Bicycle Mechanics - Crank "Q-factor": is less always better?

Hello, all.

The old loose-ball bottom bracket on my Centurion has developed some pitting in one of the cups and seems to be on its last legs. I'm thinking about replacing it with a Shimano UN-73 cartrige style. Cranks are fairly old 105.

Anyway, looking at the BB, I'm struck by how much gap there is between the crank (and chainrings) and the frame, and how long the current BB spindle is. So I'm thinking I'll get a shorter BB and reduce my "Q-factor" a bit.

Is there any possible way this could be a mistake?

Thanks.

Hello, all.

The old loose-ball bottom bracket on my Centurion has developed some pitting in one of the cups and seems to be on its last legs. I'm thinking about replacing it with a Shimano UN-73 cartrige style. Cranks are fairly old 105.

Anyway, looking at the BB, I'm struck by how much gap there is between the crank (and chainrings) and the frame, and how long the current BB spindle is. So I'm thinking I'll get a shorter BB and reduce my "Q-factor" a bit.

Is there any possible way this could be a mistake?

Thanks.

Chainline. You may run out of adjustability on your fd near the small rings, or alternatively make for more lateral travel on the smallest cog in the back.

Chainline. You may run out of adjustability on your fd near the small rings, or alternatively make for more lateral travel on the smallest cog in the back.

Right! For details on this see: http://sheldonbrown.com/chainline

As to the "subject:" heading, less is not always better. Depends on your skeleton.

For most people, less is better, but for a significant minority, particularly those who naturally pedal splay-footed, wider can be better. That's the rationale behind the pedal spacers called "Kneesavers."

Sheldon "Diff'rent Strokes..." Brown

+-----------------------------------------------+
| Who has deceived thee as often as thyself? |
| -- Benjamin Franklin |
+-----------------------------------------------+

Yup. Your crank determines Q-factor. You want to get a bottom bracket that puts the chainrings at the proper distance from the bike's centerline (usually 43.5mm for a road double, 45mm for a road triple, 47.5mm for off-road triple) so that chainrings line up with the rear sprockets. This is more important than q-factor.

Okay, I want to do this really scientifically, so I'll go with 43mm. Which chainring should I measure to?

Or, if I do it by guess/golly, which chainrings should line up with which sprockets?


I think the 105 crank is not original to the bike; the original crank may have worked better with longer spindles.

I pedal with knees in and feet pointed straight ahead... haven't noticed any problems with my other bikes, from mtb and touring triples to road doubles.

If you've got a double crankset, you measure to the middle area between the two chainrings. If you've got a triple crank, chainline is measured to the middle of the three chainrings. You should measure from the centerline of the bike (so, out from the seat tube). Don't worry about measuring the sprockets - the measurements that I listed earlier are set up for 130mm-spaced road 8/9/10 speed cassette hubs, or 135mm-spaced off-road 8/9 speed cassette hubs.

You've got a road double, so the distance between the center of the seat tube and the center between the chainrings should be 43.5mm.

Duh.

I looked at the inside of the crank - it's an FC-1055.

This:
http://www.bikepro.com/products/bottom_brackets/bb_comptable.html
sez 113 mm.


and

http://www.sheldonbrown.com/harris//cranks.html
has a few for sale

http://www.sheldonbrown.com/bbsize.html#shimano
says 113-115. I guess the 115 might be for real fat tubes - or if you tend to use the smaller cassette gears.

http://www.cyclingforums.com/archive/index.php/t-184870.html

thread also says 113mm.

Here:
http://www.cyclingforums.com/archive/index.php/t-184870.html
a guy says 107mm and someone sets him straight that it's 113.


I actually have an old UN-52 113x68 sitting around on the shelf. It's not perfectly smooth. And I seem to have misplaced its crank bolts (my crank is currently held on by nuts), so I will probably have to go grovelling back to the LBS.

Well good, you figured it out. Go with the 113mm and be happy with it. Use your old UN-52 and get some new crank bolts. Good thing with those sealed-cartridge BB's is that you can ride them into the ground - doesn't matter much if they're not perfectly smooth.

Thanks for the help!

There are easier ways to adjust Q-factor. Some pedals are available with spindles of various lengths, I know Speedplays are. Spacer washers are available if you want a wider Q. Most cleats have some amount of adjustability available on the bottom of your shoes.

Al

Hot Tip: eyeballing the center of your seat tube for outboard measurements is quite tough, and you'll probably miss by a bit. Put some masking tape on there, use calipers to mark the center, and then measure to that. Or just caliper your seat tube, measure to the side of the tube, and do the math.

I'm using a 110mm Shimano UN-71 with a set of FC-1055 cranks. No problem for me.

Crank "Q-factor": is less always better? Generally yes, but of course there can be situations with weird riders or small frames or odd crank and chainring selections that require more space than a "normal" setup.