Suspension Frequency

[Siestarider]

Through asking questions and getting answers from Jerrick at Meister R, I concluded I don't know jack about springs and dampers. Thought I did.

Understanding your Dampers: A guide from Jim Kasprzak is a 25 page white paper on springs and damping I have read several times and need to read more. The most illuminating point to me is that diagnosing a bad damper on a car is nearly impossible. If true, we could all be driving around on bad dampers and not know it.

Main point is that many of us desire better cornering performance on track. Lowering springs and coil overs offer ways to lower COG. Higher spring rates than stock are generally considered good for track by reducing weight transfer to outside wheels, ie reducing roll in corners. Higher spring rates are not as good for street. But those of us fortunate to have mostly good roads can get away with a little more stiffness on road for improvements on track.

Somehow dampers get left to the last item of interest. There used to be a section on Far North Racing web site on dampers, basically said most are junk. Claimed most dampers regardless of cost will not dyno match each other when brand new, much less after track abuse.

I am willing to do the calculations required to get into the ballpark on sprung and unsprung frequency, get to a spring rate # for F & R that seems about right for tracking our cars. From a practical perspective, Joe at 2J has already opined 6/4 or 6/5 F/R is the sweet spot for combination road and track springs. Members of this forum have reported those spring rates are a good track compromise. RAAM is trying out even stiffer combos, my words from his comments are that going higher than Joe recommends will get sketchy on bad roads.

But what to do about dampers? Does anyone dyno their dampers to find out how they behave?

 

[KKaWing]

I'm curious about the stock dampers. Tein and Racecomp shared their dynos they got from the FT86 dampers when they were developing products. I reached out the Swift springs since they dyno shocks to develop their springs. Sadly, I was ignored.

 

[Siestarider]

My interpretation of stock damper behavior is overdamped in both compression and rebound at low speed, pretty good at intermediate speed. Speed means shock velocity not car velocity. On stock spring rates.

Swift makes some spec R lowering springs (maybe thats Wing's point?) so they must have tested stock dampers. As I recall they are 212 lbs/inch both front and rear.

By the way, I found the shocks section on FNR, its under books heading at top of home page. Fascinating stuff.

 

[MeisterR]

6/4 will work also for the Fiesta ST, but we use the 5/3 for a very good reason.

The rule of thumb in vehicle dynamics is you want to stay under 2.0Hz wheel frequency.
That is the point where the car goes from a nice driving fast road car to a car that you don't want to be driving in for more than 30 minutes in a straight line.
The 5/3 setup was just under the 2.0Hz, and the 6/4 was just right over.

I have some fast road & track setup around the 2.25Hz, and they work but as expected it isn't as comfortable for a daily.
Some customer say it is perfectly fine and they like it, other say it is a little harsh and rather have something a bit more compliant.
But that 2.0Hz line is a good rule of thumb we stick to.

What about on track performance between the 5/3 and the 6/4?
I would say there isn't a huge huge difference.
The 6/4 may seems more responsive, but the 5/3 isn't going to be sloppy.
You aren't going to see 1 second difference because of the difference in springs rate.

Damper are valved to match the springs rate of course.
Having a damper where it should be makes a huge difference, everything works together as a package.

I would say a 5/3 suspension that is critical damped, will out perform a 6/4 setup that is under damped or over damped on the track.
But it also boils down to the tyres compound, driver preference, track temp / surface, etc.

Everything is link, which is why vehicle dynamic is a bit confusing because what is good for one car at one place may not necessary be good for another car at the same place on a different day.
When you design a suspension, you have to take all the possible variable into account... and that is where those rule of thumbs come from.

Jerrick

 

[RAAMaudio]

I am now running 8/7k Swift linear springs, was on 6/5k BC with the rears being progressive which is generally known to be less than ideal on track or even on the street in some cases. I wanted a bit more rate to control body roll and depend less on sway bars and the car felt a bit to soft in some places on the 6/5k setup. It felt like it could ride well on stiffer springs in most situations though if in a place with really bad roads I would of not gone this far.

The car is not a daily driver, 8k miles in 2+years, on the street it will have 600+ lbs of cargo in it most of the time.

I have pounded it around a few corners with just myself in the car to get a feel of the overall grip on 225 Rival S tires on 9" wide wheels and it is like glue but skips a little if bumpy but most race tracks are not that bumpy and I do not drive all out on the street so I think I am going to be OK with it.

The rear is a bit harsh but I wanted it a bit stiffer since the dogs and cargo add up to more than my wife and I do up front, I am going to soften up the dampers a bit, set on 22 out of 32 right now to see it it helps. I might have to go to 6k rears but hope not.

I am a long time advocate of keeping spring rates as soft as possible and using dampers and sway bars to control things more but wanted to try a different route on this car and the spring rates I am running are not super stiff.

I also have sleeved urethane bushings everywhere which means no rubber bushings that stiffen as they deflect, meaning I could get by with a bit more spring rate, at least in theory

I might have to go to a bit softer spring in the rear, possibly front but will stay with swifts if I do so I have a set of BC 6/5k springs I do not need if anybody is interested.

--------------

I am also interested in the MR higher end coilovers but no budget right now, spent a ton on the car lately so have to slow down a bit as retired.

 

[RAAMaudio]

My car is a true multipurpose car, grocery getter, site seeing, doggie hauler, track car and I know one cannot easily do it all with one setup but I have done a few others that turned out quite well and willing to try things just to see how they work out. Right now the cornering with just me in the car on sticky tires, I have race tires that are even more so for the track, there is little body roll and very quick transition from right to left, I need to get on the track and see how it does on the curbs as was very good on the softer springs and I love pounding over them so will go lighter if I have to just for that alone.

I also have a 5-way adjustable race grade rear sway bar I did a serious amount of work to adapt from an E36 to fit this car, FWD cars generally need more rear bar than fronts, my last FWD had a massive rear bar and no front and was very fast but also had IRS instead of a beam axle.

Notice: I am running -2.5 front and -1.9 rear camber, zero rear toe and a tiny bit of front toe out so what I do is not always directly comparable to more standard ST setups. Also if not for hauling dogs around and other rear cargo I might have gone with 8/6k and still might have to.

 

[MeisterR]

I'll give you guys some food for thought just to help explain springs rate a bit.

I am helping a Honda S2000 in the SCCA AutoSolo at the moment.
The car was a road & track car, but this year they want to get more focused as the competition have ram up.

Generally, the S2000 run a 9/6 setup, which is a good road & track car and does everything well.
We just made a new race focused GT1 coilovers, and the springs rate is 14/12.
So you can see how much the rate had gone up.

Reason I bought this up was because we were in the car together for the 2 hour drive down to the last event.
The full stiff damping adjustment was bang on, so we were on pace and faster than before.
That is what you would expect, so nothing special there.

The bit that concerns me was on the softer setting during our 2 hour drive to and back from the event.
The driver thought it was really comfortable, he say it is even more comfortable than the old Bilstein PSS9 setup that he had.
I on the other hand thought it was pretty harsh. It isn't rock solid and it still absorbed bumps, but over expansion joint you do feel a proper whack to your chest.

I bought this up because I think it give a good show on springs rate and damper.
A good damper could make stiffer springs rate bearable, but it doesn't change the fact that the springs rate are stiff.
So some eve feel it is very comfortable (as my driver in this case), but generally speaking it isn't something I personally would like to drive long distance in.

The rule of thumb of the 2.0Hz I listed above is what this deal with.
Below that 2.0Hz line, you get a good ride while retaining good response on stiffer setup on road tyres.
Once you start changing to very sticky semi slicks and start to get more track focus, numbers start to change.

Also, do remember that the wheel frequency is directly related to corner weight.
So a 3kg/mm springs might be good for a standard car, but if you put 100lbs in the back, you may need a 5kg/mm springs to provide the same wheel frequency that the 3kg/mm did without the added weight.
All those have to be taken into consideration.

Jerrick

 

[Siestarider]

Lots of good information being added here by RAAM and Jerrick. My research suggests that anyone who is really serious about tuning with shocks either has to dyno them, or record each wheel's behavior with a logger and analyze springs and dampers that way. I don't want to go the logging route, I would bury myself in data.

Cost to dyno is fairly cheap, around $50/shock, until you want to know about adjustments. For example I found a reputable dyno shop that will provide you 16 curves for 32 way adjustable shocks for $175/shock. My take is that unless you are racing for real, it is just not worth the cost. Especially as my particular butt dyno is not calibrated enough to tell me much about shocks on the street. On track, I believe I would be faster with a stiffer suspension than stock, but here I have to dampen my own tendencies and be realistic about it.

Maybe I ran 1000 miles on track last year, 10 track days and about a full tank of fuel for each at around 10 mpg. A lot more miles than that on street. Additional reality, mountains are over 500 miles from here, have to run interstates to get there. So even fun street driving is quite a way off. Florida particularly boring in this regard.

What Jerrick reports for street suspension frequency is consistent with everything I have read by experts in the field. I have not decided what to do, but am inclined to pursue adjustable coil overs so I can corner balance and play with adjustments on track and street.

 

[RAAMaudio]

In my particular situation I have a wife and dogs to haul around at times, go site seeing, grocery hauling, etc...not a ton of use but do not want it to radical though my wife is pretty tolerant of a solid ride and some added noise, far more than most. Then I want to be fast on an autocross course, fast on a race track and have to make barely any changes in between.

I want a do it all very very well but might not be optimal for any particular situation and I have built many project cars this way with great success.



The springs I am now running are a bit to stiff for streets with a bunch of bumps, not a big deal. I do not drive all out on the street more than a moment or two at a time and if I skip a bit over a few bumps here and there no worries, not racing or going against a clock, more practice in less than ideal situations will keep my skill set stay more honed for when things get more serious.

The only real thrill in driving to me is being on the ragged edge and going as fast as the car can go no matter what you are driving and the speeds capable. I only do that on a race track now as modern cars, cheap power, super sticky tires, cars are far to easy to make to fast for real world use in a safe manor.

 

[Siestarider]

Poking around a little on suspension frequency, I found a neat calculator http://www.racingaspirations.com/apps/wheel-frequency-calculator. Used [MENTION=2925]Rhinopolis[/MENTION] post for corner weights, assumed 1:1 spring motion ratios (probably right for front) and 0.7 for rear (guess for twist beam springs) and found 6/5 F and 4/3 R is just above and below frequency of 2.0 respectively.

Interestingly, playing with unsprung weight, the calculator is not very sensitive to it. That was surprising to me.

Fits with what Jerrick says, and Far North as well. I really want damper dyno data, may order a set of coils and have them sent directly to a dyno shop, get hard, soft and intermediate curves so I can at least suspect something.