|First some terminology:
Crank: The front chain ring assembly including the pedals, the arms they are attached to and the big front gears. It's called the crank because that is what you crank when you are pedaling.
Bottom Bracket (abbr. "BB"): It is a cylinder that is packed with bearings and located at the bottom of the bike acting as an axle of sorts, around which the crank to turns freely on the frame.
Drop Out: the slot on the frame where the rear axle "slides" into the frame.
Horizontal Drop Out: So called because the rear axle slides in horizontal to the ground - so that one can adjust the tension on the chain by moving the rear axle toward the front of the bike (to loosen chain), or pulled away from the front (to tighten the chain). Anyone who has had a single speed or a three speed growing up remembers doing this.
Vertical Drop Out: So called because the rear axle slides into the frame perpendicular to the ground; thus one cannot adjust the chain tension by moving the axle forwards or backwards, since neither is possible. If your bike has a vertical drop out, you need either a chain tensioner to adjust the chain tension, or an Eccentric Bottom Bracket.
Chain Tensioner: It looks like a derailleur, but is (essentially) just a cog on a spring that uses the tension of the spring to keep the chain taut on a bike with a vertical drop out.
Eccentric Bottom Bracket (abbr. "EBB"): A BB that can be adjusted to move forward or backward within the hollow cylinder on the frame where the BB normally goes.
Okay, if you are still reading this, it's probably because you have a bike with an eccentric bottom bracket, whose chain is now loose, and you haven't any clue how to tighten it. This post will tell you how to do so (specifically for a 2009 Giant Seek 1 bike, but the principles work on other models as well).
First, I haven't ripped the BB out of my Seek 1 (yet), but I am told that it is a Shimano HollowTech II. Here is a list of the tools you will need:
- 6mm hex tool
- pin spanner (you can use needle nose pliers in a pinch)
- a ruler or tapeline
You should start by thoroughly cleaning and oiling your chain, and getting the area you plan to work spic and span. You don't want dust, gravel or any such thing to get into the BB and grind away like sand paper when you are finished, so treat the area as though you were a surgeon, and dirt was infection.You should also fight the urge to turn your bike upside down for this procedure. The reason being that the casing around the BB opens on the bottom side of your bike, and since things (like little tiny grains of sand etc.) tend to fall downward it is worth your while to make sure they don't fall into your work.
Now, before you begin, I need to caution you. I am not a bike mechanic, I just play one on my blog. If the directions I give you cause you to destroy your bike, and kill your dog - don't hold me responsible. I am simply telling you what "I" did to adjust "my" chain tension on "my" Seek 1.
One more word of advice. Before you begin, give your crank a turning by hand - listen carefully to it. Does the chain make a scratching sound? What does the weight feel like? When you are finished you want it to sound the same and the weight to feel the same, and if you don't pay attention beforehand, you won't have any point of reference to know if that scraping sound was always there, or was added by something you did.
I will describe the whole thing in steps, and with pics - but I want to just give a quick overview. First, relax. You don't need to take off the pedals, or pull out the BB - all you are going to do is loosen the bolts that clamp the BB casing, then using a pin spanner or some needle nose pliers, you are going to turn the EBB within the casing (by hand) which is going to tighten or loosen the chain, then you are going to measure the distance you can wiggle the chain to see if it is too loose or tight - when you are satisfied that all is well, you will again tighten the bolts that clamped down the BB casing.
So without further adieu:STEP ONE (Clean, clean, clean!): Clean the area around the BB - make it spotless - use water and a mild detergent if you have to. Clean and oil your chain, there is no point in adjusting the chain tension if you haven't cleaned and oiled the chain. Remember: resist the urge to turn your bike upside down!
STEP TWO (Loosen the EBB casing): Using your 6mm Hex tool, loosen the bolts that are clamping down the BB casing, you can't miss them, as they are standing out at the bottom of your BB casing. Note1: Don't damage the rubber gasket, or try and pry the casing open - it only needs to be loosened. Note2: you may hear some grit or dirt fall out of the screw holes in casing. If that is the case, you may want to take the screws right out, and blow out the holes with air or even just with a straw and your own power. Note3: Take note of how tight the screws were so that when you tighten them again, you don't over tighten them or put them in too loose.
STEP THREE (Identify the pins): On the LEFT side of the crank you will note two pin holes in the EBB (see photo)
The scratch on mine was there from the shop, I didn't put that there (btw). These are just little holes, they don't have anything in them, the whole point of having them there is so that you can put something in them to twist the EBB in the casing.
STEP FOUR (Rotate the EBB): Rotating the EBB one way brings the BB forward, and rotating the other way brings it backward. This of course moves the crank which tightens or loosens the chain. It shouldn't be difficult to do, especially if you have a proper spanner - but it isn't going to feel all loosie-goosie either. NOTE1: You won't have to rotate it much to get a big effect, so go slow, and go careful. NOTE2: If you don't have a spanner and are using something else - be extra careful. I did mine just as I was leaving work, and because I didn't have a spanner on my, I had to use my needle nose pliers - and in my haste they slipped out a couple of times.
I can't really remember whether it was clockwise or counter clockwise, but really, this is pretty intuitive at this point, if you turn it the wrong way, just turn it the other way. No biggie. What you are trying to do is rotate the EBB within the casing by putting something in the pin holes so that you can turn the EBB (see photo):
STEP FIVE (Measure the tension): For most bikes, you should have about a quarter of an inch play up and down on the chain when measured from the top. What you want to do is rotate the EBB until you find that sweet spot where you have only about a quarter inch chain play on the chain (see photo)
Note1: Don't get all crazy about making the measurement perfect. I mean, you should be able to eye-ball a quarter inch, especially if you put a ruler as your back drop when you measure it. But don't waste your time trying to make it "exactly" a quarter of an inch. Close enough if good enough for this measurement. Note2: The measurement is taken from the middle of the chain, as opposed to it's top or bottom. From the arrows it looks like you measure from the top of the chain, but it is supposed to be from the middle. I don't feel like doctoring another photo, so you will just have to know I mean 1/4" (6.35mm) from the centerline.
FINAL STEP (close it up!): When you got the BB where you want it, tighten the 6mm screws. Note1: Don't squish the gasket by tightening it more than it was when you started. I am sure there is some torque value you have to set it to, but I haven't been able to find it anywhere. But to keep you from destroying your bottom bracket, let me say this - don't over tighten the screws. Note2: Tighten the screws evenly - don't leave one tight, and the other loose.
Once you have done this once, you should be able to do it again with ease, so long as you have the tools on hand. As I have said, I am not a bike mechanic, but because I couldn't find any info on this myself (at least not in a timely manner - and especially for this particular bike), I opted to write this quick tutorial for other Seek owners. You can ask questions, but I can't guarantee my answers.
Here are some hard to find links that may be of service to you:
Cannondale: Chain Tension Adjustment For Eccentric Bottom Bracket
2009 Giant Seek 1 (BikePedia)
Sheldon Brown's Nexus 8 manual (Forerunner and very similar to the Alfine Hub)
The Alfine Hub (SG-S501), black
This post will probably not help very many people, but those it does help, I hope it helps very well.
You may have noticed that I personally didn't do a very good job cleaning the bike. In my own defense, I had to adjust the chain tension at the office, having had the chain fall off thrice on the way to work. It was more of a necessity thing. If I was adjusting the tension at home, the bike would have been spotless.
It's been years since I rode a mountain bike and your article is proof of the fact that I've been so out of touch with all things connected to these two-wheeled technological wonders. =)
EBBs. What's next?
Deohsan, my guess is a kick stand that doesn't look dorky or get in the way... no, wait, I don't think they will ever invent one. =P
Kickstands! Good one!
Great blog by the way! =)
Thanks Deohsan! Drop by any time. Most of my posts are more substantial than this one, and I tend to be a windbag, lacking the brevity gene that makes better men, well... better.
Can you give us an idea how often the chain needs tensioning? Once a season or after so many kilometres?
I am contemplating buying this exact bike but not if its "high" maintenance.
John, it's going to depend on how often you ride, and what kind of riding you do, but (bottom line) more than twice a season for most people. It only takes a minute or so to adjust also, so it isn't a lot af maintenance - a lot less than your average dérailleur - well... Maybe just 'less' rather than a lot less. :)
Seriously, if you can confidently change a flat, you should gave no problem adjusting the tension. I haven't adjusted my tension in the last 500 miles. I can't promise, but I suspect you'll be both pleased and surprised (pleasantly) if you make the purchase.
Um that was supposed to say NO more than twice a season for most people...
thanks for this - I have a Giant Escape Sub Zero R with the same arrangement (Alfine hub and EBB) and those pics make it look nice and easy
MisterBeepy - glad to put up there. After you've done it once, it will be easy-peezy, lemon squeezy.
Thanks for this info ... I couldn't find it anywhere else!!!
Thank You Sir, a finer fount of wisdom cannot be found ANYWHERE else!
Carlos from Melbourne, Australia.
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