By John Yoder
I’ve enjoyed watching the cyclists in the Tour de France the past three weeks. Their skill on descents in the mountains at 60 mph, their power in climbing long mountain passes, their unreal (for me) speed in time trials and sprint finishes and their bicycle handling skills at 30 mph while surrounded by other riders in the peloton are phenomenal. Most of the time, I see scant connection between their riding and mine, but there is one cycling principle the tour always reminds me of: when overlapped wheels touch, a fall is almost inevitable.
By overlapping wheels I mean when the front wheel of one rider is close to and beside the back wheel of another rider. Despite their skill, riders in the tour do touch overlapped wheels. In 2015, to pick one notable example, there was a major crash in stage six when the front wheel of tour leader, Tony Martin, touched the wheel of the bicycle ahead of him, causing him to crash and break his collarbone, ultimately forcing him to retire from the race. The same kind of crash can happen on our group rides as well.
But why does touching a front wheel to a back wheel cause a fall? I remember being puzzled by this type of accident when it happened to me for the first time, though I was not the one who fell. I was on a group ride when suddenly I heard a loud thud behind me. I looked back to see a rider on the ground. I stopped to see if he was alright and to find out what had caused him to fall. He said that his front wheel had touched my back wheel, and he went down immediately.
Back and front wheels can touch in either of two ways. Let’s say I’m riding in a group, and I’m to the right and slight behind someone else so that my front wheel overlaps with their back wheel. With my front wheel beside his back wheel, I’m susceptible to falling: 1) if the other rider suddenly moves right, and their back wheel touches my front wheel; or 2) if I inadvertently turn my front wheel left into their back wheel.
Here’s my understanding of the physics that causes the fall. If my front wheel is to the right of another cyclists back wheel and that cyclist moves right, his rear wheel will push my front wheel to the right, as if I’m starting to turn right. But the bicycle itself continues going forward, resulting is an immediate fall, because the wheel is turned to the right without a corresponding lean to the right by the bicycle. In other words, turning the front wheel of a bicycle without leaning in the direction of the turn will result in a fall. Turning a bicycle is as much about leaning as it is turning the wheel and it’s something we do instinctively.
Test the validity of this dynamic sometime by holding the handlebars and walking a bike in a straight line, then turning the wheel right or left without leaning the bike in the same direction. The bike will immediately become unbalanced and begin to fall in the opposite direction from the way you turned the handlebars. It is a perfect illustration of how importance leaning is when making a turn on a bike.
In the second case where I turn my front wheel into the back wheel in front of me, I will likely fall from over reacting to the wheels touching. Instinctively, when wheels touch, I want to separate them and will jerk the handlebar away from the other rider’s back wheel. But, again that turn of the wheel without the bicycle leaning in the same direction (while the bicycle continues going forward) will cause a fall.
What should you do when your front wheel touches a back wheel in front of you? Is there a way to avoid falling? The answer is yes, but it’s not easy or instinctive. If you touch wheels (assuming you are the rider behind the other), the best thing you can do is to immediately turn your front wheel into the other rider’s back wheel – a counter intuitive move — and stop pedaling until the bike in front pulls away. In other words, avoid overreacting by turning your front wheel away from the back wheel you touched. Do that, and you will fall.
I wish I could say that I’ve tested this last point – steering into the back wheel you touch — but I’ve never personally touched wheels. Fred Matheny gives you more details though, along with a drill to practice on your mountain bike.
Stan Purdum says
Regarding bikes falling the opposite direction from a suddenly turned front wheel, somebody has come up with a “Sherlock Holmes” type mystery involving that very phenomenon. It appeared as a “puzzler” on the CarTalk radio show a few years ago. See it here: https://cartalk.com/content/paging-sherlock-holmes
Stan Purdum says
I’ve learned that the URL doesn’t work. You can get the story by googling “Paging Sherlock Holmes!” and then clicking on the story link.
The front wheel is a gyroscope and when it is deflected to the right, the resultant (gyro) force will rotate the bike to the left. You can feel this effect quite easily by simply removing the wheel from the front fork and hold the wheel (by the axle nuts) in your hands. Spin the wheel and then try rotate the axle like it would rotate when you initiate a turn. You will feel the resultant gyro reaction force in your hands/arms.
Your description of the physics involved is certainly the most common cause of the rear rider falling from overlapping wheels. In the case of my fall last year, it was a bit different. I accelerated to close the gap on a rider in front of me when he suddenly braked hard when he saw a mail carrier on the opposite side of the road pulling away from a mailbox. He thought the carrier was going to pull into our side of the road. I did not. I was not as quick to brake and ran up on his non-drive side and got trapped against his skewer to the point that my front wheel quit rotating. I did a classic end-over-end and landed on my right shoulder and back. I was fortunate to walk away with only a sore wrist from holding onto the shifters, no road rash or even scuffed clothing. Our ride mechanic straightened a shifter that twisted from impact and in a couple of minutes we resumed the ride. The rear skewer lever should be turned and locked in such a way that it doesn’t trap a rear rider’s wheel should he run up on the left side of the front rider’s dropout. In my case, I wasn’t trapped by a skewer lever. There was just too much difference in our speeds for me to get off his wheel.
Brian Nystrom says
I can vouch for the fact that turning into the contact can prevent a fall, but if you don’t react quickly enough, you’ll still hit the deck. The one time I really needed this technique, I was too slow to react and the end result was a broken hip. Be careful out there…