Why Big Brakes Won't Stop You Faster but Wider Tires Will - Friction and Surface Area Explained

In today's video we'll be explaining a seemingly  simple but often confusing conundrum involving   vehicular Brakes and Tires. If you have ever  considered upgrading your vehicle's brakes   and did a bit of research online then you have  probably found out that, contrary to intuition,   bigger brakes will not reduce your braking  distance. On the other hand, we all know that   wider tires can improve both your braking and your  cornering performance. So the underlying question   we're answering today is why does surface area  matter with tires but not with brakes? If this is your first time hearing about it  then you're probably surprised to learn that   increasing the size of your brake discs and your  brake calipers and brake pads will not reduce your   braking distance. This is contrary to intuition  because we believe that by making the pad and   the disc larger we give the pad more surface area  onto which it can grab which is going to increase   friction and improve braking performance and stop  the car sooner. We can make an analogy to a wooden   plank that we're pushing on the floor. Again, intuition  tells us that if we're pushing the plank on its   side it's going to be easier than if we had to  push the plank on its face. Again we believe that   reduced surface area leads to reduce friction  and thus we need less strength or less force to get   the object moving. But physics disagrees with  intuition and physics says this! Yes, it's a formula but don't get scared it's the simplest formula  there is, and tells us that frictional force "F" equals the Greek letter "mew" or "mu" different  people pronounce it differently it's basically   the coefficient of friction times the normal  force. Let's explain what this means    Frictional force is obviously the amount of friction. The  higher the frictional force the more friction   we have to overcome and the harder it will be  to get the object moving. And the coefficient   of friction is a constant and it depends on the  nature of the material and surface roughness   For example sandpaper has a much higher coefficient  of friction than glass. And normal force that's   the force acting on the object pressing it down  against the surface. On a stationary object that   lies on a flat surface the normal force is going  to be the weight of the object pressing it down   against the surface. As you can see there is  no surface area in the formula. Physics doesn't   care if the object is on its side or on its face.  Even if the difference in surface area is quite dramatic the frictional force remains the same and  this is because the weight of the object remains the same and the material is the same no matter  how we place the object on the surface So why is this the cas? Well, to understand this properly  we have to zoom in a bit and if you took a microscope   and looked at the surfaces of all the  objects around you you would see that what seems   as very smooth to the naked eye is not smooth  under a microscope. Instead it's all jagged and   full of peaks and valleys. Even surfaces are  like glass which which you would guarantee   that are smooth are actually all rough and jagged  under a microscope. Now when we take two surfaces   and place them against each other the peaks or  the asperities of each surface area are making contact They're rubbing against each other or  interlocking if you will, while the valleys of each surface, they they never make contact. And  this is why it's very important to distinguish   between apparent and actual contact area. Now  when we take a wooden plank and place it on a table to the naked eye it seems that the entire  face of the plank is touching the table   This is the apparent contact area. But on a microscopic  level, the amount of the plank that is touching   the table is only about five percent of the  apparent contact area. This is the actual contact area and it's so small because only the peaks of  each surface are actually touching one another    How many peaks interlock and how much they interlock  depends on the normal force. When we flip the object from its side to its face the normal force  remains the same because the weight of the object   remains the same. Although we increase the number  of peaks facing each other when we increase the surface area We are also distributing the same  force over the greater surface area which means   that the peaks interlock less, they touch each  other less. This is why stabbing yourself with a needle is far more painful than doing the same  thing with, let's say, a bottle cap. You may apply   the exact same force in both scenarios but  in the case of the needle all the force is   concentrated on an extremely small surface area  leading to a much higher pressure In the case  of the bottle cap the force gets distributed  over a larger area leading to reduced pressure   The same thing happens with our plank. Friction  stays the same because we're offsetting   the increased number of peaks with reduced pressure  on the peaks since we're distributing the same   force over a greater surface area. In other words  we're increasing the apparent contact area but the   actual contact area stays the same. But this formula  or model doesn't mean that your intuition is wrong   And that is because physics models aren't meant  to be an accurate representation of all instances of reality Instead they're just that - models based  on idealized assumptions. This model assumes that   objects are perfectly flat and equally rough over  their entire surface. In other words it's assuming   a repeating pattern of peaks and valleys over the  entire object. Objects like that do not exist in real life Your intuition isn't wrong because it's  based on actual experience of years of moving, lifting and pushing various objects. If you were to  go to a hardware store right now and buy any old plank   or whatever, you know, and push it against the  floor and then measured how much force it takes to get it moving when it's on its face versus on its  side. You would see that it would likely require more force to get it moving when it's on its face.  And this is because real life objects are neither   perfectly flat nor equally rough over their entire  surface. And this is why surface area often matters   In real life it matters because it reduces  your chances of running into various surface   imperfections or abnormalities which increase  friction. Okay so the same thing then applies to   brake discs and pads? They're imperfect surfaces so  the formula doesn't work? Actually no. Brake pads and discs are machined very accurately so they  are pretty uniform over their entire surface    And the formula definitely works. Also the way that  the brake pad moves means that the application of force is very repeatable and linear. Much more  so than a human hand which cannot apply the exact   same force twice. So the formula definitely works  and increased surface area of brake discs and pads   does not lead to increased friction.  So the question is why do then all the sports cars and race cars have these big breaks which  are obviously so much bigger than the brakes on "normal" cars? Well, the answer isn't increased  friction. The answer is heat or more precisely    the prevention of brake overheating. If you look at  brakes more closely you will see that many things   in their design have to do with heat management.  For example the brakes of cars are tucked inside   the wheel and then the wheel is tucked inside  the wheel arch or the body of the car Which means that the brakes aren't exposed to a lot of, you  know, incoming air. In contrast to this motorcycle   brakes are sitting there directly in the stream of  the incoming air and this is why car brake discs are ventilated but motorcycle brake discs are not.  Ventilation works to try and evacuate as much of   the heat as possible out of the braking system. Why  is heat such a problem with brakes? well because it   leads to break fade. When brakes overheat a thin  layer of gas forms on the surface and this layer of gas   prevents proper braking. It leads to reduced  friction and loss of braking performance otherwise   known as brake fade. A larger brake disc and  caliper is going to have a much larger surface   over which heat can dissipate into the surrounding  air making this braking system much harder to overheat So if you were to take two identical  cars: One with stock brakes and the other one with upgraded brakes and you then take them  to the track and drive them hard they would    break equally well for the first five  minutes. After that the stock system will start   to overheat and its braking performance will be  dramatically reduced. On the other hand the upgraded  braking system might retain the same braking  performance for an hour or two or maybe even longer In fact the upgrade braking system might  start breaking better when it gets really hot and   it might brake worse than the stock system  during the initial five minutes before it gets hot   And this is because different brake pad materials  are designed for different temperature ranges   A material's temperature can have significant impact  on its frictional capabilities And it's almost impossible to get a material to be equally  "frictiony" when both cold and hot For example, racing brakes might be optimized for hot breaking  performance at the expense of cold breaking    which isn't so important for race cars. On the other hand  stock brakes must offer optimal performance    from the moment you start driving, but they do so at  the expense of extreme temperature operation    which isn't achieved under normal driving circumstances  anyway. So we have to correct our statement from before Larger brake discs and pads will not  reduce your braking distance. Under normal driving conditions But if aggressive track driving  is involved or stuff like towing increased loads   especially downhill then upgrading your brakes  might be necessary. Also if you significantly   increase the power output of your vehicle making  you capable of reaching much higher speeds much   more quickly and then you find yourself in a scenario  where your have to repeatedly stop or slow down from   these much higher speeds, then again upgrading  your brakes might be a good idea So, increased surface area with brakes doesn't increase friction.  Instead it increases the surface area available   for heat dissipation with the goal of preventing  reduced friction. Now this formula works with   brakes obviously, but but it does not work with  tires. Over the years many tests have been done   and they have proven that increased tire width or  size can improve braking performance and cornering performance The question is why? Why doesn't  surface area matter with tires? Well the answer   is surprisingly obvious, and it's because brakes  are solid and rigid and tires are not    They're elastic because they're made from rubber after all.  And also brakes aren't designed to change shape or   deform under normal operation. Tires change shape  and deform all the time. The loads applied on   brakes are simple and linear because the brake  pad moves in only in one Direction. But the loads   applied on tires are extremely complex and ever changing. In fact brakes are something called a simple solid And this means that if we double the  normal force or the load on the object the actual   contact area and thus the frictional force will  double as well. In other words the relationship   between the load and frictional force is linear. Double the load is double the contact area double the frictional force   but things work differently  for tires because tires are made from rubber they   are soft and pliable. This means that the tire can  conform to the material it's contacting and even   reach the valleys which a solid material could not.  This means that even a light load can increase   the actual contact area and friction by a disproportionately high amount. On the other hand   increasing the load further leads to ever reducing  gains in actual contact area and friction    This is because the more load you apply to rubber the  stiffer it becomes until it reaches a point where   it can't become any stiffer. In other words, it  takes a tremendous amount of force to push the   rubber to reach the lowest parts of the valleys.  You may have experienced this property of rubber yourself   by trying to squeeze a rubber ball. The  more you want to compress the ball the harder it   becomes to do so. There is very little resistance  initially but the more you squeeze the stiffer the   ball becomes and you have to apply ever more force  for ever decreasing ball compression or shrinking   So with rubber and rubber tires double the load  does not mean double the actual contact area and friction but this also means that half the load  isn't half the friction. Now our load is the weight of the car pressing down on the tire and our load  remains the same. By installing wider / larger   tires you have spread out the same old over  a larger surface area leading to decreased   pressure pushing the tire into the surface. But  the reduction in pressure isn't offset by an   equal reduction in actual contact area between  surface asperities and the rubber. Because tires aren't solid a 20% reduction in pressure doesn't  result in a 20% reduction in actual contact area   The actual contact area reduces by only maybe  15 or 10 percent for example. The end result is   that a larger or wider tire has a greater actual  contact area for the same load and this property   becomes ever more desirable and even more obvious  when weight transfer and thus changing loads start   acting on the tires during cornering and braking.  But this isn't the only reason why sports cars and   race cars have larger, wider tires. Heat management  is again an important factor and tires can also   overheat leading to increased wear and reduced  performance. A larger wider tire is obviously   harder to overheat just like a larger brake rotor  is. Mechanical strength is also important    A wider tire, a bulkier tire, can be made stronger and this  can help it resist deformation better leading to   more consistent performance. Another key factor  is wear. You've probably noticed tire marks on roads or tracks or even entire chunks flying off  during races. A larger wider tire spreads out the wear over a greater surface, and this helps it  last longer. Seeing tire marks or actual parts   of a tire that have remained stuck onto the road  tells us just how important adhesion is for tires   Rubber is a material that is great at adhering   to a large variety of surfaces and the physics   of adhesion are not the same as the physics of  friction between two simple solids. x This is why  the instructions on glues tell you  to roughen up the surfaces that are about to be bonded Roughing up a surface increases the surface  area and surface area definitely matters for adhesion So wider tires can offer more grip and  traction but we also know that they usually have   reduced performance in wet conditions. Why is that? Well, it's because increased actual contact area   doesn't matter if there's a layer of water between  the tire and the road surface and when it comes to   wider tires water has to travel a greater distance  until it's pushed out from under the tire and   contact between the tire and the road surface can  be re-established. This is why narrower tires often   perform better in wet and slippery conditions.  The final very obvious evidence that surface area   matters with tires but not so much with brakes  is that race cars and sports cars often have   slots or holes drilled in their brake rotors. This  shows us that manufacturers are very much willing   to sacrifice surface area in the interest of heat  management and/or debris evacuation.   On the other hand when motorsport vehicles want to go fast  and have maximum grip in the dry they're always   going to go for slicks, and slick tires have no  thread pattern no grooves nothing zero surface area is sacrificed  in the interest of maximum  contact area and thus friction and grip    Brakes and tires are seemingly simple intuitive things,  after all it's just metal grabbing into metal and   rubber rolling on asphalt. But the science which  deals with the mechanics of brakes tires and   other forms of surface contact, friction and  lubrication is called tribology and it's one   of the most complex mind-boggling scientific  fields there is. This video barely manages   to even scratch the surface of the incredible  mechanics behind tire deformation and brakes   and friction but I hope that at least it manages  to clear away some misconceptions and explain the   basics so that you have a better understanding  of what's actually happening with your vehicle   and so that you can make better, more informed  tuning and upgrading choices. As always thanks   a lot for watching and I'll be seeing you soon with  more fun and useful stuff on the D4A channel.

Channel: driving 4 answers

Views: 1,070,498

Keywords: big brakes, brake upgrade, big brakes upgrade, big brake kit, brembo, ebc brakes, are brembo brakes worth it, do I need bigger brakes, why are larger brakes better, tires, tyres, pros and cons of wider tires, wider tires, is it ok to put wider tires on my car, api brakes, wilwood brakes, stopping distance bigger brakes, big brakes friction, big brakes surface area, coefficient of friction, friction surface area, big brakes test, grip, traction, surface area, brake fade

Id: CyH5xOcsXxs

Length: 16min 17sec (977 seconds)

Published: Sun Oct 23 2022