A question on effect of applying the front brake?

[Hawk]

Hi guys.

I've noticed something that doesn't seem correct from my experience: When you are leaned over and apply the front brake, the bikes lean angle decreases and therefore the bikes turn rate markedly decreases with it. As far as I can remember this doesn't happen in real life(Though I'm getting on a bit now, and remembering exactly what happened when I used to race is not always clear. Hehe). Don't get me wrong, I know that when you apply front brake on a bike in a turn that the bike wants to lift up, but as I remember, you can easily counter that force to stop it happening during the turn, in GP Bikes it does it and you cannot stop it until you stop apply the front brake. Is this correct?

Can anyone whose had recent real experience racing bikes comment on this action? Thanks

[Stout Johnson]

Hey Hawk,

the GPB behaviour is correct - while braking in-turn (or going off the gas), the bike will tend to stand up and decrease lean angle... when applying throttle in-turn, the bike you should notice the bike maintaining its line.... and there is no way of changing or "countering" that behaviour. Maybe you remember situations where you were relatively slow or you apllied throttle that made you have the feeling you 'countered' that behaviour...

scientifically, I cannot explain that well - either you google an answer or maybe MaX can explain?

[HornetMaX]

Have to go in 3min, but will explain later. Yes, Stout is correct, GPB is correct.

MaX.

[Hawk]

Hey Hawk,

the GPB behaviour is correct - while braking in-turn (or going off the gas), the bike will tend to stand up and decrease lean angle... when applying throttle in-turn, the bike you should notice the bike maintaining its line.... and there is no way of changing or "countering" that behaviour. Maybe you remember situations where you were relatively slow or you apllied throttle that made you have the feeling you 'countered' that behaviour...

scientifically, I cannot explain that well - either you google an answer or maybe MaX can explain?

Have to go in 3min, but will explain later. Yes, Stout is correct, GPB is correct.

MaX.

Hi Guys.

@Stout: Thanks for updating my old mind. Hehe

@Max: I will look forward to your in depth scientific explaination when you log back in later.

Thanks guys!

[C21]

It´s correctly done by Piboso 
Sometimes a bit toomuch for my taste but anyway..

I will give it a try:
- Brake force
- tyre surface contact point
- steering angle

Driving into a corner:
When increasing the lean angle the tyre surface contact point moves to the inside. (Nomally at straight drive it is in the middle of the tyre).
At the steering rack (steering and fork) the mass (momentum) force of the bike take hold of leading to a "lever arm" which tries to turn the steering to the inside.
The tyre reacts with an upward movement (decreasing lean angle).

When you turn the steering with a fictional lever further inside the bike will move upwards (lean angle decreased).
The fictional lever in this case are your front brakes.
Keyword: Centrifugal forces.

I´m sure Max can explain it better and more scientific. 

[HornetMaX]

I'm pissed off because I'm sure I have a very nice explanation of this in one of my books but I cannot find it at the moment (Alzheimer FTW).

If I recall correctly, it is exactly what C21 was saying:

• The contact patch is the part of the tire that is touching the ground. Side note, it is typically as big as a credit card: now you know what your life depends on when you take that 200 Kmh corner ...
• When  the handlebars are centered and the bike is perfectly vertical (no lean):
  
  
  
  • the "center" of the contact patch is "in the middle" (OK, it lies on the intersection of the track plane and the bike's longitudinal "symmetry" plane).
  • if you intersect the steering axis line and the track plane you get a point, let's call it point A: the distance between this point and the contact patch center is the bike trail.
• If you brake with steering centered and bike perfectly vertical, the braking force exchanged between the tire and the track (at the contact patch center) passes via point A (easy to see, they are all in the bike's longitudinal symmetry plane): in that case the braking generates no torque on the steering (as the force passes via the steering axis).
• Now if your bike has zero thickness tires (they don't exist, but you can imagine them as road bicycle tires, very thin) , when you lean the bike the contact patch center does not move a lot. But if you have real tires, it moves sideways, laterally (with respect to the bike's longitudinal "symmetry" plane). This means that the braking force direction will no longer intersect point A, hence generating a torque: as the force pushes the tire backwards, the torque, when leaning right, will make the handlebar turn right and this straightens the bike up, making it go wide.

Practical way to see it: imagine a rod, suspended to the roof with a wire knotted in the middle of the rod: the rod stays horizontal if left alone.
Now if you push the rod with your finger exactly in the middle (where the wire is), the rod will not turn, it will just move backward: you're generating a force but no torque.
But if you push it not in the middle, the rod will turn (and move backward): you're generating a torque and a force.

MaX.

P.S.
Couple of essential tools to visualize all this stuff. I actually have both: just told my wife they were for my son





P.P.S.
If the bible (V.Cossalter, Motorcycle Dynamics) is too complex, a book that can be interesting (I think neoraptor mentioned it here a while ago) is T.Foale, Motorcycle Handling And Chassis Design.
It has almost no maths (which is at the same time a pro and a cons) but covers a lot of ground in a very understandable manner.

[HornetMaX]

Somehow related, I managed to find an amazing video from the latest Bosch MSC (Motorcycle Stability Control): click on the two videos with the label "with MSC" here http://www.bosch-presse.de/presseforum/details.htm?txtID=6468&locale=en

Little explanation (and a real life test, including intentional low-side fall !) here:

http://www.youtube.com/v/bK2gENALKkM

It is already available (or will be this year, can't remember) on the KTM 1190 Adventure.

MaX.

[janaucarre]

hello,
here is a video from moto journal, the point which interest you is from 3.40 to 4.45, you can try subtitles.

http://www.youtube.com/v/KrSEmsmj8Bw&feature

[Hawk]

Somehow related, I managed to find an amazing video from the latest Bosch MSC (Motorcycle Stability Control): click on the two videos with the label "with MSC" here http://www.bosch-presse.de/presseforum/details.htm?txtID=6468&locale=en

Little explanation (and a real life test, including intentional low-side fall !) here:

http://www.youtube.com/v/bK2gENALKkM

It is already available (or will be this year, can't remember) on the KTM 1190 Adventure.

MaX.

Does this guy know how to ride a motorcycle? He leans his body away from the turn not into or with the turn, something that if you were a pillion passenger can be lethal!

However, having said that, it's a nice demonstration though I feel the rider is exaggerating the inputs a bit to get the desired response.

[Hawk]

C21/Max: Thanks for your explanations, it always great to have the in depth facts to what can seem a strange reaction while riding these bikes. Though we may not always understand exactly what you physics guys are talking about, I think we get the visual idea of what your saying, and it will help those would-be engine/bike tuners to adjust the physics files on GP Bikes. I'd love to give the physics files a try myself one day.

Thanks guys!