Why do riders hang off a motorbike when cornering?

[ihateblackbox]

Hi,

I was wondering why some riders lean their body off a bike into the corner when cornering? (a more extreme case being knee almost touching the floor for example)

The general explanation I get from riders more or less is this: When you lean your body off a bike, the bike has to lean less to have the same cornering effect. In other words, if you lean your body off your bike without changing the lean angle of the bike, the bike will corner more. It has something to do with contact surface area being more.

I don't really understand how this works.

1) How does the surface area of the tyre in contact with the road change if all you're doing is redistributing weight around the bike?

2) Assuming the contact patch does increase, this would increase the grip, that I can see. But how does it allow the bike to change its direction more? (with the same lean angle)

 

[Nessdude14]

Leaning your body won't increase the area of the tire contacting the road. The riders simply lean this way because it's the most comfortable position, and at high speeds it's necessary just to stay on the bike. Think about the apparent force that you feel when making a turn in a car: you're pushed towards the outside of the turn you're making, and you have to make an effort to hold yourself in place. Try to imagine if your car seat was able to tilt in the direction of your turn; If it tilted just right, you wouldn't need to make an effort to stay in your seat, you would just be pushed comfortably into the bottom of your seat, and you would be leaning off the edge of your seat in the direction of your turn.

When a motorcycle rider makes a turn, they tilt in the direction of their turn to get the same "comfortable" feeling of being pushed into their seat, rather than having to hold on tight to keep themselves from falling off. Of course, this leaning along with the bike allows them to tilt the bike over even further without falling off, allowing them to make tighter and faster turns.

 

[jbriggs444]

I don't buy this explanation -- either part.

It would seem obvious that a rider sitting upright in the seat is going to naturally be in the comfortable point where the sum of gravity plus centrifugal force is pressing them directly down into the seat.

As for your other point, while leaning out of the turn in order to make the bike steer more aggressively works for riding a bicycle "no-hands", your goal in a high speed motorcycle turn is to keep the bike more nearly upright, not more nearly flat on the pavement.

Mind you, I'm not a motorcyclist.

 

[berkeman]

We had a long thread about this a while back -- I'll see if I can find it with a search.

The main reason that racers hang off is to increase the ground clearance of the bike. If you look at pictures of mid-corner racers, you will see that the bike is leaned over pretty much as far as it will go before dragging the footpeg and other parts.

The other reason has to do with "body steering", but that's a whole other subject for discussion.

 

[berkeman]

Here's the old thread. It centers mostly on how motorcycles are steered:

https://www.physicsforums.com/showthread.php?t=536228

(EDIT -- BTW, the user JeffKoch was banned later for things unrelated to that thread.)

 

[bahamagreen]

May be a secondary effect, but leaning out presents more surface to the wind... it allows them to slow down during the turn without having to brake... and if the conditions are right they can actually increase power at the same time to balance the increased drag of "air braking" so as to maintain speed and alter the slip angle vectors to their advantage in the turn.

It's complicated, but they are not just turning the handle bars and leaning the bike in a turn at those speeds; they are also pulling the bike over while leaning out into the turn, altering their effective centers of effort against the bike, and air braking to add rotational torque to those centers of effort in the horizontal plane of the bike...

 

[CWatters]

"Hanging off" allows the bike to be more upright. In addition to increasing ground clearance Google found a comment that says:

The suspension works better the more vertical the bike. Part of what the suspension does is keep the tires in contact with the road and it becomes less effective at that when the bike is leaned over to a great extent

Even race tracks have bumps.

 

[CWatters]

Another thought occurs to me...

Hanging off might have an effect on the lateral moment of inertia... With the rider sat on the seat the mass of the bike and rider are close together. With the rider hanging off the mass of bike and rider are further apart so the moment of inertia is greater. When hanging out more mass is above the contact point (even if the CoG of the combination is still in same place). If you hit a bump on the corner the force acts vertically where the wheels contact the road. This would tend to try and rotate the bike about it's center of gravity towards the horizontal. Increasing the lateral moment of inertia might help stop that.