# Motorcycle physics - near crash

https://i.chzbgr.com/full/8313751296/h1A9779F0/

Can someone who knows motorcycles, physics, or better yet a combination of the two explain what is happening in this gif? It looks to me like a combination of braking at the wrong time or with the front wheel when he should have been using the rear. What does the rider feel when this happens and what did he do to recover?

As you can probably tell, I’m not a motorcycle rider.

It appears he locked up his rear brake. I don’t know what type of event this was. Locking up the rear brake is a novice error.

After pooping his pants, he made a great recovery.

Is the issue that he locked up the rear brake during a turn? I’ve locked up a rear brake on my bicycle many times and the bike didn’t react like that, but then the weight of the bike versus the weight of the rider is quite different.

On a bicycle you don’t have an engine that keeps spinning the wheel when you let off the brake.

The wobbles at the end are from the bike trying to correct itself (it wants to stand up and go straight) after he lets off the brake. The initial part of that skid (where the back tire slides toward the inside of the curve) is from braking too hard. You could replicate that on a bicycle.

Another novice error is to let up on the gas around a turn. Either touching the break or letting up on the gas will straighten up the bike and make you go over the top.
One of the thing they teach you in a basic motorcycle class is that if you feel like you’re going to fast around a turn, don’t let up, lean into it.

Something else of note…he’s pretty clearly not the first person to do that.

Also, to the OP, while it’s not exactly the case since he recovered quickly enough, he almost started tank slapping. There’s a few ways (that I’ve heard of) to recover from that, but it seems like it’s not always up to the rider.

He’s racing a bike with much more horsepower than most bikes. Part of the technique in racing is sliding both wheels just enough to corner faster than your competition. “Pushing” the front wheel is more risky, but is routinely done. “Pushing” rear is commonplace. He got on the throttle just a little too early, maybe trying to set a fast qualifying lap for a good grid position at the start, and the rear slid too much. More than that would have made him loose the rear and lowside the bike, an easy fall. He felt it going, backed off the throttle and when the rear end “hooked up” again it grabbed and resulted in the hiccup you see. If he had caught it later and it hooked up harder he may have been thrown off the “high side” of the bike and gotten launched in the air, resulting in a much more violent crash.
His reaction tells me he was on a qualifying lap rather than racing. It was one of those “damn that would have been a corker” and he just eased off and will have to accept a previous laps time as qualifying.

And look where ya wanna go.

The rear wheel should follow the front wheel thru the turn. With a little luck, combinded with fast reflexes, the rider was able to stay on top of the bike and regain control.

(I used a trial copy of Camtasia 9 to enlarge the gif and slow it down.)

61 entered the turn with his right knee extended. (They use their knees as “curb feelers” to tell them when they’re reaching the the limit of tire adhesion. He wasn’t even close.) The rear tire slides slightly to the outside of the turn, and no longer follows the line of the front tire. Why did it slide? The rider may have entered the turn to fast and lost traction, or the rider applied too much rear brake, or the rider applied too much power. Regardless, rear tire traction was lost.

While still looking at his intended line, the rider turns the front wheel to the left in order to control the skid. The front wheel is headed in a new direction, the frame flexes, but the rear wheel is slow to respond. It’s offline several degrees to the right of the new direction of travel.

(At much slower speeds, if you were to push a wheel straight down a sidewalk, the tire will travel in a straight line, at least for a while. If you were to turn the wheel 45 deg to the right of the sidewalk before you push it straight, the wheel will fall over to the left.)

Because the rider’s rear wheel is now offline to the motorcycles new direction of travel, the rear wheel attempts to fall over to the left taking the motorcycle and rider with it. Violent centrifugal(?) force tosses the rider into the air.

The rider maintained his grip on the handlebars and manages to turn the front wheel to the right, which causes the motorcycle to fall farther right into the turn, but cancels the centrifugal force. A quick stab with his right leg prevents the bike from falling over completely.

The rider regains his seat but the bike is still thrashing around a bit. A quick touch with his left foot, and reduced speed, allows the rider regain control of his bike.

Woohoo, wadda ride. A few swear words, and maybe clean shorts, and he’ll be ready to take anouther trip around the track.

Actually, that was one of the hardest things for a lot of us. I can’t tell you how many times the instructor told us not to worry, there’s nothing 3 feet in front of us and if we’d just look at him (135 degree turn, with some speed), we’d be much better off.

Even after the entire class, and some time on the road on my own, it still didn’t click until I realized that it’s exactly the same way that you drive, you don’t look right in front of you, you look way out to where you’re going to be. Suddenly, everything about my riding got much better. Just as it did when that concept clicked for me when I started driving.

Mind the gap and **doorhinge **did a good job of explaining it. In motorcycle terms, he was on his way to a high-side crash but saved it (or lucked-out).

Also at the MSF class they warn you to keep your rear wheel locked if you inadvertently overbrake it during a stop. If the locked/skidding rear wheel is way out of line and you release the brake, the wheel will grab traction and try to get back in-line with the front (like in the linked video). How violently this happens depends on how far out of line the rear is and how quickly it regains traction. On dry pavement, it can be instantaneous.

I’ve never done that on one of my scooters. Yet.

he locked up the rear wheel, then released it. what this means- a locked rear wheel in a turn can lead you to a “low side” crash; the bike slides out from under you. If your rear wheel locks in a turn and suddenly regains traction, it’ll pull the bike upright and “high-side” you (throw you off of the bike.) a low side is bad news, a high side is really bad news.

That racer is not braking, he is accelerating. When he feels the bike go, he lets off on the gas, which causes the rear wheel to grip again. Because the bike is not aligned on its axis, it jerks, quite violently in some cases.

:eek: Phew! Glad he didn’t go down.

A lowside is going to scratch up your bike, but it sets your body down on the pavement relatively gently. As long as you’re wearing decent safety gear, you’re probably going to be fine as long as you don’t hit something before you slide to a stop.

A highside is bad news from the outset. Depending on the speed and severity, the kinematics of the bike rotating itself back to vertical can launch the rider 10 feet or more above the pavement, with that initial landing potentially causing severe injury before the slide even begins. Generally speaking, the rider ends up in front of the bike, with the bike tumbling/sliding toward him, presenting the risk of another problematic collision.

Here’s a nice compilation of high-side crashes.. You won’t see any obvious broken bones or blood, but you will cringe.

Locking up the rear wheel is intentional in many forms of motorcycle racing and riding (backing it in). It’s really common in supermoto but they use it in road racing as well. Most people that ride dirt bikes learn this but it is a bit different than road riding.

I don’t know if the rider in the gif was trying to back it in or got on the gas too soon.

Search for “backing it in” and you’ll find plenty of videos on the technique: