Originally Posted by
BobW
One thing that needs to be considered besides just the Re No is the orifice discharge coefficient (Cd). For short tube orifices, which we have, the discharge coefficient varies with Re No. and the L/Dh ratio (piston thickness/orifice diameter). The graph below shows the Force vs Piston Velocity (F-V) characteristics for two pistons with the same total orifice area. One has 6 holes and one has 3. As you can see the piston with more smaller holes produces much higher force than the piston with fewer larger holes. This is a result of the Cd coefficient.
Note that this damper model does not account for fluid compressibility. What compressibility will do is significantly reduce the force the damper produces during the first half of the compression stroke. This effect will be much much more pronounced at higher velocities. Pack velocities as they are referred to.
This model is being validated along with Icecyc1's work so I am very confident the effect of hole size is real. We see this effect in the dyno results.
First I want to thank you for your work with RC Crew Chief, without it I wouldn't have made my spreadsheet.
Now, the graph shows speeds of up to 2000mm/s. Do you have any data regarding average piston speed? If not I would gladly include that in my list of projects to-do.
Originally Posted by
razo125
Thank you. All good info and the explanation of the formula modeled clears things up for me. The math now matches what we experience on the track. Based on just the math I'm taking away the following in laymans terms:
All comparisons below assume equal piston speed, equal velocity of oil passing through the holes.
1. With equal pistons but with just a difference of thickness. The longer hole (thicker piston) will pack at a lower piston speed. Tapered/thinner pistons will have a tendency to pack at a higher piston speed.
2. Given 2 pistons with equal hole area but different number of holes. (6x1.5 vs 8x1.3) the 8 hole piston will pack at a lower piston speed.
3. A piston hole shape at the entry/exit (Cd) has little affect at lower piston speeds, but has a greater affect on the RE (Pack) at higher piston speeds
I agree with all the conclusions but technically we don't want our shocks to pack ever, the best shock setup would be as close to critically damped as possible. Unfortunately our shocks are too primitive to work in a wide range of piston speeds but most of the tracks have "small" bumps and jumps with receptions. They aren't too wild in that aspect, abrupt high speed sharp bumps or jumps without reception are almost unheard of.
That's why shock consistency is important, if the track develops holes and ruts after half an hour of continuous racing and the shocks provide, say less 20% of damping, our car would be bottoming and catching more ruts towards the end of the race, something we have to account for and the test with the hair drier is important, at least to me and others who race long races (more than 15 minutes).