new tires

[92droppedx]

i'm looking into buying two new tires and buying the other two later for my 18"s. I'm thinking about running 245/45 in the rear and 245/40 in the front. I think this will fill the backs fender gap out better. what does everyone else think?

[rodtice]

might cause probs with the ABS. Due to the different revolutions per mile. Need to keep the same diameter as stock front and rear to avoid any such problems.

[92droppedx]

didn't think about that... good point

[92droppedx]

what kind of problems do you think that would cause to the abs? would it be safe?

[rodtice]

could increase your stopping distances, thats not exactly safe.

Abs measures the revolutions of the wheels, if one is slowing/stopping at an inappropriate rate than the other wheels it kicks in the abs pump and doesn't allow you to apply as much braking pressure

[ugsrich]

Operation of the ABS is a function of wheel speed and is not dependent upon wheel/tire diameter. The angular velocity of each wheel is measured independently and ABS activates when any given tire begins to skid. This does two things: First, since the coefficient of static friction is slightly higher than that of kinetic (skidding) friction a greater stopping force (between the tire and the road) is made available. Second, ABS affords the driver greater vehicle control and stability as the friction force being generated (in a panic stop) between each tire and the road is more evenly balanced (no one tire is allowed to skid while another continues rolling). This "balance" of friction force by the ABS significantly reduces the fish-tail that is often induced due to an imbalanced friction force between tire and road on the rear wheels. Further, the "balancing" provides the driver the ability to steer around hazards even in a hard stop.

I don't believe that your proposed tire size difference will have any effect on the operation of the ABS. If anything, the tire size (diameter) may well affect brake pad wear as it will create a need for a greater torsion force to be generated between the pad and the disk requiring increased pedal pressure and ultimately higher temperatures leading to faster wear. I do believe that the two sizes will induce ride and stability matters that are not in harmony with the vehicle's design. I personally believe that any vehicle intended to be used for general driving should have 4 tires of the same brand and size and that those tires be purchased simultaneously and rotated at prescribed intervals.

[rodtice]

the increase in stopping distances is because we have a single channel ABS system if I remember correctly.

Meaning if one starts to skid it reduces the braking force of all the wheels. If we had a four chanel system it would only reduce the braking forces on the skidding wheel.

[ugsrich]

The cause of the skid (independent of the number of wheels skidding) is the result of braking force at the pad/rotor overcoming the friction force between the tire and the road. As mentioned earlier, the coefficient of static friction (tire rotating over the road in this case) is greater than that of the coefficient of kinetic friction (the tire skidding over road). Any tire/road skid either independently or by all four wheels is a net loss of tire/road stopping force combined with a loss of vehicle control. Maintaining the best level of tire/road stopping force is the objective of ABS. At 30 or so pulses-per-second the ABS will ensure that every wheel is providing balanced level of friction force between the tire and the road (static coefficient of friction) and provide enhanced vehicle control. The matter of stopping distance (which of course increases as a function of the square of the vehicle velocity) is most likely negligible in magnitude and is probably about the equivalent of an imbalance of wear of the tires between the front and rear of the vehicle. In general, automobiles are engineered with pretty liberal factors of safety that include some amount of tolerance for [lack of] owner driving skills and negligence when it comes to matters of tire wear, etc.

If stopping distance is REALLY a major concern, then I suggest: 1) Slowing down, or; 2) Buying new tires every 20,000 miles or so to ensure that you have the absolute BEST friction force being developed between the tire and the road (I suspect most of us use our tires to about 45K-50K miles). I will accept the combination of balanced stopping force (between tire and road) with enhanced vehicle control and stability over the expense of frequently purchasing new tires. (Interestingly, we have not even touched on the condition of the shocks, springs, number of people in the vehicle, stuff in the trunk, etc. all of which can have an effect on stopping and control.) I will continue to operate my vehicle at a speed that is both legal, in keeping with current road conditions and with my empirical experience of the vehicle’s dynamic performance (stopping, controlling, etc.).

In short: Introducing new tires, of slightly greater or lesser diameter is inconsequential when considering all the factors that will affect collision avoidance in any automobile. As stated earlier, my belief is that tires should be purchased in equivalent sets of four (4) and maintained as specified by the manufacturer.

[kev's 96 Aurora]

And class is over....do we have to study for a test...you know your shit man...good stuff

[98AuroraMA]

I've been browsing this forum for months and that was the most thorough, educated, well thought out reply that I've ever seen!

Just thought I had to add my 2 cents...........great job!

[92droppedx]

i ended up buying 245/40 series all around but this post did bring a lot of things to my attention. thanks

[ugsrich]

As a final follow-up and to ensure the accuracy of my assumptions I have consulted the 1995 Service Manual for the G Platform (Aurora & Riviera). In section 5E2 page 3 a description of the ABS operation is provided. Here is one of the paragraphs:

“The PMV [pressure modulator valve] has two solenoid valves per individual wheel, a normally open inlet and a normally closed outlet valve. These valves work in pairs to achieve the three ABS modes. The ABS system in reality isolates, monitors and controls all wheels simultaneously….”

From this and the remainder of the operation description each wheel’s skidding condition is monitored and controlled individually. The key phrases above are “…two solenoid valves per individual wheel…”, and “The ABS system in reality isolates….”. In the parlance described earlier this is a 4-channel system on the Aurora with 4 independent wheel sensors. Most 3- and 1- channel systems are only used on pickup trucks where the rear end is extremely light and subject to higher potential wheel lock in a no-load panic stop. In a 3-channel system the two front wheels are managed independently and the rear is managed as one. In a 1-channel system the rear wheels are monitored at the differential and ABS is applied to the both wheels as if they were one.

In addition to the Service Manual I reconfirmed some of my assumptions at the following web-site: http://auto.howstuffworks.com/anti-lock-brake1.htm Perhaps you too will find it interesting and informative.

I'm glad you chose to put the same tires all around I think that's the safest overall. I'm confident you'll be happy with your Aurora, I know I have been since 1995.

[jim56]

Originally Posted by rodtice

the increase in stopping distances is because we have a single channel ABS system if I remember correctly.

Meaning if one starts to skid it reduces the braking force of all the wheels. If we had a four chanel system it would only reduce the braking forces on the skidding wheel.

Aurora's have had 4-channel ABS at least since '97

[jim56]

Im not sure how close you have to stay to the original tires circumference. Obviously they have to allow some for wear,different brand variance, etc. The 245/40 18 i looked at ( bridgestone) is 25.7"
the 235/60 16 Bridgestone Turanza that I have is 27.2. 1.5" difference.
Does anyone know how much you can deviate from stock?

[rodtice]

Then that would explain why it stops so much better than my grand am on snow and ice.

Glad to hear it.