physics of cornering

[billymtb]

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,

F= (m*v^2)/r, m = mass, F = force, v = velocity and r = radius of turn.

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.

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 .

weighting the inside grip and weighting the outside pedal just counteract each other, both change lean angle of the bike but if you are able to reach your desired lean angle, i dont see how weighting outside pedal or inside grip will allow for more cornering grip. There is still the same amount of force acting through the tyres, and thats all the tyres care about.

i will add more when i remember what else i want to say

 

[haltz]

This applys to a bike with no suspension characteristics and square tyres yeah? Sorry mate not to sound rude cos you sound like a clever bugger but if we all tried to work out the physics of every shaped corner,camber and surface nobody would have time to ride, leave the physics in the classroom, get on a bike and go fast!

 

[pharmaboy]

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 .

 

[John U]

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.

 

[pharmaboy]

On 1 i think it's aboutD 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.

Yes, eloquently put ;D

Add another thing to the physics of cornering. You have to have all your braking done by turn in to maximise grip in the corner - so many people keep on the brakes all the way to the apex, and wash out. Further, if you are washing out into the apex, the best thing to do is release the brakes entirely to try and transfer the braking grip to cornering grip, although this is counterintuitive when you think you can't make it, it's the best option.

 

[dunndog]

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.

 

[redbruce]

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,

F= (m*v^2)/r, m = mass, F = force, v = velocity and r = radius of turn.

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.

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 .

weighting the inside grip and weighting the outside pedal just counteract each other, both change lean angle of the bike but if you are able to reach your desired lean angle, i dont see how weighting outside pedal or inside grip will allow for more cornering grip. There is still the same amount of force acting through the tyres, and thats all the tyres care about.

i will add more when i remember what else i want to say

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

 

[bikesarefun]

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,

F= (m*v^2)/r, m = mass, F = force, v = velocity and r = radius of turn.

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.

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 .

weighting the inside grip and weighting the outside pedal just counteract each other, both change lean angle of the bike but if you are able to reach your desired lean angle, i dont see how weighting outside pedal or inside grip will allow for more cornering grip. There is still the same amount of force acting through the tyres, and thats all the tyres care about.

i will add more when i remember what else i want to say

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....

 

[SummitFever]

...You have to have all your braking done by turn in to maximise grip in the corner...

This is an old wives tale and is totally wrong. You need to think about the circle of forces. Applying that to your riding is the single biggest thing you can do to improve speed.

There is plenty of information from the car guys on how to eek out maximum cornering performance that can be directly applied to offroad riding. I can't remember his name, but one famous F1 driver described his single biggest cornering improvement as figuring out how to "gradually let off the brakes" through the corner.

It's all about keeping the sum total of all of the forces acting on the bike within the traction envelope of your tyres. Braking is a force and cornering is a force. Easing off the brakes as the cornering force increases keeps the total forces within the traction envelope. You phase out the braking force as the cornering forces ramp up. If you keep them separate, ie don't brake while in the corner then you are not maximising your cornering speed. All braking should be done by the time you hit the apex so all traction is applied to cornering.

 

[pharmaboy]

It's all about keeping the sum total of all of the forces acting on the bike within the traction envelope of your tyres. Braking is a force and cornering is a force. Easing off the brakes as the cornering force increases keeps the total forces within the traction envelope. You phase out the braking force as the cornering forces ramp up. If you keep them separate, ie don't brake while in the corner then you are not maximising your cornering speed. All braking should be done by the time you hit the apex so all traction is applied to cornering.

Surprisingly, what you have written is pretty much exactly my understanding of it, though with more though and eloquence than I can muster. "Turn in" point was the wrong term, because I expect, you can't instantly start the corner at maximal g, but have to work your way into it, thus using the brakes as you get there????

I have got much faster through turns when I realized I had x amount of grip and I could only apportion that grip -

So, is the apex of the turn identified by geometry or maximum g force.? And is it different for a bike than a motor vehicle - given on the bike it's a coasting situation as you can't apply power like they do in motor racing?

 

[willsy01]

I can't remember his name, but one famous F1 driver described his single biggest cornering improvement as figuring out how to "gradually let off the brakes" through the corner.

The gradual release of brake pressure is to compensate for the lower grip levels as a result of decreased downforce. Hence the brakes being their most effective at high speed.

 

[willsy01]

So, is the apex of the turn identified by geometry or maximum g force.? And is it different for a bike than a motor vehicle - given on the bike it's a coasting situation as you can't apply power like they do in motor racing?

It's the geometric centre of the corner.

 

[SummitFever]

...I have got much faster through turns when I realized I had x amount of grip and I could only apportion that grip...

Spot on. That's the way to look at it.

 

[Mr_hANky]

Does this make Sam Hill a physics genius?

 

[geoff_tewierik]

It's the geometric centre of the corner.

So, hitting the apex is good for some corners i.e. flat ones, but not so good in others i.e. bermed ones?

 

[redbruce]

It's the geometric centre of the corner.

No its not. Technically it is the point where you are closest to the inside of the turn when taking the straightest (often referred to the racing) line.

So, is the apex of the turn identified by geometry or maximum g force.? And is it different for a bike than a motor vehicle - given on the bike it's a coasting situation as you can't apply power like they do in motor racing?

That point will vary according to a number of a number of factors that influence the best line for the particular vehicle.

So, hitting the apex is good for some corners i.e. flat ones, but not so good in others i.e. bermed ones?

If you riding the fastest line through a corner there will always be an apex, even in a bermed corner.

 

[scblack]

No its not. Technically it is the point where you are closest to the inside of the turn when taking the straightest (often referred to the racing) line.

Yes, there can also be more than one apex depending on the corner.

 

[redbruce]

Yes, there can also be more than one apex depending on the corner.

Correct. Eg southern loop at Phillip Island (and a particularly scary corner on a motorbike because of the combination of speed that can be carried if you get it right and the sudden loss of track if you get it wrong = consequences).

 

[driftking]

This is an old wives tale and is totally wrong. You need to think about the circle of forces. Applying that to your riding is the single biggest thing you can do to improve speed.

There is plenty of information from the car guys on how to eek out maximum cornering performance that can be directly applied to offroad riding. I can't remember his name, but one famous F1 driver described his single biggest cornering improvement as figuring out how to "gradually let off the brakes" through the corner.

It's all about keeping the sum total of all of the forces acting on the bike within the traction envelope of your tyres. Braking is a force and cornering is a force. Easing off the brakes as the cornering force increases keeps the total forces within the traction envelope. You phase out the braking force as the cornering forces ramp up. If you keep them separate, ie don't brake while in the corner then you are not maximising your cornering speed. All braking should be done by the time you hit the apex so all traction is applied to cornering.

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.

 

[SummitFever]

... as the tires have one job and one job only which is to grip not to slow down as well...

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.