Why is my bike downshifting?
 

I just built up a drop bar MTB for commuting. It uses it's original 7speed rear cassette, and RD that came on the bike.
I swapped out the original shifters when I went to drop bars. Instead, I bought Rivendell's bar-end shifter pods, and hooked them up to the 7speed indexed DT shifters that originally came on my C-dale touring bike.

Now when I got on my MTB for it's maiden voyage, it tries to downshift into the larger gears on the cassette every time that I put down any amount of serious force on the pedals. It makes that "clack-CHUNK" sound as it tries to downshift and then falls back into it's proper gear. it probably does this once or twice on every pedal rotation. What's going on?
The shift lever is staying in place during all of this.

Thanks in advance for your advice. I know it's probably something stupid, so I appreciate your patience and advice with a noob like me.

Possibly too much tension on the rear shift cable.
Or maybe a bent derailleur hanger.

Nope. Just checked. Rear cable tension is actually a tad loose if anything. If the derailleur hangar is bent, it's news to me. Looks straight to the naked eye.

Cleaned the RD? Oiled all the moving parts? Using new cable and housing? Checked for any obstructions?

Are you a monster and does your rear d cable run under the bottom bracket ?

Is it possible that your shifters are going from index to friction mode?

Done all of the above. Gave the RD a VERY thorough cleaning and oiling. All cables and housing are new.
No obstructions anywhere.

I am hairy like a wookie but that's about as close to monster as I get. Yes, the cable runs under the BB.

I don't think these shifters have a friction mode.

All of this is very strange. I went through some of these same troublehsooting questions on my own, and I can find absolutely no reasonable explanation.

Do you have some pictures of your bike... specifically the cable routing at the bars ?

It sounds like you may be creating just a little extra pull when you lay down the power and cause a little lateral flex to the frame... the shifter cable can often bind under the bb.

I get this on my old road bikes which are fairly stiff... for a little guy I can still lay down enough torque to bend chain rings.

You want to make sure the cable moves through this guide very smoothly.

Frame flex, maybe cracked chainstay?

I recently had trouble with a bike doing that. Turns out the chain had a couple stiff links that I hadn't noticed. If that's not it, keep fiddling with the RD adjustment. Sounds like you may have it close but not quite.

How did you figure the cable-tension is loose? Here's the symptoms:

cable too tight - chain wants to rub & ride up into larger cogs. Slow shifting to smaller cogs. Loosen cable to fix (screw barrel-adjuster IN).

cable too loose - chain wants to rub & drop down into smaller cogs. Slow shifting to larger cogs. Tighten cable to fix (screw barrel-adjuster OUT).

Thanks for all the help guys. It's late and I am on trial tomorrow (assisting the attorney, not as a criminal) so I'm off to bed. I'll have to figure it out tomorrow. One thing I just noticed is that when I shift to the smallest chainring, and the smallest cassette gear, there is a little bit of slack in the chain. It's a brand new chain. I had taken out 4 links, but I guess I need to take out another 1 or 2. Could this be a cause of problems?

I'll put up pics of the bike tomorrow, and do a full analysis based on the suggestions you've given me in the last couple of posts. Thanks for all the help guys.

Size your chain properly.

Big/Big +1

I'd check it with a gauge.

Is the downshifting consistent in all gears? It might be useful to eliminate the shifter & cable from the equation. Maybe release cable tension, let the derailer fall into the highest gear, and try riding the bike. Does it still try to downshift?

I'd work some more with the cable tension adjustment. If you think that it's a tad loose, that's probably your problem. It could be that the chain is in the wrong gear most of the time and is trying to put itself into the right gear when you pedal hard.

sounds like an automatic gear box kicking down under acceleration.
have you fitted a turbo 350 from a Chevy to your bike. :)


Or is the rear mech a dreaded Rapid rise mech. That goes to the big cogs when the cables loose

DannoXYZ-I fele like it's a tad loose because when the cable is shifted into the low-tension position there is a little slack in the cable. I'll use the barrel adjusters to fix once I get things dialed in. Good technique.

JonathanGennick- It attempts to downshift in all gears but the largest/lowest. Brilliant idea about disconnecting the cable and trying it. I'll do that later. As for checking the RD hangar with a gauge, the bike never did this with it's original setup, and i haven't dropped it or banged it since then. So i don't think the hagar could be messed up. That leads me to believe it's the way I set it up, not the hangar. As much as I'd love to blame the bike, I'm pretty sure it must be something I've done.

RetroGrouch-I'll play with cable tension if the above mentioned diagnosis technique shows it to be a cable issue.

griftereck-You figured me out. I popped a 350 on there. You should see those 26X1.25 tires smoke!!! RD is not rapid rise. It's an old mid-90's SLX i believe. It's the RD that came on the bike when I bought it.

Thank you for all the help guys. I'll check the bike when I get home this evening and write back with an update.

Make sure your chain length is right first, If there is slack in it in small/small, That leads me to believe its too long. In big/big it should be 1 link bigger than it can fit (so that big/big is just possible to use) Not saying this will correct your problem, but it would not hurt, either.

Are your derailleur limit screws set up properly? top idler on derailleur should be centered with highest gear and lowest gear, use corresponding screws to get them that way. Then, recheck cable tension and adjust until it shifts smooth up and down from each gear, and each front cog combination, avoiding extremes. I usually tune in the middle chainring and shifting is smooth.

Still problems after all that, check derailleur hanger alignment. You cant tell by looking at it, either, visit a shop that has the tool, they shouldn't charge much to check and adjust if possible. If its an old frame that has seen a lot of use, Might start off by doing the alignment.

Hope ya figure it out!

Did you spin the crank and let the RD shift after moving the shifter? If not, that looseness is caused by the RD being out of position relative to the shifter's click.

You may have the "off-by-one-click" problem. Shift the lever all the way into the smallest cog and spin the cranks a couple times. Then shift it up ONE click and spin the crank. Does the RD shift up just ONE cog? If it doesn't shift at all, you're off by one click, if not more.

Next, do the same test at the other end of the cassette. With the RD and chain on the largest cog, move the lever down just ONE click and spin the crank. Does it go down easily? Continue down the cluster into smaller cogs one at a time. Does it go down smoothly and quickly each time? Does it go into the smallest cog on the last click?

In it's at-rest position of small/small, the cable should be taut - not loose. Not really tight, either. If the cable were spaghetti, it should be al dente.

Well, I'm right. I was a nitwit.

I tried all your recommendations last night. Couldn't get the darn thing to work.
I brought the bike in to the bike shop this morning.

Mechanic asks me about the symptoms rather than asking MY diagnosis. What I thought was the bike trying to upshift (i swear that's how it felt to me), was actually the chain slipping on the cassette. I had put a new chain on the old cassette. the reason it worked with my old chain was that the old chain was worn to fit the cassette. As soon as I put he new chain on, i was screwed. That cassette is pretty well worn too. Teeth are rounded off and skinny, etc...

So I mislead you all with an incorrect diagnosis based on what I felt the symptoms to indicate. Whoops.

That'll teach you for assuming that I've got a clue!! I know JUST enough to get myself into trouble, but not enough to get myself out of trouble!!! LOL.