# Work Energy Equation Help (what size shock absorber to use)

1. Jul 8, 2017

### Mechaman

Hi everyone. I'm trying to work out what size shock absorber to use. It's going under a seat so just the weight of a man really that it has to support.

I've found a website showing the equations I need but I'm not sure about one of the steps. In step 2 it asks force output of the motor/drive. For my problem would this just be the weight x 9.81?

Here is pic from website: http://imgur.com/a/expcS

Any help would be much appreciated!

2. Jul 8, 2017

### CWatters

I suspect it's way more complicated.

Presumably your shock absorber is intended to damp out motion caused by a bumpy road or similar?

Firstly it's the spring part not the shock absorber part that supports the weight.

Shock absorbers must be able to dissipate the energy they absorb. The amount of energy they must absorb depends on the vibration pattern (eg Large fast transients puts more energy into the shock absorber than short slow transients. I think you can only answer the question with data on the motion of the floor of the vehicle eg bottom end of the shock.

3. Jul 8, 2017

### Mechaman

I should clarify that the seat is attached to a base by a sort of leaf suspension set up. The shock absorber (with its own spring as well) will be mounted to the seat, connecting the seat to the base.

If I know the weight of the person and I know the G force subjected to them, should I not be able to estimate how big a shock absorber I would need to lessen the impact?

4. Jul 8, 2017

### JBA

The force that will be experienced by the rider is a function of the spring rate and deflection of the seat spring vs. the input energy. Without a shock absorber the rider experiences very little force (beyond that of his own weight) at the beginning of the impact that increases to a peak value at the point at which the spring stored energy is equal to energy input to the seat spring assembly.
The shock absorber reduces the peak force at the end of the spring travel by increasing the force the rider experiences at the start of impact, absorbing energy throughout it travel and thereby reducing the amount of energy the spring must absorb and it maximum travel and force.
As a result, the spring and shock absorber act in concert to absorb the impact energy and determine the maximum G force the rider will experience.

5. Jul 8, 2017

### Mechaman

Thanks for the explanation. Does that mean that I can take the amount of energy the shock absorber dampens and the remainder is in the spring surrounding the shock absorber? Hence, if i had a second or third spring under the seat, my damper could be even smaller each time I add a spring?

Also does this mean that the springs have stored energy so they have a tendency to spring back up where as the damper dissapaits the energy. Would the springs want to throw the person off if they outnumber the damper in terms of energy?

6. Jul 9, 2017

7. Jul 9, 2017

### JBA

In response to your question regarding the rebound of the springs stored energy, Yes, if the springs return is not damped then that is exactly what will happen. That is the reason that automotive shock absorbers are "double acting" (damp on both their compression and extension strokes) but not necessarily at the same amount by having separate internal valving and orifices for each direction.

See the attached excel calculation that I performed for an earlier thread about the design of combined spring/shock absorber seat cushioning for kayaks. In your case the Kayak's contribution due its submersion as it impacts the water is set to zero. It will allow you to determine the amount of energy the shock absorber will have to absorb for a given input energy and spring selection.

PS In that case it was shown that the planned design damping system for its application was not practical.

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