Monday, January 25, 2021

Getting Screwed

So I had a little difficulty with Blackbirb recently. The BB area felt a little loose while pedaling on a recent ride, and after I got home I dug into things a little bit and found two things.

First, I found that one of the pedals needed a service, so I serviced them both. This was not difficult.

Second, I found that one of the bottom bracket bearings was feeling a bit crunchy. This would pose more of a challenge.

The bottom bracket of a bike is basically a collection of parts that goes between the crank spindle (the shaft that goes through the bike frame, upon which the cranks are mounted, the latter easily identified as the part that the pedals are attached to) and the bicycle frame itself. Its consists of a pair of bearings, and whatever bits of hardware are involved in holding those bearings in place.

In the simplest case you have a BB30 bottom bracket, where the frame is reamed to a precise interference fit and a ball bearing is pressed in from either side, into which the crank spindle is inserted. More complicated systems use a threaded interface on the frame into which you thread a bearing carrier that carries the bearings (odd name for it, I know) and so on.

The PF30 style bottom bracket sits somewhere in the middle. It presses in like a BB30, but instead of pressing the bearing directly into the frame, the bearings are seated into a plastic bushing, and that bushing is pressed in. This makes it so that you can be slightly less precise when reaming out the frame, as the plastic will compress somewhat and make up for any minor irregularities.

But it is, nonetheless, a press-in style bottom bracket, which means I need three things.

Firstly, I need a whacking tool that's used to dislodge the old pressed-in bottom bracket from the frame. You can also get by with more makeshift options, but it's really best to use a tool so you don't bugger things up. I ordered this off Amazon and it arrived quickly.

Secondly, I need a new bottom bracket. It wouldn't do to remove the old one and then just put it right back in again, or just leave it out forever and never have a bottom bracket again. I ordered this off Amazon and it arrived quickly.

So I set to work knocking the old BB out and cleaning up the frame in the bottom bracket area. It was at this point I discovered why the BB had gotten all crunchy so quickly.

You see, the bottom bracket area of a bike tends to be a bit complicated these days. You not only have the bottom bracket itself to deal with, but also things like internally routed shifter cables, brake cables, dropper post cables, and so on. I don't have an internally routed dropper post on this bike (yet), and the brake cable wasn't causing any issues. What was causing an issue was the shifter cables, or, to be more precise, the little bracket that held a guide for the shifter cables.

On this particular bike, this bracket screws into a threaded insert moulded into the carbon of the frame. So far, so good. This insert, and the carbon of the frame, and the short bolt screwed into it, were for some reason designed a bit too long, and they interfered with the outer plastic shell of the bottom bracket. Oops. Someone wasn't paying attention in their CAD software.

So instead of completely redesigning this part of the bike, they apparently decided to just implement a manufacturing fix: they brought out the die grinder with an abrasive bit and sanded down the inside of the bolt, insert and carbon. This solved the problem of the interference, but caused a new problem: the bottom bracket area was now completely full of abrasive grit, shards of carbon fiber, and metal chips.

Apparently the manufacturer saw nothing wrong with this situation, and didn't even attempt to clean up that mess before pressing the bottom bracket into the bike, and some of this grit and detritus eventually made it into the BB bearings and fucked them right the hell up. Oh well.

I meant to post a picture of the inside of the frame here, but apparently I forgot to do that. I also forgot one other very important thing.

So, thirdly, I need a bottom bracket bearing press to install the new bottom bracket. I forgot to wait for this to arrive. But no worries, Amazon confidently stated that it would be arriving the next day.

The next day, the shipping information was updated to inform me that the package had somehow been irreparably damaged and was being returned for a refund before it even made it to me.

Oh dear.

No problem though, I'll order another one. And it's gonna take over a week to get here. Lovely.

So a week goes by, the package shows up and...

There's something not quite right. It's almost as if something is missing.

Ah, yes, the piece of threaded rod that the handles screw onto, and which forms the backbone of the bearing press. Lovely of them to forget to include it, but luckily it's only a $2.50 part at the Home Despot.

Anyway, with that finally taken care of I pressed the new bottom bracket in, completely forgetting to take a picture of the inside of the frame, as I mentioned previously, and reinstalled the crankset.

All buttoned up and ready to ride!

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