Wikipedia:Reference desk/Archives/Science/2016 November 23

= November 23 =

Any difference between braking slow and fast?
When driving, I tend to follow the car ahead at as far a distance as seems reasonable (without risking being passed, or missing a green light, for example) and tend to brake as gently as possible, since sometimes the car in front will resume motion before I reach it. I am curious whether, if one ultimately comes to a stop anyway, is there a benefit to slow breaking, such as less wear to the break pads or the drum/disk?

Assume a smooth deceleration in both cases. For instance, at a speed of 25mph, 100yds behind the stopped car in front, is there any significant difference in wear or energy consumed between immediately braking very slowly at 100yds, versus braking more heavily starting at 80yds distance, as long as neither scenario leads to a skid or a jerking halt? Thanks. μηδείς (talk) 01:40, 23 November 2016 (UTC)


 * Yes, gentle braking results in less thermal fatigue of the pads, and hence lower wear rates. I suspect you'll find some research in the world of commercial vehicles, or railways. However, in doing so you may glaze the metal parts, so when you need to brake hard they may not work as expected. I suspect much of the info out there is anecdotal, and the good stuff is proprietary. The total energy absorbed in both your scenarios is the same (more or less), but the peak temperatures are not. Greglocock (talk) 02:30, 23 November 2016 (UTC)
 * Here is an interesting take. Shock Brigade Harvester Boris (talk) 02:36, 23 November 2016 (UTC)
 * You've only mentioned braking (it's brake, not break), but for real-world driving in "conventional" vehicles, if you're going to be stopping, the "best" overall thing to do is to coast as much as possible: feet off the pedals until you need to brake. That's what I do. This saves both fuel and wear on the brakes. Quite a few people seem to have this idea that they must be pushing one pedal at all times, so they accelerate right up until they need to brake and then brake hard. Also downshift if you're going downhill. Here in the U.S. where automatic transmissions are commonplace, no one seems to know about downshifting. However, if you're in a hybrid or electric vehicle with regenerative braking, I think I may recall reading somewhere that the optimal thing is to hit the brakes early, but I'm not sure as I don't drive one. --47.138.163.230 (talk) 02:50, 23 November 2016 (UTC)
 * Regenerative brakes have a maximum power input, so they don't help stop a car as fast as traditional brakes. Cars like the Prius also have ordinary hydraulic brakes that kick in when the regenerative brakes are not sufficient to slow the car down. I'm not sure exactly how hard you have to brake to make those kick in, but all of that energy will be lost to heat. Someguy1221 (talk) 02:57, 23 November 2016 (UTC)
 * I don't think people avoid downshifting because of automatic transmissions ... it's just that transmissions are expensive, while brake jobs are routine. I don't know if it's true, but my thought is that using the transmission more might tend to wear it out, and especially, manually grabbing the automatic transmission lever and moving it back and forth while driving risks that there is some un-optimal meshing of gears, I leave it halfway, etc.  It only takes having the transmission light come on once from leaving the stick in between positions to make a person really nervous. Wnt (talk) 13:30, 23 November 2016 (UTC)
 * While you might be right about drivers' psychology, these days I think there's little issue. Automatic transmissions in modern cars are all fancy and computer-controlled, and a lot of manufacturers are moving to continuously variable transmissions, where there are no gears to shift, so there are no worries about gears not meshing and the like. --47.138.163.230 (talk) 01:29, 24 November 2016 (UTC)
 * Well yes, but who wants to pay a fortune for some newfangled car whose main selling point is that Kim Jong Un can steer it into a wall whenever he feels like it? Anyway, I'm not making up the bit about seeing the light come on from leaving the lever in a midway position, at least as of a 1998 model. Wnt (talk) 21:46, 24 November 2016 (UTC)


 * I drive similarly. Ideally I don't brake at all. Manual transmission, no coasting, use of engine braking to control speed down hills but not to decelerate. In my partner's car, which has a dashboard mpg gauge, I get 50mpg, she gets 46mpg. Some variations, but the difference is consistent. Andy Dingley (talk) 12:10, 23 November 2016 (UTC)

One more reason for braking early and gently: it means that, if one of your brake circuits (the hydraulic systems connecting the brake pedal to the individual brakes) fails, you can still stop safely by applying full force to the remaining brakes, whereas, if you wait until you're close to the intersection and then brake hard, you might not be able to stop in time to avoid plowing into the car in front of you or (worse yet) rolling out into crossing traffic if one of your brake circuits fails. Whoop whoop pull up Bitching Betty &#124; Averted crashes 15:55, 23 November 2016 (UTC)


 * You will get frictional losses from your drive train in any case when you take your foot off the gas. You also get engine braking, depending on the drive train. To the extent that these substitute for the actual brakes, the brakes get less wear. Best practice is therefore to coast to a stop, other things being equal. Other factors enter in when you are driving an EV, but in general you still get the best results with the least amount of brake pedal use. In a typical case, This usually happend in conjunction with getting your foot off the gas earlier, which of course means you have less energy to dissipate in the first place.  I drive a BMW i3, which is pure EV with aggressive regenerative braking, but the same principle applies because regenerative braking does not recapture all of the energy. -Arch dude (talk) 18:43, 23 November 2016 (UTC)


 * I second Whoop whoop pull up above, don't leave it to the last minute to use the brakes. I don't do that but even so the brakes on my car failed once and I had at least the time to plan and pressed the accelerator to get to the other side before the traffic across crashed into the car. Rather a hair raising experience. (plan might be an exaggeration, I notice sometimes I do things before I'm really aware of the problem never mind a solution) Dmcq (talk) 14:27, 24 November 2016 (UTC)


 * Never use the brake as a permanent footrest. It will overheat and fail. Brake fluids DOT3, DOT4 and DOT5.1 are hydroscopic. Solved water inside will boil and make the brake fail by missing pressure of the gased water which is compressable make the brake reach the floor. Renew the all the brake fluid in time to have less water in the brake lines. It requires bleeding all the hydraulic system. Longer gradients downward, least with trailer and fully loaded car require to shift gears down to keep brakes alive and relyable. Use the brake short time only to keep it cool and reduce the wear. But, never force a vehicle behind You to pile up. If You rent a car in other country, be careful the brake service might have least expired. Failing to follow a serpentine ends up fatally. When using a hybrid or electric vehicle which supports recuperation to regenerate electric power from decelerating, use the brake in the recuperation range. When using it stronger, the conventional brake will be applied and heat up when using longer. Have known such vehicle, but also know the batterys capacity. When it is a hybrid vehicle, it might use the combustion engine when the battery has charged full up. In the mountains, reach the top of the gradient with empty battery. Each week once, You should use decelerate stronger to clear rust from the brake disks and drums. Never force a vehicle behind You to pile up. New brake pads should not be used stronger within the first miles due less surface fits to the disks. As possible, avoid situations which require to stop immdiately until the pads fitted the disks perfectly by wear. -- Hans Haase (有问题吗) 01:19, 25 November 2016 (UTC)

Thanks for all the responses. The matter was one of academic curiosity. When I visit the main shopping area in my parents' town, I usually coast down a long hill on the way home. At the bottom of the hill is a light. It is often red, with several cars stopped, so that even if it should turn green, I will likely have to stop anyway, since those cars will have to accelerate, once they finish texting.

I could either brake slowly all the way, or brake heavily at the bottom of the hill. I normally brake slowly and smoothly, which leads to the people behind me getting annoyed, but they aren't going to get there quicker if I speed along and then slam on my brakes at the last minute. The issue of glazing the brakes was something I had never heard of, and I do not remember getting any advice on braking fast or slow, just being told only to use the right foot, never both.

μηδείς (talk) 02:34, 25 November 2016 (UTC)


 * When I was little we lived at the bottom of a hill on a dead straight Roman road.  I always knew when we were home because my father would cut the engine and coast down the hill.   I understand that driving instructors warn against this because you are not in control of the vehicle.   Also they tell pupils not to try using the engine to slow down ("brakes are for stopping, gears are for going"). 86.145.54.170 (talk) 14:10, 26 November 2016 (UTC)


 * μηδείς, did You ask for this reason? -- Hans Haase (有问题吗) 20:59, 26 November 2016 (UTC)


 * No, living in NYC I do not own a car (a parking spot costs as much as a studio apartment) and the car I drive on the weekends and holidays is an automatic, with no clutch. I do, however, take advantage of the fact that while accelerating in an automatic, you can get it to shift into a higher gear more quickly than it would on its own by taking your foot off the gas momentarily as the engine passes 2000rpm.  As the rpm rate drops, the car will shift up.  This allows to to pull ahead of most drivers "out of the gate" since automatic transmission is pretty much standard in the US nowadays. μηδείς (talk) 23:49, 26 November 2016 (UTC)
 * Yes, I like the big robust engines, too. Nice to drive when not all the time needed to stop and easy in service, more efficient than expected in shifting behavior and consumtion on freeways when having overdrive, but not made for "sailing" at all. -- Hans Haase (有问题吗) 10:11, 27 November 2016 (UTC)

Women whose work is on the circulatory or lymphatic systems?
I'm after the name of a famous female scientist or medical person who has contributed to our understanding of the circulatory system or lymphatic systems. Even though both of these articles have a 'history' section, they only list ancient developments, not more modern discoveries (where women might be more expected to have taken part). My search-fu is failing me - can anyone please help? --122.108.141.214 (talk) 07:07, 23 November 2016 (UTC)

What's the highest-walled endorheic basin on Earth?
One with the most elevation difference between "lowest land" and "filled to the brim". Sagittarian Milky Way (talk) 22:38, 23 November 2016 (UTC)


 * The Dead Sea would have to rise 660 metres before the basin overflowed (southwards, to the Gulf of Aqaba and the Red Sea) Wymspen (talk) 23:07, 23 November 2016 (UTC)


 * But is that the deepest, or just the deepest one you can think of? --76.71.5.45 (talk) 03:16, 24 November 2016 (UTC)


 * It's a starting point - if anyone can come up with a better answer, that will be fine. Wymspen (talk) 08:59, 24 November 2016 (UTC)


 * I haven't got an exact number, but casual inspection in Google Earth show that the Turpan Depression must have walls of about 1200 metres, if you take the area large enough. That is, you have to add about 1200 metres of water before it overflows into the ocean (via Ebi Lake, Lake Alakol, Lake Balkhash, Aral Sea and Caspian Sea to the Black Sea). PiusImpavidus (talk) 12:02, 24 November 2016 (UTC)


 * Cenotes may also be of interest. StuRat (talk) 17:59, 24 November 2016 (UTC)