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Let's assume I have an object with a mass of 3 kg. I want this object to isolate from vertical shocks and vibrations when I'm riding approximately 30 km/u on my bike.

Let's also assume I have an air suspension front fork modified and mounted on top of the bike's frame. This suspension unit carries the object I just mentioned (see figure below).

enter image description here

  • What should be the air pressure from the suspension unit to isolate most of the shocks?
  • Is it even possible to dial down the air pressure from an air suspension unit in order to isolate shocks for a relatively low mass (3 kg)?

My initial thoughts were to calculate the relation between mass and required air spring pressure level based on values in a product's manual. But the exponential decrease seems just too much for the fork to adjust.

Thanks in advance for any feedback.

Note: I read questions about specific products are mainly downvoted, but just for reference I included the RockShox manual with Air Spring Pressure levels with respect to a rider's weight. I'll edit the post if this isn't appreciated.

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Air pressures provided by companies are only estimates, to set up a fork you would need to find the appropriate amount of sag through trial and error, here is a set up movie pinkbike.com/news/technical-tuesday-setting-sag-2010.html . To achieve what you want you may find you end up with too little air for the spring to be effective as you think. Also, what is more important to your design is the damping effect of the suspension. You don't want oscillation. This would be more complicated for low weight and would involve oil volume and viscosity. I don't how you'd compensate for this. –  DWGKNZ Mar 27 at 18:37
    
What kind of terrain are you riding over? –  Batman Mar 27 at 19:47
    
The usual answer is a lever to bring the force up to something that the suspension can deal with. If you got a rear shock unit and had a pivot just in front of the seatpost/top tube joint that might work. Or just use an elastomer there. –  Nuі Mar 27 at 22:43
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Stiction is going to be a big issue with fork too - it may take more force than the 3kg mass can supply to get the fork to move at all. –  Nuі Mar 27 at 22:44

3 Answers 3

up vote 2 down vote accepted

It's fairly easy to compute (or at least estimate) the surface area of the piston and divide that into the weight of the object suspended to obtain the pressure needed to keep it "levitated".

Let's assume a piston with a diameter of 2" (5.1cm) -- that's (conveniently) 3.14 square inches. Divide that into 3 KG or 6.6 pounds (ignoring the weight of the piston itself) and you get about 2.1 PSI or 0.14 ATM.

Of course, if you use a piston rated at a significantly higher pressure than the above the friction in the cylinder is apt to require a pound or so to overcome, so it's unlikely that you can use a standard suspension fork effectively for this duty. Something along the line of a "gas strut" might work, though.

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...but gas struts are generally filled to too high a pressure for that. –  Chris H Mar 27 at 21:17
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@ChrisH - Gas struts come in a wide range of "strengths", and some are adjustable. –  Daniel R Hicks Mar 27 at 22:51
    
All the ones I've used have been too stiff for the sort of loads in question here. My "generally" was only meant to be a note of caution against just stuffing a gas strut from something else in there and hoping for the best. –  Chris H Mar 28 at 7:48
    
@ChrisH - Yeah, and I just tossed it out as something to consider. Can't think of any obvious "ready made" solution to this problem. –  Daniel R Hicks Mar 28 at 11:01
    
I'm sure if you could get soft enough gas struts and make something like a hexapod construction you might get somewhere - but the stiction would still be an issue. Nice idea though. –  Chris H Mar 28 at 11:27

I don't think (like the other answers) a suspension fork will do it. There's another reason as well - suspension forks only isolate vibrations in one linear dimension, as you go over bumps, as well as up and down you will accelerate and decelerate, and pitch up and down. So here are some other thoughts. We might be able to offer more help if you give some idea of what the object is.

It's not unknown to use an underinflated inner tube as a shock absorber for light loads - but that's generally a flat object on a flat surface. Providing suspension for a light object can be done by hanging it in a frame using bungee cords (shock cords). The rubber provides some damping so it's better than just springs. Might be hard to fit between you and the bars - but then if I was doing 30km/h over rough ground I wouldn't want a set of forks pointing at my chest as you've shown. Either on a modified rack behind or in a modified front basket would be better.

Whatever means you use, for best performance you need to tune the system - with some means of sensing the vibration (smartphone app these days?) and adjusting either the thickness of the cords or the mass.

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I think using a suspension fork won't work - 3 kg is way lower than the usual loads a suspension fork is designed to isolate shocks at any meaningful level (the mass on the fork would be at least an order of magnitude below the usual on a bike, so it would be hard to have enough deflection to do something useful at 3 kg without bottoming out all the time on a regular load - its just too far from the operating regime of the fork normally). In any case, if it were possible, it would likely be a very sensitive adjustment that would need to be made and maintained.

This problem has been studied to some extent in cameras for image stabilization on bicycles, where it is suggested to directly mount a camera on a bicycle and having soft tires with some suspension (on the bicycle).

The movie industry way of stabilizing a camera while it is moving is with a steadicam. Some photography enthusiasts have made home made steadicam type things which you may want to start with and modify to mount on a bicycle. Unlike a real steadicam unit, these probably need to be toughened up a bit for 3 kg since my guess for the weight of a DSLR with a small lens is closer to 1 kg - 1.5 kg. This thread also suggests a vibration plate.

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Professional grade Video cameras are larger than DLSR's so I am sure the problem has been solved in that domain. Great answer...... –  mattnz Mar 27 at 19:34
    
In the pro domain, sure. But most people building steadicams at home are probably mounting in a small DSLR on the large end or probably a compact/go pro / iphone type thing on the small end. –  Batman Mar 27 at 19:40
    
Steadicam explicitly does not solve high frequency vibration, and the manual always says "this keeps the camera pointed in one direction, it will not work if there is too much vibration". –  Nuі Mar 27 at 22:38

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