Tandem Bottom Brackets

Jim’s Tech Talk

By Jim Langley

This week’s topic comes from RBR subscriber Kevin Meyers, who asked:

“I have an 11-year-old Co-Motion Robusta tandem. I have used it for racing and training. Should the bottom brackets be replaced after so many miles? So many years?”

Because they’re such hard-working components, whether it’s time to replace bottom brackets in any road bike, be it a single or tandem, is a great question. So I’m sharing what I told Kevin.

Bottom Brackets Let the Pedals Go Round

If you’re unfamiliar with the term bottom bracket (commonly called the “BB”), it’s the bike part that the crankset is attached to. Because it’s mostly contained inside the frame it’s hard to see. 

So, here’s a photo pointing out one of the bottom brackets on a 2020 Co-Motion Robusta so that you can get an idea what it looks like and does.

The cups of the BB hold the bearings. And the bearings are what allow the crankset to turn when you’re pedaling. On a tandem bicycle, there are two cranksets and two BBs. And because two cyclists are aboard, there’s twice the load cranking on the BBs.

Takes a Licking and Hopefully Won’t Start Ticking

But, all BBs have to withstand a lot of abuse. Which is why BB noises such as ticks, clicks, creaks and clunks are among the most common bicycle glitch. 

They result from all kinds of things that go wrong from the massive force of cyclists stomping with all their power (and all their weight when standing) on first the right and then the left pedal.

Over time, and over countless revolutions (consider that in only an hour you’ll pass 5,000 revs) this force can loosen things, wear out and even break parts. So, it’s a good idea to listen for any noises and fix what causes them if they occur. 

Check Your BB Regularly

Plus, even if your BB stays silent, it’s a good practice to do a couple of routine checks at least yearly. If you ride in the snow, rain and dirt, which all attack BBs, you should check it at least twice a year.

Checking BB Health

Test 1 – feel for play

The first test is easy to perform. It’s a check to determine if any side-to-side play has developed in the bottom bracket. If it has, it could mean that it’s wearing out, or in a newer BB, it might only mean something has loosened and needs tightening. In an old bottom bracket, play can mean that the lubrication inside the bottom bracket has dried up and is no longer working.

To check for play, stand beside the bike and rotate the crank so that the pedal nearest to you is at 6 o’clock, which will put the away pedal at 12. Now, hold the crankarm closest to you with one hand, reach through the frame and hold the other arm with your other hand. 

Holding the crankarms (not the pedals) firmly, push and pull to try to rock the crank inside the BB. If there’s play, you will feel movement and possibly a little clunk. If you find it hard to hold both crankarms, you can alternatively try pushing and pulling the crankarm positioned at 12 o’clock toward and away from the bike frame while holding the frame with your other hand.

In a properly fastened BB, you won’t feel any play when you do this test.

Test 2 – Feel for lubrication

The other test is to check how the crank(s) spins. In a nice new BB, the crank will spin smoothly with a slight resistance from the lubrication (typically grease) inside the BB bearings.

The way to feel for this is to lift the chain off the chainring and move it away from the crank so that you can turn the crank freely (no drag or rubbing from the chain). When you turn the crank, hold it gently and turn slowly so that you can feel how much resistance there is in the bearings inside the BB. If there’s enough grease in the bearings, the grease will slightly resist turning.

When you do these two checks, you’re hoping to find no play in the BB and also a nice-turning crank without binding or roughness.

If you have play, binding, roughness or worse (like grinding from corroded bearings), you’ll want to remove the cranks and investigate further and repair problems. Because you don’t want to risk a failure out on the road. But, also because even if the failing BB allows you to keep riding, it’s slowing you down and making you less efficient whether or not you can feel it.

11 years on the same BBs?

Back to Kevin’s question. Since he said he’s been training and racing on his BBs for 11 years, I assumed he hasn’t had any noticeable trouble yet. Which is pretty amazing considering how tandems torture bike parts.

But, at the very least, even if there aren’t any telltale signs, I told Kevin he should try to determine if the lubrication inside the BBs is still doing its job. The crank spin test is how to feel for this. But, you can’t always feel it through the crank.

So, an even better test is to turn the bearings with your finger to feel it. To do this means accessing the BB bearings, which means removing both cranks.

When in doubt, go to the source

The fact that Kevin hasn’t had to think about his BBs for 11 years could mean that Co-Motion sourced some incredible BBs for their 2009 Robusta tandem.

But, I couldn’t find a mention of the BBs they’re using on their bikes today or when I searched for specs on their 2009 model that Kevin owns either. So, I told Kevin that he should contact Co-Motion and ask them his question.

There’s no one who knows more about his Robusta than Co-Motion. So if he needs special tools or instructions for checking his BB bearings, they’ll let him know. And, if indeed, I’m right and he should replace them, Co-Motion will surely know what he should buy and how to install them correctly, too.

Here’s Co-Motion’s Contact Us webpage: https://co-motion.com/pages/contact-us. And here’s their full line-up in case you’re in the market: https://co-motion.com/collections/tandem-bikes.

Feel free to weigh in with your tandem BB advice for Kevin by leaving a comment.