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      12-29-2013, 11:13 PM   #1
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Combating Wheelspin

As I prioritize next year's mod budget, one thing really sticks out on my Z4M - wheelspin. I thought it was bad down south in the summer, but now that I've moved to northern VA in the winter with summer tires (Michelin PSS 265's), I can't even go WOT in 3rd gear without DSC intervening. In 2nd, I usually get axle hop, even with smooth throttle application. When the power is somewhat laid down, I get all kinds of rear end shimmy. It's to the point that it's frustrating. I don't want to snap an axle.

The car has 105k miles. I've read that the stock diff can wear out and essentially become an open diff at around 80k miles. Would an OS Giken rear end be worth it or overkill? Should I have any bushings checked?
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      12-30-2013, 12:41 AM   #2
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Do a burn-out and see if it leaves elevensies.

It actually rained here, a little, a couple weeks ago.
Got the first hop out of mine going up a hill at the low end of second gear.
Kinda a butt-dyno observation, but it felt different than my MR2.
More side-to-side, rather than just one corner bouncing.
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      12-31-2013, 08:04 AM   #3
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I would make sure you know if it is actually the diff, but doing all the bushings in the rear suspension should fix the issue if it isn't the diff.

But summer tires in the winter won't cut it when it's <40...all seasons won't cut it for a supercharged car either though, so it's a bit of a catch 22
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      12-31-2013, 10:47 AM   #4
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always 2 lines when i set off..... and when i change gear..

take a video of the ar setting off and playback :-) oh while you at it post up the video!!

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      12-31-2013, 01:55 PM   #5
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i spin in 3rd on a cold summer day with pss and NA. im not surprised you spin supercharged. warm up the tires a little before or get a better tire for the winter months.
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      12-31-2013, 08:06 PM   #6
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pokey,

The RTABs do one thing. Keep the wheel straight in regards to the thrust axis (or rear thrust angle set during alignment), during load and during suspension arm travel in the X axis (up/down).

If your RTABs are worn your wheels could be canting off true with regards to the thrust axis. Replacing the RTABs will help but not be the end all, cure all.
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      01-01-2014, 06:04 PM   #7
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Thanks everyone! Ship, I'll start with the RTABs. Based on what you describe, I think it will solve a good portion of the problem. I get a bit of side-to-side rear end shimmy under acceleration. I figure it's either flex or the variable diff not immediately locking.

Most of those going for the OS Giken diff are high powered HPF turbo M3's. The initial cost of entry is only part of it. I've read, but haven't confirmed, that the fluid is supposed to be changed every 12k miles with expensive semi-exotic oil.
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      01-01-2014, 08:14 PM   #8
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Depending on which car you are talking about determines when the differential oil is changed. On your 3.0i (non-LSD) the oil is supposed to have be very long life. On your ///M, the differential fluid gets changed every other inspection, I think. Based on driving style, that would be anywhere from every 40 to 55K.

Yes, the fluid used in the LSD is pricy. The standard SAF-XJ is around $40.00 a pint. This is the OEM fill from the factory. There is an SIB that talks about changing the fluid to SAF-XJ w/friction modifier ($55.00/pint). The SIB says to change to that fluid if you get excessive noise from the differential at very low speed, in a very tight turn.

SAF-XJ w/friction modifier has a side benefit of giving the differential some added lockup over the normal SAF-XJ.

The normal SAF-XJ is only sold in 55gal drums. Most dealers will sell you some by the pint if you take your own container to them. The SAF-XJ w/friction modifier is sold in bottles.

Your differential only need two pints (1.6p) of fluid.
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      01-01-2014, 08:17 PM   #9
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You're running PSS tires and expecting them to actually grip.

RS3s, ZIIs, Rivals are the tire that offer the most grip today. However, NO tires grip at cold temperatures (aka ambient). RS3s, for example need to see a surface temp of around 130+ to achieve maximum grip.
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      01-01-2014, 08:26 PM   #10
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Quote:
Originally Posted by Shipkiller View Post
..........

SAF-XJ w/friction modifier has a side benefit of giving the differential some added lockup over the normal SAF-XJ.

.......
Interesting.....
I hadn't heard that before.
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      01-01-2014, 10:48 PM   #11
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Quote:
Originally Posted by Kgolf31
You're running PSS tires and expecting them to actually grip.

RS3s, ZIIs, Rivals are the tire that offer the most grip today. However, NO tires grip at cold temperatures (aka ambient). RS3s, for example need to see a surface temp of around 130+ to achieve maximum grip.
Even in 100 degree south Mississippi heat, I was experiencing significant wheelspin. I am fully aware that summer tires lose grip in cold temps. I'm interested in finding out if there are other factors at play here, especially considering my mileage. Yes, the car is supercharged, but I don't think I should be having traction issues at 4000 rpm in 2nd gear in the summer heat. With winter here, it's worse, but it's not a new issue. I remember test driving an M coupe back in 2007, and I was surprised at how much straight line grip it had, even on a damp road.

As far as the diff fluid is concerned, I don't think I was clear. I was talking about the OS Giken diff, which requires more frequent fluid changes than the OEM diff, and supposedly uses a different fluid. It's all forum talk, and I haven't confirmed anything with the manufacturer. I do appreciate the info though, especially regarding the additive increasing lock.
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      01-02-2014, 12:19 AM   #12
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You need to lower psi to about 28 and/or switch to a more appropriate tire for this power level. Extreme performance tires, around 120-140 tread wear

How much power are you making?

I'm eventually gonna do widebody fenders in the back, then you can run a street tire like the PSS in 315mm or 335mm and hook up
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      01-02-2014, 06:55 AM   #13
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Quote:
Originally Posted by pokeybritches View Post
Even in 100 degree south Mississippi heat, I was experiencing significant wheelspin. I am fully aware that summer tires lose grip in cold temps. I'm interested in finding out if there are other factors at play here, especially considering my mileage. Yes, the car is supercharged, but I don't think I should be having traction issues at 4000 rpm in 2nd gear in the summer heat. With winter here, it's worse, but it's not a new issue. I remember test driving an M coupe back in 2007, and I was surprised at how much straight line grip it had, even on a damp road.

As far as the diff fluid is concerned, I don't think I was clear. I was talking about the OS Giken diff, which requires more frequent fluid changes than the OEM diff, and supposedly uses a different fluid. It's all forum talk, and I haven't confirmed anything with the manufacturer. I do appreciate the info though, especially regarding the additive increasing lock.
What I'm still trying to tell you is that PSS tires really aren't that good. People think they are amazing, because they don't know better and that is all that they have used.


To comment on @ZStig post above, I ran my RS3s at 27 pounds in the front and around 29 pound hot in the rear while autocrossing. Granted, I'm stock power but launching at 2.5k leaves a couple skips and then instant grip
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      01-02-2014, 09:13 AM   #14
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Quote:
Originally Posted by ZStig
You need to lower psi to about 28 and/or switch to a more appropriate tire for this power level. Extreme performance tires, around 120-140 tread wear

How much power are you making?

I'm eventually gonna do widebody fenders in the back, then you can run a street tire like the PSS in 315mm or 335mm and hook up
Hankook Ventus TD 265/35/18 front , 285/30 rear ( stock fenders , lowered on KW V3's , -3 front camber -2 rear . Traction is not an issue lol . Best tire available period right now . This set lasted 8 Advanced group track days and 3,000 street miles while delivering near Hoosier levels of grip on track . They are even reasonable down into 40 degree temps , which has not been my experience with other DOT R comps.
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      01-02-2014, 10:58 PM   #15
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Quote:
Originally Posted by gmd2003 View Post
Hankook Ventus TD 265/35/18 front , 285/30 rear ( stock fenders , lowered on KW V3's , -3 front camber -2 rear . Traction is not an issue lol . Best tire available period right now . This set lasted 8 Advanced group track days and 3,000 street miles while delivering near Hoosier levels of grip on track . They are even reasonable down into 40 degree temps , which has not been my experience with other DOT R comps.


you would have even more straightline traction with 0 toe in the back
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      01-03-2014, 08:53 AM   #16
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I think I need Quaife or OS Giken soon...my LSD has been ticking for like a year now lol
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      01-03-2014, 01:16 PM   #17
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Quote:
Originally Posted by ZStig
Quote:
Originally Posted by gmd2003 View Post
Hankook Ventus TD 265/35/18 front , 285/30 rear ( stock fenders , lowered on KW V3's , -3 front camber -2 rear . Traction is not an issue lol . Best tire available period right now . This set lasted 8 Advanced group track days and 3,000 street miles while delivering near Hoosier levels of grip on track . They are even reasonable down into 40 degree temps , which has not been my experience with other DOT R comps.


you would have even more straightline traction with 0 toe in the back
I need to turn too brother lol . At 100 mph I like a little stability when I turn in
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      01-03-2014, 01:19 PM   #18
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Quote:
Originally Posted by mfanatic325
I think I need Quaife or OS Giken soon...my LSD has been ticking for like a year now lol
Dan Fitzgerald at Diffsonline build fantastic units too and cheaper than the OSG . I have a 3 clutch 30/60/90 variable ramp with 4.10 BMW Motorsport gears . It has been flawless on track for 30,000 miles.
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      01-03-2014, 01:43 PM   #19
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Quote:
Originally Posted by ZStig View Post


you would have even more straightline traction with 0 toe in the back
And absolutely zero traction in any corner.

I run a -1/4 total toe in the rear and has helped put down down much easier than -1/8th total toe
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      01-03-2014, 03:00 PM   #20
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corners? pfffffff
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      01-03-2014, 03:45 PM   #21
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Throw 2 50 lbs sandbags in the trunk of the car. If they fit.

Traction is merely a function of weight over the rear axle vs the friction coefficient of the tires. More weight = more traction.

That's why it's pointless to equip a Lotus Exige S with more than 300 HP when the car only weighs 2,400 lbs. Even with R-Comps standard (Exige comes with Yokohama Advan AD-08 I think) the Exige S will easily break rear traction because, well, the car's freakin' light. With a measly 240HP.

Sometimes cars with too stiff springs all around will exhibit this behavior too. Problem may be alleviated by stiffening up rebound in the rear (odd, I know...But sometimes stiff rebound promotes weight transfer).
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      01-03-2014, 06:39 PM   #22
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Quote:
Originally Posted by gmd2003 View Post
Dan Fitzgerald at Diffsonline build fantastic units too and cheaper than the OSG . I have a 3 clutch 30/60/90 variable ramp with 4.10 BMW Motorsport gears . It has been flawless on track for 30,000 miles.
Thanks for the rec. I will keep him in mind
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