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      06-25-2018, 03:21 PM   #1
dre99gsx
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Open Diff + Corner Exit traction issues

Beating a dead horse here, but looking for some additional advice.

I'm on the 3.0si setup: Open 3.64 rear diff, stock rear/front sways, BC coilovers at 29front/25rear (30=full stiff) with 504front/672rear springs, -3fr/-2rear camber.

Car feels neutral and balance so far everywhere on the track, except I am spinning the inside-rear at corner exit for almost every turn. This has forced me to cruise and wait until perfectly straight before getting on the power. I can count on my fingers the seconds I lose per lap - sad.

I know the solution is an LSD, but before going there, any other things I can do to try to increase rear-inside-tire-grip on corner exit? I've read about removing the rear sway bar, but also hear it is not good for high speed corners or high speed stability.

It's been frustrating since I have all this torque but can use it.
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      06-25-2018, 09:36 PM   #2
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Quote:
Originally Posted by dre99gsx View Post
Beating a dead horse here, but looking for some additional advice.

I'm on the 3.0si setup: Open 3.64 rear diff, stock rear/front sways, BC coilovers at 29front/25rear (30=full stiff) with 504front/672rear springs, -3fr/-2rear camber.

Car feels neutral and balance so far everywhere on the track, except I am spinning the inside-rear at corner exit for almost every turn. This has forced me to cruise and wait until perfectly straight before getting on the power. I can count on my fingers the seconds I lose per lap - sad.

I know the solution is an LSD, but before going there, any other things I can do to try to increase rear-inside-tire-grip on corner exit? I've read about removing the rear sway bar, but also hear it is not good for high speed corners or high speed stability.

It's been frustrating since I have all this torque but can use it.
What tire brand/model/size tire are you running in the rear?
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      06-25-2018, 09:38 PM   #3
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295/265 R888r
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      06-25-2018, 10:01 PM   #4
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295/265 R888r
That's a tough one then. You've got some pretty grippy tires, but a lot of power. Going to be challenging to solve that without going to the LSD.
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      06-25-2018, 10:23 PM   #5
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Remove the rear sway bar. Increase rear rebound, decrease front rebound.
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      06-26-2018, 07:35 AM   #6
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I noticed recently when running my street tires versus track tires this issue become so much more obvious. To the point where the car didn't want to put anything down, what the point of being able to carry speed through corners without being able to accelerate out.
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      06-26-2018, 07:55 AM   #7
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To add to Hack's comment, since you're trying to reduce the roll of the car and weight transfer to prevent the inside tire from unloading, stiffer springs may also help.
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      06-26-2018, 08:27 AM   #8
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Yup - thanks for the tips. I'm at 12k rear, and for these cheap (but thus far reliable and to-my-knowledge effective) BC Racing coilovers, I'll need a re-valving if I go any heavier. That is Ok, options are good.

Makes sense, simply removing the rear sway should help push the inside wheel down with more force via the spring. Although, I'm sure there are downsides to go along with stiffer springs or no rear bar. The car is very sorted and neutral right now around corners... hmm.

At this point, I already ordered a Wavetrac unit and am going to go all in. Obviously, the next phase will be re-learning how to drive with an LSD, and not get into any trouble. I've started to gas it earlier on corner exit, anticipating inside-wheel spin, but knowing it will just go straight, and just Go.

I bet I'll now have to be even more ginger with an LSD. Always learning!
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      06-27-2018, 12:55 PM   #9
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Ok, Wavetrac is a month backordered, and Quaife dealers don't have anything in stock.

Being that I'm at 672lb rear springs (fairly stiff already I would assume), I'm going to order a smaller rear sway bar. I'm a bit hesitant to remove it all together since there may be unintended consequences with high speed stability, especially when i'm going over 150mph...

My 3.0si had the M-sport suspension = 19mm rear sway bar. Just found out the non-sport has the 18mm rear sway. 1mm may not be much, but for $150, it may help. I'm also going from the stock 25mm front sway bar to the E46 Convertible 27mm front sway. It may cause more front end push/understeer, but if I get more corner-exit grip, I'll make up more time. I can always go back to the OEM bars if this doesn't work as planned.

Stiffer front sway bar should help keep the rear more planted (correct me if I'm wrong). I prefer lifting the front than rear since corner exit is my strength.
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      06-27-2018, 02:51 PM   #10
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I still think disconnecting the rear won't hurt, especially high speed stability since you have relatively stiff springs in the rear. Give that a try before you invest in a new bar. I don't think a 1mm difference in thickness will make a dramatic difference, and going to a thicker front bar may take away more grip up front. Ideally you'd want to add grip first before you subtract for balance.
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      06-27-2018, 03:01 PM   #11
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I haven't looked yet, but how feasible is it to remove one linkage only on the rear? Would I have issues with the bar coming into contact with driveshaft/arms? Being that I have a splitter down there and exhaust, would be massive surgery to remove the entire bar and later install if I'm not satisfied, especially trackside.

Secondly, do you or anyone else here have first hand experience with running fast sweeping road coarse without the rear bar? What negative effects did you notice?
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      06-27-2018, 03:03 PM   #12
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Quote:
Originally Posted by The HACK View Post
I still think disconnecting the rear won't hurt, especially high speed stability since you have relatively stiff springs in the rear. Give that a try before you invest in a new bar. I don't think a 1mm difference in thickness will make a dramatic difference, and going to a thicker front bar may take away more grip up front. Ideally you'd want to add grip first before you subtract for balance.

^WHS
I ran w/o a rear bar for the last 2 years and it was great.
I mainly run at Mosport which is a VERY fast track with several high speed sweepers.

but....I also ran aero
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      06-27-2018, 08:15 PM   #13
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Alright, I'm convinced. I ordered the bars (used junkyard) but will also try no rear bar, and go from there. Last resort will be Wavetrac. Thanks everyone.
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      06-28-2018, 07:00 AM   #14
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Be careful with the e46 front bar. Though it may technically fit, depending on the model it may be a tad too wide. I ran the m3 bar and turns out it would cause knockback so I’d have NO BRAKES after turns. Took forever me to diagnose but yea that was the reason.
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      06-28-2018, 08:28 AM   #15
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Is this because it would rest on and hit the brakes?
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      06-30-2018, 07:40 PM   #16
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Alright, slight change of plans. I went with the H&R 27mm bar for 3.0si. Those of you who used this bar, being that it is same diameter as the Z4Mm bar, would you say the outer most hole (there are only two holes on the bar) gives the same stiffness as Z4M bar? Or, is it already stiffer using the outer hole? I'm not planning on going inner hole yet.

Doing one thing at a time. With an LSD coming in next week, I'll start with stiffer front bar and LSD, and go from there. I can remove the rear bar or install the 18mm bar, but will leave that for last resort. The car rotates so well and handles great that I don't want to mess with a too much of a good thing.
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      06-30-2018, 09:44 PM   #17
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Quote:
Originally Posted by dre99gsx View Post
Alright, slight change of plans. I went with the H&R 27mm bar for 3.0si. Those of you who used this bar, being that it is same diameter as the Z4Mm bar, would you say the outer most hole (there are only two holes on the bar) gives the same stiffness as Z4M bar? Or, is it already stiffer using the outer hole? I'm not planning on going inner hole yet.

Doing one thing at a time. With an LSD coming in next week, I'll start with stiffer front bar and LSD, and go from there. I can remove the rear bar or install the 18mm bar, but will leave that for last resort. The car rotates so well and handles great that I don't want to mess with a too much of a good thing.
I can't answer the question on how it compares to an "M" bar, but I have the H&R on my 3.0si. I tried the rear holes and that was a bit too soft, and the two forward gave too much understeer. Currently I'm running one forward and one rear which is the middle stiffness setting and that's seemed to be about right.
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      06-30-2018, 11:15 PM   #18
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Whadda-u-know, I learned something new

"For advanced users, note that you can set the front and rear bar in "middle" positions. For example, the rear E36 M3 Sway Barbarian has two adjustment holes. Conventional thinking would assume that you have two positions, stiffer (holes furthest away from the ends of the bar) or softer (holes on the ends of the bar). For increased fine tuning, you can set one side to soft and the other side to stiff. The vehicle does not know the different from side-to-side when it comes to bar stiffness, so in effect, you have a "middle" position."

Thanks for the tip!
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      07-01-2018, 08:50 AM   #19
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Whadda-u-know, I learned something new

"For advanced users, note that you can set the front and rear bar in "middle" positions. For example, the rear E36 M3 Sway Barbarian has two adjustment holes. Conventional thinking would assume that you have two positions, stiffer (holes furthest away from the ends of the bar) or softer (holes on the ends of the bar). For increased fine tuning, you can set one side to soft and the other side to stiff. The vehicle does not know the different from side-to-side when it comes to bar stiffness, so in effect, you have a "middle" position."

Thanks for the tip!
One key item to make this valid is that your sway bar has to be free to rotate in the bushings. It really needs to rotate freely anyway otherwise it becomes part of the spring system. Part of my pre-event prep is to hit the bushings with some silicone spray to help make certain there's no binding.
Also, until you get the LSD, later apex. :-)

https://www.scca.com/articles/200126...st-on-the-apex
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      07-02-2018, 02:28 PM   #20
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This topic will be ever changing, but while I'm waiting for my Quaife to come in, I've been having a hell of a time removing one of the diff carrier bearings for re-use. Note that the only carrier bearings you see for sale (and locally) are too small. You need the 90mm size unit (f ag F-801298-TR1P-H79-T29), which of course is not sold in the US (or extremely difficult to find!). Tried all techniques - heat, chisel, etc.

Luckily I found David Green @ 770-324-7972 Synchro Design Works who has them new in his shop. They are about $60-$100 each depending on where you get them. He'll also send an assortment of shims and circlips since I will most likely have to readjust my backlash.

I've done transmission rebuilds and diff rebuilds in the past, mostly Toyota, but am blown away by how little information exists on BMW units, let alone spare rebuild parts. Seems like they don't want these things serviced...
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      07-02-2018, 06:20 PM   #21
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Quote:
Originally Posted by dre99gsx View Post
This topic will be ever changing, but while I'm waiting for my Quaife to come in, I've been having a hell of a time removing one of the diff carrier bearings for re-use. Note that the only carrier bearings you see for sell (and locally) are too small. You need the 90mm size unit (f ag F-801298-TR1P-H79-T29), which of course is not sold in the US (or extremely difficult to find!). Tried all techniques - heat, chisel, etc.

Luckily I found David Green @ 770-324-7972 Synchro Design Works who has them new in his shop. They are about $60-$100 each depending on where you get them. He'll also send an assortment of shims and circlips since I will most likely have to readjust my backlash.

I've done transmission rebuilds and diff rebuilds in the past, mostly Toyota, but am blown away by how little information exists on BMW units, let alone spare rebuild parts. Seems like they don't want these things serviced...
Yuck.

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      07-02-2018, 06:24 PM   #22
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Wavetrac isn't perfect either!

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