blochow2001

I have just purchased a Sun Tadpole CX, and from time to time the chaing jumps from one gear to the next in the middle of the rear sprocket. The manual that came with the bike is a useful as a termite colony to a log home owner. Can anyone tell me exactly how the chain is routed from the chain rings, over the pully deal behind the seat and onto the rear sprocket? I can't find any info anywhere that can help. These bikes are a rarity in my area and I don't have a any other owners to speak with on how their machines are set up. I did adjust the chain length after I had adjusted the boom to its correct X- seam length.

Any help would be greatly appreciated! Thanks!

ppc

Sounds like a classic case of mis-indexing to me. For info, most recumbents or tadpole trikes are no different than regular bicycles for most of the drivetrain, save for a longer chain, one or two pulleys to deflect the chain at strategic points, and perhaps one or two chaintubes. What that means is, you can follow generic instructions on how to adjust derailleurs and it will work just fine. Even better, you don't even have to worry about the chainline on a bent. What it also means is that you can bring your trike to a LBS and he'll be able to work on it without any problem (if your LBS makes a funny face and tells you he can't work on your bike, avoid him, he's an idiot).

So anyway, back to your problem: chances are the derailleur cable on your new bike has stretched and has let itself go a bit. It happens on many new bikes. You can try a quick fix by loosening the barrel adjuster, either at the gripshift or at the rear derailleur. If it shifts okay but drops to a smaller cog once in a while, loosen the barrel adjuster only two or three clicks, then one more if it does it again, until the shifting works well. If you feel the derailleur has trouble dropping to a smaller cog, you've overdone it, so tighten back the barrel adjuster one or two clicks.

If that doesn't solve your problem, raise the rear wheel off the floor and check the entire derailleur adjustment. It's not that hard:

First check that the stop screws are good (the H and L screws behind the derailleur's paralellogram), by shifting all the way up and all the way down. On the innermost and outermost sprockets, the chain should be perfectly aligned with the sprocket. If it's not, adjust the stop screws.

Then check the B-screw (the screw that pushes against the derailleur hanger and sets the derailleur's angle with regard to the frame): shift all the way up and check the distance between the sprocket and the first idler: set the idler as close to the sprocket as possible but without actually touching the sprocket. If you're too close, you'll know, the idler will ride on the sprocket, shake and go clang-clang.

Then do the indexing: start by screwing the gripshift's barrel adjuster half-way in (so you have leftover up and down clicks if you ever need to adjust it again while riding), and play with the derailleur's barrel adjuster until you can reach all gears and shift from any one to the next, up and down with equal ease. If you feel the chain goes up more easily than down or vice-versa, keep adjusting until there's no difference between shifting up or down.

Finally, if you keep trying and trying but somehow the shifting seems vague, or the adjustment seems elusive, you might have trouble with the cable and/or the housing, but that's another problem.

Finally, if all that scares you, bring the bike to a LBS, he'll know what to do. But really it's not that hard.

Like all manuals really They don't explain how to solve your problem because it's a generic derailleur problem though, not a problem specific to your bike.

Generally speaking, the chain goes from the top of the chainring, into the front derailleur cage, to the top of the sprocket, around the sprocket, into the rear derailleur cage and onto the top of the top idler, around the top idler, to the back of the bottom idler, exits the bottom idler and goes back to the bottom of the chainring and around the chainring. The run of chain that's under tension (i.e. that transmits your pedaling force) is the length that goes from the top of the chainring to the top of the sprocket.

From what I can see on a photo of the Sun Tadpole, the chain needs to be deflected under the seat (it can't run straight from the front to the back because the seat is in the way). Therefore, there's an extra idler under the seat that takes care of deflecting both the tensioned run of chain and the slack return run. Apparently Sun could have used the idler to deflect only the tensioned run and let the slack run go back to the front directly, but I guess they chose to deflect the slack run too so it's doesn't bob up and down on uneven road. And of course, you also have two chain tubes, but that's just so your pant stays clean and to keep the chain in check. Neither the idler or the chain tubes have anything to do with your gear-skipping problem however.


You might want to check that you have the correct chain length too then. Do this is as follows: raise the rear wheel. Shift to the middle sprocket, shift to the largest chainring, and *carefully* shift up to the largest sprocket, checking that the derailleur cage isn't going to be ripped apart if the chain is too short. If your chain is long enough, you should see the rear derailleur cage painfully extended to the front, but you should have some slack left in the chain. That takes care of the dangerous big front/big rear combination. Then shift down to middle chainring/middle sprocket, and check that your derailleur cage is more or less vertical (or perpendicular to the cassette). Then shift down to smallest chainring/smallest sprocket and see if your derailleur can still maintain the chain tensioned. If it can't, it's probably that you have a wide-range cassette and a large difference between the smallest and the biggest rings, but that's okay, small/small shouldn't be used anyway. Add or remove links to get the derailleur perpendicular to the sprockets in middle/middle, but never EVER let the chain be shorter than the derailleur can accommodate in big/big, or you'll make a very bad mess of the rear derailleur and derailleur hanger.

Good luck

bobkat

Excellent discussion ppc! I hit my "print" button as you explained it better than my "how to tune up your bike" book!
One other thing for him to remember is that no matter how well it is tuned on a stand, it still might need a bit of tweaking up on the road, particularly when pedalling hard or uphill, for final adjustments. You can gently adjust the barrel adjuster while pedalling down the road.

BlazingPedals

Here's what I do at this point:
At the handlebars, click to your highest rear gear - i.e. the smallest cog. The chain may or may not actually end up on the smallest cog. Then downshift one click. If your adjustment is close, you should end up on the 2nd cog. (You're starting here to ensure that you don't end up one gear off when you're done.) Adjust using the barrel adjuster on the derailleur until the jockey pulley is perfectly centered on the 2nd cog. Once you've got the jockey pulley centered on the 2nd cog, test your shifting through the entire range. Fine tune using the barrel shifter at the handlebars. Turning the barrels CCW has the effect of tightening the cable, which (unless you have that @#$% backwards XTR stuff) will move the derailleur toward the largest cog.

If your bike is new/nearly new and worn-out components are not suspected, then a repeating skip/half shift may be caused by a tight link in your chain. To find a tight link, slowly pedal backwards and observe the chain as it travels through the jockey pulleys. A tight link will not be able to make the bends smoothly. You can loosen it by carefully prying the outer link with a small screwdriver. Remember that the 'return' side of the chain, i.e. the non-drive side, is where the shifting happens in back.

Hope that adds to ppc's very good instructions!

blochow2001

Thank you for your input. I have made some adjustments and have only two gears in the middle of the rear sprocket to deal with. I did find it odd that every five rotations of the crank it skips a gear, which leads me to believe that the chain may have a permanent twist in it so I will also check that.

Pedal on!