Help dialing out understeer

[Kgolf31]

Hey all,

I'm trying to source down my understeer issues on my STX 128i. Just so everyone is on the same page, here is my set-up.

TCKline DA Suspension - #400 F/#700 R
Apex 17x8.5 ET40 w/Dunlop Z2s 245/40
Eibach 28mm Front Bar

Alignment
Front:
-3.6* Camber
7.5* Caster
0 Toe

Rear:
-2* Camber
1/8th Total Toe

I'm still on the stock e-diff, and have been trying to tune the suspension. I've been encountering some understeer at corner exit...specifically when I want to get on the power the car wants to push.

Turn in the car feels really planted, but at exit when I want to power on, and try to get the car to rotate it doesn't want to do that. It pushes.


It may seem like I might have too much front bar, because I don't know what other type of adjustments I can do on the front axle to help get grip.

Thoughts?

[fe1rx]

Since you are happy with the corner up to the point of applying power, and presumably happy with the feel you get from the stiffer front bar I suggest leaving the front axle alone.

I know you know this, but just to walk through my thought process:

1) Adding power transfers weight to the back.

2) If you are already at the limit of grip at the front, that transfer reduces the weight on the front and thus reduces front grip, introducing an understeer tendency.

3) If your rears were actually also at the limit adding torque reduces the grip available for cornering introducing an oversteer tendency.

4) Counteracting that, the weight transfer aft gives the rears more total grip, which is why adding a bit of power to a RWD car often makes the car feel more planted in a corner.

The obvious thing to try at the rear seems to me to be toe out. As you add power in a corner and transfer weight aft, proportionally more grip is available from the outside wheel. If that wheel is toed out it introduces an oversteer tendency, which is what you want.

I find it easier to work with toe in decimal degrees. As I understand it the OE rear toe total specification is 0.30°, which works out to 1/8" (your setting). I suggest trying a small amount of toe out at the rear. In fact I thought this was pretty common for an autocross setup. There is great trepidation among much of the population regarding trying toe out at the rear of a car, but in my experience the results are not as dramatic as feared. Also, there is an argument to be made that whenever you increase negative camber (front or rear), you should decrease toe in (by 0.1° of toe per side for each 1° of negative camber. I can provide the rationale if you are interested.) Basically both negative rear camber and rear toe in are stabilizing. By increasing your rear negative camber you have got a more stable car than OE. You really want more control, not more stability.

If you don't have the means to adjust rear toe yourself, this is a bit of a pain as it will mean at trip to the alignment shop. My motivation for building hub stands was so that I could do such experimenting myself.

If I were to suggest a number it would be 1/16" total rear toe out (about 0.15°) as a starting point.

[Kgolf31]

Thanks for stepping through this. I need to logically think this stuff out.

So, I appreciate the comments and have a question. Toe out would indeed help my issue but wouldn't that compromise basically every other aspect of a course (chicago boxes, bus stops, lane changes, and most importantly slaloms)?

So, with that question being said, here is my thought process.

1) Add more rebound in the front? That would reduce weight transfer to the rear and thus aid more grip in the front

2) Compression up in rear? Sure...adding more compression "could" help, but there is a limiting factor to this. Upping more compression would reduce power down basically everywhere else and make the rear a bit more bouncy.

3) Compression up front? Ehh, doesn't really effect front grip besides in braking.

4) Reduce rebound in rear? Don't really want to touch that since it would effect turn-in and high speed transitions

5) Change out front sway bar? You basically explained the negative effects. Sure...I could get more front grip but at the cost of turn in and camber.

6) Reduce front pressure. Did this and basically bottomed out at 32 PSI HOT. The tire starting rolling over too much and had to stop here.

7) Increase rear pressure. Sure...it will help a bit but won't solve the issue.

So, after those thoughts I've came to these conclusions (so I think)

1) The car still needs a rear differential. Is my open e-diff (whatever you want to call it) causing more push because of the power going to the outside wheel and/or the inside braking causing too much lockup?

2) Does the rear need more spring? I don't have any info on motion ratio, so I don't know if I would benefit on more rear spring in the back. I have M3 subframe bushings so I can handle the high spring rate.

3) Reduce rear weight. Looking at trying to shed the muffler ~30 lbs and possibly a LTW battery (say ~30 lbs there). Light rear end will help...but I don't think 60 lbs is going to make a big difference. Thing to note, the car is NOT corner weighted yet.

4) Rear bar? Ehh...don't want to touch this because I have to drop the rear subframe. I'll probably end up doing this when I go with a diff and/or another alignment next year.

I'm trying to budget for a Diff by the end of a year from Jim @ Performance Gearing. Ultimately I think this may/may not be the answer but I need a diff regardless.

[Kgolf31]

For reference, here is my run from yesterday.

Things to note, it appears trailbraking really helped front end grip. Maybe try and do that more often?

And this is the guy I need to catch. He was 0.572 (IIRC) ahead of me. Appears most time was gained in the opening first section before the crossover. His car is fully prepared for STX.

[pixelblue]

you already know the answer. it will take a little bit of all that you outlined to fine tune that ride. it's a science and a law of diminishing returns. of course a nice diff will go a long way.

[John_01]

I guess your car may be lifting the inside front wheel too much coming out of corners. I would try less front rebound damping, a softer front sway bar, or both. Just my ideas for the sake of discussion!

[Ginger_Extract]

What about a healthy lowering pf the rear tire pressure? I was noting wear of the RS3, and my rears at 32psi cold still were not using all of the tire. Just for laughs, dropped to rears to 28psi. You'd think this would effectively increase rear grip, by using more of the available rubber, but by doing so, the car got a lot more loose in subsequent runs.
Things to note: This is with 17x8.5 on 255/40, IIRC, you're on 17x8.5 245/40, YMMV. Also, our local venue out here feels similar to OVR's Buckeye lot venue, which is where many of your events are likely at this year? Don't really follow the goings-on with the OVR gang, other than little musings here and there from friends.

[fe1rx]

Originally Posted by Kgolf31

So, I appreciate the comments and have a question. Toe out would indeed help my issue but wouldn't that compromise basically every other aspect of a course (chicago boxes, bus stops, lane changes, and most importantly slaloms)?

I think your car is overly stable. Are you concerned that the car will become too tail happy? If not toe out, I believe zero toe would be beneficial regardless of any other changes. BMW tunes their cars for the lowest common denominator driver. You will certainly be able to handle a bit less stability. That said, I don't think this is the whole answer. I offered it up because you seem disinclined to consider going back to the OE front bar.

Your shock tuning suggestions are your domain, and they are fine tuning. I think you need a bit more than that. Same with tire pressures.

Originally Posted by Kgolf31

5) Change out front sway bar? You basically explained the negative effects. Sure...I could get more front grip but at the cost of turn in and camber.

Your stiff front bar gives you a crisp turn in because it decreases the time it takes your car to take a set. As long as you don't reduce your total roll stiffness, you won't lose this. In fact your total front grip will improve with a front bar that is not as stiff (because of tire load sensitivity). If you move some of your roll stiffness aft you should see a gain in total front grip and no reduction in turn in response. See rear springs below.

Originally Posted by Kgolf31

1) The car still needs a rear differential. Is my open e-diff (whatever you want to call it) causing more push because of the power going to the outside wheel and/or the inside braking causing too much lockup?

An LSD will cause push because it wants both rear wheels to turn at the same speed, which they only do when driving straight. E-diff is not functionally different, but it absorbs more power, which is to say, thrust. This is not your problem.

Originally Posted by Kgolf31

2) Does the rear need more spring? I don't have any info on motion ratio, so I don't know if I would benefit on more rear spring in the back. I have M3 subframe bushings so I can handle the high spring rate.

Yes. If you dig through my Ohlins post you will find the motion ratio measured. Assuming you want a front suspension ride frequency of about 2 Hz and the rear just a bit less, your front springs are good but you could add 100 to 200 lb/in to the rear. This would substantially increase the rear roll stiffness as well, given the flimsy rear bar (see my Ohlins post for a measurement of the front and rear bar stiffnesses, and their relative effect on total roll stiffness).

Coincident with this, I would still suggest trying going back to the OE front bar. Because of the stiffening at the rear you will still have the total roll stiffness you like, but it will be rebalanced.

At your spring rates and camber, the effect of bars should be "normal", which is to say more front bar means more understeer. The "abnormal" rule applies to soft suspensions.

I am running about 350/700 and OE bars and that works well but I think just a bit more rear stiffness might be even better. Generally though a 2:1 front/rear spring stiffness keeps front/rear ride frequencies in correct proportion from a true performance point of view.

Originally Posted by Kgolf31

3) Reduce rear weight. Looking at trying to shed the muffler ~30 lbs and possibly a LTW battery (say ~30 lbs there). Light rear end will help...but I don't think 60 lbs is going to make a big difference. Thing to note, the car is NOT corner weighted yet.

The weight reduction is a diversion. Corner weighting is not.

Originally Posted by Kgolf31

4) Rear bar? Ehh...don't want to touch this because I have to drop the rear subframe. I'll probably end up doing this when I go with a diff and/or another alignment next year.

No, for reasons mentioned above.

Originally Posted by Kgolf31

I'm trying to budget for a Diff by the end of a year from Jim @ Performance Gearing. Ultimately I think this may/may not be the answer but I need a diff regardless.

Not to say a true LSD won't be a good thing, but again this is a diversion from the real issue, which is that your front/rear roll stiffness balance is biased too much to the front.

[Kgolf31]

So, in summary you think going 800 lb/in springs in the rear would help?

I might pick up a swaybar for the rear. There is a UUC 19mm for sale on the forums that is 2 way adjustable and I can probably set it to pretty soft and close to OE specs.

I really like the front turn in, but perhaps the rear bar would help in addition to the springs?

I'm not concerned with the car being tail happy. I drove a Z4M in RTR before and it was very power oversteer friendly and I absolutely loved it. I just like how stable the car is through high speed transitions and I don't want to compromise that

[Kgolf31]

I have an event Sunday and I think I'm going to try and raise the rear by 2 turns to see if this will help at all.

Raising the rear will reduce camber a bit while reducing toe...so we will see how this works.

[John_01]

Originally Posted by Kgolf31

I might pick up a swaybar for the rear. There is a UUC 19mm for sale on the forums that is 2 way adjustable and I can probably set it to pretty soft and close to OE specs.

Congrats on your 5th place result posted in the other thread!

Compared to 12mm stock, I think the 19mm rear bar will completely change the handling balance. Theoretically its something like 628% firmer, based on diameter to the 4th power. The stock bar is weak so hopefully this is what you are looking for in Autox competition.

[02rsxpilot]

Kgolf, I thought you'd find this read interesting: http://www.bimmer-mag.com/issues/101...1#.U44GTfldWSo. It's not necessarily specific to your understeer on exit issue, but might have some broader insights that help dial it out for you.

It's several years old, so maybe everyone's already seen it. Main thing that popped for me was: "If there’s any message I’d like to get out there, it’s that the best suspension is not the one with the most spring rate, but the one with the best shock damping that’s designed to work with the chassis and has as little spring rate as you can put in that gives you the best handling."

[Kgolf31]

Originally Posted by John_01

Congrats on your 5th place result posted in the other thread!

Compared to 12mm stock, I think the 19mm rear bar will completely change the handling balance. Theoretically its something like 628% firmer, based on diameter to the 4th power. The stock bar is weak so hopefully this is what you are looking for in Autox competition.

Thanks!

I don't think it is that drastic of an improvement because you're dealing with motion ratios.

Anyways, I got the bar yesterday and going to start installing it on Wednesday. Hopefully it isn't a PITA.

Most people were telling me that I shouldn't need a rear bar, but this is coming from people who are driving E36s and their rear bar is 19mm stock. So perhaps we just don't have enough rear bar.

Originally Posted by 02rsxpilot

Kgolf, I thought you'd find this read interesting: http://www.bimmer-mag.com/issues/101...1#.U44GTfldWSo. It's not necessarily specific to your understeer on exit issue, but might have some broader insights that help dial it out for you.

It's several years old, so maybe everyone's already seen it. Main thing that popped for me was: "If there’s any message I’d like to get out there, it’s that the best suspension is not the one with the most spring rate, but the one with the best shock damping that’s designed to work with the chassis and has as little spring rate as you can put in that gives you the best handling."

Thanks for the article, it is really good information.

My spring rates are still on the "soft" side. And I am running TCK DA Shocks, which are just epic when it comes to damping. I think I'm good here.

[Kgolf31]

So despite my shitty driving, it appears that the rear bar helped cure some understeer.

I had some good steady state corners on this weekends course to really test the setup, and all seemed to work really well. Very impressed with how this bar is working.

Next weekend I'll be testing on concrete, so I'll see how that works. But this is pretty positive and I'm liking how the car reacts now:

[Ginger_Extract]

How do you have the dampers set up? Since you have DA's, with the ability to control compression and rebound, I figure that a lot of the E82 chassis' irksome handling characteristics could he dialed out.

[Kgolf31]

Originally Posted by Ginger_Extract

How do you have the dampers set up? Since you have DA's, with the ability to control compression and rebound, I figure that a lot of the E82 chassis' irksome handling characteristics could he dialed out.

Yes, I have DAs

Setting up depends on the conditions.

For example, during last event we had full rain runs, where I ran everything on soft besides 2 clicks in the rear on bump.

During that run above, here were my settings:

Front:
Rebound: 1.75 turns from full soft
Bump: 6 Clicks from full soft

Rear:
Rebound: 2.25 turns from full soft
Bump: 10 Clicks from full soft

I talked to a couple people and trying to maintain stability in high speed transitions (aka slaloms for example) I need to increase front bump and/or decrease rear rebound OR decrease rear bump and/or increase front rebound.

All depends on the surface conditions and what the car is doing real time.