JBK Posted June 8, 2016 Posted June 8, 2016 I need some assistance wrt the possible cause for all three of my chainrings not rotating 'true' - as you turn the crank all 3 rings are visibly 'bent' with the greatest movement visible on the large ring and the less on the smallest ring. I guess that the large chain ring fluctuates by about 3-4mm as you rotate 360°. Clearly is not my chainrings that are all bent, so where is the problem ? The crank is tight and both the 8mm & 10mm nuts are very tight. There is no movement of the crank arm sideways. Perhaps it's the bracket from the crank that holds all 3 chainrings that is bent ? Sent from my SM-G925F using Tapatalk
popcorn_skollie Posted June 8, 2016 Posted June 8, 2016 I need some assistance wrt the possible cause for all three of my chainrings not rotating 'true' - as you turn the crank all 3 rings are visibly 'bent' with the greatest movement visible on the large ring and the less on the smallest ring. I guess that the large chain ring fluctuates by about 3-4mm as you rotate 360°. Clearly is not my chainrings that are all bent, so where is the problem ? The crank is tight and both the 8mm & 10mm nuts are very tight. There is no movement of the crank arm sideways. Perhaps it's the bracket from the crank that holds all 3 chainrings that is bent ? Sent from my SM-G925F using Tapatalk Sounds like you might have bent the drive side crank arm. A hard enough pedal strike could do this. Happened o me.
NukeproofPeanut Posted June 8, 2016 Posted June 8, 2016 I've had this happen with a tapered BB, where the crank arm has dug into it at an angle from a fall. This makes the rings wobble as you pedal.
JBK Posted June 8, 2016 Author Posted June 8, 2016 So... New BB or new Crank..? Sent from my SM-G900F using Tapatalk
popcorn_skollie Posted June 8, 2016 Posted June 8, 2016 If its a square taper bb it could be either or both. I suggest replacing the bb first. SInce its cheaper and at some point would need to be replaced anyways. If that doesn't solve it, then its time to look for a new crank. What you could try though (this worked for me) is to try to bend the crank arm back to where it should be. I did this by putting the arm wrapped in a lappie in a vice and clamped it bit by bit. Then after it bent a little I fitting it to the bike and rode it to check till i was satisfied. Pain staking millimetre by millemetre process because if you bend it too far the other way its stuffed. It saved me from buying a new crank in the end. Worked perfectly for months after I did this before I got a good deal on a replacement. It was a shitty bike and a shitty crank. So I didnt have much to lose. Disclaimer: I take no responsibility should my advice go south. But...it might just help you.Good luck.
droo Posted June 8, 2016 Posted June 8, 2016 What kind of crank is it? If it's SRAM and has a removable spider, check that the spider is seated properly on the crank. Otherwise, something's bent.
raptor-22 Posted June 8, 2016 Posted June 8, 2016 Ok first question so we take it from the top. What is the brand and model of crank in question?
droo Posted June 8, 2016 Posted June 8, 2016 Raceface Sent from my SM-G900F using Tapatalk Eish. My money says it's been ridden with the bolt loose at some point in the past and the spline's worn. I binned one a few years back with a similar issue that couldn't be fixed.
raptor-22 Posted June 8, 2016 Posted June 8, 2016 X-type bb. Ok so its either a mounting tab that's bent or the spline the crank connects to the bb with is damaged inside the crank. is the crank slightly loose?Hold the pedal and try to wobble the crank. Is there a little play? Is that play at crank or the axle?
JBK Posted June 8, 2016 Author Posted June 8, 2016 Eish. My money says it's been ridden with the bolt loose at some point in the past and the spline's worn. I binned one a few years back with a similar issue that couldn't be fixed.I have had issues before with the bolts working themselves loose but I've always tightened again almost immediately. There is no movement if you try and move the crank sideways but you might be on the money with a worn spline... [emoji30] Sent from my SM-G900F using Tapatalk
JBK Posted June 8, 2016 Author Posted June 8, 2016 X-type bb. Ok so its either a mounting tab that's bent or the spline the crank connects to the bb with is damaged inside the crank. is the crank slightly loose?Hold the pedal and try to wobble the crank. Is there a little play? Is that play at crank or the axle?No play at all... Sent from my SM-G900F using Tapatalk
Skinnyone Posted June 8, 2016 Posted June 8, 2016 Given the age of that crank (circa 2006/7?) and the fact that the bolt was notorious for working loose I would say bin the crank and fit a new(er) Shimano/Sram system. I had cranks like that and they were fine until the bolt worked loose on a ride...after that they would come loose almost every ride and would never sit "true".
JBK Posted June 8, 2016 Author Posted June 8, 2016 Given the age of that crank (circa 2006/7?) and the fact that the bolt was notorious for working loose I would say bin the crank and fit a new(er) Shimano/Sram system. I had cranks like that and they were fine until the bolt worked loose on a ride...after that they would come loose almost every ride and would never sit "true".That's so funny because it's so true - exactly what happened with me. Was perfect for about 7 years and then didn't stop working loose. Used some locktite end of last year and that stopped it but the 'skewness' is getting worse... Sent from my SM-G925F using Tapatalk
divernick Posted June 8, 2016 Posted June 8, 2016 Upgrade to a newer setup with external BB - deals can be had for less than R1k
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