Brake Traction Control System

[TheScrutineer]

Hi all,

I have just purchased a Focus ST170. I noticed that the car has standard traction control which you can turn on and off. It also has a 'Brake Traction Control System' which is permantly on. This is not ESP, as that is another option on the car entirely.

Well, the traction control light came on for this Brake Traction Control system yesterday when I went over a rather bumpy potholed surface. Am I right to assume that its acting almost like a limited slip diff? By that I mean it is limiting one front wheel (FWD) from slipping by applying brakes to it. I would assume this is correct, and that the standard 'Traction Control' limits engine power to BOTH wheels in event of wheel spin?

Are my assumptions correct?

[Rockefella]

Were you using the brakes when it went off?

[CapnBoost]

It limits tire spin, it does not in any way act like a limited slip diff. There are a few ways of executing traction control, this way uses the brakes. I don't think that model focus comes with a limited slip so it would only be controlling the single wheel. Here's an article on traction control.

[RacingManiac]

I'd guess its there in place of an real LSD....it's similar to what a lot of RWD car that does not use a mechanical LSD.

[coolieman1220]

the traction control limits throttle. it cuts your throttle when it detects slip due to acceleration. the brake traction try's to keep to straight and safe by individually sending brakes to the wheels necessary to keep you going straight.

thats wat i believe it is. in my G35, you can turn off the VDC (traction control) and it won't cut throttle however if you rip a hard turn you will hear the brake control making noise

[RacingManiac]

Originally Posted by coolieman1220

the traction control limits throttle. it cuts your throttle when it detects slip due to acceleration. the brake traction try's to keep to straight and safe by individually sending brakes to the wheels necessary to keep you going straight.

thats wat i believe it is. in my G35, you can turn off the VDC (traction control) and it won't cut throttle however if you rip a hard turn you will hear the brake control making noise

Ditto...the newer G35 now have actual LSD as an option for both auto and manual(older G35 only on 6MT).

[CapnBoost]

Originally Posted by RacingManiac

I'd guess its there in place of an real LSD....it's similar to what a lot of RWD car that does not use a mechanical LSD.

not really, it's there to minimize slip- not maximize traction.

Originally Posted by coolieman1220

the traction control limits throttle. it cuts your throttle when it detects slip due to acceleration. the brake traction try's to keep to straight and safe by individually sending brakes to the wheels necessary to keep you going straight.

This system doesn't use spark retard or throttle control to effect traction management. this uses the vehicles brakes to slow the wheel down. It would be most accurate to describe it as the opposite of abs.

VDC is similar, but it's more focused on unwanted lateral movement than the drive tire's traction. Think yaw more than tire spin.

relevant video:

YouTube - Fifth Gear ABS, TC, ESP Full version

YouTube - Fifth Gear ABS, TC, ESP Full version

[TheScrutineer]

Hi Rockefeller, the brakes were not used.

CapnBoost, thanks for you info. However, the point about it being there instead of an LSD - If its cutting wheel spin to an individual driving wheel that is slipping then surely it is a form of LSD? When would the engine decide to use this method to stop loss of traction rather than cutting engine power?

And you are right, the Focus does not have an LSD.

[pimento]

It's not so much a form of LSD as an emulation of one -- it's not a diff, so it ain't no LSD. This would be used to stop the loss of traction to a single wheel, cutting engine power would be for a loss of traction at both wheels. At least, I'd presume as such.

[RacingManiac]

Originally Posted by CapnBoost

not really, it's there to minimize slip- not maximize traction.
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Hence limited slip differential. Traction control mainly is there to control speed differential between driven vs free wheel, LSD controls the relative speed between the driven wheels. Be it as mechanically locking or by braking the slipping wheel. IMO minimizing slip = maximizing traction. Anytime you are stopping the tire from slipping you are managing traction. Traction control should work on primarily on controlling wheel speed vs forward velocity and LSD should just be between the left and right velocity regardless if the car is losing traction against "ground speed". With modern electronic based system it can get real blurry where the line is between the 2....

Without knowing what ST170 has I don't think I can say for sure, if your TC is off, and you are not on the brake on a straight and bumpy road and its acting, then I am guessing it is a LSD function, because a mechanically locking diff would be working too...

[CapnBoost]

Originally Posted by TheScrutineer

Hi Rockefeller, the brakes were not used.

CapnBoost, thanks for you info. However, the point about it being there instead of an LSD - If its cutting wheel spin to an individual driving wheel that is slipping then surely it is a form of LSD? When would the engine decide to use this method to stop loss of traction rather than cutting engine power?

And you are right, the Focus does not have an LSD.

A limited slip differential is a performance part. It's there to maximize traction during acceleration by sending torque to both (or all in the case of awd)powered wheel hubs, but it will not minimize slip. Traction control is a safety component that minimizes slip when traction is compromised.

Basically- if you cane it on dry pavement with traction control on you will likely accelerate slower than if you had it off (after wheelspin subsides anyway). Whereas if you punch it on ice with a limited slip differential the tires will spin forever and nothing will happen.

Since this is not a pure performance variant of the Focus they went without the LSD and gave you traction control.

[Alastor]

Originally Posted by CapnBoost

A limited slip differential is a performance part. It's there to maximize traction during acceleration by sending torque to both (or all in the case of awd)powered wheel hubs, but it will not minimize slip. Traction control is a safety component that minimizes slip when traction is compromised.

Basically- if you cane it on dry pavement with traction control on you will likely accelerate slower than if you had it off (after wheelspin subsides anyway). Whereas if you punch it on ice with a limited slip differential the tires will spin forever and nothing will happen.

Since this is not a pure performance variant of the Focus they went without the LSD and gave you traction control.

Like RacingManiac said I don't think it necessary clear what the system should be classified as.

If a Focus with an open differential had one powered wheel on ice and one dry pavement, then when accelerating the wheel on the ice would just spin and the car would not go anywhere.

However, if the system in question then applied braking to the spinning wheel it would transfer torque/power to the wheel on dry pavement and the car would be able to move. In this case the system could be considered an electronic LSD.

Not to say the system isn't for traction control, just that an electronic system could mimic more than one type of traction device.

[CapnBoost]

Originally Posted by Alastor

Like RacingManiac said I don't think it necessary clear what the system should be classified as.

If a Focus with an open differential had one powered wheel on ice and one dry pavement, then when accelerating the wheel on the ice would just spin and the car would not go anywhere.

However, if the system in question then applied braking to the spinning wheel it would transfer torque/power to the wheel on dry pavement and the car would be able to move. In this case the system could be considered an electronic LSD.

Not to say the system isn't for traction control, just that an electronic system could mimic more than one type of traction device.

What you're describing is more like Nissan's Attesa e-ts or Mitsubishi's Active Yaw Control. It's an electronic/ electromagnetic center diff that constantly varies torque to each hub.

Rather than the differential distributing the power, the brakes are limiting the power to prevent slip. I suppose it could be considered similar to a differential, but instead of biasing the torque from one hub to another it's eliminating it with the brakes.

Imagine you have a 700hp firebreathing monster at the race track. With a locking rear diff you dump the clutch and get two long black streaks on the pavement. When you turn on traction control (with the brakes, like the focus) and dump the clutch the computer will sense slip the brakes will clamp down on the drive wheels and you will drive away with very little drama and no black marks. On a spark retard system (not the brakes like the focus), the computer retards the spark limiting the power of the engine until traction comes back.

Racelogic spark-retard traction control:

YouTube - Locost Racelogic Traction Control

YouTube - Locost Racelogic Traction Control

[RacingManiac]

A diff, any diff, allows wheel speed differentiation to happen. Be it as under power or just turning a corner. The difference with adding power to that equation is that you are adding energy/torque to the axle, which adds wheel speed. Now torque like electrical current in a circuit, likes to go to the path of least resistance, in the case of a powered axle, its the wheel of less traction. It could be a tire on ice, or a inside wheel that's unloaded during cornering, no difference. Open diff, all the power will go to that wheel. A LSD's job(regardless what type) is to prevent that from happening. Be it as a gearset based system like Torsen which locks up the gearset, or a clutch-disc salisbury type that engages the discs based on preload, or a electronic braking based system that just brakes the spinning wheel, the result is power transferring to the other wheel. You don't need AYC or ATTESA ETS to do that......

For modern car, you already have all the sensor and actuator for ABS operation and traction control, to add a brake-based electronic emulation of a LSD is just more lines of code, as opposed to an actual diff....Especially in most cases, there are no vehicle behavior modification, it is not yaw or input based, just a simple left-side is spinning faster than right-side and I need to brake left wheel to equalize the speed...

[johnnynumfiv]

Originally Posted by RacingManiac

A diff, any diff, allows wheel speed differentiation to happen. Be it as under power or just turning a corner. The difference with adding power to that equation is that you are adding energy/torque to the axle, which adds wheel speed. Now torque like electrical current in a circuit, likes to go to the path of least resistance, in the case of a powered axle, its the wheel of less traction. It could be a tire on ice, or a inside wheel that's unloaded during cornering, no difference. Open diff, all the power will go to that wheel. A LSD's job(regardless what type) is to prevent that from happening. Be it as a gearset based system like Torsen which locks up the gearset, or a clutch-disc salisbury type that engages the discs based on preload, or a electronic braking based system that just brakes the spinning wheel, the result is power transferring to the other wheel. You don't need AYC or ATTESA ETS to do that......

For modern car, you already have all the sensor and actuator for ABS operation and traction control, to add a brake-based electronic emulation of a LSD is just more lines of code, as opposed to an actual diff....Especially in most cases, there are no vehicle behavior modification, it is not yaw or input based, just a simple left-side is spinning faster than right-side and I need to brake left wheel to equalize the speed...

What about a spool/welded diffs?