I recently had LBS replace chain and cassette. The origional cassette was 12-30. The new cassette is 11-28. I watched the mechanic when he did the work and he cut the new chain the same length as the old one. I notice now that when on the stand if I am in the small chain ring and the smallest cog the chain will sag and rest on the chainstay. My question is, should the chain have been shortened? Thanks for your input!

Yes. Bring it back.

Did the mechanic adjust the rear derailleur? The chain should not rest on the chainstay if the rear derailleur was adjusted properly after the chain was replaced, unless there is something wrong, or the chain is too long.

Maybe he was trying to anticipate whether you'd be using another 12-30 cassette and left it long to be on the safe side (many bike riders are not as conscientious of these things as we are ). There's no reason to use the small-small combination, you know.

As noted above there could be more than one problem, or the mechanic may have made it the same length on purpose. I would note, though that going by the old chain length alone is not the right procedure unless one has checked the old chain for proper length.

We are not the people to ask but rather the shop/mechanic. I don't understand what it is that causes so many people to not just try personal interaction first. Though I understand the desire for "expert" advice coming here is only virtual, so we seldom have all the necessary information, especially with something like this that is not a purely mechanical issue.

Finally, there is only one full link diff between chain length for a 30 vs. as 28 tooth. If it were me and I were not replacing the chain, for example, I would have left it the same length.

Actually there is only one half-link (1/2") difference between a 30T and 28T cog. You only adjust for 1/2 of the tooth difference when you change gearing. Since you can't remove only a half-link and have the chain ends join, the mechanic decided to make the chain the same length. If the chain is now too long, it probably was before too.

True, I plead temporary brain check-out.

I doubt that the mechanic made an error relating to chain length. There may not have been an option to shorten since 2 teeth would only allow shortening by 1/2" and chains are cut in 1" increments. On the small side, the 1 tooth difference would only account for 1/4" of chain so, unless it was at the edge before, it shouldn't be too long now.

That said, it's possible that the mechanic adjusted the B-screw bringing the RD forward a bit and creating some slack.

In any case there's a simple, easy test you can do right now to verify that the chain is short enough.

Shift to small/small and pull the lower pulley back (not the RD itself). If you can pull the lower loop taut, before hitting the idler cages's stop the chain is short enough, and the sag you;re seeing is from another cause.

I suspect that the sag your seeing is the normal sag we see when coasting. It's compounded by the chain starting lower off the 11t vs the 12t, and can be made worse by a sticky freehub body which pushes the chain forward when coasting.

So You might start by raising tour comfort level with the chain length test, then bringing it back to the shop reporting only the sag, without suggesting a cause or that he was at fault. Let the mechanic approach the problem with an open mind and you're more likely to get the correct solution.

You have more faith than me. I recently had to take two links out of a chain on a friend's bike after a shop mechanic left it too long.

It sounds to me like the mechanic assumed that the previous chain was the correct length. Was it?

Right ThermionicScott! I don't ride there. Just discovered it when had it on the rack.

I'm still pretty new at this. The chain that was replaced was the original chain that came on the bike new. I assume it would have been the correct length.

That's my plan next time I go up to the shop. 40 mile drive in the truck. Would ride the bike up there but don't feel comfortable riding in the city.

My simple test-
Put it into Big:Big.
Take a couple "skinny" enough screw drivers or similar to insert into the links.
On the bottom chain run where you have some distance, slide the screwdrivers in a few inches apart.
Move them toward each other and see how much slack you can produce.
If 2" or more, the chain is too long. You want at least 1", but less than 2". (2 FULL inches)

Simple. Is relative.
Small:small. Chain should be able to move freely without rubbing. It doesn't get much simpler than that.

Not faith, unless you mean faith in math.

The relationship between chain length is 1" (inner+outer link) for every four teeth) so a 2 tooth change, or 1t change in the small/small isn't enough to justify cutting a chain shorter, and if cut, can result in the chain being too short if it had originally been the minimum length.



Likewise It's unlikely, though not impossible that the chain was within 1/4" of the maximum length, and that extra tooth of slack broke the camel's back.

A better explanation of the problem and proper confirmation of chain length as described earlier are needed to analyze the cause. For example if the OP was seeing upper loop slack when coasting, that's not chain length, it's a sticky freewheel.

These chain length debates are dumber than chain lube debates.

The absolute minimum length is what is what's needed to loop the big/big combination with 1" to spare.
The maximum length is based on the RD pulling up the slack in the small/small combination.

ANY length that meets BOTH tests is fine. If necessary, it's OK to exceed the maximum for super wide gearing though it will mean the smallest ring can not be used with the smaller few rear sprockets. However it's never OK to cut shorter than the minimum.

I might add that Sram RDs don't care about chain length (subject to the rules above), but Shimano and Campagnolo RDs tend to work better with longer chains. The difference being that Sram Jockey wheels are centered on the cage pivot, but the others aren't and rise or drop depending on the angle of the cage.

But the basic rules for changing cassettes are simple.

1- you NEVER have to shorten a chain if reducing the size of the largest sprocket (front or rear)
2- you never have to do anything if increasing the size of the smallest sprockets (front or rear)
3- Increasing the size of the largest sprockets MAY require more chain, and must be checked.
4- reducing the size of the smallest sprockets also MAY call for shortening the chain, though that's rarer.

Yes, simple enough unless the chain is too short to allow big:big. If that's the case the problem won't be a slack chain, it will be broken parts if the unavailable gear combination is inadvertently used. So while a usable small: small is nice, a usable big:big is essential.

Sometimes I will even cut my chain long enough so that it droops and rubs a little in the small-small, since I hate throwing away chain links that I paid for.

No, faith in some random mechanic at some random shop that you've never met or worked with.

I'm just stating that my cousin's bike came back from a shop and the chain would drag in small-small. I removed two links to stop the dragging, no issues in big-big. The mechanic just threw the chain on, without measuring or removing links or anything. It was the standard 114 link chain, mounted as-is. I don't put a lot of faith in shop mechanics, this is just one example of why.

You are making an assumption. You are assuming that the mechanic would even use any math; talking about random mechanics here, not you or me.

I don't disagree, but now we are getting into a scenario where the chainring and cog sizes are pushing the capacity of the rear derailleur when in small/small. Granted, at that point the rider is cross-shifting which should be avoided. To me, an ideal set up will allow small-small without running out of chain for big-big. When we cross that thresh-hold, the bike is no longer idiot-proof. The person riding absolutely MUST be aware of what combinations work and which won't (which is uncommon). Whatever the case, I was just laughing at the fact that @Bill Kapaun referred to his method as "simple." The method is fine, but I'm not sure I'd call it simple.

I explained my reasoning, specifically that a drop in small sprocket size would only "lengthen" the chain by 1/4" compared to a " minimum possible reduction in chain length. By the same token, if the chain was already the minimum length shortening by 1" might leave it 1/2" too short, a dangerous and unacceptable condition.

So I posted not out of faith in an unknown mechanic, but because the math indicated that not cutting was most likely a reasonable decision.

I understand that you believe a mechanic made a mistake, but unless it was the same mechanic (something you can't know) that doesn't directly apply here.

BTW- While I posted from faith, since not seeing the bike, I had to go with the odds, I also posted a simple, reliable 100% bulletproof way the OP could CONFIRM if the chain was too long or not. So it's not about faith, it's about facts.

may be cutting the chain is good way,but the way is not everyone can do .

That is a simple enough check I can do it. Could save me a trip to the bike shop giving me more riding time.

I am amazed at the knowledge you guys possess. Thanks for all the input. Trying to learn to do my own bike work. Replaced the bottom bracket last week which I'm still proud of. Found an older post on here that filled in the gaps from the directions that I had.

That scenario isn't that uncommon for riders using triple cranks with a short cage rear derailleur or for those who replace an OEM granny chainring with a smaller one as i have done many times. My insistence on being able to use big:big is based on the damaging consequences of it not working. The inability to use small:small is only a nuisance and a bit noisy from the slack chain but doesn't harm anything. Yes, you shouldn't use either combination but brain-fade does happen.