# Alternatives to shock pump?



## OrngRubberDucky (Mar 27, 2009)

I was trying to set the sag on my bike letting a little air out at a time. Well I ended up going form not much change to too much. Can I use an air compressor with the regulator set at low pressure? Or is a shock pump a must?


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## Jerk_Chicken (Oct 13, 2005)

Shock pump is a must. You're looking at high pressure, low volume. Compressors tend to run too much volume, and that leads to bad things sometimes.


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## mlepito (May 1, 2007)

+1. You must use a shock pump, don't let anyone tell you different. You will ruin your shock.


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## highdelll (Oct 3, 2008)

mlepito said:


> +1. You must use a shock pump, don't let anyone tell you different. You will ruin your shock.


WHY? Trust me, I'm not arguing w/ you...I'm just curious.
I thought PSI was PSI and atmospheres were atmospheres. I understand that shocks and forks are low volume, but if you have, say a max output of say 50 PSI on a compressor, and your shock can handle 75 PSI, there is no way for the shock pressure to go above 50 PSI because that is the max potential of the compressor.
The reason I thought shock pumps were necessary were limitations of a conventional pump - not the limitation of the shock.

Can ya clue me in?


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## belowambient (May 17, 2008)

the first time you use a shock pump you will see what we are talking about


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## mlepito (May 1, 2007)

There is no way to move that small amount of air with a compressor. Belowambient is right. When you use the shock pump you will realize the amount of air going into the shock is this tiny little squirt, but will increase the psi by as much as 5 or 10 depending on the model.


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## highdelll (Oct 3, 2008)

Dude, I use a shock pump - a compressor isn't practical for getting the pressure precise. I was talking about the physics of it all - I don't see how you could ruin a shock. Please don't make assumptions about what I do or do not do.


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## millertime1025 (Apr 12, 2007)

I can't really comment on why the compressor won't work. But make sure you disconnect the pump before sitting on your bike to determine the sag. And also remember that the pressure will decrease by a little after you disconnect the pump. 

That's all i can think of.


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## mlepito (May 1, 2007)

That's what I was telling you. The amount of air the compressor will push is too much. It's not just that it isn't precise, it's like threading a needle by putting it on the front of your car.


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## JDM (May 2, 2008)

Your compressor probably doesn't go above 125psi. I doubt that 125 psi would damage any shock. In fact, it may not be enough - I run 190 psi in my rear shock. You could try the compressor in a pinch, but it will be a hassle. it is absolutely worth spending $30 on a shock pump.


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## kapusta (Jan 17, 2004)

mlepito said:


> There is no way to move that small amount of air with a compressor. Belowambient is right. When you use the shock pump you will realize the amount of air going into the shock is this tiny little squirt, but will increase the psi by as much as 5 or 10 depending on the model.


If you have the pressure regulator set for a certain amount, of course you can move that little air. It's just going to pressurize the shock to whatever the regulator is set for. It won't pressurize it beyond that.

I would question whether it will be as accurate as you need it to be, but with a pressure regulator it won't over-pressurize it.

I think an issue would also be getting the right air valve to screw onto the shock.

I'd just use a shock pump.


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## CupOfJava (Dec 1, 2008)

Why is this thread still going on? The first answer was the correct one.


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## kapusta (Jan 17, 2004)

CupOfJava said:


> Why is this thread still going on? The first answer was the correct one.


No, it is not.

The level of misunderstanding and mininformation in this thread is pretty staggering.

Highdell is correct in his theory.

People need to understand that an air compressor (at least any one that I have used) is not just an air pump hooked up to a hose. If it were that simple, the comments here about pushing too much volume would somewhat more correct.

An air compressor has two gauges. One is on the tank and tells the compressor motor (air pump) when to turn on and off. For example, it may kick on when the pressure drops below 100, and turn off when the pressure is 150. So in this case the tank is always between 100 and 150. The _pressure regulator_ is between the pressurized tank and the hose. It lets air into the hose until the pressure in the hose reaches a certain amount. Obviously, it can't pressurize the hose beyond the pressure in the tank, so in this case the regulator would need to be set at or below 100. It can let VERY small amounts of air in. If you have ever used an air powered brad nailer, you can get an idea of this when you hook it up to the hose and it fills up the inside of the nailer.

The air pump on the compressor is not cramming the air straight into the shock. It fills the tank which supplies the pressure to the regulator. The regulator less just enough air through in order for the hose to reach a certain pressure.

If anyone does not believe a regulator can do this, I use CO2 in my lab to knock out mosquitoes. The tank is under extremely high pressure (I don't remember off hand how much, but many hundreds of pounds). I can set the regulator so that blow just enough gas on them to knock them out, but not move them from about half an inch away. That is a VERY gentile breeze.

Yes, hand shock pumps are very low volume, and that is for two reasons. One, they need to be in order for it to be easy to generate such high pressure, but more important, because *there is no pressure regulator* to control how much air is let in to the hose. Unlike an air compressor, it IS just an air pump hooked up to a hose, so the air need to be pumped in slowly so that you can manually keep track of how much has been let in.

Perhaps the air hoses at a gas station don't work like a compressor as I describe, but if you are talking about a compressor with a holding tank and an adjustable pressure regulator for the hose, you are fine.

Of course, getting the proper valve adapter to do this may end up costing you as much as a shock pump.

The other consideration is if the shock is actually deflated (compressed) then the rapid inflation could snap it back too quickly. I don't know if this is a problem, but if it is, just turn the pressure regulator down to zero, hook up the hose to the shock, and turn the regulator up slowly until the shock has un-collapsed.

One more thing: You are not loosing pressure from the shock when you disconnect a shock pump. You loose it when you re-connect it.


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## highdelll (Oct 3, 2008)

Ahh thank you kapusta for a little sanity 
I thought that was a good explanation.
It's like the moto mechanics across the street from me. They work on shocks and forks (pretty similar to MTSs actually) and they charge the shocks with a tank of nitrogen and a regulator - pretty much the same concept as a compressor (a tank of air w/ a regulator).
I just don't trust my el cheapo regulator on my compressor to get as precise - might have something to do with the fact as the closer you get to your target pressure, the slower it goes (there's a mathematical term for this - my brain isnt on yet  ) - don't know fer sher


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## vk45de (Feb 1, 2009)

Nice writeup kapusta, a bit lengthy though. Would have been sufficient to say AC fill a tank @ so so pressure. Once you connect the tank and shock and let it equilibrate, whatever pressure in tank is pressure in shock. Since AC pressure is less than shock max pressure, you will not pop your shock.


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## kapusta (Jan 17, 2004)

> Nice writeup kapusta, a bit lengthy though.


Thanks, yes I am often long winded, but when there is this much mis-information, I think it is good to try and explain thoroughly for those who are not familiar.



> Would have been sufficient to say AC fill a tank @ so so pressure. Once you connect the tank and shock and let it equilibrate, whatever pressure in tank is pressure in shock. Since AC pressure is less than shock max pressure, you will not pop your shock.


Actually, that is not the same as what I said. The pressure in the shock is NOT the same as what is in the tank if you have a pressure regulator between the tank and hose. I'm not disagreeing with you, but I was explaining it as the OP was asking about it (using the pressure regulator).


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## OrngRubberDucky (Mar 27, 2009)

Thanks for the replies. My shock is not all the way deflated. It just has about a 75% sag now, haha. I was talking about a compressor with a tank and regulator. It looks to me like the shock has a regular schrader valve. But it sounds like its actually smaller then the normal tire valve?


On a side note I was just reading on Fox shox website. "Pump to the desired pressure setting. Air pressure range is from 50 to 300 psi for the main air chamber on air shocks. DO NOT EXCEED 300 PSI IN THE MAIN AIR CHAMBER."
This to me seems as if a compressor would work as long as you dont over pressurize? Also it has no warnings of not using any other kind of device to fill their shocks.

Maybe I should be asking this question in a physics forum...... Or even directly to fox.


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## kapusta (Jan 17, 2004)

OrngRubberDucky said:


> Thanks for the replies. My shock is not all the way deflated. It just has about a 75% sag now, haha. I was talking about a compressor with a tank and regulator. It looks to me like the shock has a regular schrader valve. But it sounds like its actually smaller then the normal tire valve?
> 
> On a side note I was just reading on Fox shox website. "Pump to the desired pressure setting. Air pressure range is from 50 to 300 psi for the main air chamber on air shocks. DO NOT EXCEED 300 PSI IN THE MAIN AIR CHAMBER."
> This to me seems as if a compressor would work as long as you dont over pressurize? Also it has no warnings of not using any other kind of device to fill their shocks.
> ...


I think whatever you do for now, you really need to get a real shock pump. They are not very expensive.


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## OrngRubberDucky (Mar 27, 2009)

I am going to, but I am still interested in if you could use a compressor.


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## Jerk_Chicken (Oct 13, 2005)

I think we should all use compressors.


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## Jim Z in VT (Sep 9, 2007)

The compressor should get you started but won't likely be capable of getting you to full pressure. Home/contractor's compressors anyway....the big one we have at work will go way too high. A floor pump might work too; some go to fairly high pressures

I think most shocks have a schrader valve that should be accessible by most air chucks. Forks on the other hand often have recessed valves that need the chuck on the shock pumps, and sometimes a screw-in adapter as well. The nice thing about the shock pumps is they have a gauge, right there where you can read it while pumping, and usually a bleed valve too. 

PricePoint has a nice shock pump for about $20.

JZ


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## TiCain (Sep 25, 2008)

Forget about the compressor and buy a shock pump already, if you screw up you'll be buying a new shock "or at least having it rebuilt" and pump.:nonod:


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## kapusta (Jan 17, 2004)

TiCain said:


> Forget about the compressor and buy a shock pump already, if you screw up you'll be buying a new shock "or at least having it rebuilt" and pump.:nonod:


Assuming he can use the pressure regulator, what is he going to screw up?

He IS getting a shock pump:madman: , the question is can he use this to get him going for now?


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## JonathanGennick (Sep 15, 2006)

This question of using a compressor is interesting. The answer probably comes down to the type of compressor.

I'm thinking of the typical compressor with an air-tank that one buys at Home Depot or Sears. If the pressure in the tank is less than or equal to the maximum pressure that the shock can handle, then I can't envision a problem. If my air-tank is pressurized to 150psi, and my shock can handle 200psi, then air will flow from the tank into the shock until the pressure equalizes at some smidgen less than 150psi. There is no possibility that an air-tank pressurized to 150psi will be able to put out more pressure than 150psi.

The type of compressor I wonder about are those little, piston-driven jobbies that are sold for car-tire inflation. I do not believe those typically have reservoirs. So in the case of one of those, there probably are two things to worry about: 1) the pressure generated by the piston, and 2) the amount of air moved by one stroke of the piston. One Sears model that I looked is capable of generating 250psi pressure. If one stroke of the piston moves enough air to fill my shock to 250psi, and my shock is rated for 200psi, then I'm screwed. One stroke of the piston will overload my shock. Even if there is an automatic shut-of mechanism, that won't be enough, because the compressor probably can't measure granularly enough. 

When filling a car tire, there's plenty of room in the tire, so there is plenty of time for the compressor gauge to recognize that the tire has reached whatever pressure you've set. Two or three or a dozen more pumps of the piston just won't matter when filling a large object like a car tire. But the margin for error is very small with a bicycle shock.

Bottom-line, no way would I even consider using a tire-inflator compressor on a bicycle shock.


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## Jerk_Chicken (Oct 13, 2005)

I fill my high volume Zokes to about 30-40 PSI and my lower volume Fox to about 55 psi. Rear air shock on my gf's to about 125.

Like I said, I would do it and enjoy it.


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## TiCain (Sep 25, 2008)

I'm not trying to give the guy a hard time, it's his shock and he can use whatever he wants to use. I'm just saying there is a correct tool for the job and that's a shock pump, if that's all he has to work with right now just be careful.:skep:


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## jim453 (Oct 23, 2006)

Does anyone know if you can use a compressor to charge an air shock? Or will it move too much air?


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## kapusta (Jan 17, 2004)

jim453 said:


> Does anyone know if you can use a compressor to charge an air shock? Or will it move too much air?


No way, it moves too much air, so the air hits the shock really fast and the pressure gets super high until it sloshes back into the hose again. The wider the hose, the more pressure it builds up. Also, really big tanks push it harder as well. Red ones are the worst.


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## jim453 (Oct 23, 2006)

Very good Kapusta, that actually made me laugh outloud.


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## belowambient (May 17, 2008)

make sure and get a video plz



OrngRubberDucky said:


> Thanks for the replies. My shock is not all the way deflated. It just has about a 75% sag now, haha. I was talking about a compressor with a tank and regulator. It looks to me like the shock has a regular schrader valve. But it sounds like its actually smaller then the normal tire valve?
> 
> On a side note I was just reading on Fox shox website. "Pump to the desired pressure setting. Air pressure range is from 50 to 300 psi for the main air chamber on air shocks. DO NOT EXCEED 300 PSI IN THE MAIN AIR CHAMBER."
> This to me seems as if a compressor would work as long as you dont over pressurize? Also it has no warnings of not using any other kind of device to fill their shocks.
> ...


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## noodletips (Dec 8, 2008)

*I should'nt respond, but...*

I have a compressor _with tank_ in my garage, hooked up to PVC lines for air access, with an accurate guage at each access point. Since I already have this setup, I use it on my shock & fork. Turn on the comp., wait 'til it reaches the desired pressure, turn it off. Now connect the hose to the shock or fork to equalize the pressures. That's all there is to it.

If you don't have a compressor & tank, with an accurate guage, it's cheaper/simpler to just buy a shock pump.

jeff


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## otis24 (Apr 1, 2004)

noodletips said:


> I have a compressor _with tank_ in my garage, hooked up to PVC lines for air access, with an accurate guage at each access point. Since I already have this setup, I use it on my shock & fork. Turn on the comp., wait 'til it reaches the desired pressure, turn it off. Now connect the hose to the shock or fork to equalize the pressures. That's all there is to it.
> 
> If you don't have a compressor & tank, with an accurate guage, it's cheaper/simpler to just buy a shock pump.
> 
> jeff


This seams like it would work and be easier than a shock pump, granted you already have the air compressor and all.


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## ljsmith (Oct 26, 2007)

This has got to be one of the dumbest threads ever. I have worked on cars, bikes, houses, I repair lab equipment for a living. And there is one thing I have learned:

Use the right tool for the job. Otherwise you are asking for trouble.

Yes you can probably use an air compressor to fill your shock or fork. However this is going to be a very inaccurate way of doing it. You may want 95psi, and end up getting 90 or 100psi or maybe worse. An air compressor is designed to deliver large volumes of air. Using it to fill such a small volume is using it in a way it was not designed to work optimally. Also understand that even if your regulator is calibrated, there is still an amount of error. If it is not calibrated, or has not been calibrated in many years, there could be a large amount of error. Its kind of like asking "Can I use an axe to cut my toast instead of a knife." Yes you can, but a knife will give you a more precise and accurate cut. So, use the right tool for the right job. In this case a shock pump is designed to deliver a small volume of air, so why use something else? Its not like they are expensive or diffcult to use.


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## kapusta (Jan 17, 2004)

ljsmith said:


> This has got to be one of the dumbest threads ever. I have worked on cars, bikes, houses, I repair lab equipment for a living. And there is one thing I have learned:


The thread seems dumb to you because you haven't read it and understood what he is really asking. Everything you just said is well understood already, but it is beside the point.


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## highdelll (Oct 3, 2008)

kapusta said:


> ... Red ones are the worst.


yeah... no doubt...that's why I got a blue one. :thumbsup:


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## Jim Z in VT (Sep 9, 2007)

Damn......mine is red.

This has gone on long enough. I'm outta here.

JZ


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## ljsmith (Oct 26, 2007)

kapusta said:


> The thread seems dumb to you because you haven't read it and understood what he is really asking. Everything you just said is well understood already, but it is beside the point.


Actually I can and did read this thread, and what is being asked is pretty clear. In fact here it is: "Can I use an air compressor with the regulator set at low pressure? Or is a shock pump a must?" And the thread is stupid. Just go buy a shock pump, they are not expensive and work very well, all this back and forth about it is stupid.


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## kapusta (Jan 17, 2004)

ljsmith said:


> Actually I can and did read this thread, and what is being asked is pretty clear. In fact here it is: "Can I use an air compressor with the regulator set at low pressure? Or is a shock pump a must?" And the thread is stupid. Just go buy a shock pump, they are not expensive and work very well, all this back and forth about it is stupid.


If you read the thread you would know that he IS buying a pump, and the question is now if he CAN use the compressor to get by until then. If you don't like a discussion on whether it will work, then don't read the thread:thumbsup:


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## net wurker (Sep 13, 2007)

noodletips said:


> I have a compressor _with tank_ in my garage, hooked up to PVC lines for air access, with an accurate guage at each access point. Since I already have this setup, I use it on my shock & fork. Turn on the comp., wait 'til it reaches the desired pressure, turn it off. Now connect the hose to the shock or fork to equalize the pressures. That's all there is to it.
> 
> If you don't have a compressor & tank, with an accurate guage, it's cheaper/simpler to just buy a shock pump.
> 
> jeff


Quick question for you.

Using this method, lets say you wanted 90 psi in your fork. Would you charge your compressor up to 90 psi or 180 psi?


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## noodletips (Dec 8, 2008)

net wurker said:


> Quick question for you.
> 
> Using this method, lets say you wanted 90 psi in your fork. Would you charge your compressor up to 90 psi or 180 psi?


90 psi. The volume of the shock or fork is very small, while the volume of the compressor tank is quite large (many gallons). When the hose is connected to the shrader valve on the fork or shock, the pressure in the fork or shock becomes equal to that in the compressor tank. So little air is moved that the tank pressure is essentially (but not _exactly_) unchanged.

I posted on this thread just to show a working example of compressor usage. When I got my suspender bike, I was concerned that shock pumps would create a problem when the pump was disconnected from the shock/fork - that is, there would be a miniscule air loss, and since the fork/shock volume is so small, how could one be sure of the resulting pressure. My system solves this _concern_. It turns out (from what I've been told) that this is not a problem.

jeff

BTW: I've been using this system since '04 w/o problems. Checking the sag on my fork or shock confirms that the pressures are correct for my weight. The bike rides great, too, but that's less objective.


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## net wurker (Sep 13, 2007)

Cool. I wasn't sure how that would work, but your answer makes sense to me.


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