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Old 10-02-2012, 08:14 PM   #21
SeanC
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Originally Posted by B_Cyrus View Post
I'm having a hard time believing this. 325i's come stock with like 195/50/16
Stock tire size for a 325 is 205/55/16, which results in an overall diameter of 24.87'', not too different than that of a 225/45/17, or 205/50/17, or 225/40/18, or 255/35/18 (all stock sizes).

The correct tire size combination for a staggered 18 inch set up is 225/40/18 fronts and 255/35/18 rears (for example, stock ZHP). The overall diameters are 25.08'', and 25.02'' inches respectively, the slight difference is negligible.

OP's problem is having incompatible tire sizes in the front and rear. He could have gone with 245/40/18's all around (even though not a stock size), or 225/40/18 all around (assuming wheel width allows). Putting one size in the front and the other in the rear is a NO GO.

Let me elaborate even further:

If you install 225/40/18 in the front and 245/40/18 in the rear, rear tires will be rotating 809 times per mile, while the fronts will be rotating 829.3 per mile, the difference is quite possibly large enough to be picked up by the DSC sensors. For the correct rear tire size, i.e. 255/35/18, revs per mile is 831.3, which is much closer to that of a 225/40/18 size tire, and the difference is within DSC's operating limits.
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Old 10-03-2012, 01:05 AM   #22
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OP. As I said from the beginning, make sure you're running identical BRAND and MODEL of tires front and back as well as the CORRECT BMW STOCK SIZE for your car. Don't mix and match brands and models and wear patterns. If you buy two new tires for one axle, make sure you get new ones for the OTHER axle very very SOON.

Not quite sure exactly how someone knocks off a speed sensor when doing tires. Is that what you are saying happened?
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Old 10-03-2012, 01:24 AM   #23
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Stock tire size for a 325 is 205/55/16, which results in an overall diameter of 24.87'', not too different than that of a 225/45/17, or 205/50/17, or 225/40/18, or 255/35/18 (all stock sizes).

The correct tire size combination for a staggered 18 inch set up is 225/40/18 fronts and 255/35/18 rears (for example, stock ZHP). The overall diameters are 25.08'', and 25.02'' inches respectively, the slight difference is negligible.

OP's problem is having incompatible tire sizes in the front and rear. He could have gone with 245/40/18's all around (even though not a stock size), or 225/40/18 all around (assuming wheel width allows). Putting one size in the front and the other in the rear is a NO GO.

Let me elaborate even further:

If you install 225/40/18 in the front and 245/40/18 in the rear, rear tires will be rotating 809 times per mile, while the fronts will be rotating 829.3 per mile, the difference is quite possibly large enough to be picked up by the DSC sensors. For the correct rear tire size, i.e. 255/35/18, revs per mile is 831.3, which is much closer to that of a 225/40/18 size tire, and the difference is within DSC's operating limits.
Nail on the head. The DSC, TCS and ABS all work by monitoring the wheel speeds relative to each other. When one wheel starts spinning faster than the others, it indicates that wheel is slipping and the DSC/TCS/ABS will intervene to try and stop it from slipping. Here, your tire sizes are such that your front wheels are ALWAYS spinning faster than the rears (since the rears have a larger diameter) and the DSC is basically telling you to go **** yourself since it has no idea what's going on or how to fix it. You can drive fine without it, but I would just keep the car happy and install the correct diameter tires.

Also, your ABS is likely inop since it works off the same principle as the other systems. You may get by just fine without it, but if you are not a skilled threshold braker I wouldn't chance it (especially in rainy climates).

Honestly I would expect a service manager to know all of this since it is very important with modern cars. I think it could be considered unsafe of him and you may want to speak to someone higher up about it and let them know about the whole situation.

Last edited by Wh33lhop; 10-03-2012 at 01:33 AM.
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Old 10-03-2012, 11:08 AM   #24
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Okay it was actually just a wear pad sensor that they knocked out. And discounts tire policy will most likely take the two front tires back. I thought that was what someone meant when they said brake sensor. But now I realize they were talking about the ABS sensor. What is a close enough replacement for the fronts to get the DSC from acting up? And if this was the problem would the DSC notice it immediately? And why do you guys think this happened when I've had these sized tires for 6 months and now that they are new and the diameters are even closer due to the tire being new and having more tread is this now a more possible problem?
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Old 10-03-2012, 11:14 AM   #25
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I was actually the first to mention the brake sensor and while the other guys have good info, I'd wager you need an alignment more than the tire sizing having an effect.
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Old 10-03-2012, 11:50 AM   #26
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Alright, I'll start off with an alignment to see if that does anything. I'll see if I can find a local shop today.
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Old 10-03-2012, 12:03 PM   #27
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Okay it was actually just a wear pad sensor that they knocked out. And discounts tire policy will most likely take the two front tires back. I thought that was what someone meant when they said brake sensor. But now I realize they were talking about the ABS sensor. What is a close enough replacement for the fronts to get the DSC from acting up? And if this was the problem would the DSC notice it immediately? And why do you guys think this happened when I've had these sized tires for 6 months and now that they are new and the diameters are even closer due to the tire being new and having more tread is this now a more possible problem?
It could be a number of different things; the new tires could be running lower pressures or have a different carcass structure that would give them a slightly smaller effective diameter than the old ones which would push the dsc over the edge, your alignment could be off making the fronts spin a little faster. But bottom line you are running different diameter tires which is risky. If you want to fix it the right way you can replace the fronts (cheaper since DT will take them back) go for 225/45 or (what I would do) instead go for 255/35 rears. Otherwise you can try to put a band aid on it and hope it doesn't pop up again.

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Old 10-03-2012, 12:11 PM   #28
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man, so much misinformation in this thread.

#1 I could have told you in the beginning if I happened to see this earlier that the "brake sensor" people were referring to has nothing to do with ABS or DSC, it is just the brake pad wear sensor that indicates when the pads need replacing. Now, there is another ABS speed sensor that is bolted to the back of the brake carrier that almost comes in contact with the rotor or hub I believe. they may have damaged that sensor, which would cause your ABS and DSC lights to come on.

As far as making sure you have the exact same brands front and rear, that's ridiculous. They need to be the proper SIZE, but different brands will not make your ABS and DSC lights come on for any reason.

For example, right now my wife has 2 16" Michelin MXV4 Plus's up front with 2 Continental DWS 16"s in the back. All 4 are same size, but not same brand, and her fronts are worn down much more than her rears as those are older tires.

On my car, I have the staggered ZHP setup and have 2 225/40/18's in front that are Hankook Ventus V12 Evo 110K's and in the rear I have 2 255/35/18 Pirelli PZero Nero's. never had any issues with ABS or DSC lights on the dash with either setup.
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Old 10-03-2012, 12:19 PM   #29
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...The DSC, TCS and ABS all work by monitoring the wheel speeds relative to each other. ...
Actually, not true for ABS. ABS looks at changes in the rate of deceleration of each wheel individually, not differences in speed of one wheel relative to another. (Although one or more wheels turning at a speed significantly different than the rest will likely trigger the diagnostics to indicate a sensor problem.)

DSC and TCS do, of course, look at wheel speeds relative to each other.
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Old 10-03-2012, 12:49 PM   #30
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Actually, not true for ABS. ABS looks at changes in the rate of deceleration of each wheel individually, not differences in speed of one wheel relative to another. (Although one or more wheels turning at a speed significantly different than the rest will likely trigger the diagnostics to indicate a sensor problem.)

DSC and TCS do, of course, look at wheel speeds relative to each other.
You're splitting hairs.

ABS looks for a tire that is not turning and releases the brake pressure so it can turn.
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Old 10-03-2012, 12:59 PM   #31
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The brake sensor was disconnected. This would trigger the DSC light, right? They knocked it out when they were balancing the back two tires. If this is the case, how long would it usually take to reset?
The left front and right rear have two sensors, one monitors wheel speed the other monitors brake wear.

In the days of old, people had to -- wait for it -- _look_ at the brakes to see if they were worn out or not. BMW put in a sensor that "looks" for you constantly so you do not need to do an inspection. The Brake Wear warning comes from one of two things, the brakes are worn or the sensor is broken/disconnected. The brake wear will not trigger a DSC light.

The wheel speeds are monitored to prevent brake lock up or uncontrolled wheel spin. If you slam on the brake and lock a tire, the ABS jumps into action to release the brake pressure to that tire so it can turn -- the idea is that a tire that is turning can be aimed in a new direction while a tire that is skidding will continue on its path regardless of where you attempt to get it to go -- and if the car detects wheel spin, then it will apply the brake to that wheel and reduce the application of throttle.

Since there is a brake wear sensor and a wheel speed sensor both at the right rear corner, I suggest taking that tire off and inspecting the wires to these sensors, and inspecting the brakes at the same time. It is possible that the brakes just happened to wear out at the same time as the tire service, and it is also possible (probable) that the tire service disturbed the sensors.

In any case, the best course of action is to go back to the tire store and take all four tires off and look at the brakes and check the condition of the wires that lead to the wheels. The second best course of action is to blow the tire store off and do this at home.
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Old 10-03-2012, 01:01 PM   #32
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You're splitting hairs.

ABS looks for a tire that is not turning and releases the brake pressure so it can turn.
It doesn't wait till it isn't turning. When it senses that the rate that it is slowing down at is more than is possible, it releases the brakes until the wheel speeds back up.
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Old 10-03-2012, 01:14 PM   #33
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It doesn't wait till it isn't turning. When it senses that the rate that it is slowing down at is more than is possible, it releases the brakes until the wheel speeds back up.
Technically that isn't true, but as a practical matter, it does not matter. ABS looks for a tire that is about to skid and releases the brake pressure so it does not skid.

It has nothing to do with the rate of slowing, the speed sensor either generates a long pulse width -- the tire is in danger of skidding -- or a short pusle width -- the tire is turning. The rate of change doesn't matter.

But none of this has anything to do with the problem that the OP has.
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Old 10-03-2012, 01:32 PM   #34
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man, so much misinformation in this thread.

#1 I could have told you in the beginning if I happened to see this earlier that the "brake sensor" people were referring to has nothing to do with ABS or DSC, it is just the brake pad wear sensor that indicates when the pads need replacing. Now, there is another ABS speed sensor that is bolted to the back of the brake carrier that almost comes in contact with the rotor or hub I believe. they may have damaged that sensor, which would cause your ABS and DSC lights to come on.

As far as making sure you have the exact same brands front and rear, that's ridiculous. They need to be the proper SIZE, but different brands will not make your ABS and DSC lights come on for any reason.

For example, right now my wife has 2 16" Michelin MXV4 Plus's up front with 2 Continental DWS 16"s in the back. All 4 are same size, but not same brand, and her fronts are worn down much more than her rears as those are older tires.

On my car, I have the staggered ZHP setup and have 2 225/40/18's in front that are Hankook Ventus V12 Evo 110K's and in the rear I have 2 255/35/18 Pirelli PZero Nero's. never had any issues with ABS or DSC lights on the dash with either setup.


Don't make me put you on block again. you aren't understanding correctly when I mentioned brand.

The reason why you want to ensure you have all four brands the same is 1) because you don't want to be ghetto (main reason) and 2) a Dunlop 225/40/17 might not be the same diameter as a Pirelli 225/40/17. Every tire is just a bit different. So if you have one tire thats worn way the hell down (and that particular brand tends to run on the small side) and you have another tire thats brand new (which runs on the large side) then you have just enough size different to push the rotation speed difference over the edge.

we wouldnt be having this conversation if OP had the correct tires of same brand and all were brand new. period.
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Old 10-03-2012, 01:59 PM   #35
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Can I just add that ABS systems don't use some set logic (on if "this", off if "that") to decide absolutely when to activate. It is a control system with pretty advanced logic and algorithms to measure some parameters and estimate others using the measurements. The math is pretty complex.

Anyway, the OP's problem has to be caused by the different diameter tires. Mango is saying that the brand is doing it, but I doubt that two brands would be off by more than 1-2 rev/mile. I bought my car with 2 brands on it and it behaves fine. I will get 4 new michilens when I can, but for now, the tires work. OP already admitted he has different diameter tires on front and rear axles. The problem lies there or maybe a wheel speed sensor.

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Old 10-03-2012, 02:09 PM   #36
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Can I just add that ABS systems don't use some set logic (on if "this", off if "that") to decide absolutely when to activate. It is a control system with pretty advanced logic and algorithms to measure some parameters and estimate others using the measurements. The math is pretty complex.

Anyway, the OP's problem has to be caused by the different diameter tires. Mango is saying that the brand is doing it, but I doubt that two brands would be off by more than 1-2 rev/mile. I bought my car with 2 brands on it and it behaves fine. I will get 4 new michilens when I can, but for now, the tires work. OP already admitted he has different diameter tires on front and rear axles. The problem lies there or maybe a wheel speed sensor.
no no. I'm not saying the brand IS doing it, I said it's a possibility. OP's description of his problem and his own knowledge of cars in general is vague at best. What I am saying is his particular cocktail of tire brands/sizes/etc seems to be a mess and he needs to do the proper thing which is to buy the correct tires spec'd by the manufacturer of matching brand and model for the BEST RESULTS because I don't give advice on anything else other than HOW TO ACHIEVE BEST RESULTS.

Do not mix and match your tire brand/models even if you CAN.
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Old 10-03-2012, 02:35 PM   #37
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ha, as we're having this conversation, my buddy's wife's E46 currently has 3 17" wheels on, probs different tires on some of them, and one 16" on the front. I think she's been rolling like that for quite some time now. No idea how or why.
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Old 10-03-2012, 03:45 PM   #38
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ha, as we're having this conversation, my buddy's wife's E46 currently has 3 17" wheels on, probs different tires on some of them, and one 16" on the front. I think she's been rolling like that for quite some time now. No idea how or why.
The size of the rim shouldn't be a factor in this, only the outside diameter of the tire. Still, I wouldn't run with a mix of different rim sizes.
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Old 10-03-2012, 04:01 PM   #39
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I've had 2 instances of different brands of tires causing issues. Bought the car with 2 different brands but same size on the front and the car would pull hard to the right. Had the car aligned with no improvement. Ended up being the tires. I replaced them with a couple of used RFT's to get me by and it was fine until the back tires needed replaced, so I bought some brand new non RFTs. On the drive home I noticed when I would go through a good left hand turn at a decent speed the DSC would kick on and pulse the left side front brake. It was real noticeable. I then went back to the tire place and had them replace the fronts as well. Morale of the story, buy brand new all around unless you want to have issues.
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Old 10-04-2012, 06:31 AM   #40
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I guess I could buy the same two tires for the back but I just bought those pirelli p6's in the back for $300. What am I supposed to do with those? I'm on minimum wage guys, halp!
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