Quote:
Originally Posted by fooddude
Some n00b/basic questions about shocks and shock dynos:
Can shock dynos tell you what the highest spring rate the shock can handle?
If yes, should the shock dyno results be significantly higher than the chosen/custom spring rate, and need plenty of headroom?
If so, by how much is a safe/decent number? And where on the dyno plot/graph and why? (slow[performance handling] vs fast[street bumps] velocity)
ie:
If say you wanted a 450lb spring rate, should the shock's dyno result be higher than 450lb in the slow [0-2in/sec] portion of the graph already. Or, is it okay to be more towards the faster portion [2+in/sec] of the graph? And, by how much headroom in the shock dyno for the said spring rate, in either, or both, the slow and fast portions of the shock dyno?
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Short answer yes, the shock dyno can answer these questions.
You want to read up on damping ratios and critical damping.
Quote:
Originally Posted by Racecomp Engineering
When people say 65% critically damped... generally this means it is 65% critically damped in the low speed range (0-2 inch per second or so) for rebound and compression. It may be higher or lower in the high speed range. It isn't necessarily the same throughout (it rarely is honestly).
Some like a little higher than 65% for low speed piston velocity and then closer to 65% in the higher speed range. Some like 65% for low speed and 50% for high speed piston velocities.
I'm all about theory as a starting point but to me it is a starting point. Not a hard and fast rule. Calibrated butt dynos are excellent tools. Feel, driver preference, and driver confidence can be very important too...and sometimes drivers adapt and end up faster. But I do really like having a starting point over a wild-ass guess.
- Andy |
To actually calculate the critically damped ratio use this equation:
Critical damping = 2*sqrt(springrate*sprungmass)
Then divide by the actual damping.
Careful with units! Especially stupid english units.
So, if you're damping force is around 65% of the critical damping rate at the piston speeds you're interested in, you're in the right neighborhood. It gets way more complicated of course but this is a start.
Example for a front BRZ coilover with 450 lbs/in springs and your damping of 450 lbs at say 2 inch per sec or whatever. Corner weight (sprung) of 700 lbs off the top of my head. I'm going to roughly convert to SI units because they are better in every way.
2 * sqrt( 80000 N/m * 300 kg) = 9797 N*s/m = 9.8 N/(mm/s)
That is the critically damped force per velocity.
To be critically damped at 2 inch/sec (51 mm/sec) you need a force of 9.8 N/(mm/s) * 51 mm/sec = 500 N.
500 newtons is about 112 lbs. If your damping is 450 lbs at 2 inch /sec you are 450 lbs/112 lbs = 4.02 --> 400% critically damped. So...very very stiff.
To be the "ideal" 65% damped you'd want to be around 73 lbs at 2 inch/sec. IMO that may be to soft but it depends.
- Andy