Mountain bike trials

Pi's father had wanted his son to learn motorcycle trials by practicing on an ordinary bicycle.

Trials riding is an extreme test of bicycle handling skills, over many of obstacles, both natural and man-made.

Competition trial bikes are characterized by powerful brakes, wide handlebars, lightweight parts, single-speed low gearing, low tire pressures with a thick rear tire, distinctive frame geometry, and usually no seat.

Currently there are two official types of competition rules, enforced by the UCI and BikeTrial International Union.

Within UCI rules,[1] if any part of the bike except the tires touch any object in the course, a dab will be given.

The UCI rules were changed to this format after too many competitions ended in a draw and riders were forced to ride an extra section.

When a rider is in a section, neither tire is allowed to cross the side boundary tape even if the wheel is in the air.

Before beginning a section, a rider is allowed to walk through it, and examine all the elements, but must not enter it with their bike.

In terms of geometry, trials frames, especially those biased towards competition, tend to have BB spindles positioned significantly higher than the line between the axles.

A grind may be referred to as 'dead' if it has been used a lot and the brake pads have started to polish it back smooth.

The main downside to this method is that, similar to a smooth rim, even small amounts of moisture will drastically reduce the stopping performance of this braking setup.

Part of this is that disks are almost invariably better at controlling the speed of a bike while it is rolling, that is they can take more energy per unit time from the rider (i.e. they have more power, which is why they dominate all other off-road cycling disciplines), but most disks will not hold a wheel locked as firmly as a rim brake, making them less desirable for competition trials.

[citation needed] Rim brakes set up for trials tend to squeal or howl if they are pulled while the wheel is turning, occasionally leading to trials riders attracting undesirable attention or being considered a nuisance for noise pollution.

UCI regulations only stipulate that the bike must have a working front and rear brake.

The change in volume of a tire on a wide rim when landed on a sharp corner will tend to be greater than that of a narrow rim, which again leads to improved pinch puncture resistance, critical for trials as landing on sharp edges is a very common requirement of both competition and street trials.

135 mm spaced trials hubs originally were taken from mountain bikes, but modern designs are focused on making room for as many sprockets as possible and often don't have the torque carrying capacity needed to be safe for trials use.

Many of them also don't pick up drive quickly enough to give a trials rider the instant pedal response required for precision riding.

Trials freewheels are available with up to 120 clicks per revolution to give excellent drive pickup.

The gear ratio chosen by most riders results in the bike moving approximately the same distance per turn of the pedals regardless of the wheel size.

Horizontal dropouts at both spacings are becoming more and more common in stock bikes as they eliminate the need for a chain tensioner, making the bike lighter (The chain is tensioned by moving the wheel backwards in the dropouts; vertical dropouts are designed to hold the wheel in one position only and therefore require a separate means of tensioning the chain to accommodate wear).

Frame geometry for Stock bikes also have a high bottom bracket, around +70mm, wheelbase around 1095mm, and chainstay length around 380mm.

They are not traditionally used to compete in competitions and were originally designed to suit riders with a more "street" style.

These offer a good compromise between the length of a 26″ frame for bridging between obstacles and the ease with which a 20″ bike can be lifted to the back wheel.

Compared to stock and mod frame geometry, street trials bikes have a slightly higher bottom bracket, shorter wheelbase and shorter chainstays to assist the rider with spins, manuals and bunnyhops while still providing assistance with back wheel moves.

Geometry may vary between different models of street trails bikes, but typically the bottom bracket height is around +15mm, wheelbase around 990-100mm, and chainstay length around 360.

More fluid than competitive trials riding, it encompasses the same skills: very precise control of the bike, through jumps and balancing on very narrow obstacles.