Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

How are bikes tilted?

  1. Oct 3, 2011 #1
    I noticed recently that serious bike racers(like that of moto gp) tilt their whole body and bike while taking curves. I know that they do so to produce necessary centripetal force. But I wonder how can they tilt their bikes?
    The biker will exert certain force on bike to tilt it. By Newton's third law, the bike will exert same reaction force on biker. So, net force on system of biker and bike is 0. Then, how is the biker able to tilt bike?
     
  2. jcsd
  3. Oct 3, 2011 #2

    A.T.

    User Avatar
    Science Advisor
    Gold Member

  4. Oct 3, 2011 #3

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

    You are forgetting about gravity. The biker in shifting his position by leaning to one side, creates a torque on the bike about the point of contact of the tires with the road. This torque creates a change in angular momentum of the wheels which causes the bicycle to turn in the direction of the tilt.

    The purpose of leaning is not to provide centripetal force (this is not like banking). The centripetal force is provided entirely by friction between the road and the tires.

    AM
     
  5. Oct 3, 2011 #4

    rcgldr

    User Avatar
    Homework Helper

    That ceases to work in high speed (100 + mph 160 + kph) turns. The bike will lean, but turn so little that it's imperceptible to the rider, due to gyroscopic reactions at both front and rear tire interfering with the lean senstive steering that trail would otherwise (at slower speeds) produce . At high speeds, direct countersteering by applying opposite torque to the handlebars is needed.

    Hanging off to the inside allows the bike itself to lean a bit less in turns, allowing for more cornering clearance. This isn't needed on the skinny two stroke racing bikes, but the riders still hang off. Hanging off may make the situation more forgiving if the tires start to slide.

    One of the newer techniques done by some riders is to swing the inside leg away from the bike during approach to a corner, then put the foot back on the peg as they lean the bike over. I'm not sure why they do this, but it's something to do with the transition.
     
  6. Oct 3, 2011 #5

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

    I was assuming "bike" meant bicycle, but I now see the reference to "moto gp" means we are talking about a motorcycle.

    Yes. I suspect that at some speed, the ability of the rider to cause the bike to tilt by leaning would be lost. So to turn left and get the bike to lean to the left the rider would countersteer to the right, providing a momentary torque to the bike about the centre of mass pushing the wheels to the right. Inertia would put the centre of mass out to the left (ie. left of the point of contact between the tires and road) and create the desired left lean.

    The purpose of the tilt is really to balance the torques on the bike. If the bike was upright while turning, the centripetal force on the tires would cause the bike to flip away from the turn. The lean is needed to keep the torques on the bike balanced: ie the gravitational torque of the centre of mass about the tire/road contact balances the opposite torque of the tire/road about the centre of mass.

    AM
     
    Last edited: Oct 3, 2011
  7. Oct 3, 2011 #6
    As a motorcycle rider, I know the precise answer :) it had nothing to do with leaning...you do that for balance. Look up "countersteer"
     
  8. Oct 3, 2011 #7
    Actually, leaning doesn't accomplish anything at any speed. I've tried all sorts of crazy things on my bike, and countersteering is physically the only way to turn a motorcycle. At higher speeds, you have to counstersteer through an entire turn to not high side
     
  9. Oct 3, 2011 #8

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

    I have never driven a motorcycle but I think that it would depend on the mass of the motorcycle and the size and weight (moment of inertia) of the wheels. Leaning might have an effect on a light bike at low speeds. If the angular momentum of the spinning wheels is great enough, leaning out would not cause the bike to tilt at all.

    I do ride a bicycle. I don't think I countersteer at all. To turn left I just lean left and maybe turn the wheel slightly left. At least that is what I think. I will check a little more carefully.

    AM
     
  10. Oct 3, 2011 #9
    It feels that way doesn't it? But thats not how it works. When you turn your wheel to the left, your bike tilts over to the right. The only reason that leaning left turns you left is that, when you lean left, you accidentally push the left handlebar forward. Thus, you countersteer. If you focus very hard on not changing your pressure on the handlebars, you will note that when you lean left, your bike either does nothing or tilts over to the right slightly and continues to go straight.

    Some misguided people(not you) will tell you that countersteering is only necessary at high velocities. It is actually only VERY NOTICABLE at high speeds.
     
  11. Oct 3, 2011 #10
    NOTE: You have the impression that you turn your wheel to the left to turn left. Which is correct, AFTER you countersteer. To turn left, you lean left, which(for someone who doesn't race motorcycles) means you push the left handlebar forwar for a fraction of a second. THIS is what causes your bike to lean over. After that brief fraction of a second, if you continued to countersteer on your light bicycle you would low side(fall over to the inside) so you immediately push on the right handlebar. This keeps you from falling to the inside, and doesn't work on a fast motorcycle.
    So turning left looks like this:
    1. Turn the wheel to the right for a fraction of a second
    2. This causes your bike to flip to the left, which is where you are leaning
    3. Turn the wheel to the left to counter balance your fall to the inside shoulder(this is the part you notice, because it lasts long enough)

    The whole process can be completed without any leaning at all, and cannot be done if the front wheel is locked into place.

    From this you can see that leaning has no effect on turning, and that turning has everything to do with steering(countersteer, specifically.)

    I hope that example is clear :)
     
  12. Oct 3, 2011 #11

    rcgldr

    User Avatar
    Homework Helper

    On some motorcycles, you can do a mild turn at moderate speeds (40 or less mph 64 or less kph) by leaning. The rider leans inwards, the bike leans outwards, the trail on the bike causes the front tire to steer outwards in response to the bike leaning outwards, an indirect method of counter steering, and now the center of mass is offset inwards so the bike can lean left and make a gradual turn. This doesn't work on motorcycles with a small amount of trail, such as road racer replicas. If there's a steering damper, that will also interfere with the trail effect. As mentioned before, this doesn't work at high speeds on any motorcycle that I'm aware of.

    Keith Code runs a motorcycle racing school, and he has a test 600 cc road racer motorcycle with a second set of handlebars fixed to the frame that don't steer. The bike is stable, but while holding on to the second set of handle bars, rider leaning will only produce a tiny amount of turn response on that bike. However Keith also mentions that his dirt bikes (these have a lot of trail), can be easily turned by leaning.
     
  13. Oct 3, 2011 #12

    berkeman

    User Avatar

    Staff: Mentor

    I use body steering pretty exclusively on my liter sportbike, both on the street and at track days. I originally learned body steering at a CLASS track school by Reg Pridmore, and have also attended STAR school with his son Jason Pridmore. I prefer body steering over countersteering because it works so much better in emergency situations. Relying on your arms to do something useful when you are tensing up because something bad is happening in front of you, is not generally a good thing. But moving your body and shifting your weight on the pegs (only 20% of your weight is on the seat when steering) is very natural, even when things are getting crazy in front of you.

    The problem with Keith's "No BS Bike" is that you still sometimes use small corrections with the handlebars, even if you mostly use body steering to turn the bike. I still remember climbing the hill toward the Corkscrew at about a buck at Laguna Seca at one track day, and laughing to myself how I had to turn the handlebars into a turn (the opposite of what you would do for countersteering) because of the 30mph crosswind across the track. Goofy.

    BTW, here is a video of Reg Pridmore, celebrating his 72nd birthday on the track at VIR. Reg uses bodysteering pretty exclusively. I want to celebrate my 70th birthday like that (still plenty of years to go...)! GOOSEBUMPS! :biggrin:



    EDIT -- BTW, in the video you can see that bodysteering works just fine at 165mph+ o:)
     
    Last edited by a moderator: Sep 25, 2014
  14. Oct 3, 2011 #13
    On a motorcycle this speed is very low, like walking speed. Tony Foale has a thick book explaining some of the dynamics involved, but above about a walking speed you have to force a countersteering input to turn the bike. You can force this directly with the bars, you can force this by leaning to one side, or you can force it by leaning to one side and cutting the throttle a bit.

    Incidentally I rode Keith Code's bike like 10 years ago at Streets of Willow, and you can indeed turn it but it's a challenge to reorient your brain and just get a feel for what's happening as you move around and play with the throttle. It's still slow to turn, but you can do figure-8's in the parking lot with practice.

    Some bikes do require constant steering input through a turn, but it depends on the bike and tires - many bikes, if set up correctly, are neutral. You don't highside unless you give the bike too much throttle while leaned over, it actually has nothing to do with mid-turn steering inputs. Yes, I've highsided and lowsided many times when I used to roadrace....
     
  15. Oct 3, 2011 #14
    The thing to keep in mind is that its not the "leaning" thats turning these bikes. Its the countersteer. There are two ways of countersteering: Mechanically, using your handlebars, or by thrusting your weight one direction or another. When you thrust your body, its more than just a lean, and it is a physical chain reaction(equal and opposite the whole way through): the motorcycle tilting slightly in the opposite direction of your thrust, thus the tire turning in a direction opposite that of the motorcycles lean, causing the entire body to be flipped the direction opposite the tires turn. In either case, the action of your front wheel turning one way causes your motorcycle to turn the other.
    Once your in this lean, your back to using your handlebars.
     
  16. Oct 3, 2011 #15

    berkeman

    User Avatar

    Staff: Mentor

    I'm not tracking what you are saying, but that's okay. Bodysteering is weighting the pegs in the direction you want to turn, and getting your butt on the edge of the seat toward the inside of the turn. You don't touch the bars other than to work the controls. The bike front end finds the right angle of turn-in to match the balance and lean angle. You make mid-corner corrections to tighten up the turn by weighting the inside peg more, moving your body more to the inside, and pulling in a little bit on the outside of the tank with your outside thigh.

    There is no initial outward motion of the front tire at body steering turn-in, and you don't use the handlebars for either turn-in or to straighten back up at turn exit.

    It is true that you can quicken your transitions by using both body steering and countersteering, but because of the issues with using countersteering in emergency situations (that I mentioned above), I do my best to never use yaw inputs to the bars for steering.
     
  17. Oct 3, 2011 #16
    With all due respect, I have to disagree with your assertion. Do you feel you could use body steering to lean over a bike with a fixed front wheel? The answer is no. The reason for this is that the front wheel MUST slightly turn the opposite direction, for the most brief fraction of a second, in order for the bike to tilt the opposite way. Its simple physics, nothing more than equal and opposite reaction.
     
  18. Oct 3, 2011 #17

    A.T.

    User Avatar
    Science Advisor
    Gold Member

    Probably because when you lean one way, the center of mass of you and bike still stays centered, and the bike tilts the other way.

    Yep. People don't realize it, because it's minimal at low speeds.

     
    Last edited by a moderator: May 5, 2017
  19. Oct 3, 2011 #18

    berkeman

    User Avatar

    Staff: Mentor

    I could certainly turn it in, but would then promptly low-side. When body steering, you lean the bike with your weight shifting on the pegs, and the bike's front end turns itself in to match the balance and lean angle.

    I learned body steering at Laguna Seca at my first racetrack CLASS day. I had been using countersteering for many, many years, and thought I was pretty good at it. But I could not manage to steer through the Corkscrew no matter how hard I tried, and how quick I tried to correct my lines.

    And then in one of the classroom sessions (you alternate classroom-track sessions all day), Reg was talking about how if you used your balance to aim the bike where you wanted to go, the front end would find its own turn angle to keep the bike stable and going where you wanted it to go. So the next time out on the track I stopped using my hands and started using body weight shifts, and nailed the fall line through the Corkscrew time after time. It was a true epiphany.

    If you haven't tried using body steering at speed (and especially at a racetrack), you probably shouldn't be trying to convince others of how it works by some form of countersteering. It doesn't.

    Try this. With the help of a friend, walk your sportbike in a straight line, not holding the bars. It will probably track straight pretty well. Now lean the bike to the right about 10 degrees while still walking it and not touching the bars. See what happens to the front end? It goes from straight to turned, without any counter-turn involved....
     
  20. Oct 3, 2011 #19
    "It is important to distinguish between countersteering as a physical phenomenon and countersteering as a conscious rider technique for initiating a lean (the usual interpretation of the term). The physical phenomenon always occurs, because there is no other way to cause the bike and rider to lean short of some outside influence such as an opportune side wind, although at low speeds it can be lost or hidden in the minute corrections made to maintain balance.

    At the same time, the rider technique of applying pressure to the handlebars to initiate a lean is not always necessary, since, on a sufficiently light bike (especially a bicycle), the rider can initiate a lean and turn by shifting body weight, called counter-lean by some authors.[1][2][3] Documented physical experimentation shows that on heavy bikes (many motorcycles) shifting body weight is less effective at initiating leans.[4]"

    That's from the wiki page on countersteering. Note how it explains that it is impossible to turn without the physical phenomena of countersteer taking place.
     
  21. Oct 3, 2011 #20

    berkeman

    User Avatar

    Staff: Mentor

    Don't believe everthing you read at wikipedia, especially if it involves the word "impossible". :smile:

    Please try the experiment I outlined above, and tell us what the front wheel does.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: How are bikes tilted?
  1. Precession Tilt Angle (Replies: 4)

  2. Bike on a Treadmill (Replies: 9)

Loading...