Mountain Biking - How do you fix BB Flexing?
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02-28-03, 05:47 AM
Hi, My bike is suffering from BB Flexing. If you dont know what that is, it is when pedal hard and you shift to the higher gears (5, 6, 7 etc.) the higher gears sound like theyre groaning and they tend to "chirp". I just wanted to know how to fix this "BB Flexing" problem.
P.S. My bike is a GT Avalanche 3.0, if that makes any difference.
02-28-03, 09:04 AM
you need a new BB
02-28-03, 09:56 AM
I agree, manufacturer's usually spec very cheap bottom brackets in order ot keep the cost of bikes down. A new bb should help. Stiffer cranks, and a new bb would be significantly better. If you go the new crank route, get either a Shimano Splined, or an ISIS compatible crank. The splined bb helps reduce a lot of flex.
Regardless, you are going to get some flex no matter what cranks you use, but less expensive cranks are more noticeable. Whay kind are currently on your bike?
02-28-03, 10:49 AM
I'm unclear on the problem.
In higher gears, under load, the rear cassette makes noise?
This could be mnay things other than the bottom bracket. Worn componentry springs to mind, some sort of frame issue is another. Generally I've seen bottom bracket flex will manifest in terms of front shifting issues.
02-28-03, 12:57 PM
Don't know if this could be the problem, but on my road bike (853 Steel), when I'm in my big ring, small cog combo (chain in most outside position) and honkering down, the chainring/chain will rub on the outside plate of the front derailleur when my drive side pedal is in the 6 o'clock position (max torque to that side). Is this what you're experiencing? On my bike, it's not a chirping, but a definate scraping. I've played around with the front derailleur limit screw, but if I adjust it too far, I drop the chain off the big chainring. That, and I don't do a whole lot of honkering down!
If this is similar to what you experience, stiffer cranks will help. I've got Ultegra on mine, but I'm also 230 lbs on a steel bike!
Might not even be your BB, frames can flex a LOT in the BB area.
02-28-03, 05:17 PM
I'm not defiantely sure its the BB problem. It could be a variety of things as one of the replies said. BB Flexing just sounds like the most likely to my problem. And when I get my new BB, how long will it take to flex again? (My crank is a Suntour CW-128)
Thanx a heap!
03-01-03, 12:17 PM
If you get a good bb and crank and aren't super hard on them they could go for a number of years before any problems arrise :)
03-01-03, 12:29 PM
Square Taper BB, Octaling, or Splined?
Maelstrom is correct, a good BB will last you a long time.
03-03-03, 08:14 AM
The Suntour is pretty much a cheap crankset. If you upgraded to Shimano XT or RaceFace Prodigy or Truvative ??, it would make a significant improvement. When you do get new cranks, be sure to get the splined bb as well. With Shimano it's called splined, with others it's ISIS compatible! Same idea, different system!
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