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Old 07-20-2010, 04:53 PM   #1
davidkillion
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Default Tuning help

The car is a 1998 318ti with Bilstein PSS9 coilovers. The spring perches have been removed from the back and the coilover height in the front has been set for only a mild rake to the front (it is pretty darned low). I have changed out the control arms, ball joints and tie rod ends to tighten up the front end. I also run EBC Green Stuff brake pads on EBC sport rotors. I run the stock sway bars and no X brace. I autocross it at full stiff on the front and back shocks.

When I originally autocrossed it with 480 treadwear tires it was quite fun and most of my understeer issues could be resolved with some agressive throttle to bring the rear end around.

I decided to change tires and everything has been downhill since. I went from a 205/60-15 Bridgestone (480 treadwear) to a 225/50-15 Yokohama ES100 (240 treadwear).

So here is the problem. The overall grip is better, but the dreaded understeer still exists. I'm talking PLOWING! The worst part is that with the added traction in the rear, the little 1.9L engine cannot break the rear tires loose. At this weekend's GGC BMWCCA autocross I struggled in all 7 runs to plant the front end under braking and get the car steered and it feels like I had to come to a stop to get it to rotate. The bottom line is that I think I was faster with hard tires because I could "drive" the car ... not let it drive me.

Below is a picture of the car on Laguna Seca with the 480 treadwear tires coming out of turn 5 ... it shows the lean of the car at current state.

What should I do? The options for stiffening up the front are the X brace, stiffer sway bar and shock tower brace, right? Of these three - which will be the most effective at solving this understeer issue? Are there other tuning tips for dialing out the understeer in this car?
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Old 07-20-2010, 06:43 PM   #2
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Soften the front struts and or springs, increase rear spring rate/sway bar size and stiffen shocks. Basic tuning for getting over steer(or reducing understeer).

You might just be too low in the front and have comprimised the geometry. There are a few posts around by Gimp for the max front drop.

Completely different car (same problem however) I ran my protege full stiff rear and 2-3 on the front struts with a huge rear bar and stock front to get the car to rotate well.
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Old 07-22-2010, 05:40 PM   #3
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You may want to add a bunch of front caster as well.

If those coil overs mount to the stock front locations, dial in a bunch of caster (move the tops of the struts rearward) if you can.

The stock M3 front caster tops work well with stock (style) struts but I am not first-hand familiar with your suspension set up...
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Old 07-22-2010, 09:22 PM   #4
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i bet you i could make that thing lose traction
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Old 07-23-2010, 01:33 AM   #5
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If the PSS9's have m3 sway bar mounts you could try an M3 front sway bar.
I have heard that a rear swaybar delete is an upgrade for racing with z3's. I thnk the ti is more top heavy than a z3, but you could try that out.
Also, have you considered putting the bridgestones back on the rear only?
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Old 07-23-2010, 04:02 AM   #6
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Soften the front sway bars, tighten the rear sway bars. Keep higher treadwear tires in the rear to help compensate. Aim is to reduce the grip at the rear to help balance it out.

Someone also mentioned about the caster which may help out. You'll have to experiment with it as well as possibly camber adjustments. Tyre pressures can also play a roll in handling.
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Old 07-23-2010, 01:30 PM   #7
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Quote:
Originally Posted by RAiMA View Post
You'll have to experiment with it as well as possibly camber adjustments. Tyre pressures can also play a roll in handling.
I was going to mention Camber; however based on the picture posted I'd say Camber is just about perfect as it is... Record it so that when/if you change Caster it remains where it is.
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Old 07-26-2010, 03:11 PM   #8
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Enter the turn slower.

I'm not being rude by any means, but I want to give the right advice. How much track time/autocross time do you have?
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Old 07-28-2010, 07:22 AM   #9
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Quote:
Originally Posted by gimp View Post
Enter the turn slower.

I'm not being rude by any means, but I want to give the right advice. How much track time/autocross time do you have?
Gimp brings up a valid point. You can gain lots of time by getting on the brakes a little sooner, staying tight on your apex cone and getting on the throttle earlier. Also, make sure you finish braking before you begin turning. That is also a surefire way to induce plowing.

Assuming your driving technique is okay, here's my suggestions:

First, take the front struts off of full stiff. This will help remove understeer at turn in. Too stiff of front shocks will cause the weight transfer to happen very quickly, causing the outside tire to lose grip and slide. I would go closer to full soft than full hard, but somewhere close to the middle is probably okay for now. I would also take the rears down closer to the middle as well.

Check your front sway bar for bind. After putting an aftermarket sway bar on my car, it began pushing terribly! Turns out that most of the aftermarket bushings that are used to mount the sway bar to the body are too large and will prohibit the sway bar from rotating as it should. The best way to check this is to remove the endlinks from the sway bar (or disconnect the end links from the control arms) on both sides and see if you can easily rotate the sway bar. Think 1 finger's worth of effort. If it is harder than this, you probably need to lubricate the bushings and possibly shim the brackets that hold the bushings + sway bar to the chassis.

Get more negative front camber. Unless you are above -3 degrees, more will help. With the grippier tires, the car's body will roll more, necessitating more camber. Shims/washers are the cheap way to do it, but you might run into some fitment issues depending on how much you dial in and the offset of the rims. With your wheel/tire sizes, you should be fine. Camber plates are the better and more expensive option. -3 degrees may seem like too much, but with the suspension on the front of these cars (macpherson strut) you don't gain as much camber in compression as you lose due to the body roll. As a point of reference, the Vorshlag guys were running around -4 degrees on their e30 ST car.

Check to make sure the car isn't riding on the bump stops or going to coil bind during cornering. If you are hitting the bump stops in the front, your effective spring rate is skyrocketing, overloading the front outside tire and causing it to lose grip and slide. There are a couple ways to check this. You can attach a zip tie to the shaft of the front shock and see if it move upward into the bump stop. Since bilstein probably did an okay job designing the kit, you should have to worry about the springs going to coil bind, but just to make sure, you can put a small piece of sticky-tack on one of the middle coils. If the piece gets compressed to where the layer on top of the coil is essential see through, you know the two coils must have touched at some point. Synopsis: If your front is super low, you may very well be riding on the bump stops during cornering. Either raise the front of the car some, cut the stops down some, or some combination of both. Just don't remove the bump stops all together. You need the protection they provide.

Tire pressures can also have a significant effect on car handling. I run slighly (about 3psi) more pressure in the front tires due to the higher weight over the front axle. Too much front pressure can cause understeer. Too low on the front pressure and you'll see the tire being scrubbed over on the sidewall. You want to find the lowest pressure that keeps the tire from scrubbing down too far. Optimizing camber will help in that regard. I run my 205/45/16 b-stone re-01's at 35-36 front 32-33 rear. Too little in the front and it starts to feel mushy. Too much in the front and it loses grip. Different tires like different pressures. Do some experimenting to find out. Tire pyrometers can be a big help here, but you should be able to get close without one. You can increase the rear pressure some to help the car rotate better, but the downside to this is that you lose some forward bite. If you don't have any wheel spin issues on corner exit, this might be something to try if the above suggestions don't work. Assuming you don't have a limited slip differential, this is probably not a compromise you want to make.

I suspect addressing those issues will help significantly. If it is still more pushy than you would like a stiffer front sway bar may help. Typically more front roll stiffness (what a larger front sway bar gives you) will increase the understeer effect, however, it will reduce total body roll, which reduces the amount of camber the front tires lose, which means more front grip. Clearly there's a trade off here. At some point further increasing the front sway bar size will induce more oversteer. I'm running a 28mm eibach bar at the softer setting. Body roll is much better, but it still exists.

What to do with the rear sway bar is a bit of a loaded question. I think the right answer depends on a multitude of other parameters, such as driving style, tire grip, springs and stocks, front bar, and differential. I run the eibach rear bar set on full soft. It's great on grippier surfaces, but on slicker asphalt lots, it doesn't give as much rotation as I'd like. I probably need to go up a set of holes on that surface. The problem with a stiffer bar and to some extent any rear sway bar in general is that it limits the abilty of the inner rear spring to plant the inside rear tire. When you have lots of grip (ie race tire) the tire is hardly touching the ground at all, causing excessive wheel spin when you get back on the throttle at the exit of the corner. Removing or softening the rear sway bar helps to better plant that tire (or at least keeps it from being pulled up as much). If you're not having wheel spin on corner exit problems, a stiffer rear sway bar might be a good way to help with rotation. Without a limited slip, increasing it may cause more trouble than it's worth.

And please, whatever you do, don't put the old tires on the back to try to improve balance. That is throwing away grip! Figure out a way to make those puppies do more work!

My 0.02, YMMV, use at your own risk, etc...

Apologies for being so long winded. I hope this helps!
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Old 07-28-2010, 12:48 PM   #10
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Great post, Josh.

It brings a good point to mind: We sometimes focus too much on the hardware, and not as much on the software.

The tires are a critical link in the package, as its the only thing touching the road!

Proper tire selection, break-in and pressure are all excellent points. I will add to that with a log book to keep tract of what combinations did 'good' and which did 'bad' at each track will help you a ton as well. Using more of the 'software' in the drivers head so to speak.

Good springs, shocks, sways and mechanical set up are all excellent ways to lower lap times; however put a good driver in a bad car and he will out drive a bad driver in a good car.
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Old 08-11-2010, 05:38 AM   #11
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The two posts above nailed it with all the gritty details. To grossly over simplify it, in a racing situation, you're usually better off increasing grip to solve a handling problem than you are decreasing grip.

So, many of the suggestions above deal with increasing grip on the front end to make the rear feel looser in comparison. I'd start with the suggestions for adding traction in the front before you jump in and take any away in the rear, as that just might be enough.

When I was auto-xing, I seem to remember many guys thought running a 24mm rear bar (GC8 impreza) was a badge of honor or something. The ended up making the rear so twitchy that they lost time even though it cured the understeer problem.

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Old 08-19-2010, 04:48 AM   #12
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This is some great advice. I fell I have a much better understanding of what is going on now. There are some simple changes I can make with air pressures and shock stiffness that would likely result in removing some of that understeer. I feel now that camber plates up front as well as a rear sway bar will likely be my next purchases. I'm anxious for the next autox to try some of this out. You guys rock!
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