By Lennard Zinn
I was struck when reading this question about what a strange world some of us cyclists live in, where bodily encounters with solid objects and even riding over fallen riders are commonplace. It got me thinking in the wee hours not only about bicycle geometry, which was the original gist of the question, but also about bruising, inflammation, Tyler Hamilton’s lower back and heart attacks! Now, either these are more closely connected than we may think, or maybe I should do more sleeping and less pondering in the wee hours, as connections between divergent thoughts that seem so profound long before the sun come up sometimes look a bit far-fetched in the light of day.
I was just refreshing my understanding of fork rake, fork trail and head-tube angles when I saw something interesting in your book that reminds me of that typical racing crash where someone has fallen and someone else is riding over them. Considering the steep drops and descents I can handle on my MTB, I was a little disappointed that I lost control and went down the last time someone fell in front of me in a criterium and I rode over them.
Figure 40.5 in Block 40 (which I found via the VeloNews.com site) of Zinn’s Cycling Primer shows a bike with its rear wheel high in the air to illustrate the condition of negative fork trail by artificially creating an incredibly steep head angle coupled with a standard amount of fork rake. It shows the front wheel, as the bike is leaned over, turning away from the direction of the lean of the bike, the opposite of how a front wheel normally turns in a lean on a stable bike. Instead of turning to get the tire contact patch back under the rider’s center of mass to stabilize the bike, the tire contact patch in this extreme-fork-trail mock-up instead moves rapidly further out from under the rider’s center of mass and encourages the rider to fall down.
Figure 40.5 implies that steering geometry becomes extremely unstable in that awkward moment when the rear wheel is riding up and over a fallen rider. I remember many crash photos where the front wheel is cocked to one side away from the bike’s lean, as in your figure 40.5, while the rear wheel is still up high. It’s been my most common type of crash — get the front wheel up, get part way over . . . and still fall.
I guess the way we handle this on the MTB is to shift our weight up and back, but this is hard to do from the drops with no warning, no? Is there some pre-programming in the brain to help deal with this next time?
While in polite society we would normally not discuss so dispassionately how to ride over prone riders without falling ourselves, it’s a really great question. Indeed, pushing your butt off the back of the saddle would definitely help in that instance and is worth practicing while in the drops. Maybe preparing for riding over fallen riders is another reason for the current surge in popularity of cyclo-cross!
Dropping down over a prone rider is, however, different in two very significant ways from your MTB example, and hence harder to pull off, even if you can manage to get your weight back over the rear wheel. First, the shallower head angle of the mountain bike makes the effective fork trail greater (or at least less negative) when placing your front onto a horizontal surface while the bike is pitched downward coming over a big drop. This grants the bike more self-stability to assist you in maintaining your balance. Secondly, usually when you come over a big drop on a mountain bike, your front wheel is landing on a downward slope, not on a horizontal road. This means that the effective head angle that you would want to measure to determine the fork trail would be the angle of the steering axis relative to the slope the front wheel is rolling on, not relative to the horizontal. Even though gravity is yanking down on you perpendicular to the horizontal, it is the rolling of the front wheel on a surface that is the critical component of the fork trail, and hence of the self-stability of the bike.
When the front wheel is rolling down a slope, the effective head angle of the pitched bike is shallower by the angle of the slope relative to the horizontal than it would be if the bike were pitched up at the same angle relative to horizontal and the front wheel were instead rolling on a horizontal surface. As you can see from the formula for fork trail in Block 40 of the Primer, a reduced head angle results in an increased fork trail. And being the lever that rights the bike, greater fork trail means greater self-stability of the bike.
So, here’s the recommended method:
1. As soon as you see somebody falling in front of you, use the self-stability of the bike to generate a sharp lean and hence a sharp turn to avoid the fallen rider by applying a strong counter-steering force on your handlebar. You do this by pushing out on the hand that is toward the falling rider (and better yet by simultaneously pulling back on the hand that is away from the falling rider). This momentarily turns your handlebar toward the falling rider and forces the bike to lean away, and hence to turn away, from said rider. The self-stability of the bike granted by sufficient fork trail works to bring the forces in balance to keep you upright when you push the contact patch of the front tire out from under your center of mass. It works way better than trying to steer away from the fallen rider, which I promise you will have a low rate of success. I am convinced that avoiding running into fallen riders will be a more successful method for keeping you from falling than will be improving your abilities at riding over prone bodies and bikes.
2. Failing that, practice getting your butt rapidly back off the saddle as you shift your weight back to lift your front wheel up onto the fallen rider’s body. Bring your butt down and back as quickly as you can as you go over the top, and push straight outward on your handlebars as your front wheel comes back down toward the pavement. This will reduce the amount of weight on the front wheel, which is very important, because you know from Figure 40.5 that your front wheel will be trying to turn the wrong way when it hits. If you can reduce the amount of weight on it, you have a better chance of getting it to go the direction you want and hence staying upright.
This concludes the answer to the question and commences the pondering down divergent thought tracks. Since this column has reached its length limit, you’ll have to wait until next week to find out what it has to do with bruising, inflammation, Tyler Hamilton’s lower back and heart attacks. Even I don’t know where this is heading yet.
VeloNews technical writer Lennard Zinn is a frame builder (www.zinncycles.com),a former U.S. national team rider and author of numerous books on bikesand bike maintenance including the pair of successful maintenance guides”Zinnand the Art of Mountain Bike Maintenance” and “Zinnand the Art of Road Bike Maintenance” as well as “Zinn’sCycling Primer: Maintenance Tips and Skill Building for Cyclists.”
Zinn’s VeloNews.com column is devoted to addressing readers’ technicalquestions about bikes, their care and feeding and how we as riders canuse them as comfortably and efficiently as possible. Readers can send brieftechnical questions directly to Zinn (firstname.lastname@example.org)Zinn’s column appears each Tuesday here on VeloNews.com.