DIY Shock: Rubber vs Hydraulic Damping + Information Help
DIY Shock: Rubber vs Hydraulic Damping + Information Help
(OP)
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:

Hydraulic:
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:

Hydraulic:





RE: DIY Shock: Rubber vs Hydraulic Damping + Information Help
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.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: DIY Shock: Rubber vs Hydraulic Damping + Information Help
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: DIY Shock: Rubber vs Hydraulic Damping + Information Help
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
RE: DIY Shock: Rubber vs Hydraulic Damping + Information Help
RE: DIY Shock: Rubber vs Hydraulic Damping + Information Help
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
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.
EDIT: btrueblood, yeah, unintentional design flaw there. Damping was supposed to be from friction between the triple rings and inner cylinder surface.
RE: DIY Shock: Rubber vs Hydraulic Damping + Information Help
I think rubber o-rings would not last very long.
RE: DIY Shock: Rubber vs Hydraulic Damping + Information Help
RE: DIY Shock: Rubber vs Hydraulic Damping + Information Help
je suis charlie
RE: DIY Shock: Rubber vs Hydraulic Damping + Information Help
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.
RE: DIY Shock: Rubber vs Hydraulic Damping + Information Help
Hi ornerynorsk. We're fabricating this chair (manual, not electric) totally from scratch. Thanks for the advice though. Will pass the ROHO recommendation on.
RE: DIY Shock: Rubber vs Hydraulic Damping + Information Help
This http://softwheel.technology/theacrobat/ may also be of interest.
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.
RE: DIY Shock: Rubber vs Hydraulic Damping + Information Help
RE: DIY Shock: Rubber vs Hydraulic Damping + Information Help
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.
RE: DIY Shock: Rubber vs Hydraulic Damping + Information Help
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.
https://upload.wikimedia.org/wikipedia/commons/thu...
http://1.bp.blogspot.com/-Y7cwzj1-4Rg/T3mKrzT1CiI/...
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.
RE: DIY Shock: Rubber vs Hydraulic Damping + Information Help
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.
RE: DIY Shock: Rubber vs Hydraulic Damping + Information Help
And some space is still needed for thermal expansion of the oil.
http://www.ducati.ms/forums/attachments/apparel-ac...
RE: DIY Shock: Rubber vs Hydraulic Damping + Information Help
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.
RE: DIY Shock: Rubber vs Hydraulic Damping + Information Help
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.
RE: DIY Shock: Rubber vs Hydraulic Damping + Information Help
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: DIY Shock: Rubber vs Hydraulic Damping + Information Help
RE: DIY Shock: Rubber vs Hydraulic Damping + Information Help
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: DIY Shock: Rubber vs Hydraulic Damping + Information Help
Maybe take a peek at a truck drivers chair to see what they use.
Also these guys. They do custom units. http://timbren.com/aeon-springs/
Depends on what your deflection and load requirements are.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: DIY Shock: Rubber vs Hydraulic Damping + Information Help
I've got a write up on this from my Ariel Atom post that I will add.