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Rear brake... the most dangerous control on a bike.

Discussion in 'New Riders and Riding Tips' started by robsalvv, Jan 19, 2007.

  1. Once again, bike physics is counter intuitive... aggressive use of the rear brake reduces rear traction and can possibly even lift the rear off the deck.

    It's all here:


    and here:


    The key word is "aggressive"... which adrenalin can drive you towards in a pressure cooker moment... Food for thought. :-k
  2. The article is only partially true.

    loading the clutch with the rear brake applied will raise the rear suspension, but the article gets a few things mixed up in terms of roll centre and CoG.

    The rear does unload during braking because there is a weight transfer to the front. This is due to the fact the CoG is above the roll centre.

    But here is where the article is majorly wrong. For the same reason the back squats under acceleration.
  3. Ibast - I'd love you to get on the msgroup.org forum and tell Mr Davis what you've just said. He is sure to engage you... and I'd really like to see that discussion!

    Personally - I think you've over simplified it. The situation is more dynamic than that and I also think your point of reference is YOU (as part of the bike), rather than from outside the bike, so your perception is likely to be skewed.

    I'd be surprised if the author made such a basic COG mistake... there's miles of forum discussion about COG on the msgroup forums. This guy is the guy that other motorcycling guys go to and he has a vast understanding of motorcycle physics and dynamics.

    In regards to the squatting thing, it is a largely held misnomer that the rear squats under acceleration. It might "feel" that way to the rider (it certainly feels that way to me), but the physics say otherwise. I think the front extends more than the rear giving the squatting impression.

    TOTW cautions against snapped on hard acceleration out of corners for the very "rear rises under acceleration" reason, because extending the rear shock leaves the suspension less compliant...
  4. *looks frightened at the technical talk and darts away* :bolt:
  5. His misunderstanding is not so much based around the CoG, but the concept of the roll centre.

    The torque reaction of the rear wheel must reach a point where it equals that of the reaction of the one created by force through the CoG pivoting around the roll centre.

    this occurs at some point in the rear shock compression. For that to happen the arse end of the bike must go down.

    There is no doubt that the dynamics of a bike coming out of a courner are effected by adjusting compression damping on the rear shock.
  6. :-?


    Who mentioned damping?

    I think you just stringed a bunch of non sequitors together... for a lark.

    Torque reaction, forces... equivalent force in the shock... eh? whaa? Perhaps if you talk moments maybe it will start making sense to me.

    At any rate...

    If I park the front wheel headon against a wall and engage the clutch friction point, the rear rises. This is the example in TOTW which is used to demonstrate rear rising under acceleration - not squatting. Rear shock extended under hard acceleration is now accepted conventional wisdom of bike physics.

    On the topic of rear wheel losing traction under aggressive braking, the Davis articles describe WHY aggressive rear brake will effectively reduce rear traction. I don't beleive you've countered this meaningfully yet.

    Take it to the forum ibast. I'd love to see that discussion!
  7. Of course, this article doesn't suggest that it's not fun or appropriate to deliberately skid the back wheel around for shits and giggles every now and then. :) :LOL:

    Note: New riders please be advised that Seany is an idiot. Do not attempt to immitate childish behaviour described by Seany. He is a trained profesional twat. :wink:
  8. The bike against the wall, or on a dyno for that matter, is the example of why he is wrong.

    It does rise, but it's not the complete free body diagram. Parking the bike against the wall takes the reaction load from accelerationout of the equation.

    F=ma. Newton's second law of motion. This force dissapears if there is no accelleration, but is very much present in the real world situation of riding the bike.

    The rear wheel under brakes is a simple weight transfer situation. The bike tries to roll around the front wheel and the front forks compress.

    As a result there is less normal force on the rear tyre and thus the friction force that it can handle is less. so under heavy braking the rear wheel will lock if you are not carfull.
  9. Sorry Rob I have to agree with ibast. Those two discussions are so flawed as to not be worth reading.

    While he basic point that using too much rear brake is a bad thing is quite true (I know, I do it too often myself), his analysis is really flawed. How about "the shock absorber will suck that wheel off the ground". Well, the shock absorber will never do that. As even he said, the shock absorber will only ever extend, unless acted on by external forces, or it has reached its limit of movement. At the same time, it will resist those forces, through compression and rebound damping.

    I think what he was getting at is that if you hit the rear brake hard by itself, and the swingarm pivot point is higher than the rear axle, then the rearward braking force acting on the tyre will tend to extend the wheelbase length, which being constrained by the swingarm will rotate up. If the sudden vertical movement cannot be accomodated by the shock absorber pushing back down on the wheel, and gravitational force pushing the whole bike back down, then the vertical force at the contact patch of the rear wheel will be reduced, and this could result in loss of traction, or chirping.

    Then there is his definition of "wt". Two lengths, divide one by the other, and get an angle? Simple dimensional analysis shows that aint right.

    The author should go back to basics and analyse the situation based on the tangential forces of acceleration and braking at the wheel contact points, the always vertical gravitational forces acting on the bike, and the inertial forces from acceleration and decceleration.

    Sorry again, but I aint gunna try to explain it all here, or to Mr Davis. That would be a lot of work. :roll:

    On the good side though, most of the articles on that site are good value, and worth a read.
  10. OK, fellers, have a go at this article then.


    This article is wholly focussed on the rear squat / rising issue, and goes into more detail - in particular about the weight transfer issues that ibast bases his argument on.

    To my mind, the article is right and it follows then, that so are the braking ones in the OP.

    On the topic of shock absorber "sucking" the wheel up, it was simply a turn of phrase. If the dynamics produce a small relative upward movement of the wheel, the only place for the relative movement to go is in the shock absorber. i.e., sucking up the wheel.

    Rod, remember SOH CAH TOA in basic trigonometry? Sin = opp / hyp. Cos = adj / hyp. Tan = Opp / Adj... One length divided by another...
  11. RodG/ibast, you both referenced R Davis' lack of understanding... well in addition to the link in my post immediately above, I think you'll find the collection here will blow your misconceptions out of the water.

    If the weight transfer articles don't show that there's a solid grasp of the physics of weight transfer, COG and gravity, and address Ibasts concerns, then nothing will.

    http://www.msgroup.org/TIP074.html Weight transfer 1
    http://www.msgroup.org/TIP188.html Weight transfer 2

    Here are some other specific articles addressing the supposed flaws and lack of understanding that R Davis apparently has.
    http://www.msgroup.org/TIP106.html COG

    http://www.msgroup.org/TIP174.html Braking - weight transfer

    http://www.msgroup.org/TIP180.html Weight and traction mgmt

    http://www.msgroup.org/TIP207.html COG weight and mass

    Seriously guys, this guy knows his stuff. SOoo it follows then, that if he doesn't have a flawed understanding of mass, forces, loading, bike mechanics, bike dynamics etc etc etc then his rear braking article is dead on right.
  12. It may be a turn of phrase, but it implies that the shock absorber is in fact lifting the wheel, which we both know is wrong. It doesn't help that at the end he says: "Bad, bad, shock absorber! Behave yourself!"

    The braking force combined with the internal force in swingarm is what is tending to lift the wheel, ignoring the inertial effect of deceleration of the bike, applied at the Centre of Gravity.

    Well, I haven't looked up the actual definition of "weight transfer angle", so it could be that simple. Maybe he was oversimplifying. However those formulas only give, in this case, the value of Tan for the triangle defined by the opposite and adjacent lengths of a right angle triangle. Using the CoG height and the wheelbase would seem to give a false value, as a vertical line from the CoG cuts the wheelbase part way along. Therefore, an angle calculated using the CoG height and Wheelbase would be less than the angle of a line drawn from the rear wheel contact patch through the CoG, which is probably the important angle. Maybe he knows this, but has over simplified. Unfortunately, the Free Body analysis of a vehicle under acceleration and deceleration is not a simple proposition.

    Man, that was scarey. :shock:

    Wrong, if he does have a solid understanding, then he has written a very poor article, which is oversimplified and could lead to gross misunderstandings by readers. Image a reader using that information to try to understand, and adjust, their rear shock so that is doesn't "suck that wheel off the ground". Even the confusion that it would create for someone trying to understand how their suspension works is enough reason to criticise the article.

    I may have a look at the other articles, but all the knowledge in the world, and a bunch of other great articles doesn't excuse a poor article. As I siad earlier, many of the articles on that site are good value.
  13. The point is are they flawed?
  14. The first two article certainly are. I havn't reviewed the others.

    Basically what it comes down to is he has left a force out of his equation.

    I also havn't seen anything refer to Roll Centre, which is very, very fundemental to the way suspension moves.

    Mind you I have just skimmed.
  15. Look at it this way.

    Imagine a rear wheel sitting there with a swing arm and a chain. Now pull on the top of the chain. What will happen? Well if you pull hard and fast enough the wheel will not only come forward, but the swing arm will rotate back over the wheel.

    It is this tendency that he is arguing will raise the rear of the bike and he is right if the bike isn’t accelerating.

    But here is where he is wrong. Acceleration causes a reaction force. This force acts through the CoG. This creates a moment around the role centre of the bike.

    Here’s the important bit. This moment created by the acceleration is GREATER then the moment created by the torque reaction of the rear wheel.

    The result is the bike wants to rotate backwards. What stops it is the rear suspension. The bike will continue to rotate backwards until such time act the moment created by the rear shock equals the difference between the two moments noted above.

    In other words the shock compresses under acceleration. I do not see any mention in his article about the moment created by the acceleration force.
  16. See Ibast, I disagree.

    I'm contending the original articles aren't flawed.

    The weight transfer articles I linked a couple of posts up, expand on the bits that were sort of glossed over in the rear brake articles - the glossed over bits just happen to be the bits you and Rod seem to have issues with.

    2ndly, you contend the exact opposite to R Davis in terms of what the rear does under acceleration, but the swingarm article (linked above) explains pretty clearly why a rear rises under acceleration... and not the other way around.

    3rdly, it's been put that R Davis has missed some obvious things, or over simplified them, but in amongst his other articles (some more linked above), he focuses on some of these individual items almost exclusively.

    4thly, the articles seem like a cohesive set of articles on the topics at hand, so if one is flawed, they must all be.

    Rod, a noob is likely to have run a mile after reading the rear brake articles! Too technical.

    And any noob who takes one "turn of phrase" as the sole point of the article in their endeavour to adjust their rear shock deserves a pan galactic gargle blaster adjustment to the head. There's not a single element to the originally linked article that relates to shock setup!

    PLUSSSSS! the whole article is about NOT APPLYING THE REAR BRAKE aggressively... which the noob must be doing in order to have observed the chirping... so if they then set about trying to dial out the "suck up" to improve their rear braking, rather than just not apply aggressive rear brake like the WHOLE article is focussed on... then perhaps their pea brain is actually a dried up pea brain and they have somehow managed to avoid being darwin award contenders to date... :LOL:
  17. Is that in response to http://www.msgroup.org/TIP080.html which describes the swing arm situation?? Because he comprensively argues the weight transfer versus the forces involved at the swing arm... which i think you missed...
  18. Yes, they are. While they do put an explanation to observations, his logic in the article(s) is flawed. I don't believe that his explanations are correct.

    I just read http://www.msgroup.org/TIP080.html, and I'm quite amazed at the logic. His reliance on two artificial concepts, the "weight transfer angle" and the "chain angle" are of significant concern. Perhaps they are a simplification of a more complex analysis, but if so, he should link to the proofs. As it is, he really offers no justification for statements concerning those two concepts. He just gives his definition of them.

    He also ignores significant contributing factors, such as the tension in a chain, at a distance from the rear wheel axle, contributing to the rotation of the whole bike around the rear axle.

    Why would the chain angle be less on a chain driven bike, and why is there an imaginary "pivot point" slightly forward of the swingarm pivot? Because the Chain Angle is actually taking into account the torque provided by the chain tension, but he does not discuss that at all. It is a force he has left out.

    The thing is, he is nearly right in a lot he says, but then he makes dumb blunders. For example, he says:
    "That is, traction at the contact patch would push back against the tire with exactly the same force as the tire pushes against the road surface from acceleration (Correct), regardless of whether the swing arm is pointing up or not. (Correct) For any given length of swing arm, the higher the frame pivot point is above the contact patch, the greater will be the lifting of the rear end from acceleration. (Wrong) In other words, whether that pivot point is above or below the wheel hub, acceleration must result in the raising of the rear-end of the motorcycle. (Wrong)"
    Well, that is just wrong. He should have said: "the higher the frame pivot point is above the rear axle, the greater will be the lifting of the rear end." If the pivot point is below the rear axle height, then acceleration will tend to rotate the swingarm clockwise instead of anti-clockwise, and so the rear of the motorcycle will be lowered by acceleration.

    I think some drag racing bikes have a horizontal or slightly downward swingarm, and when they take off the bike squats down harder. Someone find a movie of a drag racer huh!

    I haven't even addressed the inertial force of the CoG, mentioned by ibast. Depending on how high the CoG is, this can be a significant force.

    His example of a wheelie on a Goldwing is interesting. Notice that they put a pillion on the back. Why? Perhaps to move the CoG backwards, which we all know makes wheelies easier. How does his "weight transfer angle" account for that?

    Anyway, you guys have been slogging away while I wrote this, so some of this has been mentioned. I was gunna draw a little diagram, but that can wait until we all have a feed at a place with butchers paper for a table cloth. Then I will draw you heaps of diagrams. :grin:
  19. :popcorn:

    i don't understand so someone tell me who wins... K
  20. OK this article helps me to understanding where he is making his mistake. He is considering only weight transfer and the torque load on the rear wheel.

    He's still forgeting our freind Newton.