The drive - all assembly work

BIKE Magazin

· 07.05.2008

Photo: Daniel Simon

The drive - all assembly work

Everything you need to know about cranks and bottom brackets, rear derailleurs and cassettes - from assembly to disassembly, right through to retrofitting chainrings. Plus: all the important data and dimensions.

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1. SCREW CONNECTION

The crank arm is pulled onto the multiple gearing of the crankshaft with a central screw and fixed with 40-60 Nm. A cover ring serves as an integrated puller.

2. REDUNDANT CLAMPING

The crank arm is pulled onto the fine-toothed shaft using a central screw to adjust the bearing play. Clamping on the shaft is achieved using two screws.

3. screw connection WITHOUT puller As in the first picture, an "XTR" crank is a splined shaft screw connection. However, an additional crank puller is required for disassembly.

BOTTOM BRACKET WIDTH Spacing according to housing width and front derailleur: Apart from downhill bikes with 83 or 100 millimetre wide bottom bracket housings, all bike manufacturers limit themselves to 68 (pictures 1+2) or 73 millimetre wide housings (pictures 3+4). All standard cranks are compatible with these two widths. In addition, front derailleurs with bottom bracket mounting (E-Type, pictures 2+4) can be clamped with all bearings. Spacers compensate for the different distances. Three spacers are required for a 68 bottom bracket with standard front derailleur (1). A 78 with an E-Type front derailleur does not require spacers (4).

CURB LENGTH AND Q-FACTOR Two parameters and a wide range of opinions: As soon as the terms "crank length" or "Q-factor" are mentioned, there is a dispute. What is the optimum crank length for my height or leg length? Is it better to ride with a wide crank or as narrow as possible? Scientific studies on this are rare and do not provide complete clarity. In general, however, the following facts can be stated: The length of the crank and therefore the length of the lever used to apply the leg power influences the power output. In addition to the overall seating position, it determines whether the rider's available energy can be utilised in the best possible way. A length of 175 millimetres has established itself as the industrial standard, with which a large number of riders can cope. Tall riders over 1.90 metres and shorter riders under 1.70 metres should use longer or shorter cranks. In addition, longer cranks are recommended for bikers with a low cadence because the torque is higher, but the distance travelled also increases. The following applies to the "Q factor" (see image): Narrower cranks are more in line with the human musculoskeletal system and touch down later when cornering. If you train a lot on a road bike, you should make sure that the cranks are of a uniform length with a narrow width so as not to strain your economy of movement. PREPARING THE BOTTOM BRACKET To remove paint residue from the bottom bracket shell and make the thread smooth.

25 Images

Photo: Daniel Simon

Bottom bracket dimensions and variants of MTB cranks

There is hardly a component on the bike that is not subject to constant optimisation pressure. Cranks and bottom brackets are no exception. The era of heavy square shafts is over and even since the introduction of the hollow shaft, a lot has happened. After moving away from the square mount, Shimano initially launched a rigid hollow shaft with so-called "Octalink" gearing. Other manufacturers such as Race Face and Truvativ followed suit and also introduced a system with a hollow shaft and multi-tooth connection (ISIS) between the crank and shaft. Almost all crank manufacturers now offer a system based on Shimano's "Hollowtech II" standard. The special thing about this is the higher rigidity compared to the old systems. The new cranks are also more durable and lighter. Whereas previously the bearings were housed in the bottom bracket shell, the current cranks have external bottom bracket shells. This increased the support width, the crankshaft was able to grow to a torsion-resistant 24 millimetres and the bearings have become more generous - and therefore more durable. Another special feature of the "Hollowtech II" system is the crankshaft-shaft unit, which is fixed on one side. This eliminates the need for an unstable screw connection. Almost all manufacturers connect the right-hand crank inseparably to the shaft. Only Race Face does it the other way round. The reason for this is not so much a technical necessity, but the fact that Race Face's crank packaging can be smaller. Another speciality is the Truvativ bottom bracket. Normally, lateral load is used to adjust the bearing clearance. With Truvativ, the left-hand bearing is clamped between the shaft and the left-hand crank arm, while the right-hand bearing remains axially free to move. This benefits durability.

Photo: Daniel Simon This is how you determine the crank length and the Q-factor of MTB cranks.

MTB cranks in comparison

All important data and dimensions of the most important MTB cranks at a glance:

Photo: Screenshot Crank length, chainline and Q-factor of the most important MTB cranks at a glance.

Photo: Screenshot Overview of housing widths, weights and chainring dimensions of the most important MTB cranks.

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The drive - all assembly work

Bottom bracket dimensions and variants of MTB cranks

MTB cranks in comparison

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