I've been tracking my 2001 with no ABS for a while now. On Kumho Victoracers, and with HP+ pads, the fronts lock first every time.

 

Andrew, it is possible that you are confusing the deformation of the tread with the deformation of the tire structure as a whole (sidewall / cross sectional shape). Snow tires and tires that feel sloppy and slow to react have compliant sidewalls. That does not necessarily match the way the rubber on the face of the tread stretches.

 

These graphs are related to lateral slip (and may not be very accurate as they are generalize tires in to two groups) but they still show that the race tire achieves peak grip at lower slip angles / percentage. What Andrew said is completely in line with every book I've read on the subject (Which isn't too many but I got a good idea of whats going on). I just never thought about it in terms of longitudinal deceleration and what slip the ABS system engines programed the unit to become active at and how all this relates to the type of tire that is being used.

I could be mistaken but if you ask me that was some of the more insightful point of views on the matter I've heard



Edit; Graph are from Ross Bentley.

 

I don't understand how slip angle / steering comes into play in this discussion. Weren't we talking about stopping the car? ABS takes data from your wheel speed sensors to determine if the wheels are locking up or not and adjusts braking accordingly. Regardless of what tire, ABS thinks everything is all good until it senses a lock up; one of the sensors dropping in speed.

 

That's cause you got confused We are talking about slip percentage in longitudinal deceleration. The graphs I posted were only to illustrate that race tires do in fact require less slip to reach maximum traction than street tire which was something that was debated (I thought that I made that clear in my previous post).
The notion being that, since ABS systems are designed to engage at a higher rate of slip where street tires achieve their maximum traction (not simply when a wheel locks up) you have already surpassed the peak traction of a race tire since the maximum traction is at a lower slip percentage.

 

I'll take minor exception to this statement. While I agree that there are differences in the various surfaces and methods of braking to achieve shortest stopping distance, I've experimented with ABS vs no ABS in snow. I'm sure it varies with the exact system and perhaps with the tire and weight distribution of a vehicle but my informal tests have shown that I can stop faster in snow by keeping the wheels at the point of the ABS wanting to kick in, modulating to stay at that point, than by allowing the BAS to work. I can surmise that this is due to the wheels locking and then not having enough friction to start rotating again. I don't see how locking the brakes would provide a shorter stopping distance - sliding friction vs. static friction and all that.

That said, locking the brakes may well prove better in certain cases when you simply can't modulate properly in the first place (panic stops, etc.). Of course, ABS will do the modulation for you...

 

The ABS computer can't see slip rate unless it has a ground speed radar sensor, it can only look at the relative differences in speed between the four wheels.

There is a maximum deceleration rate programmed into some ABS systems, if all the wheels slow at a greater rate than the set rate it assumes that all four are locking up and releases some pressure. If the wheels don't speed up again it returns the pressure. Each brand/system is slightly different and has a different take on this.

I have data showing better braking while threshold braking without engaging ABS and a slight drop in G-force if you engage ABS. This is with stock (or Axxis Ultimate) pads and Hoosier A6 tires. The S2000 system has quite an aggressive allowable deceleration curve, or it only looks at the relative wheel speeds. In other words you can brake just as hard with or without ABS, but the ABS is there to save you if you aren't perfect.

You couldn't pry ABS from my hands, even for a healthy weight savings. It's awfully nice to have it there in case you make a mistake or there's a bump in the braking area. Pull the fuse for a few events and then make your decision. You'll learn a lot about threshold braking but you run the risk of ruining tires if you screw up.

 

I must say I prefer tracking the car without ABS (which I didn't intentionally disable, I think they musta whacked the sensor when I had the rear wheel bearing replaced).

For two reasons (neither of which is that I think I'm faster without it):
1. If I totally lose it, I can lock all four wheels which is infinitely preferable to ABS allowing the tires to hook and propel you across the track into traffic.
2. Keeping braking skills in practice. I'm sure there'll come a day when a computer can totally drive your car around a track faster than you could do it, but where's the fun in that (unless you're the system designer/architect)? Even if the computer could do it better, I'd rather do it myself...

I haven't had any issues with tracking the car with no ABS other than the occasional lockup (which I have to recover from myself). Fronts always lock before rears on my car, wet or dry, though I know there are cars out there that lock rears first with no ABS (which is an exceedingly dumb idea! Not failsafe...)

 

That's what I thought initially but then I read "slip angle" and Andrew Wojteczko mentioned steering input, so I was like

 

It still is a slip angle really. It's just facing in the direction of travel.

Your tires achieve max grip at x% slip. Should be slipping them sideways or forwards for the most part doesn't matter.