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I The physics of braking on a motorcycle (manual transmission)

  1. Apr 21, 2017 #1
    When trying to abruptly stop a motorcycle that is in motion, can engine braking actually have a negative effect on the rate of deceleration?

    Scenario: a motorcycle is travelling down a straight highway at 100mph in 6th gear and needs to perform an emergency stop. The weather is clear and road conditions are perfect (i.e. no oil or debris on the road).

    Question: will the bike stop faster if the rider pulls the clutch in AND engages the front brake? OR will the bike stop faster if the rider only engages the front brake?
    (We'll say that the motorcycle's speed in 6th gear while the engine is idling is 15mph so, for the purposes of this scenario, we will consider 15mph as "stopped" because below that speed the idling will certainly produce forward force)

    Fact: front brakes are most effective in slowing down a motorcycle due to weight transfer to the front wheel.
    Engine braking is also effective but the force is exerted on the rear wheel only and the rate of deceleration provided is not nearly as high.

    So, let's say maximum front brakes slow the motorcycle down at a rate of X.
    And let's say engine braking slows the motorcycle at a rate of Y.
    X > Y

    So, is it possible that Y has a 'negative' impact on X and, therefore, the bike would actually stop in a shorter distance if the rider clutches IN and only uses front brakes? Or will Y (engine braking rate of deceleration) always provide additional deceleration (in 6th gear which at lower speeds does not provide a large rate of deceleration) and actually has a 'positive' impact on X and therefore will always stop the bike in a shorter distance when using both front brakes and engine braking ???

    Someone please enlighten me. Initially, I thought engine braking *could* have a negative impact the rate of deceleration of the bike under heavy braking (using front brakes) but now I'm not so sure

    Also please consider the difference characteristics of a sport bike vs a cruiser if that has any bearing on the response.

    Thanks!

    EDIT: Perhaps a better fitting scenario is:
    2 riders on identical bikes accelerate to redline in 3rd gear, hold redline for 3 seconds, then perform heavy braking. Rider 1 clutches in and only uses front and rear brakes. Rider 2 leaves 3rd gear engaged and uses front and rear brakes. Assuming there is no skidding or loss of road contact with the rear tires, who stops faster?
    (If it's not clear, basically, I'm trying to determine, from a physics standpoint, if adding "engine braking" can actually cause a bike to take longer to stop in certain circumstances. i.e. when braking force (from use of front and rear brakes) is greater than engine braking force. Because, of course, if a rider is only applying a little bit of brakes, engine braking will *assist* in slowing down the bike)
     
    Last edited: Apr 21, 2017
  2. jcsd
  3. Apr 21, 2017 #2

    BvU

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    here's my thoughts -- shoot at them
    (not a racer, just a slow cruiser -- but still very happy with ABS)

    Use both brakes -- the front does the work, the rear keeps the rest of the bike from overtaking the front in some uncontrolled way.

    So in that line not engaging the clutch is better.

    And now for the theoretical case: assume a straight line no matter what. Front brake does the work -- can't be too much or you still go over forward. Any rear wheel normal force that's possibly left over can contribute to braking force. If nothing is left over it doesn't matter if you engage the clutch or not.
     
  4. Apr 21, 2017 #3

    A.T.

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    And what was your reasoning behind that?
     
  5. Apr 21, 2017 #4
    I thought that maybe, because the deceleration rate using engine braking was less than using brakes alone, engine braking might (in some scenarios) have a negative impact of the total deceleration rate. But when I think about it now, it seems like it would only help deceleration.

    Say it takes 15 seconds to decelerate from 10,000rpm (120km/hr) to 2,000rpm (30km/hr) using only engine braking.
    Say it takes 7 seconds to decelerate from 120km/hr to 30km/hr clutching in and only using the front and rear brakes
    I don't think leaving it in gear and using the brakes will shorten the 7 seconds, I hypothesized that it will take 9 or 10 seconds to slow from 120 to 30 using front+rear brakes with the engine left in gear. But my bike has a "ride by wire" throttle system (electronic signal, no physical cable) so it's possible that the bike's electronics could be affecting the deceleration by giving some throttle when I have not twisted the throttle.
     
  6. Apr 21, 2017 #5
    I could, and probably will, try to test this theory once I insure my bike in about a month but think that my braking will likely not be uniform and so the results of the field experiment will not be, for lack of a better word, valid. My experience/feeling thus far makes me believe that engine braking *can* have a negative effect on total deceleration rate but am looking for someone with a specialty in physics to correct me and say that engines with no throttle engaged will undoubtedly assist in slowing down the motorcycle at any given speed/rpm.
     
  7. Apr 21, 2017 #6

    BvU

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    I have one of these (same color too :cool: ) and I wouldn't dream of trying to test its braking to any limit whatsoever. 30 m at 60 mph means learning to jump off is more effective than learning how to brake 1 or 2 m less.
    And if you're a beginner you shouldn't try it either. Certainly not at 100 mph. Establish a comfort zone and stay in there.
     
  8. Apr 21, 2017 #7

    sophiecentaur

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    If it takes you any time at all to 1. Start considering how you should best be stopping or 2. Introduce any possible delay in applying the brakes then your effective 'thinking time' will increase. This involves travelling at maximum speed for whatever small length of time you have wasted. Being in neutral during the stopping process means you have fewer options, if circumstances should change during the process. That could be bad news.
    The initial effect of braking by the engine will be the greatest because the internal friction / loss mechanisms are maximum at high revs.
    I can't think of any reason for emergency braking in neutral - except for the possibility of the wheels being driven after you have possibly come off the bike. That could be bad, too. I have seen videos of rogue bikes getting up to all sorts of antics when the rider has come off. (There should be a return spring on the throttle, in most cases.)
     
  9. Apr 21, 2017 #8
    - the limit on how fast you can decelerate is dependent on the tire since brakes are powerful enough to lock the wheel.
    - the bike slows down whenever the wheel is experiencing braking, this can be from engine braking or regular brakes.
    - modern brakes are powerful enough to lock the wheels, thus the need for ABS to keep the wheels spinning and not sliding.
    - engine braking works because throttle body closes and creates vacuum in the intake stroke which resists the engine spin.
    - engine braking always helps normal braking. But that doesn't mean you stop quicker, it just saves your brake's wear.
    - in op's example, the two bikes stops at the same time because the brakes are good enough to provide braking force to the wheel regardless of engine braking, and it is up to the best tire to deliver the stopping force to the moving bike.
     
  10. Apr 21, 2017 #9

    berkeman

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    On a sportbike or a superbike, full braking with the front wheel is enough to lift the back tire off the pavement. It is common at the racetrack (I ride track days) for the back end to be "skipping" off the pavement as you are braking hard for a corner. It's a little un-nerving at first, but you get used to it and it helps you to "feel" when you are at max braking. Any less and the back tire is not skipping, any more and you are lifting the back end too much and need to ease up a bit to get it back near the ground.

    The technique that is taught at racetrack schools is to downshift through the gears while using max front brake. You need to either blip the throttle at each downshift to get the revs to match the wheel speed in the lower gear, or you need to learn to ease the clutch out on each downshift to let the tire spin up the engine smoothly to the matching RPM for that gear at that speed. If you do neither, then the back tire skids as you let out the clutch at each downshift, which can be very distracting if it gets you sideways as you are braking into a turn. Do not ask me how I know this so well...

    You can try to use the rear brake under hard braking on a sportbike, but again you risk skidding or worse yet stalling the engine if your rear tire is off the ground and you tap the rear brake. It's usually safer to use engine braking for the rear, since it is skipping off the ground anyway.

    And learning and practicing full-on braking at a racetrack is the best way to learn the techniques. I have had to use full-on emergency braking a few times in my street riding, and knowing how my bikes act at the racetrack has helped to save me several times on the street.

    Ride safe!

    https://s-media-cache-ak0.pinimg.com/736x/b1/04/c4/b104c464b706844139ebb130d5b2fb37.jpg
    b104c464b706844139ebb130d5b2fb37.jpg
     
    Last edited: Apr 21, 2017
  11. Apr 21, 2017 #10

    berkeman

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    And for a little fun for your Friday, here is some video from my favorite racetrack school (CLASS) at my favorite racetrack (Laguna Seca). Put on your headphones and turn up the music! :biggrin:

    http://www.classrides.com/video.html
     
  12. Apr 21, 2017 #11

    CWatters

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    +1

    If the front wheel has enough grip to allow the rear to liftoff the ground then engine braking can't contribute.

    If the front wheel didn't have enough grip to do that then yes any braking force the rear wheel can provide will help (be it engine or rear brake).
     
  13. Apr 21, 2017 #12
    These are all good replies and I appreciate the time spent.
    There was the assumption that there is no skidding or loss of contact with the road of the rear tire so these scenarios aren't necessarily maximum emergency stopping which could result in skidding or loss of contact; these scenarios are *heavy braking*
    The idea is, under heavy braking (using front and rear brakes), can the engine winding down (causing "engine braking") be at such a lower rate than the heavy braking force that it could actually hinder the deceleration of the bike produced by the front and rear brakes alone.

    Originally, I had thought engine braking Could hinder deceleration in certain scenarios but after more consideration I think it's likely that the deceleration force produced by the "engine braking" can only help to decelerate the bike (and not, in a way, work against the brakes)
     
  14. Apr 22, 2017 at 3:10 AM #13

    A.T.

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    This idea is correct. If you apply both brakes then it's always better to decouple the engine, so you don't have to brake the engine, just the bike (less energy to dissipate by the brakes).

    But the original question was about using front brake only, with or without clutch:
    This is one is tricky, because it depends how fast the engine slows down due to internal friction, compared to the front brake slowing down the entire bike.
     
  15. Apr 22, 2017 at 4:44 AM #14

    BvU

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    Not a very useful post in the context of this thread :wink: eh ?

    At least the guys here don't agree with my wimpy post #6 -- and it's about braking. But they do agree on using both brakes :smile:. Hard.

    Probably not a high physics standing in PF, this link, though....:oops:
     
  16. Apr 22, 2017 at 11:48 AM #15

    berkeman

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    Yeah, what I went looking for was a CLASS video that demonstrated engine braking (via the engine sounds), but that was the first video in his list, and I gave in to temptation. The music makes it hard to hear the engine sounds, but it's still a great video of my favorite racetrack. Riding down the corkscrew at speed is incredible.

    So here's a more on-topic video that demonstrates how engine braking is used for high-performance riding. The music goes away after about 30 seconds, and you can hear Reg's engine very well... :smile:

     
  17. Apr 22, 2017 at 1:30 PM #16

    rcgldr

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    One issue is that applying the front brake can reduce the normal force on the rear tire to cause the rear tire to hop, chatter, or skid due to engine braking. Some sport type motorcycles (and most racing motorcycles) have a (one way) slipper clutch that limits the amount of engine braking. Wiki article:

    https://en.wikipedia.org/wiki/Slipper_clutch
     
  18. Apr 22, 2017 at 3:36 PM #17
    With regard to your first statement; having to brake the engine; what laws of physics, more specifically 'forces', would be propelling the engine 'forward' that would require the brakes to have to displace that energy too? Once the throttle is released, does the engine start and continue to slow down to idle RPM?
    My initial thought was that there is a certain rate that the engine "winds down", and if you are braking hard and slowing down the bike at a greater rate than that, yes, the brakes would have to slow the engine down further as well.
    However, what I'm unsure about is if the engine "winding down" is always going to be at a rate faster than what the brakes are doing, hence the whole purpose of this post.

    I've posted this question on a Facebook Motorcycle group and many who chimed in (supposedly skilled and even some so-called instructors) believe the engine engine braking will always further assist the brakes in slowing down the bike... but I'm very unsure and wanted to pose the question to more physics-minded people, not know-it-all so-called skilled instructors :)
     
  19. Apr 22, 2017 at 4:08 PM #18

    A.T.

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    Nothing is propelling the engine, but it has rotational kinetic energy, which needs to be dissipated too, if it's not decoupled.
     
  20. Apr 22, 2017 at 6:43 PM #19

    rcgldr

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    For normal brake application, engine braking would reduce some of the wear on the brakes. For near maximum braking, where there's very little normal force on the rear tire, engine braking can hop, chatter, or skid, which is why "slipper" clutches are used on race bikes and some sport bikes. See post #16.
     
  21. Apr 22, 2017 at 10:37 PM #20
    So as the op asked the question the simple answer is no
    The long anser
    The engine brake is inversely proportional to the effect of the front brake
    but never reaches negative effect as long as road speed is in excess of RPM
     
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