5

My FSA Mega Exo BB is about 2 years old and came on the bike when I bought it new. The right-side bearing is fine. However, the left-side bearing is gritty and resistant when installed in the frame, but when off the frame it turns freely.

The left-side bearing feels fine until it's just past "snug", at which point it feels mildly gritty. Once tightened to factory torque specs, it can still be turned with a finger, but requires notable effort and feels quite gritty.

Do I need to replace it? It seems weird that it'd need replacing after less than 2 years (I started noticing resistance about a year ago, but it was minor so I ignored it).

FYI - torque specifies 40-50Nm. This was my first time removing it so I don't know what it was tightened to before.

UPDATE - I took it to my LBS. LBS employee felt it and said it was notchy/pitted, instructing me to press a bit when testing with my thumb. I felt the notches too. I ended up purchasing a new BB for $100 (Wheels Inc. with replaceable cartridges). LBS employee said he was surprised it was in the condition it was. The new BB works much better (smooth, with minimal drag), even when tightened to specs. So I suppose problem solved for now. But, I'll leave this up for a few days in case anyone else has any insights or similar experience.

Here's a pic if it helps:enter image description here

7
  • 1
    2 years old could mean anything in the range of 0km-15.000km, so it is hard to tell if it is reasonable for it to be worn out. Other factors(riding conditions/rider torque) also affect what one can expect. Current shimano cartridge BBs can't be expected to last for more than 5000km in my experience just for comparison. I don't own a torque wrench, but it is my impression that cartridge BBs also seize if one applies the full 50-70Nm of torque on both ends.
    – WornChain
    Commented Apr 13 at 22:17
  • 1
    Can you estimate your mileage on this BB? And what kind of conditions are you riding in? Clean road riding is way different to mud-plugging/cyclocross.
    – Criggie
    Commented Apr 14 at 7:37
  • 2
    I've ridden 4000-4500km. ~70% pavement, 15% flat dirt/gravel, 15% bumpy/rocky terrain. Only about 5% of miles in muddy conditions. Not racing.
    – Darren C.
    Commented Apr 15 at 1:10
  • 2
    Mine lasted ~10Mm in cyclocross conditions. I’ve read that the precision of the frame can be very important for bottom bracket life. If the faces are not parallel and the threads not perfectly in line it’s bad for the bearings. A well equipped LBS should have a facing+tapping tool.
    – Michael
    Commented Apr 15 at 5:33
  • 1
    @DarrenC. 4000km+ with 15% dirt/gravel is more than enough to ruin a cartridge BB. It seems from your update that your old BB did no better. GL with the new one!
    – WornChain
    Commented Apr 16 at 1:02

1 Answer 1

2

I asked in comments if the bearing felt notchy when you turned it off the bike. You indicated in an update that this was true.

Consider all the physical aspects of the bottom bracket: how wide the BB shell is, how round the bore is, how off-center are the bores on each side (I think this is more a carbon bike thing), how flat are the edges of the shell, how far apart are the edges of the shell, etc. Each of these will have a nominal dimension, e.g. a standard British BB shell for road bikes should be 68mm wide.

If you took very accurate calipers and measured the actual shell widths of a sample of bikes, most likely no bike would be 68.00000mm. There will be some variance. Tolerance is the amount of allowed variation in a critical dimension. If the dimension is out of tolerance, things won't work optimally. If it is too far out, things will fail. But tighter tolerances are more expensive to menufacture to, as you might expect.

Obviously I don't have my hands on your bike. It is possible that the faces of the BB weren't parallel, like Mike mentioned in comments, or that the left bore wasn't round, or something similar (all these factors can be expressed as physical parameters with some tolerance). If the bearings get pinched at a certain point in the rotation, like if the shell wasn't round and it was too tight at some point, that could damage the bearings and I think it would account for the notchy feeling. If it were me, I'd have asked the bike shop to check that the shell is faced and that the bores are round, or to check for something else that could account for the damage. You already installed the BB - next time, I'd recommend giving the BB a spin after you install it, and if you detect notchiness, that corroborates that the problem is the shell's tolerances. You should definitely do further checks if the problem recurs. I will point out that you should be able to remove the cranks fairly easily and give the BB a spin with your fingers.

Say you had turned the bearing and it felt gritty but not notchy. I believe that's just a sign that the bearings or the races (the metal surfaces the bearings turn on) are worn. The only solution would be replace the BB (or in other places you can extract the bearing and replace it). If you were pressure washing your bike, then spraying a jet of water right at the bearings will get water and other stuff in past the rubber seals. Don't do that. Other than that, I wouldn't expect a bearing to wear out in that short a time if the shell was in tolerance.

In theory, items that are out of tolerance should be scrapped or repaired in production, then the production process corrected. If a carbon frame has a bore that's out of round when it arrives at the shop, a skilled shop can sand it or ream it with a specialized tool. Similar story for disc brake mounts, which must be completely flat. A frame seriously out of spec could be warrantied. The problem is what happens if it's slightly out of spec, or the tolerance issue is obscure and the customer service rep has no idea what you're talking about.

Anyway, now you know about tolerances. Mapdec cycles, a UK bike shop, has a YouTube channel, and they will sometimes post videos of them them diagnosing and possibly addressing tolerance issues. You can search for them if interested.

More specifically on bike washing. Do not do what Peter Sagan did in this video. He is standing facing the bike and he sprayed directly at the BB. Simon of GCN has a video demonstrating the effects of pressure washing on bearings. He's using a crankset mounted outside of the bike to a fixture. However, the way he's standing, he would be facing the bike from fore or aft, and the spray won't really affect the bearings. Spray from that direction.

8
  • If you were pressure washing your bike, then spraying a jet of water right at the bearings will get water and other stuff in past the rubber seals. Simply hitting the area directly with water from a garden hose is enough to get water into the bearing and wash out the lubricant. Pressure washing isn't necessary, although it does make it much more likely. Especially if the washing is, "I'm getting EVERY LAST BIT OF DIRT OUT OF EVERY LAST TINY NOOK AND CRANNY!!!" That will ruin bearings fast. Commented Apr 16 at 11:24
  • 1
    how off-center are the bores on each side (I think this is more a carbon bike thing) I can confirm it's also a steel bike thing :/
    – Paul H
    Commented Apr 16 at 18:37
  • @AndrewHenle if a garden hose is pushing past the seals in your cartridge bearings, that's a problem with the bearings. You should have to get real close with a pressure washer for any actual water ingress in any quality bearing.
    – Paul H
    Commented Apr 16 at 18:39
  • The last case might be a problem with just the seals, rather than the bearings as a whole.
    – Weiwen Ng
    Commented Apr 16 at 18:51
  • 1
    Folks, if there are disagreements, please take them up in chat. Thank you both for your understanding.
    – Weiwen Ng
    Commented Apr 16 at 20:22

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.