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The Effect Of Damping On A Shock Absorber

 
Researched by Briston T. 
1999-2000 


PURPOSE

The purpose of this experiment was to determine the effect of damping in a shock absorber when the piston has different numbers and different size of holes, but keeping the total area of the holes constant.

I became interested in this idea when I was reading an article about the dynamics of damping and it mentioned that people do not know if fewer big holes or more small holes would work better.   It also mentioned that finding the answer would be difficult.  It was then when I decided to do a project about it.

The information gained from this experiment will help car designers ensure a much safer and smoother ride.  On a smaller basic it will benefit snowmobile enthusiasts with better control and reaction on the racetrack. 



HYPOTHESIS

My hypothesis is that the time that it takes for the piston to compress of the single  and multiple holes will be very close if the total surface area is kept constant through out all the tests.

I base my hypothesis on the research that I have done and the experiments on damping that the people at "Team Losi" have done and if the total area of the single  and multiple of the holes drilled in the damper the compression time should be the same. 
                          

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EXPERIMENT DESIGN

The constants in this study were: 
* Same level of viscosity of the fluid in the piston 
* Same surface area of holes drilled 
* All the material in the shock absorber are the same 
* Same temperature of fluid in the piston 
* Same weight for compression

The manipulated variable was the number of holes drilled in the damper that equal a certain same total open area.

The responding variable was the amount of time it took for the piston to compress from completely extended to completely compressed to time the compression with a certain amount of weight on top.

To measure the responding variable I would measure how long it took for it to compress when using a stopwatch in seconds.  
                          

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MATERIALS
QUANTITY ITEM DESCRIPTION 
 1  Fox shock absorber
 1 3.81 cm hole saw drill bit 
 1   0.9525 cm drill bit
 1  Electric drill
 1  Birch plywood 2.5mm x6x12"
 Stop watch
Vise grip
Lid to 35mm film canister
200 ml. cooking oil for shock fluid 
 X-Acto hobby knife
Tube of epoxy 
Paint Brush
1.59 mm drill bit
2.78 mm drill bit
3.18 mm drill bit
3.97 mm drill bit
 4.37 mm drill bit
5.95 mm drill bit
 6.35 mm drill bit
202.4 grams weight
3.1 cm hole saw drill bit
 0.9525 cm drill bit 

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PROCEDURES

I.  Setting up the shock absorbers for the tests. 
    1.  Dismount the shock absorber, 
    2.  Take the damper off. 
    3.  Take the bushing off. 
    4.  Pull the O-rings out of the bushing. 
    5.  Place the bushing on the shaft.

II. Setting up the damper for the tests. 
    1.   Place the piece of birch plywood in the vise grip 
     2.  Drill out as many disks as you can out of the plywood because some  might get damaged in the drilling with the whole saw drill bit.( at        least seven) 
    3.   Drill a hole in the center of the disk were the pilot hole of the whole saw drill bit using a 0.9525 cm drill bit. 
    4.   Drill a 0.9525 cm hole in the center of the 35. 5mm film cap. 
    5.   Take the Ex-Acto knife and cut off as much excess plastic, but still leaving an area for it to attach to the shaft and make sure the film cap     doesn’t cover any of the wholes. 
    6.   Apply the epoxy on the disk on both sides. 
    7.   For damper number one single drill one 6.35 mm hole in the disk. 
    8.   For damper number two single drill one 5.95 mm hole in the disk. 
    9.   For damper number three single drill one 4.37 mm hole in the disk. 
   10.  For damper number four single drill one 3.97 mm hole in the disk. 
   11.  For damper number five single drill one 3.18 mm hole in the disk. 
   12.  For damper number six single drill one 2.78 mm hole in the disk. 
   13.  For damper number one multiple drill three 1.59 mm holes in the disk. 
   14.  For damper number two-multiple drill one more 1.59 mm holes in the disk. 
   15.  For damper number three-multiple drill two more1.59 mm holes in the disk. 
   16.  For damper number four-multiple drill two more 1.59 mm holes in the disk. 
   17.  For damper number five-multiple drill eight more 1.59 mm holes in the disk. 
   18.  For damper number six-multiple drill two 1.59 mm holes in the disk for a total of sixteen holes. 
   19.  Attach the right damper for the test to the piston rod with the cut out film cap last to stop the flow of excess fluid and to make a better fit in the   cylinder. 
   20.  Add the shock fluid. 
   21.  Put the shock absorber back together. 
   22.  Expand the shock absorber to its full amount.

III. Setting up the test area. 
  1.    Place the shock absorber in the vise grip. 
   2.   Make sure the shock absorber is fully expanded. 
  3.    Attach the U-bolt to the bottom of the shock absorber 
  4.    Drill two holes in the piece of aluminum for the U-bolt to go in. 
  5.    Tighten the bolts on the U-bolt. 
  6.    Place the shock absorber so it is being bottom loaded.

IV. Conducting the Experiment. 
  1.    Place the weight on the top of the shock absorber.  
  2.    As soon as the shock starts to compress record how long it takes for  the shock absorber to fully compressed. 
  3.    Record the data.  
  4.    Repeat the steps in I. 1 and 2, in II.10, 11, 12, 13, and in III. 1, 2, and 3 eleven times to do all the experiments. 
  5.   When doing the multiple, record the data and drill the required amount  of holes. 
 

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RESULTS

The original purpose of this experiment was to see if one big hole drilled into the damper in a shock absorber has the same reaction time to compress as a many small holes drilled into the damper of the shock absorber and keeping the area of the hole drilled constant.

The results of the experiment were that the larger the holes are the less amount of damping takes place and the smaller the holes are the greater amount of damping takes place.  It also doesn’t matter if there are fewer or more holes in the damper because the compression time is too close to matter in a simple shock absorber.

See the tables and graph below for more information on the times of the shock absorbers. 
                          

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CONCLUSION


My hypothesis was that the time for the shock absorber to compress with the single and multiple holes drilled into the damper would be very close when the total area of the single and multiple holes are the same.

The results indicate that this hypothesis should be accepted because the amount of time it took the shock absorber to compress was close comparing the time of the single and multiple hole drilled in the disk with the area kept constant.

Because of the results of this experiment, I wonder if the material used in the damper would have different times to compress because of the texture of the material.

If I were to conduct this project again I would use a drill press to drill out the holes to ensure that the size of the hole would be more accurate. 
 

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RESEARCH REPORT

INTRODUCTION

The technology in a shock absorber has greatly advanced sense they were first introduced to early automobiles in 1900’s.  Shock absorbers can be very simple or very complex pieces of machinery.  They range from shock absorbers in radio controlled cars to NASA’s shock absorbers in their spacecraft.  The most common place shock absorbers can be found in an automobile’s suspension system.  Viscosity is the level of thickness or thinness on a liquid.  The higher the level of viscosity the slower the liquid will move.  The lower level of viscosity the faster the liquid will move.  Damping controls the amount of fluid or gas that goes through the damper and into the other side of the shock absorber.  Altering the surface area of the whole or slots in the damper can change damping.  Friction is a major concern in shock absorbers because the friction in the shock absorber causes it to slow down.  The manufactures are trying to reduce friction as much as possible by making the inside of the shock absorber smooth. 
                                      

SHOCK ABSORBERS

 A shock absorber is a device that reduces shock caused by motion.  It is used mainly in automobiles, airplane landing gear, and doors.  The main purpose of a shock absorber in an automobile is to get rid of unwanted vehicle motion caused by uneven road surfaces or by turning or breaking maneuvers.   It also provides better safety while riding in the vehicle.  If vehicles didn’t have shock absorbers turning would be very difficult because the weight of the car when it is turning it wouldn’t distribute the weight evenly and this would cause a lot of stress on the vehicle.  Most shock absorbers consist of a piston inside of a cylinder containing a certain viscosity level of oil or gas.  The way a shock absorber in an automobile is when the vehicle goes over a bump, the force is transmitted to the shock absorber that pushes the piston upward.  Then the oil or gas in the cylinder of the shock absorber resists the piston. This resistance absorbers the shock and offsets the force f the bump.  The shock absorber was invented to reduce the force of the bump caused by the motion of the vehicle. The shock absorber is a device to control shock or stress.  The shock absorber was introduced in 1900’s.  The shock absorber is very common today and the technology of it advances as time goes by. 
                                      

VISCOSITY

Viscosity is very crucial for shock absorbers response and control.  If the oil inside the shock absorber has a higher viscosity level it would take a longer amount of time for the shock absorber to reduce the shock.  If the oil has a lower viscosity level the shock absorber might respond to quickly to the shock and it might cause the vehicle to not respond without any control.   Viscosity is the degree to which fluid resists flow of an applied force.  The temperature affects the level of viscosity.  The higher the temperature is the lower level of viscosity.  The lower the temperature the higher level of viscosity the fluid has.  Viscosity in a shock absorber is used when there is oil in a cylinder, which different levels of viscosity is used in different conditions for safety and control.  

DAMPING

Damping is also a very important feature in a shock absorber.  Damping is the resistance generated by the damper.  A damper is another term of a piston, but it is a shock absorber.  The damper controls the amount of fluid or gas that goes to the other side of the shock absorber.  When oil or gas goes through the damper it is called damping.  The force required to push and pull the piston through the fluid is called the damping force.  If there was no damping force in a car’s shock absorber the suspension springs would release almost all the energy they stored after decompressing over a bump, which would cause the car to rebound into the air.  If there is to much damping in the shock absorber, the shock will "pack up."  It means that the springs cannot extend the shock absorber before another bump and it will not operate effetely.  If the damping is too light, the shock will be able to compress quickly in respond to bumps, but it will rebound harshly and cause the vehicle to loose traction and may cause the vehicle to bottom out on large bumps/ jumps/ or sharp turns.  Altering the surface area of the holes or slots in the damper can control damping.  Another way to change the damping is to change the level of viscosity of the fluid in the shock absorber. 

FRICTION

 Friction is generated inside a shock absorber.  When the damper moves through the cylinder it rubes on the insides of the shock absorber.  Friction inside the shock absorber is reduced by having a smooth surface inside the cylinder or by having oil inside the shock absorber.  Friction in a shock absorber can cause wear and unwanted heat that can damage the shock absorber.  The less friction inside the shock absorber the better it would work.  The more friction inside the shock absorber the more stress is forced in the shock absorber and the vehicle.  Friction is also a major factor in a shock absorber and important to have the best response and safety. 

SUMMARY

 Shock absorbers very important for the safety and control of a vehicle.  Shock absorbers reduce shock caused by vehicle motion.  Shock absorbers distribute the weight of the vehicle.  Viscosity is the degree to which fluid resists under an applied force.  Certain levels of viscosity can change the response of a shock absorber.  Damping is a very important feature in a shock absorber.  Damping controls the amount of fluid that goes through the damper to the other side of the shock absorber.  By altering the surface area of the hole or slots in the damper you can change the response time of a shock absorber.  Shock absorbers are a very important feature in a vehicle because if there was no shock absorbers the vehicle wouldn’t turn with control and the motion of the vehicle would cause a lot of stress on the vehicle.  Shock absorbers make riding in a vehicle safer because the vehicle could respond to an obstacle that may endanger the passengers in the vehicle.

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BIBLIOGRAPHY

"Arctic Cat Performance Manual", no publishing information given 

"Fluid Power", online at http://www.machinedesign.com/, 12/2/99 

Fox Shox ? "Off Road Racing Shock Absorbers", at http://www.milnerconv.co.uk/foxshox.htm

"Shock Absorbers", World Book Information Finder, CD-ROM, 1996

"Shocks rebound in importance", at http://www.nascar.com/ 11/30/99 7:55 A.M.

Staff of R/C Car Action, "Dynamics of Damping," Radio Control Car Action, Vol. 14-Number 9, September 1999, pages 143,145, and 146 and can be found at http://www.rccaraction.com/

"Viscosity", at http://www.machinedesign.com/

 



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