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แถมอีก 1 บทความครับ จากวารสารข่าวจักรยานประจำอาทิตย์ RBR #589

New Chain Doesn’t Always Require New Cassette

My last two columns were about chainrings, how to tell when they’re worn out and how to replace them. I also offered some tips on chain and cassette wear.
One of these tips piqued Winchester, Massachusetts, roadie Carl Kaplan’s interest, who asked me to elaborate. Carl wrote,

“Hi Jim,
Can you expand on what you wrote in the tip in part 1 of your chainring series:

‘Tip: Since I mentioned worn-out chains, cogs and skipping in the opening paragraph, I need to take a moment to dispel the myth that’s out there that it’s mandatory to replace your cassette when you replace your chain. That’s not true. You only need to replace a cassette if the cogs are worn out and they skip. In other words, a new chain will work fine on an older cassette, just so long as it isn’t worn out.’

The reason I ask is that every bike shop I have ever dealt with tells me I have to replace the cassette if the chain is worn (it is being done as I type). I have a very good relationship with my local bike shop, and they have never given me any indication of trying to sell me things I do not need. In fact, it is usually the other way. I don't want to challenge their knowledge, but would appreciate a little more knowledge to have this discussion. I could do it myself, but by the time I've screwed up my bike it's usually too late.

Thanks in advance.”

Carl isn’t the first one to tell me that he’s been told he needs to replace his cassette at the same time he replaces his worn-out chain. That’s why I provided that tip in the first place. So this week, I’ll take a closer look at the issue.

Chain versus cog wear
The first thing to know is that just because a chain wore out doesn't mean the cassette wore out. If you think about how you use a chain and cassette, the chain gets constant wear, however, only those cogs you use the most get a lot of wear.

So in flat Florida riders don't use their easy/big cogs very much, and in vertical Vermont they use them quite a lot. Likewise, a heavier rider or pedal masher wears components faster than a lighter, smoother pedaler. And some riders switch out wheels for different types of rides, typically with different cassettes on each wheelset. So they don’t put the same amount of wear on their cassettes, while putting wear on the chain no matter which wheelset they use.

So, it varies a lot how the individual cassette cogs wear, while the chain is always wearing with every pedal stroke – no matter the type of riding, or the type of rider. Also, if you keep your chain clean and lubed and don't ride in the rain too much, the wear is less on the cogs and chain, too. So that plays a part in it, as does the quality of the components.

Testing cogs for wear
When you replace a worn out chain, you definitely want to carefully check the cassette cogs because it’s possible that you wore them out. You can look at cogs and notice wear marks on the teeth, but that won’t tell you if they’re simply worn -- or worn out. You have to test them.

To do this, once you’ve replaced the old chain with the new chain, you test ride the bike with the cassette that's on it. And, I mean a real test ride -- shifting through the gears and climbing hills in the climbing cogs and even jumping and sprinting a bit in the higher gears to really test the new chain on the old cassette and on every cog.

Tip: When test riding any bicycle you’ve just repaired, be careful. Gradually increase the pressure on the pedals, hold onto the handlebars securely and make sure everything on the bike is tight and adjusted and working before you sprint or jump or climb hard. Then be aware that if the chain skips (keep reading), it will startle you, which could cause a wobble or crash if you’re not ready for it.
Cogs wear differently than chains

When you test new chains on old cassettes with a few chains and cassettes over the years on your own bike, you find that it's more common not to need a new cassette than to need one. Also, if you have a skipping issue from a worn cassette, it's almost always on only 1 cog or 2 at the most -- the ones that wore out and are skipping.

Tip: What’s skipping? It’s when, during hard pedaling, instead of engaging the cassette cog fully and driving the bike forward as normal, the chain rides up off the cog and skips a tooth, jumping forward and slamming down onto the cog again. It feels like a violent hiccup in your pedal stroke. It can be caused by a stiff link in the chain, but in that case it would happen on all the cogs. With a worn-out cog, it only happens on the bad cog(s).

Back in the day
When I started working in bicycle shops in 1971 and right up through about 1990, companies like Shimano, Suntour and others provided cog boards with every size cog on them. That meant you could get replacement cogs and just replace the worn cogs, which was a great thing because for $25 or so, you could replace a skipping cog and be back in business.

But today it's hard to impossible to find individual cassette cogs to replace worn ones, so bike shops don't have that option. This is the case because component manufacturers have gone to cog sets that are designed to be used as a unit to optimize the shifting performance. Also, because some of the cogs are usually attached to each other and not made to be taken apart. We can debate whether these innovations are worthwhile or not, but it won’t change the availability of cogs.

Still, I would never replace a cassette unless it really needed to be replaced. They’re just way too expensive. If you take care of your chain and drivetrain, keeping it lubed and clean and don't run the chain until it's so stretched it's completely ruined, there's an excellent chance you can get more miles out of a cassette than you can out of a chain.

The basic rule of thumb is that if there's no skipping on any of the cogs during your test ride, and the shifting is still fine, you can keep that cassette. You don't need a new one.

Why do shops say you need to replace the cassette?
The bike shop is surely well-meaning, and I’m sure they believe they need to always replace the cassette with the chain because someone told them they had to – like a senior service manager or some other influential person that the shop looks up to and trusts (even component manufacturers say such things).

They also may not know that it used to be common practice to replace individual worn-out cogs. And, it’s one of those things that sounds logical. So, I understand how they can believe it and do it. But, unless you’re really hard on your equipment, I bet you will be able to get more than one chain’s use out of your cassettes, and that can save you some real money these days since cassettes have gotten so expensive.

Tip: This article raises the related question, Do you need a new chain if you get a new cassette? For example, maybe you decide to go with wider range gearing and purchase a new cassette. Will your old chain work with it? The answer is yes. Even if a chain is well worn, it will run on a new cassette and won’t skip. Depending on how worn the chain is, it might run a little rough, but it will work in a pinch.

Jim Langley has been a pro mechanic and cycling writer for 38 years. At RBR he's the author of Your Home Bicycle Workshop and moderator of the technical forums on the Premium Site . Check his "cycling aficionado" website at http://www.jimlangley.net ,

[pskorat]

และอีก 1 บทความ (ถ้ายังไม่เบื่อที่จะอ่านกันนะครับ) ของผู้เขียนท่านเดิม จาก RBR #587

Things to Know About Chainrings.

One of the first things you learn when you get into road riding is to keep your chain lubed and to expect that it will wear out sooner or later and need to be replaced. When it does, you learn that you should check your rear cogs (the freewheel or cassette), too, because if you wore out any of the cogs, that new chain will skip on it, which is highly annoying and, worse, could even cause you to crash.

So, you probably know about chains and cassettes. But it’s possible you’ve never heard anything about chainring maintenance (the chainrings are on the crankset). That’s because, unless you log lots of miles, you might never have worn one out and had to replace it. Or, you might have actually worn one out but not realized it, because it didn’t act up the way worn-out cogs do.

Chainrings do wear out
You might also have had a 10-speed during the ’60s and ’70s and never had to deal with your chainrings, giving you the idea that they last forever. But most of those were made of steel. Most of today’s lightweight road bikes have aluminum rings, and, while durable and long-lasting, they can and do wear out if you ride enough miles.
To help, this week I’ll go over some things to know in order to check chainrings, and next week I’ll explain how to replace them.

Tip: Since I mentioned worn-out chains, cogs and skipping in the opening paragraph, I need to take a moment to dispel the myth that’s out there that it’s mandatory to replace your cassette when you replace your chain. That’s not true. You only need to replace a cassette if the cogs are worn out and they skip. In other words, a new chain will work fine on an older cassette, just so long as it isn’t worn out.

How to tell if the chainrings are worn out
The best way to track chainring wear is by inspecting the teeth every time you replace a chain. If you look at the teeth when they’re new, you’ll see that most have a nice symmetrical shape. Now, modern chainrings often have shifting gates in them, which are specially shaped teeth designed to let the chain move between the rings more easily/quickly. Those features can fool you into thinking a new chainring is worn out. So look at the other teeth -- the regularly shaped ones -- and keep an eye on them.

Compare the Big and the Small
On a double (two chainrings) crankset, usually we ride in the smaller ring more than the larger ring, so that one is more commonly the one that wears out. And that also lets you compare the shape of the teeth on the larger ring to the ones on the small chainring to recognize wear. The teeth can become shorter and even damaged from bad shifts.

You might also see wear on the sides of some rings if the shifting isn’t working as well as it should or if the bike has been shifted with too much pedaling pressure for a long time. This can also happen if the chain hasn’t been kept adequately lubricated. Keep in mind that the chain is steel and much harder than the aluminum rings.

Tip: To get the most life out of your chainrings, ease off the pressure on the pedals when you shift. The chain is making a pretty huge jump from the small to big ring, and the more rigid and inflexible you make the chain by pushing hard on the pedals, the more difficult it is for the front derailleur to kick the chain onto the ring. When you pedal lightly, the chain can flex and the derailleur can smoothly shift it into place.

Check the fit
Another way to sport a worn-out ring is that on a new chainring, the chain fits tightly. When the chainring teeth are worn, play develops between the chain links and the teeth. You can actually see this if you stand to the side and look at the light coming between the chain and the chainring teeth.

You can also feel it by pulling up on the chain at 3 o’clock on the chainring and seeing if you can lift it off the ring and pull it forward. If you can create a gap between the chain and the ring, and you have a chain in good condition, there’s a good chance the chainring is worn out -- or almost worn out.

Rumbling and skipping are other signs
Worn-out chainrings can also run rough so that you feel like the pedaling isn’t as smooth as it used to be. Not everyone is in tune with their bikes enough to feel this, but some roadies do and describe it as a “rumbling” feeling at the pedals.

Also, while it’s rare, chainrings can cause skipping, just like worn-out cogs can. So that’s one way you might recognize a worn-out chainring, from the sudden lurch in your pedaling when you try to pedal hard on a climb or hill one day. I’ve never seen a large chainring do this, but have had it happen with small rings on doubles and triples occasionally.

Bad shifts from big to small are another indication
While he was editing my column, John Marsh sent me another good indication of a chainring wear. Here’s what he wrote: “For me, a telltale sign of a worn small ring has been bad shifts from big to small at the transition from downhill to uphill (especially when the uphill isn’t real steep, and I tend to shift quickly with quite a bit of pressure still on the pedals). With a small ring in good shape, no problem. Bad shape, problems “catching” the chain on the way down.” John’s right, and that’s another thing to pay attention to on your rides.

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