Things come apart

Disassembling a dirt bike

HOP ON THE BIKE. Make sure the engine stop switch (29) is in the run position; it’s a kill switch for emergency situations, meaning the key will do you no good if it’s in the Off position. Turn the ignition to On. Put the bike in neutral or pull in the clutch lever (32) to disengage the bike’s multiplate clutch (9), a friction-based clutch that works like a car’s, but uses many small plates, rather than one big plate, because of space limitations. Press the start button (26) to fire up the engine.

On the CRF250L, you can go just about anywhere. It’s a dirt bike in a street-legal bike’s clothing, headlight (22), turn signals (24), and mirrors (25), dressing up bigger wheels (1) that offer more clearance and aggressively knobby tyres (2) that improve handling. Before you release the clutch lever, use your left foot to push the shift lever (10) down, popping the bike into first gear. The shift lever connects to ratcheting mechanisms that turn a drum (18) that moves shift forks (17) to engage gears (16). Now goose the throttle; rolling the throttle grip (27) toward you pulls the throttle cable (23), opening a valve in the throttle body (4) and letting more air into the engine.

The engine revs up, its single piston (14) displacing 250 cubic centimetres per stroke as it pumps up and down in sync with a counterbalancing shaft (12), which smooths the harsh vibrations of a reciprocating engine with an odd number of cylinders. (An even-numbered bank can be timed to balance out.) The engine is mounted transversely, meaning the crankshaft (15) is perpendicular to the bike, so it’s easy to string the drive chain (8) between it and the sprocket (3) on the rear wheel.

Each wheel has a unique suspension system. The front wheel is at the bottom of a telescoping fork. This bike uses an upside-down fork, with the slide pipes (20) on the bottom and the fork tube outers (19), which receive the sliders and contain springs, dampers and lubricant, on top. That improves handling because it lowers the bike’s unsprung weight (the portion connected directly to the wheel, not carried by the suspension). The sliders are connected by a clamping system called a steering stem (30) to the bike’s frame (13) and handlebars (31). On the other end, the rear wheel is connected to the swing arm (6), a horizontal connector that pivots up and down to allow wheel travel; it’s stabilised by a single shock absorber (11) behind the engine.

Upon reaching your destination, squeeze the brake lever (28) to move pistons that hydraulically clamp callipers (7) on to the disc brakes (21). Come to a stop. Turn the ignition to Off, then turn the handlebars all the way to the left and turn the key to the lock position. That’s your theft deterrent. Lock your helmet to the helmet holder (5). You’ve arrived.

NOTES: In a car, you control the throttle with your foot and shift gears with your hand. On a motorcycle it’s the opposite. And with good reason. Even if you could fit a car-stylecar- style shifter on a bike, it would be way too dangerous s to take a hand off the handlebars to operate it. That pretty much sums up two-wheeled motor vehicles, es, vis-à-vis the four-wheeled ones: the same main parts are there, but configured for a smaller, lighter,er, riskier space. Which makes the engineering all the more fascinating.