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Old 08-18-2008, 03:16 PM   #1
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Default Shock absorber "pack"?

If we begin to translate RC jargon into what we learned in our high school physics class, these RC suspension optimizations can be solved pretty easily.

For example, for "slow piston speeds" you can get the same damping with "heavy oil and a large hole piston" as you can with a "light oil but small hole piston".

HOWEVER, at higher piston speeds (i.e. like landing a jump), another effect comes into play, in the RC world we like to call it "pack". What is this called in the of physics? I think it may be the "laminar to turbulent flow" transition based on the Reynolds number.

On the RC track, there may be two general states to optimize for:
1. On the track: piston speeds operating within the laminar region.
2. Onto the track (landing a jump): piston speeds operating within the turbulent region.

There are things that I know that I do not know, so I can't even ask the questions!!...

Discussion, please...
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Old 08-18-2008, 03:39 PM   #2
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There are many ways to get shocks to react in a similar way.
  1. There is the oil viscosity. Thick moves slower giving more dampening.
  2. Piston hole size. Smaller allows for slower movement.
  3. Spring tension. Heavier recoils back from dampening faster. Also has some to do with dampening.
  4. Angle of shock. This is a "simple lever". This controls the amount of force from the suspension movement to the shock.
So as you can start to see, you can arrive at almost the same shock solution with different settings.

There seems to be a trend again for heavier springs with lighter oil. This "progressive" setup is something we used a lot back over a decade ago in R/C. This lets the shocks react quickly to a rough track. The dampening is light and recoil is fast. This is how Losi has their stock setup on the front with their XXX-TCR. This also makes for a very active front suspension when cornering. Weight transfer causes the front suspension to load going into a corner. This will also cause the truck to push a little coming out of a corner. Not a bad setup if it matches your driving style.

There really isn't one suspension style for everybody. Driving style should determine more of your setup.

These little cars are infinitely adjustable! The key is to make it go straight, able to turn and land jumps... If you can drive that, then make slight adjustments (1 at a time) and drive.. and drive.. you change too many things you will never know what worked.. and what really didn't!

Have fun!

Jerome
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Old 03-13-2016, 11:33 AM   #3
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Default Shock absorber pack

Pack simply refers to the ability of the shock to minimize the effect of shock.
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Old 03-13-2016, 11:51 AM   #4
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Quote:
Originally Posted by kingwillie206 View Post
Pack simply refers to the ability of the shock to minimize the effect of shock.
The technical term for "pack" is high-speed compression damping. This is independently adjustable from low-speed compression damping on high end motorcycle suspension.
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Old 03-13-2016, 03:31 PM   #5
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I always took "pack" to mean a condition where the piston is traveling faster than the oil can pass through the holes, causing it to reduce speed quickly...like on the edge of hydrolock, in layman's terms. Is that correct?
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Old 03-13-2016, 04:51 PM   #6
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Quote:
Originally Posted by bubbaslash View Post
I always took "pack" to mean a condition where the piston is traveling faster than the oil can pass through the holes, causing it to reduce speed quickly...like on the edge of hydrolock, in layman's terms. Is that correct?
Yes, in our simple dampers high speed compression (pack) refers to the damping created by turbulence as the orifices/blowby are overwhelmed by the piston's speed. It is not a fixed value, however, it is a range. Less high-speed damping ("pack") = less bottoming resistance relative to static damping. With large piston holes/heavy oil, it is possible to have a heavy/slow setting that still doesn't resist damping well. More high-speed damping can be viewed as the opposite.

I don't know enough to say at what point hydraulic lock occurs. I think it's really rare with shocks that are in the ballpark. Most often we'll blow through the travel and bottom out.
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