How to replace an asymmetrical bottom bracket?
#1
Senior Member
Thread Starter
Join Date: Mar 2011
Location: Seattle
Posts: 89
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Likes: 0
Liked 0 Times
in
0 Posts
How to replace an asymmetrical bottom bracket?
Hi. The old bottom bracket in my bicycle (mid 80's trek) has a spindle that's about 3mm longer on the drive side. My impression is that the Shimano UN-54s are the same length on both sides. I put in a replacement of the same overall length (118mm) and now my chainline is a little bit closer to the frame. So what size do I use the get the same chainline? Thanks.
Last edited by nicoth; 04-25-11 at 02:47 AM.
#2
Senior Member
Join Date: Aug 2010
Location: Washington
Posts: 472
Bikes: Serotta Davis Phinney, 1992 Serotta T Max,1984 Specialized Allez, Olmo, 1974 Strawberry,Redline bmx, ect.,
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Likes: 0
Liked 0 Times
in
0 Posts
I don't think it will hurt but the next size up is a 122.5 and then 127.5
#3
Senior Member
Thread Starter
Join Date: Mar 2011
Location: Seattle
Posts: 89
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Likes: 0
Liked 0 Times
in
0 Posts
Ok. I think maybe it's not as complicated as I suspected. A symmetrical spindle would increase the spacing equally on each side, so if I want +3mm on the drive side, I would want a +6mm spindle, which means 124mm total, right?
#4
Shop Wench
Join Date: Mar 2009
Location: California
Posts: 284
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Likes: 0
Liked 0 Times
in
0 Posts
What cranks do you have?
#5
Senior Member
Thread Starter
Join Date: Mar 2011
Location: Seattle
Posts: 89
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Likes: 0
Liked 0 Times
in
0 Posts
My cranks are are a mystery: SR SXC-200 according to old trek brochures. Sheldon Brown's BB database doesn't have any info on it.
#6
Banned
Priced upwards, Phil Wood, is adjustable in offset, get the length right
and the spindle is a press fit, so can pressed to change the offset.
+ the mounting rings are entirely in the shell, no fixed cup edge.
and the spindle is a press fit, so can pressed to change the offset.
+ the mounting rings are entirely in the shell, no fixed cup edge.
#7
Senior Member
Thread Starter
Join Date: Mar 2011
Location: Seattle
Posts: 89
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Likes: 0
Liked 0 Times
in
0 Posts
Sheldon says that a particular washer sort of thing can be used to add space; maybe I'll try that. Everyone says that Phil Woods BBs are good, but I'm not sure they're a good solution since they are worth about half as much as my bike.
Last edited by nicoth; 04-24-11 at 10:42 PM.
#8
Fred-ish
Join Date: Aug 2007
Location: Rhode Island
Posts: 1,800
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Likes: 0
Liked 1 Time
in
1 Post
No, he simply needs spindle 3mm longer. He had 57.5 nds/ 60.5 ds = 118. He needs the long side of 60.5 x 2 = 121. FWIW, 122.5 should fit and shift just fine.
#10
Fred-ish
Join Date: Aug 2007
Location: Rhode Island
Posts: 1,800
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Likes: 0
Liked 1 Time
in
1 Post
Wrong. He needs to add 1.5mm to the each side relative to his symmetrical 118 that he bought. His asymmetric 118mm spindle is 3mm longer on the drive side. To achieve symmetry, he just needs 3mm on the non drive side.
Last edited by rogerstg; 04-25-11 at 12:14 PM.
#11
Senior Member
#12
afraid of whales
Join Date: Apr 2009
Location: Front Range, CO
Posts: 4,306
Mentioned: 6 Post(s)
Tagged: 0 Thread(s)
Quoted: 347 Post(s)
Likes: 0
Liked 5 Times
in
5 Posts
If he goes with 121, his drive side will be 1.5mm closer than his stock asymetric, not enough to matter most of the time, but without pictures and not knowing how far the old crank will install in the spindle, I'd go for a little extra to be safe.
#13
Senior Member
#14
Senior Member
Thread Starter
Join Date: Mar 2011
Location: Seattle
Posts: 89
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Likes: 0
Liked 0 Times
in
0 Posts
My cranks have plenty of room, by the way; I'm just interested in getting optimal chainline. 121 mm sounds right. My error originally was that I thought the symmetrical spindle would be 3mm shorter on the drive side, when it would actually only be 1.5 mm shorter.
#15
Fred-ish
Join Date: Aug 2007
Location: Rhode Island
Posts: 1,800
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Likes: 0
Liked 1 Time
in
1 Post
If that's what you want rather than the same chainline (as first posted) I'd work toward narrower rather than wider on a road bike. On the road, riders typically spend most time in the outer chain rings. The problem you may encounter would be that the FD does not retract enough to drop the chain onto the inner most chain ring. FWIW, 1.5mm is not much so I'd try to set up what you bought.
Thread
Thread Starter
Forum
Replies
Last Post
gatto karma
Bicycle Mechanics
9
02-15-17 04:04 AM