# Bicycle seat post shock absorbtion.

1. Mar 9, 2013

### mr pizzle

Hi i am currently studying a bicycle seat post suspension unit. It is one of the lower end of the range models which consists of a spring and a rubber dampener. Basic outline in cad below.
[URL=http://imageshack.us/photo/my-images/407/drawing1layout1.jpg/][PLAIN]http://imageshack.us/a/img407/1720/drawing1layout1.th.jpg[/URL][/PLAIN]

I would like to know the calculations behind the spring and damper selection. Should i be using the spring mass damper ones found on the forum in other threads. I have read alot about shock absorber and suspension systems and have got lost in the vast amounts of information.

As i understand it (but unsure if it correct) the load from the user is initially taken up by the spring compressing. The damper (rubber/elastomer) absorbs and dissapates some of this energy through rebound resilience? (of the material used as the damper). Some energy is returned back through the spring.

If the user is 80kg and the travel of the unit is 50mm how does one go about calculating the spring and damper?

Do i need to find a spring with a constant which equals the load at given length of compression and then select the dampening to suit the amount of energy to loose from the spring? So also must need to know the relationship between when the spring is doing one thing what is the damper doing.

I think as i write this im slightly unsure of to what going on in the system to actually understand what to calculate.

I apprieciate any feed back and would be greatful for a point in the right direction.

Regards Sean

2. Mar 10, 2013

### Bobbywhy

Sean, Welcome to Physics Forums!

Your proposal seems non-obvious and possibly unique.

Before you decide on the type/characteristics of the spring and dampener it seems to me that you need some real data to use for calculations. Have you measured the real-world conditions a rider encounters on typical and varied surfaces at various speeds? Each “shock” delivered to the rider’s bottom via the seat has its own unique set of characteristics. You may consider installing some instruments (accelerometers) and recording method. Then send a rider out and gather this data. This way you can know exactly what you need to dampen and or absorb.

One other method, called “guess and test”, would be to make your best guess about the shock absorber system, test by actual riders. Then modify the setup and test again. Riders would report the ride “quality” and you would compare the results. You may discover a commercially available shock absorber is ideal.

Cheers, Bobbywhy

3. Mar 11, 2013

### mr pizzle

Thank you for the reply Bobby.
I will certainly look in to obtaining some real world data. The real reason i was looking in to this type of shock absorber is to possibly implement in to a forearm crutch because it is quite compact and linear to absorb some impact of the cructh hitting the ground. Its for a college project and i thought would work quite similar. So i will definatley be looking to get the data from crutch use.
Once again thank you for the response.

4. Mar 11, 2013

### etudiant

The idea seems a logical follow on to the current wheel suspensions on mountain bikes, Rack Shocks etc. The complaint to watch out for is that the pedaling effort not be diverted from the wheel to bouncing the rider up and down.
The crutch shock absorber idea seems very good. Crutches are no fun to use, so anything that makes them a bit more comfortable would be welcome.

5. May 17, 2013

### Curve Shifter

For crutches, I wouldn't think that spring rate, per se, is a concern as the impacts will be fairly evenly spaced and much farther apart. You just don't want it to rebound too quickly or they'd turn into pogo sticks. Probably going to be a tradeoff between building in enough travel/cushion to dissipate the shock and not having so much travel that the shock feels "squishy" to the point that it would be a safety/comfort issue. I would think some type of adjustable-pressure gas ram with some kind of rebound damping would be ideal in terms of damping characteristics and adjustability. Maybe look into some of the lower-end gas frame-shocks for bicycles using 4-bar suspensions?

If you use a coil spring you'll have to play with various stiffnesses (spring coefficients) to get something that will give enough to absorb shock, but not bottom out and vary it based on individual user weight. Same thing goes for elastomers. You'll need to find something with a spring coefficient that will fully dissipate the max force without going through all the available travel in the design. With a gas shock, you just pick one of an appropriate length for the amount of travel you want, and adjust the pressure.

Maybe contact one of the people in SRAM's suspension division for sourcing parts? I was fortunate enough to work on a couple trails projects with them. Good bunch of guys/gals.

6. Jul 31, 2013

### Baluncore

I was under the impression that underarm crutches carried the entire body weight, while forearm crutches were more an aid to balance. I would therefore expect that shock absorbers in forearm crutches would be a real disadvantage as they would reduce positional sensitivity and immediate control.

Underarm crutches are quite a different problem. In effect the user is “pole vaulting” along the path. The longer the step the bigger the height variation and the greater the shock of take-off and landing. We bend our knees when walking and so reduce vertical variation of body position. Energy is stored and recovered from the elasticity of tendons. A recent sporting example shows a curved spring blade performs this function more efficiently than a knee, without the alignment complexity of an enclosed axial coil spring.

So why do underarm crutches not have a curved blade with an elliptical ground contact to moderate the height fluctuation? Maybe because it would obstruct the forward movement of the crutches?