Damping Ratios

[ritwikdas18]

Hello,
My damping ratios are as follows


Front low speed compression damping ratio 0.67
Front low speed rebound damping ratio 1.50
Rear low speed compression damping ratio 0.67
Rear low speed rebound damping ratio 1.50

Front high speed compression damping ratio 0.35
Front high speed rebound damping ratio 0.75
Rear high speed compression damping ratio 0.35
Rear high speed rebound damping ratio 0.75


Knee Speeds:

Front Compression 12.50 mm/s
Front Rebound 12.50 mm/s
Rear Compression 12.50 mm/s
Rear Rebound 12.50 mm/s

Mass of car :275 kg
Ride Frequency Front: 2.83 Hz
Ride Frequency Rear : 2.69Hz
Front Weight %- 35
MRF- 1.30
MRR-1.64
Front Spring Rate: 22 N/mm
Rear Spring Rate: 63 N/mm
No ARBs
I wanted to know if the above ratios are in the ballpark. I am using KAZ QM dampers.

Or should I use the following ratios:

Front low speed compression damping ratio 1.35
Front low speed rebound damping ratio 1.15
Rear low speed compression damping ratio 1.50
Rear low speed rebound damping ratio 0.92

Front high speed compression damping ratio 0.70
Front high speed rebound damping ratio 0.56
Rear high speed compression damping ratio 0.70
Rear high speed rebound damping ratio 0.54

KAZ has suggested less damping ratio in rebound in their damping calcs which is the major reason I am all confused now.Please help. This is an old calc I have from KAZ.
965

The link to the excel sheet is below.

https://docs.google.com/spreadsheets/d/1AFlmX_naDcl80clZFQqB6dIyF4hQGcPOV46t12gsqU4/edit?usp=sharing

[CWA]

Which settings do you think will be the best? Why?
Why do you think KAZ recommended the settings that they did?
Who do you think knows more about the application of these dampers, your team or Kaz?
Clue - nobody on this site is going to attempt to answer these questions for you until you've at least had a go yourself

[Claude Rouelle]

Ritwik,

Not a bad question but definitely not the best way to ask a question. Looks entitled, needy...

Anyway...

I wish there would be an easy answer for your question. And if the answer was complicated and I had the answer I would be very rich!

You need serious good track profile data with the shape of each bump and curb, car data, tire data, inertia, masses, masses CG location, compliances (the compliance is a biiig issue in FS) , the real installation stiffness (again compliance: the motion ratio is most probably not the one you think it is) etc…. And/Or you need 4 or 7 post rig testing.

In other words simulation has its limits. Especially for dampers.

A few side but important perspectives I can share with you.

1. Can you be pretty sure that what you have in your calculations is pretty to what you have in your dampers> in other words did you put your dampers on a damper dyno to see if your logged damper Force Vs Speed do match your prediction. If you do not have a damper dyno do you have data that you can trust. If you do not have such damper dyno test results your calculation will be irrelevant and will not impress any design judge.

2. i am very skeptical about damper setup (piston, shims, preload, needle, low and high speed bump and rebound "clicks") given by damper manufacturer as I never saw 2 dampers with the same setup giving the same damper curves. I saw as much as 20 % of difference in the force at a given speed of 2 dampers with the same setup. The issue is manufacturing tolerance, bleeding procedure....etc... Some FS damper manufacturers are noticeably bad for that.

3. In any case if you record data from damper linear potentiometers and you create damper speed histograms I bet you would see that you are 95 % of your lap the time between -50 mm/sec and 50 mm/ sec which is what people usually call low speed (some define low speed between – 25 and +25 mm/ sec. It is just a convention.) Therefore the high speed adjustment is probably irrelevant in FS. In Baja for sure but in FS most probably not.

4. The # 1 influence on car performance is tire grip. The # 1 influence on tire grip is tire temperature. The #1 tire temperature influence is tire temperature. I have seen competitive teams using damper ratios as big as 3 (Yes three) to get their tire at temperature.

5. As an engineer and a design judge I should be only rational and scientific so I hate to say the following but here we go... Remember this rule of thumb: bump is for the tire and rebound is for the driver. Not scientifically proven (to do that you would need many things, first one being a thermal tire model) but my experience seems to confirm this is rule of thumb is working. Tire too cold more bump. To hot less bump. Too much tire wear less bump. Driver chicken more rebound. Driver confident less rebound until he chickens again. More damping for Autocross (quick grip) than endurance (less wear). Do not be afraid to have asymmetrical left and right dampers setting if it is what your tires ask for. And do not come to design telling other judges you did that because “Claude told you so”.

6. It is good that you made the calculations but at the end you need to test. Find a damper dyno and get your damper “real” curves. On the test track try different damping ratio, try different low-high speed break, and different bump-rebound ratio. Contrarily to what some book say (and I said 10 years ago – I was not wrong but I wasn’t that right either- I just keep learning) the rebound doesn’t need to be smaller than the bump. I have seen good cars having twice the amount of bump than rebound. Again try and follow what your driver, your tire visual inspection (graining) and temperature and most of all your lap time and even more your lap time consistency.

*****

35 % front weigh distribution? Is this real or is there a typo? Same tires front and rear? And (here we go again) no ARB?

[J-rob68]

Doing the basic calculations is great, but at the end of the day as Claude has stated your going to have to tune the dampers appropriately. If you have access to any damper dyno data for these dampers you should see an extremely broad range of adjustment available to you. So much adjustment in fact that the extreme positions may or may not actually be useful to you.

Its true that some teams may use extreme damping to overcome certain track conditions such as very cold track temperatures, or perhaps suspension kinematics that are not aggressive enough (roll center heights which are too low etc) to heat up the tire properly, but in most cases you hopefully wont need a damping ratio of 3 ;)

I would agree with Claude. Get your dampers in, get them in early. Test them on a damper dyno to see what the actual curves are, and label each damper for reference. Then go out and tune each one based on tire temps and driver skill. Get a good data harness on the car so you can start drawing data based correlations between damping ratios and lap times, yaw rates, tire temperature and rates at which the tires build up heat. Then make your own educated decision on which damping ratios are the best for your car, driver, track on that day.

[darges]

Last year, we were able to put our Ohlins on a shock dyno down at RCR. I was shocked to see how different they were, side-by-side. There was absolutely no consistency in dyno plots when they were at the same settings. As Claude said, yes the manufacturer gives you some plots, and yes they might have been right for 1 damper they tested. But getting all 4 to all match the damping you want would almost certainly require a dyno to know what the shocks are actually doing.

[Claude Rouelle]

You probably do not need the same setting for 4 dampers. The front and rear probably require different settings.

And another one; if you change your springs or ARB stiffness you should change your damper settings...