PIAGGIO HAVE DEVELOPED A STATE OF THE ART SYSTEM WHICH INTEGRATES ABS AND ASR, AN ABSOLUTE FIRST IN THE SECTOR. THE SYSTEM, WHICH IS THE FIRST OF ITS TYPE IN THE WORLD, COMES AS STANDARD ON THE BEVERLY E X10, THE APRILIA SRV 850 AND ON THE NEW VESPA 946
In the last edition of Wide, we saw how the Piaggio Group made key strategical choices by acquiring expertise in the electronic technology sector to both build and maintain its competitive advantage. The Group can also use its own know-how in electronics and create advanced solutions entirely from within. One of these creations for its own scooters is the first system in the world to integrate ABS (Anti-lock Braking System) and TCS (Traction Control System) also known as ASR (Acceleration Slip Regulation). The system made its debut in late 2011 on the Piaggio Beverly and has since been fitted to the Piaggio X10, Aprilia SRV 850 and Vespa 946. Needless to say, many of the motorbikes made by the group have ABS and ASR – such as the Aprilia RSV4, Dorsoduro, Caponord and the Moto Guzzi California – and are also fitted with sophisticated dynamic control systems to which we will dedicate a specific chapter.
Base dynamic control systems
Everyone knows what the initials ABS and TCS stand for from their widespread use cars. But do we really know what they mean, what they do and above all, how they work?
Let's begin by remembering that cars and bikes have very little in common from the vehicle dynamics point of view. This means that you can't simply transplant devices from one to the other without a lot of adaptation work if not a complete redesign.
As you know, cars have “static stability”, i.e. they don't fall over when standing still. Motorbikes on the other hand have what is known as “dynamic stability”, i.e. they are only stable when in motion. It might seem an absurd statement but has vital implications. One of these implications for example is as follows: the locking of the front wheels on a car under braking increases the stopping distance and makes steering difficult; if the same thing happens on a motorbike, even for just a second, a fall is inevitable and dangerous regardless of who's at the controls. When all's said and done, the motorbike is much more "delicate" and any safety devices therefore have to be more sophisticated.
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A game of force
ABS and TCS systems for motorbikes and scooters are very important for both safety and performance. To better understand how they work, we need to go back to basics i.e. the patch of contact between the road and the tyre. It is here that the forces which allow a motorbike to remain upright, takes bends, accelerate and brake come into force. The tyres generate large and little "slipping" forces with the ground.
Let's focus on the longitudinal forces which are those that come into play when accelerating and braking. If a wheel simply rotates and nothing else, it does not transmit forces and there is no slip; this is the case of a motorbike front wheel going along a straight at a constant speed. If the same wheel transmits a force, for example by braking, it will slip a little i.e. it will have a rotational speed that differs from that which it would have were it just rotating and nothing else. If it is braking, it will have a slightly lower speed. The slip is usually expressed as a percentage of the pure rotational speed.
The first thing to do to make a safety device that controls traction or braking work is to define the maximum allowable degree of slip. This depends on the tyres and the road surface; as it is impossible to predict how much grip will be available to the machine at any given time, choosing the degree of "objective slip" between safety and a limited loss of performance is always a compromise.
Both ABS and ASR decrease the performance of the machine: the former brakes a little less but more safely while the latter decreases the power available but improves acceleration and safety. In both cases, their effects should not be harsh but gradual and almost undetectable.
Two tyres, four wheels
So, the first thing to do is to measure slip so as to ensure that it stays within the safety limits. Machines fitted with ABS and TCS have a sophisticated system for detecting the speed of each wheel. This system consists of a sensor and a phonic wheel. That gives us two real wheels and two phonic wheels. The sensors measure the actual speed of the real wheels and their variation compared with the speed of the phonic wheels. They are connected to a 2 channel hydraulic control unit which activates the ABS system at the moment a wheel is about to lock under braking (increase in slip). The ABS Electronic Control Unit (ECU) adjusts the degree of braking through servo valves which lift or reapply pressure to the brake calipers to optimise braking efficiency.
The system also prevents the wheels from locking on surfaces with little grip, ensures stability and optimises the performance of the brake system.
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A “magic” formula
As mentioned before, under braking micro-slips between the tyre and the ground take place: the greater the braking force applied, the more slip occurs. If the tyre is simply rotating, the speed at the point of contact with the ground would be zero and the speed at the middle of the wheel would be 𝑉= 𝜔𝑟 where 𝜔 is the angular velocity and 𝑟 is the radius of the wheel. If the wheel is braking, the slip footprint and the speed of the point of contact is not zero but is . The relationship between speed of the middle of the wheel and the speed of the point in contact with the ground is known as slip ratio or sleep and is indicated with the letter σ:
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Experimental evidence shows that the coefficient of friction of a road wheel ( increases with up to a maximum, after which it decreases. If any of you remember Coulomb's law of friction, put it aside for a minute until we reach the end of the article: tyres and tarmac simply refuse to accept that law.
The second section of the curve is unstable and the wheel quickly tends to lock. Normal drivers and riders do not use all the grip available on the road as this limit is near the unstable section and braking even just slightly too much can lead to the wheel locking up.
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Under braking, the ABS system compares the peripheral speed of the wheel with that of the car or bike wheel. If the speed of the wheel drops rapidly, this means that the wheel is about to lock. At this stage, the hydraulic control unit decreases the pressure on the brake caliper until the wheel picks up speed again which is an indicator that it has regained grip. Following this, pressure at the caliper is restored to make the maximum braking force available. The cycle repeats with high frequency increases and decreases in pressure until the machine is completely stopped.
ABS increases and decreases
Pressure increase stage.
Valve 1 is open and Valve 2 is closed. The pressure generated by the brake pump acts directly on the caliper and increases the braking force.
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Pressure decrease stage.
Valve 1 is closed and Valve 2 is open The pressure downstream from Valve 1 expands the low pressure accumulator. Pressure on the caliper drops. The pump is activated when the accumulator is drained sending brake fluid upstream of Valve 1 causing typical pulsations of the ABS system.
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The twin brother: TCS
As a concept TCS (or ASR) plays a very similar role to ABS even if in a completely different way: its task is also to control wheel slip in real time, though just the rear wheel in this case, and to cut in rapidly but not harshly should the amount of power being fed to the rear wheel need to be decreased.
In Piaggio Group machines fitted with ABS/ASR, both units use the same sensors (phonic wheels); the ASR control algorithms are implemented directly by the engine management ECU. Sophisticated programs have been developed to optimise machine behaviour under acceleration. The technology was developed from experience gained from racing and adapted to allow for the characteristics of scooters to ensure maximum safety under all riding conditions.
System communication is conducted along a CAN line which is the most evolved protocol for digital automotive communication. The ABS ECU sends wheel speed readings to the engine ECU when can then constantly monitor any rear wheel slip (compared with the front wheel readings which are taken a reference for road speed) and can immediately decrease engine power by adjusting the ignition timing advance setting and, if necessary, by cutting off the fuel injection system.
The ASR system provides riders with great help in controlling the machine on slippery surfaces or, more generally, when accelerating while close to the limits of grip available.
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