Thursday, October 3, 2013

Why you cannot corner a mountain bike like a sport bike

Two days ago, I think I finally figured out why you can't corner a mountain bike like you would corner a sport bike.  I then drew it out to make sure that it made sense.  Afterwards, I decided I should share it with my biking friends.  The drawing was somewhat too big to put as a picture on facebook, so I made this, pretty much with the sole purpose of being able to spell this out, for anyone who is interested.  I will probably never post on here again.  

Some short background:  A few months ago I smashed my head into a berm really hard at Crested Butte resulting in a mild concussion and some rather hilarious pictures.  The crash was familiar... I go around a corner, my wheels wash out, handle bar drives into the ground, my bars cross and I slam into the dirt.  It was so familiar, and had happened so often, that I decided I was absolutely doing something wrong.  At the next race, a friend explained that I should corner with my bike, and not with my body (Fabian Barel can explain this much better than I can).  I did this, and not only did I fall much less, but my cornering improved immensely.  However, intuitively I struggled sometime trying to figure out why this made sense.  

Well I think I figured it out, and here it is:  note that I am not a physics expert, any physics experts who read this are welcome to correct any inaccuracies, force vectors are not drawn to scale… 

We’ll start off with slab climbing.  It’s related, I promise.  Think of it as somewhat of a simple case.  Here is a diagram of someone trying to slab climb incorrectly.  Note that they are leaned in against the wall, this is what you want to do, because it’s less scary, but it is wrong and will cause you to fall.  Also note, that the point of contact with the rock (the feet) is not at the same point as the center of gravity (say somewhere around the body).  The climber’s hands touch the wall as well, but let’s just assume basically all the weight and therefore friction rests on the feet. 

Now, let’s think of some forces applied upon this climber.  Gravity, of course, which acts essentially on the center of mass.  The Normal force, which opposes gravity (think pressure on your feet when you stand), and Friction, which holds the climber to the wall.  These next two forces, act on the feet.  Both forces act vertically but in opposite directions.  

Assuming  the there is sufficient friction, these forces will be equal.  However they act on different places.  The combination of the friction and normal forces push the feet up, while gravity pushes the body down.  This results in a kind of rotational force I think.  

This rotation will pull the climber’s feet away from the rock, thereby reducing friction and causing him to fall. Make sense so far?

If we think of a more experienced climber, he will remain vertical on the slab.  The friction/normal force combination work directly against gravity and the climber can then balance. 

This, is quite similar to what happens cornering.  First, we’ll take a mountain biker cornering properly.  The effect of keeping your body upright and leaning your bike, is to keep your center of gravity above where your tire hits the ground as much as possible.  Here we have centrifugal force working with gravity to push you down and out of the corner, while the normal force and friction work to push you up and in.  If your center of gravity is not too far horizontally from your tires, you should be in good shape. 

Now, let’s say you’re cornering like I was in Crested Butte; leaning full on into the corner like you’re on a motorcycle.  In this case, your center of gravity is further out horizontally.  Much like in the case of the unskilled slab climber, the forces acting on your tires and on your body do not match up.  Once again a rotational force is created that pulls your tires out of the corner and slams you most ungraciously down into the earth.  

Now you may be wondering.  But Rick, people on sport bikes lean so far down into corners with their bodies that they need knee pads?  Why can’t I do that with my mountain bike?  Well, because you are not on a sport bike.  Fat slick tires on smooth asphalt and much smaller knobby tires on dirt are two very different realities.  Mainly, you’re dealing with less friction.  So, let’s think about forces acting on a sport bike.  

Now I’m take my guessing to another level here, but it seems that with a huge increase in friction, that if the sport bike is creating any kind of rotational force, it is actually going the other way, which if anything might force the tires into the pavement harder and allow for better cornering?  Either way, there’s enough friction there, that the situation is entirely different. 

And that is my take on why you cannot corner a mountain bike like a sport bike. 
sport bike.  

EDIT:  Some friends pointed out that this is also probably an aspect (or entirely a result of) the fact that this method of cornering employs the cornering knobs of mountain bike tires more effectively.   I had thought of this, but forgot to mention it.  I feel like both explanations likely come into play.  

I suppose the next step is to find a bike with slick tires and a particularly loose corner. 


  1. I was thinking about all of this myself and I got the idea: centrifugal force on heavy and fast machine is much BIGGER than on light and slow vehicle. With heavy/ fast You can lean A LOT into the corner. I think that friction has not so much to do with it. I was comparing myself scooter 160 kg vs motorbike 260 kg on roundabout. On scooter (weight 160 kg) I could not lean into the corner, because I felt unstable. However with the heavy bike (weight 260 kg) I felt very safe when leaning into the corner. There is also gyroscopic effect which is hard to obtain with bicycle. Wheel is trying to keep itself straight, but this gyroscopic force depends on revolutions of the wheel per minute. For example try to rotate angle grinder with slow ( say 5000 rev/ min) and fast speed (say 12 000 rev/min). You will feel the difference of gyroscopic effect.

  2. Macief,
    on motor bikes, mostly all the weight comes from te bike : rider position is much less important for the balance.

    motor cycles tires are also quite special : they are very large wit a very round section. Wen leaning the motor bike, contact point on tire moves inward much more than on a mountain bike.
    At the end, on both cases, due to different factors, forces are aligned and balance is then OK.

    Yes spikes are important to KEEP adherence whatever the changes on the trail. On many mtb bikes, due to limited width, tire design is somehow tricky : in order use full potential of cornering traction, ture must be leaned a lot... yes cornering spikes are very far away from the center of the tire.

    And lastly, ny leaning the mtb bike a lot, center of gravity is then closed to the ground which increase stability.