I Bicycle and tackling inertia at higher speeds

VVS2000

Summary
While going on a bicycle, how can I take a quick turn when there is a obstacle ahead of me. I cannot slow down so while not lowering your speed, and in terms of inertia, torque centripetal force and other relevant quantities which you may choose, how can I take a turn ?

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phinds

Gold Member
How do you think you might do it?

A.T.

While going on a bicycle, how can I take a quick turn when there is a obstacle ahead of me.
Unplanned direction changes with single track vehicles are difficult:

metastable

I would lean forward above the handlebars so the bicycle becomes a freewheeling unicycle, and then stick out a leg to one side while counter-steering by the appropriate magnitude to avoid the obstacle. This simplifies the equations to the forces acting on only one wheel.

phinds

Gold Member
I would lean forward above the handlebars so the bicycle becomes a freewheeling unicycle, and then stick out a leg to one side while counter-steering by the appropriate magnitude to avoid the obstacle. This simplifies the equations to the forces acting on only one wheel.
Wow. I would not have though of that but if you could pull it off (I'm confident I could not) I think you are right that it would be a possible solution.

metastable

That’s how fixed gear cyclists stop all day with no brakes, they “unweight” the rear wheel by leaning forward, lock their legs (the bike has no freewheel mechanism so doesn’t coast), let the rear wheel fall back down, initiating a controlled skid which turns the rear tire into a brakepad. These skids can be arbitrarily long depending how far they lean forward.

berkeman

Mentor
Summary: While going on a bicycle, how can I take a quick turn when there is a obstacle ahead of me.
In addition to the other tips, keep in mind that different bicycle geometries will be able to maneuver more quickly than others. The "rake" and "trail" of the front forks will have a big impact on how quickly the bicycle can steer.

When I bought a mountain bike a few years ago, I went to a bicycle shop that allowed test rides in the parking lot, and had very knowledgable people there to help me find the right MTB for me and my planned riding. Since I have a background in motocross, I wanted a bike that I could jump with and was quick on tight single-track trails. I test rode 3-4 bikes, and picked the one that had the quickest steering and lightest front end (for lofting it over obstacles). It was also a bit unstable, but that is the tradeoff you make when you want quick steering.
Summary: I cannot slow down
That is not a good thought to have. You can pretty much *always* slow down some, even while you are turning hard. It takes practice (especially on a street motorcycle), but it should become a muscle-memory reaction to get on the brakes hard (with good braking balance front-to-back) while you are taking evasive maneuvers. At the very least, if you end up hitting the thing you are trying to avoid, the energy of the impact decreases as the square of your velocity, so hitting it at half speed will hurt you a lot less than hitting it at full speed.

Practice your emergency braking and evasive maneuvers before you need them, and ride safe and smart!

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sophiecentaur

Gold Member
Practice your emergency braking and evasive maneuvers before you need them, and ride safe and smart!
There is always the advice that you should never be going so fast that you can't avoid hitting 'unexpected items'. That's impossible to do without going at walking pace - but it is possible to cycle (and drive) with that advice in mind, taking blind corners slowly *slower) etc. etc.. Unfortunately, the majority of fast cyclists tend to be young men who have testosterone-enhanced force fields around their cycles and also, immortality. (Times ten for motorcycles.)

VVS2000

How do you think you might do it?
I mean I was actually looking for a mathematical point of view, like cornering your way around and therefore making an angle with the ground and resolving forces... like how you do it in a banked road situation

metastable

I think unless you plan on modeling a metal wheel on a metal road, first you’ll have to define the 3D microstructure of the road surface and the 3D structure of the tire surface and how it deforms under dynamic loads. Next you’ll have to model the human body and how movements of various muscle groups translate into forces acting on the wheel, while factoring that the wheel is also acting as gyroscope.

metastable

Perhaps you’ll have an easier time modeling a tricycle that isn’t allowed to tilt taking evasive action, as it isn’t inherently unstable, and therefore doesn’t depend on the human for dynamic balancing. Then you make it a motorized tricycle with automated steering. Assuming no wheel slip, perhaps the 3 wheels will follow predictable tracks for a given steering input.

metastable

For reference:

The Schwalbe Fixed Gear Longest Skid Competition London

& the winner's posture:

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cjl

That’s how fixed gear cyclists stop all day with no brakes, they “unweight” the rear wheel by leaning forward, lock their legs (the bike has no freewheel mechanism so doesn’t coast), let the rear wheel fall back down, initiating a controlled skid which turns the rear tire into a brakepad. These skids can be arbitrarily long depending how far they lean forward.
That's really not what you want to do though if you want to stop quickly. Fixed gear cyclists do this to show off. If you want to stop quickly with no front brakes, you need to shift weight as far back as possible so you have as much weight on the braking wheel as you can, and then you want to hold the brakes at just the right level so the wheel almost starts skidding (but doesn't actually lock up - grip levels are higher for wheels that are rolling).

If you do have front brakes, you still want to lean back, but you want to get on the front brakes hard. Ideally, if you have enough grip, you want to be on the front brakes right to the limit of picking up the rear tire. If you don't have enough grip for that, you want both front and rear to be just on the limit of skidding, though as I mentioned before, you don't want to actually skid.

As for the OP's question? The only way to turn at speed on a bicycle is countersteering, so you'll have to do that. I second the statement that you want to be on the brakes as well though.

rcgldr

Homework Helper
If the obstacle only exists at the surface of the road, such as a pot hole, you can positive steer around it quickly, but this is an uncoordinated turn, and the bike will be leaning after passing the pothole, requiring correction to straighten it back up. For example steer right so that the wheels pass by the right side of the pothole, which results in the bike leaning left, which you correct after passing the pothole.

If the obstacle requires turning the entire bike (as opposed to just the contact patches of the tires), then the harder you counter-steer, the faster the rate of change of lean and the sooner you can start turning. Note while counter-steering to initiate a lean, the center of mass of the bike is turning the "wrong way". I don't know if there is an ideal amount of counter-steering torque for obstacle avoidance, versus just using a lot of counter-steering torque.

"Bicycle and tackling inertia at higher speeds"

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