Hey! 
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I know a few DH riders that replaces their roadside bushing with metal washers. Stable as heck but you’d be lucky to make a left hand turn on a 4 lane road
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I’m not really ‘design a working piston’ good im more good at crappy robotics.
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Ain’t that the mood my friend 
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So looks like Cad is like riding a bike.
If anyone wants to help with the modeling of this, I can send a Grab Cad link
There are still many details to be hammered ou (note the lack of threads due to thin wall)
But let us math first
AS for the forces we know we need at most 4K lbs on the bushing.
We know that we have RC linear actuators that can do ~ 112 lbs (500N)
So we get the 50mm travel with 20mm per second (loose to tight in 2.5 sec)
That leaves us with a Master:Slave area ratio design of about 35:1. ( Do I have that the right way @BluPenguin?) How does the travel distance figure in?
Ball park Values Collected so far:
- Force required by Ring Piston : no more than 4000 lbs
-Riptide Krank bushings to be used - Bushing compression delta length ~ 10mm
- Master cylinder travel ~ 50 mm
- fittings / hose to be used (will likely be these)
Few things to be engineered:
- Area of slave piston (sits at 0.434 square inches in this draft)
- Area of master cylinder , this will be an off the shelf hydraulic piston. (15 square inches?? cant be right , I asume this is where the travel length works in)
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Am I right in guessing that the volume remains the same?
So area x travel length is a cosntant for master and slave?
And the force multiplier I want, forces me to select a certain travel length ratio and do an inverted area ratio?
You have it backwards, the slave piston is the bigger one. The master is trading travel length for surface area. But the rest of it looks good! Although those walls look way too thin. By my rough calculation for aluminum, the outer walls needs to be about 5mm thick and the inner wall about 3-4mm to hold pressure without deflecting to the point of possible leaking.
If we are using steel then can remove roughly 30% thickness. The more material we can fit the better the longevity (need to account for the part being close to the ground and getting scratched up, so need to use more than the bare minimum thickness.)
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Im a big fan of ballparks. Ill use those values.
So that makes the master cylinder area very small in area, but needs a longer travel lenght?
L= travel length
A = Area
units in mm from now on because mixing the two is apaaaiiin
So Bushing force is now 17.8 K Newtons.
Linear Actuator Force is 500 N
Gives us a ration of ~35:1
So:
Slave is more or less fixed at : 10mm = L , A = 280 mm2
Factor is : 35~
so
Master needs to have 350 mm = L (10*35) and 8 mm^2 (280/35)
Is that correct? The actual values will change now. Sounds like I want to make the Slave Area much larger , since a master cylinder with an area of 8 mm^2 is going to be hard to achieve haha
Not quite. 8mm plunger is perfectly reasonable for this. I imagine the master plunger will be a steel rod, perhaps 2 seals. I think 5mm of travel for the slave is enough. The entire piston ring depth needs not be more than 15mm, thus the outer sleeve perhaps 22-25mm max (accounting for a ring of reinforcement material where the kingpin nut will press against)
Hmm ok. Thanks for the tips
About travel I think youre right.
Krank bushings are good for 2 turns past snub. at thread to thread distance of 1.75 (M12 course ) or 2 mm (1/2" coarse) I dont actually know which it is
2 full turns of 2mm each turn is 4 mm , so 5 mm is reasonable
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5mm is kind of an overestimation, realistically I think we could remove up to 5mm of thickness total, a 10mm piston with a 17-20mm body… Anywhere in that range would be capable of fully compressing the bushing to the point of unusable. (keep in mind the bushing is going to have pre-compression already even before the piston extends)
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I dont think its that far overboard. At teh very least its a good design constraint, and we can always adjust things down the line.
As far as the stack height goes, 10 mm seems low if we plan on using 5 of that fully extended. My thoughts are that we design teh sleeve based on teh piston. and teh piston based on the O ring spacing. Back from my break caliber rebuil days, Id guestimate, a 15mm piston with rings spaced as follows.
2mm- ring(2mm thick) - 2 mm space- ring(2mm thick) - 5mm swept wall ~= 13mm piston
I think its better for the piston to be just a ring of polished steel and the o rings be in glands on the body. The ring is going to see some side loads from the bushings flexing, and pushing against a steel washer. 1:3 extension is a good rule of thumb so 5mm extension would give a 15mm piston
this would require machining grooves on teh interior of the sleeve
Yeah, fairly simple on a lathe, plunge cut and groove it with an internal boring bar.
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I don’t machine anything myself
Outsource it
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To whom?
taking @BluPenguin’s word for it that its doable, Ill cad up an alpha draft and 3D print it first.
Its a secret
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It’s for sure doable. Look at the photo of the brake caliper I posted previously. I think on almost all brake calipers, the ring is in the body not the piston
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