Miradaman

Howdy folks
So, after reading here that a chain is a wear item (something I never knew) I thought a new chain for my '89 Trek would be an easy affordable upgrade. Got an appointment at my local Trek dealer (sadly, the only game in town when it comes to LBSs) and rode over. The tech took a look and said putting a new chain on what was otherwise an old stock drivetrain would actually hurt performance, as a new chain would constantly slip off the worn teeth of the original crank and cassette gears. He said either replace the whole drivetrain, or just keep riding as is. So I'm curious, is this true and did he save me some money? Or did he sell me a bill of goods in hope I'd consider buying a new bike (he did recommend that over continuing to upgrade my old bike). Thoughts?

Andrew R Stewart 

It depends. If the chain were just old but not worn out he is misguided. If the chain is worn beyond about 1% (as in measured length) then he's got a good point, although the chainrings tend to outlast the rear cogs by a factor or three. Did he measure the chain wear? Did he test the rear cog wear (we use a Rolhloff device)? Did he show you, on your bike, the worn shapes of the teeth?

As to his suggestion of a new bike I have missed feelings. A new bike is new all over, not just the links and teeth of the proposed repair. This newness will show in both a smoother running bike but as well as allow you to enjoy 30 years of product/feature/function development. One aspect that many don't initially understand is that the dealer wants a happy customer many months later. They will make more money on the repair in the short term but the customer will generally be happier months later with that new bike. That happy customer will enjoy riding more and return for both service and possibly service over the years.

So pick your poison. Andy

JohnDThompson 

The drivetrain wears as a unit, but the smaller sprockets on the rear cassette tend to wear more because the wear is concentrated on fewer teeth. Putting a new chain on a worn cassette can result in the new chain failing to engage securely with those smaller sprockets, so standard practice is to replace both chain and cassette at the same time. The large chainrings on the crank can last a very long time compared to chains and sprockets, but if you do a lot of riding in abrasive/corrosive conditions and neglect routine maintenance you may eventually need to replace them as well.

Bigbus

I've replaced several chains without replacing gears, front or back, and haven't experienced any problems directly related to one being more worn over the other. My chains seem to wear out pretty fast but then I might not be cleaning them often enough for the conditions I ride in..

Miradaman

He put it actually put it on the stand and went over it pretty well for me. He measured the chain with some tool I didn't recognized and said it was defintely stretched. He didn't show me the specific wear points on the teeth, I just took him for his word.

MudPie

There are many tools offered to measure chain wear and this is one of them. https://www.parktool.com/product/cha...dicator-cc-3-2 You can also use a tape measure. Chains don’t really “stretch” but will elongate with wear. Ideally the chain link measures 1” exactly. As the pins wear away, the measurement becomes greater than an inch, and when you add up this wear for each of ~114 links, it adds up. As the chain elongates, it starts wearing the rear cogs to match the elongated chain length, thus altering the cog teeth profile. This happens without you knowing it. Now, if you install a new chain, the new chain links are at 1” exactly. But unfortunately, the old worn cogs no longer match the chain. This mismatch causes the chain to slip on the cogs because the chain and cogs don’t mesh properly. So the wrench does have a good point about leaving it alone.

One way to minimize this is to replace your chain before it reaches a certain amount of elongation, and avoiding excessive wear on the cogs. I can usually get 3 chains for each cassette.

Andrew R Stewart 

MudPie says the common understanding. But there's more then this. Cog/ring teeth don't change their pitch as they wear, unlike the chain. They do change their shape and thus where in the "valley of the tooth" the chain seeks it's point of engagement. The more tooth wear there is the less the verticalness of the valley wall and the more the chain will want to ride up that wall, till it can no longer pull the tooth without slipping over it's top. This is the skip and grab we seek to avoid. As the chain wears and it's pitch does lengthen the more the last tooth on the cog will bare the forces we apply when pedaling. The more that tooth will wear at it's upper part of it's "wall".

This is why a partially worn cog can agree with a new chain, to a degree. But too much cog tooth wear and the tooth is more like a ramp then a wall. With too much pedaling pressure the chain will want to climb up that ramp shaped wall. Thus on the stand all seems well but in real life the chain skips over the cog with each pedal stroke. This point is the often misunderstood one. Cog teeth don't change their pitch, chains do.

Another aspect not well understood by some is that chain wear tends to be fairly linear referenced to miles (of similar conditions) but the tooth wear tends to be more geometrical with miles. So if , say, a chain and cog have a 2000 mile life and the chain is measured to have 75% of wear the cog will not have the same amount of wear. But in that last 25% of chain life the cog will see the rest of it's wear. Thus many chain checkers have a .75 and a 1 gage. Replace your chain before that 75% point and the cog will last for a few chains, extending it's life.

What some also miss is that chain performance is not solely restricted to cog skip as it wears. the chain's lateral flex increases too. This affects shifting, especially on the crankset. But this is much harder to measure and thus is not talked about much. With no number to post why bring it up Why is because improved shifting is had with less worn chains. Since front shifting is more technique dependent this loss of performance dur to side to side flex increasing is, again, lost in the "shadows". Andy

cpach

TLDR: The mechanic is almost certainly right in this case. Wear it into the ground, or be prepared to replace the chainrings, freewheel, and chain together at this point. That's an insanely long period of time to go without a chain replacement assuming the bike has been ridden at all regularly.

He's probably making a fair suggestion regarding a new bike, particularly given the amount of labor required at a common shop rate. Still, many of those older Treks are fine bicycles, and while I might explore the idea of a new bike with a customer, I would definitely also lay out clearly a plan for repair. I'd repair it if it were mine--I like most Treks of that approximate vintage.

dedhed

My rule of thumb for things like this.
Put on the new chain see what happens.
89 Trek - rear is likely a freewheel costs $15-25 to replace. Chainrings are very likely just fine. For under $50 you should be able to replace all the "needed" drivetrain (chain & freewheel) even at LBS prices.

vane171

I gather your chain is not skipping cogs and so, why change it. You won't notice any upgrade with new chain, only upgrade will be in looks and the good feeling you got new chain, that you done something about your bike.

There is a whole spectrum of bike users here, from casuals and commuters to very serious bikers. Those discussions that would lead you into thinking you should do something about your chain pretty much weekly and change it altogether every season (or whatever) are pertaining to those serious bikers.

Those people have fancy bikes bikes with many gears and so correspondingly thin chains and they put serious miles on them and those folks tend to be in very good shape too (riding hills and sprinting on their bikes like you probably are not even remotely capable of). Couple that with the trend to lighter chains, even with hollow rivets and what not (like intricately machined side plates) and the result is, the chain (really the whole drivetrain) is like a baby that needs care if you want it to last.

davidad

https://www.sheldonbrown.com/brandt/chain-care.html
Don't bother with a chain checker unless it is the Pedro's or the new Park. All of the others are not accurate and have you replace a chain before it's worn out. https://www.parktool.com/product/chain-checker-cc-4.
https://pedros.com/products/tools/ca...ecker-plus-ii/

Miradaman

Yep, I deferred to his opinion and I'm just going to continue riding it as is with the all-original drivetrain. It certainly rides good enough as it is now.

Miradaman

Yeah, usually I avoid the pretentious bike snob threads since they really don't apply to me, but somehow somewhere along the line I'd picked up the idea that regular riders should replace their chain too now and again. Either way, lesson learned and I saved some money by leaving things as is.

vane171

You got here some definitive info regarding chain wear, very worthwhile reading, but like with everything, one's own context is what determines how it applies to you. Myself I always like to know a bit more than what I really need to know because only then I know what I am doing, if that makes sense.

Homebrew01

Get some sort of chain lube and apply it every now & then.
I usually use a spray, pointed at the rear cogs, while pedaling backwards with the other hand.
Wipe off the extra with a rag, while pedaling backwards.
Be careful not to get it on the rim, or your rear brake won't work.
If you do, you'll need to clean it off.

EarlGrey

I would like to add the very basic information that the lube is *not* needed for the chain-cog interaction, but to lubricate the inner roller/bush of the chain


NipponBill / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)

(I say this for sake of completeness, since you explained already the very basic of lube, one may wonder why you need to clean the chain)