What can you learn from a shock dyno chart? Or shock dyno 101

[Racecomp Engineering]

Shock dyno charts have more and more become a topic of discussion in the aftermarket suspension world over the last decade. They can also be complicated and are not always well understood. This is intended to be a brief overview that may help you at least know what you're looking at when you stumble across a shock dyno chart in the wild. I'll talk about exactly how much compression and rebound you may want as well as how each of them affect the car in another thread.

Note: I'm not a shock engineer. I've learned a few things over the last decade from reading books and technical articles and listening to very smart people. This overview is relatively basic and hopefully free from error. I'll add some links for more advanced stuff. If I've made a mistake, please feel free to let me know without being a ****.

1. Force vs velocity chart basics
2. Damping ratio
3. Digressive vs. linear vs. progressive vs. regressive
4. Comparison example
5. Should I require a chart?

__________________________________________


1. Force vs velocity chart basics



The x-axis is velocity of the piston in the damper. This is not vehicle speed. This is the speed of the shock shaft moving up and down. Generally, up to about 100 mm/sec is considered "low-speed" and this is where we look when we want to see how the shock behaves during body roll. Hitting bumps and kerbs will result in piston speeds greater than 200 mm/sec and we consider this the "high-speed" range. Some will also look at a "mid-speed" range between the two and that can be useful too.

The y-axis is damping force. Compression damping, also known as bump, is the top part above 0 and is the force the damper generates when you compress it (or the wheel moves up when it's on the car). Rebound damping, also known as extension, is the part below 0 with negative values and is the force the damper generates resisting extension back to normal. On the car, you might think of this during body roll as the damper pulling the body down or the force keeping the wheel up in the wheel well instead of back on the ground after you hit a bump.

*You will sometimes see compression and rebound flipped. This will depend on who's making the chart. IMO rebound should be on the bottom with negative force values. You may also see compression with negative velocity values and on the left side of the y-axis.

** Sometimes you'll see velocity in inches per second and force in lbs. Different strokes, but I'm a scientist and I like SI units for a lot of reasons.

*** Another type of dyno chart is a force vs. displacement graph. I won't get into those in this post.

**** Sometimes you'll see a force vs. velocity chart that's a lot more "raw" and not smoothed or adjusted. This can be really helpful, but I won't get into that right now either.


2. Damping ratio

The question of how stiff should the shock be (how much force) can be an article on its own. We want a good firm platform for handling and quick response, but we want to be able to hit bumps without being launched off the ground. So we want stiff low-speed and soft high-speed then right? Yes, kind of. The actual amount of force you want depends on a few factors...sprung mass (everything but the wheel/tire/brakes and the shock), the spring rate, and the piston velocity. With these values you can calculate a "critical" damping force; this does not mean this is the correct amount of force but it is a useful reference point. The damping ratio is the force your shocks are generating divided by the critical damping force. 65% to 70% of this critical damping force is often seen on the internet as recommendations for low speed damping. Opinions will vary greatly. There is a lot more here to talk about and I hope to get to it later...including some math!

Looking at how the damping ratio changes with velocity can tell you a lot about how a shock will behave.


3. Digressive vs. linear vs. progressive vs. regressive.

We talked about how we want a stiff low-speed and soft high-speed damping right? Well, what we really want are higher damping ratios in low-speed and lower damping ratios at high-speed piston velocities. The same ratio throughout all piston velocities would be a linear shock...the force curve would be a straight line.

A digressive shock will have a lower damping ratio in the high-speed range. This is generally a good thing. The force curve will have a "knee" after which the slope of the line is reduced. Forces will still be higher than the low-speed range, but the damping ratio is not.

A progressive shock will have a higher damping ratio in the high-speed range. That's generally not desirable, but some rally shocks will get a little progressive in the very high piston speed range.

A regressive shock will have a knee similar to a digressive shock, but the high-speed forces will actually be lower than the low speed forces. This is pretty rare...and expensive.


4. Comparison example

Here we have an shock dyno chart comparing 3 different front shocks for 2015 STI. Blue is OEM, red is a track oriented coilover, and green is a street coilover. Spring rates are different on all three and obviously an important part but we don't necessarily need to talk about them here.



What do we see? Well, the green street coilover is probably fairly comfortable (assuming it has enough travel) since it does not have a ton of high or low speed damping. However, this is an example of a coilover that may not be a handling improvement over stock. It might feel a little sloppy in comparison and probably won't handle transitions as well.

Blue (OEM 2015 STI) has a nice digressive curve for both rebound and compression. Plenty of low-speed rebound for good handling control with high-speed leveling off a bit. It's still a lot of rebound overall though.

The red track coilover has a TON of high speed rebound. That isn't great for bumps or street ride and requires you to use firm springs to keep the car from "jacking down." That occurs when you have too much damping and the strong rebound force prevents the wheel from quickly returning to normal. Over repeated bumps that can be very uncomfortable since you'll eventually run out of travel. At the same time, you have less low speed compression than stock. So it's basically the worst of both worlds. Uncomfortable and probably not great on the track.

Both of the coilovers are adjustable and generally you'd want to test them at multiple settings. Let's say we want to make the car firmer in the low speed range, so we turn the knob up on the red one and increase rebound damping. You end up with even more high speed rebound (which was already too high). You can turn it down, but you already have less low speed than one of the other options so less low speed control. Kind of the worst of both worlds. So, adjustable or not...it's not what I would choose.

This is a case where a shiny adjustable coilover may not be all that great. There's always been the assumption that a coilover is automatically better than stock but that's not always the case. Yes, these 2 coilovers are height adjustable and may have firmer springs, but they may not be an improvement other than that.


5. Should you require a chart with your shocks?

For me it depends. I never did dyno the Konis on my MINI but it's a street car and I have enough experience with Koni to know what to expect. I'm still curious, but it would not change anything for me. For a coilover on a car that will see track time and I know I'll be considering swapping spring rates at some time, yes I want to see a shock dyno. I trust Ohlins, I trust KW, and I trust JRZ, and I know they'll be great off the shelf and work well. But sometimes I need some data to work with if I want to get the most out of them. And if I'm hearing about some brand I've never heard of before, then yes I want to see a shock dyno in conjunction with some driving.

The shock dyno is not the definitive gauge on how good a shock is or isn't. But it's one important tool in the toolbox that can be used if you know what to look for.


6. Links

Roehrig shock dyno training
This video series shows how the actual tests are done and have a lot of detail. A little slow and dry but informative.
https://www.youtube.com/channel/UCyZ...vS6MuGA/videos

OptimumG
The Springs and Dampers section is awesome. Lots of great detail.
http://www.optimumg.com/technical/technical-papers/

I'll add more some other time.


- Andy

[Calum]

This looks like a great way to kill some time at work tomorrow.

[djliquidsteele]

Holy Crap!!! Thank you!

[Shankenstein]

Thanks for posting up some good information and advice!

I've been sifting through the videos from Fat Cat Motorsports. While some may disagree with the speaker's opinions... I'm a fan of their product, and his theory jives with my understanding.

[ame]http://youtu.be/x0hdJXwQYI0[/ame]

[ame]http://youtu.be/xOtFJ-NcsHQ[/ame]


Far North Racing links:
Shock Dyno Plots
Common Inefficiencies
How to Choose Knee locations
Dyno Plots of Different Shock Brands

As you stated, Optimum G is an amazing resource if you'd like to understand what each zone of the shock dyno is good for.
In Ride
In Roll
In Pitch

[Racecomp Engineering]

Forgot to add the Penske shock manual:
http://www.penskeshocks.co.uk/downlo...TechManual.pdf

Another great link.

[Racecomp Engineering]

I like FCM. Spoke on the phone with Shaikh once. Cool guy and I agree with a lot of what he says.

I also like Dennis Grant/FarNorth Racing. I agree with a lot of the things he says too. Not everything, but most of it.

Love OptimumG. They really connected a lot of the dots for me.

- Andy

[Shankenstein]

Originally Posted by Racecomp Engineering

Forgot to add the Penske shock manual:
http://www.penskeshocks.co.uk/downlo...TechManual.pdf

Another great link.

Speaking of Penske, what about their shock dyno plots would be an indicator of quality? (Penske 8100 pictured below)

What I see? Probably a dedicated track car with Optimum_G-style response.

• The initial slopes match. Minimal jacking down and consistent response during steering inputs (low frequency inputs).
• The adjusters make uniform changes across most of the meaningful shock velocities.
• Compression is slightly digressive, without fully blowing off. Good compromise that won't bottom easily or generate unnecessary driver discomfort.
• Rebound builds steadily, rather than a steep digressive response. Probably not designed for the street (but still would feel better than many coilovers).

What else should we be seeing?

[Racecomp Engineering]

Originally Posted by Shankenstein

Speaking of Penske, what about their shock dyno plots would be an indicator of quality? (Penske 8100 pictured below)

What I see? Probably a dedicated track car with Optimum_G-style response.

• The initial slopes match. Minimal jacking down and consistent response during steering inputs (low frequency inputs).
• The adjusters make uniform changes across most of the meaningful shock velocities.
• Compression is slightly digressive, without fully blowing off. Good compromise that won't bottom easily or generate unnecessary driver discomfort.
• Rebound builds steadily, rather than a steep digressive response. Probably not designed for the street (but still would feel better than many coilovers).

What else should we be seeing?

Yes those things, but with Penske you can have your shock built any way you want. They'll make you a linear rebound, regressive compression shock if you ask. Or digressive on both. Whatever. And the adjusters will be perfect.

- Andy

[Calum]

Andy, that's awesome, great job putting everything in language that's easy to read and follow. I can't wait for the next episode.

Any chance you could post the dynos you guys took of the twins and any of the RaceComp Engineering coilovers?

[Racecomp Engineering]

Originally Posted by Calum

Andy, that's awesome, great job putting everything in language that's easy to read and follow. I can't wait for the next episode.

Any chance you could post the dynos you guys took of the twins and any of the RaceComp Engineering coilovers?

Thanks for reading it.

Yes, I have them all somewhere. Need to dig. The RCE coilover dynos were posted somewhere on here, need to find them. The raw data is on my home computer.

[Racecomp Engineering]

OEM BRZ/FRS shocks

[RBbugBITme]

Originally Posted by Shankenstein

What else should we be seeing?

You're looking at high speed adjuster affects only, no low speed bleed adjuster changes were made in this graph which would dramatically change the 0-3 in/sec range. Combining the two adjusters would show a large total adjustment range which is good for marketing but bad for pressure balancing/hysteresis.

Also, do you have gas pressures/rod force from a gas test on the other shocks? Rod force is rarely discussed and is a way for a poorly designed shock to make a driver feel like he has good low speed platform when there is very little low speed damping. In other words it feels good to those that expect a go kart feel but doesn't actually improve grip in power down or cornering scenarios.

Is the above BRZ dyno from a 2014 or 2015 shock? I'm told the spring rate and shock curves were changed.

[Racecomp Engineering]

Originally Posted by RBbugBITme

You're looking at high speed adjuster affects only, no low speed bleed adjuster changes were made in this graph which would dramatically change the 0-3 in/sec range. Combining the two adjusters would show a large total adjustment range which is good for marketing but bad for pressure balancing/hysteresis.

Also, do you have gas pressures/rod force from a gas test on the other shocks? Rod force is rarely discussed and is a way for a poorly designed shock to make a driver feel like he has good low speed platform when there is very little low speed damping. In other words it feels good to those that expect a go kart feel but doesn't actually improve grip in power down or cornering scenarios.

Is the above BRZ dyno from a 2014 or 2015 shock? I'm told the spring rate and shock curves were changed.

On the 2 aftermarket coilovers, no gas or rod force unfortunately. I should have it for the OEM stuff. I'll try to find it later.

The BRZ dyno is a 2013 shock. Shocks did change in 2015 but I don't think springs did (or at least not by much).

- Andy

[Calum]

Can someone define platform in this context?