LSD and Traction control.
 

I understand both individualy.

I also know you can switch the TC off if you want (I assume it resets every time car is switched on/off).

What happens with TC on with a LSD? dont they kind of work against each other?

If the TCS simply detects slip of both rear wheels and then cuts power then a LSD and TCS will be happy mates.

If the TCS also applies differential braking if one rear wheel slips then I too see incompatibility issues.

The VDC must apply differential braking to accomplish its task. Of course the VDC will not be active until the yaw sensors trigger it.

So I am postulating that the TCS only cuts power if both rear wheels are turning faster than the front wheels. No differential braking is used. Once the yaw sensors detect a slide then VDC applies differential braking and possibly a power cut.

I do not like driver's aids so I am happy that the RX-8 TCS can be disabled. I just wish I could get an 8 without ABS. Yes, I know ABS does wonders but threshold braking is a fun thing to do. Isn't a sporty car intended to be fun!

If you like to threshold brake, just pull out the ABS fuse when you're at the track. Not as simple as pushing a button on the dash, but it's not too difficult to disable ABS temporarily.

Before you do something like that, just remember that there are some cars which have a couple of things on one fuse. You might be disabling more than just the ABS...

---jps

this thing comes with LSD? :p

What about the brake force distribution? I am assuming that since the RX-8 has electronic brake force distribution that there is no proportioning valve. If so, disabling the ABS will mean that the brakes will not be biased correctly.

vipeRX7,

Yes, the RX-8 with 6 speed manual has LSD standard in the USA market. Availability may be different in other markets.

I was referring to the other LSD. :p Thanks anyway :)

It's possible, but I'm not sure how EBD works. I assume there is a default brake bias to work from since the brakes still need to work even if your electronics go out.

Good point. I just wonder how hard the engineers would work to optimize hydraulic brake bias when they know that an electro brain is gonna be default 99.99% of the time.

Of course, this doesn't apply since the car isn't out yet, but the FC and FD RX-7's both had ABS on a single fuse, not shared with anything else.

So instead of braking to the threshhold of wheel lockup, brake to the threshhold of ABS engagement. What's the big deal? It's still threshhold braking, only you don't get flatspots if you get it wrong! ;)

Regards,
Gordon

yeah, but flat-spots are part of the fun ;)

HA!! riiiight. "oh sweet!! now i have this killer vibration on my brand new $200 front-right tyre!!"

You gotta be pretty damn ham footed to flat spot a street tire. If the tire locks up for a few feet the flat spot is neglible. Racing slicks are a completely different story since their abrasion resistance is way lower.

The problem I have experienced with braking to the limit of BAS is that when ABS has actuated you have to back way off the brakes before above takes a break on the brakes.

;) spell that five times fast ;)

good job Lou :D

True,

but depends on what kind of limited slip, and how limited it is. Viscous limited slip takes a second to tighten up. So long as the TCS isn't trying to correct a Major SNAFU, then differential braking could be used for a short time. I think with power limiting control it could be feasable. OEM's are going away from just differential braking because of the abuse the brakes take when drivers keep the power on.

"and how limited it is"
I say that because limited slip clutches can be tuned for a certain percent of lockup/torque transfer. I don't think OEM's really use anything other than 100%, however.


How would things work out with Torsen limited slip?

Tom

P.S. I was 'evaluating' several new 2003 Lincoln LS's with TCS, and in dirt the TCS wouldn't let the car get out of hand enough to take a long time to recover. It would kick in immediately, and quickly recover the vehicle. Didn't get opportunity to test it on snow.

...i guess you couldn't try to spin it with a pendulum turn, eh??

after reading this thread it seems the question about how this works with the abs/ebd leaves out the fact that the abs/ebd is working the front brakes as well as the backs. or have i missed something?

Nope, you saw all of it, just that applying the TCS to the front doesn't have to deal with mechanism that tries to keep the wheels spinning the same. since the spin of the front wheel isn't connected to anything else, the brakes can do what the computer tells them to do.

Tom

oh so i actually was missing a major thrust of the discussion which had to do with how the braking programs work with or against the lsd. or vica/versa. ok i'm on track now. you may continue ;)

Sideslip,

The RX-8 uses a Torsen type LSD so differential reaction is instantaneous.

I will not be getting an RX-8 with the TCS or DSC so I guess I do not have to worry about any interactions amongst these systems. I will leave it to the rest of you figure it out. Good luck.

Nice thread.

A couple of points. TC only works on the driver wheels, so in the RX case the rear wheels only.

TC's only job in life is to keep both drive wheels spinning at the same rate with each other and the fronts. It can do this through brake application and/or cutting the spark to the engine. USUALLY the system is designed to go to the brakes first as most customers find the engine suddenly dying a bit unrefined. With a manual it can, and has to me, stall the engine under very slippery conditions.

As far as the LSD and TC, the TC doesn't care or look at what the LSD does. It simply looks at the wheel speed of each rear wheel and the front wheels. If either the rear wheels aren't spinning at the same speed or the rears are spinning faster than the fronts it kicks in.

But, a smart development engineer will be sure to set the TC threshold to work best with the LSD so as not to give the car odd feeling handling. This is easy since once the diff does it's job the the rear wheels can no longer spin at different speeds. So all the TC needs to do is compare the rear to the front and it can then brake both rear wheels together.

However, like all else ------- junk in -----------> junk out!

A fellow engineer has a very good site on ABS and EBD which will explain both in laymen terms:

http://www.geocities.com/nosro/abs_faq/

An excerpt from the site on EBD:

What is Electronic Brake Distribution?



"EBD" is also sometimes called "Dynamic Rear Proportioning" or "DRP". When a vehicle brakes, weight shifts forward so that the front wheels are carrying more weight than the rear wheels. As weight is removed from a tire, its ability to grip is reduced. Under braking, since the rear wheels are carrying less weight, they are more prone to skidding (locking up under braking). This is bad. Very bad.

Some people expect that locking up the rear wheels should be preferable to locking up the front wheels. After all, at least with the front wheels unlocked, you'll be able to steer. This is true. However, locking the rear wheels causes the vehicle to become unstable. That is, with the rear wheels locked, the vehicle will tend to spin out in the same way that an upside-down broom balanced on the palm of your hand will tend to tip over. Sure, you can try keep the inverted broom balanced on the palm of your hand by moving your hand around, but you have to be quick. The same goes for steering when the rear wheels are locked. The vehicle will tend to spin out and you will be able to steer, but you must be extremely quick. As the vehicle speed increases, the tendency to spin out becomes even greater and you need to be even quicker to steer out of a spin. Eventually, you reach a speed where not even the best driver can react quickly enough. Therefore, it is always preferable to lock up the front wheels, or at least to lock up the front wheels before locking up the rear wheels.

So what is done about this? In the days before ABS, a "proportioning valve" was used to reduce rear braking as you braked more. For example, this mechanical device might reduce the rear braking force by 10% if you applied 50 pounds to the brake pedal and reduced the rear braking force by 20% if you applied 70 pounds to the brake pedal.

EBD uses the ABS hardware to take over this task. The mechanical proportioning valve is removed. Instead, ABS uses the wheel speed sensors to determine when the rear wheels are spinning minutely slower than the front wheels (we're talking about less than 1% here). This would be due to the rear wheels being comparatively over-braked relative to the weight on the tire. ABS uses this information to cap or reduce the brake pressure to the rear brakes, using the same hydraulic mechanisms as ABS. You won't hear this happening, however. When ABS is working, you hear a buzzing sound - this is the ABS motor which turns the pump. However, in EBD mode, not much fluid is being moved around, so the motor is not turned on to save you the annoyance. You also will not likely feel anything in the brake pedal since so little fluid is being moved.

EBD has become extremely popular for the reasons that it is inexpensive (after all, it is just some extra software) and actually does a better job than the old mechanical device. While the mechanical device worked in a fixed pre-determined way, EBD can adapt to any particular situation, including a loaded trunk, or tires with different stickiness.