Bike Building: The Bottom Bracket and Chainset – Shimano Hollowtech

Ok, I’ve finally got moving on my aforementioned bike build, otherwise known as, “Turn a 4 year old Specialized Enduro S-Works frame into my dream steed!”

So far I’ve wasted many hours drooling over sexy looking forks, mean looking riser bars and sleek, sleek saddles (the Body Geometry Avatar is looking nice…) but really, where we have to begin is the bottom bracket. Without a drive-train the whole thing’s pointless so I decided to start here and make sure that works before continuing on with the more desirable (and expensive…) parts.

The first measurement for getting yourself a bottom bracket in the width. The housing on your frame for the bottom bracket will (hopefully) be in the range of around 165mm to 180mm wide. This is most easily measured on the bottom so get out your ruler and measure up.

Now, these days if you’re building from scratch you’ll be pleased to find that a few of the top chainset comes complete with bottom bracket. I was going for a shimano XT drivetrain and I had heard great things about the Shimano XT Hollowtech which shipped with a bottom bracket. I considered a few other options, particularly the Raceface Evolve, but eventually decided on the Hollowtech simply due to the sale on at the time. £90 for a high quality chainset and bottom bracket seemed to good a deal to miss.

I worried initially about fitting the bottom bracket as I’d heard some nasty things from friends and on the web about the number of different variables and problems that can creep in, but the only variable I could seem to find with the XT Hollowtech was the width, so I measured up my Enduro at 170mm and put in the order.

A couple of days later, the kit arrived and I ripped open the box with excitement. I was surprised to find how light the bottom bracket components were, and the lack of any bearings, but the brand new shinyness overcame any reservations.

Obviously at this point I discovered that I was a tool down and had to go to the bikeshop to buy a new fat spanner. There’s a special tool required for this that is of no use to anything but bottom brackets (that’s a money spinner and no mistake), and you can see the one I ended up buying here: bottom bracket tool.

My fears were utterly dismissed when I read through the instructions and got to work installing the bottom bracket. The process was a breeze – simply screw one side on with your brand new tool, placing your spacers as required, and then screw the other side on. The frame itself has threading to take the bottom bracket. Then, the rod protruding from the chainset itself slides into the hole that’s left and rotates freely inside. The axle is ribbed so that you can only put the opposite crank on the right way, and then you’re left with a plastic black cover that goes on the end of the axle. The last thing to do is simply tighten the nuts on the opposite crank to secure it to the axle (in a rotating order but clearly explained in the instructions) and you’re done!

I have to admit I was amazed at how easily this was acheived, for a component I believed was one of the most complicated to service and fit. Admittedly that’s probably due to it’s slightly mystical, hidden nature (I’d never even glimpsed an open bottom bracket before) and as is often the case, the reality was far less complicated than I’d imagined. I understand that older bottom brackets can be harder to work with, but the XT Hollowtech was as easy as… well, someone very easy.

So, bottom bracket on: next stop, the headset – my other ‘most feared’ component. Although, again that’s probably because it’s the other part I’ve never seen free of the frame. What makes me think this wont be so easy…

[tags]bottom bracket, specialized enduro, build build, chainset, shimano, hollowtech[/tags]


Colin started mountain biking in the early 2000s and has haphazardly, and with barely increasing skill, dragged his bike around the majority of Scotland's trail centres since then. Colin has oodles of hard earned experience in how NOT to do things - listen, be warned and don't repeat his mistakes...   More about Colin... Google+ Colin

2 Responses

  1. Shinno says:

    Hi there. . .
    I think you’re confusing o/a Axle Length with bb shell width here?
    “The housing on your frame for the bottom bracket will (hopefully) be in the range of around 165mm to 180mm wide.”
    Ain’t no way any frame has a 165mm wide bb width!
    I know what you’re on about (…but others might not)
    anyways, keep up the good work.

  2. Colin says:

    Hi Mark,

    You’re quite right, thanks for noticing that mistake! Yep, the bottom bracket shell will be more like 68mm or similar. I’ll get on a change that soon as!



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