Don't forget the driver behind you who will still want to go faster than you. I wonder how that works at the speed of light? I've spun a few vehicles, through a combination of modification (weight bias of body changed, brakes unchanged. Rear brakes then were to powerful for the rear of the car) and junky tires, and in all situations the point the tires get enough grip back on the road is always the point at which you've just stopped. Not entering the spin in the first place seems to be the way to not be in a spin.
Hopefully, red blue shift. Blue red would happen at impact and I wouldn’t be writing this. Unless of course I discovered a worm hole then I could kick back and watch the spectacle. Nanu nanu
LOL…No, just something I saw on MPT. Studying impact craters and the effects on quartz. Probably got it wrong. Raman shift, wave lengths of light. Edit: should I turn in my pocket protector
Its probably easier to think about this if you look at a vehicle that only has two wheels. Its pretty obvious that the majority of the braking comes from the front wheel......even though on a motorcycle the front wheel is often narrower (smaller contact patch) than the rear wheel. Simply riding on a wet surface and applying only the front brake.....and then getting up and trying it with application of just the rear brake will quickly verify which causes more problems when applied to the point of lock up. Motorcyclists quickly learn to never lock up the front wheel, (except for pro racers) yet many times they successfully over apply pressure to the rear wheel. I think that it basically has to do with the leverage provided by the trailing wheels position. The road in front of my house is a pretty good hill. In the winter we sometimes get freezing rain and ice and cars get stalled before they reach the top. Last winter a young lady got her Mustang marooned in no mans land about 3/4 the way up. Since I was outside I walked over to help her back it down the hill. I had her let off her brakes, and roll very slowly till she was straight with the hill. She was nervous and kept riding her brake, and each time she touched them the car would start to slide sideways. Since she was rolling backwards, the front wheels with the more effective brakes would immediately lock up. The front wheels were serving as rear wheels. Now the car was only moving maybe 1 mph. So as she tried to get straight, she kept that foot lightly on the brake and each time the front(rear) wheels stopped rolling she skidded slightly sideways. Finally got her to keep her foot off the brake and she immediately got straight. Once properly aligned, she was able to continue rolling straight with just light taps of the brake to keep from increasing speed. That showed me that the rear braking allowed far more control if the wheel was turning instead of skidding. So I have to say that I agree with keeping the rear wheels rolling. Did you ever wonder how the police measure skid marks for cars with anti-lock brakes ?
I'll go along with this, however, just once try riding a dirt bike down a steep rocky & rutted hill with the rear brake only. IF you make it down in one piece, with all your skin intact, you will immediately appreciate the value of a front brake. But other than that, yeah, I accept your explanation of the physics involved with loss of traction on the rear while the front is still grabbing.
Backing down a hill can get really hairy, I had an experience in a Ford F800 service truck off road, following a road cut for a pipeline job across the hills in the middle of nowhere. I knew the water trucks went up and down the road, so I figured my F800 would make it; I was wrong. I hit one hill it just wouldn't make it up. I could get just almost to the top before losing traction and spinning out. Backing that thing down the hill, with a 30 foot trench off to the side was sketchy as hell. Applying the brakes caused a weight shift to the rear, which unloaded the front, causing the tires to lose traction and the front to drift toward the ditch. ****! I got it stopped before going into the ditch, but getting it back headed in the right direction was sketchy as hell. I was sweating bullets. Every move I made slid the whole thing closer to the ditch. Once I finally got it straightened out and heading the right direction I just let off the brakes and let it go, did my best to keep it straight down the hill till I got to the bottom. That was one hell of a ride! Then I just sat there until somebody came along and found me; and went back and got a 980 wheel loader to pull me u the hill, and that wasn't easy either, but we made it. He was spinning all 4 wheels of the loader and I was spinning my wheels, but we inched up the hill and made it. I guess the water trucks had a lower differential gears, and the weight of the water on the back helped with traction. Anyway, I think most states rules of the road say that if 2 cars going different directions meet on a hill where the road is too narrow to p***, the one heading down hill is supposed to back up the hill until they reach a spot where the car headed uphill can get around. Backing up hill may be hairy, but not as bad as trying to back downhill.
"Y" is also affected by the camber of the lane you are in. Also consider it likely the automobile in question is not loaded equally side to side, so even if the front wheels have equal "X" force, the uneven loading of the vehicle will impart a force in the "Y" direction. The best way to avoid the endo swap: Well adjusted brakes, a calm demeanor, driving within the safe parameters of road and weather conditions, and most importantly situational awareness.
"ABS brakes, by virtue of preventing front wheel lockup, allow the driver to maintain directional control of the vehicle. Rear wheels that do not lock up prevent spins from occurring. The question becomes, “If there is no wheel lock up and therefore no skidding, how can the accident reconstructionist/investigator determine the speed lost by the vehicle during emergency/panic braking?” Vehicles equipped with ABS brakes will in fact leave faint tire marks on the roadway when the ABS system is activated. A trained accident reconstructionist/investigator can actually visualize, measure and do***ent ABS tire marks. However, these marks are highly perishable and can become undetectable in a very short time on a heavily travelled roadway. This short time frame is measured in hours or minutes as opposed to days and weeks for locked-wheel skid marks."
If you want to further confuse your self, start looking at the Heavy Truck world. It is the exact opposite from the automotive side of things. It was common in the 70's and 80's to order new semi tractors without brakes on the steer axle. With big rigs, you want as much anti-dive built into the suspension as possible, so the weight transfers to the rear axles (bigger brakes and more tire contact), and the tires do not unload. If the drive axles unload (I.E. Pro-dive, and transfer weight to the steer axle), the trailer starts deciding what direction things are going to move, and that normally doesn't end well.
Seems you have eluded to a few clues. The difference in traction or grip (Co-efficient of friction) is continuously changing both at the front and the rear contact patches. The physics comes into play when a Polar moment occurs during yaw. When the front brakes lock up first the car may yaw some and create a polar moment but it is restrained because the rear tires still have grip. When the rear brakes lock up first a great deal more yaw can be generated and create a much stronger polar moment causing the car to swap ends. Imagine the general public in the rain with the rear brakes locking, loosing rear grip, swapping ends, creating quite a mess on the highway.
Vehicle Dynamics engineer here…. The Milliken book is fantastic and has wonderful theory with illustrations to help you understand suspension/ tire systems. Id recommend it to anyone looking to geek out a bit on ch***is design. Ha- Also have the Gillespie book and took his cl*** during my MSME. Small world!
I’ve come across a few vids/articles over the years in which they say modern science/ physics still cannot adequately explain why a bicycle does not fall over. True?
Uneven loading, weight transfer, road surface and angle are inputs I was ignoring, trying to simplify. There are many variables in this dynamic system. Even each of those has its own list of variables to be explained. Agreed. A well functioning system, don’t panic, and safe driving techniques should prevent the panic stop / brake lockup.
Hey Hi Ziggster I Love your Avatar! Trust me here, or ask how I know. A bike will fall over if the skid gets ahead of your weight. As I said yesterday, That business of bike control starts with the spine and gut. You don't get that with a car.
If the front brakes have to break first why do the combination valves have a hold of feature set at approximately 150 pounds before the front brakes are applied?
Exactly! My comment was tongue in cheek as well........... I think that the subject of braking and the forces involved, will always arrive at different results when you consider just theoretical results vs real world results which include so many variables. There are lots of topics on the Hamb where different results occur when comparing theory to real life situations. The reason engineers have to test their conclusions is because they don't always know all the variables and how their interaction may affect the final product or action. One last comment here........I think the process of "downshifting" or "gearing down" on an icy hill is a good example of rear wheel control when you don't want the front wheels to stop turning. Great question though.
yes, so if you are using 11 by 2'' brakes on the front from an F1 and an 8'' axle with 10 by 2'' should I switch out the rear 2'' shoes for 1.3/4 then someone says, what are you talking about it will be alright. then you follow with - then why do they exist in the different widths....
Most of this discussion appears to be base on straight line. Where it really gets hairy is when you are in a bit of a curve or turn and have to apply the brakes quickly. Y goes up a bunch and around you go, if the rear locks first! You don't have to be going that fast either.
Yes, I addressed that in the first post. Winter driving in Illinois will teach you all about rear wheel traction and going sideways. If done on purpose, this can be fun. If it happens unexpectedly, you’re in trouble almost before you know what’s happening.
Rear brakes are often self energizing drums. Fronts are discs. These work differently and at different line pressures. The combo / proportioning valve is trying to activate both front and back for good braking, while providing more braking on the front to prevent rear lockup. The two common ways to do this, using all parts from an OEM system of similar weight and balance, or using and adjustable valve, are both imperfect hot rod solutions to a complex engineering problem.
This brings to mind the highway patrol turn around. Slam on the emergency brake and do a 180* turn.brodrick Crawford was good at it!!
I think I had his cl*** in about 2000 or 2001. It’s a bit fuzzy 20 years later! General is doing good
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