RodsterinFL

We were discussing the difference between these three and I was unsure. I read the information in the sales brochure on the DSC as adjusting power and braking and understand limited slip differentials but what about TCS and why do you need it with limited slip?

Mat

ABS: when you brake, some onboard electronics will detect if your wheels block - if so they will decrease braking pressure so they unblock. This is to take maxium advantage of static friction

TCS: Works in a very simiar way but when accelerating. Electronics detect if your (back) wheel spins - if so, they will decrease acceleration power so they stop spining. Again, we want static friction.

Limited slip: power is distributed (possibly unevenly) between your rear wheels. If your left wheel starts spining, say, because it is on a surface with very low friction, then more power will be applied to your right wheel.

Note that TCS would only brake that wheel, not give more power to the other wheel.

DSC: Is relatively new for non high-end cars. It will detect some unstable states such as understeer and oversteer and will apply braking to individual wheels and/or use throttle to correct that state.

As you see, all of these security systems complement each other. I probably missed a few details but google or howstuffworks is a great tool to find details

Hope it helps!
-Mat

TheColonel

The traction control system I believe works not by braking. Instead, it reduces the throttle to cut the power output of the engine until it feels that traction has been restored.

jwitzer

ABS (Antilock Braking System):

A magnetic or optical sensor near a toothed gear at each wheel measures the speed of that wheel by counting the number of teeth per unit time. By comparing the speed of each wheel with the others, a locked (or locking) wheel can be detected and the hydraulic pressure of that one wheel's brake line can be reduced (sometimes to near zero) momentarily, then reapplied. Repeat several times per second.

ABS DOES NOT IMPROVE BRAKING DISTANCES. It only improves your ability to steer during heavy braking by ensuring that the tires are still turning. In fact, ABS will usually worsen braking distance when compared to threshold braking, but allow you to steer around a hazard. The decrease in braking performance is minimized by quicker ABS cycling (think analog wave vs. its digital approximation).

I believe the RX-8 uses a 4 channel system (one for each wheel). The FD used a 3 channel system (4 sensors, but if either rear wheel locked, both rear wheels cycled hydraulic pressure).

Dynamic friction is stronger than static friction. Static friction occurs when a wheel is locked and sliding along the road. While this will slow the car down, the highest braking loads occur when the wheel is still turning, but turning a bit slower than the vehicle's road speed.

As an aside, the best acceleration occurs NOT when the driven wheels are spinning (static friction against a static road) but when they are turning a bit faster than the vehicle's road speed - which is why good TCSs allow *some* wheelslip.

Without going into each tire's friction circle, the more turning you ask of a tire, the less it can brake, and vice versa. Therefore, the best braking happens in a straight line.


TCS (Traction Control System):

Using the wheel speeds determined by the ABS, a spinning rear wheel can be detected. The brake for that individual wheel can be applied (shifting the torque through the LSD to the other rear wheel) and the engine power can be reduced. Good systems will use the brakes first and then feather the throttle as necessary. My Q45 just dumped the throttle at the first sign of wheelspin, which was NOT good in corners.


LSD (Limited Slip Differential):

The differential ("diff" or pumpkin) takes driveshaft torque as input and divides it between the rear wheels, allowing different speeds for each rear wheel which is needed for turning. An open diff will send torque to the wheel that accepts the most (usually even, but if a wheel spins, it will accept more). An LSD will send torque to the wheel that resists the most (the one that has grip). The FD had a Gleason Torsen LSD, which is very good. I believe the RX-8 does as well.


DSC (Dynamic Stability Control):

Using information from the TPS (throttle position sensor), the steering wheel angle sensor, a yaw sensor, and a linear accelerometer, DSC attempts to determine which way you want to head and compares that against the way the car is going. If these two differ, DSC can apply brake pressure to one wheel to attempt to influence the direction of the car more towards your intent.

For instance, if you are going around a right-hand corner and the car is understeering (pushing), DSC will apply a bit of brake pressure to the right rear wheel to try to bring the nose around. If you were oversteering (loose) in the same corner the left front wheel would get a bit of brake pressure. Again, the LSD and TSC may come into play if you’re driving ham-fisted.


Question: With a fly-by-wire throttle, is there still a TPS (usually a potentiometer) or is it a different technology now?

None of this technology will revoke the laws of physics, so the responsibility to stay on the road is yours, not the car’s computer.

Most good drivers will turn these babysitters off on the track because they can usually out-perform them in the extreme applications found at the limit. I usually left ABS on and used threshold braking anyway. This helped when running sandy tracks (like Sebring) when one side of the car had significantly more grip than the other.

www.howstuffworks.com is a great site.

Mat

Good writeup. I would simply like to add that static friction coefficient is almost always higher than the dynamic coefficient, at least for most material combinations (including asphalt and rubber). This is why we don't want the wheels locked.

You can validate this by doing a simple experiment. Or by looking up google or howstuffworks (http://auto.howstuffworks.com/brake4.htm).

jwitzer

Mat,

Thank you for the correction. I had the terminology for dynamic and static friction reversed.

The logic still holds, that we want the wheel to still turn under braking.

While we're throwing out links: http://www.geocities.com/prohibition_us/friction.html claims 10 - 15% slippage provides maximum braking.