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      08-05-2020, 11:06 AM   #23
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Quote:
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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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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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:
Originally Posted by DRLane View Post
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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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Originally Posted by dparm View Post
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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      08-17-2020, 08:56 AM   #34
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Quote:
Originally Posted by tsk94 View Post
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.
I know you're saying that, I've said it before too, but noticeable and real are not the same thing, and I trust Mayfield's take on this more than most people's so I'm looking forward to seeing what else he can find. I'm not going to fool myself that I know this platform as well as somebody who is actively trying to go faster in it AND has the perspective from outside the BMW bubble that I don't

I mean, I personally have noticed earlier interventions on a square setup from the stability control. If ABD and ABS are also running on high alert (meaning, just running along closer to an intervention threshold) because the rear wheel speed is also a little quicker than it would be on the stock staggered setup, it makes perfect sense to me what he's saying about how the system would react and what the consequences would be.

But he's right on the limit of what the car can do on any given lap, and is ALSO somebody who knows a thing or two about development, and most people just aren't coming at a car from that angle, so for most people, square just makes more sense as it's a very good, very easy button to smash, and you can set up a fast racecar around a square setup too
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      08-17-2020, 10:25 AM   #35
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Quote:
Originally Posted by Richbot View Post
I know you're saying that, I've said it before too, but noticeable and real are not the same thing, and I trust Mayfield's take on this more than most people's so I'm looking forward to seeing what else he can find. I'm not going to fool myself that I know this platform as well as somebody who is actively trying to go faster in it AND has the perspective from outside the BMW bubble that I don't

I mean, I personally have noticed earlier interventions on a square setup from the stability control. If ABD and ABS are also running on high alert (meaning, just running along closer to an intervention threshold) because the rear wheel speed is also a little quicker than it would be on the stock staggered setup, it makes perfect sense to me what he's saying about how the system would react and what the consequences would be.

But he's right on the limit of what the car can do on any given lap, and is ALSO somebody who knows a thing or two about development, and most people just aren't coming at a car from that angle, so for most people, square just makes more sense as it's a very good, very easy button to smash, and you can set up a fast racecar around a square setup too
I understand that noticeable and zero aren't the same thing. But for me, if there is no discernible difference, then even if there is a real difference, it doesn't concern me.

If I run the 275/295 combo again this season, which I likely will, I will compare my Aim data to that of running the 275 square to see if there is any difference in ABS intervention between the two.

This is also assuming that it would be a negative impact, until both sets are analyzed it could that it's actually a benefit. Remember, the stock wheels came with a diameter offset, perhaps running a square setup is more detrimental to ABS performance than a diameter offset between the front and rear.
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      08-17-2020, 02:48 PM   #36
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Like DRLane my Aim data doesn't register ABS activation . Thus I'm not sure of a way to compare (easily) ABS differences between the two sets. If anyone knows of a method let me know and I'll look into it once I get the data from running the 275/295 stagger again in 2 weeks.
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      08-17-2020, 03:31 PM   #37
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Originally Posted by tsk94 View Post
Like DRLane my Aim data doesn't register ABS activation . Thus I'm not sure of a way to compare (easily) ABS differences between the two sets. If anyone knows of a method let me know and I'll look into it once I get the data from running the 275/295 stagger again in 2 weeks.
I was hoping i was just doing something wrong. Whomp...

I'm at 16 heat cycles on mine. When these cycle out I'm going to replace the 295 with 305/35s and send this diameter variance in the opposite direction.
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      08-17-2020, 06:43 PM   #38
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I would expect to see a difference in Longitudinal G-force more than any difference in ABS activation, or I guess more accurately I would expect to see ABS activation at a lower Long G threshold. The slip thresholds and targets for ABS are not very big, and are tuned to the stock tires. This is why some cars are super susceptible to "Ice Mode" and others are not, even when modified. It also means that when you change stuff, you may be using up a lot of that threshold so that when it does engage it's falsely detecting wheel slip because of the difference in wheel speed from diameter as opposed to wheel slip.

I work on the OEM side of things so I have a little more perspective on the capabilities (and limits) of the stock road-legal ABS systems. I've also experienced proper motorsport ABS and that's so far ahead of what we get to deal with on our street-based cars, it's astonishing.
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      08-22-2020, 04:17 PM   #39
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In the context of ABS activation, I was wondering if 275/30/19 and 275/35/19 maintain square contact patch but allows for "staggered" diameter. I was using this combinations for the first two trackdays this year it wasn't bad
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      08-29-2020, 12:49 PM   #40
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@tsk94 Hi man, what pressures were you running with the RRs? We're you relatively close to stock weight?
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      08-29-2020, 12:58 PM   #41
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@tsk94 Hi man, what pressures were you running with the RRs? We're you relatively close to stock weight?
I'm currently running 34-35 hot, still experimenting a bit. I've tired 32-38 and found 34-35 to be the sweet spot for my car given its alignment, weight, etc.

My car is ~3350 with driver. A bit lighter then stock but not much.
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      08-29-2020, 12:58 PM   #42
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@tsk94 Hi man, what pressures were you running with the RRs? We're you relatively close to stock weight?
I'm currently running 34-35 hot, still experimenting a bit. I've tired 32-38 and found 34-35 to be the sweet spot for my car given its alignment, weight, etc.

My car is ~3350 with driver. A bit lighter then stock but not much.
Super helpful ! Thx.
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