# MTB: choosing trajectory (line) through a turn

Consider a 3 meter wide track, the incline is 30 degrees, the terrain is dirt road with some sand, rocks, maybe mud. An at least 90 degree turn is approaching, and you have clear visibility of the whole turn.

How do you choose consciously (and with practice subconsciously) your path through the turn, in order to minimize loss of speed? What factors affect the decision and in what way?

TL;DR

So here is the scenario. The incline is constant 30 degrees. A and B indicate the terrains on both sides of the track.

What effect on the decision has the estimate how much is A and B dangerous. For example, if B is a 100m drop, and A is lush field.

Here is an example of taking the turn entirely on the outer edge. This has the effect of increasing the turning radius. On the other hand, on slip and the bike leaves the track.

On the other extreme, here is an example of taking the turn as tightly as possible.

Further, if there is another turn very near, the exit line of the first turn directly affects the attack line of the second turn. What are the strategies to deal with a double turn?

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By "incline" do you mean the banking of the corner? – andy256 Oct 14 '13 at 12:05
@andy256, the track inclined along it's axis, there is no banking (as in one can fly off to zone B easily). I wish I could indicate this in the sketches, but everything became a mess. – Vorac Oct 14 '13 at 12:13
Typically auto racers will roughly follow the 3rd pattern (although they'd hug the outer wall a bit longer before beginning the turn). But of course auto racers rarely have a sheer drop on either side. (In practice the 3rd pattern -- as modified -- has a larger radius, because it makes use of the width of the track to increase the radius going in and coming out.) – Daniel R Hicks Oct 14 '13 at 19:47
I've always suspected that, ignoring issues of traction, etc, the best path through a racecourse is a Bézier curve, constrained by the "walls" of the course. – Daniel R Hicks Oct 14 '13 at 21:22

Cornering is an essential mountain biking technique, regardless of the type of turn you're going into.

Scenario 1: Turn to obstacle
With clear visibility you should make the turn so the exit lines you up for how you plan to ride the upcoming obstacle. Usually this means taking the turn wide (high) so you can pull the bike back into line easier.

Scenario 2: Single turn
A single turn leaves you with only one decision: fast or slow? The lower you go through the turn, the tighter the radius and the more speed you will carry (conservation of momentum). But like any type of riding, maintaining control at speed can be difficult, which may cause you to take a wider line.

Scenario 3: Turn to turn
Consider an S-curve. You'll want to kind of approach it with the idea of slaloming through the turns. This means picking whichever line will bring you into the entrance line of the next turn. Again, this usually means riding further out, then coming back in to the entrance of the next turn.

Lastly, as you mentioned in example 1, there are times when the other side of the berm isn't so pleasant if you happen to overshoot. Here it's about how much courage you have! If you have the skills and want to ride high then go for it, otherwise scrub a little speed and take the tighter line. And really, there's always the ideal situation, then there's what happens on the trail, which is often very different from how the ideal works.

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Maintaining a tight radius does not conserve momentum. It does shorten the track ever so slightly, but at the expense of more momentum lost to sideways friction, and possibly to braking. – Daniel R Hicks Oct 14 '13 at 19:49
@DanielRHicks I meant in the physics sense, under ideal conditions. I don't really want to break it down to the point of coefficients of friction and then a tire debate following that! – Aaron Oct 14 '13 at 20:41
But in the physics sense a tight radius means nothing. Momentum is mass times speed, and so long as you maintain the speed you maintain the momentum. The only thing that causes you to lose momentum is friction. – Daniel R Hicks Oct 14 '13 at 21:13
This guy seems to always choose the outer diameter of the track, even going over the banking. (Singletrack, but still.) – Vorac Oct 23 '13 at 15:55

@Aaron has a great answer, but he missed probably the most significant point ..

Almost always, take the line most traveled. Unless you are a track builder (I hope not, as I would hope someone building tracks would not need to ask this question), there will be a line that others have used, or you are on "wild" tracks...... If its dirt/sand/mud it will be obvious.

Rarely there is actually a faster but less obvious line thorough the corner that the unwashed masses have not found. This is where some real fun and learning starts to happen - find it. Even if there is not a better line, searching for it will teach you why the line is where it is. Ride the line everyone else takes, follow some really good riders into the corner, get really good at riding this line as well as you can (within your limits of skill, time availability, boredom etc). Now, go back, and pick a new line, ride it, and see how you go. Feel the difference. Follow some good rider through, and see if you make ground against them.

I once spent an hour on one corner with a coach- a never forgotten lesson, if you can, do the same - pay someone who is really good to show you. Get lessons - a few hundred dollars will speed you up more than a multi-thousand dollar bike, and unlike a bike - the lessons learned last a lifetime.

Be aware that you own speed and fitness plays apart - a top level rider may carry enough speed to get around the top faster than going tight, but you may be better staying tight. A weak rider on a heavy bike may be better to maintain speed, where as a powerful rider on a super light bike can accelerate quicker, so may be able to loose a bit of speed for a tighter line.

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I agree, especially the part about spending a couple of bucks for a coach. But get a good one. Everyone and his dog calls himself an MTB coach nowadays. – arne Oct 15 '13 at 6:14
+1 for the most traveled line. I read the OP as being in ideal case where all parts of the turn are equally ridable. – Aaron Oct 15 '13 at 13:45