Cornering on a mountain bike
January 1, 2019 3:18 PM   Subscribe

I’m moving into mountain biking from motorcycling, and I want to know who should lean more: me, or the mountain bike? And why?

I’m used to cornering on a (road) motorcycle, where the goal is to keep the bike more upright and shift the rider’s weight to the inside of a corner. This, as far as my understanding goes (from reading half-forgotten blogs and watching half-forgotten youtube videos) is because you can use the contact patch where the rubber meets the road for changing speed, i.e. braking into a corner and accelerating out of it, or for leaning, and it’s a zero-sum game - the more that the bike leans, the less control you have over speed, so it’s preferable to use the rider’s weight instead of the bike’s. (This way also gives you more ground clearance, and you’re not in danger of scraping a peg on the ground.)

To learn good mountain bike technique, I’m using the (generally really excellent and highly recommended) “Skills with Phil” series of videos. For flat and off-camber turns, Phil recommends a technique where the bike leans underneath the rider, and the rider stays upright with their weight over the tyre. In other words... the exact opposite of cornering, on-road, on a motorbike.

In this article (scroll down to the “Countersteering” section of “Entering the corner: Leaning in”) the cornering technique for a motorbike analogous to what Phil’s recommending is referred to as “motorcross style”.

I’ve never ridden motorcross, but it’s suggestive to me that the “motorcross” and “mountain bike” techniques are apparently similar, and that the bike geometry and the terrain they’re used on are also similar.

So, is Phil right to say that this is the correct / optimum way to corner a mountain bike? If so - why is the on-road technique (for motorbikes) the precise opposite of the off-road technique (for mountain bikes)? What are the advantages in terms of physics, ground clearance, safety or whatever?
posted by chappell, ambrose to Sports, Hobbies, & Recreation (8 answers total) 1 user marked this as a favorite
I have never considered this question and I guess it's totally foreign to my intuitions about riding a bike. I think that much like tennis rackets become just a longer part of your arm, when I'm cycling, the bike's one more limb*, and I'm much more concerned with keeping good control and contact with it than I am about the small change of centre of gravity from adjusting my position relative to it.

In particular, I always descend in (or slightly above, if it's bumpy) the saddle.

*weight wise, I guess it's about the same as a leg, right?
posted by ambrosen at 4:11 PM on January 1

Phil is right.

Leaning the bike under you (off road) allows your CoG to stay over the bicycle's centerline and you will not fall over even in a low traction environment. Your turning radius will be larger. Too much speed will result in you laying the bike down underneath you as the bike slides out of the corner.

The reason why has to do with the desired cornering speed, amount of traction, and what the rider wants to happen when they exceed the available traction. On road, with plenty of traction you can lean to your hearts content and maximize speed through the turn. However, if you exceed the limits you will low side/high side. In snow, sand, or with a slippery road you switch to the off road technique so that loss of traction results in the bike sliding but you remain upright.

Notice that a mountain bike on hard pack or similar can be ridden with the "on road" style. And vice versa, a road bike should be ridden "off road" style on a loose surface.
posted by pdoege at 4:50 PM on January 1 [2 favorites]

The reason why has to do with the desired cornering speed, amount of traction, and what the rider wants to happen when they exceed the available traction.

This makes a lot of sense to me. So it’s a risk mitigation strategy? Something needs to lean, or you won’t get around the corner. “Rider leans” is optimum but the failure mode sucks. “Bike leans” is less ideal, but the failure mode is much more survivable. So “bike leans” is used when the friction might not be there.
posted by chappell, ambrose at 5:58 PM on January 1

Here’s a quote from the Brian Lopes book “Mastering Mountain Bike Skills” that pretty much lines up with your conclusions above. I highly recommend the book if you’re looking for another resource.

“The safest default is leaning your bike more than your body. You can never go wrong that way, for these reasons: (1) You get more bike lean and cornering force. (2) You set a harder edge by driving down into the pedal. (3) If the bike bounces or slips, you're above it where you can control the situation, rather than going down with the ship.”
posted by doctord at 6:38 PM on January 1 [1 favorite]

Keep in mind that there is a trade off between leaning vs speed and radius of turn. The lean angle defines the speed and radius of the turn. What matters in the turn is the lean angle from vertical formed by the line through the center of mass and the contact of the tire with the road. The center of mass is the combination of your body weight and the bike's weight. So if you lean the bike but not so much your body, then the center of mass will be higher because your body mass dominates the mass of the bike. This means less lean angle. A smaller lean angle means that you either must reduce your speed or increase the radius of the turn. So you are sacrificing speed and distance by not leaning as far.

But, on the other hand, if you are on the edge of losing traction, you can't take the turn any sharper or faster anyway. You have maximized the safe angle of lean for the speed and radius.
posted by JackFlash at 7:34 PM on January 1 [1 favorite]

Omgosh, I hear you. As a motorcyclist who's taken up mountain biking, the instinct to lean over the bike caused a couple of low-sides. Phil Kmetz is a gifted national-level rider and I would listen to him. The Lee McCormack and Lopes "Mastering Mountain Bike Skills" book just mentioned is also great.

The reason for the difference in technique, so I understand, is that a road bike weighs more than you, where as you weigh more than a mountain bike, so there's a big difference in how you manipulate the centre of gravity.

I also really like the concepts of "inclination" and "angulation" for explaining cornering. Angulation is how much you tilt the bike inwards under yourself (or your legs, for skiing). Inclination is how much total leaning over you do. To corner, you need enough angulation to keep traction, and enough inclination to counteract the centrifugal force. You need both to corner well.
posted by other barry at 7:37 PM on January 1 [2 favorites]

Thanks all - this makes a ton more sense to me now. (Now I just need to practice cornering without following my previous instincts!)
posted by chappell, ambrose at 9:23 PM on January 1

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