Wheel size and ABS

[tmoretta]

I am reading with great interest everyone's opinion and FFR suggestion on wheel/tire size for 818. Not to throw in a new wrinkle, but, I want to maintain the WRX ABS system on my 818. As far as I know, the system requires that all four corners have the same rolling circumference. How can this be accomplished while mounting wider tires in the rear?

[JeromeS13]

By keeping the same/very close overall tire diameter.

[Goldwing]

Yep, use a wider tire in the rear with the same (or very close) circumference as the front. There are tire size calculators around the Internet or app stores to help verify dimensions. I haven't figured out the front hub setup yet, but definitely don't delete any toothed rings up there as the ABS uses that to measure speed with a closely mounted sensor. Most systems kick ABS in with a certain, say 10%, difference in wheel speed wrt other wheels. I have no idea where Subaru kicks in, but you would need to keep diameter differences within that threshold or look into changing the tooth count on the toothed ring to show the same rolling wheel speed to the system. My plan is to use ABS as well.

[Xusia]

Hi tmoretta, and welcome! That is my plan also (to use ABS). I also want the speedo to remain accurate (I want to avoid re-calibration). These choices have led me to try and find wheel and tire sizes that are close to stock. As a result, I put this together:

https://docs.google.com/spreadsheet/ccc?key=0AhG99PnZiZFHdGZJVFhJVm5hanp0QkxONTAwNXQwd 3c&usp=sharing

[RM1SepEx]

Actual tire dia do not usually match the theoretical numbers... In real life tires from diff mfgs vary greatly even tho they have the same size molded into the sidewall. So you need to check the tire on the rim to find the true rolling dia.

also using a tire on a wider rim makes the diameter smaller and on a narrow rim the diameter larger. Add that to the handling diff related to tire width and rim width and we have a ton of evaluation work to do...

[tmoretta]

Hi tmoretta, and welcome! That is my plan also (to use ABS). I also want the speedo to remain accurate (I want to avoid re-calibration). These choices have led me to try and find wheel and tire sizes that are close to stock. As a result, I put this together:

https://docs.google.com/spreadsheet/ccc?key=0AhG99PnZiZFHdGZJVFhJVm5hanp0QkxONTAwNXQwd 3c&usp=sharing

Good work!! Thanks so much.

[Frank818]

Xusia, could you put the diameter (in inches) in your google chart? Using OEM tires as much as possible, though certainly a couple of brands have been used. I can tell only one, the Bridge Potenza RE92 (what a **** crap tire loll) on 05 NA Impreza RS in Canada.

[Xusia]

Yeah, sure - I'll do it later today. Keep in mind (as RM1SepEx pointed out), these numbers are theoretical/calculated - meaning actual numbers may vary somewhat. Still, I think it's a good guide.

EDIT: Done!

[Frank818]

Tnx buddy!! :)

[tmoretta]

Hi tmoretta, and welcome! That is my plan also (to use ABS). I also want the speedo to remain accurate (I want to avoid re-calibration). These choices have led me to try and find wheel and tire sizes that are close to stock. As a result, I put this together:

https://docs.google.com/spreadsheet/ccc?key=0AhG99PnZiZFHdGZJVFhJVm5hanp0QkxONTAwNXQwd 3c&usp=sharing

Anybody know how much variation in rolling circumference of tires the Subaru ABS system will allow before sensing a problem? It is pretty difficult to choose a staggered front/rear tire and wheel combination for the 818 maintaining speedometer accuracy and allowing the ABS to work properly.

[Bob_n_Cincy]

Anybody know how much variation in rolling circumference of tires the Subaru ABS system will allow before sensing a problem? It is pretty difficult to choose a staggered front/rear tire and wheel combination for the 818 maintaining speedometer accuracy and allowing the ABS to work properly.

In the 2006 factory service manual it says the ABS tries to maintain 8 to 30% slip.
I read that as it does nothing with less than 8% error.

if you had a 24" tires in front and 26" in back the 2/24= 8.3% (problem)
if you had a 24" tires in front and 25.8" in back the 1.8/24= 7.5% (no problem)

I don't know how ABS handles corners because you could easily have 20% difference left to right.

I will run ABS with a key swith to turn it off at the track.
See below
Bob

21399

[RM1SepEx]

ABS does each wheel independently... so wheel stagger doesn't matter.

When you start using the wheel speed sensors for traction control... you have a problem and even slight variations in tire wear are an issue. Chew a tire on a newer BMW IX and a ruined tire means an entire set of new tires. (they use the brakes to control traction with an open diff.) No wonder rear brakes are high wear items on new traction controlled cars.

[tmoretta]

ABS does each wheel independently... so wheel stagger doesn't matter.

When you start using the wheel speed sensors for traction control... you have a problem and even slight variations in tire wear are an issue. Chew a tire on a newer BMW IX and a ruined tire means an entire set of new tires. (they use the brakes to control traction with an open diff.) No wonder rear brakes are high wear items on new traction controlled cars.

I too was thinking/recalling that the ABS system senses impending lockup, not difference in rotational speed of the four wheels. Anyone have any official literature/answer to this basic question.?

[Frank818]

Yup, looks like the ABS doesn't check the difference between wheels.

http://auto.howstuffworks.com/auto-parts/brakes/brake-types/anti-lock-brake1.htm

The controller monitors the speed sensors at all times. It is looking for decelerations in the wheel that are out of the ordinary. Right before a wheel locks up, it will experience a rapid deceleration. If left unchecked, the wheel would stop much more quickly than any car could. It might take a car five seconds to stop from 60 mph (96.6 kph) under ideal conditions, but a wheel that locks up could stop spinning in less than a second.

The ABS controller knows that such a rapid deceleration is impossible, so it reduces the pressure to that brake until it sees an acceleration, then it increases the pressure until it sees the deceleration again. It can do this very quickly, before the tire can actually significantly change speed. The result is that the tire slows down at the same rate as the car, with the brakes keeping the tires very near the point at which they will start to lock up.

[Goldwing]

The depth of knowledge and info in this community truly impresses me sometimes! What an incredible resource this forum is for our builds.

[ssssly]

If it is just a straight ABS system, it solely monitors wheel deceleration speed. So the only concern with mismatched wheel sizes is the actual effect it will have on handling and breaking. ABS doesn't care about it though.

If it has the EBD system, your rotating circumference comes into play. Because the EBD system monitors wheel speeds in reference to the other wheels. Specifically it averages them to compare against vehicle speed and steering angle to calculate slip and under vs oversteer.

As long as they aren't radically different, +/-10% shouldn't have much an impact if any.