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36 to 28 cog

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36 to 28 cog

Old 03-26-16, 07:04 PM
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36 to 28 cog

So I bought a bicycle with a 11-36 cassette, 28-42 chainrings. So I bought a 12-28 cassette for riding in flatter areas. My LBS tells me to buy a separate chain for the tighter cassette. So 36 minus 28 equals 8. Is that the number of full links that I need to remove from the length of the new chain for the tight cassette? Or is there something I'm missing?

Another question: I drop a chain now and then on the chainrings. I just grasp it and put it back on. But I've never paid attention to the wide half link or the narrow part. Should I be putting the wide half link on the chainring teeth or does it matter at all? My guess it matters a bit.
Your opinion?
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Old 03-26-16, 07:35 PM
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If the derailleurs can properly handle the 11-36 cassette with the current chain then they should also work fine with a 12-28 cassette using the same chain since it's just like shifting on the middle part of the original cassette. That would also allow you to easily switch back to the wider range cassette if you want to do a ride with some steep hills.

On the second question it seems to me that when putting a dropped chain back on a ring you'd usually drop at least a full link or more on the teeth of the ring. BTW, it should usually be possible to get the chain back on by just trimming the front derailleur while pedaling forward cautiously without any need to touch the chain with your hand.
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Old 03-26-16, 08:57 PM
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Sounds like your front derailleur needs a tiny adjustment. The high limit screw (big chainring ) or low limit screw (small chainring ) needs to be screwed in 1/4 turn or so to prevent the derailleur from moving quite as far.

As said, no need to change chains. When this chain needs replacing you can size the new chain to the larget cog you will be using. If you will sometimes use the larger cassette, size the new chain to your current one. Better a bit too long than too short, ripping you rear derailleur off the frame.
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Old 03-26-16, 10:06 PM
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Your chain needs to be long enough to safely cover the big.big sprocket combination. At the same time, you'd like for it to be short enough that your derailleur will take up the chain slack in the little/little combination. Does your present chain meet both tests?

Assuming it does, Your derailleur will still be able to take up the slack in the new little/little combination so there's no reason to shorten your chain.

However, there's still this: Chains and cassettes wear together. As your chain wears it becomes functionally longer. When it does that, it wears the cassette teeth to match it's new, longer length. Bottom line, a worn chain will wear your cassette teeth prematurely. A good rule of thumb is to install a fresh chain whenever you buy a new cassette.

If it was my bike, and the chain had any significant wear on it, I'd buy a new chain to go with the new cassette. Since I was buying the new chain anyway, I'd size it to the new cassette using the big/big sizing method. I guess that's a kind of long way of saying that I agree with your LBS>
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Old 03-26-16, 10:15 PM
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I wouldn't remove any links. I would buy a chain to go specifically with the 28T cassette, and keep the existing chain with the 36T cassette. If you're going through the trouble of swapping a cassette, swapping the chain should be no trouble at all. You'll see much better life out of both chains and cassettes.
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Old 03-27-16, 03:46 AM
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Originally Posted by DrIsotope
I wouldn't remove any links. I would buy a chain to go specifically with the 28T cassette, and keep the existing chain with the 36T cassette. If you're going through the trouble of swapping a cassette, swapping the chain should be no trouble at all. You'll see much better life out of both chains and cassettes.
Except you have to break and rejoin the chain which these days really needs a pair of missing link pliers.

I honestly don"r see the need for alternative cassette anyway. I think it should already have a 28T cog, and the 12T on the 28-12 won't give the same top speed as the 11 which the OP is alluding to. The Trek 920 the OP has should be quite capable of being ridden with the seven gears between 28 and 11 on the standard cassette.
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Old 03-27-16, 06:52 AM
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The change in chain length is 1/2 the teeth difference so going from a 36T cog to 28T cog, you could shorten the chain by (36-28)/2 = 4 "half links" or 2". That assumes the chain was long enough to allow big-big in the old cassette. As noted the old chain length could be left unchanged with no problems since it worked before.

However, there may be another reason for changing to a new chain. If the old chain and cassette have a lot of miles together, a new chain should be used with a new cassette.
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Old 03-27-16, 07:05 AM
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As mentioned before try the big / big then go small & small & make sure there is still some tension in the derailleur arm, i.e. its not folded back on itself so the chain is running across it.

The basic rule is in the big - small tooth combination then the arm on the RD should point straight down or be 90' degrees to the ground, (between the fixing bolts of the dolly wheels) if its leaning forward its short, if its leaning back then the chain is too long. The Shimano web site has excellent references for adjustments etc.
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Old 03-27-16, 08:24 AM
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I do exactly the same thing. I have MTB wheels and road wheels and swap the cassettes back and forth.
I work on the assumption I will be careful on the cross chaining. I'd just check at what point the chain gets too slack for proper operation and don't use those gears.
I don't worry about the chain, I assume I'll just change it before it gets too worn.
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Old 03-27-16, 08:39 AM
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Originally Posted by HillRider
The change in chain length is 1/2 the teeth difference so going from a 36T cog to 28T cog, you could shorten the chain by (36-28)/2 = 4 "half links" or 2".
Not that it matters in this case, but losing 8 teeth is equivalent to 4" of chain. Bike chains are made with 1/2" pitch.
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Old 03-27-16, 08:50 AM
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Originally Posted by Rowan
I honestly don"r see the need for alternative cassette anyway. I think it should already have a 28T cog, and the 12T on the 28-12 won't give the same top speed as the 11 which the OP is alluding to. The Trek 920 the OP has should be quite capable of being ridden with the seven gears between 28 and 11 on the standard cassette.
It's marginally off topic but you make a good point:

Back in the days when "10-speed" meant 2 on the front and 5 on the back, we had to squeeze out every possible gear combination. We still an occasionally digit head going on about the wonders of half step gearing. Today, with 10 cogs on the back alone, we have such a plethora of stock gear combinations that I have a hard time understanding the need for customization.
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Old 03-27-16, 08:55 AM
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Originally Posted by gregf83
Not that it matters in this case, but losing 8 teeth is equivalent to 4" of chain. Bike chains are made with 1/2" pitch.
No, the 1/2" pitch is per "half link". A full link is a male and female pair or two half-links or 1". You want to remove the equivalent of 4 teeth or 4 half-links (2"), not 4 full links. 4" of chain is 8 half-links.
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Old 03-27-16, 09:11 AM
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more mechanics than math .. bag the chain & cassette together.. .. new cassette; new chain .

+1 _ 1/2" pitch is pin center to center but except for half link BMX chains ,

roller chain has both rollers on the inner link, the pins are fixed in the outer links..

in the 80's 52-36t chainring 13~28t freewheel .. 36:28 .. was my hill climbing gear

I was lighter then..

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Old 03-27-16, 10:38 AM
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Originally Posted by HillRider
No, the 1/2" pitch is per "half link". A full link is a male and female pair or two half-links or 1". You want to remove the equivalent of 4 teeth or 4 half-links (2"), not 4 full links. 4" of chain is 8 half-links.
Ah yes. i neglected the fact that the chain is only in contact with half the cog so you only lose 4 teeth not 8.
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Old 03-28-16, 07:22 AM
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Thanks folks! I'm gonna go with separate chain for each cassette. Not really necessary but should wear longer. And since I already own it.
Thanks
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