Keep in mind also that ABS will be relatively more effective than a human in situations where different wheels have different levels of traction, like when braking on a slanted surface.

The flip side that I've heard asserted that since factory ABS systems are tuned with street tires, they're not capable of getting the most out of R-compond tires. It's not clear to me why that would be the case, but an argument about the breakaway characteristics being different could be plausible.

 

an R compound or race tire will run less slip percentage to achieve maximum decel. This means that if the ABS was tuned to start activating at say 8% slip but the race tire makes peak grip at 6%, the ABS won't start working until the race tire has already gone past its peak. In my opinion this is an advantage as the ABS becomes less intrusive when used with an R compound or race tire.

 

^ That is a really interesting point.

 

except that you have to be a really good braker to modulate that 2% better than the abs. Almost no one on this forum is that person.

 

Are you sure you have that right? I'm pretty sure R-comps generate max forces at HIGHER slip angles than street tires, not LOWER slip angles.

Ie., your ABS system may be set up to get 6% slip, based on assumptions about tires and pavement. However, your max grip with race slicks may come at something more like 9% or 10% slip. So your ABS restricts you from getting to that.

But overall, the ABS tends to outperform human drivers in almost every real world situation.

The one exception is in gravel or deep snow. In that case a good driver can get shorter stopping distances and more control by intentionally locking up the wheels.

 

For what it's worth (and I hope this is useful when it comes to discussing stickier tires):

I've noticed that running 255 PS2's all the way around with Hawk HP+ front and HPS rear pads, I have a pretty difficult time engaging ABS on the street - basically, it has to be wet or I really have to try.

It's night and day compared to my last set of tires, GS-D3's (225/255). The harder compound of the GS-D3's made ABS almost a daily event - and not because I was driving the car harder than I do now...just because it was that easy to break those tires free while stopping hard.

The stickier tire, theoretically (correct me if I'm way off here), keeps the wheel turning against the pavement. A harder tire will start to slide on the pavement sooner (which allows the brake to lock-up sooner), making ABS easier to engage. This explains why braking in the wet more easily engages ABS - the tires are not sticking to the pavement as easily.

Extrapolating to R-comps, ABS should allow a higher brake g-load before activating - i.e. you can brake harder before ABS kicks in.

 

^ If you are not able to engage ABS with the HAWK HPS+ your overheating the pads (out of the temp range). I bet if you went around the block you'd get ABS easily but once your on track you'll start to experince brake fade, then probably biol the fluid. These pads won't last more than a day under these conditions. You need to step up to a more aggressive pad. You should be able to easily engage ABS at any time.

I ran them and had the same issues and I burned through them in about 4 1/2 sessions on track and had to go home early. These pads are not designed to handle these conditions regardless of what Hawk says...they are garbage.

-Rob

 

We had a heck of a time with the rear coming out when we lost an ABS sensor. It was not fun.

 

Sorry, I should have clarified - this is my street setup. I can engage ABS. My point was that it takes more with these tires than with the previous, harder tires.

I'd step up to better pads for use on the track (HP+ were good for my HDPE track time on FK-452's, but that's not as hard on pads as full on racing)

 

I am quite confident about the race tires requiring less slip angle for peak grip.
If we look at the lateral force versus slip angle curve, you will see there is an elastic portion where minimal sliding occurs. During this phase the tire is deforming elastically. Take a snow tire, with a large soft sidewall, and it will require a large steering input, or slip angle before it reaches the non linear portion close to where peak grip is reached. You can see that the snow tire will require a much higher slip angle than the race tire before building peak grip. The snow tire is obviously an extreme example, but the stree tire that our S2k's are designed for will be between the race and snow tire example.

Also, think of it this way. Racing tires are designed to allow our cars to change direction as quickly as possible. The higher the slip angle required to make peak grip, the longer the transition time will be between peak braking to peak lateral g to peak accel.

One other important point is that the race tire has a smaller window of peak grip, and grip will fall off more dramatically than a street tire. This narrow window of peak grip makes the ABS's job more challenging and I'm confident that the higher frequency, higher quality ABS systems will see less performance degradation when using high grip tires.

The 2% difference in my previous example means that if you do engage ABS with the race tires, you are already over the peak longitudinal force and grip has fallen off, so you aren't getting the most from the tire. But it does mean if you are attentive to your decel you can reach peak decel force without any ABS interference, yet still have the safety net to prevent flatspots, and prevent individual wheel lockups over irregular/rough/dirty surfaces.