Cornering Problems 2 - Cornering lines, stability and the 'Point and Squirt' technique
You may have heard this statement:
"A motorcycle is most stable when it is upright and travelling in a straight line at a constant speed."
You'll find this in quite a lot of writing on advanced riding. Unfortunately, it's not quite correct. A motorcycle is most stable when it is lying on its side. That's not a great deal of use to us if we're hoping to ride it.
So I'll make an alternative statement for you to ponder:
"If we're not actually crashing, the bike must STILL be stable - crashing is the definition of UNstable."
All of our bike control skills achieve just two results:
- changes of speed
- changes of direction
So have a think about this too:
"If we don't make steering inputs the bike will ultimately become unstable and it will fall over."
In essence, as soon as we're moving, the machine is generating is a different kind of stability which is sometimes called 'meta-stability', and by feeding in steering inputs we're constantly maintaining a meta-stable state.
So what we're actually interested in is not 'stability' per se, but understanding how stability is affected by the INPUTS we make to the machine change speed or direction, where we sit within the LIMITS of stability at any one moment, and whether our inputs (or even lack of inputs) in terms of changing speed and direction are pushing the machine towards those limits. Specifically we need to know if we are about to create an unstable state, because that's when we are going to crash.
So let's think about limits of stability. It's determined by two things
- how much input force we apply through the controls
- tyre grip against the road surface - even the best tyres offer zero grip if the road surface can't deliver its half of the bargain
Let's start with the input forces. There's applying a braking force (either via the brakes or a closed throttle) and there is a driving force (from opening the throttle), and usually forgotten is that steering also applies a force (which makes the bike change lean angle).
So what pushes us towards the limits of stability? Most of us will usually think of the consequences of an excess of force combined with insufficient grip. Big handfuls of brake or throttle risk tyre lock-ups (or triggering ABS) or wheelspin (or triggering traction control). Big lean angles risk sliding tyres. And these limits are 'mix-and-match' too. If we are braking or accelerating, we compromise our ability to lean the bike, and if we are leaning, we compromise our ability to brake or accelerate. This is the basis of the advice to keep braking or throttle use away from corners. If we're upright, we can brake or accelerate as hard as the tyres' limits. But once leant over, some of the grip is being used to maintain our curved path, and so we have reduced grip to brake or accelerate. This is the basis of what's known as the 'traction pie', where we 'slice up' grip.
Less obviously, instability can be result from the rate of change of one of those forces. we can destabilise the bike by braking too hard (and pulling a stoppie) or accelerating too hard (and pulling a wheelie). Ultimately we could loop the bike. But we can also destabilise the bike if the rate of change of direction is too high - we can generate a big wobble or weave. Bumps or gusts of wind are external forces and can also destabilise the machine. To reduce the rate of change we just back off the input and reduce the force being applied.
So having said all that, if the machine is upright and with only just enough power being applied via the rear wheel to keep it moving at constant speed, then what we have are the biggest RESERVES of stability.
So how could we try to stay away from the limit of grip in a corner?
A common suggestion is to maximise the radius of the turn, instead of simply staying in the middle of the lane all the way round, which is - more or less - the approach taught on basic training. So on a right-hander, we'd start by kerb, cross the width of our lane to come close to the centre line mid-corner, then drift out again so by the time we leave the corner, we're back over kerb. This way we fit the maximum radius line compared with the 'middle of the lane' line. In theory, we are "working our tyres less hard". That very advantage is mentioned in an early 2000s BikeSafe video from West Midlands police which I have in my collection.
But let's think about that a little harder.
That's only true if we keep our speed the same. Let's think about cornering physics. If we accelerate at the same lean angle, we'll spiral out onto a wide line. If we slow down at the same lean angle, we'll spriral inwards. Corner grip - and thus stability - isn't just lean angle OR speed. It's a combination of both - angular momentum.
When we corner, we almost all develop a 'comfort lean angle' where rider and bike are at their happiest. Now, a moment's thought should tell us that if we adopt our comfort lean angle at the same time as we follow the maximum radius line, then the wider line does NOT reduce the need for tyre grip at all because we'll be riding the corner faster. So we've not actually increased our tyre grip margins at all because we've increased our angular momentum. We actually need to reduce our lean angle (and angular momentum) to increase our margins and to "work our tyres less hard" as claimed in the video.
But in reality, wide cornering lines are nearly always used to carry more speed at our comfort lean angle. A clue to that is its other name - the racing line.
There are other problems. Compared with the middle-of-the-lane line, the maximum radius line starts earlier and finishes later. Or to put it another way, we're leaned over for longer. When we're leaned over, our ability to brake or accelerate is compromised. And that means on the way into the corner, we can't brake as late. On the way out of the bend, we cannot get on the gas as early.
You might have noticed that many bends in the UK are blind - that is, we cannot see our way out of the bend from the point where we start turning. Taking a line that commits us to an early turn-in makes it even more difficult to see through the corner, which means it becomes more difficult to assess it. And if we do get the line wrong, committing ourselves onto this maximim radius line commits us to a wide exit line. If the bend goes on just a bit longer than we expected when we committed to the maximum radius line, there's a risk we'll run out of room on the way out of the corner. And that's the main reason I only ever use the maximum radius line when I have 100% vision, right through the corner and out the other side.
So... if the maximum radius line is a potentially risky one, what are the alternatives?
Ever since CBT was introduced in 1990, basic training has taught riders to follow a middle-of-the-lane line, or even keep a little to the left of centre. It may be 'basic' but it keeps us away from the extreme edges of the road. It has some pluses:
- it keeps us away from the centre line on right-handers (and gives us a slightly better view ahead)
- it keeps us away from the nearside on left-handers (and gives us a slightly better view ahead)
- it keeps the bike upright slightly later on the way into the bend (we can brake closer to the corner if we need to)
- it gets the bike upright slightly earlier on the way out of the bend (we can get on the gas sooner if we need to)
Whilst we trade off a bit of mid-corner speed at our comfort lean angle, because we're reduced our angular momentum, we actually need a bit less grip, even though the lean angle is the same as on the wider line. And this bonus tyre grip is useful in case we need to brake or swerve to avoid an unseen hazard.
A second option is to push this 'follow the bend' line out closer to the edge of the lane. Not so close we're at risk of meeting an oncoming vehicle on a left-hander, but far enough towards the centre line that we get a little more view still. Likewise on a right-hander, a line closer to the verge would also improve our view ahead.
But remember I said that if the machine is upright and with only just enough power being applied via the rear wheel to keep it moving at constant speed, then what we have are the biggest RESERVES of stability?
What if we mix and match these ideas? How about this - what if we use that 'follow the bend line' shifted to the outside of the corner around the first part of the bend, then use a quick steering input to straighten out the final part of the corner and get the benefits of the maximum radius line when we can see where the bend goes?
What are the benefits? By avoiding turning in on the maximum radius line initially, we do deeper into the corner and stay upright longer before we change direction. Mid-corner, we've trade off the sweeping line's mid-corner speed which gives more reserves of grip, which in turn allow a more positive steering input to change change direction more rapidly at the delayed 'turn-in' point which happens only when we can see where we're going next. The more positive steering input gets the bike turned faster, and pointed where we want to go sooner, which means we can open out the final part of the bend on that maximum radius line. Finally we get the machine upright sooner, which means we're back on the power earlier.
This is the technique I've been teaching ever since I launched my Survival Skills advanced cornering courses, and I call it the 'Point and Squirt' line because what we effectively do is 'point' the bike first, THEN 'squirt' the power on to drive the bike clear of the corner.
It's slower INTO the corner, but gives us a better view THROUGH the bend before we start to turn, and that means we're more likely to spot mid-corner obstructions like a parked vehicle, a pothole or a fuel spill, as well as identify a double-apex corner before we're committed. The wider line means we can keep away from extreme left or right positions in the lane and we can build in a bigger clearance to entrances to our left and oncoming vehicles to our right. The lower speed at the point where we steer to open out the corner means we've more reserves of grip. And whatever speed we lose INTO the bend we more than recover on the way OUT. In a short sentence, managing corners to keep both the time and the distance where the bike is learnt over as short as possible is a good thing, not bad.
Despite the explanations, this line still comes under fire, with critics saying:
"it's slower" - yes, it's slower mid-corner - is that such a bad thing - but we're upright sooner and faster out, which fits with the well-known 'slow in, fast out' advice. But we're also upright a little longer into the corner which allows us to decelerate slightly later, and because we're upright much sooner, we regain speed whilst the rider on the maximum radius line is still leaned over. Yes, that rider will catch us MID-corner, but we'll leave them behind on the way out of the bend.
"it needs more grip because you're steering harder" - a misunderstanding of the angular momentum issue. The reduced mid-corner speed more than compensates for the more positive input needed to make the quicker change of direction.
"it's less fun" - well, if having your head removed by a passing truck is fun, carry on. If I want big mid-corner lean angles, I'll go to the track where I'm unlikely to have to take evasive action halfway around. The only real difficulty is that we have to get used to a late, postive steering input. If we've used to turning in early on a sweeping line, it can take some adapting to.
"if it's so good why doesn't everyone teach it" - as it happens, Andy Ibbott, the former racer writing as the UK director of the California Superbike School, featured exactly this approach in an MCN article some years ago, so there are other rider coaches who recognise the advantages.
Kevin Williams
Survival Skills Rider Training...because it's a jungle out there
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