Bicycle Mechanics - New to chain transmission

Hello everybody!

New to this forum and also new to chain transmission bikes.
You see, I just bought this (inexpensive) 26" Aluminium MTB with front fork suspension. It runs great and with all the cheesy decals removed and glue polished away, it looks very expensive!

Now to the point. It has a 3x8 Shimano chain transmission. I just wonder - how "exact" are chaingeared bikes supposed to change their "fine" gears (the rear spoked wheels) when you ask them to?

For example, if I set my gear to 2x8, sometimes it will change all the way to 2x8 but sometimes only to 2x7 and so on. It is especially the eigth gear that is occasionally refusing to "click in". Tried to tense/release the gearwires but actually with little success.

I don't care much about this really (who needs to be as exact as 24 steps?!), but just to educate myself it might be good to know about it.)

Replies kindly appreciated!

Regards,
Tobias Radeskog of Sweden

The bike should shift crisply into each gear. Try this; shift into the middle chainring (the ones in front are chainrings), and shift into the 3rd from smallest gear in the back (should be 6th gear). Get someone to hold the rear tire off the ground and pedal it while screwing the barrel nut adjuster (where the cable enters the rear derailleur) counter-clockwise till the chain starts making noise in preparation to shifting. Now screw the nut back in till it stops making noise, this should do it. If this isn't clear or you still have trouble, check out http://www.parktool.com/repair_help/FAQrindx.shtml

What Buzz said.

Note that less expensive shifters tend to be less crisp than pricey ones, but they should still be precise. As Buzz, suggested, cable stretch may be at fault. As cables are used, they get stretched out and affect shifting quality. If you haven't ridden the bike much, it may have been set up poorly to begin with.

Originally posted by grapetonix ...
For example, if I set my gear to 2x8, sometimes it will change all the way to 2x8 but sometimes only to 2x7 and so on. It is especially the eigth gear that is occasionally refusing to "click in". Tried to tense/release the gearwires but actually with little success.

Possible causes of not reaching the smallest cog: 1) rear gear shift cable too tight; 2) rear derailleur high gear limit screw advanced too far; 3) rear derailleur bent slightly inward.

How about when they shift perfectly on a stand but jump a gear when under load?

Cheers

Originally posted by Simon Ed
How about when they shift perfectly on a stand but jump a gear when under load?

Cheers

It's not a new bike, is it?

My best guess is that it's time for a new cassette.

If everthing else works ok (no skipping or jumping) and it just does not want to go into the small cog, you should try to clean or replace the shifter cable.

Sometimes the spring in the rear derailer can not overcome the friction from the crud that gets built up on the cable. The small cog position is were the spring in the r.d. (unless you have a dual spring r.d.) has its least pulling power.

Personally my mechanical sympathy for the chain wouldn't let me run middle ring at fron (2) to 8 at back. Too much chain deviation.

I have now adjusted both the TOP and BOTTOM and TENSION alignments, and it now works perfectly... it seems :D

The way to use 3x8 systems is to use the middle ring for normal riding. You can use all 8 rear cogs, but the chainline at the ends of the range are a bit too diagonal (increased wear) for some people
The large and small rings have a lot of overlap in their range with the middle. The large is for downhill and tailwinds, but you should not use the larger rear cogs .
The small is for uphills and rough trails. Again, to avoid cross-chain positions you should not use the smallest few cogs.
In reality a "24 gear" bike has about 14 separate usable gear ratios

Originally posted by grapetonix
I have now adjusted both the TOP and BOTTOM and TENSION alignments, and it now works perfectly... it seems :D

:thumbup:

Regular Guy,
No it quite new actually. Its a 7 sp that only has 1K KM on it. I dont grind and I look after the transmition religiously. I clean and lube the whole thing weekly. I can get it to shift great on a stand but under load it will sometimes jump a cog.
Should it perform the same under load as it does on the stand or is there something I can tighten a 'wee' bit to fix it?

Cheers

Originally posted by Simon Ed
Regular Guy,
No it quite new actually. Its a 7 sp that only has 1K KM on it. I dont grind and I look after the transmition religiously. I clean and lube the whole thing weekly. I can get it to shift great on a stand but under load it will sometimes jump a cog.
Should it perform the same under load as it does on the stand or is there something I can tighten a 'wee' bit to fix it?

Cheers

Hmmm...misaligned derailleur hanger maybe? I'm guessing. now. I think we need to get Rev. Chuck in on this one.

Is that like the hmm a doctor makes when he knows you are for the big LBS in the sky or hmm I think there is a screw he can tighten up a 'wee' bit to fix it :)

Have a great weekend all!!

Si