UN72 Shimano XT Bottom Bracket

...in my case the first UN72 I fitted was to replace an old bottom bracket with worn bearings which made my bike sound like someone had filled the frame with a handful of gravel when I pedalled it. The difference in this case was huge. If you have a bike that you're thinking of upgrading, provided that the current bottom bracket is causing you no problems, changing to a UN72 is going to bring you no benefit, bottom brackets not being the sort of thing ...
...of the assembly. As the UN72 is sealed then it's pretty much toast once the grease has been washed out, you can't strip and regrease it. Provided you don't commit the cardinal sin of pointing a hosepipe straight at it though, this shouldn't happen easily.

I've got no complaints about this part whatsoever. It's well enough sealed to last for ages and it's long life means the fact that you can't maintain it isn't an issue to me. It may ... more

Before I go and review a bottom bracket, it might be worthwhile if I explained to everyone what exactly a bottom bracket is. Let's start at the pedals. The pedals are attached to the cranks, the cranks are then attached to a spindle that goes through the bike frame. This spindle and the bearings that surround it form the bottom bracket. As you may have guessed, it's not a very exciting thing to write about, so my apologies in advance for a very dull opinion.

There's two times that anyone is likely to come into contact with a bottom bracket, if you're building up your bike from parts, or if there's a nasty grindy noise when you pedal and you need a new bottom bracket. In this case, many people will take their bike to a shop to get it looked at as although changing a bottom bracket is easy enough, it requires a few specialised tools which are expensive if you don't plan on doing regular maintainance on a bike. Replacing one involves removing the bolts that hold the cranks on with an allen key (which should be part of every rider's toolkit) and removing the cranks with a crank extractor, the first specialised tool which is available from any good bikeshop at around £10. Having removed both cranks, we need to take the old bottom bracket out using either a bottom bracket tool (£8) or a set of pin spanners (£10) depending on what sort of bottom bracket you have installed already. With an outlay of around £20 in tools just to get the old one out, it's obvious why many people have this done at a bike shop when compared to the £20-£25 for the part itself.

Once the old one is out, you'll need to put the new one in. They're available in different sizes, if you don't know, take the old one to the bike shop with you. If you're having the job done for you, then the shop will figure it out for themselves. The new one will be a silver cylinder (as opposed to the brownish rusty cylinder that's just come out of your bike) with an axle poking out of both ends. This then screws into your frame with a little dab of grease on the threads using the aforementioned bottom bracket tool. Cranks sit on the end of the bottom bracket and the bolts that you took out earlier can be screwed back in. All in all, a fairly simple (but messy) job, although the fact that you have to spend more on tools to do it than the cost of parts may well force many peoples' hands in deciding to get a bike shop to do it.

So, now that it's fitted, what difference are you going to notice? Well, in my case the first UN72 I fitted was to replace an old bottom bracket with worn bearings which made my bike sound like someone had filled the frame with a handful of gravel when I pedalled it. The difference in this case was huge. If you have a bike that you're thinking of upgrading, provided that the current bottom bracket is causing you no problems, changing to a UN72 is going to bring you no benefit, bottom brackets not being the sort of thing that can make a huge performance difference to your bike.

In everyday use, I've had these bottom brackets last for ages and ages. I've seen five year old UN72s still going strong. Mine has been through hell and (literally) high water with me. It's been through all sorts of mud and we've been in plenty of rivers together and it's not dead yet. The bearings haven't developed any play and it's not making any nasty noises. High pressure water can penetrate the seals though, which will wash the grease out of the assembly. As the UN72 is sealed then it's pretty much toast once the grease has been washed out, you can't strip and regrease it. Provided you don't commit the cardinal sin of pointing a hosepipe straight at it though, this shouldn't happen easily.

I've got no complaints about this part whatsoever. It's well enough sealed to last for ages and it's long life means the fact that you can't maintain it isn't an issue to me. It may not be very exciting, but your bike won't go without a bottom bracket, and the UN72 is a good 'un. 

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