Why is lifting a fast travelling bike harder?

In summary, on a bike, lifting yourself up is more difficult when traveling at high speeds due to the drag of the wind.
  • #1
pivoxa15
2,255
1
If you ever rode a bike, you might notice that trying to lift yourself up (while sitting on the saddle and riding) when traveling at high speeds is more difficult than when you are traveling at low speeds. Why is this?

I think that the reason is more to do with the fact that when you lift your bike up while riding, you are pulling backwards somewhat and so naturally when you are traveling fast, you are pulling against a lot of force so you tend to spread more of your energy on pushing the bike back rather than upwards. The net result is that you don't lift the bike up as much. Do people agree?
 
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  • #2
I think so, when you do a "wheelie" (I presume this is what you're talking about) it isn't a straight vertical lift and you would have to go against an amount of force. Even if you DID do a perfect vertical lift (like in the paragraph below) it would still be "harder" (ie take more force) then when still because of friction and drag.

If you were in a vacuum (ignore ground friction too) and there was a bike traveling along and you were running along side it at the same speed then you tried to lift it straight up it would be just as easy (or hard, force wise) has doing it while sitting still, but the instant air friction is bought into the equation it would become harder.
 
  • #3
Wind resistance seems to be the answer. If you present more of an area to the moving air then you will be subject to more drag, this has the effect of pushing you down.
 
  • #4
I don't understand the original post. Do you mean lifting your upper body, or doing a wheelie, lifting the front end?

Unless your leaned downwards, (not a usual position on a bicycle), the faster you go, the more the wind pushes your upper body upwards (and backwards).

If you're referring to doing a wheelie, then it's a case of how much rear wheel torque you can apply. You'd need to be in a very low gear (slow speed) in order to do this without jerking your body in order to initiate the wheelie.
 
  • #5
I got to say, I'd like the OP's post clarified too. If he means 'popping a wheelie' then I'm with Jeff.

Like in a car, acceleration is highest at low speeds - try burning rubber in a car from 0mph, then try doing it from 40mph - much harder. Burning rubber and popping a wheelie are essentially the same forces applied to a vehicle.
 
  • #6
I think I mean a wheelie where I try to lift the front wheel up while riding. I assume that I am not pedaling but moving when lifting up the bike.

What do you mean by burning rubber in car? You mean setting it on fire?
 
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  • #7
pivoxa15 said:
I think I mean a wheelie where I try to lift the front wheel up while riding. I assume that I am not pedaling but moving when lifting up the bike.
If you're not pedaling, than it gets easier to lift up the front wheel the faster you go, as the wind is pushing against your body, and the most drag occurs above the center of rotation. If you are pedaling, then it's because you can't suppy enough torque at the rear tire to assist with lifting the front end, due to the limit of how fast your legs can supply this torque, or if on a multi-geared bicycle, because you're in a taller gear with a lower torque multiplier.

What do you mean by burning rubber in car? You mean setting it on fire?
Spinning the rear tires, which causes them to smoke.

Sample video, of a car spinning the tires (even at 100mph while turning):

z065g.wmv
 
  • #8
Don't forget about angular momentum of the two bicycle wheels increases with velocity. This is why it is easier to ride with no hands the faster you go.
 
  • #9
civil_dude said:
Don't forget about angular momentum of the two bicycle wheels increases with velocity. This is why it is easier to ride with no hands the faster you go.
The amount of acceleration component required to do a wheelie (the main component is due to body movement for a typical bicycle), is independent of the wheel speed affecting momentum. The rate of acceleration will require the same change in moementum regardless of the initial momentum state.

Riding with no hands works because of steering geometry. The axis of the pivot point intercepts the pavement in front of the contact patch of the front tire. The distance between these points is called trail for a bicycle or motorcyle, and called caster for a car. Angular momentum resists a change in lean angle, and at the front there's some correction factor, but the main effect is to slow down the self recovery reaction as speeds increase.
 

1. Why is it harder to lift a fast travelling bike compared to a slow travelling one?

When a bike is travelling at a high speed, it has more kinetic energy, which is the energy an object has due to its motion. This means that the bike has more resistance to being lifted as you have to overcome this energy in addition to the weight of the bike.

2. Does the weight of the bike affect how hard it is to lift when it is travelling fast?

Yes, the weight of the bike does play a role in how hard it is to lift when it is travelling fast. The heavier the bike is, the more force is needed to overcome its weight and kinetic energy to lift it.

3. Are there any other factors besides weight and speed that make it harder to lift a fast travelling bike?

Yes, air resistance also plays a role in making it harder to lift a fast travelling bike. As the bike moves through the air, it creates a force in the opposite direction, which adds to the resistance of lifting the bike.

4. Is it easier to lift a bike that is travelling on a smooth surface compared to a rough surface?

Yes, it is easier to lift a bike that is travelling on a smooth surface compared to a rough surface. This is because a smooth surface creates less friction, which means there is less resistance to lifting the bike.

5. How does the angle of the bike affect how hard it is to lift when it is travelling fast?

The angle of the bike can also play a role in how hard it is to lift when it is travelling fast. If the bike is at an incline, it will have more gravitational potential energy, which adds to the resistance of lifting it. On the other hand, if the bike is at a decline, it will have less gravitational potential energy and may be easier to lift.

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