Why is lifting a fast travelling bike harder?

[pivoxa15]

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?

 

[Gelsamel Epsilon]

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.

 

[AntiMagicMan]

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.

 

[rcgldr]

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.

 

[DaveC426913]

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.

 

[pivoxa15]

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?

 

[rcgldr]

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

 

[civil_dude]

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.

 

[rcgldr]

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.