Hollowtech II: chain line and Q-factor
Hollowtech II is a type of bicycle bottom bracket (BB) manufactured by Shimano. They are very easy to remove and replace, which meant it was also very easy to alter the chain line on my Ridgeback touring bike. It was simply a matter of fitting some spacers in different positions to those indicated in Shimano's instructions.
Hollowtech II BBs have become very popular despite a traditional preference for the square taper type. Hollowtech has external bearings with smaller balls. They are sometimes alleged to be prone to premature failure due to the reduced ball size. I haven't experienced this, and I prefer the external bearings because they are further from the bike's centreline (more distance between the bearings – a good thing) and the circumference of the bearing surface is greater (must be a good thing too). True, you cannot maintain a Hollowtech BB – replacing the balls and greasing etc – but they are cheap, and easy to replace if they fail. Anyway, as mentioned, mine haven't failed so far after many thousands of miles.
My Ridgeback Panorama (fitted with Hollowtech II, although later models have apparently reverted to square taper) had two spacers – plastic rings – between the right-side bearing and the BB shell, and one on the left – pic above. This is how they should be, according to Shimano's tech docs, which also refer to four spacers: 0.7 mm, 1.8 mm, and 2.5 mm x 2. There are actually three: all 2.5 mm.
I removed the crankset when upgrading from Deore to Deore XT, supplied with a Hollowtech II BB (I've used the old Hollowtech II BB on another bike to replace the square taper it came with). The only tools required are Allen key, a torque wrench (preferably), a special spanner for the external bearings – Shimano refers to them as 'adapters' for some reason – and a small plastic ring with teeth on to turn the preload cap. In less than two minutes the crankset is off and you can clean inside the BB shell with a rag (the browny colour is copper grease I put on after).
Chain line and Q-factor
Chain line: (i) the direction the chain runs from a chain ring to a sprocket and (ii) the distance of the chain from the bike's centerline. For maximum efficiency and minimum wear it should be parallel to the centerline. With a derailleur geared bike, of course, this won't always happen – so as parallel as possible by gear selection and also by setup. The distance from centre depends on various factors: number of chain rings, thickness of seat tube where the front mech is mounted, and the type of crankset and BB.
Q-factor, also known as tread factor, is the distance between the outside of the cranks at the pedals. It measures 180 mm on my Ridgeback's Deore chainset – on the wide side and would be less with a road crankset. There's nothing to be done about it with a Hollowtech II BB because the spindle that goes through is part of the right crank and the left crank sits in a pre-determined position on the other end of the spindle. So the Q-factor is fixed. To move the chain line inboard or to centre the middle chain ring with the middle sprocket, the only scope seems to be placement of the three spacers.
Mrs Taylor's Scott Sportster Solution also has a Hollowtech II BB and is fitted with two spacers on the right. Like the Ridgebacks the chain line is too far outboard so the two spacers should be on the left! It was my gear ratio analysis that led up to this conclusion. It's only a small modification but it gives a better placed big chain ring and less time wearing out the smaller ones. And the pedals are centralised.
Added the morning after... the chain line distance from bike centreline can be a question of preference and type of cycling as well as the setup one is faced with – Q-factor etc. An MTB might benefit from the middle chain ring further outboard to allow the smaller chain rings to engage more sprockets, and vice versa with a road bike ridden in flat terrain where you'd want to maximise the use of the bigger chain rings. Either way, it seems worth checking the setup (mine are all 27-speed; I've no experience of 30-speed or double chain ring bikes).
How to remove a Hollowtech II crankset
- Unscrew the two left crank arm fixing bolts using a 5 mm Allen key (alternately in stages)
- Unscrew the preload cap using the special TL-FC16 plastic ring
- Remove the left crank arm after lifting the little black stopper plate
- Lift the chain off the chainwheel and rest it on the BB shell (protect with rag)
- Slide out the right crank arm, chainwheel and spindle (all one unit)
- Unscrew/remove the Hollowtech bearings and spacers (anticlockwise on the left, clockwise on the right) using the special spanner; the cable guide screw may need unscrewing to allow the plastic inner cover to slide out freely
Refitting the crankset is more or less the same sequence in reverse but the use of a torque wrench is desirable to achieve 12-14 N-m on the left crank arm fixing bolts. The preload cap is tightened first but not too much or the bearings will be over-compressed and the cranks won't turn freely. If in any doubt read Shimano's technical service instructions but it's all pretty straightforward and much easier IMO than with a square taper bottom bracket.