Cornering Lessons: The Limits of Grip

This is part II of the BikeBlogorDie series on turning. For part I, click here. And for the precursor, click here.

Think of grip in percentages. 

For example, you're cruising along with slick tires on smooth pavement. We'll call that 99% grip, because there is basically no way that you can slip out on your own, so long as you keep going in a straight line.

But everything you do, every movement you make, every rock you roll over, reduces that grip percentage. The more modifiers you put on, the less grip you have, until you get to 10%, which is where you start to slide, or 0%, which is where you lose all control.

So lets look at the influences that affect grip, and give them an arbitrary number. For the rest of this article, we'll assume the rider is in a racing situation, so all movement will be on the limit.

Grip is affected by things outside your control, and things within it. The things that you cannot change while riding through a corner are called "static modifiers" and the things that the rider can change are "dynamic modifiers"

First lets look at the static modifiers:

Tires:

For our purposes, tires have 3 important characteristics: size, softness, and tread. Wide tires provide a larger contact patch, soft rubber grips better, and dense tread is better for dense ground, while wide tread patterns (like the one in the picture) are better for mud, snow, and sand.

For this article, lets assume we have 25c racing slicks.

Road Conditions:

The grippiest road type is paved and smooth. Let's assume that's what we're working with for this article, but add a descent into it for some excitement.

Now let's examine the dynamic modifiers:

Lean Angle:

Again, we're assuming a fast descent, so the lean will be at least as hard as this guy in the photo. Maybe harder.

Speed:

Since this article is about performance handling, we'll assume the rider is entering the turn at the highest possible speed.

Braking:

Let me amend the previous statement about speed. The goal is to enter the turn at the highest possible speed, then exit the turn alive and still riding. Braking, then, is an important aspect of the turning process.


So if we are to assume that a rider using racing slicks on dry pavement has 99% grip, then put that rider on gravel and she now has, say, 60% grip. She's still in control, but the more modifiers she puts on, the closer to slipping she'll be.

Now, lets put her back on cement, but have her enter a turn. Let's say it's a turn that she's done before, and she knows that at 25mph she's in total control. We'll say that she has 80% grip. So, at 40mph, the forces acting on her tires are all greatly increased, specifically the inertia that wants to push the bike to the outside of the curve. To counter inertia, she increases her lean angle, but that cuts away from her grip. So, instead of 80% grip, she's now at 30% grip.

Remember, she won't start skidding until 10%, so at 30%, while in mid corner, she still has traction.

But what if we add one more modifier?

Lets say that she's looking through the corner, but realizes she's actually going a little too fast, and she's not going to make it. She has two options: brake, or increase lean angle. Both will take away from her grip percentage. In short, she has run out of grip. No matter what, she's going to skid. At this point, if she's not panicking like a normal person. She'll either stand the bike up and grab full brake and hope it stops her before the road ends, or she'll brake while still leaning and enter the strange world between 0% and 10% grip, where she's sliding, but still might have a tiny bit of control. The best thing she can do in this situation is maintain balance and drift on two wheels. Eek.

The worst thing is for her tires to suddenly gain traction, like here:

 The most likely situation is that the bike will slide out from under her, and she'll accept the red badges of courage up and down her legs, hip, elbows, and hands.

We see, then, that the goal of performance cornering is to keep just above the skid point, 10%, but to never exceed it. Every movement, every brake grab, every tiny deviation decreases the grip percentage.

The tire's ability to grip is limited, therefore a rider should always limit the number of tasks a tire should have to do.

Proper cornering technique can be reduced to three distinct phases: Braking, Leaning, and Exiting. 

The correct flow of a corner will look something like this: 

This is from a cornering guide for motorcycles, but the principles are similar. 

For each phase, the tire has only one job. That way it is less likely to run out of grip by being overburdened. Let's take a look at the individual phases and see what's going on. 

Braking:

A rider should always brake before the turn. Any braking being done while the bike is leaning should be avoided, unless it's deliberate – like in the case of trail braking, where you gradually reduce braking pressure with the increase of lean angle. We'll save the trail braking discussion for another time. 

Warning, Digression: There is a better way to brake. Because of inertia, when you hit the brakes most of a rider's weight is transferred to the front wheel. This means the front wheel has more traction than the rear, which means you can apply more braking force on the front without losing grip. The concern for most people, however, isn't that the front will skid out, but that the body on top of the bike will flip over the handlebars. To avoid this, get your body weight as far back, and as low as possible. This means: be in the drops (if it's a road bike), be off the saddle (behind it, actually), and use your legs to brace yourself against the braking forces, like this guy:

Adopting this body position while braking has the added bonus of setting the rider up for the leaning portion of the turn, because the rider will already be off the saddle and in the drops. 

For fun, here's a guy over-cooking it a little: 

And here's what it would look like if that guy tried to make a turn: 

Leaning:
Full lean angle – that is, the closest to sideways the rider intends to get the bike – should occur directly after the braking phase. Again, this has to do with getting the most out of the tire's available grip.

Proper technique dictates that you want to flick the bike into a turn quickly, then spend the rest of the turn gradually uprighting the bike. The reason for this is that a high lean angle eats up a lot of the tire's grip percentage, so the best time to lean the bike is while it's moving the slowest – i.e. just after braking. With all that said, the perfect line through a corner, won't be a perfect parabola, but will look more like a curved elbow.

The object, as the below image shows, is to spend as little time at maximum lean angle as possible, so that you can accelerate more quickly after the turn – or if safety is important to you (nerd), a quick, sharp lean will give the rider more traction later in the turn, to better avoid obstacles, deal with speed changes, or cope with changing road conditions.

If I may steal a gif from motorcycling, here're some bikers executing perfect turn technique. Notice how quickly they get their bikes sideways, then spend the rest of the turn uprighting themselves: https://www.reddit.com/r/oddlysatisfying/comments/3bpz5l/the_first_turn_from_assen_this_past_week_xpost/

Exiting:
Here's where I admit that there are some differences between motorcycles and bicycles, one of the biggest being that motorcycles can quickly lay on the throttle after reaching a turn's apex. It therefore behooves a motorcycle to get the braking and leaning part of a turn over with as soon as possible so they can spend more time on the throttle. Ahh, but here is where a bicycle is not so different. Because pedaling significantly reduces the a bicycle's ground clearance, a bike rider who wants to pedal through a corner should time their maximum lean early, while the inside pedal is up, then get the bike upright asap. Same goes for if the rider is descending, because gravity will make the bike accelerate.

But I will admit, if a rider is turning on flat land or up hill, and wants to exit a corner going much slower than when they entered it, they don't have to do a quick, hard lean to enter the turn, and can take a more parabolic approach, or even a reverse elbow, because the slowest point will be at the end of the turn. Mountain bikers riding up blind switchbacks make use of this technique if they're unfamiliar with the trail.

So yeah, braking and initial lean are the points of greatest force on the tires, and are the moments when the bike is most likely to lose grip. This is what you want because the braking phase and the initial lean phase are when the bike is traveling the slowest, so a slip-out then is much preferable to a slip-out at the exit, when speeds are higher.

Let us return, now, to the lady from earlier, who botched that first turn:

This time, she sees the turn coming, so she gets off the saddle and into the drops to put her weight as far low and back as she can, so that she can get the most out of her brakes, while giving the front tire a little more grab-action than the back.

Then, still off the saddle, she eases off the brakes and leans the bike, making sure to have her weight divided between the outer pedal and inner hand grip, with her eyes focused on the turn's exit. Her lean is swift and decisive. She achieves maximum lean angle moments after releasing the brakes, then gradually un-leans the bike just as she reaches the turn's apex.

From that moment on, she's sprinting. Her tires, having regained the traction lost through braking and turning, now have full grip availability, and she's dedicating it to acceleration.

And thus, having exited the corner with speed and confidence, she wins!

Now that you know how to turn, practice. Remember: look through the corner, brake before entering, lean the bike in a separate phase, and exit with speed and confidence.

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