Vehicle dynamics question for the engineers

[Boobietrap]

I have a question regarding spring rates. Apart from track smoothness and aero, what is the purpose of having stiffer springs (or vice versa)? I'm not talking about F/R balance, my question is what is the reason of having higher (or lower) frequencies on all four corners, from the perspective of physics/engineering, preferably backed up with some math. Please don't respond with anecdotal or "common belief" type of answers.

Similarly, why have higher damping rates, say, 1.1 of critical vs 0.7? Again, not asking about transient balance here. And how should a damper velocity histograms look and be used to tune overall damper stiffness?

Much appreciated!

 

[PhilS13]

Au contraire, I've done plenty of reading but have yet to be able to answer the questions at the start of the thread - as you'll notice that my comments in the latter posts are exactly why I posted the original questions .

Anyway. Still interested in what you meant by your comments on dampers .

You probably know slow damping is a compromise between overshoot and response time(response here in terms of how long the car body takes to reach final state, not load transfer response). Low damping means big overshoot, high damping means slow response. Correct damping minimizes both overshoot and response time.

Around 0.7 is often said to have the quickest response time but comes with a little bit of overshoot which is fine in most cases. It is considered a good baseline in ride, meaning it supposed to be doing a great job at keeping tyre loads as constant as possible when going over square road ondulations. Square for me being when an axle hits an ondulation without any ARB action.

I've read a few times but not checked that the problem with 0.7 in race cars is that often you have pretty stiff ARBs installed but no exclusive "roll" damper coming with it so you can find yourself underdamped whenever the ARB is working. That means as you roll the car in and out of corners or when you hit road ondulations that involves ARBs then you can have big overshoot on the roll motion and you might not want too much of that.

http://www.optimumg.com/docs/Springs&Dampers_Tech_Tip_1.pdf

through

http://www.optimumg.com/docs/Springs&Dampers_Tech_Tip_6.pdf

is covering a lot of it with math looking at roll and pitch stiffness and damping for it exclusively.

 

[Boobietrap]

Thanks. I've actually read those before.

It sounds like the approach is still start so where near 0.7(or higher depending on arb), then tune it for more responsiveness or energy dissipation? What about for high speed damping, is 0.7 still a good baseline?

What if we start considering different bump and rebound rates?

 

[RasmusP]

You probably know slow damping is a compromise between overshoot and response time(response here in terms of how long the car body takes to reach final state, not load transfer response). Low damping means big overshoot, high damping means slow response. Correct damping minimizes both overshoot and response time.

Around 0.7 is often said to have the quickest response time but comes with a little bit of overshoot which is fine in most cases. It is considered a good baseline in ride, meaning it supposed to be doing a great job at keeping tyre loads as constant as possible when going over square road ondulations. Square for me being when an axle hits an ondulation without any ARB action.

I've read a few times but not checked that the problem with 0.7 in race cars is that often you have pretty stiff ARBs installed but no exclusive "roll" damper coming with it so you can find yourself underdamped whenever the ARB is working. That means as you roll the car in and out of corners or when you hit road ondulations that involves ARBs then you can have big overshoot on the roll motion and you might not want too much of that.

http://www.optimumg.com/docs/Springs&Dampers_Tech_Tip_1.pdf

through

http://www.optimumg.com/docs/Springs&Dampers_Tech_Tip_6.pdf

is covering a lot of it with math looking at roll and pitch stiffness and damping for it exclusively.

Thanks for this Post!
Could you give us a proper explanation about soft vs stiff springs and grip?
I'm studying mechanical Engineering but sadly never really learnt about this stuff yet. And i sadly don't have the capacity to read a book about it. Very very interested in this stuff but to figure out what to read is already a Problem... which book is correct, which book really contains and focuses on what I need...
So maybe you could enlighten us as I trust you to definitely know what you're talking about!

 

[PhilS13]

Thanks. I've actually read those before.

It sounds like the approach is still start so where near 0.7(or higher depending on arb), then tune it for more responsiveness or energy dissipation? What about for high speed damping, is 0.7 still a good baseline?

What if we start considering different bump and rebound rates?

Yes it pretty much always is a good starting point, excluding aero. High speed needs to be way lower.

I like more rebound overall for many reasons but we are getting into transient and personal preference tweaks. Whatever works works.

Thanks for this Post!
Could you give us a proper explanation about soft vs stiff springs and grip?
I'm studying mechanical Engineering but sadly never really learnt about this stuff yet. And i sadly don't have the capacity to read a book about it. Very very interested in this stuff but to figure out what to read is already a Problem... which book is correct, which book really contains and focuses on what I need...
So maybe you could enlighten us as I trust you to definitely know what you're talking about!

What the others responded to you is correct and I don't really want to get into that here. This is standard roll stiffness, lateral load transfer and load sensitivity stuff which is explained very well in many places.

You need to make a difference between

- The original question which was stiff vs soft on all four corners completely disregarding track smoothness. That's a highly theoretical thing and the question is pretty much only interesting to look at if you already know a decent amount about car dynamics.

- Being able to go faster on soft springs in the real(or sim) world, which is something that does happen, even if not for the reasons you thought.

 

[RasmusP]

Lesson learnt. I am clearly wrong and need to build some knowledge. Always fun to see that all the lectures and exams successfully done still let me suck at this stuff...
Before I search and read a lot of not properly researched documents or outside of what I can understand right now:
Can you recommend a book for me?

The optimumg links were an interesting read but apart from some general information and equations to calculate a baseline, I couldn't really learn things like how a softer suspension really works or example calculations to really see the resulting values that would need a compomises to be made and how these compromises influence each other.
Like perfect values for roll vs perfect values for pitch.

 

[Boobietrap]

Yes it pretty much always is a good starting point, excluding aero. High speed needs to be way lower.

I like more rebound overall for many reasons but we are getting into transient and personal preference tweaks. Whatever works works.

Ok got it .very helpful. I have a second question for you if you have the time, or for rest of the forum for that matter.

Let's say you have front and rear at similar frequencies, and you have to tune for balance. What is the rational to tune it using different spring rates, or ARB? Is it really important to have sightly higher frequencies in the rear so it will catch up to the front, or does it not really matter in practice?

 

[PhilS13]

Lesson learnt. I am clearly wrong and need to build some knowledge. Always fun to see that all the lectures and exams successfully done still let me suck at this stuff...
Before I search and read a lot of not properly researched documents or outside of what I can understand right now:
Can you recommend a book for me?

The optimumg links were an interesting read but apart from some general information and equations to calculate a baseline, I couldn't really learn things like how a softer suspension really works or example calculations to really see the resulting values that would need a compomises to be made and how these compromises influence each other.
Like perfect values for roll vs perfect values for pitch.

My starting point was skip barber's going faster : mastering the art of race driving. It's not a vehicle dynamics book, it's mostly a driving book. There is no math in there. It only contains a few chapters on car balance, tyres, suspension that make it clear why soft at one end means more grip, why low CG is good, why wider car is good etc...No equations or anything. Just a bunch of "oooooh okay I see". It is useless to go into Gillespie or Milliken kind of books if you don't have that base first. I would stop looking for perfect values, they rarely exist. The goal should be to understand how things work, not to find equations or guides leading to perfect values.

Ok got it .very helpful. I have a second question for you if you have the time, or for rest of the forum for that matter.

Let's say you have front and rear at similar frequencies, and you have to tune for balance. What is the rational to tune it using different spring rates, or ARB? Is it really important to have sightly higher frequencies in the rear so it will catch up to the front, or does it not really matter in practice?

I'm pretty sure that this need for the rear to catch up the front comes from the road passenger comfort side. I see no point in doing that on race cars. Honestly if you give me a car after a roll stiffness change at one end I will feel it but also have a hard time telling if you did it using springs or ARB, unless the aero has been affected. Usually I ask myself if the resulting change in pitch stiffness and/or ride height would help or hurt aero. If it helps I use springs, if it doesn't I use ARB.

 

[Emery]

Let's say you have front and rear at similar frequencies, and you have to tune for balance. What is the rational to tune it using different spring rates, or ARB? Is it really important to have sightly higher frequencies in the rear so it will catch up to the front, or does it not really matter in practice?

If the rules (or simulation) allow, ARB can be adjustable by the driver. Fuel is burnt and tires heat/wear, so giving the driver the ability to compensate the balance is an advantage for races longer than 20 minutes (if the driver knows how to properly use it).

Ride quality (slightly higher frequency in the rear) is more important for the vehicle occupants than it is for handling.

 

[Boobietrap]

What about the proportion of roll resistance coming from springs vs ARBs? Should they be relatively similar, or does it not matter even if, say, ARBs contribute to 80% of the wheel rate/roll resistance?

 

[Emery]

If you think of ARBs as defeating independent suspension, then the answer surely is as little as possible of the roll resistance should come from ARBs. There is probably a more nuanced explanation of the tradeoffs, but does it matter?

 

[Kek700]

Had a mate who was a top suspension design engineer, who happened to be a fanatical FF 1600
weekend racer and was also fanatical setup exponent too. ( no aero dynamics involved ) .
I sometimes used to go testing with him.
He always used to say to me , the first rule of car setup is, there are no rules.
I had always assumed that included the line ( do not take that to literately )
Also, on his last lap on a practice day I attended with him at LLyden race circuit, he matched
the lap record in his FF1600.
But got quite upset when I pointed out that his rear left tyre had started to visibility deflate
on the last lap.

I am not "certainly" trying to prove anything from the above. But from that point on I realised that
when getting into suspension setups, it can sometimes be highly subjective.

And that does not for any reason take away the skill needed from good suspension design
experts , especially with all the sensors and computational outputs available to assist his
intelligent observations in today's racing Vehicles.

What am i trying to say,
is that there are certain black and white areas's were there are definitive answers , dampers
are not that easy, unfortunately.
Maybe if miracles exist , one day I will see the light , and make adjustments to ARB's,
springs and dampers with skill rather that objective quess work.

 

[Mr Deap]

My starting point was skip barber's going faster : mastering the art of race driving. It's not a vehicle dynamics book, it's mostly a driving book.

Sadly this book doesn't have the bomb diving method past the apex...
​

I am not "certainly" trying to prove anything from the above. But from that point on I realised that
when getting into suspension setups, it can sometimes be highly subjective.

Yes, the very basic which is suspension geometry, alignment, inertia, tire load sensitivity(compound) & contact patch. Setting change will differ in all the car. The wing setting, air temp interact in those area, but in most cases the default presented in a straightforward way to understand what to fix to win easy & effortless.

For the most part spring rate is adjusted for straight line stability & how quick the car return to neutral when meeting chicane. Damping & ARB is mostly to adjust the alignment change during cornering to meet the target contact patch & load sensitivity.

If you loose the spring(reduce rate) it will result higher amplitude from interaction & to get close to default value you have to increase damping to get the same target contact patch & load sensitivity... Reduce damping value if you increase rate.

ARB kind of represent that single suspension setting X2, because they connect both side. Changing this setting result a massive change in contact patch & load sensitivity during cornering.

If done right, the suspension travel shouldn't change & allow you to keep the same toe setting.

 

[PhilS13]

R.I.P. thread
2019-2019

 

[Boobietrap]

R.I.P. thread
2019-2019

LoL

 

[Mr Deap]

Race car is already built for race spec the car is already low as it is, so for CG & setup change is a bit misleading . See post #8 which is a fact.

I suppose... with higher rate & lower change in suspension under compression is to get that load sensitivity quicker. The higher is the CG the more time(load transfer) it will take to get that load sensitivity(optimal load on contact patch). Although uneven surface would result instability & result loss of traction/contact patch from the suspension movement(wrong camber/toe change from the suspension geometry).

​

 

[Ghoults]

Arbs can have many purposes. I think in nascar the cars do or at least used to run very soft front springs on some tracks to get the splitter as close to the ground as possible (coilbind). In such case you probably want to run very stiff front arb so you don't get weird stuff happening when the car rolls and one side of the car lifts off of their bumpstops for example. Not to mention the aero effects that could create.

Some formula cars have suspension on the front called monoshock where you may not have any kind of springs for the suspensions. You only have an "arb" which is the bar connecting the left and right side suspensions. In that kind of suspension your roll spring is undamped and you may not have any kind of normal springs and dampers. Some cars use z-bars which is basically an arb that is z-shaped so it does not affect roll at all but works as a spring with bumps. I think these are used on the rear on swing arm suspensions and you can essentially increase your spring rates without affecting your roll rates.

All this matters because if you want to get technical with setting up your springs, dampers and arbs you want to know what kind of system the car has. Ac does I think support monoshock for example although I don't know any sim which support z-bar. That being said I don't know any real car either which runs it. Maybe some old formula cars which use the wv beetle chassis..

Some cars have completely decoupled suspension roll and heave: http://theracingline.net/2018/race-car-tech/race-tech-explained/porsche-919-front-suspension-part-1/

 

[Tennisbiki]

Not sure how much I can contribute to this discussion. My background is BS in mechanical engineering, a few years of club autoX/Solo, and a couple of years in driving Assetto Corsa. I used to be more familiar with vehicle dynamics from books such as Carrol Smith's Tune to Win. Any was a silent student of the no ARB setup of C production AW11 and X19 on MR2OC forum. I am attractive to this thread because I have similar questions as the original post.

From my memory, the original proponent of the no sway bar setup was used on SCCA highly modified Fiat X1/9 then using the same calculation and applied to AW11. He appeared to have lots of success with his setup: essentially high to low front/rear spring with no swaybar. He said the car has excellent grip and easy to drive. Essentially point and shoot. Basically a reverse of front wheel drive, stiff rear swaybar setup except using spring instead of sway bar. He said sway bars reduce grip.

I confess that I was fascinated by the number of responses on the thread but since I only raced E stock MR2 Mk1 with R-comp tires. I was only able to put a large front sway bar without rear bar. I love the way the car drives.

I wanted to test out the no swaybar setup in AC, but my coding skill was lacking at the time. But I went the other way and went softer, lower ride frequency.

Sorry for a long winded introduction. And no, my comment is not mathematical based.

I took a MR2 SC mod and changed the wheelrate and AWB. In about a year, I arrived at a ride frequency about 1.5 or 1.6 Hz with about 10,000 (what's the unit N/cm?) and I love it. I found that I can control the car, getting it to the racing line better than any car I have in AC. Yes, I know I am slow and I need a lot of help from the car and I think I know why.

At this lowered ride frequency the car respond slower, or matches my timing, my "feel". My input, the car's response-output is in-sync'd. So going back to the basics of frequency, it is about time, how fast something happens. And I found that going toward 1.7 or 1.8, I started loosing that connection-ness. And everything like a higher ride frequency modern high downforce cars became more like a video game that feels digital and frantic with not much feel.

Sorry if I went off-topic, but I wish more people can enjoy driving a stock 2CV around GPK Spa like I did. I think the ride frequency can go even lower from 1.6 to 1.0 Hz. Yeah, the suspension configuration is completely incorrect but the thing carves around the kart track so nicely.