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DIY Shock: Rubber vs Hydraulic Damping + Information Help 1

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ChrisDanger

ChrisDanger

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May 15, 2012
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I'm helping a friend design a coilover shock for a wheelchair for his nephew. He suggested using an o-ring or seal friction damper, but after not finding much information to aid in design I wondered if using hydraulics would be viable, but I couldn't find much information on that either. I'm wondering two things: Which would be cheaper in the long term, and where can I find information about design details? He works in a machine shop and will be fabricating everything himself.

The way I see it, a rubber damper might suffer from wear and need friction elements replaced fairly frequently (also giving a declining performance), whereas the hydraulic damper might be (slightly?) more expensive to produce but last longer between maintenance schedules (and give more uniform performance over time).

Both designs have stumbling blocks. I can't find any guidelines on gland dimensions, seal material, or required surface finish for any friction dampers. And for the hydraulic shock how is oil inserted and topped up? I imagine some kind of valve but again, I couldn't find any information on this either.

Preliminary designs are below for reference.

Elastomeric:
URL]


Hydraulic:
URL]
 
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In the top one, are you saying the damping will be from the friction of the rubber O-rings against teh internal walls of the cylinder? Does seem like you may have wear issues. It will also be acting as a pneumatic spring won't it - unless you have a relief valve I don't see.

If you're going to use rubber, why not use the rubber as both the spring & the damper? As I recall the DeHaviland Mosquito used a stack of O-rings as it's suspension. Googling 'rubber spring suspension' got me some hits that explained the idea.

Posting guidelines faq731-376 (probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​
 
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ChrisDanger,

A lot of commercial and military anti-vibration mounts are stuffed with wire mesh to provide damping.

How badly do you need damping on this thing, or an automotive style suspension? Wheelchairs do not go very fast. If I were designing one, I would go for sturdy, light weight, stable and cheap. I don't see a suspension helping with this.

--
JHG
 
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In the first drawing, you would probably get some leakage past the triple o-rings, thus a pneumatic damper. If you deliberately created a leak path for the air (via an orifice, or cut gaps in the o-rings) you could actually control/tune the amount of damping, just like you could by sizing the orifice(s) in the hydraulic fluid version's piston.
 
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Hi KENAT. Thanks, yeah, I should've added some kind of pneumatic relief to the design.

Greg, I was thinking of just filling with oil and screwing in the plug but wasn't sure if an excess pressure would damage the seal if it was overfilled. I'll look at putting a valve in, and at mountain bike shocks for ideas. Thanks. (PS Thought I recognised your name from f1t. Searched and first result was a thread on dampers/shocks [smile])

drawoh, I've used a wheelchair without suspension and it's not fun. I wouldn't want to subject the little guy to any more discomfort than is possible.

Thanks for your help guys. [thumbsup2]

EDIT: btrueblood, yeah, unintentional design flaw there. Damping was supposed to be from friction between the triple rings and inner cylinder surface.
 
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You should be able to find a coil over damper to suit your application at far lower cost and fewer tears than DIY. Mountain bike sounds like a great starting place.

je suis charlie
 
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My dayjob is currently in medical equipment, so I deal with a lot of these parts and components. Presuming that your friend's wheelchair is electric, have you absolutely verified that there is no suspension currently in the chassis? Many (most) chairs do have some suspension. Get ahold of the manual, and barring that, talk to the technician at the dealer. There are also some really good "flotation" cushions available to reduce pressure concentration, ROHO being one of the better ones.



It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.
 
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Hi gruntguru. Yeah, we're looking at maybe producing a few of these for the other kids at the school, so it may not be a 1-off. My friend is keen to make everything himself for that custom look. He only has to pay for material, and tears we can handle. Hopefully we get it more or less right first time, although clearly at this stage we don't really know what we're doing. I have plenty of free time though, and am eager to apply some of my engineering knowledge.

Hi ornerynorsk. We're fabricating this chair (manual, not electric) totally from scratch. Thanks for the advice though. Will pass the ROHO recommendation on.
 
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You can buy an off-the-shelf MTB shock for less than $50. Considering what's required to make a shock that actually works (high-end materials, hardened shaft with very fine chroming, intricately machined parts with fine surface finishes, etc) I doubt your buddy will be able to beat that, even only paying for materials.

A shock designed for a mountain bike, if used on a wheelchair, is going to last probably until the end of time. They are low maintenance as it is, and a shock on a mountain bike takes a LOT of abuse.

If you want to take it on as a design exercise then that's fun, but for cost and performance you won't be able to beat something off the shelf.
 
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Will this damper be mounted shaft ~ vertical?
When the damper compresses, the volume of the shaft reduces the space within the shock.
If the shock is full of oil the damper won't be able to compress at all.
Real shocks provide a volume of gas to compress so the displaced oil has some place to go when the shock is compressed/shortened.



A few include a volume of gas that was intended to mix with the oil and form an emulsion, and the valving in the shock was designed to perform with the emulsion, not "pure" oil.
British manufacturer Girling offered an emulsion shock for off road motor cycles.
 
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Riiiiiight. I did see the gas-filled chamber but I thought this was to regulate oil pressure to prevent cavitation which I figured wouldn't be a problem in this application. So that shock will not work. That's quite a serious oversight, thanks!

I'm starting to lean towards buying one. They're not so cheap or available here in South Africa but I'll see what I can do.
 
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Anything on Bid or Buy? Any local bike shops? Ebay (US) has scads of them, most shipping from China at fairly reasonable prices. Just a thought. Best wishes on the project!

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.
 
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Interesting, thanks Tmoose.

Hi ornerynorsk. Nothing on Bid or Buy. Found some places in China but trying to get prices and see if they'll do small volumes.
 
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