 |
|
Questions:
When a wheel sensor senses lockup does it trigger brake pulsing to only that wheel or to all wheels?
What is your opinion on just how well our ABS works vs. threshold braking? I’d always thought that threshold braking beats ABS braking. But I read a thread by an automotive engineer some months ago at BF.C and he felt very strongly that ABS worked so well that no human could match it.
Maybe modern ABS works better then humans, but did 1987 ABS work better then humans?
Yes in general but some of the older ABS system where three channel design where both of rear wheels were treated as one.
In wet situations or in turning situations, ABS will out perform (that is the car will come to a stop in a shorter distance).
In a straight line braking, assuming brake bias is set up correctly, a skilled driver can out perform most normal street car ABS. I’m sure at the F1 level, they can program the ABS to mimic the best driver out there (that is to lock up just enough to get the most traction).
The ABS system in an E30 is pretty primitive as these systems go. My guess is that you probably can get another 15% with ABS disable if you can hold the car in threshold braking.
jlevie wrote:
That’s a big guess. We don’t know what the trigger is for ABS deciding that lock up has occured, and we don’t know precisely how ABS responds. There’s folks out there that do know. Our challenge is to find them.
I did some more poking around and I think that my '87 is the 3 channel ABS system that Chi described. So when a rear tire locks up, both rear wheels are pulsed.
That’s right, the number is a guess and I think I made that pretty clear.
When I did the Skip Barber school the numbers they cited were 80-85% for first gen 3 channel systems (what an E30 has) and 85-90% for second gen 4 channel systems. A tuned race quality systems was said to be about 95% of max possible. When you are in true threshold braking you are keeping the tire in the 105-110% range. I.E., the tire is in the slip range. In the Formula cars it was easy to see this happening once one could get the car into threshold braking, which was the first thing they teach. You could see the front tires alternately slowing down and speeding up as you rode the slip. And the car exhibited an oscillation in yaw through the braking zone. Both are the result of being in the slip range and the tires seeing variations in the track surface.
While the initiation of ABS could be tuned a bit higher at the expense of nuisance activations, like any control system there has to be some hysteresis. The desire to avoid nuisance activations, hysteresis, and the necessity to accommodate a range of tires and vehicle loading means that the system can’t be made as effective. A race system is a different beast in that the tires are a known factor, front to rear weight balance is well known, brake bias is adjustable, track surfaces are generally very smooth, and nuisance activations can be tolerated. Or so I was told.
I could threshold brake a lot better in a Formula car than I can in an E30. The weight difference (1300lb vs 2700lb) makes a big difference as does the tuned brake bias and lack of boost. Even though I can’t do it all that in an E30 well and tend to be a bit shy of full threshold braking I do see a reduction in braking distances with ABS disabled. My experience is that the 85% number probably isn’t that far off the mark. Of course the down side to no ABS is the risk of locking up a wheel and flat spotting a tire. I occasionally get a squawk from a tire, but so far I’ve avoided flat spotting one.
Great post.
Didn’t intend criticism with the use of the word guess.
The Skip Barber info, albeit general in nature, is better then any other estimate I’ve read on the subject. So thanks for the info.
ABS didn’t work on my car until a couple months ago. I was flatspotting a lot of expensive tires. I was trying like the devil to detect the onset of lockup, but I found that sometimes the variables were such that I couldn’t always detect it fast enough.
No offense was taken Scott. I didn’t mean to sound defensive and was just trying to emphasize that my information should not be taken as factual.
In a like manner I can’t say how well researched and factual the numbers from Skippy’s school are. They tend to agree with what I see in my car, but that’s all the support I have. There’s lots of information available about what ABS is and how great it is, but I’ve found very little information about design criteria and trade offs.
In my opinion, learning to threshold brake with ABS disabled is going to be a bit of a challenge in one of our cars. The exercise is probably best done in a manner similar to what’s used in the pro race schools. They set up cones to use as a brake point and have the students accelerate up to what felt like about 40mph (speed is deceiving in a Formula car). The goal is to find the point at which the tires aren’t quite locking up, which means getting in to lockup a time or two. Since the speed is low, there’s not too much risk of flat spotting a tire.
As I mentioned earlier, there’s a lot more control and feedback in a Formula car, which makes it easier to learn where the threshold point is and what the car feels like when you are threshold braking. Once I knew what it felt like at low speeds, it was pretty easy to do the same thing at speed in the Formula cars. It seems harder to me to accomplish the same thing in a Spec E30. There’s more affect from vehicle dynamics (weight transfer) less pedal feel from the boost, and just a lot more weight.
I recently fixed my ABS, (actually at the RRR race) before that Ive never driven on track in my E30 with ABS.
I was fine with no ABS and felt good on the brakes…but, in the rain at CMP it was a major disadvantage with no ABS.
Ive also been pretty good about not flat spotting tires but it was always a $$$ concern before I fixed it. Now I have more confidence going deeper in the braking zones, especially with someone 2 inches off my outside corner. I’m not worried about locking up the tires and collecting someone because of no ABS.
With our early systems maybe it would be more efficient on a dry track if you could effectively and consistently engage threshold braking when needed but it is really worth it considering the chance of tire damage etc?
From an anti-flatspot perspective ABS is very useful in keeping tires round.
Also, while a theoretically amazing driver might be able to extra 100 percent of a car’s braking potential, that assumes that the car has PERFECT braking balance fore to aft and side to side. If the bias is to the front, you’ll never be able to get to 100 percent since the rears won’t be doing their share of the work until the fronts are over their maximum threshold.
ABS evens this out some; as you overcome the friction capability of the front, the ABS engages, but the rear ABS won’t engage until they’ve stepped over the maximum, too. You’ll notice that the best F1 drivers are constantly fiddling with their brake bias from corner to corner to extract maximum deceleration without the aid of ABS.
We all know that a track surface is rarely a scientifically neutral environment. The right side of the car might be on marbles while the left is on super-grippy weathered-in asphalt. I don’t think anybody has come up with a right-to-left biasing control (other than ABS itself, of course) or the ability to precisely predict how much grip each tire will have in a given corner.
Even though a busted ABS box was the most probable source of my rear brake lockup woes, I wouldn’t have considered disabling it on the car as long as it’s a legal option.
Scott you just described what I have been saying for ages except I didn’t know why it was so. But seriously turn off your abs you will be much faster ;). Honestly I’ve given up trying to help, if you think you will be faster without the abs more power to you.
Simon , I am with you. I have given up on arguing about this. Were are not driving F1 cars and I like the safety net ABS gives me. When I can feel that it is holding me back then I might try turning it off, but I just don’t see that day coming anytime soon.
On other boards as soon as I say this I normally get: “why don’t you just Drive an Automatic”. So I just gave up trying to convince anyone one else.
It is a personal choice. If you are faster than me because you don’t have ABS than you are a better driver than I am and you deserve to be faster. If I pass you while your on your roof, who is the better driver then?
Then again I am here to have fun and drive the car on to the trailer at the end of the day. If I can do that then it was a good day.
Don’t get me wrong, I’m not suggesting running w/o ABS. 1) I think that inop ABS breaks the rules. 2) At my skill level inop ABS prematurely kills tires, and that’s not affordable.
My intent was simply to better understand how our ABS works. Success in my brake biasing via brake pad experiment will mean that sometimes my rear wheels lock up. If the rears lock up occasionally instead of the front, then I’ll know that I’m somewhere in the ballpark of balanced braking. Right now my fronts lock up first every single time. At least I’m pretty sure that’s what is happening. And there’s nothing balanced about “every single time”.
But because of ABS it’s not just a matter of locking up…It’s actually a matter of triggering ABS (stating the obvious). And that means I have to understand how ABS works. It was only yesterday that I learned that front ABS is pulsed individually but the rear as a pair. Last week, for all I knew ABS was pulsed to all 4 wheels if any wheel triggered action.
And let me make one further point. If you’re racing against me and you end up on your roof, I’m gonna pass your ass. Ok, landing on your roof is a technique, but I know the counter. I call it the “Holy $hit, dodge around the upside-down guy” trick. I’m even prepared to do it twice should the upside-down guy have lapped me.
So if you’re on your roof, you’ve not a prayer against me. Remember that.
Ranger, this is from memory, so it would need to be fact-checked before it ran in GRM, but:
Each of the front wheels has a magnetic sensor floating just above a special circumferential portion of the hub or CV joint area. The special portion is carved evenly with high and low bits at right angles.
Here’s a crude ASCII drawing of what the grooved section looks like in profile since I don’t have a picture handy. Imagine the top part of a castle wall.
INSIDE OF AXLE, FLAT HERE BUT ROUNDED IN REAL LIFE
| || || || || || ||
/\ <SENSOR HEAD
||
So the sensor floats above these ridges and as the wheel spins so does the hub and the axle, and their passage under the magnetic tip of the sensor generates a simple signal. (Dunno how the rear wheels are handled in our cars, might be based on a sensor in the diff) The faster the ridges go by, the higher the frequency of the signal generated.
The ABS computer watches the signal from each wheel, and depending on the complexity of the computer compares it to other wheel speed sensors or simply to itself. If the speed goes from 50MPH to zero in an instant, the computer assumes that the wheel has locked up and triggers the ABS solenoid to pulse for that particular wheel, applying and releasing pressure rapidly to prevent lockup. That’s the pulsing sensation you feel through the pedal.
Hope that helps get you started, I’m sure Google knows more than I do.
Back in my prime I could crank out ASCII art with ALT-### codes like a damned ninja. This stoopid Mac doesn’t even know what to do with 'em.
Scott, fyi, you can repeat the scenario for the rear. There is a similar machined surface, but it is on the outboard end of the half shaft, one sensor on each side.
Ed
Ranger wrote:
:blink: I searched the CCR and the SpecE30 rules for “brakes”, “abs”, “lock”, and “anti”. I can’t find anything that says whether or not ABS can be disabled… or if the ABS unit can be removed. Am I missing something?
(My ABS is disabled and likely will be for the life of the car.)
http://spece30.com/component/option,com_kunena/Itemid,86/func,view/id,14989/catid,9/#14989
… is why you can’t.
Dang - I’ve read the rules several times but this one slipped by me.
Well, that seals the deal, then. I’ll have to throw in the towel on this class for now. Other things in my hoped-for “hybrid” race car could be switched out for different classes/clubs. But the ABS hasn’t worked on my car since I bought it and I’m not spending any dough to fix it. I’ve driven happily without for 2 years.
Maybe I’ll see some of you at another NASA DE. Otherwise, maybe we’ll meet in ITS or GTS. Good luck to all and cheers! 
ABS didn’t work on my car either. Worried me because the ABS unit is pricey. Ultimately it turned out to be two problems. An ABS relay under the dash was blown and my ABS light was disconnected in the dash. The light really does have to work for ABS to work. Total cost <$30.