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Double Adj Shocks setup: Theory vs Practice

Car: 2016 GTPP
SUSPENSION: MCS Doubles with 450/750 springs
Use: Autocross (CAMC)

RE: Double adjustable shocks/coilovers, I've seen that the "recommended" ratio Rebound to Compression is 2:1....
Is this a good ballpark to stay within (assuming surface and what not). I know this is a generality and is good in theory, but what about in practice?

Been playing around and I'm at 11R/3C front and rear and it feels good (def stiff) when I drive it to work. Haven't tried this particular setting at an event, yet.

Should I try a higher compression setting?
 
75
49
Exp. Type
HPDE
Exp. Level
Under 3 Years
TX
Car: 2016 GTPP
SUSPENSION: MCS Doubles with 450/750 springs
Use: Autocross (CAMC)

RE: Double adjustable shocks/coilovers, I've seen that the "recommended" ratio Rebound to Compression is 2:1....
Is this a good ballpark to stay within (assuming surface and what not). I know this is a generality and is good in theory, but what about in practice?

Been playing around and I'm at 11R/3C front and rear and it feels good (def stiff) when I drive it to work. Haven't tried this particular setting at an event, yet.

Should I try a higher compression setting?

2:1 and 3:1 is recommended for a couple reasons. 1) Rebound controls sprung mass and Compression controls unsprung mass. Since the sprung mass is higher than unsprung you need more force to control it. 2) Rebound is used to "tie down" 1 or more corners of the car, think of shocks as a timing device to place the car into a position most optimal to achieve predictable results for a given setup.

More total damping (stiffer) means that the primary goal is body control, this is most typical on very flat tarmac with lots of aero on the car. It is more common for high aero cars which are sensitive to pitch. Otherwise with less aero you can allow the body to move around more and take more advantage of mechanical grip. Less stiffness in the system gives more mechanical grip, technically you want as little spring and damper as you can get away with. What that is depends on what your goal for the car is.

JAJ's post is a good starting point, my caveat to knoi's dated approach is that you can run more compression than you think you otherwise could.

Lastly, I got hung up on theory vs practice a lot myself. I am actively experimenting to understand what that difference is for my own fun. One thing I have learned to be true and I have read and heard the same from engineers is that; The car wants what the car wants, the difference between theory and practice basically go out the door depending who is behind the wheel. Pick a target or a goal and try to achieve it, turn some knobs on the way and see what the car likes or doesn't to get you to the goal. From there you will know what you need to do first should that "goal" ever change.
 

ArizonaBOSS

Because racecar.
Moderator
8,552
2,392
Arizona, USA
Honestly, you're going to need a test day. Sweep up and down the ranges (or at least start in the middle, then do runs with significant incremental changes in one direction) to understand what the car does and doesn't respond to or like. Then you can zero in on your optimal setup with smaller changes. It will take time and effort but it will be worth it.
Also it would be worth knowing if these are high-speed or low-speed adjustments, in both cases. This is with respect to shaft speed, not vehicle speed.
 
Honestly, you're going to need a test day. Sweep up and down the ranges (or at least start in the middle, then do runs with significant incremental changes in one direction) to understand what the car does and doesn't respond to or like. Then you can zero in on your optimal setup with smaller changes. It will take time and effort but it will be worth it.
Also it would be worth knowing if these are high-speed or low-speed adjustments, in both cases. This is with respect to shaft speed, not vehicle speed.
Both low speed.
 
2:1 and 3:1 is recommended for a couple reasons. 1) Rebound controls sprung mass and Compression controls unsprung mass. Since the sprung mass is higher than unsprung you need more force to control it. 2) Rebound is used to "tie down" 1 or more corners of the car, think of shocks as a timing device to place the car into a position most optimal to achieve predictable results for a given setup.

More total damping (stiffer) means that the primary goal is body control, this is most typical on very flat tarmac with lots of aero on the car. It is more common for high aero cars which are sensitive to pitch. Otherwise with less aero you can allow the body to move around more and take more advantage of mechanical grip. Less stiffness in the system gives more mechanical grip, technically you want as little spring and damper as you can get away with. What that is depends on what your goal for the car is.

JAJ's post is a good starting point, my caveat to knoi's dated approach is that you can run more compression than you think you otherwise could.

Lastly, I got hung up on theory vs practice a lot myself. I am actively experimenting to understand what that difference is for my own fun. One thing I have learned to be true and I have read and heard the same from engineers is that; The car wants what the car wants, the difference between theory and practice basically go out the door depending who is behind the wheel. Pick a target or a goal and try to achieve it, turn some knobs on the way and see what the car likes or doesn't to get you to the goal. From there you will know what you need to do first should that "goal" ever change.
When I posed this question to a well-known National Champ Autocrosser, he stated people don't needs much in terms of compression damping.

Truly I guess it is all driver dependent.
 

ArizonaBOSS

Because racecar.
Moderator
8,552
2,392
Arizona, USA
Both low speed.
OK, that's good; both adjustments are controlling pitch/roll/dive, in that case.
I would try gradually creeping up the compression adjustments to find how much you can run without making the ride too harsh in autox conditions.
For rebound, just understand that this is going to help control how much weight transfers as the car unloads the front as you go to throttle, and unloads the rear as you go to braking.
 
75
49
Exp. Type
HPDE
Exp. Level
Under 3 Years
TX
When I posed this question to a well-known National Champ Autocrosser, he stated people don't needs much in terms of compression damping.

Truly I guess it is all driver dependent.
A lot of what determines results is the driver for sure. I can say as anecdotal as my experience is, in speaking with many engineers, suspension tuners, and other kinematic resources. There is a lot of consensus that you can run more compression than what was previously thought. Jan Zuijdijk (JRZ's founder) has a book where he talks about his own discovery of running more compression after having been an engineer for both Koni and Bilstein and participating in the suspension design for one of the various incarnations of the 911.

I'm not saying it is a requirement or that it is a one size fits all solution, merely that should you choose to, you can run more compression as it resembles additional spring rate which in the most simplest examples more spring rate will confer more grip to a point. It is way more complicated but I was generalizing since the subjective experience and the best stopwatch time crossover will be different person to person, car to car.
 
A lot of what determines results is the driver for sure. I can say as anecdotal as my experience is, in speaking with many engineers, suspension tuners, and other kinematic resources. There is a lot of consensus that you can run more compression than what was previously thought. Jan Zuijdijk (JRZ's founder) has a book where he talks about his own discovery of running more compression after having been an engineer for both Koni and Bilstein and participating in the suspension design for one of the various incarnations of the 911.

I'm not saying it is a requirement or that it is a one size fits all solution, merely that should you choose to, you can run more compression as it resembles additional spring rate which in the most simplest examples more spring rate will confer more grip to a point. It is way more complicated but I was generalizing since the subjective experience and the best stopwatch time crossover will be different person to person, car to car.
I've always heard "Compression increases grip until it doesn't".

I'm definitely going to play with some things for sure. I have 3-4 more events this year so I'll treat each as a test/tune.

I thought for sure 11 would be too much on rebound, but in some street "testing" it seems to be good.

Thanks all.
 

Dave_W

Cones - not just for ice cream
418
404
Exp. Type
Autocross
Exp. Level
20+ Years
Connecticut
I've seen that the "recommended" ratio Rebound to Compression is 2:1....
...
Been playing around and I'm at 11R/3C front and rear and it feels good
Not sure, but are you thinking that the 11R/3C means a 3.67:1 rebound-to-compression ratio? The bulk of the shock damping is set by the main valving in the shock (deflective discs on the piston for a typical monotube), and the adjusters change the damping from that main valving baseline. Depending on brand and parts used, the adusters can vary between a wide and narrow adjustment range. Did MCS provide dyno plots with adjuster sweeps for your shocks (not plots of generic models, the 4 dyno plots for actual 4 shocks you bought)?
 
Not sure, but are you thinking that the 11R/3C means a 3.67:1 rebound-to-compression ratio? The bulk of the shock damping is set by the main valving in the shock (deflective discs on the piston for a typical monotube), and the adjusters change the damping from that main valving baseline. Depending on brand and parts used, the adusters can vary between a wide and narrow adjustment range. Did MCS provide dyno plots with adjuster sweeps for your shocks (not plots of generic models, the 4 dyno plots for actual 4 shocks you bought)?
This is all that I have.Screenshot_20211015-055954_Chrome.jpgScreenshot_20211015-055937_Chrome.jpg
 
1,012
947
In the V6L
Okay, so the sets of charts indicate that the compression and rebound settings are strictly for the low speed damping setup. High speed damping, the slope of the line past the breakpoint, is fixed in whatever dampers these were. Four-way adjustable shocks make that adjustable too, but they're pretty exotic for a track day setup.

Low speed damping is about controlling the sprung mass (the chassis with driver) relative to the earth; high speed damping controls the motion of the road wheel relative to the sprung mass. Low speed damping adjustments affect the way that weight transfers with driver inputs that change the attitude of the car - accelerating, braking and turning. High speed damping is essentially a blow-off valve that acts when you encounter a suspension input from the road - a bump, a curb, whatever.

Handling is about weight transfer. Setting a high level of compression damping on the front dampers will slow the compression of the outside spring, effectively increasing its spring rate until the suspension stops moving. Set it too high, the tire is overloaded briefly and grip suffers. Set it too low and not enough weight is transferred and grip suffers. Setting a high level of rebound damping on the front dampers will slow the rise of the inside spring, taking weight off the inside tire more quickly. Whether this matters depends on how much grip the outside tire can generate - it could help or it could hinder. The same logic applies in the rear suspension.

So, that's why instructions like the ones from Koni make sense - if you aren't lucky enough to have a four-post test rig, then you have to drive the car and follow an organized and disciplined approach to finding the sweet spot for that car with those tires on that specific track. There is no magic ratio or theory that will find it for you. A ratio might get you close, but you're leaving something on the table.
 
Okay, so the sets of charts indicate that the compression and rebound settings are strictly for the low speed damping setup. High speed damping, the slope of the line past the breakpoint, is fixed in whatever dampers these were. Four-way adjustable shocks make that adjustable too, but they're pretty exotic for a track day setup.

Low speed damping is about controlling the sprung mass (the chassis with driver) relative to the earth; high speed damping controls the motion of the road wheel relative to the sprung mass. Low speed damping adjustments affect the way that weight transfers with driver inputs that change the attitude of the car - accelerating, braking and turning. High speed damping is essentially a blow-off valve that acts when you encounter a suspension input from the road - a bump, a curb, whatever.

Handling is about weight transfer. Setting a high level of compression damping on the front dampers will slow the compression of the outside spring, effectively increasing its spring rate until the suspension stops moving. Set it too high, the tire is overloaded briefly and grip suffers. Set it too low and not enough weight is transferred and grip suffers. Setting a high level of rebound damping on the front dampers will slow the rise of the inside spring, taking weight off the inside tire more quickly. Whether this matters depends on how much grip the outside tire can generate - it could help or it could hinder. The same logic applies in the rear suspension.

So, that's why instructions like the ones from Koni make sense - if you aren't lucky enough to have a four-post test rig, then you have to drive the car and follow an organized and disciplined approach to finding the sweet spot for that car with those tires on that specific track. There is no magic ratio or theory that will find it for you. A ratio might get you close, but you're leaving something on the table.
So...high rebound damping on L hand turn shifts wt toward outside tire quickly and unloads inside tire quickly....how does that affect the Torsen diff? Dependent on tire/surface I guess?

How does one correct for different surfaces such as asphalt vs grippy concrete in autocross so to speak? Can't test every event. I guess take an event at each surface and "test"?

Sorry if this doesn't make sense...I'm still trying to figure things out
 

Dave_W

Cones - not just for ice cream
418
404
Exp. Type
Autocross
Exp. Level
20+ Years
Connecticut
To help the Torsen, you want to increase the inside rear tire load on corner exit (or reduce the overall unloading in a corner), or increase the speed at which the load transfer takes place. You can decrease the inside rear unloading by either reducing the rear roll stiffness or increasing the front roll stiffness. You can affect the rate of load transfer with low-speed shock adjustments. But shock adjustments only work on shocks that are moving - a shock that's not moving develops no force for the suspension.

As the car transitions from steady-state corner at the apex to power-on exit, the suspension movement is going from roll to squat. The outside front corner is losing load / coming up / in rebound, and the inside rear is gaining load / moving down / in compression. To increase the speed of that load transfer, you want to increase the compression stiffness of the rear shock, and/or decrease the the rebound stiffness of the front shock. These same adjustments promote understeer on corner exit, which helps counter the oversteer being induced by a RWD car's increased longitudinal grip use of the rear tires under acceleration.

So both the sway bars (and springs) and shocks can change the percentage each axle sees of the total lateral load transfer, which then tunes handling. But the bars & springs change it for all phases of a corner, whereas shock adjustments can change it for specific phases of a corner.

This has a good explanation and a chart of shock adjustments - http://www.ozebiz.com.au/racetech/theory/shocktune1.html
Note that the same shock adjustment can affect the car's handling differently in different phases of a corner.

For different surfaces, the easy assumption is that you're changing the maximum grip level, so you can attain more lateral & longitudinal g-loading. The same adjustments apply, just with more grip they have more affect - as more load is being transferred, any difference in the load transfer percentage front-to-rear will be more pronounced. Bumpy vs. smooth is a whole different matter, where you can work with high-speed damping, but for very bumpy you want to make sure your rebound/compression split is not so great that you "jack down" or "jack up" the shock - soften all-around to let the shock find it's balance more easily.
 
Last edited:
75
49
Exp. Type
HPDE
Exp. Level
Under 3 Years
TX
split is not so great that you "jack down" or "jack up" the shock
Note* Jacking will always take place when the damper is not force matched. More rebound will cause the car to jack down or tie down. This means the shock is slower to extend to ride height and over successive bumps will become shorter and shorter until you hit the bump stop or run out of travel. The more rebound the quicker this happens (most passenger cars do this off the lot). The opposite is true with too much compression, you get a launching type reaction and the car can feel skittery over successive bumps. In practice you will more than likely not run a force matched damper, you can and it is on paper what an ideal damper is. However as mentioned before chassis stiffness, tyre, road surface, etc all play a part in what the final ratio split will look like.
 

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