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« Last post by Andre Jute on March 29, 2025, 04:53:05 AM »
BICYCLE LEVER ARMS FOR FACEPLANTS AND NOGGIN KNOCKS
There are all kinds of lever arms and associated pivot points on bicycles. Almost everything you operate or change on a bicycle has an associated pivot point, either fixed or wandering around dynamically. Many of the pivots and associated lengths of lever arms are crucially important to your safety in operating the bike, and almost all are taken into account when you judge a bike as you ride it, even if you don't know their names and positions.
You can search for lever arms implied in discussions about the steering geometry led by Dan over the years, and discussion of the centre of gravity (CoG) of the bike plus luggage and rider, and the centre of aerodynamic pressure (CoP), on which I've written over the years, including just recently as a reply to a question in another thread, as well as more general discussion passim of tyres and other forms of suspension.
Here are a few more important pivot points, less obvious but all reasons for having a long wheelbase on a touring or utility bike, where stability and security of the rider should be the first consideration. Also good reasons for having larger wheels rather than smaller.
THE FRICTION TORQUE ARM BEHIND THE FACEPLANT
Almost all regular cyclists have at one time or another made a sudden hard stop and flown over the handlebars towards the sudden stop on the tarmac which we call a faceplant. It's a reason not to set up your bike's brakes to be ultra sharp, or even to prefer a certain class of brake over all others, as I prefer the weakest of the Magura rim hydraulic brakes over smaller discs with greater clamping force. (The Magura hydraulic rim brake is just a disc brake using the rim of the wheel as its disc, but the larger circumference makes it more progressive.)
The faceplant (and its show-off lesser relative the stoppie) has three lever arms coming sequentially into play.
The first is the arm between the hub and the contact patch of the tyre, which with the brake clamped has suddenly become a hard point so that whatever is behind it on the bike angles upwards.
The second is the lever arm between the hub and the centre of gravity. Here we see good reasons for taller wheels rather than lower, because generally the luggage on a touring or utility bike is at roughly the same height regardless of wheel size, because on purpose-designed bikes (not repurposed mountain bikes, for instance) the bottom bracket is at approximately the same height regardless of wheel size and thus in big-wheel bikes the lever arm from the hub to the CoG is more advantageously angled to resist the faceplant upset; the greater angle also gives you a fraction of a second longer to react and let the brake go.
The third is the lever arm between the centre of gravity of everything on the bike and the stiction-point between tyre and road. The mass of the bike now gains a pendulum effect, no longer around the hub but around the stiction patch.
The pendulum effect on its long arm probably reaches greater importance than the hub-stiction point before the hub-stiction-point vector reaches the halfway mark at 45 degrees. From perhaps a rotation of 15 degrees it is already too late to let go of the brake and recover the bike -- the whole bike is now joining the effort to throw you off. You have perhaps 0.15 seconds to respond, which is impossible. (In my youth I was a participant in several bloodsports -- once after a motor racing incident I caught a glass full of water without spilling a drop when a resuscitation nurse knocked it off the cabinet beside the patient trolley as the trauma specialist pulled the sheet over my face after declaring me dead. Even late in life my reaction time is still 0.45 seconds. Still not enough to save you from a faceplant once it is initiated. The same incidentally applies to washouts on a bike: once the front wheel starts sliding out from under you, you are gone because there is not enough time to react.)
This leads to an argument for a bike with a long wheelbase and a low centre of gravity in its load design.
Notice that, at least in theory, relaxed (touring) or steep (racing) steering geometry creates another, if short, lever arm that may aid or hinder the other forces. I have some doubts about measuring this, as it was proven conclusively as long ago as the 1950s that turning the steering tube around 180 degrees in the head tube, thus reversing the elements of the steering geometry, does not reduce the bike's innate stability! (I don't say "gyroscopic stability" -- it's tempting but wrong as nobody has proved it, as far as I know.) But the theory is sound -- it's just the practice that is baffling. Fortunately, we're talking here about a minor contribution to a total effect that is in absolutely no empirical doubt.
THE TORQUE ARM THAT THROWS YOU OFF BACKWARDS ONTO YOUR HEAD
Imagine you're riding on something with an ultra-short wheelbase like a Surly Karate Monkey, with your bottom well over the rear wheel. On a very steep hill you stand up on the pedals, and suddenly the bike casts you off, backwards.
It is, in the light of our discussion of the faceplant above, pretty obvious what has happened here.
This too is a torque effect, a rotation around a hub and then around a friction point, caused by the application of power through the pedals. The difference is that here your feet on the pedals os driving the whole patootie, including you, backwards around the wheel because the tyre isn't find enough grip to accommodate all the power you're putting into it.
We can identify the lever arms, which are from the pedal axle to the rear wheel axle, actually hard rather than virtual as it is a chain under high tension, and the virtual swing arm from the centre of gravity to the rear hub.
It is important for us to recognize that the steep hill adds to the angle of the lever arms. The road thus makes the pendulum effect of the loaded bike's centre of gravity greater and more sudden in its effect because it has less distance to travel before reaching the tipping point, which is likely here to be reached well before the CoG is vertical above the hub. In this regard, remember that Gravity itself, the elephant in the corner of this discussion, works perpendicular to the core of the Earth, regardless of local undulations.
Once more, we have a good argument for larger wheels rather than smaller, and of course for a low centre of gravity being designed into the bike from the start.
Finally, a long wheelbase also makes it easier to arrange for the centre of aerodynamic pressure to fall well behind the centre of gravity, which is a major help in designing a bike whose stability under all conditions you can trust. This too is a lever arm, as you will discover if the centre of aerodynamic pressure ever gets in front of the centre of gravity, when in a moment your bike will become unstable.
Copyright © Andre Jute 2025 Free for reproduction in full and with this notice intact on non-profit sites. Others should ask permission.
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