What is a rear wheel lift (jackknife accident)?

This phenomenon mainly occurs on sports bikes when you apply the brakes suddenly, such as when a traffic light changes at an intersection or when a person or car suddenly jumps out from a side street.

The front wheel locks, the rear wheel lifts, and if you're unlucky, you could fall along with the bicycle and injure yourself.

On MTBs, "jackknifing" is a technique where you intentionally lift the rear wheel after practicing.

Of course, you practice not to fall, but this kind of technique is not normally done on road bikes or cross bikes.

Therefore, if you actually experience it, you will panic.

If you're going too fast, you might fall along with your bike and have an accident.

Furthermore, once you're in a jackknife state, the rear wheel is airborne, so there's nothing you can do to control it.

You'll only be able to protect your head with your arms when you fall.

Causes of rear wheel lift

The cause of a rear wheel lift is "inertia."

This is due to the "law of inertia" that you learned in junior high school physics, but to put it more simply, "a vehicle cannot stop suddenly."

Even if you've been traveling at 20km or 30km per hour, you can't stop suddenly just by applying the brakes hard.

Also, sports bicycles (road bikes, cross bikes, etc.) are lighter than light-duty bicycles and city cycles.

This makes them more prone to lifting.

Furthermore, sports bikes have a higher center of gravity compared to mini velos.

Mini velos refer to bikes with tire sizes up to 20 inches, so the saddle position is naturally lower.

The center of gravity is in a low position.

Therefore, jackknife accidents are almost unheard of with mini velos.

Road bikes and cross bikes, with 700c wheels equivalent to 27 inches, have higher saddle positions.

The center of gravity is inevitably high, and a high center of gravity makes them more prone to falling.

There's also the issue of braking timing.

The right brake lever generally controls the front brake.

Since most people are right-handed, the right hand reacts faster and grips with more force, so the front brake engages sooner and more firmly compared to the rear brake.

Due to these combined reasons, jackknife accidents occur.

Are the left and right brakes determined by JIS (Japanese Industrial Standards)?

One of the reasons for the rear wheel lifting phenomenon (jackknife accident) was that the right brake is for the front wheel.

So, there's also the idea of simply switching the left and right brakes.

However, JIS (Japanese Industrial Standards) includes the following passage:

1. Brake lever placement. Brake levers are generally placed with the front brake lever on the right side of the handlebar and the rear brake lever on the left side of the handlebar. (JISD 9301)

If you want a bicycle that complies with JIS, you must have the front brake engage when you squeeze the right brake lever, and the rear brake engage when you squeeze the left brake lever.

The established method for brake levers also affects the position of holes in the frame where brake cables pass through.

If you swap the right and left, the cable routing will become quite awkward.

Honestly, I don't really recommend it.

How to prevent rear wheel lift (jackknife accidents)

• Practice braking with the left brake first

The simplest prevention method is to practice applying the left brake (rear wheel) first on a daily basis.

While it often comes down to awareness, in an emergency, your daily habits will dictate your actions.

If you keep your fingers on the left brake lever, ready to brake at any time, and firmly grip the handlebar with your right hand, there will be a one-beat delay in applying the right brake.

If the rear brake engages first, you can avoid a jackknife with a high probability.

• Control your speed

The reason the rear wheel lifts in an emergency is that you're going too fast in the first place.

If you commute to work or school, you're likely to be riding in the city a lot.

If you normally ride at a reasonable speed, the likelihood of a jackknife decreases.

Not only when a light changes at an intersection, but if a bicycle ahead suddenly stops or slows down, emergency braking is unavoidable, so slowing down beforehand can help you avoid it.

I think it's also good to differentiate: ride normally on large roads where you can go faster, but slow down in crowded areas.

• Practice jackknifing

While it might not be suitable for road bikes or cross bikes, you can practice jackknifing using an MTB, for example.

Once you master jackknifing, you'll also develop a feel for how much the rear wheel lifts, and you'll naturally be able to prevent it in advance.

The scariest thing is to panic and become unable to move.

By practicing and becoming friends with the jackknife, your body will react and you'll be able to brace yourself in an emergency.

External writer: Koichi Okuno

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