I just saw a question in Tires & Wheels section asking if larger tires in rear than front would affect the ABS. My instant thought was no. Then I began to think about it and just over this past w/e, I had the darn ABS chatter on me and felt I was no where near locking up (could have been some gravel or dirt on road and surface was pretty rough as well or maybe I was about to lock up) I know the ABS measures the speed of each individual wheel and "compares" them so the differential is not to great.


Will the ABS be adversly affected with the already different rotating speeds of staggered front/rear diameter tires?

Jazzer

 

I am assuming that by staggered you mean wider rears compared to fronts. I think it will only be effected if the wheel and tires are a different overall diameter then stock. As long as you are within like 3% of the stock height it wont be an issue. Im not expert, the tech guys will probably have a more refined answer then mine.

 

Ya, I wasn't too clear

I am referring to differing diameters, rather than widths, of front VS. rear. Say a 25' diameter front and a 26' or 28' diameter rear. I am wondering if the ABS will more likely engage the front wheels as they are assuming the brunt of braking while also spinning faster (would this negate each other?). Seems to me, if the wheels are spinning faster to begin with, although not much, is the ABS closer to the point at which they will engage?

The more I think about it, the more it intriques me. I am running 25.1" diameter fronts and 25.5" diameter rears (doesn't seem like a big deal and S197 Saleens have at least that much difference from the showroom). In my above post, I mentioned the ABS kicking in and didn't feel as though it should.

Is this .4" enough difference to create an 'imbalance' between wheel rotational speed and make the ABS engage more easiliy? Does it make any difference of the diameter, but rather ONLY when the rotational speed is a large %? What happens with the Chrysler Prowler? That thing has like 14" fronts and 22"s in the rear. Does the ABS read all the wheels and ONLY compare the difference under braking so size doesn't matter? (Did I just say that? )

What gives?

 

It is at least theoretically possible for an ABS system to activate on any corner of the car that has asmaller diameter tire, if the diameter difference is great enough and your deceleration is sufficiently high. What is happening is that the smaller diameter tire, which runs at more revs per mile, has to drop more revolutions per second in order to remain at a slip that is consistent with the larger tires. The ABS would interpret the different rates of wheel rpm loss as incipient lockup at the "little" tire,because it doesn´t know that that the tire size there is different from the others. If the difference is great enough, the ABS will then activate the antilock behavior at that corner or corners. That´s the theory.

In practice, there is apparently a 3% or slightly greater margin built in, presumably to allow you to replace tires in less than full sets of four, and to cover for mild cases of under- or over-inflation that could result in a tire or tires spinning a few more revolutions per mile than another (or others).

Edit - I think that covers most cases where all four tires are the same size (or at least the same diameter). If the car was originally equipped with different diameter tires, the ABS would be calibrated accordingly (and could activate the end that originally came with the larger diameter tires if you then made them all the same, since it is still expecting the end that originally had the "littles" on it to lose revolutions faster).


Norm

 

So, in theory, if I installed 215/25/15"s up front and 345/75/20"s out back, I would not even be able to drive as the wheels would just lock-up 100% of the time .

I like questions like this and figured Norm would toss a little rocket science this way and sho' nuff

Until next time......Same forum, more bad puns

Jazzer

 

No. Those little ones would be into the ABS about . . . all the time, and would contribute very little to the overall braking effort, while the rears would work normally (except for having to supply almost 100.0% of the stopping). Your stopping distance would increase considerably. What ABS does is release the brakes from the [default] applied condition so that they can regain rolling.

This topic is probably worth doing a little poking around for in the feature articles over at www.brakeandfrontend.com. Lots of good stuff there, and in the printed magazine and in the e-mailed newsletters, even though it is primarily aimed at the OE replacement business.


Norm