motoDNA: Getting a Grip on the Mechanics of Trail Braking

03/07/2014 @ 2:44 pm, by Mark McVeigh18 COMMENTS

motoDNA: Getting a Grip on the Mechanics of Trail Braking motoDNA trail braking 05

On the track, racers are either on the throttle or on the brakes – no free wheeling – this wastes time. Trail braking is a technique which racers use to slow the bike as quickly as possible from one speed (on the straight) to another (corner apex speed).

In applying this technique, a racer will approach a turn and at their braking marker, apply full braking force, normally with the bike being upright.

As the rider begins to turn in, they reduce brake pressure, easing off the brakes. Decreasing or “trailing” the brake lever force as the bike lean angle increases until they gets to the apex, the rider then releases the brake and applies the throttle.

Sounds easy enough in theory, but proper execution is complicated because it comes down to feel — and remember these guys are doing this seamlessly, every lap on the limit.

As Freddie Spencer once said, “fast riders have slow hands” so all this is done smoothly, progressively and powerfully.

motoDNA: Getting a Grip on the Mechanics of Trail Braking motoDNA trail braking 01

Also consider a MotoGP machine has so much front tyre grip when upright, that you would easily endo the machine if excessive brake lever force is applied too aggressively, so a certain amount of finesse is required.

Normally only the front brake is used for trail braking as the rear brake contributes little braking power, has less feel, and is normally reserved for mid-corner fine adjustments or to stabilise the bike.

To comprehend the dynamics of trail braking, ignoring any aerodynamic effects, some understanding of tires and grip is beneficial, with the amount of grip from the tires depending on various factors.

motoDNA: Getting a Grip on the Mechanics of Trail Braking motoDNA trail braking 02

The main contributor to grip is the weight or load on each tire. The ratio between the maximum possible grip and the vertical load is called the coefficient of friction. This coefficient is not constant and normally decreases relative to the vertical load.

MotoGP tires go beyond the normal theoretical coefficient of friction value of 1.0, hitting values over 1.3. These tires behave more like gum, melting into the track surface to create exceptional levels of grip questioning Newton’s model of friction.

Also as the brake is applied, torque is transferred through the wheel to the contact patch, which creates a horizontal force at the track surface.

The road pushes back on the tire, and equally the tire pushes forward on the track surface. You can thank Newton for this mechanical grip, as for each force there is an equal and opposing force.

According to Brembo data, deceleration forces over -1.6g are common with riders applying around 8kg of lever pressure at speeds of 350 km/h.

For example, at the Sachsenring circuit, whilst braking into Turn 1, the MotoGP machines decelerate from over 300 km/h to 100 km/h in a distance of 253 metres within 5.8 seconds. As riders spend almost 20% of this lap braking, trail braking is an essential skill to master.

motoDNA: Getting a Grip on the Mechanics of Trail Braking motoDNA trail braking 04

Also to consider is the significant grip increase experienced as the front tire contact patch pressure multiplies due to the load transfer when braking. This grip effect decreases as the lean angle increases, and the load transfers off the front to the rear.

As the brakes are applied and the weight shifts forward, the forks are also compressed. This compression of the forks alters the motorcycles steering geometry, reducing the rake and trail.

This decreases stability but increases manoeuvrability in a fashion that makes the motorcycle lean and change direction at a higher roll rate.

The tire temperatures also increase from this weight transfer and subsequent tire loading, with tire temperature windows critical for optimum grip without tire degradation.

motoDNA: Getting a Grip on the Mechanics of Trail Braking motoDNA trail braking 06

Another important factor is the reducing road speed, which decreases the motorcycle’s cornering radius. Other factors such as track surface characteristics and other elements between the track and the tire such as water or oil play an important part in trail braking efficiency.

These guys are close to the limit, sometimes stepping over, notably demonstrated by the small difference in the Respsol lad’s track position at the last corner on that weekend in Germany, which resulted in Casey Stoner’s defining Championship moment of sliding down the road.

Mark McVeigh is a former international motorcycle road racer and MotoGP engineer who now works as a moto-journalist and development rider.

He currently is also the Director of Coaching at the motoDNA Motorcycle Academy. Read more of Mark’s work on the motoDNA blog, and follow motoDNA on Twitter and Facebook.

Comment:

  1. twoversion says:

    “racers are either on the throttle or on the brakes”

    #turning
    #tractioncircle

  2. damn says:

    Not by hand or foot. ECU!!!!

  3. Newton says:

    I know this isn’t a science site, but……you lost me with pseudoscience about the theoretical coefficient of friction being 1.3 was something abnormal.

    First the COF is a system property and is dependant on the two surfaces in question. It is not a fixed value. Secondly materials such as rubber generally can have a COF value of 1-2 (dry eraser on paper for example) and lastly a value above 1 merely implies that the force required to slide an object along the surface is greater than the normal force of the surface on the object. Where Normal force = Mass x Gravity. Aka it takes more energy to slide the object across the surface than the mass of the object as affected by gravity.

  4. Chaz Michael Michaels says:

    Too difficult in theory and in practice.

    So much easier to be a fan in the stands drinking beer and eating nachos.

    Mass of my gut x gravity as I pour beer in= 10x amazement at what these guys can do.

  5. Chaz Michael Michaels says:

    I gotta say, others have said this too–I absolutely love the photos. In this article: front forks depressed, tire deformed under pressure, positioning of the rider on the bike in a moment of laser sharp concentration.

    Amazing photos every time.

  6. MajorTom says:

    Here’s what I don’t understand. Using the picture of #26 as an example, how is it that he can apply enough front brake to lift the rear tire and still have his arms bent, and presumably, relaxed. His legs must be doing most of the work holding him on the bike and yet they are not “pinched” to the sides of the bike.

  7. 2ndclass says:

    In that pic he’ll be using the inside of his outside leg to support his weight against the tank and then using core strength to support his upper body and take the weight off his arms.

  8. crshnbrn says:

    …and that is one of the many reasons motorcycle racers are considered athletes.

  9. Mitch says:

    MajorTom: I ain’t no pro, but when I brake at the end of a straight into a bus stop or tight section, I typically brake hard enough that I slide forward on my seat and end up crotch to tank, so that I can let my legs and waist absorb the braking forces while my arms are unloaded for fine control.

  10. Chaz Michael Michaels says:

    Their technique is utterly brilliant, they are incredibly athletic not only entering the corners but also blasting off from the apex, then there’s the mental part of their race-craft, if that weren’t enough they are the bravest souls on the planet.

    The motoGP website video pass gives footage from the racer’s POV after each race. I always look forward to watching that after each race. It is absolutely mind blowing. In fact, if you show it to friends who aren’t into bikes it basically just scares them.

  11. john says:

    @MajorTom-

    Its the pucker of the sphincter which keeps them on their bikes….:)

  12. Norm G. says:

    re: “In fact, if you show it to friends who aren’t into bikes it basically just scares them.”

    exactly, it’s not the universal draw we think it is.

  13. Westward says:

    @ Newton

    1. “First the COF is a system property and is dependant on the two surfaces in question. It is not a fixed value.”

    Is that not answered in the statement by the author, “This coefficient is not constant and normally decreases relative to the vertical load.”

    2. “Where Normal force = Mass x Gravity. Aka it takes more energy to slide the object across the surface than the mass of the object as affected by gravity.”

    Are we not talking about motorcycles? What is normal force? (F=MxG) Inertia is force, and the engine of the bike coupled with the fuel determines the amount of force the bike will generate.

    So, what was your point?

    The only real thing that caught my eye by the author was the statement. “You can thank Newton for this mechanical grip, as for each force there is an equal and opposing force.”

    I thought that was Einsteins theory of relativity which supersedes Newtons theory of mechanics…

    E=MC2

  14. Kalle says:

    California Superbike School recently posted a transcript on this issue from an interview with Keith Code. I think it’s much better for understanding the issues at hand than this article which is just vague.

    https://www.facebook.com/californiasuperbikeschool/posts/734032623295378

  15. jzj says:

    Simply, what I don’t get is how the rider knows where the limit is. Obviously with the rear tire it can move around a little and not get you into too much trouble. But, I’ve found out the hard way a couple of times that when the front end is loaded (i.e., braking has necessarily shifted weight to the front of the bike and so the suspension is compressed), and one is turning or starting to turn, and the front tire loses traction due to a road imperfection (which is why I’m a knucklehead for pushing on the street instead of on the track), the front suspension then unloads and pushes the front tire even further out of shape and bam! half the time I’m down with little warning or opportunity to catch it. Obviously on the track they have better suspensions, better surfaces, and much better tires, but they are also dealing with much higher speeds and commensurately greater forces, and I just can’t imagine how they can “feel” the edge of traction and/or react fast enough to save it when it goes over the edge.

  16. Newton says:

    @Westward

    “So, what was your point?”

    My point being to someone that isn’t clear on the subject the description makes it sound like the COF thats being achieved is something so extra ordinary that it doesn’t appear else where in nature. Thats the pseudoscience described in the article I have/had issue with. Levels of COF even higher occur all the time there is nothing extra ordinary special about a COF of 1.5.

    In a simplified system using a free body diagram it wouldn’t be out of the way to use Newtons second law.

    To me it was akin to watching an episode of CSI and all of a sudden some scientific fact is incorrectly used to add to the drama, I donno just bothers me.

  17. Bluey says:

    nice aritcle, thanks. i live by trail braking!

    direction arrow shown for the sachsenring circuit is backward tho, just for info.

    -B

  18. Hahahaha, what a comical this YouTube film is! I’m still laughing, thanks to admin of this site who had posted at this web site.