Tire size and ABS

[tmoretta]

I am using 17 inch Team Dynamic Motorsports wheels on my 818S - (8" and 9"). I am considering using 215 40 17 front tires and 255 40 17 rear because they are pretty widely available in various brands and will provide a nice stagger. I am concerned about the potential effect that the difference in diameter might have on my ABS system. The front tires will measure 23.8 inches diam. and the rears 25 inches. Is this difference likely to cause my ABS system to engage? I really want the safety margin that the system will provide and it has been a lot of extra work to install. And yes, I know how to threshold brake - I tracked a BMW 2002 for many years.

[Hindsight]

I posted this question a while back. Through replies, found out ABS in Subies activates at 8% differential so do the math and add a little padding for differential speeds due to corners and you'll get the answer.

[Bob_n_Cincy]

I am using 17 inch Team Dynamic Motorsports wheels on my 818S - (8" and 9"). I am considering using 215 40 17 front tires and 255 40 17 rear because they are pretty widely available in various brands and will provide a nice stagger. I am concerned about the potential effect that the difference in diameter might have on my ABS system. The front tires will measure 23.8 inches diam. and the rears 25 inches. Is this difference likely to cause my ABS system to engage? I really want the safety margin that the system will provide and it has been a lot of extra work to install. And yes, I know how to threshold brake - I tracked a BMW 2002 for many years.
This is out of the of Subaru manual. Your difference is 5%
Bob
http://thefactoryfiveforum.com/attachment.php?attachmentid=33328&d=1378845093

[tmoretta]

So then, does difference in diam. of tires front to rear have any effect on slip angle?

[Bob_n_Cincy]

So then, does difference in diam. of tires front to rear have any effect on slip angle?
Yes, On your setup it will think that the 2 front tires are 2.5% faster than average speed and the rears are 2.5% slower than average.

[tmoretta]

Being as 5% is less than 8%, am I then safe? This is such a conundrum. Guys have experienced rubbing with 215 45 17 tires, so what is the biggest 17" tire I can use in the rear, in conjunction with the 215 40 17 front size that will not effect my ABS system?

[Hindsight]

I would think so. I'm going to be at 4-5% with my current planned tire sizes. It doesn't leave much for differences in speed when going around corners but then again, you shouldn't be hard on the brakes in a very sharp radius corner anyway. Most ABS-assisted braking would be done pointing straight ahead (either in a panic situation or before a turn entrance).

Biggest issue right now is that no one has proven it works or doesn't. Maybe one or two people have 818's on the road with ABS but they haven't had time to do an ABS stop check and report back, and I don't know if either one has the ABS module powered up right now either. Should have an answer within the next 3-4 months though, as more of the folks wrap their builds up and have time to report back.

[bbjones121]

different tire diameter does not matter for the ABS system

[bbjones121]

http://thefactoryfiveforum.com/showthread.php?9650-Keep-the-ABS-brakes-on-the-818R

[Hindsight]

different tire diameter does not matter for the ABS system

I am interested to understand this. I understand the ABS systems determine wheel slippage by measuring the difference in pulses between each of the four wheel speed sensors. If you have a small diameter tire up front and a big one in the back, the wheel sensors will detect this and at a certain percentage of difference, the ABS computer will determine this is caused by slippage and will activate accordingly. If what you are saying is true, how can the computer tell the difference between tires rotating at different speeds due to different diameter vs due to slipping?

[Boog]

different tire diameter does not matter for the ABS system

The question being asked is not if the ABS system will engage and work, but rather if the ABS will activate during normal driving conditions.

Random simplified example: Going around a curve with a 50ft radius on a car 5ft wide, the inner wheels are turning 11% faster than the outer. If in addition to the difference in inner to out wheel speed, you had front tires that are 5% smaller in diameter than the rear, the speed difference between the outer rear tire and inner front tire becomes 16%. Obviously the ABS system wont activate unless you were trail braking into the turn, but the system could potentially turn on when there was no loss of control and just a speed mismatch.

People (like myself) are wondering what difference in front vs rear diameters are acceptable for real world spirited driving without having the system engage when undesirable. I would like to have a larger rear to get a lower cruising RPM, but the front seems to run into sizing problems pretty quickly.

BTW, I enjoyed the thread you linked. Great discussion on what happens during engagement.

[bbjones121]

All four wheels are spinning different in a turn. The ABS should easily be capable of watching for large differentials in rate of pulses by looking at each wheel independent of one another.

[bbjones121]

The system engages when it senses pulses from one wheel changing at different rate than others. I believe diameter deference should be insignificant.

[Boog]

The system is based on rate of change of velocity (acceleration) for each tire, looking for an outlier?
That makes a lot of sense, thanks.

[bbjones121]

Here is good info.https://www.google.com/url?sa=t&source=web&rct=j&ei=i9oPVIXZK8_KgwTWqoCICA&url=http://www.autoshop101.com/forms/brake09.pdf&cd=6&ved=0CCgQFjAF&usg=AFQjCNFCs-i0kMYsWe8Rhw0uktsUGOVbLw&sig2=xWi5TylcAWrTEi68Rpm1OQ

[Evan78]

So which one of us is going to make and sell new ABS tone rings to fool the computer?

[AZPete]

Why try to fool the computer? ABS activates when a wheel quickly stops turning, regardless of any other wheels. ABS would work on a unicycle.
It is a Traction Control system that is activated by differentials between wheels.

[Evan78]

I read the "Keep the ABS brakes on the 818R?" thread that bbjones121 linked to both when it was active and just skimmed it, but I don't see any obvious conclusion regarding ABS operation with regard to wheel sizes. Did I miss it?

The Subaru doc that Bob posted says peak friction occurs at 8-30% slip and the Lexus/Toyota doc (http://www.autoshop101.com/forms/brake09.pdf) that bbjones121 linked to contains the following (emphasis mine):


The chart below shows the slip tolerance band (shaded area) in which the
most efficient braking occurs. From a slip ratio of zero (0), at which the
wheel speed and the vehicle speed are equal, to a slip ratio of 10, braking
is mild to moderate and good traction between the tire and the road
surface is maintained. Between slip ratios of 10 to 30 the most efficient
braking occurs. This is where the tires are at a point where they may
begin to lose traction with the road surface. This is also the band in which
ABS operation occurs. Beyond a slip ratio of 30%, braking efficiency is
reduced, stopping distance is increased and directional control is lost.

This illustrates that ABS is not as black and white as releasing a locked wheel. There is a range of slip between full traction and full lockup. This also implies (to me anyway), that differential tire sizes may have a noticeable impact on ABS performance. Of course it will still function, but how far from optimal will it be?

This brings up an old memory of one of my first experiences with ABS. I was in a parking lot in a 1991 Mitsubishi 3000GT VR-4 when I hit the brake pedal while making a fairly sharp turn at low speed. The ABS kicked in at a much lower threshold than I expected. Perhaps this was due in part to the large differential in wheel speeds, even though I was only going somewhere between 5-15 mph I believe. I thought "That's weird, dry asphalt, low speed, why the heck did ABS engage?" I'm sure steering angle is part of the logic in at least some systems now, but probably not at least some of the Subaru systems of the 2000's.