Bosch has launched its Safety Control System with ABS and sway mitigation for caravans and heavy trailers and we went along to see how well it works.

TRAILER TOWING is far more difficult and dangerous that normal driving.

Sure, it is normal for people to claim they “don’t know the trailer is on the back” and manufacturers to crow about how good their trailer tows, but the simple fact is that when you tow heavy trailers you fundamentally change the dynamics of your vehicle, and not for the better. This truth will only really become apparent when the vehicle’s limits are approached and the trailer begins to wag the vehicle.

There are two disturbing trends in the caravan world. First, over recent years trailers have become bigger and heavier, with little thought given to dynamics, total weight, and most importantly, weight distribution. Second, tow vehicles have become more powerful but not more capable.

Car makers are engaged in a marketing war where they all want to claim the highest tow rating in their class, so they headline figures like a 3500kg towing capacity when in fact that figure is only achievable in very specific loading circumstances which would almost never be the case in real life.

Yet the complexities of trailer weights are such that the buyer wouldn’t know the difference, and trusts the earnest looking fellow in the sales shirt because well, they are an expert… aren’t they?

What makes a good tow vehicle isn’t power, it’s controllability which generally comprises of: long wheelbase; strong chassis; short overhang; all-wheel drive; and weight relative to the trailer. Many tow experts say you shouldn’t tow a trailer more than the weight of the towcar, and I agree – yet we have 2.2 tonne utes trying to tow 3.5 tonne trailers. It is no wonder that there are so many trailer accidents – take a look at the Clayton’s Towing Facebook page, or listen to their podcast for details.

It doesn’t help that according to one insurer the average age of a caravanner is 63, and many are relatively inexperienced at towing. Now, it is a maxim of the Australian male that they are born with an inherent knowledge of all things automotive, but sadly that’s not true and towing is not only a distinct skill, there’s a lot of technical knowledge required before you even turn the key.

So, it is into this climate that Bosch has launched its trailer stability control and anti-lock braking system (ABS) product, and we went to the Australian Automotive Research Centre to check it out earlier this week.

First, what is Bosch? It’s the world’s largest automotive technology supplier and one of the leaders in driving aid systems such as stability control, ABS and traction control. There are only a few makers of such systems worldwide supplying all the car makers, who then help calibrate and re-brand the electronics. That’s why stability control is called DSC by one carmaker, ESC by another and VSC by a third… it’s all the same base electronics.

Bosch’s Australian arm is a major research and development facility which focuses on off-road systems as well as being Bosch’s centre of competence for trailers, and using its decades of experience with cars, it has designed a trailer stability control and ABS system. To explain how it works, first a brief recap on the fundamental technologies.

When a car corners it will, ideally, track in an arc precisely defined by the angle of the steering wheel. If it runs wide that’s understeer, if the back end is lost and the car starts to spin that’s oversteer. Neither is desirable when driving. A stability control system takes input from many different sensors – wheel speeds, steering wheel angle, yaw, throttle, brake and more – to decide what the car should be doing, and then it’ll help the car do it by varying the braking pressure on individual wheels, and even where necessary reducing the throttle. Basically, stability control keeps the car pointing where it should be, and there are all sorts of statistics to prove that is has a massively benficial effect on road safety. That’s why it’s been mandatory across Australia since 2008.

ABS is an anti-lock braking system. When a wheel is braking so hard it slides over the road you lose steering and directional stability, so it’s better to keep the wheel rotating. In the dry you still get pretty decent stopping capability, but in the wet as the tyre isn’t rotating it can’t clear water out of the way and stopping distances are vastly increased. ABS simply detects when a wheel is about to lock (stop rotating) and fractionally reduces the brake pressure, preventing the lock.

The image below shows Bosch’s test commodore in four states; both wheels rotating, fronts locked, front and rear locked, then still front and rear locked but in a spin.

Both ABS and stability control have been around on road cars for many years, but their introduction to trailers is new. Well-known trailer accessory manufacturer AL-KO has had a system on the market for a while, and now we have Bosch. 

The Bosch trailer system works in a similar way to its car system. There are wheel speed sensors on each of the trailer’s wheels, and when the system detects one wheel is slowing down significantly relative to the rest then it will reduce that brake pressure, preventing a wheel lockup – that’s the ABS function. As with cars, this means more effective braking, particularly in the wet. It also means effective split-surface braking, which is when the left wheels of the trailer are on a high-traction surface and the right wheels on low-traction. Without ABS both wheels get the same amount of braking, which leads to a yawing moment (spin), not great for trailer control. With ABS the braking pressure is modulated so there’s no wheel lockup. ABS alone is a huge boon for trailer drivers, but there’s more.

The Bosch system also include stability control, for when the trailer starts to snake behind the towcar (sway), or otherwise fails to follow the towcar as it should, for example when braking hard and it jacknifes, or when the rig corners and the trailer runs wide.

Again, the principle is the same as car stability control – individual wheel brake pressure is varied to keep the trailer on track. However, there is one difference. With a car you have the steering wheel angle as an input, but not with the trailer. So how does the system know the difference between what would normally be a cornering input, and the onset of a problem?

The answer is the rate of change of yaw (turn rate). Let’s say you’re cornering – the build up of yaw is relatively gentle, but when the trailer starts to lose traction the rate of yaw suddenly increases. As the system is monitoring things 25 times a second then it can very quickly detect and react to problems and fix them even before they’ve fully developed. While it reacts instantaneously, it does so based on a short history of what’s just happened so problem events can be detected.

Some of the components in a stability control system. There’s no steering angle sensor for the trailer though!

The Bosch system is completely independent of the towcar, and requires only electric brakes with an approved brake controller. Bosch say it has tested it with most of the mainstream units on the market, and it is compatible with all but a few. When the product is released onto the market next year there will be an approved list. 

There is no driver calibration required or input, you just drive the trailer as you would normally, and the brakes work as they would for any other comparable system. The system is always active, but does not interfere unless it detects the onset of a problem situation. The system can work on trailers with up to three axles (six wheels), and up to weights of 20 tonnes. It will be able to be retrofitted to older trailers, provided the brake controller and electric brakes are suitable. 

The system is independent of, but compatible with tow car safety systems such as stability control, and trailer stability control (TSC) the variant of stability control specifically designed to mitigate trailer sway. It’s a bit confusing, but TSC on towcars works by detecting sway through the forces on the towbar generated by a swaying trailer, and then brakes individual wheels on the towcar to cancel it out – that’s nothing to do with the trailer itself. This new system, also called trailer stability control, works on the trailer itself to do much the same thing. The best solution is to have both. TSC is also called TSM, or Trailer Sway Mitigation, and you should look for it on your next towcar.

The Bosch system can work with any trailer and doesn’t require specific calibration for different tyre sizes or trailers. That is because it is a closed-loop system, establishing a baseline and then looking for relative deviations from it, as opposed to needing an absolute to work off. For example, the ABS system looks for relative deceleration of a wheel to the rest of the wheels, not a specific speed that wheel should rotate at.

A big question for off-roaders will be what happens in the event of a system failure, for example loss of a wheel speed sensor. The answer is that the axle on the failed wheel is disabled by the system and the rest keep working. The failure can be notifed to the driver, but exactly how that is done hasn’t been determined as the system hasn’t completed its development life-cycle.

Robustness seems to be designed in. We had a good look underneath and the electronics look pretty well sorted on the de-logoed Jayco Basestation used as a test rig behind a manual Ford Ranger PX2 3.2 4WD. The Basestation has a tare weight of around 2200kg, and there was a fair bit of test equipment on board including the outriggers. It has four-wheel independent suspension as seen below.

Underside of the Jayco Basestation test caravan.

Pricing for the system has not yet been released, but Bosch indicated it would be in the order of $1000, and it should be available in 2018.

So that’s all the theory, and now, how well does it work? We didn’t get to drive or ride in the test rig, but to be honest you’d learn more watching outside. Bosch did two demonstrations to prove the system’s value. The first was split-surface braking test at about 50km/h where the left wheels were on high traction and the right wheels on low traction. Without the system enabled the trailer jacknifed under braking. Without it enabled the braking was straight and true.

The second test was two high-speed lane changes at 80 and 100km/h. These were done only with the system enabled due to safety concerns. The trailer tracked direct behind the towcar, and impressively so. We believe there is no way that could have been done without the trailer stability system enabled as there would have been a jacknife or significant onset of sway, maybe even a rollover.

In my opinion…

There are two types of trailer tower – those that have experienced a moment like trailer sway, and those that haven’t but will. When you do have such a moment then, in a fraction of a second, the value of systems like this become very clear and whatever the price is suddenly becomes extremely good value. So, based on what we’ve seen, this system will be definitely recommended for all trailer towers, and we think there’s a good case for making it legally mandatory for trailers, much like stability control was made mandatory for cars. It will be interesting to see if the caravan insurance industry offers discounts for its fitting as well, given than Bosch says 80% of loss of control incidents are a total loss.

There is however a significant danger with this system because it’s so good. Right now, we have, on the roads many overweight trailers with poorly-distributed loads pulled by towcars operating beyond their design limits. These inherent trailer/towcar set-up problems are often masked or mitigated to some extent by weight distribution hitches, extremely stiff suspension, sway devices and more. This new system from Bosch has the capability to mask poor trailer set-ups, but it can only do so much. Owners still need to pay careful attention to trailer setup, and ensure this fantastic safety device never needs to work its magic. 

AL-KO vs Bosch

Trailer aftermarket specialist AL-KO has had a trailer stability system on the market for some time. Its system is less advanced than the Bosch version, as it only applies trailer brakes to all wheels in the event of sway. The AL-KO system does not brake individual wheels, and nor does it have an ABS function. Nevertheless, AL-KO has proven the value of the system in numerous demonstrations, and they have worked with Bosch on their system.


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  1. The trailer sway system being developed by Bosch sounds excellent. I trust that action is already in hand to educate road associations (eg. RACQ, RACV, NRMA, et al) and other relevant parties of the system’s benefits. It seems that legislative / regulatory bodies also need to embrace and mandate such new technologies. Great work by Bosch!

    1. Yes it is, but they weren’t the first to market with a system.

      The bigger issue is driver education on towing and better designed caravans from a dynamics perspective, plus less misleading information from carmakers and trailer makers alike.

  2. Why was the comparison made with AL-Ko and didn’t include Dexter. A true comparison would have included all three offerings that are available in the market today.

    1. It’s not a comparison, it’s a report on the Bosch system. Al-Ko helped develop it so that’s why the extra bit about their system was added. The Dexter system is part-way between the two – it doesn’t have ABS, but it will brake wheels on either side of the trailer. The Bosch brakes individual wheels, the Al-Ko brakes them all.

  3. Very informative article Robert. Nice to read about why and how the system actually works. I would be in the group of people who haven’t experienced a serious trailer sway but not naive enough to say it will never happen.

    The estimated price of the system balanced against what it potentially protects your family and other road users from seems like a very reasonable investment.
    How difficult do you think this system would be to retrofit to an existing van? In our case a 20’ tandem axle with 12” electric brakes.

    1. It will definitely be possible to retrofit. Bosch have not provided details, but based off what I saw and knowing how it works I would expect it to be a wiring job not dissimilar to trailer electrics, but you’d also need a compatible brake controller (most are) and perhaps new brakes as well to allow them to be individually controlled.

      Once you have experienced trailer sway then you will not hesitate to fit one of these systems. It’s a bit like that expensive 4WD recovery gear suddenly being very cost-effective when you are bogged 200km away from help!

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