Traction is probably the most important thing to master... your dirt bike only has two wheels and most people need both to work :).
Traction is a function of weight: a loaded tire will grip, while an unloaded tire will not grip. It's as simple as that, so most of it is discussed at Weight Transfer.
Grip? Why do we need grip? Grip/traction is what you can use to support all the other forces acting on the tire:
Remember that all the forces acting on the wheel sum up (with some angles discounted) but at any point in time, the sum of all forces acting on a wheel cannot exceed the grip available. This has huge implications:
Traction, beyond Weight Transfer, is affected by the shape of the course:
The one obvious factor when talking about traction is the type of tire/ground etc... but that's so obvious we won't address it here.
When cornering on a berm, the forces push through the bike and into the berm at the right angle (90), so this is the best grip available while cornering. You already know that you can go much faster in a berm than otherwise and this is why: plenty of grip because all the forces act directly through the bike and push both wheels into the berm.
This is familiar to tarmac racers as well, as 'on camber'. Same idea - you can corner faster.
Ruts, once you get both wheels in them properly, are much like a berm: they push back on the bike so it's the same idea. The major difference here is that ruts are thin and very line-critical: must get both tires in the rut, otherwise you get none of the benefits and all of the pain!
When you corner off camber, the road is sloping away from the direction of the turn. This tends to 'grab' your wheels and make you slide. You have to go slower and more carefully here, since the forces grabbing your tires are greater and gravity is working against you. What this means is that there is less grip left to cope with the braking, acceleration and cornering here.
A classic off-camber corner in the woods is turning towards the uphill, on a slope.
So, you basically have to take it easy in an off-camber corner, slow down and be careful with the brakes and throttle. And yes, you may prepare that inside leg for eventual support...
While cornering, the front tire must be planted and tracking in the direction you want to go. If you sum up all the forces acting on it, you will find that the faster you go, the more the wheel is pushed to the outside, consuming more grip. This means that your options to brake or accelerate are now limited by the amount of remaining grip.
On camber helps with more grip while in an off-camber corner or going over a crest, you have less grip available.
It is usually recommended to countersteer more and get on the gas more instead of braking, if you find yourself having gone too fast into a corner and that does have some logic behind it. At the same time though, some instructors advise that it is possible to slow down as well instead.
If you find you entered the turn too fast and start braking, you have to keep a fine balance between the rate of decreasing the speed and the amount of grip available:
It really all depends on how well you can master braking and leaning - and that's all experience and training. It is not easy being smooth on the brakes or throttle when you're stressed or freaked out.
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