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      08-05-2020, 11:06 AM   #23
pbonsalb
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Originally Posted by dparm View Post
There are several different tire sizes on the car from the factory (18", 19", and the 19" GTS). Is that programmed into the ABS computer anywhere?
I think you are confusing tire sizes with tire diameters. You can have different tire and wheel sizes but the same diameter. For example a 265/40/18 and a 265/35/19 are the same diameter. ABS reads wheel speed, and it wound not change with those two examples.

However, I think you can change diameters without consequence as long as you maintain the same front to rear differential.

And again, the differential % to trigger activation may be enough that you are unlikely to experience any issues unless you have a pretty significant difference in diameters.

I suspect the GTS may have different programming if its diameters are different.

In addition to ABS, traction control would be similarly affected. Not an issue for track people who turn it off.

Interesting thought on BMWs recommended winter tire size — that it is chosen to trigger activation sooner for safety reasons. I don’t know what the recommended winter sizes are — I run 255/40/18 rear and 235/40/18 front snows on stock wheels.
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      08-05-2020, 11:12 AM   #24
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Quote:
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Originally Posted by dparm View Post
There are several different tire sizes on the car from the factory (18", 19", and the 19" GTS). Is that programmed into the ABS computer anywhere?
I think you are confusing tire sizes with tire diameters. You can have different tire and wheel sizes but the same diameter. For example a 265/40/18 and a 265/35/19 are the same diameter. ABS reads wheel speed, and it wound not change with those two examples.

However, I think you can change diameters without consequence as long as you maintain the same front to rear differential.

And again, the differential % to trigger activation may be enough that you are unlikely to experience any issues unless you have a pretty significant difference in diameters.

I suspect the GTS may have different programming if its diameters are different.

In addition to ABS, traction control would be similarly affected. Not an issue for track people who turn it off.

Interesting thought on BMWs recommended winter tire size — that it is chosen to trigger activation sooner for safety reasons. I don't know what the recommended winter sizes are — I run 255/40/18 rear and 235/40/18 front snows on stock wheels.
You saw his follow up post with the literal diameter measurements?
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      08-05-2020, 11:31 AM   #25
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This setup has less than a 3% differential in overall tire diameter between the front and rear. The modern ABS modules are sophisticated enough to compensate for this small change without being detrimental to performance. As some have mentioned, the stock wheel and tire packages for this car had different diameter front and rear tires - just the rears being larger. But, the car also has a recommended square winter setup which eliminates this stagger in diameter.

We're talking about <5% change in diameter, it's not going to be an issue. I ran this exact tire setup for most of last season and coming from a square 275 NT01 setup there were ZERO noticeable affects on ABS performance.
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      08-05-2020, 12:36 PM   #26
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Quote:
Originally Posted by DRLane View Post
You saw his follow up post with the literal diameter measurements?
Yes and I looked them up as well and found the factory 18s and 19s to be virtually the same:

265/40/18 is 26.3”
265/35/19 is 26.3”
245/40/18 is 25.7”
245/35/19 is 25.8”

The percentage or mph differential for the stock sizes are what will be in the BMW programming for ABS.
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      08-05-2020, 12:40 PM   #27
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Quote:
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Originally Posted by DRLane View Post
You saw his follow up post with the literal diameter measurements?
Yes and I looked them up as well and found the 18s and 19s to be virtually the same:

265/40/18 is 26.3"
265/35/19 is 26.3"
245/40/18 is 25.7"
245/35/19 is 25.8"
Gotcha - You were emphasizing OEM sizes per the 18,19, GTS comment.

Obviously, he was speaking to the original question as it relates specifically to the RR in 275/35/18 and 295/30/18.
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      08-05-2020, 12:59 PM   #28
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Does the AIM Solo DL have access to wheel speed sensors? You could probably run some pretty simple tests on a deserted road with various wheel/tire sizes to see how the ABS responds differently. Maybe something along the lines of five panic stops from 60mph on the same stretch of pavement with each set of tires; check the graphs to see how soon ABS engages, how the wheel speeds vary during the stop, etc.

Even better if you can see the brake pressures at each wheel.

I really do think that a 3% difference, as tsk94 said, is negligible and the ABS computer will be fine.
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      08-05-2020, 01:00 PM   #29
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Quote:
Originally Posted by dparm View Post
Does the AIM Solo DL have access to wheel speed sensors? You could probably run some pretty simple tests on a deserted road with various wheel/tire sizes to see how the ABS responds differently.

Hell, go pump the pressures up to like 40 psi on the stock wheels and run some tests.
Yes it should. But it should also have the ABS triggers and it's not registering that activity. Need to do some digging... Thx man
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      08-05-2020, 05:17 PM   #30
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Quote:
Originally Posted by DRLane View Post
Quote:
Originally Posted by dparm View Post
Does the AIM Solo DL have access to wheel speed sensors? You could probably run some pretty simple tests on a deserted road with various wheel/tire sizes to see how the ABS responds differently.

Hell, go pump the pressures up to like 40 psi on the stock wheels and run some tests.
Yes it should. But it should also have the ABS triggers and it's not registering that activity. Need to do some digging... Thx man
Do you see anything when looking at brake pressure?
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      08-05-2020, 06:15 PM   #31
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Quote:
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Quote:
Originally Posted by DRLane View Post
Quote:
Originally Posted by dparm View Post
Does the AIM Solo DL have access to wheel speed sensors? You could probably run some pretty simple tests on a deserted road with various wheel/tire sizes to see how the ABS responds differently.

Hell, go pump the pressures up to like 40 psi on the stock wheels and run some tests.
Yes it should. But it should also have the ABS triggers and it's not registering that activity. Need to do some digging... Thx man
Do you see anything when looking at brake pressure?
Yes - And I acknowledge both the sloppy brake pressure and coasting at turn 4 after the straight 😳. I'd like to blame it on traffic but can't be certain.
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      08-05-2020, 06:42 PM   #32
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Dumb question but was ABS maybe not engaging? With good tires it's very possible you won't get into the ABS because you have tons of grip available.

And yes, do you even trailbrake, bro? ;-)
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      08-05-2020, 07:54 PM   #33
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Quote:
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Dumb question but was ABS maybe not engaging? With good tires it's very possible you won't get into the ABS because you have tons of grip available.

And yes, do you even trailbrake, bro? ;-)
burn, haha

Well, I'm certainly not trying to engage ABS. There were a couple spots in other laps that I def did engage ABS. I'll try to fish out an example.
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