wanklin

iTrader: (2)

It is common advice for those considering new tire purchases to choose wheels that will maintain near stock tire height because not doing so will cause ABS issues.

To this I ask, on what is this advice not to deviate from the stock 24.7-24.9" tire diameter founded?

From the very limited reading that I have done on Sumitomo patent relating to ABS system design (linked below) and sensory value derivation, I find nothing supporting this advice not to deviate tire heights (and thus the sine wave frequency per given speed). Keep in mind that it really doesn't matter who the manufacturer is because the determinants are still the same - that is that the 3rd gen RX-7's sensory capability is confound to 4 abs sensors linked to a computer, which controls a pump/mod assembly. In all honesty, I have not come to a final conclusion yet on the threshholds that determine which wheels are "accelerating" relative to derived vehicle speed (taken as a weighted average of samples from all 4 wheels), but I am pretty well convinced that tire overall diameter means nothing to the ABS computer, because how can it? Rather, disparity of diameter is the issue to be concerned about.

Consider my scenario: I have decided to use 315s in the rear and 275s up front in my vehicle (with proper inner fender mods). The rear wheels will be 25.66" and the fronts will be 25.68" tall; large compared to stock heights, but very similar relative to each other. The common advice for my scenario would sound something like, "don't do it!" Fair enough ;o)

Now, it's pretty obvious that each revolution of the sensory wheel now reresents a larger distance of travel than stock. IOW, my new wheels tuning at 500 RPM now travel further in .6 seconds than the stock wheels would have.

As exciting as that is, the above really means nothing to the ABS computer, because it is not a speedometer. 24 pulses (a guess of how many teeth are on each wheel) still represent one revolution of the toothed sensor wheel, regardless of how far the car travels in that one rotation. The sine waves coming from the sensors are all that the computer has to go by, and since the computer is not attached to an acceleromoter or GPS, it knows no difference.

However, that is not to say that the ABS does not care about rotational speed and acceleration. From what I gather (being careful not to assume that I know everything about every system) from my very limited reading, the RX-7 ABS computer only uses wheel RPM data and pressure data in making its decisions. It derives a weighted average of the vehicles velocity by taking frequency samples from all 4 wheel sensors and then uses this sensory information to analyze the road conditions and establish a dynamic weighted avergage speed as a threshhold to measure individual wheel speeds against. A wheel/s that is/are found to be accelerating (slowing down - locking up) will be addressed by the ABS computer after it performs an analysis of the cylinder pressure and utilizes an algorythm to decrease pressure to the locking wheel in open loop (keep in mind that the earlier 3rd gens are 3 channel so the rear wheel signals are mixed and treated as one). Again, the computer doesn't care how fast the vehicle is traveling, it just cares how quickly the wheels are rotating relative to one another.

Accepting this (which I don't expect anyone to based off my word alone), it seems clear that larger diameter tires make no functional difference in regards to ABS, as long as you are not staggering your tire heights excessively.

Now, if one were to put monster truck tires in the rear and skate board tires up front the rear wheels would necessarily rotate at a lower RPM than the front wheels and would therefore convince the ABS computer that near lockup is occurring in the rear. With this being said, it seems that most of the algorythms and sophisticated valve functions are more concerned about right vs. left disparity than front rear, but that's getting into an area that is beyond this discussion.

My thoughts as of now (which are always open to criticism) are that you can use whatever tire sizes you want as long as they are all roughly the same diameter. Ofcourse I am basing this on sparse data, but unfortunately that's all we really have to go by. Your thoughts?

One of the sources:
http://www.patentstorm.us/patents/5403075-claims.html

rynberg

Well I agree with your findings but I don't think that is something that people actually think around here. I've never dissuaded people from changing diameter due to ABS issues, and I've never seen any other knowledgable people in this forum doing so either.

The real problem with increasing diameter is dealing with fender clearance issues.

Damn, you are running some big meats!

wanklin

iTrader: (2)

Hmmm maybe I'm on crack, but I know I've seen this advice many times over through the yrs, but all I can find is this now: lol - https://www.rx7club.com/showpost.php...3&postcount=28

ptrhahn

iTrader: (9)

I've run virtually every combo out there (except for yours Rob), and I think everyone is WAY too **** retentive about diameter relative to ABS/speedo issues. It just hasn't proved to be an issue.

You can go up to 25.7" (275/40/17), particularly in the rear, with no issues. The only real problems occur up front, as Rynberg says, with fender clearance.

trickshot

Doesn't anyone care about unsprung weight and rotational inertia with these big tire and wheel combos? I don't follow the bigger is always better school of thinking.

mad_7tist

do not confuse traction control with abs. the abs will not care if the backs spins faster or slower as long as there are pulses coming in from the sensors. no pulses = no rotation = abs activation (so long as the abs comp senses brake pedal application

a traction system will crae about rotational speed from side to side etc... brake pedal application is an important for abs.
three states of abs
isolate (senses lock up and will isolate the affected wheel from further pedal pressure)
dump( will bleed off pressure to allow the wheel to turn slightly)
reapply(allow master cylinder pressure to the brake in order to modulate pressure at the skid threshold)

wanklin

iTrader: (2)

That's not correct. The ABS starts working long before lockup occurrs and it does care about rotational speed. If the rear wheels rotated at a rate that was slow enough to surpass the systems algorythmic slip threshhold then the system would respond by decreasing pressure to the rear cylinders. Also, There are status signals, triggers etc that are constantly changing as the car rolls about and the ABS computer is always monitoring wheel RPMs to analyze the road surface (ie: split road surface). The only time anyone pays attention to ABS is when the pedal is being stomped, but it is actually at work every time your foot touches that brake pedal.

wanklin

iTrader: (2)

You're absolutely right, but it isn't always ideal to follow the OEM is always optimal school of thinking either (not saying that you necessarily believe this).

I am actually in agreement with you to some extent and that is part of the reason I settled for 315's and didn't go with some extreme petit flare 335+ setup w/ 315's up front.

I like the 275s because they have nice side wall height and plenty of contact patch surface area to remain effecient during hard cornering and braking. The 315s offer no more "friction" (achieved on a molecular level) than a 255, but the added traction (acheived through tread grip - IOW the tires pushing off of the tiny protrusions in the road surface) through makes sense for the amount of torque that my LS6 will be putting down. Is increased unsprung weight a tradeoff? yes it is, but at the end of my cost-benefit analysis a 275/315 setup will return a net gain (in my scenario). Certainly I wouldn't recommend it for a 255RWHP car....

The setup is dialed in and balanced, now I just have to finalize my front offsets to get beck to zero scrub.

mad_7tist

you are right for a more modern ebtcm (abs with trac)but our simple abs system just looks for tone pulses. the car does not have the ability to learn road surface, steering input, tps etc. more modern cars do but i was under the impression this was about the fd not a general abs/trac thread. if you hit the brakes (without lockup)and intercepted the tone ring signal for the front left wheel the car would dump all the pressure to that wheel because it would "think" it was locked up. the slip threshold is there but it is still a near binary system.

wanklin

iTrader: (2)

Where did you get that information? I would love to find the technical publications for our specific system.

azn akira

abs computers compare the wheel speed between wheels. if one wheel is slowing down faster than another then it releases the pressure to that wheel until its slowing at the same rate as the other wheels. so if you increase your wheels sizes it wont affect your abs system since they are still rotating at equal speeds.

wanklin

iTrader: (2)

Did you even bother to read the first post? lol

azn akira

i glanced over it, you rambled on too much for such a simple question. i was just agreeing that changing the tire size does not affect the abs system...

wanklin

iTrader: (2)

Thanks for the info

Nathan Kwok

I agree Wanklin. The only problem I could see is if the left rear and right rear are different diameters which would never occur anyway. The bigger issue when it comes to brakes is having big brake kits and how that affects brake bias. I've seen lots of tests (unfortunately not much on RX-7s) where the big brake kits actually hurt stopping distances.

azn akira

im really curious to see how the racing brake bbk performs as it retains the stock brake bias.

DamonB

From the Suspension and Handling Links sticky:

ABS Brakes. A great article that explains how and why ABS braking systems work. Written by a Bosch ABS system engineer.


Long story short an ABS system is not the "perfect" braking system we tend to think it is. When the controls and software are designed some suppositions must be made; tire diameters being one. Just because the ABS system won't allow a wheel to lock does not mean the car is still stopping at it's optimum rate. You can easily continue to have a functional ABS system that doesn't lock tires and yet have your stopping distances increase because the suppositions built into the controls and software are now no longer valid.

The ABS sensors compare wheel speeds via axle RPM. If the rolling diameter of the tires are changed from what the software was designed for the ABS system is now not properly tuned. It's no different then performing engine mods without remapping the ecu.

ptrhahn

iTrader: (9)

"these big wheel/tire combos" often don't weigh that much more than the stock wheels, if you get good forged wheels... and particularly if you get lightweight tires like Toyo or Pirelli that are the outermost portion of the rotating mass.

The increase in grip more than makes up for the slight weight penalty. 225/50/16 just doesn't cut it for a modern 400+ horsepower car.

ptrhahn

iTrader: (9)

Stop Tech already has piston configs to maintain the stock bias w/ either the OE or '99 rears. No need for a 929 master, bias adjustment, or different pad compounds.

wanklin

iTrader: (2)

Ahh, fair enough, thanks for the link Damon. It seems that just about every braking or suspension mod under the sun will affect ABS. One factor to take into account is the increased braking leverage offered by a BBK which will counteract some of the added wheel inertia. I think are units are made by Sumitomo (maybe someone can pop their hood and confirm) and it is conceivable that they take a different approach in designing their algorythms, though I don't see why they would.

Chris C said that his ECU dealer also sold aftermarket ABS units from Bosch at $10K a pop which I'm assuming are programmable. I guess I'll just take my chances with my big wheels, BBK and stock ABS tune.

wanklin

iTrader: (2)

Another factor to take into account I suppose is that the ABS my start bleading pressure from the cylinders early assuming that the longitudinal/lateral aggregated slip is reaching its stability threshhold when in fact the wider and perhaps softer tires are not even close to slipping.

Lol. This can give you a headache because the above assumption ignores the monitoring of RPM and acceleration.

My basic understanding at this point is that the sensors are used in making the decisions when and how to react through hydraulic manipulation, and the programming inside the box predetermines how much pressure should be varied (IOW it controls the valving) in accordance with some approximated reaction matrix for all of conceivable input levels and scenarios.

At the end of the day I like ABS because it keeps me from spinning out or losing control, not necessarily because it makes me stop 3-10 feet earlier, or whatever.

ptrhahn

iTrader: (9)

^^^^^^^

I can get a mild chirp/squeal of tires under braking indicating that they are near the adhesion limit (similar to what you'd hear if you were to threshold brake w/ no ABA)... and that's with virtually all factors changed... 500# springs, big front brakes w/ race pads, and 285/30 Hoosiers. The system may not be truly "optimum" for the new set up, but I think it's still pretty close.

I think we're overthinking the issue.

The system is built to work in a variety of conditions in which traction would vary due to the stopping surface... which has the same net effect (less or more grip) as any of the mechanical factors.

wanklin

iTrader: (2)

I can agree with that. lol

Well put Pete, but I think the article agrees with what you are saying. It's more of a timing lag issue because the sytem runs in open loop and it takes more tries for the sytem to accomplish what it is trying to accomplish when you switch up the factors. It'll still work regardless, it's just a matter of how quickly.

If you're happy with the way it's performing then I say you are giving me the kind of feedback that I was hoping to hear. It may not be optimal, but it's not like 3 channel (8 bit? lol) is exactly state of the art to begin with....

2MCHPWR

iTrader: (2)

i run tires with the approx height you mentioned with no issues at all. 245-45-17 up front and 275-40-17 out back (v710). ABS works perfectly. 275's up front and 315's in back should work just as nicely.

Eggie

I just want to add that the 929 MC upgrade does not affect bias. While the 929 has larger pistons, they are still the same size for both circuits.