Strange traction control behavior

iminhell1 · 01-02-2017, 02:58 AM

So a month back I replace my rear tires with a snow tire, larger than was on there. So I currently have 205 / 50 - 17 in the front and 215 / 55 - 17 in the rear.
Front - 25.07" diameter
Rear - 26.31" diameter

Now what I found is that the traction control/stability control does not like this large of a discrepancy in the rolling diameters. At low speeds I don't notice any problems. But as speed increases the car starts to feel very unstable, dangerously so above 70MPH. The rear wants to sway and if I input steering it gets worse.

Tonight I turned DSC off and went for a long drive. Car felt 100% normal. No swaying and what I had attributed to a big tire and soft sidewall wasn't there.
Seems that the sway was the rear brakes intervening because I'd passes some threshold for something.

Anyone happen to know what to log to find what is activating? (with Carley probably)

atr_hugo · 01-02-2017, 08:47 AM

FWIW, here is a link to a tire calculator - suspect you're right that it's a revs per mile issue. Given your setup the front tires rotate 805 times per mile driven, the rears only 767 times.

https://tiresize.com/calculator/

Motohip · 01-05-2017, 10:25 AM

That's a huge difference. What made you think that would work???

MightyMouseTech · 01-05-2017, 10:38 AM

Winter tires and increasing the sidewall height is a bad combination.

The Wind Breezes · 01-05-2017, 04:24 PM

Just do what I do and hold the DSC button before you even put the car in gear. Problem solved, better hope you have the skills and judgement to not wreck the car.

iminhell1 · 01-05-2017, 09:54 PM

Quote:

Originally Posted by Motohip

That's a huge difference. What made you think that would work???

I don't know how the DSC works. I've had T/C before and that just looks for a difference in 2 wheel speed sensors. A system that looks at 4 is new to me.

... and when I asked before no one seemed to have an answer. So I did the testing for everyone. You're welcome.

The Wind Breezes · 01-05-2017, 10:48 PM

For real, unless you're already educated on the system why would you think it WOULDN'T work? How difficult would it be for the DSC controller to figure out a constant ratio between the wheels in the event there are different ratio tires? Ridiculous that it doesn't do this.

sandlion · 01-05-2017, 11:00 PM

nah, you're silly.

Matticus91 · 01-06-2017, 02:35 PM

Question: Does the issue persist if you do a single press of the DSC? Do you need to do a long-press to totally turn it off before things feel normal again?

iminhell1 · 01-06-2017, 02:51 PM

Long press only.
Short press may reduce the feeling a little, but it's obviously still there.

I'd still like to know what to log and with what program so I can nail down an error ratio where this takes place. Maybe those with wider rears than stock get this same feeling but just don't know it? What is an acceptable error ratio?

01-02-2017, 02:58 AM	#1
iminhell1 C2H5OH 3915 Rep 2,144 Posts Drives: 2010 SG 135i auto Join Date: May 2015 Location: Darwin, MN iTrader: (1)	Strange traction control behavior So a month back I replace my rear tires with a snow tire, larger than was on there. So I currently have 205 / 50 - 17 in the front and 215 / 55 - 17 in the rear. Front - 25.07" diameter Rear - 26.31" diameter Now what I found is that the traction control/stability control does not like this large of a discrepancy in the rolling diameters. At low speeds I don't notice any problems. But as speed increases the car starts to feel very unstable, dangerously so above 70MPH. The rear wants to sway and if I input steering it gets worse. Tonight I turned DSC off and went for a long drive. Car felt 100% normal. No swaying and what I had attributed to a big tire and soft sidewall wasn't there. Seems that the sway was the rear brakes intervening because I'd passes some threshold for something. Anyone happen to know what to log to find what is activating? (with Carley probably)
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01-02-2017, 08:47 AM	#2
atr_hugo No longer moderate 325 Rep 4,401 Posts Drives: '13 135i Join Date: Jun 2007 Location: - iTrader: (0)	FWIW, here is a link to a tire calculator - suspect you're right that it's a revs per mile issue. Given your setup the front tires rotate 805 times per mile driven, the rears only 767 times. https://tiresize.com/calculator/
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01-05-2017, 10:25 AM	#3
Motohip Lieutenant 37 Rep 435 Posts Drives: 128i Join Date: Mar 2012 Location: Washington iTrader: (5)	That's a huge difference. What made you think that would work???
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01-05-2017, 10:38 AM	#4
MightyMouseTech Major General 4338 Rep 6,196 Posts Drives: 13 135i 6MT LeMans Blue MSport Join Date: Feb 2013 Location: Ottawa, Canada iTrader: (0)	Winter tires and increasing the sidewall height is a bad combination.
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01-05-2017, 04:24 PM	#5
The Wind Breezes Lieutenant Colonel 912 Rep 1,850 Posts Drives: 135i N55 DCT Join Date: Apr 2015 Location: USA iTrader: (0)	Just do what I do and hold the DSC button before you even put the car in gear. Problem solved, better hope you have the skills and judgement to not wreck the car.
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01-05-2017, 10:48 PM	#7
The Wind Breezes Lieutenant Colonel 912 Rep 1,850 Posts Drives: 135i N55 DCT Join Date: Apr 2015 Location: USA iTrader: (0)	For real, unless you're already educated on the system why would you think it WOULDN'T work? How difficult would it be for the DSC controller to figure out a constant ratio between the wheels in the event there are different ratio tires? Ridiculous that it doesn't do this.
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01-05-2017, 11:00 PM	#8
sandlion Private First Class 18 Rep 100 Posts Drives: 2011 135i Join Date: Jun 2016 Location: Dallas, TX iTrader: (0)	nah, you're silly. __________________ 2011 135i DCT Space Gray Metallic \| TR Motorsports 17x8 ET40 Wheels \| Koni Yellows \| FMIC \| Charge Pipe \| MHD Stage 1 \| Riken Raptor 245/40ZR Tires
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01-06-2017, 02:35 PM	#9
Matticus91 Brigadier General 3475 Rep 3,722 Posts Drives: 2013 135i Join Date: Aug 2015 Location: Washington DC iTrader: (6)	Question: Does the issue persist if you do a single press of the DSC? Do you need to do a long-press to totally turn it off before things feel normal again? __________________ "Tobias" 2013 135i ///M-Sport 6MT • Pure Stage 1 • XDI 35 HPFP • 404whp/440wtq IG: msteelio91
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01-06-2017, 02:51 PM	#10
iminhell1 C2H5OH 3915 Rep 2,144 Posts Drives: 2010 SG 135i auto Join Date: May 2015 Location: Darwin, MN iTrader: (1)	Long press only. Short press may reduce the feeling a little, but it's obviously still there. I'd still like to know what to log and with what program so I can nail down an error ratio where this takes place. Maybe those with wider rears than stock get this same feeling but just don't know it? What is an acceptable error ratio?
	Appreciate 0 Quote