On a BMX bike, correct drive-chain tension is essential. Improperly set, a drive chain can lose its grip on the gears. If too tight, the bike can be hard to pedal and cause unnecessary wear on the chain and the gears. To set the correct tension on the drive chain, BMX bikes are often equipped with chain tensioners, which fit on the bike frame and help to fine-tune drive-chain tension.
Design
The frames of BMX bikes are designed differently than other types of bicycles to withstand the stresses and impacts within the sport and provide a small and low frame size. In particular, the dropouts, the slots on the bike into which the rear axle fits, run straight out and away from the frame, rather than downward, as in larger multispeed bikes. This design makes for a stronger frame. Also, BMX bikes do not have a rear derailleur that adjusts the tension of the chain automatically. For these reasons, chain tension must be adjusted manually on BMX bikes.
Location
To adjust a BMX bike drive chain, tensioners are fit onto either side of the axle and the ends of each of the dropouts on the bike frame. This location allows easy adjustment of the chain without interfering with riding the bike or attaching foot pegs to the axle. Chain tensioners are typically low-profile devices that conform to the bike frame.
Function
When adjusted, a threaded bolt in each chain tensioner pushes or pulls one side of the axle within each dropout on the bike frame. Two separate chain tensioners, mounted on either side of the frame, provide a way to adjust the axle evenly to keep the wheel true within the frame of the bike. Most tensioners are designed to be adjusted with an Allen wrench, though some will take an adjustable wrench.
Adjustment
To adjust the chain tensioners on a BMX bike, loosen the axle nuts to allow the axle to move freely in the dropouts. Rotate the bolts in the chain tensioners clockwise to increase tension or counterclockwise to decrease tension on the chain. Adjust the tensioners evenly to keep the rear wheel true in the bike frame. Once the adjustments are completed and the chain is at the desired tension, tighten the axle bolts again to lock the wheel in place on the frame.
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