It is widely known accepted wisdom that brakes have to be set up such that the fronts lock up before the backs. Every thread on brake proportioning, whether fixed stock or aftermarket adjustable, every thread on brake design, whether disks or drums, eventually includes the warning that the fronts must skid first or the rear end will try to come around. The OEMs set their brakes up this way, and early anti-lock systems were rear only. The question isn’t whether this is true, the question is why? What is the physics that makes this happen? Working from a simple case first, a basic four wheel braking system. If the car is traveling in a perfectly straight line, only X has a value. Y is 0. If we brake, and remain straight, then X changes, Y does not. As we brake harder, approaching the maximum braking that the available traction supports, X changes faster, but that doesn’t change Y. At some point, we run out of traction, and wheels begin to slip. Braking is reduced at the slipping wheel. If Y is non zero, then locking the rear wheels would obviously induce a slide. So how does locking up the rear wheels first change Y? I spent a lot of time on a bicycle as a kid. One brake on the rear. No matter how many skid marks I left on the driveway, the rear end never came around on me unless I added steering input to make it happen. I think the answer is that there isn’t one X here, there are two. As we brake, the front wheels are not experiencing exactly the same traction at any given instant. If the rears are locked and sliding, whichever front side gets better traction will induce a change in Y causing the rear to slide toward the other side. Once Y starts to change, braking is no longer in a straight line, and the feedback of Y changing induces more change in Y. This is experienced by the driver as a sudden uncontrolled spin that may be catastrophic.
I don't understand, was in Metal Shop class when "Figurin' " was studied. I'm a drum brake guy, adjust fronts and backs out till you can barely turn each wheel by hand, then get in and drive, you're all set.
Well Dave... if I'm reading right, to answer your question think about opening a book at your local library and do some reading. But if you want my take on it, it has something to do with the three laws. Three Laws of Motion First Rule: An object will remain at rest or in a uniform state of motion unless that state is changed by an external force. Second Rule: Force is equal to the change in momentum (mass times velocity) over time. ... Third Rule: For every action in nature there is an equal and opposite reaction.
Yes, you are (probably) correct. If all four wheels are perfectly straight and all four adjusted perfectly the same and tyre wear is the same, with same loading on each, and road surfaces the same under each tyre it should stay straight. Now throw in the biggest variable, - the driver - who panics and swerves or even just a slight twitch of the wheel and then you may start to understand why anti locking brakes are required….. I always believed (prior to anti locking brakes), that in an emergency braking situation that you would stop in a shorter distance by putting the vehicle into a controlled spin……… (something about momentum and increased resistance to car not travelling as designed). The downside to this is that the car behind you would probably then hit you because it can’t stop as soon…….. is that a ‘39 or ‘40 in your picture ?
If you really want to know the dynamics of braking, here are some books that you're going to want to read. The green book, The Fundamentals of Vehicle Dynamics was the text for a graduate level course at the University of Michigan (go Blue). The first book by Milliken and Milliken is a very complete treatise on race car vehicle dynamics, and explains tire fundamentals in great detail. Which needs to be understood first.
I spent a lot of time on bicycles as a kid, and motorcycles as a teen and adult, and in my experience if you lock up the rear brake the bike will try to swap ends, for sure, every time. Especially in the dirt. Downhill. Try it, you will appreciate how important a front brake is.
A few decades ago. Racing a 1964 Olds 442 on the expressway on my Kawasaki triple. 92 mph, the engine locked up as did the rear tire. The rear of the bike was definitely kicking to the left. I instinctively steered into it and compensated and everything was stable. Only problem... I was in the left lane, and steering into the slide was bringing me closer and closer to the left edge of the pavement... almost into the grass, now. Major sphincter clench. I pulled the clutch lever. The rear tire caught the pavement and the whole bike did a little fishtail dance. That's when I thought I might actually fall. But it straightened up and I got 'er stopped. There's some braking physics for you. LOL
This and this ^^^, plus a little bit more. Locking up the rear brakes only applies drag/friction to the rear tires of the vehicle only (bicycle / drag car/ etc). The front is still traveling freely. This will keep things straight, as long as the steering stays straight. When there are front brakes applying at the same time, if the rears lock up first, they actually lose effectiveness compared to the front. Therefore the front is trying to decelerate more than the rear, and the rear can try to overtake the front. All over....... you've lost it. If you want anything to lock up (which you actually do not) it needs to be the fronts. Once they skid and lose braking capacity, the rears are decelerating more, and holding things relatively stable and straight.
Stupid questions time: How come rail type dragsters only run rear brakes? Why do they not have the rear end try to come around? I'm talking brakes only, not counting parachutes....
OK, you got me thinking. If your traveling at Mach one do you need a horn? Do the headlights work at the speed of light?
A spin is bad. It won’t help you stop, at least not until you hit the ditch, guardrail, or tree. Then the sudden deceleration will probably hurt a lot. With a spin, most of the time you’ll have all four wheels sliding, so will be at less than threshold braking. I’ve spun cars. I can’t recommend it. The driver has to react to the situation, and doing that successfully takes practice. Mostly doing it wrong, and learning what wrong feels like until doing it right becomes reflex. Image is a 37 Chevy. I wanted an overhead line art view, and had that one handy.
I never had a front brake. Spent a lot of time riding and racing on dirt. BMX in the 1980s. But I’m In Illinois, we don’t have hills here.
That’s what I’m thinking, basically. Sliding is less friction between tyre and ground, so the front is trying to decelerate more than the back. With any side motion, then the rear comes around. And the imbalance between the fronts acts as a lever to rotate the car. Exactly right. Greatest braking is just before lockup. You don’t want lockup, you want to stay just this side of lockup.
When I first built my Willys it was set up horribly. I had the stock front 9" brakes that I had converted from cable operated to hydraulic with a junkyard slave cylinder. Virtually no front brakes. Then I had 12" Buick drums on my Olds rear end, great brakes. To top it off I was running farm implement tires on the reae (no tread) 'cause they were cheap and looked like slicks. The rear brakes locked up easily on the featherweight car. In dry weather, the car would wobble a bit on a hard stop but I handled it with some fancy stearing. However in rainy weather the car wanted to swap endo instantly. A true 100%, gold-plated, E-ticket ride!!! Trust me, the books aren't fooling when they call for fronts locking up first.
Now we can wait for the comment about riding "hands free" and explain that, even then, you are applying steering input through lateral weight transfer and lean angle.............
Correct, You got the right answer Johnny, buuut the words got spun out... 'The Path of Least Resistance' is the phrase. The Big truck school I went to splained it this way, 'The tire that skids wants to lead the parade'. Now I see your concern. I've skidded both ways. Slid backs and slid fronts. Never fun. First time tho was unreal. A first edition Chevy II six did a complete 360, in the rain, and rested next to the curb. That business of bike control starts with the spine and gut. You don't get that with a car.
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