Bent bottom bracket?

duckson

Got a new MTB few days ago, first bike in 15 yrs since my previous one got nicked and so I went for a 12mile ride today to get used to it (i was knackered!).
I noticed it was hard work getting onto the inner chainring so following parktools guide i reset the front derailleur as it was mounted to high above the outer chainring (i could get my finger under it rather than a 1 pence piece).
Anyway while doing this setup i noticed when spinning the chain backwards and trying to get the L and H settings right the chain would move in and out quite alot. I checked the chainrings themselves (looking from the top while pedalling with my hand) and they dont seem to spin true (almost like a car steel wheels plastic cover not clipped on properly).

Is this likely to be a bent bottom bracket or something worse? Is it likely it could be fixed (remounted) or will it require replacing?

Any help/advice much appreciated...i understand that taking it back to the shop is the best solution but i want to go armed with some facts in the face of trying to be fobbed off (you never know!).

nicklouse

there is some movement in he rings as this is to aid shifting. so the movement you see could be normal.

But if you are not happy pop back to the shop and ask. (or look at some other bikes and see if it is the same).

duckson

If the slight movement is "normal" then it should be fine then....my wifes 10yr old £70 at the time MTB doesnt do it though, then again if its cheap old trash then maybe it wouldnt?

nicklouse

10 years ago they did not think about shaping the rings.

if the bb axle was bent you should feel it binding.

duckson

The bike feels perfectly fine to pedal. :?

duckson

Anyone anymore more input? Could someone check their bike, look at the front chainrings/chain and rotate the cranks backwards and observe the movement of the rings/chain and tell me if they spin straight or if they "wobble" slightly?

(Chap at work who is into MTB wasnt sure it was right and is going to lend me some tools to take off the crank arm and rings to see if they are straight but if its supposed to be like this i dont want to spend hours "fixing" it only to find theres nothing wrong in the first place!)

nicklouse

what is the bb and crank set?

duckson

Truvativ is the crankset, not sure on BB....orwould it be the same?

nicklouse

which truvative crank set? could be any of many.

supersonic

Could be a bent spider. Only true way to find out is fit a new crank: if it still oscillates badly, then the BB axle is most likely bent. If not, then the old chainset was at fault.

But as nick says, many cranks do it to a slight extent. Without looking at yours in person, I can't really judge.

duckson

Hmmm Its a Truvatic Xflow which has apparently non-removable chainrings.

http://www.sram.com/_media/techdocs/200 ... m-e-r2.pdf

Looks like i'm stuffed taking the rings off to check if they are bent.

I'll try and take a video of it on my digi camera later to see if you can shed any light on if it looks ok or not.

supersonic

Taking the rings off wouldn't show anything - if one was bent, they it would move differently to the others. It's the bit they are attathced to that could be the prob.

Pics would be good!

nicklouse

so they are eith power spline or square taper fitting.

if you can see nothing really wrong try tightening the crank bolts.

But as you say it works OK just use them.

duckson

Scratch what i said, its a Truvativ Isoflow not Xflow (checked the bike, the sales ad said Xflow)....so the rings are removable.

I'll get a small video sorted out if my digi cam is playing ball.

nicklouse

still the same bb options though. check the bolts are tight.

duckson

Here we go:-

http://www.duckworth.myzen.co.uk/chainring.AVI

If you look at the outer chainring on the right of the video and use the floor as a light background you can see the slight movement.

So is this normal?

nicklouse

i would still be heaving on the bolt. but before that i would raise the issue with the supplying shop as that is a bit too much movement and is not normal.

duckson

Cheers. Which bolt exactly? The main hex bolt in the middle of the crank?
It cant be the rings or i wouldnt of thought it likely as all 3 rings dont spin 100% true.

nicklouse

yep as if you look at the vid the whole cranks set is moving together (in and out), but the bb sounds free and you say it does not bind.

duckson

Sorry added this bit to my last post after your reply:-
"It cant be the rings or i wouldnt of thought it likely as all 3 rings dont spin 100% true."

Sorry being thick here but what do you mean by "bind" exactly?
everything seems to move freely and i cant feel/tell anything thats odd.

nicklouse

bind = not move freely.

they do all seem to move together. or at least the outer and the trouser protector do.

supersonic

Just seen it, and it seems the trouser guard is moving a little more than the rings, whicg they often do as are flimsy pastic and rarely straight. Can see a wobble on the outer, but doesn't seem too bad. Remember not to use extreme gear combos like small/small cog. as the chain goes at a steep angle accross the derailer.

duckson

LOL ok...so 1 vote for not to bad and 1 vote for not right.
Yes the guard does maybe make it look worse than it is.
Guess i'll see how it goes, see if the 8mm hex bolt is not 100% tight but if i'm honest i generally keep it on the middle ring at the front and it seems ok clearance wise.

supersonic

Lol, I can see it is a little out, but not by too much ;-)

duckson

Well i've borrowed a crank extraction tool so i'll take it off and have a look at the chainrings, i do think either they might be mounted on the crank spider incorrectly as where the outer ring meets the crank spider arms some are flat and on 1 (or maybe 2 but less so on that one) there is a 1-2mm lip or that the crank spider is bent from a fault in manufacturing as the bike hasnt been ridden in anger so it hasnt come from riding.

duckson

Oh well turns out its the wrong extraction tool! I was lent a CCP-4 but my "powerspline" crank needs a CCP-2 because of the small hole in the spindle.
Dont really wanna buy a the extraction tool, might take it to the LBS if its bothering me still and see what they reckon, i'm thinking its a slightly badly manufactured crank arm.