Last post on Aug 21, 2013 at 5:16 PM
You are in the Toyota Sequoia
What is this discussion about?
Toyota Sequoia, Toyota Highlander, Toyota 4Runner, Toyota Land Cruiser, Toyota RAV4, Toyota Tacoma, Truck, SUV
#578 of 2493 Even easier...
Aug 10, 2002 (11:31 am)
Just look at all the car magazines. Whenever they test a vehicle with ABS and one without, you will always see one thing...the one with ABS stops significantly shorter (on dry land).
In addition, not just MB, but more and more SUVs nowadays are putting in different ABS modes for on-road and off-road driving. Toyota Land Cruiser has had one since 1999. I believe the '02 4Runner has one too. Nissan has one in it's Pathfinder.
BTW, MB's 4wd system is POS! You cannot turn off the stability system's "cut-engine-power" function. In the 4Runner, all you need to do is LOCK the center diff! Same goes for the Land Cruiser. So, please no more comparing our system with that of Mercedes...it is not fair for Mercedes!
Aug 10, 2002 (12:40 pm)
That's completely incorrect. There is an "ESP off" switch which turns off the torque reduction feature altogether. Unlike Toyota's system, the torque reduction isn't so sensitive. There is no need for any locking differentials in the ML since the traction control takes care of everything. The added benefit of this is that low range can be used for towing on dry surfaces, for example. Very beneficial for towing up hills.
MB's special low range ABS feature has been on all MLs since 1997, and it works only when you're off-roading at low speeds. Nissan/Toyota have some sort of ABS sensor which is supposed to read the road surface, however, it's not the same thing as it doesn't allow the front wheels to lock up.
Heh, who do you think Toyota copied the system off of? Just as Lexus claims to have invented dual threshold airbags in their commericals :-p
Aug 10, 2002 (2:13 pm)
I don't know, but a physician I know swears that MB steals all their breakthroughs from BMW.
Also, on the Toyota, the locking diffs are in addition to the traction system. So you can use 4WD Low while towing on dry sufaces on the Toyota too.
Aug 10, 2002 (2:35 pm)
...the maximum traction point of a tire is when it is revolving at 15% slower than it would be if it were freely rolling over the surface.
I'm not sure I understand that. If the tire isn't revolving at exactly the free rolling speed, then it would have to be slipping or skidding. Perhaps you could clarify.
SUVs; Aftermarket & Accessories
#582 of 2493 Wow...CONTENT!
Aug 10, 2002 (2:47 pm)
You should have included the next paragraph in your quote.
Okay, in same instances ABS is good, and in some instances no-so-good. The insurance company statistics are indicating a slightly higher injury accident rate with ABS than without. They're still trying to figure out why. The most current best guess is that people with lot's of non-ABS life's experience have learned to lay into the brakes until the very last minute and then release the brakes for last minute manuvering.
What they get with ABS is longer stopping distances during which they could steer to avoid an object, or avoid leaving the roadbed on a curve (into which they entered too HOT).
Just what good is ABS if it modulates the brakes so I still have steering capability but I sit there like a dummy, frozen with panic, and hold the steering wheel rigid?
Do you realize that the RATE at which a fully braked tire slows can be used to determine the coefficient of traction of that wheel with the roadbed?
In any case something is clearly wrong, maybe in the driver's head, but until we figure out just what it is and apply a fix maybe it would be better to sacrifice the need for directional control until there is some indication of need.
What the world needs is a huge fleet of driving simulators to teach people how to properly react when they encounter a real emergency, and in the process we could maybe learn from the huge mass of people with life's experience how they react and thus maybe learn why ABS is not fullfilling it's expectations.
We could even put a simulator in every dealer shop and they could use it to teach people what to expect and how to properly "react" to that vibratory brake pedal (push harder!).
#583 of 2493 Split-mu
Aug 10, 2002 (3:03 pm)
This one is where the shear idiocy of ABS designers stands out so grandly. I'm driving along on a snowy and icy mountain pass roadbed at night, with my right tires on the graveled shoulder of the road and the left tires are on pavement covered with black ice. Suddenly I see an Elk just standing in the roadbed.
Without ABS I can apply the brakes fully, well knowing (I'm 62) I will need to quickly respond to the expected yawing motion, and I end up stopping a few feet away from the elk, who now proceeds to jog merrily away into the woods.
With ABS in order to prevent that "yawing" the engineers have designed it so I get no better braking HP applied to the high-traction side than the low traction side. Pity the poor elk, pity my poor Porsche.
#584 of 2493 Actually, farfegnugen, you're wrong!
Aug 11, 2002 (6:02 am)
The Mercedes M-class does indeed have a "ESP OFF" feature. HOWEVER, even with pressing this, the engine-cut-feature is STILL on!! That is why, in ALL tests to date by various magazines, the ML has FAILED MISERABLY (!!!) in off-roading terrain. Yes, it has a traction control system (LIKE THE 4RUNNER). It does NOT NOT NOT have a center differential lock like the 4Runner, which helps in tough off-roading terrain (see my comments above). If you turn ON the center diff LOCk in the 4Runner, there is NO engine power reduction! Right there, the 4Runner system is already superior to ML's. But, read on....
In addition, the ML's 4wd system is NOT very sensitive to wheel spin. Therefore, you actually have A LOT of wheelspin BEFORE the ML system becomes active (that is, brake the spinning wheel). This is OK in snow...however, in off-roading, if the system waits THAT LONG, you will have lost momentum already! This is bad! You are stuck! The Land Rover Discovery system is very good at this. Toyota's system is somewhere in between Discovery's fast-acting system and ML's SLOW-acting 4wd system.
farfegnugen, in the 4runner, you can have HIGH or LOW on DRY land too! ML's system was a good idea when it first came out; however, the newer systems (Land Rover and Toyota) are better and with more features (aka center diff lock).
I can go on and on about how the ML is really a POS in general. Cheap interior materials (yes, even worst than 4Runner). Thin-sounding doors. NO wheel articulation. Sure, it has LOW range, but it has NO FREAKING SKIDPLATES! That is helpful! It is low to the ground, with not very good aproach and departure angles.
In conclusion, it is a good & fast minivan. Unfortunately, BMW X5 kills it in every performance tests! If i were to buy a fast wagon, the X5 would be it. Who wants to buy a Mercedes ML??? The G500 is another story...i respect that ancient thing!
#585 of 2493 Tidester/15% slower
Aug 17, 2002 (11:55 am)
I'll shed some light on that. A tire is nearly always "sliding" relative to the ground. When you're cornering, the tires are sliding a bit. When you're accelerating, the tires are rotating a bit faster than the ground is passing under them. And when you're braking the same is true.
I don't know if the figure is 15%, though it would certainly vary by tire design, pressure, surface and dowforce. This may be a rough average number but it's in the range of what I'm familiar with. The max braking is provided when you are at what is called "incipient lockup" for the tire. In this range, it is slipping a bit but not locked up - which would reduce the braking force as noted above.
A variety of things are happening in this zone. The contact patch has elongated in the direction of travel, the rubber is heating rapidly, the tread blocks flex to their max and a variety of other factors beyond this list contribute to max braking. On a graph the free rolling tire contributes negligible braking force, a gently braked tire increases it, the max is at incipient lockup and then the braking force tails off rapidly when the tire is locked.
Aug 17, 2002 (12:49 pm)
Thanks for trying to explain it to me! I appreciate it.
I'm still not sure I understand it, however, at least not the 15% figure you and farfegnugen mention. I wonder if you aren't thinking of "torsion" rather than speed. Let me try to explain.
Imagine drawing a straight chalk line from the center hub of the tire to a point on the rim of the tire along a radius. Now, as you're moving along with the vehicle, you can visualize the motion of the line. When you decelerate (or accelerate), that line will be distorted when the end of the line is nearest the road surface. This is because the counter-torque from the road stretches the rubber.
As end of the line approaches the surface, it's speed will be decreased briefly during braking due to azimuthal compression of the rubber. But, after it has passed the vertical (on it's way back up) it's speed will be increased as the rubber decompresses azimuthally. The average speed should still be the same as the speed of the car - decelerating or not!
Also, the point of contact should be moving with zero speed relative to the road! If it's not, then the tire is slipping and the coefficient of friction drops drastically from its static or rolling value. Then you're in a full-blown skid!
Does that make sense?
SUVs; Aftermarket & Accessories
Aug 18, 2002 (9:23 pm)
You made me dust off my "Fundamentals Of Vehicle Dynamics" book to confirm the 15% figure. According to this book the figure is actually between 15 and 20% tire slippage to generate maximum braking force, so it's even higher.
Here's what goes on in the contact patch of a braking tire. A tread block touches down at the front of the contact patch, then travels rearward through the contact patch, finally leaving out the rear as it rolls back up and away from the street. When it first touches down, it has very little downforce on it so it cannot generate much friction or braking force. When it firmly touches down it generates braking force by deflecting the rubber. This is key. The flex of the tread block means the ground is passing under it faster than it is rolling. Otherwise there can be no deflected rubber, and thus no braking force. Throughout the center of the contact patch, the tread block is fully flexed. As the contact patch arrives at the rear of the contact patch, the downforce on it reduces as it starts to roll upward and away from the pavement. This is the area where most slippage occurs - the rear of the contact patch where flexed rubber rebounds by slipping against the road. It happens here simply because as you reduce the downforce on the tread block, at some point the tread block will no longer have enough downforce on it to hold its flex and it will slip. And that's precisely what happens at the rear of the contact patch.
So, your line on the tire is a perfect illustration. As the line touches down at the front of the contact patch, it begins to distort into a curve to indicate the tread is moving slower than the sidewalls. As the line gets to the back of the contact patch, it will suddenly straighten as the tread briefly accelerates faster than the ground under it to regain its orientation versus the sidewalls. As it does so, it slips against the ground and of course wears off some rubber as a result.
For those of you still reading, the reason lockup generates less friction is that this zone of slippage starts to extend further and further forward in the contact patch as the tread's speed versus the ground gets further and further apart (tire obviously slower). Meaning that progressively more and more of the contact patch as a percent of it's area is slipping until you finally arrive at the 100% slippage of a locked tire.
Hope this helps a bit.