Bottom bracket problem

scatty

Hi everybody
I am a new member living in chessington in the UK
I am trying to do up my old bike and wondered if any one can help me with my problem.
I have replaced all the parts with shimano alivio front and rear mech and a Un72 bottom bracket 122mm.
The original bottom bracket was a chinhaur with the markings CH52-68 122L BC137-24T.
The problem is the front mech will not reach over the large chainring, I think I may have fitted the wrong size bottom bracket?
The bike is an MTB giant boulder sports series and the original front mech was a suntour FD-TP16 and the rear mech a shimano altus and the crankset SR Suntour XR180.
Has anyone have any suggestons.

Many thanks
Richard

gclark8

If you are using big chain rings, 28/38/48 tooth, use a FD CO51 Shimano Front Derailleur. Your new FD may be for a 42 or 44 tooth big ring.

Most original cranksets use 122-124mm BB, I dont have the specs for Suntour. How far out from the BB face is the face of the inner chainring. You should be able to eyeball it.

__________________
Cheers,
George.

alfeng

Two things could be happening ...

But, let me just begin by saying that when I was updating the BB for one of my old square taper cranks several years ago, I found that a 113mm spindle was the "correct" length for MY steel frame bikes with Shimano's MTB cranks ...

Regardless, without knowing your specific crankset, there is a definite chance that it uses a Campagnolo/Sugino taper (someone told me, I believe incorrectly, that Sugino and, SunTour had changed to a Shimano compatible taper ... that was NOT my observation with a 4-arm 104BCD SunTour crankset that I worked with a few years ago -- it still used a Campy/Sugino taper) ... you can use a Shimano-compatible BB with a Campy (ouch) or Sugino-compatible ... so, you PROBABLY need a BB with a shorter spindle (either 113mm or 115mm) to continue to use your SunTour crank if you can only get a Shimano-compatible BB.

PLEASE NOTE: Some square-taper cranks use asymmetrical spindles; but, I think, most "newer" (for example, post-1998 Campy Record) square-taper cranks use symmetrical spindles! Place your OLD BB's fixed cup against the bottom bracket shell, and see whether the end of the OLD tapers are equally visible on both sides OR if it looks as though the end of the non-drive side taper is further away from the frame.

scatty

Hi

Thanks for your reply
I am using a shimano bottom bracket 122mm and a crankset with 42/32/22 teeth type shimano 2006 MC20.
The front mech is a shimano alivio and the distance from bottom bracket to the inner chainring is about 17mm I am still not sure which part is at fault, I still feel it may be the bottom bracket with the spindals out to far? or Have I used the wrong size front mech, I am new to changing bike parts.
Many thanks
Richard

scatty

Hi

Thanks for your reply

I forgot to mention I have replaced the old suntour crankset with a shimano 2006 MC20 WITH A 42/32/22 Teeth crankset, it has a square drive taper and my front mech is a shimano alivio with a rear mech also alivio.
I still feel it may be the bottom bracket at fault the spindals seem out to far, or it could be the front mech not compatible with the crankset?any more input would be most grateful thankyou.

Richard

alfeng

Presuming that your bike has a steel frame, then I would recommend a bottom bracket with a 113mm spindle (I guess, that is to say I would USE a 113mm because it what I know that I have on hand and which I know will work as I mentioned, previously) -- THAT's a long way of saying that the shop sold you the wrong size BB because they probably presumed you had a bike with an aluminum frame -- the longer spindles are often for bikes with aluminum frames, now ...

As noted, with SOME cranks, they need an asymmetrical spindle ... and, most 113mm (I think, but could be wrong) ARE asymmetrical.

If the asymmetrical spindle results in the left crank arm being further away from the frame's non-driveside stay, then you can insert a bottom bracket spacer (I have used an EXTRA cog spacer from an old cassette ... with the inside circumference notches removed BECAUSE none of my local shops carry bottom bracket spacers!)

It is OKAY to have a spindle that is a little shorter than the so-called ideal as long as the crankarms clear the chainstays ... I used the same 112.5mm XTR BB for both an Ultegra triple (which is supposed to use a 118mm BB) and a Dura Ace double; but, as you have discovered, if the bottom bracket spindle is too long, then the front derailleur cannot swing out far enough to move the chain onto the outer chainring.

I don't know what the "ideal" gap is between the crankarms and the stays, but if the inside face of the pedal end of your crankarms are more than 7mm away from the chainstays when they pass by, then the ~113mm spindle should work for you ...

gclark8

I think the Front derailleur is right for the crankset. "Alivio" with 42 tooth big ring will be OK.

However I cannot find the "MC20" crankset in the book, is that the full number?

Looking down on the bottom bracket from the top tube, 17mm clearance is too much, I normally see 1-3mm of spindle between the BB face and the inner chainring face. Turn the bike over and measure how much spindle is showing.

__________________
Cheers,
George.

scatty

Hi

Thank you for your prompt reply
After reading your comments I am getting more certain that I have fitted the wrong size bottom bracket.
You are correct the frame is steal and the crankset description is shimano 2006 FC-MC20.
What led me to fit 122mm bottom bracket came from the information on the old one I took out, it was a Chinhaur CH52-68 122L BC137-24T and I took the 122L to be the length? I a found out that this bracket was manufactured in taiwan.
So you feel confident that a shimano 113mm length bracket will be ok?.
This is the first time I have undertaken such a project and I very much welcome your comments
Once again many thanks
Richard

alfeng

Well, I feel confident that a 113mm will work for your situation because that is what I remember using with my square taper Shimano MTB cranks (570?/LX & 750?/XT) ... but, as they say, your results may vary.

BTW. If you bought the BB from a local shop, I presume they will take it back for an exchange ... if so, consider getting a slightly BETTER BB ... LX or XT quality ... it will cost you a little more, but it will be worth it because it will be SMOOTHER and weigh less -- a lot smoother, BTW.

gclark8

I just got off the Phone to Shimano.

The FC-MC20 is Alivio, now superseeded.
The BB Length is 113mm,
A 'high end' BB will be UN73, they suggest using the new UN26.

__________________
Cheers,
George.

scatty

Hi

I am delighted with your comments and I am getting more confident that you are correct in saying that a 113mm bracket will fit.
What led me astray was the information on the bracket I took out, it was a Chinhaur CH52-68 122L BC137-24T AND I thought the 122L was the length but then of course I renewed the crankset and relised this can have a big variation on spindall distance I think?
I am using a new crankset marked shimano 2006 FC-MC20 WITH A ALVIO FRONT MECH.
Once again than you for your comments
Richard