ABS is quiet a common word and specs in today running cars, but ever wondered what is ABS how it helps increasing cars performance? In this blog we will try to understand what is ABS in depth.
The name itself gives us the insight about how it works. ABS prevent the wheels from completely locking up to improve the stopping distance and increased braking force. You may be wondering? Wait What? " How not locking the wheels will improve the performance of braking , This is thrash" . But indeed in real it really helps improving braking force. To understand this we need to get little bit into " THE DARK SCIENCE" called TIRE DYNAMICS.
When we ask someone in road . What is the shape of tire? 98.99% will say it is circle. Some smart kiddoos that sum up to 1% says it is cylinder. But intellectual like me say "CAN NOT BE DETERMINED" ( SARCASM INTENDED).Basically tires look like cylinder , but it is not a perfect cylinder because tires are squeezed on the road that makes it have contact patch area. This is because of the elastic nature of polymers used in tires. Since this tire is not a perfect cylinder , now it will have some resistance , and this resistance is known as "ROLLING RESISTANCE" .
NOTE: Rolling resistance for pure circle and cylinder is 0.
Rolling resistance is caused primarily by 2 phenomenon in tires.
1. Surface Adhesion
2. Hysteresis
Surface Adhesion:
Surface Adhesion, as the name suggest the polymers are not smooth neither the road does. So the micro level imperfection in the surface lock with each other and adhere to surface , causing some energy to be lost at overcoming this adhesion . so the energy lost in this because of some resistance between road and tires.
SOURCE : phys.org
Hysteresis:
To understand Hysteresis we will do a experiment . Take a rubber bar of any shape and length. Measure the initial length . We assume it to be 10 cm. Now compress it with some force , say 50 N Note the length of rubber bar. Now exert force more that that , say 100N Note the length. Now bring the force back to 50 N from the position where it was 100N. You will observe that the length of the bar in the second case of 50N applied load is lesser than the first case. This phenomenon of visco-elastic material is called Hysteresis. This shows that when rubber is compressed some energy is lost which stops the rubber from coming to the same position in every case of 50N applied load, so some energy is lost in the tires due to compression and relaxation of tires , which resist the car from moving . When a new contact patch of tires comes in contact with the road (loaded patch area), tires get compressed for a time until the new contact patch replace it. So coming back from contact patch means they are getting unloaded .
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Also surface adhesion locks the tires with the road , the applied torque on wheel deforms the tire by some extent ,making it to undergo hysteresis which is the primary cause of rolling resistance in tires.
SOURCE: www.carthrottle.com
NOTE : This tires belong to a dragster car , which transmits around 10,000 HP to wheels . Thus making it obvious to see how tires deform under dynamic condition. A similar process happens in our road cars as well , due to way lesser power compared to dragster this deformation can't be observed in naked eye. If you notice rubber of the tire goes back elongating the rubber. This is because the downforce increase the surface adhesion of tires as a result tire gets locked in road, thus the torque elongates the rubber , which is a significant reason that we will use to study ABS.Understanding Slip:
Due to the longitudinal deformation in tires in dynamics condition, the rubber in tires has this urge to come back to its relaxed position, so the rubber slides to reach there and it is called longitudinal slip. This often happens at the back end of the contact patch of tire moving in front. This is a insignificant , because the amount of energy lost in this is comparatively lesser than hysteresis and surface adhesion.
The resistance that is caused by slip is because of slide , so sliding resistance occurs at this place.
SOURCE : Fundamental Of Vehicle Dynamics, Thomas D. Gillespie
Understanding tire under Dynamic condition:
Under dynamic condition , the loading in tires is not even through out the contact patch. They are high in front middle and low at back. So when the fresh part of tire comes in contact with road the vertical load is quiet low as well as the deformation of tires is low , so the resistance offered at the front of tires is low. While we move little back , where the loading is maximum . The hysteresis is also high due to maximum deformation at that place ( can be seen at the dragster tire image). Which means resistance offered at the place is high. Coming to back part of tire ,where the loading is very less ,so less traction. All the elongation caused at the middle of tires tends to relax ( as explained before ) at this position by slipping .
Rolling resistance is contribute by these three factors , but since slip is insignificant , for all calculation and engineering purpose , the hysteresis and surface adhesion is considered ie. the front and middle part of tires. Where the resistance is high , thus friction is this region is known as peak friction , while the friction in slip region is known as sliding friction.
NOTE: Friction force of tires along the length of contact patch moving in right direction.
UNDERSTANDING ABS:
When tires are locked under braking the tires gets deformed initially , and the tread ( contact rubber ) is stretched to an extent where it can neither further extend nor relax, thus cutting out the energy lost because of hysteresis. So the tire simply slide at the back of the contact patch , where the friction is less (Sliding Friction) . In order to increase the resistance in the tire , we need to maintain the tire in the area of peak resistance , for which tire has to rotate , so that new contact patch can come and undergo hysteresis and energy is lost again. This process makes the car to loose much energy so the car stops even faster.
So by letting the brakes to lock the wheel at intervals helps increasing the braking force much better than locking wheels , thus the brake helps in reducing the wheel RPM and tires themselves help in increasing the resistance together hastens the braking performance.
No of times the wheel has to be locked is completely dependent on wheel RPM and Tire . Tire will take sometime to go back to initial position it was before deforming . This is about 1/2 the rotation for current generation tires. So if a wheel is rotating at 10 RPS , that means tires can deform and reform 20 times in a second . That means the relaxation time is 0.05s , if the wheel is locked for 0.05s, so at 20 RPS the car can be locked and unlocked 10 times (1s/(0.05+0.05s) in a second.
This actuation of locking brakes in done by a microcontroller attached to brakes.
So the conclusion is , a complex engineering lies behind the concept ABS. It is known to many simply like not locking the wheels or braking at continuous interval by a computer , which was earlier done manually by racers. Hope you find it interesting , Please leave a comment about your reading experience down, and we will meet again with one more interesting topic in next week
Very informative, gud one 👏🏻. Keep going
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