Bicycle TutorBicycle Tutor

Show off your bike!

Related video tutorials:
How to Adjust Your Front Derailleur How to Adjust Your Front Derailleur
* Now with 3D interactive instructions
Chain rubs on frame near smallest front gear ring
#1
Hello! I am very newbie, and not extremely mechanically inclined, so I will try to describe the problem as succinctly at possible. Forgive me if I do not use the correct jargon.
I just purchased a Craigslisted GT Outpost Trail bike for my son. The seller states that it is 6-7 years old, and was purchased from a local bike shop. It has 3 gear rings in front, and 6 in back (18 speeds total). When I shift the front derailleur, the smallest and innermost ring is so close to the frame of the bike, that the chain hits the frame. It is clear to me that the space between the frame and the smallest gear ring is too small for the chain to even fit. One can see where the chain has hit the frame for years because the paint is all chipped off.
Here are the questions:
1. Is this just a simple adjustment, or a design flaw of the bike? Is there some way to just move the 3 front rings out away from the frame a small distance so that there is room for the chain?
2. If the above cannot be done, is there a way that I can just put a single, large ring on the front and have it be a 6 speed bike? He is only 6 and doesn't really need all those gears yet. We only ride on sidewalks and towpaths, and it's flat here! This will be his first bike with gears and hand brakes, so I just need something for him to learn on. He will probably outgrow it in a year or two, anyways.
The shifters are Shimano SIS and they click into numbered positions when you shift them. The derailleur (if that is what you call the thingy that moves the chain) is also Shimano. The bike frame is a "triple triangle" design, if that makes any difference!
Thank you in advance for any help or advice you can offer.
Jaybee

Reply
#2
I'm not sure how this is happening?
Do you mean it's catching the seat tube??
I don't see how the chain can catch this without the small ring rubbing on the "chainstay".
GT bikes are generally good quality. I doubt that it's a design flaw.
The only way I can imagine this happening is if the frame or the chainset is bent?
Take the bike to your local bike shop and ask them if it's O.K. - or ask another cyclist to have a look.

[font=Trebuchet MS]Ride hard or ride home alone![/font]
Reply
#3
The big question is exactly where the chain is hitting the frame. On the top of the "chainstay" (the thinner tube that runs back to the rear wheel), on the bottom of the chainstay, or somewhere else? It is actually normal for the smallest gear to come very close to the frame at the point where the teeth are pointing straight back. If you look at how the chain runs, it is never on the teeth at that point, so it doesn't need clearance there. It is possible that the front gears creeped a little closer to the frame over time due to tightening or if a wrong part was put in, but I doubt that's the problem. An old chain will also sometimes catch on the teeth a little and get sucked up and hit the frame instead of rolling off the teeth like it is supposed to.
You could get a single speed crank for the front and replace the triple, but there's no reason to spend the money usually. Having the extra gears up front won't hurt anything. 6 in back is usually plenty unless you've got real hills. Try him on the middle one in front first. If that isn't high enough gearing, move the chain up to the largest one.
good luck
But I like your idea of disabling the front gearing anyway. Simplify the bike a little and resolve this problem. You can do it a couple ways.
The full way is to remove the front derailleur and shifter completely. Just put the chain on the middle or larger gear up front and it should stay there by itself.
The next way is to try to "lock" the front derailleur into one position with the two little screws on the top of it. One may have an "L" and the other an "H" stamped by them. These stop the derailleur from moving in (L) or out (H) too far. If you screw these in farther, they will hold the derail in a certain position. If there is enough length on these screws to hold the derail in place, then you can still remove the front shifter and cable just to get the extra stuff off the bike. The chain would normally stay in place fine anyway, but having the derailleur there will help make sure it doesn't bounce off if your son is really thrashing around on it.

Reply
#4
An easier way to solve this problem, assuming the chain rings or frame aren't bent somewhere, is to replace the bottom bracket with one that has a longer spindle. This would move all three rings and the crank arm away from the frame a little more. The front derailleur would most likely need to be adjusted as well. I'd recommend taking it to a shop so they can rule out any serious problems and then swap out the bottom bracket.

Reply
#5
I think perhaps the best advice is to get it looked at, but there's no point replacing the bits on it given it's for a 6-year old who will grow out of it very quickly.
If the LBS says you need to replace something to prevent it rubbing I'd go with DaveM's solution of removing the controls for the front Derallier, besides the granny ring isn't vital. I do MTB in mountainous Wales (and i'm not desperately fit) yet I've never used it so I doubt your son will miss it on the flat area you live in. I would however modify DaveM's solution. find a small piece of wood and trim it so that it sits between the Derallier and the frame, forcing the Derallier permanently into the middle Chainring, and fix it in place with Duck Tape or something (I use this technique to set the cable tension in my front Derallier when I set it up).
If it's something like playing with the adjustment then you might as well do it yourself, though if your not sure post what you were told to do and post relevant pictures so we can help you.

Reply
#6
Thank you all for your responses. I have taken close-up pictures of the part of the bike that is in question, and will try to post them this weekend. DaveM, you said,
" It is actually normal for the smallest gear to come very close to the frame at the point where the teeth are pointing straight back. If you look at how the chain runs, it is never on the teeth at that point, so it doesn't need clearance there"
.....and after I read that, I realized that the chain really doesn't run in that spot!! After saying, "DUH" to myself a few times because it really seems so obvious, I set about trying to figure out what else could cause abrasion on the chainstay right next to the smallest chain ring. So here is theory number two: perhaps the previous owner greased the chain right in that spot, and let the grease run down on the frame and the wear I am seeing is caused by the grease eating away at the finish of the chainstay right underneath the smallest ring in front. When we picked up the bike, the owner had just greased the chain for us before we got there, so I don't know how neatly he did it. Will chain grease eat away at the paint on a bike? With that said, you will see when I do post a photo that this actually looks more like mechanical abrasion, though.....so maybe the actual chain ring at one time was rubbing on the chainstay (it doesn't rub now).
Matt, I don't know how to determine if the chain rings or frame is bent, other than it looks OK to my untrained eye. If either one is bent, will it be obvious?
JonB, I will consider the piece of wood to hold the derailleur in place, or maybe I'll take the shifter and derailleur off comletely as DaveM suggested...not sure yet. By the way, I have ancestry in the Rhonnda Valleys of Wales (Ystrad, to be specific). My ancestors were coal miners in the 1800s there. I've never been there, but I imagine that it would be a fabulous place to MB!
As far as taking it to a LBS, I am trying to avoid that if possible because I want to teach my children how to problem solve and fix things. They used their Christmas money to buy these bikes, and we already talked about how much better it is to get a used bike of good quality, than to go to Walmart or Target and spend more money on a new bike that is of poor quality. It is true that I don't know much about bikes, but I envision this as a project that we can take on this summer sort of as a hands-on, educational kind of thing. And if that fails, we can always take it to a LBS as a last resort!
I appreciate your help, and I will post photos as soon as I can download them!
Take care, everyone!
Jaybee

Reply
#7
Obviously the photos will help.
But there is a possibilty of chain "slap".
If the bike is used over very rough ground the chain "bounces" and slaps down on the chainstay. This causes quite a mess to the paintwork.
Anyway look forward to the piccies.

[font=Trebuchet MS]Ride hard or ride home alone![/font]
Reply
#8
Unlikely grease would dissolve the paint, it was probably mechanically removed by the chain. If the paint is scratched on the outside of the chainstay right under the chainring, then it is likely that the chain either fell off the chainring or got sucked up from the bottom at one point and wedged between the chainring and frame. Shouldn't happen repeatedly, but it's not uncommon. If it is not happening again, problem not worth worrying about.
The test for bent chainrings is to stand directly above them and turn the pedals backwards. If you can see the chainrings move in and out as they rotate, something is bent. If all three move in unison, it may be the axle or crank itself. If just one wobbles, it is probably the ring itself. A little bit of wobble won't hurt anything, though it can making adjsuted the shifting trickier.
Yes! Teach your kids how to diagnose and fix problems. Skills that will serve them well through life.

Reply


Possibly Related Threads...
Thread Author Replies Views Last Post
  Front derailleur won't shift into third (largest) gear bruin1989 1 5,266 08-01-2016, 07:14 AM
Last Post: Joe_W
  Remove chain from front derailleur rider2 1 3,628 05-13-2016, 04:16 PM
Last Post: DaveM
  Chain Coming off Randomly / Front Derailleur Jlpicard440 12 6,064 09-07-2015, 06:29 PM
Last Post: painkiller
  Chain skipping one gear while shifting aswinskumar 11 6,158 01-01-2015, 04:33 AM
Last Post: aswinskumar
  Triban 3 Microshift front deraillieur rub in largest ring Ryanconway 8 4,389 08-20-2014, 07:06 PM
Last Post: DaveM
  Front gear goes down but going up is problem tapash 6 3,914 07-30-2014, 05:03 PM
Last Post: cny-man
  Automatic bicycle chain keeps jumping off back gear larryq 5 4,371 01-11-2014, 07:46 PM
Last Post: painkiller
  Front gear shift stopped working... zeusthunder 4 7,128 05-27-2013, 06:32 PM
Last Post: haykong
  Rubbing on second smallest cog gecici90 14 8,693 05-17-2013, 03:40 AM
Last Post: Bill
  Chain wont stay in Gear 1 Patricmurphy 1 3,452 09-25-2012, 02:00 PM
Last Post: Bill

Forum Jump:



ISSN 1918-3445 © Copyright 2007-2010 Bicycle Tutor / Privacy Policy / Created by Alex Ramon

feed