physics of cornering

[billymtb]

Physics hasn't got on holiday.

Look at your tyres. On the edges they have raised aggressive knobs and in the centre they have flattened faster rolling knobs and sometimes sharp edges for braking - they wear quickly.

Those outer knobs are also softer rubber ion many tyres because they see so little use, thus the tyres have a better coefficient of friction on the edges that don't see use when travelling straight up and down.

Second - if you are laying over naturally and the bike starts to slide enough that you slow down - you will overbalance the bike and go down to. If however, you are more vertical, and the bike starts to slide and you slow, you can lift the bike more vertical to hold balance and the natural forces at play.

Now why aren't any of these techniques used in moto? Well the bike is too heavy for the rider to handle as easy, and 2, when they slide and slow, they simply apply more power .

On 1 i think it's about moving your centre of gravity over the bike so when/if the tyres do lose grip then you slide and are able to gain traction again instead of going down like a bag of shit.

Yup, but my argument wasn't about lean angles are their effect on grip, although it plays a big role. I agree, a more upright position will allow for a better reaction to sliding out and thus staying in control. But this does not mean more grip! alot of websites preach that it results in more grip, but actually the correct reason is that it puts you in a position to respond to sliding out with much more efficiency.

I'm no physics expert, but I apply a similar principle to cornering my bike as I do to skis.
If you just lay over and stay parallel to the bike you are then unable to apply any pressure to them, and you will eventually be detracting from their traction. If you stay above them you can apply pressure therefore enhancing their ability to grip. Same with skis, keeping your body upright allows you to load up the edges with pressure far more than laying over with them, allowing real bitey cutting turns.
Saying this, I believe this applies more to shorter turns, it gives you a tighter whippier turn. Linger more drawn out corners it doesn't apply so much as you're not asking as much of your equipment.
Try riding your bike down the middle of the road slaloming the white stripes, keeping your body upright and central and kinda laying your bike out side to side, you'll feel it whip back in underneath you. Same thing with slalom skiing.

That's my take anyway.

this is more true for slower corners where the centripetal forces are much lower, if you stay parallel to your bike, the weight that is pushing through the tyres is nearly all centripetal, and very little of it is your weight force. If you take a fast corner with your body over the bike, you have a much higher chance of flipping as your centre of gravity and pivot point has changed. Testing the body over bike position in a parking lot is very inaccurate, you are far from demonstrating the actual forces of a high speed corner. I have written alot of notes and drawn alot of diagrams about this theory, and the more i think about it, the more i am confused.

If only life were so simple.

It is in fact quite a complex interaction between bike and rider and includes the geometry of steering, centre of mass in relation to the bike (ie height, fore-aft and lateral), reaction through the tyres, as well as the cornering forces. All whilst maintaining control..

http://www.coachcarl.com/training_articles/cornering.htm

http://genjac.com/BoomerBiker/Two Wheeled Physics.htm

You're going to need a load more physics (and maths) before you'll get anywhere close to understanding cornering on a bike. I've for a degree in mathematical physics, and know for certain that I'm not even close to having the skills/knowledge to explain all the complex systems interrelationships going on.

In relation to your points, you're forgetting to factor in the other ways in which a heavy object distant form the balance/pivot point is affecting forces in other directions. By your logic, a low vs high center of gravity would also have no effect. There's a load of friction effects to take into consideration, and everything needs to be considered the three dimensions rather than two. Get out your vector calculus book....

Yes i know it is 3 dimensional, but the only reason i would have to look at all dimensions is if it were all dependant on one another, i have drawn alot more diagrams for this problem, and have left out alot of my calculations because it would be really hard to show on a forum. low centre of gravity vs high has a slight effect on grip, but the main difference is a lower centre of gravity provides more stability in the corner, such as a hitting a rock. The rock won't damage your course as much if your centre of gravity is low and thus aiding stabillity. In a perfect world, centre of gravity doesn't have a pronounced affect on grip and mass is still equal to m. Thats why i didn't mention it, but now i see that it will affect cornering in real life situations.

But doesn't braking also employ the tires to do two job in layman terms. The tire it now trying to deal with a braking force and a traction force, also braking tends to force the bike to want to stand up and not down like it should it corners, it will also shift lots of weight forward resulting in a loose rear end. we cant compare cars here because they have completely different make up of design and control. better off looking at moto gp.

Now my layman understandings because it appears they haven't been outlined for us simple none physic genius's is

1.Braking should be done before the turn, this allows proper cornering position and more speed to be carried through the corner as the tires have one job and one job only which is to grip not to slow down as well.

2.Cornering over the bike.
This concept is that the center of mass stays up right and therefore the angle in that the inertia mass is affecting the tires is very different. When you place the weight up higher the angle is less acute from the ground which puts less force on the tires to slide out. It simultaneously puts more force into the tires from above keep them planted in the ground with down ward pressure and less sideways pressure which again results in more grip and less inertia pushing the tire sideways.

2. Outside foot down.
This lowers the position of the center of gravity again, because your weight is placed on your the lower the pedal the more weight you are placing against the force applied to the inside of the tire trying to push it out.

3.Berms there is less need for these concept to be taking into account some corners you should apply them but other its not necessary and you can get away with leaning the body and just ripping the bike around the corner

4.Braking
Unweights the and essentially makes it unstable while taking pressure off the tires which reduce grip, When braking it introduces a force that wants the bike to stand up in the corner, braking employs the tire to to two jobs at one, slow down while griping.

The point of this post is to highlight my layman understanding of the concepts as I believe these are the main reasons shared by many and open up the discussion on them.

 

[scblack]

1.Braking should be done before the turn, this allows proper cornering position and more speed to be carried through the corner as the tires have one job and one job only which is to grip not to slow down as well.

Maybe here your definition should state: The BULK of the braking should be done before the turn. Some braking can always be done in a corner, but you are not going to be cranking on the brakes in the turn itself.

If you brake hard too early you are going to be coasting along for a section into the corner, and consequently losing time.

 

[driftking]

The only time the tyres have one job only is at the very apex of the corner. At that point all force is cornering, before that point it is a mixture of braking and cornering. After that point it is a decreasing amount of cornering and possibly an increasing amount of acceleration force (if you start pedalling or if its downhill). Getting on the power as soon as possible after the apex is just as important on a bike as it is on a car.

Few follow ups.
1.while the tires only have one job at the apex how, during the other times how is grip effected through braking, ie what is the tires grip level without braking vs with braking, I'd expect it would still have more grip without the brakes engaged, in which case would allow you to carry more speed into the first half of the corner and carry more through the apex and out?

2.What about the forces of braking on the bikes lean angle and any extra force it applies to the tires, that would have to come into play?

 

[thecat]

The more time i spend at uni, learning about physics, the more I am confused about why some people preach certain cornering techniques. I'll post a couple examples of what i mean and i hope someone can prove me wrong (so i can actually try and corner better!) or post some alternative ideas.


1). Putting your body over the bike, so you put more weight over the tyres and thus more grip.

This is crazy, the reason you slide out on a trail due to lack of grip is because the friction force acting at the tyres is LESS than the centripetal force required to change your direction and thus make it possible to corner (or to accelerate you as you are changing your velocity direction for those who did/do physics). Putting your weight over the bike does not change this, in theory,

You are not changing the force but the direction of force and contact.

Your mass doesnt change if you put your weight over the bike. Yes, you lean the bike over to be able to turn faster, yes, you are able to lean the bike over further if you shift your weight, but if you dont move your body as you lean the bike further, you will simply pivot at the tyres and fall on your side. This crash is NOT due to loss of grip, the shifting of your body weight (laterally) does not affect grip.

Again it's the line of force + the fact it is a loose, uneven surface which makes the dynamics far more complicated than your original post allows for

2). weight the outside hand/pedal

i weight the outside pedal and it lets me corner better, i dont know why, i haven't thought about it alot so it would be good if someone can explain this aswell .

two things to consider here. 1 getting the force down through the corner knobs as perpendicular as possible to the ground so you are pushing into the loose surface not across it.

And 2 ground clearance on the inside pedal

as the quote goes, With theoretical physics we can predict exactly what the sheep will do, well not all the sheep but all the ones that are perfectly round and standing on a frictionless surface, out of the wind.

Practical experiment for you.

Find some loose gravel stand next to your bike and lean it over. Now push on the inside pedal parallel with the leant over bike. Bike will slide easily across the gravel.
Now push on the outside pedal at a more vertical angle. Less chance of bike sliding.

 

[scblack]

as the fquote goes, With theoretical physics we can predict exactly what the sheep will do, well not all the sheep all the ones that are perfectly round and standing on a frictionless surface, out of the wind.

And didn't physicists PROVE that a bumble bee cannot fly?

 

[thecat]

And didn't physicists PROVE that a bumble bee cannot fly?

and its perfectly possible for an elephant to hang off a cliff by holding onto dandelion with it's truck

 

[driftking]

2). weight the outside hand/pedal

This is something that appears to not be the norm. Most technique or trainers will tell you to weight the inside hand forward to counter steer. I think your talking about a different weighting here though more about pushing downward with the outside hand.

 

[billymtb]

This is something that appears to not be the norm. Most technique or trainers will tell you to weight the inside hand forward to counter steer. I think your talking about a different weighting here though more about pushing downward with the outside hand.

Sorry this was a type on my part, I did mean for it to weight outside pedal and weighting inside hand. I wrote it correctly in a later part of my post.

 

[thecat]

Sorry this was a type on my part, I did mean for it to weight outside pedal and weighting inside hand. I wrote it correctly in a later part of my post.

Depends on the corner. If it's flat or off cambers I tend to weight outside pedal and get outside elbow high to get weight on the outside hand. Bermed corner I'd try and stay relatively even on both...

 

[brutasauras]

And didn't physicists PROVE that a bumble bee cannot fly?

An urban myth dating back to the early 1900s, since then physics has gone on to perform as one of the fundamental tools for the development of all our modern technology and certainly did predict the atomic bomb :nerd:. A good bit of light reading on the subject can be found in Vittore Cossalter's book Motorcycle Dynamics if any ones interested.

 

[thecat]

An urban myth dating back to the early 1900s, .

Or at least to a German dinner party in the 1930s... Where it was shown a simplistic mathematical model is not adequate for calculating complex real world scenarios, like the the flight of bumble bees. Much like the OPs attempts at describing the dynamics of cornering a mountain bike on loose surfaces using very basic mathematic principals with a limited range of input data.

 

[billymtb]

Or at least to a German dinner party in the 1930s... Where it was shown a simplistic mathematical model is not adequate for calculating complex real world scenarios, like the the flight of bumble bees. Much like the OPs attempts at describing the dynamics of cornering a mountain bike on loose surfaces using very basic mathematic principals with a limited range of input data.

That myth actually proved that a bee is unable to glide, so the calculations achieved the result they were not looking for. Also the bee example used real values, i can only imagine the degree of uncertainty whilst calculating on a dinner table. I am not trying to calculate whether i am able to complete a turn at a certain speed, im trying to theorize lateral body movement. So, i fail to see how this is much like my 'attempt'.

Issac Newton didnt think about wind resistance values, or height about sea level when he theorized the concept of gravity. So please explain what other input data i need to be able to calculate.

 

[wilddemon]

Or at least to a German dinner party in the 1930s... Where it was shown a simplistic mathematical model is not adequate for calculating complex real world scenarios, like the the flight of bumble bees. Much like the OPs attempts at describing the dynamics of cornering a mountain bike on loose surfaces using very basic mathematic principals with a limited range of input data.

But the sauerkraut was delicious.

 

[brutasauras]

Like most things that seem simple it is really just a matter of perspective, like expanding the scale of satellite images to measure the coast line of Australia. The more you expand, the more accurate the measurement becomes until the point when the coastline is always changing and no longer definable. Simple models can still be valuable tools for occupying your consciousness until the next time it becomes a passenger on another ride.

 

[thecat]

That myth actually proved that a bee is unable to glide, .

an the tabletop calculation, no myth, involved calculating up thrust based on flat, smooth wings that beat vertically only at a set bpm. The basic calculation failed to take in the complex input data needed for a thorough description of how a real bumblebee achieved flight.

Similarly without taking the complexerties such as lines of force, leverage, angles of attack and surface dynamics you orginal assumption of cornering will fall short of what is actually happening when you lean your mountain bike over and weight the outside pedal.


These dynamics differ a fair bit from those of, say, a GP bike where you have a relatively smooth steady surface, rounded profile tyes with even grip spread and completely different weight distabution. GP bike = weight inside foot and hang off inside of bike. Do that on a mtb on dirt and you are in a bit of trouble.

 

[SummitFever]

...These dynamics differ a fair bit from those of, say, a GP bike where you have a relatively smooth steady surface, rounded profile tyes with even grip spread and completely different weight distabution. GP bike = weight inside foot and hang off inside of bike. Do that on a mtb on dirt and you are in a bit of trouble.

Its all the same thing just with different levels of grip. Even with the foot weighting. You basically are always wanting to get your centre of gravity as low as possible. This means weighting the foot that is closest to the ground. The speed which GP bikes travel at means that the surface is anything but smooth and probably changes quicker for the rider than any mtb track. There's lots of rider input and cat-like reflex responses needed to get a GP bike through a corner. I was certainly shocked with how brutally direct everything was and how unsettling the slightest surface undulations were.

 

[driftking]

Its all the same thing just with different levels of grip. Even with the foot weighting. You basically are always wanting to get your centre of gravity as low as possible. This means weighting the foot that is closest to the ground. The speed which GP bikes travel at means that the surface is anything but smooth and probably changes quicker for the rider than any mtb track. There's lots of rider input and cat-like reflex responses needed to get a GP bike through a corner. I was certainly shocked with how brutally direct everything was and how unsettling the slightest surface undulations were.

Lower pedals make sense when we speak of one type o force but keep in mind he varribles of front and rear grip sometimes the best pedal position is back to weight the rear end more.

When it comes to cornering a dh bike the only answer is.
Do what gets you around the corner fastest, even if its none conventional.

 

[haltz]

Do what gets you around the corner fastest, even if its none conventional.

A bloody men! This should mark the end of this stupid thread

 

[billymtb]

A bloody men! This should mark the end of this stupid thread

Sorry mate, i should have made a thread tailored to your interests