Damping ratio & Ride Frequency questions

I think there are too few setup & physics topics swirling around and I have a few questions floating in my head.

1) A higher front than rear ride frequency is preferred for race cars vs passenger cars, which do it exactly the other way around for maximum comfort. But are there cases, especially in high downforce cars, where you might want to do it the other way around as well?

2) In most cars again, an undercritical damping is desired with higher rebound than damping. But there seem to be suspension layouts which have ratios bigger than 1 and have much higher damp than rebound values. Sometimes only in the front as I noticed in some GT3 cars. Is it possible to determine from numbers, what the right layout should be like, without having the actual technical manual from the manufacturer?

3) The F138 shows significantly lower ride frequencies than GT3 cars, which should be the other way around by my understanding. Is this because the car engineer app doesn't take into account the heave dampers and with those values it'd be >4hz?

4) How to use MoTec or similar software to analyse what happens with the dampeners and how to draw proper conclusions out of it?

 

1) nope, either equal or front > rear
2) predominantly in GTE and GTLM cars, go by car engineer and try Bump=1.4, Rebound=0.8, else trail & error
3) yep
4) install + compare to RL damping curves and telemetry

http://www.optimumg.com/technical/latest-articles/
http://www.rqriley.com/suspensn.htm
http://www.fsae.com/forums/showthread.php?9062-Damping-Rates
http://www.kaztechnologies.com/downloads/kaz-tech-tips/ (loads of useful stuff)
http://home.scarlet.be/~be067749/58/c2/
http://white-smoke.wikifoundry.com/page/Suspension+Geometry
http://www.vsusp.com
http://racingcardynamics.com/
https://www.slideshare.net/billharbin/vehicle-lateral-load-transfer-parts-iiiimar13
https://www.assettocorsa.net/forum/index.php?threads/how-to-adjust-setups-for-gt3s-in-v1-9.38177/

 

Thank you a lot for all this useful information. This helps really quite a lot and should be pinned as must-read material.

Though some question remain. The older GT3 cars have bumpers with low kN/m damping rate. Thus I can use low wheel rates and available damping is still in the theoretical optimal range. However in the newer ones, e.g McLaren 650S GT3, their lowest bumper rate is 10,6 kN/m. This is only sufficient for wheel rates of >= 140kN/m. Should I compensate these hard springs with more travel range and softer ARB? Or would I have to start using compression based suspension here in order to go for softer springs?

Are there also some applicable formulas for optimal travel range and packer rate in AC or is this pure trial&error without estimated guess possibility? And no, I don't plan on building a mathematical simulation with high order differential equations.

Is there a reason for B=1.4 and R=0.8 or just an example for "working" compression based damping?

When setting up a high downforce vehicle with heave springs/dampers, I'd calculate them using series & parallel circuits to get onto my desired rates. Going by intuition, I'd say heave springs/dampers should be stiffer than the wheel suspension. So there's better mechanical traction and still a high aerodynamic stability. But what should the split be? Based on the F138 default setup it seems to be about 2-3x more than a single wheel.

The more I read about all of this, even more questions arise.

 

I would be more worried on how the car respond to changes. I think when people say "it is over dampened" it just means that the majority doesn't use that setting. It doesn't mean that it is wrong either. Not factoring chassis flex, driveability, comfort.. I think we don't have to follow those things they tell us, especially in simulator.

To what exactly you need to do?

 

In real life there may be limitations to corner spring softness due to limited travel but in AC you can't really break the suspension by compressing the packers too much (metal-to-metal is completely missing step). There have been modern F1 cars that didn't run corner springs at all at rear and used only heave spring + arb setup (https://scarbsf1.wordpress.com/2010/12/03/spring-less-rear-suspension-a-quiet-revolution/).

IIRC when I drove the F138, I ended up running softest front corner springs and maximum front heave stiffness and that allowed low front ride height while not stalling the front diffuser on braking zones. IIRC I setupped the rear heave stiffness to minimize straight line drag (body AOA) after I had figured out the ride height. The car had rear diffuser bug back then, I dunno if it's fixed or not (Aero element for coanda simulation added so much AOA to the diffuser that it actually reduced the downforce so the car would actually have had more downforce without the effect). I reported couple of other bugs on the car and they really didn't seem to care (no response, no fix), and this one might not be even reported, not by me at least.

The ride quality and mechanical grip gain from it (rear frequency > front frequency) is not the most important thing in the aero-car setup. Often you want to optimize heights of the floor aero elements, front being often more sensitive than the rear. Also more torque demand there is from the rear tyres, more the mechanical grip gain from soft rear setup will help (depending on the track ofc).

 

Just a good & fast setup to drive, but I want to reduce the trial & error approach. So at least I have a starting point.

Following Pictures were taken with speeds @ 226-228 km/h on Silverstone International straight at 26 degrees, no wind, optimum track, clear weather @ 12 o'clock. Default setup. Only height has changed.
Front height - Rear Height
30 - 30
30-50
0-50





Because of the different heights, the wings and diffusers work with differences in CL & CD. But it seems not to take into account the rake angle (body) itself to increase CL&CD. Most of the higher CD in the high rake configuration comes from the not-adjusted rear wing angle. It does take into account the AOA of the body. Is this worthy of an issue post in the physics forum or is there something I don't understand yet? I remember reading high rake configuration is free CL in F1 cars, that's why they run these setups in RL.

When you have a higher front than rear frequency it means the rear has a softer setup, while the front is stiffer, thus stabilizing aerodynamics.

 

Yeah those images made me remember a few things.

1) Rear wing drag is crazy and that's why you get way more efficient aero by running it "low". I ran something around 16 front and 7 rear wing settings to get center of pressure close to CoG (as you can see from the images, your CP is around 46-47% but drag is not included in that number so the real CP is around 5% rearwards (41-42% front).
2) Body AOA is quite negligible (in matter of suspension setup) and you should aim to run rear diffuser close to 2 degrees on corner exits where you need the downforce. Diffusers can stall on straights under heavy aero load but it's ok as long as they work close to optimal height in corners.
3) Floor rake effect doesn't exits as itself because the floor is not a "whole" element but rake effects still are implemented by having AOA curves for different floor elements (like the rear diffuser at 2 degrees is optimal). Basically you don't gain downforce from the floor itself (there's no downforce vector middle of the floor), but it's a sum of the front and rear elements.
4) The engine's powercurve is quite weird, because it's like limiter hits way too soon (you'd want to rev above) so there's quite a lot of tenths to be gained on tracks like monza by choosing gear ratios carefully.
5)The car is quite hard to manage at high speed corners with balanced aero setup because the differential locks too much (it's not really curable by aero, unless you want slow car with understeer).

 

How did you estimated/calculated those additional 5% rearwards? I can imagine a GT3 or GTE car being affected more or less than those 5%.

Indeed in terms of suspension it won't play such a big role. Does the AOA change the upper body's function as a "wing" or is this effect so neglible it doesn't need to be calculated, as it seems to be the case as there's no CL value. Front and rear wings and diffusers play the most important role in downforce, though doesn't the upper body give some downforce as well, if so how much, with increased AOA?

Yup, it's modeled already with the mentioned parts.

I think there was a forum post somewhere criticizing the unability to change this. In the real car it's possible I read, correct me though if I'm wrong.

So far, pretty awesomely informative answers in this topic^^

 

It's a ballpark figure and in my tests it often ended up close to the 5% figure (for F138, F3 and GT3 Huracan at least), of course it varies a bit from car to car and varies also with the event (straight line and max downforce vs. start of the braking zone for example). I calculated the center of pressure by taking vector sum of the lift- and drag force vectors for all the aero elements using RaceStudio.

It's possible to make body_AOA_lift curve in Assetto Corsa but this car doesn't have it as can be seen from the 0.00 CL value from the wings app. However drag of the "body-element" is the most significant part in the car's total drag which transfers the center of pressure rearwards.