jleiningj

Hello everyone -

I am new here (and yes, I have used the search function so try not to be too hard on me my first time!). I recently embarked on the (life altering) quest to build a road bike. I had ridden bikes a lot in the past, but this is the first time I have really gotten my hands dirty and done any DIY work instead of just taking it to the shop. There have been some hiccups along the way, but countless trips to my LBS, several online orders (seemingly a part at a time), many different bike forums, numerous youtube videos, and even a night in the hospital have me to where I am now: still having a few problems getting everything just right.

The problem I am most frustrated with is that the chain rubs on the inside of the larger chainring on the bottom 3 cassette cogs. Now I have looked into it, but my problem seems to be a bit different then what I tend to read. All my components came from the same 105 groupset - and while they were used, they seemed to work fine on the last bike. This makes me believe it is something I am doing wrong with the build, and while that is entirely possible, I feel like I've been pretty careful and done a solid job so far. I am running a 52/39T with a 12-27 cassette. Its a GXP BB. And the frame I am building up is a 2002 Fuji Team.

Now I have had a lot of fun building it and I enjoy working on the bike - so I would like to avoid having to take it in, but I was hoping for any sort of advice.

Anyway - I appreciate the help guys. And be careful while doing bike maintenance!

Best,
Jake

Al1943

Yes, this sounds like a chainline problem.
How long is the BB spindle?
What is the dropout spacing on the rear axle, 130, 135?
What is the rear hub, make, model?
How many cogs on the cassette?
Sometimes a bottom bracket spacer can cure this problem.

FBinNY 

You're describing something I call "shadowing" because it's sort of like how a short building can end up in the shadow of a tall building as the sun arcs past. It isn't rare for it to happen with the outermost sprocket or even the two outermost. BUt 3 is a bit muh and I suspect your chainline may be off.

Start by doing a quick check.

You need a straight edge 18" long or so (if you posted where you lived, I might have done this metric to accommodate you). Place it against the outer face of the outer ring, and carry it back to the cassette. It should come to the cassette about 7mm or 1-1/2 sprockets outside of the center. (allows for the difference between the outside of the ring, and the center between the sprockets).

I suspect you'll find that your chainrings are more inboard than that, so the angle of the chain is shifted to the outside.

How you change the chainline depends on your crankset and BB, it could be as easy as moving a spacer, or it could need longer spindle. It's also possible that the crank is in the right place, and the rear triangle is offset to the left. But start with first seeing how the chainline is before worrying about why.

zukahn1 

Also it seems a given but have your dropout hanger alignment checked a couple of mm off could result in your problems.

FBinNY 

This would apply if the chain were rubbing on the bottom of the rings, but the RD has no control on the top chord's straight run from cassette to chainring.

BTW- we rarely hear of lower chord rub because most hangers are bent in (bike falling over) and this would solve rather than cause the issue.

hueyhoolihan

because it's a 105 component group, it means you have cup and cone bearings in the rear hub. it's possible to inadvertently minimize, when reassembling the hub and retruing, dishing the hub, (it that has even been done) the distance between the smallest cog and the driveside dropout, leaving enough clearance for the chain, but causing an excessively acute angle between the smallest chainring and the cassette, especially when using the smaller cogs on the cassette.

as alluded to in previous posts, if the BB is not the same as the one you were using when working properly, i would check the width of the crank when mounted to the new one as compared to the previous one. the new one may be wider, which would cause the problem. also check that no additional spacers were added to the rear wheel between the freehub and the cassette.

if the chainrings were removed check that they, especially the large chainring, were reattached in the proper orientation. some chainrings, i have noticed, have an intentional dish, which could theoretically, anyway, cause the problem if mounted reversed (it would probably interfere at all times, but maybe not).

it can also happen if the size of the big chainring gets bigger when retro fitting. or the small chainring gets smaller, i suppose.

if all else fails, a small spacer between the driveside GXP cup and the frame could be the answer. i'm not very familiar with external BB's so i could be full of it on this one. good luck.

fietsbob

Derailleur drivetrain Basics , in few words: chain line .. the middle on both ends, line up.

CTZunow

It could also be that your rear triangle is out of alignment... Hope not. Otherwise I would say the hub is the most likely culprit.

It is worth saying that for effective, even wearing of your chain, chainrings and sprocket teeth, you would never really want to be in that particular front/rear sprocket combination. If you were say in the biggest (lowest) gear in the rear and the biggest (highest) gear in the front, it would be the same problem going the opposite way; the chain is going at such an extreme angle that the chain has to change its path to connect putting greater lateral force on the sprocket teeth. Depending on where your 'center line' is, there will be specific steps in the back that you should be in for both your small and big ring that you should adhere by.

jleiningj

When I performed this test the straight edge pretty much was right over my 4th (from the outside) cog. The cassette has 10 cogs in total. The center of a 10 speed is at 5.5, so at 4 this is 1.5 from the middle. Is that logic right?

And I did figure that would occur on the outermost cog, but is it normal for the second outermost cog as well? (since it is happening from the 3rd as well it seems like a bit much).

And thanks for all the insight guys. I really do appreciate the help.

FBinNY 

OK so we now know you're chainline is within bounds and just about on center with the cassette (the outer ring's outer face, should line up 7mm outside of the center of the cassette).

I agree that shadowing on the 3rd sprocket is a bit much, so here are other causes and options.

1- if it's easy, move the crank out 3mm farther. I prefer outboard chainline anyway, since most of my riding is done with the outer ring and the outer half of the cassette. If moving the crank isn't as easy as moving a spacer, don't bother.

2- chainstay length. The shorter the chainstay, the greater the range of angles, There's nothing you can do about this, except adjust your expectations.

3- chain width, a narrower chain, with flat outer plates may solve the problem.

4- chainring separation. Adding a 1/2mm or so spacer between the rings might buy you the added clearance, or at least reduce the problem to only the outermost sprocket.

There's no miracles or rocket science here. It's simple geometry. Narrow the clearance needed by moving the outer face of the chain inward, or add clearance by moving the outer ring out. With a bit of finagling you should be able to get a good workable system.