Is our suspension setup correctly by aftermarket strut MFR's? Is the bias correct?

[sergechronos]

No coil overs don't mean a shitty ride necessarily, but with the rates he runs and the rates under discussion here, it is similar and both would be very stiff springs indeed. My question was more one of would this knowledge still be applicable to the rest of us if we reduced spring and dampening rates down 10-25% (which are probably still significantly stiffer than factory) so long as the ratios between the corners is the same?
Example: Car weighs like the one previously listed at
1082 893
586 752

Spring rates found to be appropriate are (as an example) let's say
12k 11k
13k 14k

Could those numbers be reduced by a percentage, thus keeping the ratios fairly similar and compromising on the bleeding edge of performance (but still markedly better than most suspension options currently) but allowing a more streetworthy ride? Say a 25% reduction (which would probably still be significantly stiffer than factory)
9k 8.25k
9.75 10.75k


Given most of our coil over and spring options now are, as pointed out previously in this thread, stiffer in the front than the rear while more modern suspension concepts for the Evo have reversed that and made the rear stiffer than the front. What I'm looking at is with this change to a stiffer rear set up, as well as a particular spring and dampening rate that is set up for our cars explicitly for tracking purposes, could be adjusted either via spring rate or dampener force to maintain the vast majority of that performance increase while being a bit more usable in regular driving.

Short and dirty of it: of how much value is this information going to be to the 3/S community who can't afford to spend 3k+ on suspension? Given that manufacturers are unlikely to change their spring rates just on our calculations, would these calculations and rates be able to be applied to some degree for the rest of us by getting springs that are closer to the ideal spring rates J is working out, or would it be so little of a difference versus off the shelf suspension options not to be worth the cost?

[CoopKill]

Also realize that J may have a 15-25% weight reduction, so his rates would give a stock weight car exactly what you are asking. The answer in short is yes. The closer to optimal spring weight, the better performance can be had. Where things may get sticky is with the valving vs. spring weight vs. car weight.

[sergechronos]

Also realize that J may have a 15-25% weight reduction, so his rates would give a stock weight car exactly what you are asking. The answer in short is yes. The closer to optimal spring weight, the better performance can be had. Where things may get sticky is with the valving vs. spring weight vs. car weight.

Oh no doubt, not even a comparison between the average 3/S and his in terms of weight. However, if what he finds holds true for the platform as a whole, then SCE might see more usage of their Evo adapters and us raiding EvoM for used coil over set ups that have rates closer to our needs, or maybe someone tracks down (or does the math) for springs that will bring the cars much more in line with the ideal. Since all suspension options for the car currently (just off hand) have stiffer springs in the front, or no listed rate for it. The Megan coil overs for instance have a 12k/8k set up, and even the Apex'i S1 that is allegedly better for the US roads uses 8k/6k set up. So this would require either new products that recognize the benefit of the stiffer rears, or a raiding of Evo parts.

[2fnloud]

Or try Tein pillow-mounted ECS Struts with Ground Control sleeves with spring rates that balance out:

corner weight info:
ride hight
ect
ect.

[J. Fast]

This post right here is absolutely brilliant and will save me hours of my life in explanation...

Selecting Spring rates for coilovers. Theory and How to

I'm writing up another one of my little bits that provides some math, some questions, and hopefully some answers. This time I will be talking about suspensions - spycifically choosing spring rates for your coilovers. I know BC Racing lets you choose your spring rates for a fair price, but have not heard about other companies offering this same service.

Now, the only spring rates posted for coilovers that I've thus far been able to find are BC (8k front and 6k rear) and Megan (8k front and 10k rear). This is what really peaked my interest two different companies producing their springs at the exact same plant with the exact same front rates, but one is producing springs 2k stiffer in the rear and one is producing springs 2k softer in the rear. So my question is which one theoretically would provide better handeling for our car?

Now, I know the BCs are a better quality (read larger monotube) than the Megans, and I've only heard good things about the BCs and mixed reviews about the Megans - most people find the megans too stiff, and a little bit low in quality for their purposes, but this isn't a discussion about brand names rather we're here to select spring rates.

I previously had asked this question in a couple other threads because I've always heard you want to stiffen the suspension in the rear to reduce understeer (which we have pleanty of) and was told at the time that the BCs would still be the way to go despite the softer rear spring rates because there is less weight in the rear so it works out.

I still wasn't completely satisfied so I did a little bit more research and here's what I have come up with: Take it as a grain of salt because I'm still a newb, this being my first car worth modding, and feel free to provide constructive feedback if I've gone off course anywhere. Without further adeau:

The first thing you want to do when setting up a suspension is to decide on your ride quality aka stiffness. The easiest way to compare the stiffness of a car across platforms seems to be by looking at the natural frequency of the cars suspension measured in hertz (cycles per second) this would be how fast the car bumps up and down when you continually press down and release on it. As a general rule of thumb it seems that around 1 HZ is standard for OEM 1.5 HZ is what a good sports car runs and 2+ Hz is typical for race/track cars. Just to give you an idea a stock STI natural frequencies are 1.6 Hz front and 1.8 Hz rear so if you've driven one of those you know about how rough/smooth these ranges are.

For my setup I'm sort of the weekend warrior who likes to take a couple corners/canyon drives a little more agressive and doing about 1 track or autocrosse a month so I wanted to end somewhere between a sports car and a track car so I'm going to be shooting for 1.75 Hz on my setup and just deal with a little bit rougher ride on my daily driving.

The equation I found to calculate the wheel rate is WR = (Wheel Frequency/187.8 )^2 * Sprung Weight

I found the curb weight to be 3010 lbs for the manual GTS so when you throw my fat butt and a little gas in there I decided to base my weight estimates on 3333.3 lbs for the car.
The GTS manual has a 60/40 front/rear split for weight distribution, and in my newbness I forgot to subtract the unsprung weight (usually 80-90 lbs per corner). I also assumed that the weight balance left to right was equal. I chose these numbers to make my calculations a little bit easier so the weight on each of the front springs is about 1000 lbs, and each of the rear tires is 2/3 of that or 666.6 lbs.

To do this for a real suspension setup you would want to get corner weight scales and measure the actual weights on each tire. These weights may also change if you use your coilovers to adjust the height of each corner (higher=more weight on that corner). I also assumed an equal drop all the way around which would keep the 60/40 split. Ideally you would want to adjust the heights and weights (relocate battery etc.) to get as close to 50/50 as you can.
You will also have to convert the frequency you desire from Hz to cycles per minute for the given equation (multiply by 60)

Plugging some numbers into the equation for the front tires you find Wheel Rate = (105/187.8 )^2 * 1000 = 312.6 lbs/inch = 5.551k
Now, if you were to take the rear through the same process you would get 2/3 of the wheel rate which is pretty close to what BC is employing with their 8k/6k springs (2/3 would be 8k/5.3k)
But there are an additional couple of equations to consider.

The first is that you don't want the front and rear suspensions to have the exact same frequency. This would create oscillations (bouncing up and down) from the front to the rear. Generally it is accepted to stiffen the rear 10-20% to combat this.

Lets take it a step further and tune in exactly how much stiffer we want the rear than the front. This is going to involve a little more math so hang on.
When coming upon a bump the front tires will hit it first and the rear tires will hit it second. A little trick is to have the car settle at the same time for a smoother ride. Exactly how far appart do they hit? We'll need to figure that out for a predetermined speed. I'm going to use 70 mph since that is my typical cruising speed.

Now, the wheelbase of the GTS is 103.7 inches = 8.64 feet. At 70 mph you are traveling 102 ft/sec so it will take you .085 seconds from when you hit the bump with your front tires to when you hit the bump with your rear tires now we need to make them settle at the same time.

To do that we actually are going to increase the frequency of our reas suspension (remember how the subaru had 1.6 Hz front and 1.8 Hz rear) to find out exactly how much we're going to take our front rate and tune that in for the rear wheel rate. So we invert 1.75 to find our front takes .5714 seconds per cycle, subtract the .085 seconds difference to find our rear rate will be .486 seconds/cycle and invert that again to get 2.06 Hz (track car rates here people)

Now we'll plug these numbers into our equation from before to find the rear wheel rate = (123/187.8 )^2 * 666.6 = 285.9 lbs/inch = 5.078k (nearly equal front and rear rates even though the back end is lighter. This will definately help us eliminate some of that understeer)

But we're still not done. You might have noticed I said wheel rates, but what we're looking for are spring rates. There's one more step before we can know the spring rates, and unfortunately I do not have an accurate measurement to base these values off of. The question is called motion ratio aka when the wheel moves an inch how much does the spring move?

Macphearson suspensions (which our front suspension is) generally have a motion ratio of 1:1. Meaning when you push the spring down one inch the wheel moves one inch. The back suspension, however, is multilink and this is where I don't have accurate numbers. I've read that the motion ratio of the rear suspension is anywhere from 1:.9 to 1:.6. This means That for every inch the wheel travels the suspension actually travels less.

Let me explain why this matters so much. What we have found so far are the wheel rates aka how many pounds of force will cause the wheel to move one inch. When you put the wheel further out than the suspension and do not place it at the proper angle you give yourself a lever so you need a stiffer suspension to counter this fact. The textbook example would be a wheelbarrow. To lift a rock that is 100 pounds one inch with your hands you have to lift with 100 lbs of force for one inch. To lift a 100 lb rock one inch with a wheelbarrow you will have to lift with less force (maybe 25-50 lbs) over a further range of motion (2-4 inches). This is essentially what we need to do with our suspension. We want to lift with 100 lbs one inch at the end of the wheelbarrow. To do this we would need to put a heavier rock in the wheelbarrow (200-400 lb.) and lift the rock only half as far (1/4-1/2 inches).
This is the equation that will help us to solve for our spring rates: SR = Wheel Rate * Motion Ratio^2

I plugged in a dummy value (1:.85) to finish off my calculations, but if somebody would be willing to measure these values that would be great since my wife has the car. The easiest way would be to measure the wheel and suspensioin on the ground then jack it up and measure how much the spring changed and how much the wheel changed. I think ebiach also had a tutorial online that involved distances from the suspension and the hub where the wheel contacts the ground and the angles and a ratio between the distances, but my way is easier and doesn't involve eyeballing it.

Contunued...

[J. Fast]

...Continued from above

To finish off:
Spring rate = Wheel Rate * Motion Ratio^2
Front = 5.551k * 1(ish) = 5551 kg/m
Rear = 5.078k * .85^2 = 7028 kg/m

So, based upon those results it seems that the Megans are set up for a more neutral stance though understandably they would be quite firm. The BCs appear to be set up to leave some of the stock understeer in the vehicle, but if you're increasing the rake of your vehicle they could prove to be a very potent setup nonetheless. Either way I've read that their damper fine tuning provides approximately +-2k adjustability so theoretically the Megans could be set up to be 10k up front and 8k in the rear or the BCs set up to 6k up front and 8k in the rear if you went full soft and full hard on the opposite ends to essentially switch their values. hence the value of a coilover setup.

Now, like I said I'm still a newb and have never set up a suspension before and fining the motion ratio will be critical to fiding accurate spring values because if it is closer to .6 then you might be looking for a suspension setup with 12k in the rear (remember the wheelbarrow example).

So what set is best? well it's all up to personal preference. 3 different professional drivers in the same car would prefer 3 different setups and could achieve very similar times.
As for myself I think 6k up front and 8k in the rear would be a pretty good setup for myself assuming I keep the stock rake, but I would imagine I will probably try to redispurse the weight a little bit better in a year or two when I actually will be buying my own suspension setup; I may even end up in a 7k/7k setup with a 50/50 weight distribution and some good lightweight parts; who knows, but it's all up to you and your preferences.

Also on a side note are there any other companies that let you choose your own spring rates?

Hopefully this will help you in selecting your own coils, but like anything else on the internet take it with a grain of salt. Sorry it was such a long post. I'm an engineer and tend to get carried away in theory and design. Good luck and cheers!

Note in this explanation how the corner weights being discussed are uniquely similar to the referenced 3/S corner weights. 1000lbs in the fronts 660lbs in the rears .

[2fnloud]

Awesome info, will read completely...now your 1000 front and 660 rear.

For what struts?
How much drop in ride height?
Other factors that decide on the rates that you listed?

[J. Fast]

Awesome info, will read completely...now your 1000 front and 660 rear.

For what struts?
How much drop in ride height?
Other factors that decide on the rates that you listed?

	Spring Rate
kg/mm	3	4	5	6	7	8	9	10	12	14	16	18	20	22	24	26	28
lb/in	168	225	281	337	393	450	506	564	674	787	899	1012	1124	1232	1344	1456	1568

Let's see here, assuming proper corner balancing and a speed of 70mph, Known Front Motion Ratio of 1:0.98, Rear Motion ratio of 1:0.84 @ 2" drop and MR of 1:0.86 at stock ride height. Estimated 600 lbs rake/frontal downforce and 500lb downforce in rear.

1. Stock 3/S Ride Freq of 1.60Hz corners weights of 1270lb fronts, 720lb rears
2. Stock 3/S Ride Freq of 1.75Hz w/1.25" drop, corner weights of 1270 fronts, 720lb rears
3. Modified 3/S Ride Freq of 2.05Hz w/1.5" drop, corner weights of 1000lb fronts, 650lb rears
4. Because Spec Racecar 3/S w/ Ride Freq of 2.2Hz w/2" drop, corner weights of 965lb fronts, 585 rears

Wheel Rate = ((60(Hz))/(187.8))^2 x (corner weight)

1. Wheel Rate:

• FRONT ((60(1.6))/(187.8))^2 x 1270= 332lb
• REAR ((60(1.87))/(187.8))^2 x 720= 257lb

2. Wheel Rate:

• FRONT ((60(1.75))/(187.8))^2 x 1270= 397lb
• REAR ((60(2.02))/(187.8))^2 x 720= 300lb

3. Wheel Rate:

• FRONT ((60(2.05))/(187.8))^2 x 1000= 429lb
• REAR ((60(2.32))/(187.8))^2 x 660= 363lb

4. Wheel Rate:

• FRONT ((60(2.2))/(187.8))^2 x 965= 475lb With Aero: FRONT ((60(2.2))/(187.8))^2 x 1265= 625lb
• REAR ((60(2.47))/(187.8))^2 x 585= 364lb With Aero: REAR ((60(2.47))/(187.8))^2 x 835= 519lb

Spring Rate = (Wheel Rate) / (Motion Ratio)^2

1. Spring Rate:

• FRONT (332)/(.98)^2= 346lb
• REAR (257)/(.86)^2= 347lb

2. Spring Rate:

• FRONT (397)/(.98)^2= 413lb
• REAR (300)/(.86)^2= 405lb

3. Spring Rate:

• FRONT (429)/(.98)^2= 437lb
• REAR (363)/(.84)^2= 514lb

4. Spring Rate:

• FRONT (475)/(.98)^2= 494lb With Aero: FRONT (625)/(.98)^2= 651lb
• REAR (364)/(.84)^2= 515lb With Aero: REAR (519)/(.82)^2= 735lb

According to the ref table above it's looking like beacuse racecar wants... 11K fronts and 13K rears.

I'd say GT Worx and Meullerized are about spot on with the bias.

That's enough for today.

[DK77]

Wait, when did we determine that the motion ratio was for sure less than 1?

[J. Fast]

Wait, when did we determine that the motion ratio was for sure less than 1?

Yep, I measured today .

And for giggles in the archives...

I addition to spring rate, wheel rate is also a factor. I'm not as much of a suspention guru as DG but wheel rate is MUCH MORE important than spring rate. Natrual frequncy is the most important, as is takes more things into acount, but wheel rate might be a bit easyer starting point.
Wheel rate is the effective force of the spring on the wheel with the mechanical advantage of the suspention taken into account.
On a 3S the rear wheel rate is over 1. The spring is acted upon using a third class lever. Spring---- wheel---- pivot. The spring travels a longer distance than the wheel, so the effective rate is higher.
On a 3S the front wheel rate is less than 1. The spring is acted upon using a second class lever. Wheel ---- spring---- pivot. The spring travels a shorter distance than the wheel, so the effective rate is lower.
For EVO's the front AND rear wheel rates are LESS than 1.
So to sum it up, even though the spring rate is diffrent, it doesn't mean the effective force is different, since the mechanical advantages are different.

*sigh*

On a 3S the rear wheel rate is over 1.

No, it is not.
Unless you are looking at a pushrod/pullrod suspension with a ratio-changing rocker in it - which if you look at page 3-6 of the 3000GT Tech Manual, it clearly is not - the only way to get a motion ratio greater than 1 is to mount the shock/spring outboard of the wheel. That might be difficult to package.

Think about it - the load is outboard of the spring. What class does that make the lever?
My rough eyeball puts the rear motion ratio around .8 +/- .075 or so. Front motion ratio, being a McStrut, will be .98 -0.1.
Once I find time to model the suspension, I'll know for sure.

Only as a stepping stone to natural frequency.
Which is stiffer, a Lotus Elise with a wheel rate of 500 ls/in, or an F350 with a wheel rate of 750 lbs/in?
DG