Motorcycles and physics

bike would rise. law of conservation of angular momentum.

(saw it on a recent Trek rerun. forgot the name of the episode, but in it Kirks gets the girl)

 

Actually, the front end would drop (momentum blah blah blah)
Braking in the air is a trick we always use in motocross.

 

Depends on where the front brake caliper is positioned, if my force diagram is right....

 

It depends of nothing, the front end drops. I still have the scar to prove it!

The front wheels turns in such a way that when you brake, the momentum is transfered to the whole bike ...

 

A rotating wheel has an angular momentum. If you apply the brakes, regardless of it you 'feather' them, a transfer of angular momentum must occur. The answer most certainly is not "doesn't change", Art . The bikes wheel will pitch downwards. I did an experiement too, and it confirms this.

Now, I wait to be told I am wrong, and all about the assumptions I made.

 

We did it too ridding dirt

 

Drops. You lose the gyro scope effect

 

Don't guys who jumb bikes, (like Robbie Kneivel) hold the front brake while in the air to keep the front of the bike "down?"

 

That was YOU on the bike and your wife was in the 360...Correct?

 

I'm no help I can't do wheelies.

 

braking a spinning front wheels will pull down the front end....did it on dirt bikes as a kid, same on my Ninja.....never could get the Harley's off the ground!!!!!!!

 

The front end would drop. On dirtbikes when jumping you never grab the front brake because the motorcycle would dive and there is no way to bring it back up because once the wheel stops spinning in the air it doesn't start spinning again. That is why motocrossers hit the rear brake to bring the front end down, they can get that wheel spinning again.

 

I guess I had the front and rear mixed up...knew they hit one of the brakes though.

 

definately drops. Now try hitting your back brakes hard during a wheelie and report back.

 

Art,

I just took my bike out to try it out to get an answer, and due to it being 14 degrees out and ice on the roads, i crashed.

I was just wondering where to send the repair bill, or would you prefer just to send me your duc as payment? Your choice

 

Bingo! According to most modern sportbikes, the caliper is placed along the side of the wheel. So by applying the brake, the linear force would change making the bike "dip" first but then compensate and rise forcing the wheelie to "increase". So when applying the brake, the bike will "dip" causing the wheelie to decrease.

However there are several factors to this problem. Also, isn't it easier to figure out by doing the linear force diagram instead of using angular momentum? (the latter requiring the ang. momentum of the whole system etc.)

 

I agree with Art, Front end dips regardless of where the brake is located. (front of forks, behind forks ) or inside(drum) all the same effect.

 

Not much different than hitting the front brake when on the ground...the bike will dive front end down due to the momentum of the bike continuing in the forward rotational direction.

 

Now the hard part is getting the bike up in a wheelie in the first place!!!!!!!!!!
Best technique is to hit front brake to compress the front fork shocks then gun the throttle, weight of rider backward, lift handle bars and WHEELIE!!!!!!!!

 

Your out of your mind if you stab the brake in order to lift the front wheel, and makes me think you have no business being on a bike at all. All you need is throttle and maybe a little clutch. Even on drastically underpowered bikes you can lift the front end by just leaning back.

And yes stoping either wheel from spinning while in the air will most definitely make the bike rotate forward. Now generally you don't grab the front cause 1, it's sketchy when you land, and 2nd there is no way to make it start spinning should you need to.

You would really be amazed how much effect it has on the bike when you have a 15+lb wheel spinning at 40mph and stop it instantly.

MX is an excellent teacher of physics, when you get it wrong, it hurts and usually a lot.

 

???

Hard part? The vast majority of the wheelies I've done have been unintentional. If the bike won't wheelie under it's own power, than it's too dang small.

And yes, under braking the front end will drop, makes no difference at all where the caliper is located, etc etc...