On the track, racers are either on the throttle or on the brakes – no free wheeling – this wastes time. Trail braking is a technique which racers use to slow the bike as quickly as possible from one speed (on the straight) to another (corner apex speed).
In applying this technique, a racer will approach a turn and at their braking marker, apply full braking force, normally with the bike being upright.
As the rider begins to turn in, they reduce brake pressure, easing off the brakes. Decreasing or “trailing” the brake lever force as the bike lean angle increases until they gets to the apex, the rider then releases the brake and applies the throttle.
Sounds easy enough in theory, but proper execution is complicated because it comes down to feel — and remember these guys are doing this seamlessly, every lap on the limit.
As Freddie Spencer once said, “fast riders have slow hands” so all this is done smoothly, progressively and powerfully.
Also consider a MotoGP machine has so much front tyre grip when upright, that you would easily endo the machine if excessive brake lever force is applied too aggressively, so a certain amount of finesse is required.
Normally only the front brake is used for trail braking as the rear brake contributes little braking power, has less feel, and is normally reserved for mid-corner fine adjustments or to stabilise the bike.
To comprehend the dynamics of trail braking, ignoring any aerodynamic effects, some understanding of tires and grip is beneficial, with the amount of grip from the tires depending on various factors.
The main contributor to grip is the weight or load on each tire. The ratio between the maximum possible grip and the vertical load is called the coefficient of friction. This coefficient is not constant and normally decreases relative to the vertical load.
MotoGP tires go beyond the normal theoretical coefficient of friction value of 1.0, hitting values over 1.3. These tires behave more like gum, melting into the track surface to create exceptional levels of grip questioning Newton’s model of friction.
Also as the brake is applied, torque is transferred through the wheel to the contact patch, which creates a horizontal force at the track surface.
The road pushes back on the tire, and equally the tire pushes forward on the track surface. You can thank Newton for this mechanical grip, as for each force there is an equal and opposing force.
According to Brembo data, deceleration forces over -1.6g are common with riders applying around 8kg of lever pressure at speeds of 350 km/h.
For example, at the Sachsenring circuit, whilst braking into Turn 1, the MotoGP machines decelerate from over 300 km/h to 100 km/h in a distance of 253 metres within 5.8 seconds. As riders spend almost 20% of this lap braking, trail braking is an essential skill to master.
Also to consider is the significant grip increase experienced as the front tire contact patch pressure multiplies due to the load transfer when braking. This grip effect decreases as the lean angle increases, and the load transfers off the front to the rear.
As the brakes are applied and the weight shifts forward, the forks are also compressed. This compression of the forks alters the motorcycles steering geometry, reducing the rake and trail.
This decreases stability but increases manoeuvrability in a fashion that makes the motorcycle lean and change direction at a higher roll rate.
The tire temperatures also increase from this weight transfer and subsequent tire loading, with tire temperature windows critical for optimum grip without tire degradation.
Another important factor is the reducing road speed, which decreases the motorcycle’s cornering radius. Other factors such as track surface characteristics and other elements between the track and the tire such as water or oil play an important part in trail braking efficiency.
These guys are close to the limit, sometimes stepping over, notably demonstrated by the small difference in the Respsol lad’s track position at the last corner on that weekend in Germany, which resulted in Casey Stoner’s defining Championship moment of sliding down the road.
Mark McVeigh is a former international motorcycle road racer and MotoGP engineer who now works as a moto-journalist and development rider.