[stretch]

A brief history:

I bought a used 2005 STI with low miles from an old guy. I'm not sure what this guy's deal was, but he owned his own business and CarFax showed he registered it as a business car. It was Aspen white, but with pinstripes and door ding protectors- how dorky. He claims to have driven it regularly for a few months before switching back to his BMW 7-series as his preferred highway cruiser.

The previous owner took the car to a tuner shop, and they put an intake and catless turbo-back exhaust on it with no engine management. It had horrible boost creep and was obnoxiously loud. The car had Nexus gauges ($$$) that were improperly wired and reading 22-24lbs of boost. All four tires were at a low 27psi. Since it was a great deal and I saw no signs of engine damage, I agreed to buy it anyway. It was a gamble.

The car was two hours away, so I bought a one-way airline ticket to pick up the car. A few days before my flight, the previous owner called and asked where I was. He had been waiting for me all day for me at the airport on the wrong day, was pissed, and hung up on me after a few choice words. Then, he checked his e-mail, found that I did give him the correct date and that it was his secretary's mistake, and called to apologize. Things definitely weren't going well with this guy.

A few days later, I picked up the car. In hindsight, I can't believe that I even went through with the deal, and I remember agonizing over it until my flight. But, it gets better- when I picked up the car, there was a hand gun in the center console. I was checking the car over just prior to saying goodbye, and, "Whoaaaaa!" I thought this rotten, suspicious deal was going to end with me being robbed.

It didn't. I drove home safely and began reversing all the crap this guy had done to the car.

I've since put the stock intake and exhaust back on, and switch to a quieter Cobb downpipe with a proper Cobb stage 2 tune. After an oil change and the proper tuning, the idle of the car became extremely smooth. It burns almost no oil. I gambled, and I'm glad this car has so far turned out to be a gem.

My previous car was a 4cyl Mazda6 with no options- deliberate to keep weight down. It was a great car. I put an aftermarket suspension on it and raced it in DSP autocross. I tuned the car to oversteer as much as possible. It was great- the car would oversteer in a turn at anything less than full throttle. It proved to be very fast and taught (forced) me to brake early and late apex. I never once lost to another front-drive vehicle, although the BMW's in my class usually won by a large margin. I'm done with front wheel drive.

Now, I plan to race the STI.

I've read several books on suspension tuning and a few books on turbo engine tuning. I enjoy suspension stuff more since I've always been one to enjoy tight roads. My favorite books include:

• The Secrets of Solo Racing (best intro to driving there is)
• Going faster! (the driver's bible)
• How to Make Your Car Handle (sadly a bit dated, but still preferable to Carrol Smith's "Tune to Win" IMO)
• Forced Induction Performance Tuning (this is much more informative than Maximum Boost)

[stretch]

Here are what I think are some of the more interesting discussions I've started or participated in. I'm a suspension guy, as you can probably tell.

Suspension Discussion:

• TiC Coilover Development Thread
• Critical Damping Analysis of many coilovers for the STI
• All lowering springs (and even stock springs) ride on the bump stops!
• Another major problem found with many lowering springs: coil bind
• Can you pick up more suspension travel with new coilover spring? Coil bind revisited.
• The complexities of tires in suspension design and tuning
• How stiff are your sway bars really?
• Sway bars and total spring rates (why sway bars sometimes do too much)
  
• Damper tuning and dyno graph discussion
• Damper design, twin-tube versus monotube
• The stock struts are perfectly damped (to everyone's surprise?)
  
• Neat trick to improve your suspension geometry
• Lifting the inside rear wheel- who cares?
  
• Spring frequency calculator
• Suspension Calculator! Weight transfer, spring frequency, damper, body roll calculator, and more!
• I don't like progressive springs, why do you?

Engine Discussion:

• List of UOA's (Used Oil Analysis)- find an oil that works!
• Intercooler discussion (front mount v. water-to-air)
• Your thoughts on the boxer engine design?
• Directing airflow to a larger TMIC

[stretch]

Goal: create an STI that handles well enough to be competitive in regional STU autocross while being perfectly comfortable as a daily driver (and to do it as inexpensively as possible)




Current engine modifications:
Cobb downpipe (stock catback)
Cobb Accessport (version 1)
Cobb Street Tuner (...untouched)

Current suspension modifications:
Turn-In Concepts single-adjustable coilovers
GT Spec anti-lift kit
Cobb 25mm hollow front sway bar
Whiteline 24mm rear sway bar
Kartboy rear endlinks

Other products I've tried:
Prodrive bump stops (stock front struts)
Whiteline camber plates
eBay Top Mount Intercooler
Ground Control coilover sleeves (stock struts)
Ground Control custom built Koni coilovers (front)
Koni inserts into WRX strut housings (rear)
Strano 31mm front sway bar
Whiteline 27mm front sway bar
AEM cold air intake
HKS catback exhaust

[wdb]

Nice start, but how about some pics of those door guards? They're JDM I hope!

[stretch]

I'm so happy with the way my car handles that I don't think I'm going to modify it anymore. Sure, there are improvements I could make, but I think I've reached the point of diminished returns. I'm so content right now that I've lost the desire to tinker- and I never thought I'd say that!

Trimming the car's bump stops is the magic modification, and it doesn't cost a dime. I don't mean for this to be a tuning secret, but I sure haven't seen that advice spread via word of mouth.


On Lowering Springs:

Everyone pushes lowering springs. Why? As I've illustrated, the car does not have any suspension travel to spare. In stock configuration, my 2005 STI was riding on its bump stops.

How many times have people complained about their STI pushing on corner entry? It's because you're hitting the bump stops. Huge sway bars alleviate this by limiting roll, explaining their effectiveness. Lowering springs, however, make this issue much worse. It doesn't matter who makes them- the more they lower the car, the worse off you are unless you do something to elongate your your bump travel.

The problem is- and I've been fooled by this myself- that it's easy for a suspension modification to feel fast when it's not actually fast. Any perceived difference must be an improvement, right? Often not.

Everyone learns some day. For me, it was the day I took lowering springs off my Mazda6 and went back to stock. When I did so, my lap times dropped and the car was more predictable. Lowering springs caused the car to be jittery at the limit, which felt fast to me, but measurably wasn't fast at all. I then took apart my old suspension and figured out the cause- lack of suspension travel. One bump stop had even split from the stress.

The STI is worse off. The drawbacks of this are numerous- more than I care to delve into here, but see some of my other threads for explanations. In short, hitting the bump stops while turning will cause an extreme and rapid spike in spring rates, leading to a change in handling bias and a potentially large jacking effect. The former leads to terminal understeer while the latter leads to a raise in center of gravity, a moment of lightness, and general unpredictability. Furthermore, the spring rate spike causes the suspension to be momentarily underdamped, leading to what many here call the "bobblehead" effect.

I trimmed the bump stops up front on my STI and it's made a clear improvement in ride and handling. The car doesn't plow on corner entry anymore. It absorbs large bumps much better. These are real improvements that springs cannot physically deliver. Lowering springs have the exact opposite effect- the car rides further on the bump stops. To make matters worse, lowering springs make trimmed bump stops an impossibility since lowering springs depend on the bump stop to offload some of the car's weight.

I love the way my STI handles- it behaves as predictably as I could ever hope, turning in every time I ask it to. I didn't have to spend much to get this. However, OE springs with trimmed bump stops aren't exactly going to make vendors money. And lowering springs no doubt feel fast. Besides, I'll admit, lowered cars look good. That's why keeping stock springs will never be a popular option for the vocal majority on this site- but I'm an advocate of the STI's stock spring rates. They're great for street use- about as firm as what you'd see in street coilovers for most four door cars.


On Stock Springs:

More spring stiffness isn't necessarily desirable unless you're going to be on a smooth track, especially once the stock springs are paired with 24mm or larger sway bars. This is based on my subjective opinion as much as it is the mathematics of weight transfer for a normal 1" bump. Drastically firmer springs can be run in lieu of larger sways, but this degrades ride quality much more severely, so why do it?

I think the sweet spot on typical roads is in the 23-26mm range up front with a 24mm rear bar set as desired. The arm length of the bar is huge, so don't look at just diameter alone. I find more stiffness can improve turn-in but at the expense of the car getting jittery mid-turn over bumps. Getting similar roll stiffness with firmer springs and stock sways would necessitate getting springs roughly twice as firm as the firmest aftermarket springs currently sold.


On lowering:

Now, I do think lowering the STI (more specifically: reducing weight transfer) has its benefits, but it should be done in ways that do not effect suspension travel. RCE's lowering camber plates accomplish this. Shorter bump stops do this. And shorter struts also help, which may necessitate moving to a twin-tube design. KW coilvoers and Koni struts both use twin-tubes, which allow physically shorter designs than monotube struts of the same stroke.


What To Buy:

I think one reason expensive coilovers work well on the STI is because they increase suspension travel (via a shorter strut or shorter bump stop) at the intended ride height. However, buying coilovers just to get shorter bump stops is overkill. Sure, nice coilovers come with better-than-stock dampers (the performance of which should not be underestimated), but it's still overkill for those merely seeking better ride quality. There's much more value for most users in lowering camber plates and/or shorter bump stops. Lowering springs, on the other hand, are for show cars.

I'd recommend the Prodrive bump stops up front and the stock rear bump stops with the firmest progression trimmed. The combination will outperform $300 springs. Try it for free: you can cut off the firmest progression or two from your stock bump stops instead if you just want to experiment.

Please, leave comments here.

[stretch]

More On Lowering:

Lowering your car accomplishes a fantastic thing: it reduces the car's center of gravity (Cg). Reducing the car's center of gravity is one of a very few ways to reduce weight transfer. Reducing weight transfer keeps your tires as evenly loaded as possible mid-turn which increases grip.

Here is an article discussing why weight transfer decreases:
Why is lowering a car so desireable?

Tires work best when load is evenly split among them. In other words, the coefficient of friction of a tire goes down as it is asked to support more weight. This is probably the absolute most important concept of suspension tuning because everything comes down to making the most of your tires!

However, there are many other considerations to lowering. Some are obvious, like the reduced ground clearance. Others are intuitive, like the change in aerodynamics. Yet there are still some other concepts that seem are less than obvious.


Camber Curve:

This isn't what I intended to discuss here, but there's a great thread on STI camber curves here:
6Gun Racing Ball Joint Extension Kit - Car Part Reviews - NASIOC

There's a lot of talk about how lowering a car will destroy the car's camber curves. It does. All cars with struts have worse camber curves the lower they get- a reason why double wishbone suspensions are very desirable!

I agree that camber curves are important, but an STI with very stiff sway bars need not worry much about them. Body roll will be reduced to under three degress (sometimes under two degrees), and most of that will be coming from tire and bushing deflection. The actual amount of suspension bump travel (where dynamic camber comes from) is often under one inch. So, static camber is really the only type of camber that matters, and it's needed to compensate for the squish of your tires. Your suspension won't compress enough for dynamic camber to make much change.

Camber curves matter a lot when your suspension can compress more than an inch. This generally happens on stock cars, but at the stock ride height, the camber curve is decent.


Roll Center::

I'll have to immediately refer to page 2 of the article above, as it has a great illustration:
Why is lowering a car so desireable?


The roll center of a car is the axis around which the car's body rolls. When you experience body roll in your car, it's rolling as if it were hinged at the roll center axis.

On a lowered car, the center of gravity (Cg) is lowered, but so is the roll center. The roll center, in fact, lowers farther than the center of gravity. The distance between them is the lever arm that causes body roll, so when lowering a car, we've increased the car's roll couple.


Roll Couple::

This is where lowering gets interesting.

The measurement of your vehicle's roll couple is a measurement of your vehicle's desire to have body roll. Cornering force on a car acts on the car's center of gravity, as we've discussed. Cornering forces are resisted by your tires. This resistance from your tires acts on the car's roll center. Because cornering forces happen at the Cg, and the Cg is above the roll center (the hinge the car rotates upon), the forces acting on the Cg are what tip and rock the car.

Imagine holding a baton straight up in the air, gripping it with one hand at the bottom. Where you grip the baton would be the roll center. Now, if someone pushed on the middle of the baton (its center of gravity) while you tried to hold it still, it'd be hard for you to keep the baton from rotating in your hands. This is because the other person has a lot of leverage over you- the distance from his hands to yours (the roll couple) is large. If you moved your hands to the middle of the baton while a friend pushed in the same location, the baton would not want to rotate in your hands. A roll couple of zero will result in none of the car's weight transfer being turned into body roll.

Next time you're driving behind an SUV, look at how the lower control arm of the SUV is angled steeply upwards. This is because SUV's need a high roll center to resist body roll. A Corvette, on the other hand, has a much lower roll center (again, easily visible when driving behind one).

When lowering the STI, you are increasing the car's roll couple. This means the car is going to want to have more body roll, and you'll need firmer springs to resist this tendency to roll. So, your lowering springs will have to be firmer than stock just to maintain a stock amount of body roll. This isn't a huge deal by itself- just run larger sway bars to compensate. The total amount of load transfer on the car is reduced by lowering the car, so usually overall grip is increased.

However, what is a big deal is that your roll couple changes with body roll. If your roll couple is constantly changing mid-turn, the car will not be able to settle properly and load the tires correctly. This is very bad. The quicker your roll center migrates, the less stable the car will feel mid-turn. This is why cars with double-wishbone suspensions can feel so good despite having a roll center at the same height (at rest) as a strut car: it migrates less. The general rule here is that the lower your roll center on a strut-equipped car, the quicker it will migrate and the worse the problem gets.

While we see kits for the WRX and STI designed to raise the front roll center, I personally believe it's best raised in the rear, but unfortunately no kits exist to do this. (They'd be complicated and expensive to make.) Still, this is worth discussing- why the rear? The above image is misleading; roll couple is actually measured as the distance between the CG (which is roughly where the shift knob is) and the spot directly below it along the roll axis. Roll couple should not be measured at just the front or just the rear axle; instead it's essentially an average of the two. Engineering textbooks tell us that the roll center should be lower up front and higher in the rear to get a desirable feeling in vehicle yaw. I think the larger benefit is to have reasonable overall roll couple while still having a low roll center up front and thus a low jacking force up front. We want a low jacking force at the steering wheels. What is a jacking force?

Jacking Force::

I've seen this force described in many books, but never described well. It is very, very important, so I hope my explanation is easy to understand.

Cornering force on a car acts on the car's center of gravity, as we've discussed. Those cornering forces are resisted by your tires. This resistance from your tires has to push back on something, and it pushes back onto the car's roll center.

However, if the roll center is above ground, that means the tires are pushing upwards on the car.

Think of a pole vaulter. A pole vaulter uses his pole to convert his horizontal force (inertia from running) into a vertical force. That higher a pole vaulter holds his hands after sticking the pole, the higher the vertical acceleration. This is why pole vaulters launch with the pole over their head.

It's the same with your car- having a high roll center is akin to the pole vaulter holding his or her pole over their head. There is a greater vertical component of the force coming from your tires.

This vertical force will cause the body of your car to rise as you enter a corner, and this is known as a jacking force. When entering a turn rapidly, your car will rise rapidly, and the car will experience a moment of increased grip followed by a moment of lightness (and the associated lack of grip). This causes unpredictability in transients and is simply intolerable in a sports car. It is for this reason that race cars have a roll center around or even below the height of the asphalt- this is to eliminate jacking forces.

However, having such a low roll center generates a large roll couple and thus race cars must use very firm springs to resist all the body roll. Such firm springs are not practical for a street car, thus most street cars have roll centers many inches above ground.


On Progressive Springs:

There is another source of jacking forces, and this is the one I touched on in the above post: progressive spring rates. With progressive spring rates, your car's center of gravity will have a similar rise mid-turn. The outside of your car will compress less than the inside of your car will decompress, causing the center of the car to rise up.

Even cars with linear coil springs have a progressive final spring rate if they are hitting their bump stops since the spring rate skyrockets once on the bump stop. It is for this reason that Porsche "S" models use much smaller bump stops than the standard models- to decrease jacking forces and increase predictability at the limit.

If your car runs 200lb/in linear springs but is touching a 200lb/in bump stop, that means the spring rate on the outside of your car is actually 400lb/in while cornering. So, for 800lbs of weight transfer, the outside of spring will compress just two inches while the inside spring will decompress four inches! Since your car has sunk two inches on one side but risen four inches on the other, your center of gravity has now risen an inch. That's an increase of roughly 5%- an increase that will happen suddenly every time you transition into a turn! It's like your car is being un-lowered every time you turn and your ride height is always changing.

So, when modifying the STI's suspension, I suggest as linear of a spring rate as possible. The car should not experience a spring rate change mid-turn ever. Bump stops and progressive springs are OK so long as the spring rate changes are outside the range your car would see under normal driving. Thus, make sure your bump stops are short enough that you rarely (if ever) hit them.

Stock, my 2005 STI rode on its bump stops up front, probably to prevent wreckless drivers from oversteering on throttle lift. This is why the car had immediate, terminal understeer on corner entry, and why I recommend that everyone replace or cut their bump stops!


Final Thoughts:

Lowering an STI makes many complicated changes. Generally speaking, I think you trade predictability for peak performance, though there are exceptions. There is no right way to do things, and even one person's ideal setup can change drastically with a new set of tires. Be sure to experiment, but know how many changes each experiment makes!

[nice-rice]

Thats really well thought out, and helpful. As I dont like my ride TOO stiff.

[lackskill]

Great read, thank you!

[stretch]

On Coilover Development and Tuning:

This is what I've read, and honestly they're quite simple rules:
- Run a higher natural frequency in the rear
- Use twice as much rebound resistance as bump resistance
- Aim for roughly 65% critically damped (average between bump and rebound) below 3in/second, less thereafter

I'm coming up with slightly different ideas although they're in the same ballpark. I haven't tested them all, yet:
- Run a higher natural frequency in the rear
- 50-100% critically damped in the very low speed is OK so long as there is strong digression
- It is OK for digression to happen as late as 6in/sec, does not have to be at 3in/sec
- Use only slightly higher bump valving than rebound up front
- Use a lower damping ratio in the rear than the front for bump valving (thus requiring a higher damping ratio for rebound) to compensate for a higher rear spring frequency
- Run as little high-speed bump as possible, not much high-speed rebound either

Some things I've learned:
- Firmer springs make your average ride quality worse since the car will respond to bumps more abruptly
- Firmer dampers make your peak ride quality worse since it'll resist the suspension motion required to absorb large bumps

[stretch]

On choosing spring rates:

Manufacturers rate their springs in kg/mm or lb/in, but neither is how you should be making your spring rate decision. (For reference, though, one kg/mm is equal to 56lbs/in.)

No, instead you should be looking at the natural frequency of the spring. This magical number takes into account the suspension geometry, weight, and weight distribution of the car- which just so happens to mean that natural frequencies can be fairly compared between cars. Want your car to ride like a Cadillac? Copy its spring frequencies, not its spring rates.

The STI's stock spring frequencies are roughly 1.6hz front, 1.8hz rear. This is actually pretty aggressive. For comparison, I've read the following:
1990 Mazda Miata ~ 1.15hz front, 1.01hz rear
Mitsubishi Evo VIII ~ 1.3hz front, 1.2hz rear
Honda S2000 ~ 1.3hz front, 1.4hz rear
Lotus Elise ~ 1.8hz front and rear
C4 Corvette ~ 2.0hz front and rear

Most experts and textbooks would suggest use around 1.6hz front, 1.8hz rear for a sports car and 2.0hz front, 2.25hz rear for a race car. Of course, there's some wiggle room in those numbers, and we see coilovers for the STI extending all the way to roughly 2.3hz front, 2.5hz rear. The important thing here is that you do not go too stiff for the road you are driving on or you will lose grip. Remember that on a perfectly flat surface, you wouldn't need a suspension- the suspension is there for the bumps, so optimize for them accordingly.

It's interesting to me that most of the cars listed above use softer or equal spring rates in the rear because I believe that is the wrong thing to do. Subaru's stock spring rates are great by comparison. Let me show you why.

Let's say our front spring frequency is 1.8hz. If we make our rear frequency 1.8hz too, our chassis motion will look somewhat like what you see in the following graph:


The two lines represent chassis movement at the front and rear axle. Any time they are different, the chassis is pitching fore and aft, creating an unpleasant rocking sensation. (And the speed at which this happens determines ride harshness.)

You can see that equal spring frequencies and (with dampers matched identically to the springs) will produce two bumps that feel of roughly equal harshness, but the car will see substantial pitching due the front and rear of the car not settling into a rhythm with one another. We can fix this by running a higher spring frequency in the rear, which will cause the rear of the car to oscillate faster and catch up to the front oscillation:


Notice how the front and rear of the spring settle together, very nice! However, now the front and rear bumps are of different magnitudes due to the firmer rear springs. While there is less chassis motion in this simulator, the change in direction is more abrupt and that's what determines ride harshness: rate of acceleration. We can soften the dampers to allow more of the bump to be absorbed by the spring, and in doing so we get this:


In this graph I've reduced the rear damping to 36% critically damped, down from 50% critically damped from the previous graph. Notice the bump is now of equal magnitude front and rear, except we clearly need more damping for the rear! This is what being severely underdamped looks like- lots of body motion. We don't want this. So, what we want is that same low bump resistance, but higher rebound resistance. This graph was made in Photoshop, but the end result is intended to look somewhat like this (the particular program I was using did not support variable damping rates):


What you have there is an ideal spring and damper combination for stability. Stability leads to increased grip from the tires, too. You can achieve this reduced pitching regardless of how soft or firm your springs are (to an extent). Pretty neat, eh? This is why I recommend firmer springs in the rear with less bump resistance relative to the front.

[stretch]

I've been working hard on simulating suspensions using Microsoft Excel. I'm quite proud of how far I've gotten doing this. Here are some pictures from my virtual 4-post shaker which I'll make available publicly some day.

Before I go any further, a HUGE thanks goes out to Wes (4banger on these forums) and Turn-In Concepts for helping get and analyze the data that has led to this program.

Each one of these screenshots below will show you three graphs. The first is chassis movement at each corner of the car. This shows you how much overall chassis movement you are getting. The bottom graph is the height of the asphalt the car is riding over. You'll notice that while the simulated bumps are very abrupt, the chassis motions are much smoother. While the least amount of body motion might at first seem desirable, what we really want is the height of the lines in that graph to change directions as slowly as possible. This is the acceleration of the chassis, which is also shown in the middle graph.

Consider this: a one inch change in asphalt will always result in a one inch change in chassis height- eventually. What matters is how long it takes for the chassis to make that change and settle. The quicker the chassis height changes, the more uncomfortable the ride will feel. However, a chassis that settles with little excess motion will feel more composed. What you want, then, is low chassis acceleration that settles again with little excess movement. This also results in high grip since this same formula keeps the most consistent load on your tires.

That middle graph is plotted as G's. 1 "G" is simply the force of gravity. 2 G's is twice the force of gravity, meaning that if you are experiencing 2 G's, your body will feel twice as heavy as it normally does. This creates discomfort. I've also provided RMS (root-means-squared) chassis acceleration numbers, too. This number is sort of an average deviation from 1G. This is an industry standard way of mathematically measuring ride quality. Lower means less harshness.

With those things explained (hopefully in an easy-to-understand method), here are some graphs I've produced.

The difference in ride quality between 7k and 5k springs (the simulated damper had changes primarily in low-speed valving only).


Here are some valving revisions I've been working on for my own front dampers:


Based on my simulated results and LOTS of reading, I've come up with this experiment in damper valving. Turn-in Concepts has agreed to try it out- we're both going to have a set of front dampers valved this way.


Compared to Ohlins Sportlines, the TiC dampers look much better. (But this is no surprise, the current valving is too.)


Note: in that last graph, the TiC coilovers look much softer than the Ohlins- HOWEVER, what you do NOT see is that they ALSO have more resistance in the critical "car control" region (measured at 50mm/second damper speed). Thus, you're gaining comfort but- in theory- also gaining performance. My virtual shaker rig isn't yet a 7-post simulator though, just a 4-post, so I can't simulate that just yet.

[stretch]

One last placeholder!

[topend]

Wow, your journal has a lot of great reads *subscribes*

[BigSky]

pics not working in your 1/04 post

[MGizzle]

Stretch, the graphs are X'd dude!!!!