Quiz: Why Not Equal Spring Rates?

[randy 77zt]

my set up for my 77z with zxt engine-12" coil overs 225lb front 250 rear.front sway bar is 15/16".rear is 7/8.tokico illumina struts stock length.front control arm pivot points have been moved up 1" and out 3/4".camber -3degrees front,-1.5 rear.car could use stiffer springs on track but it is a work car .i change to track alignment at work.i work on cars for a living and where i work we use the equipment for our own cars.for tire on track i run kumho but just because they are cheaper and wear good.

[Guest Anonymous]

Dan, I am actually collecting an assortment of coilover parts. I have 10 inch 165 front and 180 rears on the (lightweight)Tomahawk project. The 185 # x 12 inch chrone Carerra set were anticipated for the GTO project. I am looking for some lighter weight springs to pair with the 185#ers I just brought home some used 8 inch Bilstein threaded tubes that will need to be bored out 2.08 and shortened slightly to fit 280 rear struts . There are a lot of used coilover springs availble but usually in 300# plus range.. Carreras are my favorite since they are guaranteed as to weight.

[johnc]

OK, back to basics...

 

Spring Rate

 

Force per unit dispalcement for a suspension spring alone. For coil springs (which we are discussing) this is measure axially along the centerline of the spring.

 

Wheel Center Rate (Wheel Rate)

 

Vertical force per unit vertical displacement at the location along the spindle corresponding to the wheel centerline, measured relative to the chassis. This rate is generally lower then the corresponding spring rate due to Installation Ratio (or the lever arm length from the spindle to the spring and control arm pivot). The wheel spindle vertical travel is almost always greater then the corresponding displacement (vertical travel) of the spring.

 

Installation Ratio

 

A geometric concept that relates to the change in length (or angle) of a force producing device (e.g., spring, shock, anti-roll bar ) to a change in vertical wheel center movement. Its the inverse mechanical advantage of the wheel center to the force producing device. More simply, its the rate of change of spring compression with wheel movement.

 

-----

 

On a McPhereson strut equipped vehicle the coil spring center axis is essentially in line with and moves almost the same vertically as the wheel center. For purposes of our discussion there is no installation ratio to be calculated to determine the actual force AT THE TIRE (which is what we are really concenred with here folks).

 

On a control arm suspension the spring is mounted inboard of the wheel center so there is an installation ratio that MUST be factored into determining the force at the tire.

 

Assuming a linear change in installation ratio (no rising rate in the suspesnion) installation ratio is a standard calculation of the mechanical advantage of a lever expressed as a ratio.

 

Here's the math: Kw = Ks(IR)squared

 

where:

 

Kw = wheel rate (lb.in.)

Ks = spring rate (lb.in.)

IR = installation ratio

[johnc]

> Can someone please explain why the Z cars use

> such a high rate in the rear so favorably?

 

Now that I've posted the basics, I can answer this question: Stop comparing spring rates from different cars!

 

A 600 lb.in. front spring on a Pony car probably works out to a 300 lb.in. wheel rate which is probably only slightly higher then the rear wheel rate. Vehicles with 50/50 weight distributions tend to run fairly equal wheel rates at each end of the car. Vehicles with 60/40 or 40/60 weight distributions tend to run more disparate wheel rates.

 

Often, spring rates are "adjusted" from the best calculated rate to achieve some kind of handling trait that the driver or track prefers. Some people prefer a Z car that leans more towards oversteer so you "might" see a higher rear spring in those cars. Other drivers or tracks might prefer a more understeering Z so in those cases you "might" see a slightly higher front spring rate.

[Mudge]

Excellent...

 

With a sway bar removed from the rear, you would also use a higher rate to "compensate" as well, yes?

[Dan Baldwin]

Originally posted by BayAreaZ8:

Doesn't natural frequancy = resonant frequancy.

natural freq. = sqrt of k/m

 

I think damping does not change the natural frequancy, it changes the system response.

Damping DOES reduce the natural frequency/resonant frequency of a spring/mass system.

 

Go here: http://ccrma-www.stanford.edu/courses/150/vibrating_systems.html

 

From that site: "The natural frequency [omega d] is lower than that of the mass-spring system [omega o]."

[johnc]

> With a sway bar removed from the rear, you

> would also use a higher rate to "compensate" as

> well, yes?

 

Anti-roll bars are a tuning tool. Ideally you use springs to achieve the overall balance and basic response you want and then add anti-roll bars to adjust response for driver style and track. In the real work, anti-roll bars tend to be relied on for balance more then they should.

[CruxGNZ]

Time to revive this thread.

 

I am kind of lost trying to figure this out. My 240Z bucks and I don't know why. I have not had my car corner weighted yet, but the entire car was weighed at 2380lbs. if that helps. I have not purchased "Race Car Vehicle Dynamics" yet, so I'm posting this here to see what you guys think. Here is my suspension setup:

 

Coilovers-

175 lb/in front

200 lb/in rear

 

Tokico Illumina's-

set at 3 in the front

set at 2 in the rear

 

Suspension Techniques sway bar's- (going by memory here)

1 1/8†front

7/8†rear

 

The wheel/tire combo is pretty heavy. I can’t recall the exact weight.

 

Now, when I’m driving, the front will hit a bump and absorb it very well, but when the rear hits that same bump, it launches me almost out of the seat. I’ve tried adjusting the Tokico Illumina’s, but this only makes it worse. Going less with them just makes the car handle sloppy and going more with them makes the ride almost unbearable. This is a street car that will see some track events. I would say 90% street use.

 

Would changing the spring rate in the rear help with my bucking problem? I can only go to a higher spring rate, because I am running an 8†spring. Would it be best to get my car corner weighed before I make any changes? In that case, I would have to wait until my Buick engine is in the car, which will be very soon

 

!M!

[gramercyjam]

Ah, but what if the front and rear springs are different sizes (length, mass) and the same "rate"?

[CruxGNZ]

Fronts are 10" length and rears are 8" length.

 

Now, lets talk about just the rear springs:

If you have a 10" spring with 200 lb/in rate and a 8" spring with 200 lb/in rate, these springs should be identical from what I understand. One will just have less travel before coil bind, correct? But now you got me thinking, after the first inch of compression (how spring rates are measured), what is the second inch of compresion compared between the two springs? We will use the 10" and 8" springs for the question.

 

!M!

[johnc]

Now, when I’m driving, the front will hit a bump and absorb it very well, but when the rear hits that same bump, it launches me almost out of the seat.

 

Keep in mind where your butt is in relation to the front and rear wheels. Its about 18" in front of the rear wheels and about 60" behind the front wheels. The shorter lever arm tot he rear wheels exaggerates the force of the rear wheel impacts in comparison to the front - probably by about 300%.

 

Also, how low is your car? Are you hitting the bump stops when hitting large bumps?

[CruxGNZ]

John,

The car is pretty low. About 1" of fender to wheel gap with 17" wheels. I don't know off hand how low the chassis is to the ground. The strut housings have been sectioned 1.5", forgot to mention that in my post above. I have checked the bump stops many times and even used a dab of white grease on the bottom of the bump stop to see if they hit the gland nut. They (all 4) do not bottom out. The roads here in Wisconsin are pretty crappy and expected a harsh ride, but did'nt expect the suspension to act as it is.

 

Knowing that the car is low, but does not hit the bump stops, raising it up a little will have no effect on my bucking problem, correct?

 

!M!

[Forrest]

Not sure how accurate this is but if it helps, here is a spreadsheet I found online. I can't find the page I got it from now, but the guy had a 2650lb s30 (exact same weight as mine) so the default values are close. I am assuming the unsprung weight and distribution is roughly the same until I can do a better job of weighing mine.

 

http://members.cox.net/forrestosterman/springrateselector.xls

[CruxGNZ]

Forrest,

I can't get that link to work. I've tried everything to get it to run, but no dice. What program runs ".xls"?

 

!M!

[TimZ]

Something else to consider - most people that run 8" rear springs are running higher spring rates than that - higher rates generally limit the amount of suspension travel.

 

Assuming you have Eibachs (the specs will be similar even if it's a different brand - the dimensions are dictated by physics), your rear springs have 5.33" of travel before they bind.

 

For a ballpark number, let's assume your car weighs 2600lbs with you in it, and has 50/50 weight distribution. This would put 650lbs of force on each spring. which would compress your rears by (650lb)/(200lb/in) = 3.25 inches. This means that at your nominal ride height, you have 5.33 - 3.25 = 2.08 inches of suspension travel before your rear coils bind.

 

If you've sectioned the struts by 1.5" and lowered it by the same amount, you should have ~3" of compression travel available from the suspension. So, it's very possible that your coils are binding before the suspension bottoms, which would explain why your aren't seeing marks on your bump stops.

[CruxGNZ]

Hmm, that's very interesting Tim. Just curious how you figured out that an 8" spring would compress 5.33" before coil bind. I suppose you just need to know the diameter of the spring's coils to work the math.

 

I thought if my rear springs were compressed to the point of coil bind, I would feel it as a hit, like a sledge hammer on the strut tower, which I don't believe has happened yet. How the heck am I going to figure out if my 8" rear springs are compressing to the point of coil bind before my bump stops are doing thier job? Would it be wise to up the spring rate to see if my bucking problem goes away? If so, how much? I can't go any higher of a spring rate than my Tokico Illumina's can handle, which I believe is 250 or 275 lb/in springs.

 

I'm wondering if anybody else has experienced this same problem I'm having. I know there is quite a few running 8" rear springs for wheel/tire clearence for an unflared car, but what rate are they? I'm the lowest I could go in the 8", so I could keep the ride from not being "spine compressing".

 

!M!

[TimZ]

Hmm' date=' that's very interesting Tim. Just curious how you figured out that an 8" spring would compress 5.33" before coil bind. I suppose you just need to know the diameter of the spring's coils to work the math.

 

I thought if my rear springs were compressed to the point of coil bind, I would feel it as a hit, like a sledge hammer on the strut tower, which I don't believe has happened yet. How the heck am I going to figure out if my 8" rear springs are compressing to the point of coil bind before my bump stops are doing thier job? Would it be wise to up the spring rate to see if my bucking problem goes away? If so, how much? I can't go any higher of a spring rate than my Tokico Illumina's can handle, which I believe is 250 or 275 lb/in springs.

 

I'm wondering if anybody else has experienced this same problem I'm having. I know there is quite a few running 8" rear springs for wheel/tire clearence for an unflared car, but what rate are they? I'm the lowest I could go in the 8", so I could keep the ride from not being "spine compressing".

 

!M![/quote']

 

Actually, I just looked it up on Eibach's website:

 

Eibach Application Guide

 

As I recall, most of the applications that I have heard of with an 8" spring were upwards of 300lb/in. Unfortunately, if you want to drive on the street and your roads aren't perfect, this is going to be pretty stiff, not to mention being a bit oversteer-y unless you change the fronts to something stiffer, too.

 

I think that the first thing I would do would be to figure out whether or not binding the coil is possible with your setup. Pull the spring and compress the rear suspension until it bottoms, and measure the distance between the spring seats. If it's less than 2.67" you have a problem.

[260DET]

Mat, whatever is causing your bucking problem it is not the springs being too hard or mis-matched front/rear. My car has had various springs between 220 and 300 before settling on the present 275/275 combination and it never bucked.

 

It could be the bump setting on your rear shocks but it seems more likely that it is a suspension binding problem somewhere.

[CruxGNZ]

Thanks for the information guys. With my eyes opened up a bit more, I'm going to check out if I have an issue with running to low of a spring rate for my rear springs causing them to bind. When I have time, I'll pull a spring out and check the travel. If the distance between the two spring seats is 5.33" or less when the suspension is fully compressed, then I will know where my problem is. If I remember, I'll post what I find out.

 

!M!

[TimZ]

Thanks for the information guys. With my eyes opened up a bit more' date=' I'm going to check out if I have an issue with running to low of a spring rate for my rear springs causing them to bind. When I have time, I'll pull a spring out and check the travel. If the distance between the two spring seats is 5.33" or less when the suspension is fully compressed, then I will know where my problem is. If I remember, I'll post what I find out.

 

!M![/quote']

 

5.33" is your free travel. The bound length is 2.67" That's the number you need to look for. As a quick check, if your lower spring perch is considerably more than 2.67" below the top of the strut, this is probably not an issue.

 

While you are at it, make sure nothing else is binding, like the sway bar grounding against the a-rm, for instance (you'll have to disconnect the arm at the other side for the work).