Suggestion for motor

[Cooperd3]

Hi,

I am looking to build a custom piston that calculates how much pressure needs to be pumped into a shock. I was wondering what type of compact motor I could use? I have been finding some that are outrageously priced or too powerful. I would need to be able to push at least 300 psi. The motor will have to be as small as possible. Any suggestions?

 

[MikeHalloran]

Pistons don't calculate.
Where did the 300 psi come from?
How does the motor have anything to do with the shock or the piston?
What are you trying to do?

You will need to supply a lot more information to get useful help here.


Mike Halloran
Pembroke Pines, FL, USA

 

[Cooperd3]

Autonomous adjustment of shocks in mountain bikes. I want to build my own system that adjusts the pressure in the shocks using accelerometers. The psi is the max amount of psi that can be in a shock for a mountain bike. The piston is going to control the psi in the shock chamber by either releasing or pushing the already existing air back into the shock. The calculations will be down with an on board raspberry pi with accelerometers attached. I have applied for a patent already on the system. I am just a freshman mechanical engineering major and slightly lost on the motor I should use for this system I want to create. Future applications will be for off road vehicles. It should increase efficiency, handling and the overall wear on the user should decrease.

 

[waross]

Linear actuator pushing a small piston. I just bought one for $70 CAD. (about $52 USD). It will push 135 Lbs. The stroke is 8 inches. It runs on 12 Volts. Hook it up to a 3/4" diameter air cylinder.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[MikeHalloran]

Shocks are not charged with air; they are charged with nitrogen.

There is a good reason. When pressurized to 300-ish psi, and loaded with finely dispersed oil mist, and rapidly pressurized, as happens in a mountain bike shock, the air gets confused about whether it's in a shock absorber, or in the combustion chamber of a Diesel engine. ... and every so often, decides to ignite in a Diesel combustion event, which will cause a shock absorber to explode, and attract trial lawyers.

For the same reason, hydraulic accumulators are charged with nitrogen, not air.

I have not found documentation of the event, but I have heard an assertion that a loss of life event at the West Milton, NY site, where submarine reactors were developed in the Fifties, was due to the explosion of a hydraulic accumulator charged with air instead of nitrogen. I assume that any reports were classified just because they were associated with the Navy Nuke program, even though the event itself had nothing to do with nuclear stuff.

So if you are going to add gas to a shock, it should be oxygen-free. There are several ways to do that; have fun exploring that issue.


Additionally, I'm not sure how changing the gas pressure will have a noticeable effect on the shock's performance, unless there is some magic portion of the hydraulic valving that responds to the pressure environment. I suspect shocks were originally pressurized in order to shrink the size of any gas bubbles in the shock oil, and hence reduce 'fade'. You may find appropriate papers at sae.org. Last time I looked, the index to SAE papers wouldn't even fit on one CD. Ask if your school library has an account with SAE.


Third, you may not need a motor. There exist self-leveling shocks that pump oil in and out of the core of a shock absorber in such a way that the shock tries to maintain a fixed length, using a piston separate from the shock piston itself, driven solely by the motion of the shock ends relative to one another. You may be able to adapt something like that to your purposes. Again, sae.org should be a good source of information, if you look hard enough.

Welcome to engineering.




Mike Halloran
Pembroke Pines, FL, USA

 

[Cooperd3]

Thank you very much for all the help.

 

[MikeHalloran]

Beware of using the word 'simple' in an engineering context; it usually means you don't understand the problem.

You need to disassemble some old shocks and do some comparative anatomy, then make a simple shock dynamometer and test your assertions about changing the gas content to affect the shock's behavior. At least read up on the subject.





Mike Halloran
Pembroke Pines, FL, USA

 

[waross]

You may be better served by using a good quality "Off the Shelf" shock. Don't try to modify it. Use the motor to adjust the linkage so as to vary the mechanical advantage of the shock.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[John2025]

I don't know about the nitrogen on a mountain bike, I've got a Trek that comes with a small air pump and a chart that shows rider weight to shock pressure. Some of my wipe-outs have felt pretty explosive, though.

 

[MikeHalloran]

Consumer products are often 'designed' by the cheapest labor available, which usually means inexperienced engineers or experienced drafters. ... until they lose a lawsuit; then they hire some experienced people to help for the duration of the crisis, or go out of business, and reappear under another name.



Mike Halloran
Pembroke Pines, FL, USA

 

[ScottyUK]

John2025,

My Specialized gets the same treatment as yours - shocks get a pumped up with a freely-available 78% nitrogen mix. ;-)

I haven't had any really spectacular getoffs on this bike. The previous one, a Giant, ended up with a broken top tube held together with ty-raps, a branch and gaffer tape to get back to the car. Not my finest moment.