- 1 Are all bottom brackets the same diameter?
- 2 How do I determine bottom bracket spindle length?
- 3 How do I know what size crankset to buy?
- 4 Do I need spacers on my bottom bracket?
- 5 Are bottom brackets reverse threaded?
- 6 Are press fit bottom brackets good?
- 7 Are 68 and 73 bottom brackets interchangeable?
- 8 Does spindle length matter on bottom bracket?
- 9 Are bottom brackets universal?
- 10 Are longer or shorter cranks better?
- 11 Are shorter cranks better?
- 12 Does crank length really matter?
Threaded bottom brackets shells have been made over time in various thread standards that are not interchangeable. The common threaded bottom bracket uses cups or adaptors with the thread specification of 1.37″ x 24 threads per inch (approximately 34.8mm diameter).
The best way to find out which spindle length you need is by looking up the specs. (The alternative is trial and error…) Spindle length and the resulting chainline have some leeway. If you are within 2-3 mm of the “correct” 43.5 mm or 45 mm, you are doing quite well.
How do I know what size crankset to buy?
The crank length represents the distance between the centre of the bottom bracket and the centre of the pedal axis. The most common lengths are 170, 172.5 and 175 mm, but it is possible to find cranks between 165 and 180 mm in the market.
Much like GXP, you don’t need to add any spacers to the bottom bracket if you have a 73mm shell, but you will need to add a 4.5mm spacer to the right-hand side of the crank axle, unless you’re using a chain guide, in which case a 2mm spacer is required instead, along with a 2.5mm spacer on the right-hand side of the
BSA bottom brackets are reverse threaded on the driveside to counteract this.
There are benefits to press-fit bottom bracket cups. They are lighter than traditional threaded cups because there’s no metal sleeve required in the bottom bracket shell. They can also allow for wider shells and correspondingly bigger frame tubes, for improved stiffness without an adverse effect on pedal-stance width.
Multiple spacers are included in order to fit both sizes meaning if you see a BB for sale that says 68/73 it means it will fit both but it includes a couple spacers in the box to make it fit the 68, those spacers (or fewer of them) are not needed if the shell is 73mm wide.
The critical thing is your spindle length: as long as you can set up the correct chainline you will be good to go. But, again, best never to mix standards if you can at all avoid it. If you are looking to replace a cottered crank setup with a square taper setup, then look no further than a cartridge bottom bracket.
However there are many different sizes and iterations according to manufacturer – BB30, PF30, BB90,PF86/92 to name but a few – and not all are interchangeable.
Are longer or shorter cranks better?
Crank length changes may help solve long-simmering aches and pains, but they’re most effective when you’re doing everything you should be doing to be strong on the bike. It changes gearing. If you do end up changing crankarm lengths, it will change your gearing as well.
Are shorter cranks better?
Crank length can be used as a tool to improve fit related issues impacting comfort, power, and aerodynamics. Moving to a shorter crank can improve: Comfort: A shorter crank length reduces range of motion at the knee (extension and flexion), hips, and low back.
Does crank length really matter?
‘As far as maximal sprint power and metabolic cost are concerned, crank length can be anywhere from 145mm to 195mm and it really doesn’t matter. ‘A longer crank is basically a lower gear ratio. It might allow you to climb better, but its effect is tiny compared to shifting up two sprockets on your cassette.