SPRING FREQUENCY 5

[willeng]

We have come across some suspension guys who bounce cars up & down for 1 min,shocks,sway-bars etc disconnected to get a spring frequencey figure. They say they are getting cycles per min of different rate springs. They are using this as a basis for setting up there suspensions.
Is there someone who can shed some light on this subject as to me the more i think about it the more it seems like BS.

 

[SusTestEng]

bouncing a car is not uncommon. Actually, when I am chosing the spring and swaybar combination for a vehicle, I usually drive the car with NO damping force. If the vehicle is a strut suspension, I just drill a hole in the damper and drain the oil. This way you even get an idea of piston friction in the system. I typically do handling manuevers and generall ride events. You would be surprised how well a car handles with no damping force. And for ride, you can tell if the ride freqency is to high right away.

As for the critical damping questions. I typically work on very sporty production based vehicles and it is very common for me to chose a damping force very close to critical, especially in the very low piston speed areas to keep the steering very crisp. Typically the damping value drops off to .4~.45 in the mid to high speed area of the damping curve.

 

[willeng]

Seeing things are a bit quiet, i found this in a book from one of the worlds great race car builders & thought i would share it to see what people think & to comment on!

Quote:

Most of the books on vehicle dynamics tell us that we should be vitally interested in the natural frequency of our springs. I have never figured out why?
I will admit that if the natural frequency of the front suspension were equal to that of the rear, the car could get into pogo stick mode, but with the natural harmonic frequency of the unsprung masses modified by the tyre hop frequency & the whole mess dampened by the shocks, it becomes a real mess to calculate, and the odds against the front and the rear ending up at the same harmonic frequency are negligible.
I (ignore) spring frequency. I also ignore the fact that, if the spring or shock is too stiff, then the tyre hop frequency will be undamped and the frequency of the spring & tyre can theoretically combine to cause trouble and the fact that the torsional frequency of the chassis itself must be well above the tyre hop frequency--it always is!
Thats all i have to say about the various frequencies associated with rates-we DON'T need to know about them!!

 

[Warpspeed]

That is probably true up to a point, but as others have already said, for practical reasons the relative suspension rates really need to fall within some broad range to be useful.

Wheel (spring) rates by themselves do not tell you much, but bounce frequency ties everything together and gives a figure of merit that gives some idea of where you are, especially when starting out with an existing vehicle.

At least on a road car, keeping the rates within the 1Hz to 2Hz range, and with the rears slightly faster will give a pretty fair indication what the ride will be like, and if pitching is going to be a problem. There is a lot more to it, but you have to start somewhere I suppose.

 

[red300zx99x]

I have a problem of figuring out ride rates when 2 different sized springs are on a track, anyone know the correct calculation for figuring out this ride rate?

 

[GregLocock]

What do you mean by "on a track"?



Cheers

Greg Locock

 

[red300zx99x]

Sorry, on a axle, say on the rear or on the front. We have a tendancy to use very stiff springs(~4000lb/in) along with some very soft springs(~100lb/in)

 

[MoreWing]

Quote:

Most of the books on vehicle dynamics tell us that we should be vitally interested in the natural frequency of our springs. I have never figured out why?

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Carroll Smith wrote this and the following sentences in a book published in 1978. I think it's fair to say that racecar engineering has progressed a little bit since then. While his statement was completely reasonable for his time, and probably the next decade and a half, there have been a couple of things that happened that made frequency anaylsis pretty stinkin' important.

First and formost, was data acquisition systems were made small and robust enough to live on a racecar. That was a huge advancement. Secondly, damper technology progressed to the point that we could adjust damping in very specific areas and have the ability to measure with very good accuracy where and how these changes were being applied.

On an aerodynamic racecar, you don't shoot for the 'flat ride' criteria that you do on a street car. If you run the rear at a higher frequency than the front, then as the aero pushes the car down at high speed, it will increase the rake in the car and shift your CoP forward. Not much good is ever said about high-speed oversteer. For this point alone, people tend to run the front quicker than the rear, even if they don't know they're doing it.

The second point is that in most racecars you are constantly looking rear traction at corner exit. Stiff rear springs usually give you all sorts of wheelspin problems. To use a great quote from Carroll, "A car is like a primate, it's got to squat to go."

_The Shock Absorber Handbook_ has a graph on page 61 that is replicated in many other books. No one explains it's massive implications very well, though. It's a graph of transmissibility vs. frequency ratio (impressed freq./natural freq.). We know that if we bounce the system at it's natural frequency the thing bounces out of control. What you should note, though, at higher frequencies the zero damping case is optimum! Who woulda thunk it? We spend all this time on dampers just to find out we'd be better off without them.

So, you might pose the question, what type of a scenario would be good with no damping? Well, the answer is that any road input that comes at the car quicker than Nat. Freq.*(sqrt(2)) will be felt less by the driver if the dampers have no damping. At highway speeds, that means pavement seams and pot holes, but it also means that in your sportscar with its 2Hz ride frequency, anything that takes less than about 1/3 of a second to happen will upset the car less with no damping and more with more damping.

Ever driven an old car with shocks that are shot? You can hit a rail-road track and its ride over the rails deceptively smooth. You get out on the highway and over a little undulation in the pavement wants to bounce into the next lane. This is exactly the case of zero (or very little) damping. In some ways, I like driving bad cars just to feel this.

With a data system on a modern racecar you can look at bumps that the driver complains about and determine what frequency they are coming in at and what direction to go with the dampers to make the car ride them better. You can also see if maybe it isn't a shock problem at all, but the overall spring rate is too quick or too slow for the road input. Adjust the rates accordingly and the problems will sometimes just disappear.

Carroll was a very 'from the hip' sort of guy. He had a huge wealth of experience and would know what to do with the car by using that experience. He wasn't a mathmatics or computer wizz by any stretch. He would flat out tell you that he didn't know a hell of a lot about shocks. He only had to deal with them in his last couple years of race engineering and that meant pretty much delegating the job to other people. I can promise you, though, if I could have sat down and shown him that by knowing the spring frequency and damping ratio the data system will tell me which way to do on the shock clickers, he would have been all over it.

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SuspTestEng, I bet you can pull out a bunch of the low speed damping (where you have it near critical drop down to 0.6 or so) without losing the crispness in the steering, give the car a better overall ride, and better overall grip. I used this exact same reasoning to keep low-speed damping in a racecar. When I finally took it out, we went way faster and never lost any response as long as the front springs were anything above marshmallow level. Piston bleed is your friend.

 

[GregLocock]

Springs in parallel are additive, so that would be the same as a 4100 lb/in spring, if they have the same motion ratio. If they don't, work out the contribution from each separately at the wheel and add them.

We do the same with 'spring aids' and other similar devices.

Cheers

Greg Locock

 

[red300zx99x]

Thats what I thought, but Milliken on pages 588-589 makes that assumption odd. He uses a ride rate of 271.2lb/in at first, then changes that to 384.4lb/in. But then in his roll rate calculation he uses the same value.....as the ride rate for each spring. If the front ride rate is 384.4lb/in and we went by the rule that springs in parallel are additive then a value of 192.2lb/in should have been used in the roll rate calculation, what gives?

 

[NormPeterson]

That 271 lb/in is simply a first cut at wheel rates, taken from the change in wheel load and an assumed 2.5" of allowable travel. The resulting frequencies were defined to be unsatisfactory for this particular problem (front too low relative to the rear or rear too high relative to the front). Since the rears could not be made softer without exceeding their travel limit, the fronts were made stiffer to provide a not-entirely-arbitrary front frequency. That's where the 384 came from. All that means with respect to the initial 271 front rate is that the front will not use the entire 2.5" of available travel in satisfying the relative frequency criterion.

Then it goes on using the 384 front rate and the 225 rear rate to solve for sta-bar rate(s).

Understand that this example was specifically for a race car; consequently the flat ride considerations that have been discussed earlier in this thread were ignored.

Norm

 

[red300zx99x]

If you note on page 589 the first equation Kf equation then the third equation K. Now set Kf with the front ride rate of 384.4, now for K use 384.4 for the left side and the right side rate. These 2 equations are equal, showing that the right and left side have indivdual rates. Now under the assumptiom that the ride rate on an axle is the sum of the 2 rates on the axle as they are parallel spring forces, the front ride rate would be 768.8, but thats not what Milliken uses.

Sorry, this involves my off topic question of finding the ride rate for an axle with non-symmetric left-right ride rates.

 

[red300zx99x]

hope you can decipher that.....i need sleep

 

[NormPeterson]

That whole section in RCVD is dealing with roll rates as functions of ride rates, not really the ride rates themselves. And except for that one formula that addresses the roll rate given unequal ride rates, symmetry is assumed.

But it seems to me that the general case is where you have (left vs right) unequal wheel rates and/or unequal sprung mass magnitudes, where you will have unequal left vs right ride frequencies. That in turn suggests coupling between ride and roll motions.

Norm

 

[ricdat]

Quote:
"Most of the books on vehicle dynamics tell us that we should be vitally interested in the natural frequency of our springs. I have never figured out why?"

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When racecar or sportscar builders ask me how stiff their springs should be*, I can't answer, "20N/mm for a nice ride" or "500N/mm at the front of your heavy, high downforce car and 600 at the back" or "1000lb/in front of V8 supercar and 300 at the back". I can't because I don't know leverage ratios or the front or rear weights of the car.
On the other hand, I can say "2Hz for any non-downforce racecar" or "1Hz for any limo" or "3.5Hz for any F1 car". It gives you a universal starting point. Of course you add detail like, "make the rear freq 10 - 30% higher for a low damped road car" or "F/R freqs can be the same for a well damped car" or "make it 50% higher because you're using swing axles".
If someone says they have a car with 600lb/in front springs, then no-one has any idea how comfortable it is, how much suspension movement it needs or how responsive it might be. But if they said the front ride frequency was 3Hz........

Rick Dathan
*OK, so I admit it has only happened a couple of times!

 

[MoreWing]

Can we give brother Rick an amen? AMEN!