Disabling ESC, torque vectoring, curve control & changes to handling characteristics

Disabling ESC, torque vectoring, curve control & changes to handling characteristics

As the title states I've been curious what the ST would behave like with some of the electronic driving and chassis aids disabled. Due to how a lot of stability control, the torque vectoring and curve control features are implemented using the ABS (anti-lock brake system) controller I doubt there will be any way in the short-term to disabled them via current tuning solutions that interface with the Bosch powertrain controller.

So how can they be disabled and what do the different modes and features do? The "sport" mode on the ESC (electronic stability control) raises thresholds where stability control will intervene but apparently doesn't alter the torque vectoring and doesn't seem to change the curve control characteristics if you dive into a corner a little too hot. If you use the full ESC "off" mode it turns off the stability control, which can help rotate the car by braking individual wheels when grip is exceeded, but the torque vectoring and likely curve control are still active. My temporary solution to test how the car behaves without these electronic features was to disable the ABS as all of them utilize the brakes or a combination of braking and power modulation to bring about desired wheel rotational characteristics (i.e. slow individual or multiple wheels in order to rotate the car and/or transfer power to the other wheel in the case of the front wheels while accelerating).

To go off on a tangent for a moment so everyone understands what the electronic systems are doing, torque vectoring can be thought of and is often referred to as an electronic limited slip differential, also sometimes referred to as a brake-lock or brake-limited (limiting) differential. The six-speed Ford MMT-6 transaxle used in the ST has an open differential (diff) which means when traction is lost due to poor road surface conditions or when exceeding available traction at one or more front wheels the diff will still attempt to send torque to the wheel that has lost traction and because it can't send more torque to the wheel that hasn't slipped than what it can send to the spinning wheel it results in a loss of forward acceleration. A conventional limited slip or automatic torque biasing diff uses internal clutch packs or gears inside the differential to ensure engine torque is also transferred to the wheel with traction instead of an open diff having one wheel spinning and little to no torque going to the other wheel.

The torque vectoring or electronic, brake-based "diffs" work by acting on a spinning wheel at the rotor and slowing it to ensure torque is transferred through the open differential to the wheel with traction. This generally isn't as seamless in operation, isn't as quick to respond since the computer has to sense an impending spin condition then apply the brakes in order to transfer power, and because of how it uses the brakes it does utilize some of the brake capacity to scrub off wheel speed which increases rotor and pad temps. For daily driving and an occasional spirited drive or for short track events they can work pretty good but usually aren't preferred for heavy track use.

Electronic stability control (ESC) works by using yaw (rotational) and lateral accelerometer sensors in addition to a steering angle sensor to determine if the vehicle goes over a preset limit where the controller determines the car is sliding and/or watches the desired steering wheel angle and can tell when the car is understeering or oversteering and not going in the intended direction of travel. It then engages brakes at individual wheels at each corner to help rotate the car into the intended path of travel or correct oscillations that might cause the vehicle to spin. There are three modes on the ST, full on which does allow for a little vehicle slip before it intervenes, a sport mode that allows you to track the car but still have the system step in if you exceed the chassis and tire limits if the car is about to spin and full off mode that completely disables this safety net.

The Curve Control feature was a little more difficult to figure out what exactly it's doing on a technical and system level. The intent is that if you enter a corner or curve too fast, such as a highway off-ramp, the system will intervene to help maintain the desired path of travel. Ford also advertises an electronic understeer control feature but this appears to be primarily provided as part of the torque vectoring if the car is accelerating. During braking or when the controller detects there is impending deceleration coming into a corner the Curve Control or stability control functionality appears to be responsible for actively reducing understeer (front end "push" or the tendency to continue in the direction of travel vice turning into the corner).

Curve Control does this by using the stability control system (which again is likely based primarily in the ABS controller programming) to determine you're possibly about to enter a corner or curve by looking at sensor readings such as the steering wheel angle and other sensors and how readings start to ramp up as the vehicle begins into the turn. The moment you start to brake or request more steering angle than you'll have traction available it immediately applies full braking force before the car has started to rotate into the turn (curve control), then applies individual braking at each corner (ESC, and as necessary) as a preemptive stability control feature to help reduce understeer before the need for full stability control to intervene and correct for a maneuver that has already exceeded the vehicle's dynamic limits. Ford quotes the system as being able to scrub off ~10 mph as soon as you first apply the brake pedal.

All these features are made possible by sensor data and computer algorithms to determine desired chassis and handling characteristics. The controllers then utilize engine torque management and ABS to modulate individual wheel speed to change directional behavior characteristics of the vehicle. Basic traction control for example uses the ABS wheel speed sensors to determine when one or both wheels are spinning (compared to each other and/or the rear, non-driven wheels on a front wheel drive vehicle) and can reduce engine torque to bring the spin under control by closing the throttle, altering ignition timing and/or reducing boost. If that isn't effective it can also use the braking system to apply just enough brake pressure to stop the wheel over-speed (spin) condition.

The anti-lock braking behind most of these features uses highly accurate wheel speed sensors and an integral hydraulic control unit (HCU) that contains a hydraulic pump to build brake pressure without having the driver press on the brake pedal, a controller module that provides the logic to drive the systems, and a valve body assembly that has a valve for controlling brakes pressure to each wheel (with that pressure provided by the hydraulic pump and/or driver brake force from the brake master cylinder). The valves work by opening to allow brake fluid to pass, increasing braking force or closing and allowing fluid to return to the master cylinder/ABS HCU and therefore reduce pressure to a specific wheel. Older ABS used to have a fixed duty-cycle that could only switch these valves on and off a set number of times per second but some newer systems use pulse width modulation (PWM) on the valve solenoids to be able to continually vary braking pressure to each wheel. This is why you don't feel the ABS buzzing in the brake pedal when these stability or torque vectoring features are active on the ST.

A few other features made possible by ABS are panic brake assist, electronic brake force distribution (EBFD, often called EBD) and hill start assist. Panic brake assist uses algorithms that watch how quickly you let off and close the throttle (i.e. how fast you get off the gas) and then how quickly you get on the brake pedal. If the system determines you're possibly attempting a panic or emergency stop that requires full brake force it will automatically use the ABS hydraulic pump to apply full brake pressure up to the anti-lock modulation threshold before you've had time to fully depress the brake pedal. This can have a significant impact on the time required to fully engage the brakes and bring the vehicle to a stop. It doesn't seem quite as noticeable on the ST compared to implementations in some earlier vehicles but if you do experience it you can easily disable or cancel it by slightly letting off on the brake pedal. These systems normally assume you aren't left foot braking which may cause premature application of the feature under certain circumstances.

Electronic brake force distribution (EBD) uses the ABS system in place of (or occasionally to supplement) a conventional rear-brake proportioning valve. On older cars prior to the advent of modern ABS they would use a mechanical proportioning valve inside the hydraulic braking circuits between the front and rear brakes to ensure the rear wheels didn't receive too much pressure in a stop that would cause them to lock up. The brake master cylinders normally provide a variable amount of fluid between the front and rear circuits depending on how hard and fast you get on the brake pedal, but under most stops the rear brakes only provide a small portion of the total braking force (often in the 15-25% range). In a panic stop when the nose of the car is diving due to weight transfer, if you were mid-corner and the rear brakes received too much pressure they would lock up and it would be easy to lose control.

The brake master cylinder provides a very rough amount of brake pressure differentiation between front and rear depending on how you apply the brake pedal but it's the proportioning valve that limits how fast and how much brake pressure can ramp up in the rear circuit/wheels. EBD takes this further and instead of a somewhat fixed response a mechanical valve provides the EBD can dynamically use the valves in the ABS HCU to limit and control changes in pressure to the rear wheels on a continually adjustable nature. This means it can adjust for things like different vehicle loads, surface conditions, changing chassis dynamics, etc.

If you're coming to stop slowly it can use more rear brakes which helps increase front pad life. In a heavier braking application it can apply the exact amount of needed rear brake force without upsetting the chassis, such as during mid-corner braking. It does this by using precision, four-channel ABS (where each wheel has it's own wheel speed sensor and control circuit) to watch the rear wheel speed and can tell when a wheel is nearing impeding lock-up under heavy braking. It also continually compares rate of deceleration changes between the front and rear wheels and under light braking can use more rear bias than a conventional proportioning valve would provide when lockup is unlikely to occur. Some EBD implementations also integrate into the stability control system but prior to the US federal mandate for ESC there were quite a few cars with ABS and EBD but no ESC (and I won't use anymore acronyms in this sentence).


Back to the question at hand, how does the ST behave with ESC, torque vectoring, and curve control disabled? Since they all use ABS it would seem simple to just kill that system. We can't disable it via software (i.e. tuning changes to the ABS controller) so that leaves us with removing the ABS fuse or disconnecting a wheel speed sensor. I chose to disconnect a front wheel speed sensor as the system won't be able to compare wheel speed differences across the front axle and therefore can't provide torque vectoring. Disconnecting a rear sensor may very well disable the entire system as well but will require further testing. In any case if any of the four wheel speed sensor aren't working it will disable stability control and anti-lock braking. If I had chosen to pull the ABS fuse this would also have disabled EBD and caused the system to default to a fixed front-to-rear bias ratio that might cause the rear wheels to lock up prematurely.

The initial thing I noticed after I disconnected the front, driver-side wheel speed sensor was an increase in steering effort. It took me a moment to realize that the speedometer was also inoperative which means Ford is using the ABS controller's front wheel speed sensor to provide the powertrain controller with vehicle speed information vice using a separate vehicle speed sensor (VSS) on the output shaft of transaxle or the drive axle. When the car can't determine vehicle speed it appears to default to a fixed power assist level that makes for slightly heavier parking lot speed steering and the assist also seems to remain linear. Normally you get more assist at parking lot and lower speeds and the assist decreases as the vehicle speed increases.

I ended up taking the car on about 15 miles of twisty two-lane backcountry roads that I'm very familiar with and drive on a weekly basis. The speed limit in this canyon and hilly area is around 45 mph in most sections and it includes some decreasing radius, hairpin and various other turns as the road rises and falls. The surface ranges from fairly smooth to areas that have rougher, broken pavement.

Without the above-mentioned stability and torque vectoring aids the character of the car changes quite a bit. It's very apparent the chassis was tuned as a complete system and it's assumed these electronic systems will be available and working when driving the car at higher speeds and approaching its limits. One of the first things I noticed when diving into a corner a little on the fast side was that the slightly unnatural, low amount of understeer was gone. In its place was what I'd expect from a sporting, nose heavy front drive car and it started to push into slight understeer much more readily. The limits are still very high and the tires provide a commendable amount of grip but the video game like nature of how the car rotates was gone.

Getting back on and off the throttle and using it to modulate power and weight transfer still provided a means to help rotate the car but isn't quite as natural and the chassis didn't feel anywhere near as balanced without the electronics. For example if you come into a corner a little quick or too low and start to understeer, if you reducing braking to transfer weight back to the front wheels, the rear end that normally feels very controllable and lively starts to feel twitchy and like it might step out. I think a lot of this is due to the super-quick steering ratio once you move off-center and are approaching the 10.1:1 range. You can still control the car but it doesn't feel as cohesive.

When you get through a corner and are getting back on the throttle the next surprise appears. If you romp on the throttle with the electronics active I've still seen a little bit of wheel spin which led me to wonder how affective the torque vectoring, braked-based limited slip function works. With it disabled it becomes pretty apparent that it does work even if not to the level of a true mechanical limited slip. Coming out of a couple corners where I applied nearly full throttle it roasted the tires and I had to significantly back off the throttle reducing forward momentum.

This significant reduction in power and the time it took me to make that change would take a lot longer and slow the vehicle much more than just letting the torque vectoring step in. I've seen some concerns that by using the brakes to control wheel speed it is slowing the vehicle and while that might technically be true the system is primarily controlling a wheel that already isn't contributing to forward motion and transferring power to the wheel that can, and does so much faster than a human could intervene by backing off the throttle in an attempt to restore traction.

As the road straightened out and I had a chance to do some acceleration from a stop in a straight line and acceleration from a stop going into slight turns and torque steer seemed about the same as it was with the electronics enabled. I've been driving the Focus for many years and even my stock SVTs have twitchiness to them when accelerating hard on undulating surfaces (i.e. mild torque steer). The European C1 cars haven't been a lot better in this regard and we've seen how this chassis has done for the Mazdaspeed 3 and most press noting how that with significant engine torque applied the wheel tugs quite a bit. The Focus and it's platform provides great handling but has always seemed a touch prone to torque steer. Whether that's due to one or a combination of suspension geometry, tire sidewall, axle design and alignment or another number of factors is unknown.

Accelerating through a corner the torque steer didn't seem an issue either but I suspect that was because the inside wheel was lit up spinning and not providing enough traction. Should there have been enough grip it may have tugged at the steering wheel more but even with ABS disabled the electronic controlling the electrical assist motor on the rack are likely still working. It's difficult to say but when someone installs a true limited slip diff we'll have a better idea how the car will behave. I'd also like to see how the car behaves with the electrical steering disabled but want to verify with TRW that the belt-driven motor system won't be damaged if driven for an extended period without power.


Overall I summarize the experience as indicative of the true nature of the chassis. There's lots of grip but it does rely heavily on the electronics working as a team with the chassis tuning to produce the best results. Without them the steering becomes twitchy as you're wondering if (when) the rear steps out too far if you'll be able to perform super precise counter-steering since any small change in steering angle moves the front wheels so much. The steering, which has receive minor criticism as feeling a touch numb doesn't feel better with increased effort. It just feels heavier with no increased feedback or road feel and you can't tell what the contact patch is doing. Without the electronics it can't effectively put down power and won't be able to pull the car out of corners without lighting up a tire.

When everything is working together it works well. Quite well. And therein lies the next question for owners who are always looking to improve their vehicles. Ford spent a lot of effort getting the ST to ride somewhat smoothly, absorb impacts and bend into a corner with haste. It does this by using some electronic wizardry that purists might not appreciate but the end results are impressive. For someone wanting to drop suspension parts onto the car they'll need to evaluate if handling is a priority or if they want the appearance changes. For those wanting to make the car turn even better or improve ride quality and maintain stock levels of handling it will require careful upgrades and likely some trial and error in hopes of improving while working with the systems Ford spent countless hours developing and refining.