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Old 09-17-2007, 11:34 AM   #1 (permalink)
 
Registered: Sep 2005
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Lowering the front creates oversteer or understeer?

Okay guys, here's the deal:

theshadow and I are having a discussion around the effect of lowering the front (hence creating front biased rake) of the car, and we are not sure if doing so to a car that is corner balanced at 49.3% will create more understeer or more oversteer.

What do you guys think?

By the way, my street tires have seen 6 track days so far, including a number of time attack laps. It has plenty of thread left (surprisingly), but is it possible that the tires have gone hard (vulcanization?) from the repeated heat cycles and as a result lost their stickiness?
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Old 09-17-2007, 08:44 PM   #2 (permalink)
 
Registered: Jul 2007
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Corner-balancing equalizes cross-corner weights, so saying that you lower the fronts does not give enough information. You probably know this, but lowering the fronts equally does not necessarily mean you retain the same 49.3% balance.

Lowering a corner (and thereby an entire end of a car) will cause it to take more weight. Depending on what caused the understeer situation to begin with, lowering the front could cause either add to the understeer or produce a more neutral setup.
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Old 09-17-2007, 10:01 PM   #3 (permalink)
 
Registered: Sep 2005
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That's where the problem lies. The F/R weight distribution is already 61/39 and I have absolutely no clue what is causing the understeer. I'm suspecting tires, but I'm not about to go out and buy a whole new set until I can figure out what is wrong.

The way I see it, if i'm hearing tire squeal from the front, it's understeer. if I'm hearing tire squeal from the rear, it's oversteer. Right now, my car is set up such that going into a turn at a good clip will have all 4 tires squealing lightly at the same time.

As you suggested, I'm not exactly keen right now to run out and mess up the balance, and I'm trying to figure out what is causing the understeer.

Here's my alignment setting at the moment:

-3F/-1.2R
0 toe all around
Tire pressures are 32psi all around.

I use this setting for street as well since I take part in track days every 2 weeks.
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Old 09-18-2007, 09:57 AM   #4 (permalink)
 
Registered: Jul 2007
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Quote:
Originally Posted by ghoonk View Post
That's where the problem lies. The F/R weight distribution is already 61/39 and I have absolutely no clue what is causing the understeer. I'm suspecting tires, but I'm not about to go out and buy a whole new set until I can figure out what is wrong.

The way I see it, if i'm hearing tire squeal from the front, it's understeer. if I'm hearing tire squeal from the rear, it's oversteer. Right now, my car is set up such that going into a turn at a good clip will have all 4 tires squealing lightly at the same time.

As you suggested, I'm not exactly keen right now to run out and mess up the balance, and I'm trying to figure out what is causing the understeer.

Here's my alignment setting at the moment:

-3F/-1.2R
0 toe all around
Tire pressures are 32psi all around.

I use this setting for street as well since I take part in track days every 2 weeks.
We're in no shape to diagnose the cause of your understeer on an internet forum. Tire sounds are certainly not a way to diagnose it though, IMO due to how much variation there is between different compounds. Understeer and oversteer conditions themselves are pretty easy to diagnose (I know what you are talking about, just figured I'd clear that up).

Ask others who run the tires what pressures they run at. What are your spring rates and swaybars? In what phase of cornering does the understeer take place? Is it possible you are simply overdriving the car?
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Old 09-19-2007, 04:49 PM   #5 (permalink)
 
Registered: Mar 2007
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Reputation: boostinpsi is an unknown
lower front will put your roll center height underground and cause you to get a huge amount of understeer. the whiteline kit will bring it back up but from what i have heard the front should be atleast 1/2" (3/4" would be better) higher than the rear in terms of rake measuring from the ground to the fender. this will keep your geometry where you want it and help get the car more neutral.
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Old 09-19-2007, 06:09 PM   #6 (permalink)
 
Registered: Jul 2007
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Quote:
Originally Posted by boostinpsi View Post
lower front will put your roll center height underground and cause you to get a huge amount of understeer.
That's a big generality to make. How much does the front have to be lowered before your roll center is below the ground?

Quote:
the whiteline kit will bring it back up but from what i have heard the front should be atleast 1/2" (3/4" would be better) higher than the rear in terms of rake measuring from the ground to the fender. this will keep your geometry where you want it and help get the car more neutral.
Hmm I wasn't aware that front/rear height ratios played any role in a car's suspension geometry...
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Old 09-19-2007, 07:13 PM   #7 (permalink)
 
Registered: Mar 2007
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by reducing the factory rake you greatly affect handling. the ride heights i have read that are the lowest you want to go before the RCH goes underground is about 26" ground to top of fender opening front and 25 1/2'' ground to top of fender opening in the rear.

read throuhg this thred theres lots of good info in there.

http://forums.evolutionm.net/showthr...ht=ride+height
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Old 09-19-2007, 08:11 PM   #8 (permalink)
 
Registered: Jul 2007
Posts: 17
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I'm not about to read through the entire thread, but I think it's a safe bet to make that nobody has measured the CG location or the front RC location on the Evo, let alone determine at what height the front RC drops below ground level. Which isn't to say that the *theory* on which people are basing their claims is incorrect, but you can theorize until you're blue in the face and never come to a solid conclusion (in this case "Why am I understeering?") unless you hunker down and get some concrete measurements, beginning with the above-mentioned CG and FRC locations. It's not that theorizing is bad, but at some point, someone is going to have to say "Look guys, here is the x, y, and z of the car's suspension. Lowering it a-amount will change your RC by b-amount, therefore resulting in so-and-so handling characteristics." I'll save my pennies on roll-center adjuster kits until then.

As far as my comment regarding suspension geometry, I meant to lead you to the concept that a car's geometry rarely changes, unless a control arm is bent or a pickup point is altered. The geometry stays the same when you change a car's height, you simply change the point at which the suspension starts and stops traveling. Those roll center adjusters aren't fixing anything, because nothing is broken.

I see a lot of people throwing out the "lowering a mac-strut car = bad" rhetoric, without a lot of evidence beyond a crude block-and-line diagram depicting some super-extreme example of a mac-strut car being lowered. RC below the ground? 1.5x longer roll couple? Oh noes! Again, all theory, no solid measurements for the car in question. And again, theorizing isn't bad, but I'm going to need more than that to get scared into buying "fixes" for by "broken" suspension geometry, which is exactly what I think is happening now to others, when it's very possible that no fix is necessary.

case in point: high spring rates countering the effects of an increased roll couple. Say Joe Blow goes out and buys a set of coilovers, with rates of 10k all around. With 10kg/mm at the front wheels, that is a solid amount of spring for the front end of an Evo. Obviously Joe lowers the car on the coilovers, and he reads a few threads like this and goes "Uh-oh, my roll center is out of whack. My car will understeer now!" What Joe knows from reading the boards is that an RC below ground will result in understeer. So one of a few things will probably happen:

1. Joe panics and buys a roll-center adjustment kit
2. Joe raises the car up to stockish height under the assumption that Mitsu engineers designed the car with a desirable above-ground RC, thereby avoiding a perceived poor-handling condition
3. Joe drives the car as it is, understeer be damned.

What Joe doesn't know is that a lower RC will also result in
a. less geometric weight transter, that is load applied directly to the RC, and
b. more elastic weight transfer, that is load applied to the sprung mass of the car at the CG

Elastic weight transfer is controlled by the springs, shocks, and ARB's, which Joe Blow has recently upgraded. And, nose-heavy cars don't like a lot of geometric weight transfer for obvious reasons (if it's not obvious: "Hey, I can't control all this weight with my springs and shocks and ARB's! Now, the only thing I can do is fiddle with my linkages and pick up points, that costs $$$.") By lowering the RC, that nasty geometric weight transfer has been reduced, and now the bigger player in weight transfer becomes elastic, and Joe can control elastic weight transfer with his new, stiffer springs and ARB.

Joe also does not know:
c. A roll center closer to the ground reduces jacking forces.
d. A roll center closer to the ground reduces lateral forces acting at the tire.

Joe only knows the one disadvantage of lowering his mac-strut car as told to him by the internet pundits: lowering the car will cause his RC to drop below ground, which results in understeer which is undesirable. While this isn't untrue, Joe most likely does not know of the benefits of lowering his car, or of the other factors that can offset this effect of lowering. These factors can often turn the tables in favor of lowering the car, as the benefits then outweigh the downsides.
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Old 09-20-2007, 07:38 AM   #9 (permalink)
 
Registered: Sep 2005
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Wow.

Okay, having read through all that, it just goes to show that it does take a good suspension engineer to make the most out of coilovers. Question is, in the absence of such a person who is able to set the load against RC correctly, where can I start learning about suspension tuning? Clearly, this is something that will require more reading and having a coach to teach me to apply these principles would be great

Cheers

Kenneth
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Old 09-20-2007, 09:57 AM   #10 (permalink)
 
Registered: Jul 2007
Posts: 17
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It takes a knowledgeable tuner AND a consistent driver to get the most of any car. Not many drivers or even teams have the resources to bring on a full-time suspension guy. Given all the possible factors that can affect how the car + driver system ultimately handle at the track, you will *need* to have someone who can work closely with the driver on a regular basis, who knows the car and the driver inside and out.

But in the absence of someone like that, IMO the best thing you can do is simply drive the car and get more seat time so that you can more accurately recognize problems that arise. It's not really necessary for anyone to know some of these principles unless they plan on doing their own work on the suspension, and I don't mean cosmetic height adjustment and/or DIY alignments.
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Old 09-20-2007, 10:03 AM   #11 (permalink)
 
Registered: Sep 2005
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I agree. To add to what you just said, a friend of mine once said, no matter how you set up the car, the true measure is in the timing and speed. For track, which my car is set up for, it is all about best lap timing and highest average speed achieved. Without a good setup, neither of this will be good.
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Old 09-20-2007, 04:55 PM   #12 (permalink)
 
Registered: Jul 2007
Posts: 17
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It's easy to get caught up in all the work that takes place off the track, such as choosing mods, installations and making adjustments, and totally forget about what you're doing it for in the first place. For some it's improving lap times, and for most other its about just plain having fun at the track.

I think that for the vast majority of people purchase aftemarket coilovers for their Evo are not capable of driving the car within 3/4 of it's *stock* limits, let alone in a modified state. For those people, I'd say worry about squeezing that last 10% out of the car AFTER you learn to drive past the first 90%. Nobody listens, but I'd say it anyways.

Then again there's a lot to be said for a corner-balanced and properly aligned car, both of which are relatively simple and basic procedures and can be done by a knowledgeable shop. Which brings us back to where you started - the car is already corner-balanced, why mess with that?
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Old 09-20-2007, 09:38 PM   #13 (permalink)
 
Registered: Sep 2005
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To be honest, the reason why I wanted to tinker around wit it was due to the fact that the front end grip on the street was downright terrible. The front end would feel like it was floating, and while I got pretty decent grip/balance going around high speed corners, the car had practically no front grip going around roundabouts, and I suspect it had something to do with the rear settling in a couple of mm (I had it corner weighted just after I installed the coilovers)

In the end, I tried raising the rear by 5mm, but while corner entry understeer was lessened, it was still pretty bad. Raising the rear by 10 mm made the car understeer badly, and in the end, I had the rear raised by 5mm, and the front dropped by 10mm to 13.75" from the hub centre to the arch. This setting has pretty much improved overall grip and I feel more confident going through corners now, where in the past I had to be cautious since I had much less feel for the front end.

The local shop here charged around USD600 just to get my car on the flat patch and told me that cross-corner weights were 49.3%. I am seriously considering investing in a set of corner weight scales and start learning to do this myself...
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Old 09-21-2007, 06:58 AM   #14 (permalink)
 
Registered: Aug 2007
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I agree with noob4life that alot of people undervalue the stock suspension setup, which already has alot of R&D effort put into it. Only when you feel you have mastered the behaviour of the stock setup can you intelligently go about the process of evaluating further mods.

Case in point - my IX RS comes stock running the supposedly not-so-good KYB dampers but thus far, aside from getting alot of body roll, I have found that the car actually handles very well straight out of the box with the alignment set to 0 0 0 0 all around! This leads me to question my recent purchase of a set of KWV3s and a bunch of whiteline goodies because my stock car is very capable. Needless to say, I will be starting off with a very conservative base tune for the new setup and tune it gradually from there over time.

In the absence of a proper suspension engineer, the best thing to do is troll the internet for advice but test everything yourself. Trial and error is the name of the game - pays to record your settings and make notes on each change in setup - there are alot of variables to consider such as tire presures, alignment, ride height, plus spring rates, ARB settings etc. Recording changes to these is the only way to make sense of it, otherwise it's easy to simply change things for the sake of it without any quantifiable benefit, possibly ruining the car's handling in the process. Also, without consistent driving any data collected is quite questionable at best.

I suggest:

1. record all settings
2. drive the car extensively in different situations
3. alter one variable and drive drive drive
4. record the result
5. return to previous settings
6. go back to step 1


One thing I like to do is change one thing to an extreme amount (i.e. raise the rear tire pressure waaay up) and record the result, then dial it back in steps until arriving at the original state. This way you can gather info on how different things affect your car. It takes time but it's the best way IMHO. Tweaking combonations of different things at one time will only give you major headaches trying to unravel what setting is doing what.

There's no "magic killer setup" out there, since we all drive in different ways and in different conditions. Gotta find your own mojo.

PS ghoonk it pays to have some decent rubber to begin with
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Old 09-21-2007, 07:38 AM   #15 (permalink)
 
Registered: Aug 2007
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One thing I didn't emphasise enough is driving style...I only got my EVO recently (raced a Golf r32 before the EVO) so I have been driving the nuts off it trying to evaulate its limits.

Since I autocross I decided to find a big empty parking lot once I finished the break in preriod and have at it. What I found was that the biggest variable was ME - I set up a basic course and repeated it over and over, and found that the way the car handled depended hugely on my driving - corner entry speed, braking, lines, shifting, when the power was applied etc.

I was nowhere near 100% consistent by the end of the evening but I learned alot and got quite comfortable with the car - drive it properly and it will be quicker no matter what the mods are. Determine the line you want, feel out your desired braking and shifting points, then drive drive drive!

Any hiccups in driving would see my lines, corner speeds and exits (and overall times) varying hugely - too quick in and huge understeer, wide exit, panic shifting. Good heavy earlier braking, then heel and toe shift, settle car and power out worked much much better, entry was slower but the exit line was tighter by a good few FEET and I would shoot out of the corners under power as intended - big difference! By getting more consistent I was able to get alot more out of the car than any simple mods would have done.

Also, no matter what your setup is, driving style can still vary your lap times by a good few seconds - which is as big a difference as any single mod aside form slicks will likely ever get you.

Point is, you have to drive the same route many many many times and nail the driving before dissecting car settings.

My 2c
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