Can shock-dynos tell you what the highest spring rate it can handle?

[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?

[solidONE]

Short answer is yes. And to ur example at the bottom 450 lbin coupled with 450lbin of damping force at 1-2insec, u might as well use a solid steel rod for a damper...

I'd suggest google or find some books with all the formulas and math to make calculations. This is definitely not as simple as I think ur imagining it is.

[fooddude]

Originally Posted by solidONE

Short answer is yes. And to ur example at the bottom 450 lbin coupled with 450lbin of damping force at 1-2insec, u might as well use a solid steel rod for a damper...

Stiff as a steel rod.. How so? Are you able to explain the basics in calculating a general/rough estimate for certain rates, or summarize?

I've seen a few digressive race shock dynos well beyond 450lbin at 2 sec.

What's the recommended for a 450/8kg spring rate then?

Basically just want to know how to determine the max spring rate a damper can handle from a dyno. It would be of good use to know, if the time comes when people want to quickly swap/experiment spring rates without a rebuild.

[solidONE]

I was exaggerating with that comment. The force required to compress a spring 450 lb/in an inch is 450 lbs. A damper that has a resistance of 450lbs. at 1 in/sec velocity will not allow the 450 lb/in spring to be compressed at all with that amount of force, thus my steel rod damper comment.

Recommended damping rate for a spring rate will depend on the application and your target bounce frequency on your setup or how much you are willing to compromise. The higher the bounce frequency the harsher the ride. The lower the frequency, more compliant it will be. This is a very VERY simplified description I just made of just one single factor to choosing a damper for a spring. There are many more factors to consider.

I'm not going to pretend I understand this fully, as I myself am in the process of learning this stuff and have been for a while. There a many other members on this board that better qualify to answer your question. And by the lack of response you're getting, this should give you an idea of complicated this subject is. So much so that nobody, besides myself, has been willing to comment thus far.

I will share a couple of links to some literature to try to help you. For the sake of not wanting to give bad or wrong advice and steer you the wrong direction, this is as much as I'm willing to say. I'd love to see what some of our more established suspension nerds have to say about this subject as well. I have my own thoughts about this, but I'd rather keep to myself until have confirmed my own 'theories' and math to be applicable.

This one from a suspension nerd on a different board.

This one is not auto specific, but much of it can be applied to auto apps. Dampers, at th end of the day, are dampers regardless of what you are using it on moto, quad, snowmobile, mountain bike etc.

Hope this is more help than it is confusing. Cheers!

[solidONE]

Oh and see how much of the info on this thread you can digest: http://www.ft86club.com/forums/showthread.php?t=26661

This is where the most hardcore suspension nerds hang out on the board.

[fooddude]

deep stuff indeed

[solidONE]

Another thing you can do to get a better idea is to look at the rates on existing setups that work well. note the spring rates they are using and the damping curves. Look at as many as you can find and try to find similarities. Conversely, you can look at the shitty setups and compare them with the good ones to get an idea of why the shitty ones are not up to par. Although dyno graph may not tell you the entire story about a damper. Hysteresis and the effect of temperatures may cause a damper to perform rather different than what it looks like on a dyno graph.

I know you asked a simple question. I wish I knew of a simple answer to you question. I hope some of the other guys will chime in.

[Racecomp Engineering]

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?

Short answer yes, the shock dyno can answer these questions.

You want to read up on damping ratios and critical damping.

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

[fooddude]

Originally Posted by Racecomp Engineering

To be the "ideal" 65% damped you'd want to be around 73 lbs at 2 inch/sec. IMO that is too soft for what it's worth.

- Andy

A bit thick for me, I am following a little, hehe..but thanks!

So...even with all that calculation to find that "65% ideal damping".. then why is 65% "ideal" in the first place?...especially when the outcome you got resulted in something obviously too soft and maybe wrong?? Or is this just the bare minimum? Is this where preference and custom settings takes place?


Hmm, maybe instead of n00bs like me trying to figure it out lol,, we can just ask you/others what the max spring rate is that certain dampers can handle, hehe ...like the new B8s or other future dampers/coilovrs

[Racecomp Engineering]

Originally Posted by fooddude

A bit thick for me, I am following a little, hehe..but thanks!

So...even with all that calculation to find that "65% ideal damping".. then why is 65% "ideal" in the first place?...especially when the outcome you got resulted in something obviously too soft and maybe wrong?? Or is this just the bare minimum? Is this where preference and custom settings takes place?


Hmm, maybe instead of n00bs like me trying to figure it out lol,, we can just ask you/others what the max spring rate is that certain dampers can handle, hehe ...like the new B8s or other future dampers/coilovrs

So too soft and the damper will overshoot and oscillate too much and feel poopy. Too stiff and it can actually take longer to return to normal. And it's just painful.

This image might help visualize it. You can see how the 1.5 case (150%) takes longer to stabilize at 0 than the 0.5 (50%)case. Lots of people stick with 65% because it's a good balance between absorbing bumps, quickly settling, and not being too stiff.



We'll provide all the technical support we can with what we offer, but you'll have to read up for everything else. I like talking about this stuff but even I have my limits.

- Andy

[solidONE]

Originally Posted by Racecomp Engineering

So too soft and the damper will overshoot and oscillate too much and feel poopy. Too stiff and it can actually take longer to return to normal. And it's just painful.

*bla bla equations and numbers bla bla*
- Andy

Word! u don't want no 'poopy' suspension. Lol

[fooddude]

Thx for the simple example. Makes perfect sense - want the damper to catch the bounce/oscillation at a decent balance.

[Racecomp Engineering]

I kinda feel like doing a shock info sticky.


- andy

[solidONE]

Originally Posted by Racecomp Engineering

I kinda feel like doing a shock info sticky.


- andy

Please do. It's so much clearer reading from your posts than the complicated mess I have. Also include some real examples, as in existing product you guys are selling (with dynos), how you guys decided to pair dampers to chosen spring rates and how/why those spring rates were chosen for the application.