it is the whole point). Low speed rebound is taken care of by a bleed screw on the main shaft, which is also shimmed to take care of HsR. It also have a lock-out for climbing which close off the feed circuit of the base-valve. Pretty basic mono-tube design really other than the base valve arrangement.
KYB and showa have similar piston port designs and you can model that in restackor, look to the right of the Bvc tab and you will see an example.
The weird part is where is the check for reb stroke?
Rule of thumb is for the base valve in a shock to be about 10-15% of main piston dampening force. That will keep it in the range to combat cavitation and not be excessive. It's better to do the majority of your tuning from the main piston, at least that is my experience with tuning shocks.
I believe your note on the number of ports and shim bending is correct. Across the range of two to five ports with wider port spacing, there's sure to be a significant change in shim bending. At 15 tightly spaced ports, I think it's probably too close to a single continuous port to matter much. Then, add that groove and you probably get shim behavior equivalent to a single continuous port.
There are also edge effects and friction along the "boundary layer" which would be the walls of the ports in this case.
By edge effects I mean the disturbances in the flow as it rounds a corner such as the port edge. Many small ports have more edges than fewer large ports, assuming the total port area is held constant.
There's a phenomena referred to as "no slip boundary condition" in fluid mechanics which refers to the essentially zero flow velocity along the port walls. Looking at a round port you can picture a velocity gradient from zero at a point on the wall up to maximum flow velocity at the center of the port and back to zero at the opposite wall. The surface area of port wall is increased when your valve has many small ports compared to a valve with fewer large ports, assuming both designs have equal total port area.
I have read that Van der Waals force plays a significant role in the initial resistance of the shim to opening. So the surface area of the piston and shim that are in contact plays a double role there. That groove should hypothetically reduce Van der Waals force too.
I've attached a photo of some vintage KYB base valves below (circa late 1990s early 2000s). The dark gray is a KYB OEM valve, you can see that they have minimal shim contact surface area, even though the port is a tiny hole. The brass valve is a replacement from RaceTech with a larger port flow area and similar shim contact area. Race Tech's philosophy seems to be to use larger ports and get most or all of the damping from shims. The smaller light gray valve has a much larger surface area in contact with the shim, so it would have more Van der Waals force and less active port area to initially lift the shim than the other two designs. The large diameter valve is an aftermarket valve for a Showa 49mm A-Kit fork. It has a lot of finely detailed design elements to reduce contact surface and probably edge effects while trying to prevent shims from being deformed on the three port design.
Cdubya's suggestion to check out Restackor seems like a good one to me.
I want to try out that software on some KYB shock settings to learn from it... It seems very useful for comparing settings if you give the software accurate data.
The Shop
I wonder if the small plentiful ports are to fill the groove more evenly. Flow rate of the diameter of the hole versus how far the oil has to travel between holes to fill the groove?
Cwtoyota, it's obvious you have knowledge about suspension. I don't want to highjack this thread. Is it okay if I post a thread requesting your input about some old school mini forks? Thank you.
I studied mechanical engineering (fluid mechanics and aerodynamics) and I am am always trying to learn more about suspension so I do a lot of reading and experimentation. I sell a suspension product and help some local guys with valving & tuning, but I am far from being a highly experienced valving guru...
Post your question and I'll give you my best answer, even if that answer is "I don't know"... PM me a link so I don't miss the thread.
My assumption is that you use smaller shims between piston and face shim to set preload (or create some bleed).
That design allows a lot of tuning range. Clever.
Absolutely. We do tune mid as well, but the base becomes the focus for us to create a usable profile which holds up nicely in the stroke, with low speed control, and higher speed compliance. I've been more into my toyota project lately, but still getting some sets here and there to do.
The juntion between the body and the piggyback (shock's head) is pretty subsential and would easily accomodate for a return circuit with a check valve so the oil return would be ensured, rebound being taken care on the main piston like any normal monotube design.
i believe the influence of Basevalve is mostly dictated by the body/shaft diameter ratio. The formula Mod as a rather thick shaft compared to most bike shocks and so the ratio is in favor of more base valve damping options. The reduces the risk of harshness due to mid-valve damping. Keep in mind Mtb has virtually no sprung weight so too much LsC and overall compression can very quickly become a problem. Though nowadays most shocks on the market use a mid-valve damping. The Mod is an exception with the DVO Jade and previously the Fox RC4 which both have a very low ratio of shaft/body diameter.
I just checked the number of port and sizes for the softest and mid base valves and they are respectively 15x1.4mm and 12x1.2mm. Groove size and profile is identical. The hardest mid valve has less holes and I suspect those will be less than 1.2mm which would explain the compression profile on their graph, with a kind due to orifice size starting to throttle the flow. That very scientific chart is all we get from Formula, very precise and scientific lol, Marketing > that hard data right ?
Sounds like this shock works the way I understand an older ohlins TTX to? A solid piston on the shock shaft and all of the valving is done on the adjuster stacks? In this case I would figure the port size/qty limits high flow (large hits) and that lets you use a softer stack for a more plush ride through the small stuff without blowing through the stroke?
I think a piston of the same design with bigger ports would need a slightly stiffer stack to maintain the same lsc but would then be softer on the big hits? Or if bottoming is an issue would smaller/fewer ports with a slightly softer stack again maintain lsc but not blow through the travel as fast due to the piston restriction?
Seems to me that piston would let you pretty well keep the hsc and lsc separate as far as tuning goes - works good on big landings but rattles your teeth on small stuff? Just soften the stack. Works good on the trail but bottoms hard enough to collapse your spine on big hits? Restrict the piston ports
I have nothing to base this on other than thinking alone in my shop and trying to learn about suspension. Travis
Yeah exactly thats typical garbage marketing pic. The base valve is dependent on shaft diameter only, body size doesn't matter. You can have smaller shaft bigger body or vise versa It all comes out similar as long as valves and shim stacks are taken into consideration. On KYB shocks there are a couple different base valves, for a 16mm shaft currently you will find 6 2.15mm holes and for the 18mm shaft there are 6 3mm holes. That valve is used for cavitation control as well.
That is indeed a nice easy way to change the dampening, kinda like a step above the external clicker adjustments but I'd bet if you want some bigger changes it'd be best to make them on the main piston.
I was about to comment the same about damping vs shaft diameter...
Body size doesn't matter unless it's a twin tube shock.
It does tho, that's basic physics. Say you take a 10mm shaft that will travel 100mm. Put it in a 11mm diameter body and your oil level will basically rise of 90mm or something like that which will create a lot of flow through your base valve at the top of the body. Do the same with a 20mm body and it will flow less than half of what you got in first place. Now total volume is the same, but flow isn't and having more flow helps to manage damping and have setting that make more noticeable differences. At least this was the theory of Fox and DVO (formerly Marzo guys which is a brand you'd know) when they are/were building shocks with they damping mostly focused on the BV. I think I also read something along those lines on Restackor blog page but don't quote me on this.
Think in terms of displaced volume, not velocity or oil level.
We consider oil to be incompressible, so conservation of mass and volume both apply here.
Let's take a 1000mL graduated cylinder marked in mL (or cc) with 500mL of oil in it.
In your hand is a perfect steel cube that measures 10mm on each side (it is 1mL or 1cc).
Drop that cube into the graduated cylinder and the oil level will rise to 510mL (510cc).
Now take a 100mL graduated cylinder marked in mL (or CC) with 50mL of oil in it.
Drop that cube into the graduated cylinder and the oil level will rise to 60mL (60cc).
In both cases, (large cylinder and small cylinder) the oil level only increases by 10mL (10cc).
That 10cc is the displaced oil volume of the steel cube.
Do not take my word for it. Get out a science textbook or do this experiment using water with a large and small measuring cup as long as both have accurate and equivalent volumetric units marked on them. You can also replace the cube with a 16mm shaft marked off in 10mm (1cm) increments.
The shock shaft displaces the same amount of oil independent of the shock body diameter or the type of shock.
If you push 10mm of stroke with a 16mm diameter shaft into the shock body, you displace a volume equal to the shock shaft and stroke out of the shock body.
V = π*r²*h Where r [cm] = D/2 and h [cm] = shock stroke.
V = 3.14159 * (1.6cm / 2)² * 1.0cm = 2.0106cm³ ≈ 2.0mL
There are at least two types of dampers which could be considered exceptions to this.
1) Emulsion shocks mix the oil and a compressible gas (nitrogen) so technically the working fluid (an emulsion) must be considered compressible. In this case, compression occurs instead of displacement.
2) Balanced cylinder dampers (common in steering dampers and steering actuators) have a seal head on each end of the main body bore with a continuous shaft that travels through both, effectively giving zero displaced volume for the extent of the useful stroke. (I attached a photo of a balanced cylinder, note the lack of a reservoir).
Twin tube shocks are in their own category here... They have a reservoir because the shock shaft displaces oil volume as I described above, but the flow through the base valve is more complicated and not easy to calculate except when the main body piston is solid.
Great demonstration but you basically just explained what I said with the proper math behind it while I didn't believe it was necessary to illustrate my point. Read again, I will quote myself "Now total volume is the same, but flow isn't". So yeah I agree with you.
Now what I have a hard time understanding, is how/why the difference of flow should be dismissed. My (limited) understand of damping is that we try to control the flow of oil through a port for a given impact. More flow at a specific piston (BV or MV) would give a wider range of speed through the range of possible impact which would give more room for tuning. Said another way, you would need to add/remove less shims at the BV to achieve similar results with a big shaft small body than if you were to use a small shaft with a big body.I don't deny you could achieve the same result with a high ratio than with a low ratio, I just said it would be easier since you would get more flow at the base valve.
Also I remember reading somewhere than thicker shaft/smaller body was a good solution to reduce the effect of mid-valve as the flow is reduced which makes it easier to not choque and become harsh, especially when using the MV as a check valve rather than it being used to create damping. Which leads back to damping being created at the BV more than the MV with smaller body for a give shaft. But then it could all be BS that I gathered on different forums of marketing documentation, I just don't know any better.
Now if you are willing to explain to me why flow is not an important data I am all ear as I'd really like to understand more about suspension in general so all added education is welcomed.
Your confusing flow through the base valve vs the mid valve. A bigger body/main piston would have more oil flow through the mid piston but the base valve is pretty much shaft diameter dependent.
Pit Row
If the ticker shaft is displacing oil with more velocity as the piston moves along, wouldn't that also reduce the relative speed of the oil vs piston ? The way that if you drive 50kmh with a 40kmh tail wind, the relative wind speed you are experiencing is 10kmh.
It's correct what you remember, a bigger body will have more dampening through the active (mid) valve and will have more control. In SX it's better to have a bigger diameter cartridge. An example is PC A kit Showa forks have a 27mm cartridge vs stock (25mm) cartridge. But in most cases you can achieve dampening forces that are sufficient for most applications through modifying the shims stacks but SX is a special case.
Post a reply to: The influence of base valve piston profile in the damping curve