Spring Frequency

A couple days ago I was reading a thread the mentioned spring frequency and I have to admit that is not something I'd ever given any thought to.  I'd heard it and knew what it was but never much cared and after a couple days of reading a few things I'm still not sure I care...but I have got myself a little confused.  I have Eibach spring so when a link to their tech page came up I clicked on it

https://eibach.com/us/p-101-suspension-worksheet.html

hmmm.....they don't include the shock angle in motion ratio???

but they have angle correction, so there's the angle, lets see where they use it:

Ok....quick spread sheet and for my car I get a spring frequency of 1.378hz.

go to an online calculator

https://www.drtuned.com/tech-ramblings/2017/10/2/spring-rates-suspension-frequencies

and get a different answer...1.7hz.  Their formulas are different, their answer is different but these are the formulas most seem to use.  They don't say how they define motion ration but since there is no separate angle I assume its the simple lever ratio x cos(angle) measured off vertical or sin measure of horizontal.

Who's right I wonder? 

Which brings me back to not really knowing how much to care about this?  Its a 1 wheel calculation but doesn't include antiroll bar spring rate so for sure its not a real 1 wheel frequency, its more 1/2 the 2 wheel bump frequency.  I guess as a quick idea if f/r spring choice makes sense as a starting point maybe its useful but not sure how I'd use it beyond a "gut check"? The guideline ranges I see listes around the web result in 50%-100% spring rate increase from the bottom of the range to the top......doesn't seem very helpful?

The way I've traditionally picked springs was more about how much travel to I have or think I need for the application and then do math like this:

https://www.hypercoils.com/spring-rate-calculator/

around how much total spring compression do I want and how much of that in preload at planned ride height...stuff I can measure but it just feels so old school with this new fangled spring frequency I don't understand what to do with.

What am I missing in life by not caring even a little bit about spring frequency?

Unless you're very serious about performance suspension tuning it's not worth caring about. I've done more detailed suspension tuning than most and I don't have much use for the number - its function seems to be just a numerical evaluation of the effective stiffness of a car's suspension. Good for comparing cars perhaps, not so useful for tuning an individual car, at least in this hobbyist's experience.

When calculating the number it's very easy for the calculations to be greatly thrown off by incorrect inputs.

Edit: BTW, I'm assuming you're talking about what's more commonly referred to as suspension frequency or wheel frequency. You need unsprung weights and corner weights to calculate it.

My short answer is that the concept of spring "frequencies" doesn't really matter that much. Yes it can be used to reasonably compare the overall suspension stiffness between different vehicle but it's only a rough guide. Beware of anybody telling you that there is a magic number or perfect frequency for your car. There just too many variables to narrow it down to that one number.

Another way to directly compare suspension stiffness between different setups/cars without needing to know spring rates, motion ratios, etc is by measuring how much sag there is at the wheel. Note that this assumes zero spring preload, but if you know exactly how much prelaod you have you can factor it in as well. If you charted spring frequency vs. sag you'd notice that that there is a direct relationship between the two.

therieldeal said:

I've had good luck using this calculator in the past

http://farnorthracing.com/autocross_secrets16.html

A lot of info there....I have some reading to do.

a couple things caught my eye quickly:

1 -  the sheet requires several inputs it say get from WinGeo...which appears to have gone away with the death of the author.  Is this program still available or is there something else people now use and love?

2 - the spring frequency...he claims to have the magic number :)  which is OK but this is the first time I think I've ever seen anyone want the rear stiffer than the front and its about 10%....I think everything I've seen like ever says the rear should generally be 10-20% softer than the front.  

3- I don't think I've ever thought about measuring the spring rate of a tire.  It has to matter I guess but its honestly never occurred to me to try to measure it.

This thread is not what I expected. I thought this was going to be about the changing of the seasons and the answer to spring frequency was going to be "it comes once a year."

Spring? It happens once a year.

mke said:

2 - the spring frequency...he claims to have the magic number :)  which is OK but this is the first time I think I've ever seen anyone want the rear stiffer than the front and its about 10%....I think everything I've seen like ever says the rear should generally be 10-20% softer than the front.  

Hmmm...years ago I was told the full race front spring rate for my car is 800lb/in....which is interestl\ingly his 2.2hz ,depending who's math you use.....its only 1.8 in the eibach math that has the shock angle separate from motion ratio which I honestly still don't understand  but I haven't tried to go through the full derivation thinking this should be pretty standard stuff.  I think I need to shoot them a note and inquire.

mke said:

2 - the spring frequency...he claims to have the magic number :)  which is OK but this is the first time I think I've ever seen anyone want the rear stiffer than the front and its about 10%....I think everything I've seen like ever says the rear should generally be 10-20% softer than the front.  

The higher damping ratios of a racecar diminish the need to tune for a "flat ride" and a higher front ride frequency in a racecar allows faster transient response at corner entry, less ride height variation on the front (the aerodynamics are usually more pitch sensitive on the front of the car) and allows for better rear wheel traction (for rear wheel drive cars) on corner exit.

Only worry about spring frequency on valve springs and guitars.  

Everyone I've talked to agreed that it has more relevance to OEs. For them it's important to know what the system is doing so that they can design in the 10-20% softer thing to avoid oscillations on the road as well as maintaining family "feel" for their cars. They can also tune their suspension to certain tires, so they're working a slightly different game. 

red_stapler said:

mke said:

2 - the spring frequency...he claims to have the magic number :)  which is OK but this is the first time I think I've ever seen anyone want the rear stiffer than the front and its about 10%....I think everything I've seen like ever says the rear should generally be 10-20% softer than the front.  

The higher damping ratios of a racecar diminish the need to tune for a "flat ride" and a higher front ride frequency in a racecar allows faster transient response at corner entry, less ride height variation on the front (the aerodynamics are usually more pitch sensitive on the front of the car) and allows for better rear wheel traction (for rear wheel drive cars) on corner exit.

That's what I've always read and done which is why I was surprised to see the opposite recommended on this one site that seems to have tons of good info.  I guess most OEM cars are higher rear rate in the rear so they ride flatter on the highway then they use the front antisway bar to get it to understeer but I'd never seen that recommended for a race car.....maybe because autocross is low speed without a lot of braking (at least the way I do it  ) he has better luck with a more OEM spring setup? 

Anyway, though it interesting and definitely not a direction I was even considering as I struggle with rear grip with the rear about 10% softer than the front, going stiffer in the rear seems a mistake....and the simple fact that I've now calculated the freq and know the rear is already about 10% softer than the front perhaps I've answered my own question :)

In reply to mazdeuce - Seth :

I'm kind of in the "where to start' mode at the moment and was hoping to find some magic :)

I bought th car in ...2000? and it promptly set itself on fire so its 1st autocross was 2001 and it was awful, worth experience I'd ever had at an autox....sliding's fun I guess but going slow isn't.....so the factory shocks were adjustable koni soI decided to make new spring perches and install new springs to try to get things behaving better.  I remember being stunned at how bizarre the factory spring choices seemed trying to recall is was 120lb front, 180 lbs rear which with a 35/65 weight split on the surface that made sense but the motion ratios are 0.58/0.98 which explained why the front had like 8" of preload in it.   Others on the forums were doing stuff like 250/200 or 300/250 which seemed a lot better math wise so 300/250 it was and it was certainly better but certainly not right. ..so I pulled off the OEM coilovers and replaced them with single adjustable Ohlins STJ shocks and 450/400 springs which is how the car sits today.  It ​​​​​​s WAY better (or was before I took the engine out but as the engine gets closer to working properly once again) and was capable of finishing well at an autox as long as no one who knew what they were doing showed up in my class.....so still not really right......which beings me back to looking for some magic as I try to decide how much more spring to add and then how much antiroll bar....but I'm feeling kind of back to relaying on setup info from what the fast guys were doing back in the 80s when theses cars were new and requested race tracks (1984 ferrari 308 btw).

Remember that we had different ideas in the 80's. A lot of race tires were still bias ply and not as sticky as 200tw tires are today, shocks weren't very good, we didn't have data to back up our suspicions etc. 

You want to have fun, go ask Keith Tanner about how the "ideal" NA Miata setup has changed from about 1992 until now. The idea of what's best is a moving target. 

Front vs rear stiffness needs will depend a bit on the car's suspension geometry (as far as roll center heights, etc.) as that will alter behavior.  And it depends on how you're using the car.  Autocross and other low speed, tight turning environments will want a looser, more rear-biased setup than you'd want on a fast track or probably even on the street.  

Interestingly, the factory setup on my BMW has stiffer springs in the rear than the front, but that's also with a decent size front sway bar and no rear bar at all.  What's more interesting is that after lots of experimenting, I settled on a similar setup for the Jeep.  More rear spring than front, large front bar, no rear bar.  It works pretty well on the street for being able to put down power coming out of corners and keeping the car lively feeling as well as being able to deal with load changes on the rear suspension well.  In the Jeep's case, the decision was partly made due to the stock rear end links having major binding issues, so the suspension is much better behaved without the bar (unless I wanted to build a custom setup).  

mazdeuce - Seth said:

Remember that we had different ideas in the 80's. A lot of race tires were still bias ply and not as sticky as 200tw tires are today, shocks weren't very good, we didn't have data to back up our suspicions etc. 

Thats exactlymy concern with very old setup info....but looking at it the front spring numbers I have are 550-800 which is 1.9-2.2hz but then the info was rear should be 50-100 less which seems to work out about right if the tuning point is about 1.8hzfront and you want the rear a tad stiffer, a 550/450 setup is 1.8/2.0ish. but then is getting stupid in the rear at say 2-2hz.front (the 800lb spring), a 700lb rear is 2.6hz and  clearly not what anyone would do today.  This is using an online calculator, the eibach formuls give a different answer....I emailed them for an explanation.

Then comes the swaybars....I have old info and have found literately nothing anywhere on the web that would help me pick baseline bars.

This has me thinking about heading back to numbers based on travel and sag....like around 4" of travel and about 1/3 sag with near zero preload is pretty usually good​ the the lightest swaybar(s).  This works out to about 700F/800R and the eibach math says 1.7/1.7hz but the online math is like 2.1/2.7 hmmmmmmm​​​​​​

I wouldn't read into the numbers too much. If you have the means just try to experiment with as many setups as possible. After a lot experimentation I found that my s2000 works really well with quite stiff (2.5ish hz) front springs, softer (1.8ish) rear springs, no rear bar, and an insanely stiff front bar. On paper it sounds ridiculous but it works beautifully!

In reply to freetors :

autocross? track? street?

And here's one more...tested where?  

mke said:

In reply to freetors :

autocross? track? street?

 

And here's one more...tested where?  

Autocross. But my point is that it doesn't matter. Find what works best for you and your car.

In reply to mke :

Tires do come in but on a Cup Car 10lbs.of air pressure can become a 25 lb. spring, Putting the same amount of air in Alll 4 tires Negates the effect somewhat, But on our short track car 800 lbs front 200/225 rear we were playing withless than 10 llbs. difference and made a huge diff. in tire temps

 

freetors said:

Autocross. But my point is that it doesn't matter. Find what works best for you and your car.

My testing where question was about testing time is pretty hard to come by so trying everything is not only expensive but takes seasons...so I was hoping to hear something like "oh our club has a skid pad we use for development, most do" and I would be happy to find a club to join :)  

I've reached out to a couple other owners I used to know before the engine ordeal lock me way......one who autoxs got back to me so far and he's gone from I think it was 350/300 10+ years ago to 580/425 today.....question still out on swaybars and how he arrived there.  Hz wise this is going in the same direction you're liking but still a bit softer.

In reply to GTXVette :

When I raced bikes we used play pretty close attention to cold pressure vs hot pressure and temp rise to think we knew what pressure to set when....not sure I ever actually got it right like I knew real teams did but I always felt like I was being a serious racer :)

mke said:

In reply to freetors

My testing [...] was about testing time is pretty hard to come by so trying everything is not only expensive but takes seasons...so I was hoping to hear something like "oh our club has a skid pad we use for development, most do" and I would be happy to find a club to join :)  

I see that you’re in PA, but I didn’t go to the trouble to see where in PA... but I do know that there’s a skidpad that’s in the pit area of Summit Point’s “Shenandoah Circuit”.  Summit’s ownership just changed hands, iirc, so I don’t know if you can rent the pad separate.  Might be something to look into?

Otherwise Pittsburgh International Raceway (formerly Beaverrun) has a pretty sizeable chunk of pavement you might be able to rent... then you just need to buy a bunch of cones and set out a circle/course? 

mke said:

freetors said:

Autocross. But my point is that it doesn't matter. Find what works best for you and your car.

My testing where question was about testing time is pretty hard to come by so trying everything is not only expensive but takes seasons...so I was hoping to hear something like "oh our club has a skid pad we use for development, most do" and I would be happy to find a club to join :)  

I've reached out to a couple other owners I used to know before the engine ordeal lock me way......one who autoxs got back to me so far and he's gone from I think it was 350/300 10+ years ago to 580/425 today.....question still out on swaybars and how he arrived there.  Hz wise this is going in the same direction you're liking but still a bit softer.

 

Ahh gotcha. In my case it took a few years of trial and error. Our region doesn't do test and tune events.

I read the thread title and thought to myself that spring seems to happen far to infrequently here in Massachusetts. 

Spring frequency calculation is a tool to get you in the ballpark on suspension tuning. As with all spreadsheets, it's not good for much more and it has a tendency to avoid edge cases like "oops, those are bumpstops". 

You can measure the natural frequency of your suspension pretty easily: http://www.autospeed.com/cms/article.html?&title=Measuring-wheel-suspension-rates&A=113057