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      09-19-2019, 07:24 PM   #1
GreyAggie
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What does the total diameter difference need to be?

I think I remember reading somewhere that the rear tire total diameter had to be slightly larger than the front for the car to not think the traction is slipping. Does anyone know what that ratio needs to be? I'm looking at a setup that is 26.2" in the rear and 26" in the front.

Would that work?
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      09-19-2019, 10:58 PM   #2
PilotxERAU
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Quote:
Originally Posted by GreyAggie View Post
I think I remember reading somewhere that the rear tire total diameter had to be slightly larger than the front for the car to not think the traction is slipping. Does anyone know what that ratio needs to be? I'm looking at a setup that is 26.2" in the rear and 26" in the front.

Would that work?
There is a chart with all the math in one of these threads. I saved a screenshot somewhere. 275/30 295/30 matches rolling diameter of stock setup.
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      09-20-2019, 11:40 AM   #3
GreyAggie
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Quote:
Originally Posted by PilotxERAU View Post
Quote:
Originally Posted by GreyAggie View Post
I think I remember reading somewhere that the rear tire total diameter had to be slightly larger than the front for the car to not think the traction is slipping. Does anyone know what that ratio needs to be? I'm looking at a setup that is 26.2" in the rear and 26" in the front.

Would that work?
There is a chart with all the math in one of these threads. I saved a screenshot somewhere. 275/30 295/30 matches rolling diameter of stock setup.
I'm not trying to match the overall diameter. I'm trying to duplicate the diameter difference ratio. If the tires fit, they're good. But if the ratio is off and the car thinks it's losing traction during normal driving, that's a problem.

I'd love to see that chart if you find it.
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      10-13-2019, 04:21 PM   #4
doughboy
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Quote:
Originally Posted by GreyAggie View Post
I think I remember reading somewhere that the rear tire total diameter had to be slightly larger than the front for the car to not think the traction is slipping. Does anyone know what that ratio needs to be? I'm looking at a setup that is 26.2" in the rear and 26" in the front.

Would that work?

1. That's not right.TC uses the dynamic change of relative wheel speed, not absolute static relative speed.

2. When you reset your TPMS that zeros out wheel size differences due to wear and pressure.

i.e. worn rears / new fronts = 12mm diameter diff (1/2 inch) so talking about 0.2" pointless.

....and that's before you think about pressure which changes diameter even more.
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      10-14-2019, 01:39 PM   #5
MetalMK
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Many people run square setups and I have not heard their TC kicking in during normal driving. What matters more should be avoid rubbing.
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      10-15-2019, 06:35 PM   #6
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This is a great question, and and there is a lot of confusion going around as some people get this confused with the xDrive requirement, while others think you can only stray so far from the stock diameter/ratio before running into problems.

In general, RWD F-chassis BMWs don't care what your rolling diameter is. If you change it to something more than ≈2% different than stock (larger or smaller) you may feel more DSC intervention at first, but the car will eventually adapt and drive like normal. This is common when installing a square setup for the first time on a car that originally came with a staggered setup using a larger rear rolling diameter than the front.

The only thing that will actually change is the speedometer reading, which isn't typically a big deal since they read high from the factory anyway. I'd recommend using a GPS speedometer app on the first drive to get an idea of how far off the reading is at 30 and 60 MPH and keep those readings in mind.

--Dylan
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      10-16-2019, 03:35 AM   #7
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2% is nothing when new to worn diameter can be around 2% even on matching tyres.

Then add in uneven pressures and vehicle loading will be even more, coulf be over 5% mismatch easy even on same sizer tryes.

The car will just suck it all up when you reset the TPMS.
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      10-17-2019, 06:51 AM   #8
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Quote:
Originally Posted by doughboy View Post
2% is nothing when new to worn diameter can be around 2% even on matching tyres.

Then add in uneven pressures and vehicle loading will be even more, coulf be over 5% mismatch easy even on same sizer tryes.

The car will just suck it all up when you reset the TPMS.
OP,

FYI - the M2 systems are able to cope with varied tyre combos F&R.

IMHO its a non-issue, as OEM seasonal tyre swaps demonstrates:-

summer - staggered tyre sizes ie 245/265;
winter - either square or staggered sizes ie 235F&R or 235/255F&R.

Just my 2c worth,

BP
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      10-17-2019, 09:19 AM   #9
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Quote:
Originally Posted by Bee Pee View Post
OP,

FYI - the M2 systems are able to cope with varied tyre combos F&R.

IMHO its a non-issue, as OEM seasonal tyre swaps demonstrates:-

+1
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