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04-10-2016, 09:21 PM | #1 |
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Understeer at autocross
I did my second autocross of the season today, and I've got 6 more to go. I spent my entire day battling absolute gobs of understeer and I'm hoping I can get some pointers on what I might be doing wrong.
If you asked me where I was running into it, I'd say that it was from turn in to apex. Holding steady speed didn't seem to help and I tried trail braking in, slow in fast out (best result) and diving in hard after braking (no good, just plows hard). I'm not quite capable of maneuvering the car consistently with the throttle to counteract the understeer. Our local autocross pad has a lot of small transitions between pavement sections and the ground sort of rises and falls like small waves, attempting to throttle steer in a big way had me running the course backwards on one go at it. My best runs were hard braking, gentle turn in, and wrestle on the edge of understeer through the turn with a hard throttle out and some oversteer through exit. My times were not great (good, not great. Competitive for my class) and controlling the back end was difficult. Local shop aligns in degrees/minutes, BMW specialist and they strongly advised not going too far negative. Not sure I agree my e30 is around -3°/-3° and behaves well on track but I've been trying it as they suggested. Front specs -1°25' camber / 6° caster / 0°05' toe (per wheel, 0°10' total) Rear specs -0°53' camber / 0°15' toe (per wheel with 0°30' total) / 0°00 thrust angle Tires Michelin Pilot Super Sports 245/275 front and rear, have tried various pressures between 30/32 f/r to 35/36 f/r. Higher pressure seemed to have the best result in feel from the seat but wasn't totally utilizing the tire width (barely touching the michelin man. Low pressures had massive sidewall flex that had the fronts scrubbing the tire "shine" off half way down the Michelin letters. Suspension Bone stock, zero mods except for RTAB limiters when I re-did the rear end bushings. Car has 100,000km, I suspect my shocks may be a strong factor in this at this mileage. I'm open to suggestions on things I can change in driving style, or replacement parts/alignment settings. I am not afraid to run large camber if need be but would like to keep my suspension street-able (max would be H&R street performance at max ride height)
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04-11-2016, 01:43 AM | #2 | |
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Many of us have found going with stock rear toe range towards the max toe-in and using RTAB limiters helps put down the power exiting the corners. -1.7 to -2.0 rear (YYMD) also works nicely. -1.7 is more street friendly with the toe settings for wear as the rear isn't easily swapped between settings like the front. Now, with AutoX (I run fun only, not comp) what you can do and stay in your class is another question. You may be limited there or going with changes may bump a class--in which case stock might be it. You know the rules better than I! Search on alignment with Ship, Ian, The Hack, or me--lots out there if you want more detail. Here's a few. http://www.zpost.com/forums/showthread.php?t=706698 http://www.zpost.com/forums/showthread.php?t=422123 http://www.zpost.com/forums/showthread.php?t=536407 |
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04-12-2016, 12:57 PM | #3 | ||
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That's a lot of rear toe-in in comparison to the front. The more the rear toes in, the less "loose" the rear is going to want to be.
You mentioned the E30...Did that have squared tires? The next logical step, is if the "understeer" is so much and so undesirable that it's making it impossible for you to get decent time in the car, then giving up rear grip may not necessarily be a BAD thing. Go 255/whatever on the rear and see if you can get better results. LASTLY. Since you're on MOSTLY stock suspension, keep in mind that it takes smoother inputs, both hand and feet, to get the best results. You mentioned: Quote:
Quote:
Now, not having the opportunity to sit in the passenger seat, and not having video to review, whatever I have to say here is all hearsay (and wild @ss guess). But I suspect either your brake release prior to turn-in was too abrupt, causing the front end to come up too quickly and losing grip to allow you to transition into the turn smoothly, or the timing between brake release and turn-in is off leaving you in a perpetual state of understeer. My standard disclaimer applies here though. I'm not an autocrosser. Heck my CR buddy who's NEVER AXed beat me by nearly 1 full second on a 27.5s AX course in the same identical car. So take my advice with a giant grain of salt. But I suspect, fix the braking and 90% of your understeer problems may go away. If you're still willing to take my unqualified advice, I have 2 things for you to try. 1: Brake earlier. I know it's counter intuitive, but it's the BIGGEST issue that most drivers driving heavier cars than what they're used to have a trouble grasping. And on this chassis it's imperative you don't try to be "Mr. Brembo" or play "hero" with the brakes. 2: Brake LESS. Again, counter intuitive. And this might be specific to this chassis, since it's short-ish compared to your average car, and the fact that you sit over the rear axle making the pendulum of momentum swing faster and harder, but the two combined makes it easy to overbrake and overwhelm the front end. Of course, I'm going to tell you that if neither one of the above works for you, try braking later and brake harder.
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04-14-2016, 12:02 PM | #4 | |
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Thanks to you both for the input so far!
To get my excuses side of things out of the way I have gone and ordered a Bilstein B12 Pro-Kit after much debate between them and entry level coilovers like the H&R. With the mileage on my car I felt it was worthwhile, and at $800 shipped for the whole set I couldn't complain! I figure if I'm going to play with my alignment, I may as well do it after replacing my 60 thousand mile shocks. With that in mind, what should I expect my current alignment to do when I plunk them in? I understand that I'll get more negative camber as a result of the drop, will it also have the effect of messing up my front and rear toe? The kit estimates a 20mm front and 10mm rear drop. How long can I comfortably drive on it before the alignment? I get mixed reports on the suspension settling after a few hundred miles on most new spring installs which would of course change the alignment specs. When I do the alignment, I think I'll be going this route. Will I need camber washers with the new suspension height to achieve this result? Front: -2.00° camber, 0.00° toe, stock caster (tire wear aside, should I go further negative camber? What about the trick of marking and adjusting for track days? I am not going to be using camber plates) Rear: -1.50° to -1.8° camber, 0.10° toe in Quote:
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04-14-2016, 12:35 PM | #5 |
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It's a classic traction circle challenge. The tires (typical street tires) can hold on to about 1.0-1.2 G's of total combined force. If you use all of that up with braking, there's nothing left for turning and vice versa, and the Z4 is particularly sensitive to this. If you come off hard on the accelerator and jam on the brakes you get more weight transition than desired and you either overload the front tires and get understeer, unload the rear and get oversteer, or as it sounds like happened to you there was initial understeer until the front tires got some bite and then when they did the rear was unloaded so it looped on you. As difficult as this sounds to do, you need to allow just a bit of a pause to allow the suspension to recover from the acceleration and then roll onto the brakes. You can brake really, really hard but just don't stomp on the pedal initially. Learning to left foot brake will definitely help once you get the hang of it since you're not having to move your right foot back and forth frenetically between the gas and the brake.
If you download the free Track Addict app to your phone, it tracks G force data and if you use the Race Render app it will overlay a G circle on your video so you can see when and how much force you were generating. Just some food for thought!
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04-18-2016, 08:24 PM | #6 |
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When I autocrossed my Z4, the car needed to be driven like Corvette.
On my 128i I run 14 minutes to total toe and it gives me just enough to get traction on power down but don't create off-throttle push, which might be some of the issue you're having right now. What I found was trailbraking the car to set the front was almost always needed, get thru the apex and roll onto the throttle once you can get the front set. Trying to carry too much speed like my 128 would of resulted in massive push and a front end that would have wiped out. Also, the tires you're running aren't the greatest, but a lower grip tire is easier to learn on since it doesn't hide your mistakes with more grip. Keep in mind top crop tires like Rival S and RE-71Rs offer much more lateral grip which can help mitigate the issues. The RE-71R have much stiffer sidewalls too which will help a camber challenged car. If this is any help, here is my autox run from 2 years ago in my Z4M. Keep in mind this is when I was still slow but kind of helps show how I drove the car. Then you get stupid and build a full blown STX car... |
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04-19-2016, 08:09 PM | #7 |
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More advice always welcome. True words about the quality of the Michelin tires, but the thing I need to remember is this car is a street car first and autocross or track car second. That's not to say it won't see a good deal of track time this summer, but primarily it's meant for the road. Besides, the less extreme tires keep me in a much tamer class where I can continue learning.
The PSS are nice, very nice. They work great as a dual purpose tire but as is typical of every Michelin I've ever owned (especially on the bikes) they seem to have super soft sidewalls when compared to a similar performing tire from other brands. Going back to my alignment specs, anyone have input on this? Will I require camber washers to achieve these numbers up front with my pulled strut tower pins and the 20mm/10mm F/R drop? I would prefer a setup that I can daily drive, I'm not concerned too much about camber wear (I run -3.0 daily on the e30) and I won't be autocrossing competitively enough in the Z4M to bother making regular adjustments before and after trackdays. I can live with the car being slightly more "darty" than it is to steering input right now, I love when the car responds in a hurry to my inputs even if it means a bit of extra tramlining. I've not looked yet, but I suspect with my mileage I might be due for new anti-roll bar bushings front and rear, most other bushings are already new. Are OEM good enough for these or do most go aftermarket like Powerflex? Are upgrades like the Rogue shock mounts a wise investment or are the B12 kits on the softer side for damping and ok with an oem set to keep things street comfortable? Front: -2.00° camber, 0.00°-0.05° toe in, stock caster Rear: -1.50° to -1.8° camber, 0.10° toe in
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04-20-2016, 07:00 PM | #10 |
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I had my first Autocross with the Z4M this past weekend. I have autoX'ed several Mustangs and an STi, so this was not my first dance.
Prior to my First Outing I added the RTAB limiter kit, Camber Bolt & washer kit, pulled the alignment pin and added the BMW Strut tower brace. I Had the car realigned to -2.5 front and -1.9 rear, zero toe up front and 1/16 in the rear. I also acquired an offset set of Apex Arc 8's to loose 6 lbs per corner and clad the rims with some Extreme Summer Rubber (RE-11's) 235 front 265 rear. everything else is stock. I had an amazing experience, the car handled like a Champ!. I went into hard corners hot and never had a bit of under steer. This car was built to run hard!! Last edited by Bossdog; 04-20-2016 at 07:10 PM.. |
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04-28-2016, 10:33 PM | #11 |
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The Z4M is tricky when it comes to getting the most out of it in terms of front grip but the recipie I had some success with was pretty much what everyone else has said; -2 degrees or more of camber and some real tires. I ran it stock for a few events on 225/45 Hankook V12's and it was miserable. I eventually moved to a full Ground Control suspension and 245/40 RE71R's which made a huge improvement. I'm now autocrossing my E90 M3 in FS full time but if I decide to go back to the Z4M I'll probbaly try to fit the 265/35 fronts for my M3 in the front of the Z4M. I imagine that will be the ticket.
It's not an easy car to get the most out of but damn it's fun.
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04-30-2016, 12:20 AM | #12 |
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Suspension is installed, booked for an alignment on Thursday to dial it in to spec. The old front struts were toast, they had very VERY little resistance to compression and could be squished in one handed with almost zero effort. New setup rides firm and stable, steering feels much better in response but I didn't push it yet (tested through some parking lot maneuvers).
Slight quirk is that the eibach's seem prone to sitting really tall in the rear from time to time on my car, I've verified they are installed correctly and the shock mounts (upper and lower) were torqued with the car sitting at static height to eliminate the chances of them "binding" it in a raised position. Seems to be getting better the more I drive though, will have to watch if it changes. Question two, what is my SCCA2016 class now? I thought it was going to be BSP but it's not on the list for registration. ST something? They've had me running BS so far even with my wheels and tires. 18x10 with 275/35 Michelin PSS rear 18x8.5 with 245/40 Michelin PSS front Suspension, lowered 20mm on eibach with Bilstein B12 dampers All else is stock |
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04-30-2016, 05:42 PM | #13 | |
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04-30-2016, 11:56 PM | #14 |
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I'll have to get in touch with them to see what they say about it, they offer no BSP for registration in the dropdown box.
On a sidenote, here's a quick comparison I made today... First up, front strut from the Z4 after removal Second, rear shock from the Z4 Third, a random old bilstein strut insert from my e30. If memory serves it has around 70,000km of usage (with multiple trackdays in the mix) Call me crazy, but that front couldn't have been helping my understeer issues
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05-01-2016, 04:25 PM | #15 |
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Yeah, not sure why us M owners get boned into being in BSP but it is what it is. Hopefully they'll listen to some letter and open up STU or something. I love driving my car in ST trim. It would be nice to get back into it and actually be somewhat competitive.
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05-01-2016, 04:54 PM | #16 |
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Give it a try, but my experience with SCCA is they have a hard on for BMWs and tend not be very flexible even on simple things. As an example I have Rogue Engineering rear shock mounting plates on my car with OEM shock mounts. The only thing it does is turn the mounting studs upside down so that you can remove the shock from the wheel well to change the setting. I thought it would be no big deal to get a clarification that these met the rules for street, and they came back and very firmly stated they were illegal for street class use. They also don't consider the Z4M a variant of the Z4 series, they consider it a unique model by itself, so you can't legally use any part from an M on a non-M car and still be legal for street class.
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05-01-2016, 05:59 PM | #17 | |
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05-06-2016, 12:58 AM | #18 |
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Well, street is that way no matter how you slice it. It has the most strict rules (rightfully so, in my opinion). Running in FS is kind of a breath of fresh air. No more sweating the rules as much, virtually no car preparation before an event, etc.
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05-06-2016, 09:38 AM | #19 |
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Alignment is done, front and rear camber are maxed for what was available in stock trim running my suspension and the thinnest of the camber shims in the bimmerworld kit. I think I'll be plenty happy with this and the new springs/shocks on the next autocross weekend.
Final specs Front: -2.0° camber / 0.0° toe / stock caster Rear: -1.5° camber / 0.1° toe in
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