Quiz: Why Not Equal Spring Rates?

[johnc]

Say you've got a car with equal weight distribution (50/50) and equal wheel rates (struts all around). Why wouldn't you run equal spring rates at all 4 corners?

 

No prize for correct answers.

[Tim240z]

Large forward shift of weight under braking.

That's my story......

Tim

[Mudge]

Thats a good one, plus you usually need traction from the rear (and often more travel is used as well) due to the rear being your drive wheels, your turning, etc.

 

The Z car you guys are already using extremely high rates in the rear, and for most cars it is very different. My 94 Camaro has 600# fronts and 140-160# variable rears, weight distro is about 57/43.

[johnc]

Hint: springs oscillate at a specific frequency depending on rate.

[CruxGNZ]

I'm still a newbee at suspension stuff, but my guess is if you have the same rates, then you would be bucking like a bronco if you hit a bump at a certain speed. Am I even close?

 

!M!

[Drax240z]

John, are we also assuming that unsprung mass at each corner is equal?

[TimZ]

Mat73GNZ has it right. In order to prevent bucking when traversing bumps, the rear needs to be stiffer than the front (i.e., higher natural frequency).

 

So in effect, when you drive over a bump, the front of the car rises more slowly than the rear. Ideally, the entire car rises from the bump at the same time, instead of the front pitching up and then the rear (bucking). If the rear rises first the effect is much less annoying.

 

The spread in spring rates (actually it's the spread in natural frequencies, but John stipulated a 50/50 weight distribution so it works out the same in this case) determines the threshold speed below which bucking will occur. The larger the spread, the lower the speed.

[Guest AlsoRanFPrepared]

Lots of reasons. Depends on the roll center heights front and rear, the roll stiffness imparted by the swaybars if any. The tire size difference if any as mandated by the rules/regulations for a class you are racing in. In passenger cars the front is purposely set up with a higher roll stiffness because the typical driver is more apt to be able to handle understeer than oversteer in an emergency situation. The higher roll stiffness may be a result of the swaybars or the springs. As mentioned above the reduction of bucking in passenger cars is also an issue. These are the first that came to mind. If you are looking at the total lateral load distribution mathmatically then the spring rates will be different.

 

Oh, and I forgot about the installation ratios. Unless you meant that the struts were coilover. Many a Mustang has struts and very few has coilover style springs (mounts between LCA and frame).

 

-Mark

[johnc]

You guys got it figured out. You want the front and rear suspensions to oscillate at different frequencies. In many cases shocks can dampen short moments when the springs oscillate in opposite phase, but you want to keep that from happening.

 

If you drive a car with this problem it often feels as if it needs more rebound in the shocks. Sometimes this leads you down the path of stiffening rebound so much that the car is picking wheels up off the ground.

[Guest Anonymous]

John: Quiz for you. For my second project I got this hot deal on 4 185 pound coilover springs for the second project. How do I get them to "oscillate" if mounted on the same car? or should I look for another full matching set and spread them out to the 3rd project/ (There was a time I was on eBay more than Denny 411 during his spring planting lol).

[Mudge]

Spring osciliation means moving up and down. You have the option possibly of cutting a coil, which will increase the spring rate, take the # rate and number of coils.

[John Scott]

So...what determines spring rate for a given car that is 50/50? Why are the "lowering" springs from MSA, Tokico ect. so drastically different in front to rear rate? We can simulate our engine components, gear ratios,shift points. Any tips on choosing springs? Not talking race here as the majority are going to see plenty of street time on street tires. The range of spring rates in use, from low 100s to 300, leaves me pretty cluless.

John

[Mikelly]

AHHH.... There in lay the problems with setting up suspensions, and the reason guys like Longacre are in business... Corner weights... If you pull your car onto a set of scales, you will then see that not all corners are equally distributed... Further, you should be IN the car when the car is weighed and then adjusted... Takes time and money!

 

JohnC, How'ed I do?

Mike

[Guest Anonymous]

Mudge the springs are coilovers and cutting a coil would definitely upset the spring's riding ability on the aluminumn adjusting nut or top hat depending on which end got the brunt of the cut steel coil. It would cost more than the spring's worth to get the end professionally flat and square to ride the coilover part correctly without undue contact damage. I cannot do that.Good idea though.but if my front half is heavier than the rear , would I have the oscillating action even though all 185 pound 12 inch springs are equal? They were chrome Carreras... how could I resist bidding ?

[John Scott]

So spring rate controls chassis movement, strut damping the time it takes for movements. Like everything it will be a compromise for the average enthusiast. Who built their primary tube diam and length headers to the exact head flow, cam profile, engine RPM...or did you settle for something else? Custom grind your cam to match your head flow #s? My point is I'm want to end up with what I hope is the best choice with what I can afford. I just don't want to be way off base. Like anything, $1000s can be spent perfecting any element of a Hybrid. I don't have unlimited time and funds to design a street car, I'd rather learn from someone who knows with experience than my trial and error. We have some incredible talent on this site. Any recomendations for choosing spring rates for a V8 high hp near 50/50 street Z?

John

[Mudge]

Gotcha on the pigtail, in that case yes I'd buy new springs I suppose.

 

Again I am completely unfamiliar with such equal spring rates being used on a car, would love to try it out on a simulation to see how it works out on this kind of car, anyone know what the stock spring rates were?

 

In general you want one oscilation for the spring/shock assembly at the most, if you really wanted you could sit down and calculate all that info based on weight and speed per track you were going to visit.

[Guest Anonymous]

Track? and $1,000.00's I wanted the quick easy simple $4.49 solution from Home Depot or JC Whitney LOL. I will mount all four and find out when I hit the RR track or concrete expansion joints. (or wash board gravel road) Coil overs are easy to change. I will be prowling E Bay for more. Beware Denny 411 if ye see me monicker as high bidder

[johnc]

If you're starting from scratch, calcualting the correct spring rate for a particular end of a car is pretty complicated. It involves much more then just static corner weights and wheel rates. Other things like:

 

Transient Stability and Control

Ride and Roll Rate

Suspension Geometry

Wheel Load

 

and the big nasty one:

 

Aerodynamic Load

 

Luckily, the OEM engineers figure a lot of this stuff out for us, so a quick and dirty way is to double the spring rates at each end, maintaining the same percentage relationship as the stock spring rates. This will get you into the ballpark (or maybe the parking lot of the ballpark).

 

To do it right, you need to do a lot of math. Buy "Race Car Vehicle Dynamics" by the Millikens and use the formulas they provide (I stole the list above from some of the chapter headings). There are also a number of suspension design programs that will show you the numbers but generally won't tell you which ones are right.

 

For the Datsun 240Z a lot has been figured out so looking at what people have been using for years is a good practice. I doubt someone will come up with a new winning spring package that hasn't already been run by at least 20 other people.

 

One thing though, no one to my knowledge has applied any of the cutting edge downforce generators to a 240Z. When that happens you will see significant changes in spring, shocks, and anti-roll bars. Be careful about copying those setups (and be wary of someone trying to sell you that setup) unless your car generates similar downfoce numbers. This is an example of where "Monkey-see, Monkey-do" can get you in a wreck.

[Dan Baldwin]

Nothing magical about having the same spring rates at all four corners. Hopefully (assuming there's not a TON of aero load), you'll be pretty neutrally damped (not underdamped bouncy or overdamped skatey), so bucking shouldn't be much of an issue. I would have no reservations at all about trying it, and I might next year. I'm currently at 160F/200R spring rates, 1" front bar, and no rear bar (disconnected). The faster ITS guys run more like 350F/300R (something like that, anyway), big front bar, and no rear bar. I might go to something like 225/225.

 

The reason a lot of cars (like my Z28) have MUCH stiffer front springs is that they have highly leveraged front springs relative to the rears. Wheel rates aren't nearly as drastically different front/rear on those cars.

 

As far as modeling suspension behavior on a Z, accounting for variable damping with speed an position, tire stiffness/damping, and chassis twisting, I think one would do better in terms of time, money, and effort spent just going to the track with different springs and adjustable dampers. He'd have a lot more fun, too!

[johnc]

Dan,

 

On a 240Z you do not want equal spring rates at all 4 corners. You will get some opposite phase oscillations depending on the bumps on course which feels as I described above.

 

I just spent some time tuning a friend's WRX and his car exhibits the same tendency. Until we can increase the front spring rates we have to run so much rear rebound that the inside rear tire picks up occaisionally.