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Cause of chain skipping

Old 12-06-20, 11:18 PM
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joeswamp
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Cause of chain skipping

I finally got my old Peugeot PH10 back together and the chain skips on the small cog. The freewheel has been disassembled and cleaned, the chain is new, and the derailleur is newly cleaned and rebuilt (by me). Shifting is fine and it only seems to skip on the small cog, and only under significant load. I can't get it to skip on the stand, even when holding the wheel or the brake. The derailleur seems lined up vertically but the hanger is twisted so the derailleur isn't perfectly in line with the chain when looking down. I guess I need to invest in a derailleur straightener at some point.

I got my daughter to film the skipping using the slo-mo feature on the iphone. Looks like this:


If you watch this video carefully, you can see that between 3-4 seconds the chain seems to pop away from the back of the cog, then when this popped out part goes around to the top of the cog it suddenly skips. It almost looks like the chain is being pushed from below. I'm trying to understand the mechanism by which this is happening and if it could be due to the bent derailleur hanger. I always thought that bent derailleur hangers caused more problems with indexed shifting than they do with skipping, but admittedly I'm not much of an expert here.

Thanks and let me know what you think!

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Old 12-06-20, 11:40 PM
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I couldn't tell whether the chain was lifting off the small sprocket on it's own, or if it was snagging the next-bigger sprocket and so getting lifted by that sprocket's teeth(?).

When a new chain is under tension and lifts off on it's own, this is caused by worn sprocket teeth. Perhaps this bike was ridden extensively on the small sprocket, often the case when a shift lever can't maintain cable tension by it's friction mechanism, or when a newbie rider finds gear selection too difficult using friction levers.

The exact mechanism by which a worn sprocket allows the chain to skip is the failure of a chain roller to fall between two teeth, because the sharp corner above where the wear occurs on the tooth (the base of the driven side of the tooth) is preventing the roller from falling between teeth. The driven-side corners of the sprocket's teeth can be ground away to fix this problem completely, but the bevels must be kept small!

Showing here a sprocket with one tooth's corner ground away. This is an over-generous cut so keep the cuts very small to avoid causing skipping of another sort.
Also, the cut should be more vertical than this one, a bit closer to the length of the tooth or 35-degrees instead of the 45-degrees shown.

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Old 12-06-20, 11:44 PM
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Can you remove the wheel and post a photo showing the condition of the smallest sprocket from the right side?
This will indicate if the teeth on the smallest sprocket are hooked from the pocket wear at the base of the driven side of the teeth.
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Old 12-07-20, 12:25 AM
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The chain is trying to shift to a lower gear (larger cog). Adjust the derailleur.
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Old 12-07-20, 12:40 AM
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No, it's definitely not trying to shift -- that was the first thing I thought and I adjusted it to guarantee that wasn't happening.

So dddd -- you're saying that hooks on the drive side of the sprocket teeth are preventing the rollers from getting into down into the tooth valleys as the chain feeds off the jockey pulley? That makes sense. I looked at the sprocket yesterday and I couldn't really see hooking, but I could feel some subtle hooks with my fingernail. I will bust out the Dremel and see what happens.
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Old 12-07-20, 02:16 AM
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You put on a new chain, but that’s the original freewheel? How “stretched” was the old chain?

Sounds like a classic case of worn cog teeth. Have you tried a new freewheel?

You might also want to look closely at the chain ring teeth
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Old 12-07-20, 07:45 AM
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The old chain was stretched but not wildly so -- the chain checker measured 0.5% but not 0.75%. The chain was really dirty though, which probably accelerated the cog wear.

I just realized that a new chain is going to really exacerbate the skipping problem. The increased pitch of the old stretched chain would make it easier for the rollers to make it past the hooks on the teeth and lay into the sprocket. I'll get some pictures of the sprocket and post them here, I think the teeth are hooked but it's really subtle and is hard to see visually.
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Old 12-07-20, 07:51 AM
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Maybe a chain line issue? Time to break out the front and rear measurements.
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Old 12-07-20, 08:07 AM
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My first suspect would be small cog wear. It was fairly common for new riders to suffer from derailleur anxiety. They would find a gear they liked and leave it there. This was usually the small cog and small chainring combination. Consequently, the small cog would receive a high amount of wear and when you replaced the chain, it often wouldn't mesh properly with the worn small cog but would work fine with the others. This is most common with entry level, boom era bicycles but I still see it on used,1980s models.
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Old 12-07-20, 08:20 AM
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It doesn't sound like it is terribly worn.
Sometimes they skip real easy and there is nothing to be done.
But in your case and as it's your biggest gear which you probably don't use too often I would wait a while and see will it wear in.
And it is definitely only that gear?
Don't crank on it in that gear but use it gently for a while and see how it goes.
No harm will be done so long as are carefull and don't decide too sprint out of the saddle.
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Old 12-07-20, 08:23 AM
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I’m glad to see this thread and the slo-mo video. My ‘82 Puch/Austro-Daimler Alpina does the same thing on the small cog. Simplex long cage derailleur. Been a while since I’ve ridden it this year but the skipping seems to happen more often when bearing down more pressure on the pedals.

added: I missed the two preceding, posts as I was posting but both make sense.

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Old 12-07-20, 09:25 AM
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Skips in one or two of the most used cogs = worn cogs

Skips in all cogs = chain (many times it's a stiff link. Inspect all links to see if they return to straight freely after being "bent" around the jockey wheels. A worn chain will also skip on all cogs and will do it more often than a stiff link.

OR

Keep tabs on this thread until it devolves into the "fork is bent".
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Old 12-07-20, 09:32 AM
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IME stiff links can be pretty easily found- you generally see the derailleur cage shake a bit as the link passes through the pulley wheels.

Also those should pretty much always happen regularly once per chain revolution, since it's the same link every time. That video doesn't seem like the case.
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Old 12-07-20, 11:22 AM
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It's a simple case of the small cog being worn too much to mesh with the new chain properly.

As @T-Mar pointed out, many bikes have spent their life primarily in the smallest freewheel cog, resulting in premature wear of that particular cog. This skipping is a common result when a new chain is installed without changing the freewheel (or spinning on a new small cog onto the existing freewheel, if one is lucky enough to have a freewheel that can be dismantled + a new replacement cog). Also, since these cogs have the smallest amount of chain wrap, they're more susceptible to wear than their larger counterparts. Plus, the chain is under the least amount of tension in the small/small combo.

This isn't unusual, especially when throwing a new chain on an old freewheel. To make it even more difficult, sometimes it is next to impossible to see any wear on a worn small cog. It seems to baffle quite a few mechanics.

Also, to address a few alternate diagnosis here and why they're not the problem:
  • It's not a stiff link. The chain wouldn't rise off the freewheel the same way, and you'd see the derailer reacting to the stuck link as it threads through the pulley cage. It would also require a full revolution for a second pop.
  • It's not chainline. By the time the chain gets fed through the upper jockey wheel, said jockey wheel is negating the poor chainline. Also, the chain would be trying to pop off the cog at around 12-o'clock on the top if so. It's currently trying to pop at 9-o'clock.
  • The chain is not trying to shift into the larger cog. There's absolutely no side-to-side play visible when it pops. Plus, that's a 5-speed, non-ramped freewheel being run with what appears to be a modern 7-speed chain. There are oodles of space between cogs. Any attempt at ghost shifting would be painfully obvious.
Replace the freewheel and you'll be fine.

-Kurt
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Old 12-07-20, 11:46 AM
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Originally Posted by cudak888 View Post
It's a simple case of the small cog being worn too much to mesh with the new chain properly.

As @T-Mar pointed out, many bikes have spent their life primarily in the smallest freewheel cog, resulting in premature wear of that particular cog.
I think that's not the most likely case. One would expect the usage of the gears to be somewhat normal in distribution, even given the bi- or trimodal distortion caused by double or triple chainwheels, and so the smallest and largest cogs would thus get less use than those in the middle. Never the less, the smallest cogs do seem to wear the fastest of a set, and the largest the slowest.

I think it much more the case that

a) smaller cogs have fewer teeth to share the load, and the wear; and
b) the link articulation over a smaller cog is greater, so wear occurs over a greater range of pivot motion.
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Old 12-07-20, 12:09 PM
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Only part of the solution I didn't mention in my first post is that the wheel doesn't even need to be removed from the bike in order to correct a worn sprocket.

On more than two occasions, following a bike build and brief "maiden voyage" (that may be severely time-constrained as I am on my way to an out of-town ride start), I have discovered high-gear chain skipping, only to U-turn it back to my apartment or house and whip out the Dremel (with circular stone worn down to about 3/8" diameter).
After all of two minutes dressing the sharp tips of the driven side of the small cog's teeth, I hop on my bike and do the usual time-trial 18 miles down to Coffee Republic for the start of our best regional training ride, with the chain's engagement supporting all of my spirited efforts atop the pedals.

Lastly, when it's a chainring having this same sort of wear problem, the symptom won't usually be chain skipping. Instead, since the upper run of the chain feeding onto the chainring is under such tension so as to force the rollers into mesh between the teeth, the symptom typically is merely a rumble heard and felt through the pedals which gets worse with increased pedaling force. The solution is exactly the same but for the use of a file instead of a stone on the softer, gummier aluminum (if it's an alloy ring).
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Old 12-07-20, 12:12 PM
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Originally Posted by oneclick View Post
I think that's not the most likely case. One would expect the usage of the gears to be somewhat normal in distribution, even given the bi- or trimodal distortion caused by double or triple chainwheels, and so the smallest and largest cogs would thus get less use than those in the middle.
Here in flat lands Florida, not so. The "fear of gears" thing is particularly prevalent, as someone can get by treating their bike as a singlespeed. Small-small is often what I see out there when bikes are being ridden for simple commuting or leisure.

Originally Posted by oneclick View Post
I think it much more the case that

a) smaller cogs have fewer teeth to share the load, and the wear; and
b) the link articulation over a smaller cog is greater, so wear occurs over a greater range of pivot motion.
I did mention the reduction in the number of teeth carrying the load in the original post, but that's a very interesting point about link articulation. I'd say the link articulation and chainwrap is also affected by how far or close the upper jockey wheel sits from the smallest cog. Helps to get one more tooth engaged.

-Kurt
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Old 12-07-20, 12:17 PM
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Out of curiosity, does the issue occur when the chain is on either of the front chain rings?
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Old 12-07-20, 01:16 PM
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I've long suspected that, because of newbie's known tendency to ride 10-speeds with both levers slammed forward, the entry level bikes were annoyingly given longer and more heavily-offset bb spindles, so that the chain wouldn't rub against the inner side of the big ring when the bike was ridden endlessly in "fifth" gear. It's something I've been compelled to correct in the name of sane chainline on any bike that became a regular rider.

On a parallel note, seems that the 165mm cranks were fitted to these same entry-level bikes to ease concerns of newbies "digging" a pedal, or (if the bike even had toe clips), riding "toe-clips down" for perhaps the duration of their ride.

My Steyr Clubman (that I bought from a seller near to the flat part of the American River bike trail) had a heavily-worn small cog, but no wear apparent on the bigger cogs.
I noted that the right-side Simplex shift lever was broken.
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Old 12-07-20, 01:34 PM
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Possible the rear mech simply needs to be adjusted to rest outboard farther from the wheel, just a quarter or half a turn on the adjustment screw.

There's only a couple culprits here, from least common to most common (with the top two and bottom two neck-and-neck IMO):
  • Freewheel teeth are worn or internals sticking/slipping
  • Chain is worn or wrong length
  • Mech needs adjustment inboard/outboard or is mech tweaked out of shape, mounted off-thread
  • Either rear mech is on bent hanger or front mech mounted too high/off-angle depending on where your noise is (front mech won't apply here, only rear)
  • Wheel (thus FW/FH) not centered in dropouts
In this case, I'd have already tried moving the rear mech outboard a little bit, because it looks like it's trying to jump up a cog. You'll know if it's not mounted square (I hope) because it's chattering like hell at one or more points in your shifting, it will often pull at the jockey, which can easily break teeth on a Simplex derailleru.

I have a box of old freewheels, so the first thing I'd after checking that is toss on another freewheel with the same range, at least on the small end. Does it persist? If not, you know it's likely something specific to the freewheel. You might try this 1st if you're not comfortable w/ adjusting mechs. This is probably the single easiest and fastest sanity check you can make.

There are tools to measure chain stretch. Calculators to measure chain length. That's the next cheapest thing I'd check. It's always good to have a spare chain on hand for this exact reason.

Most people don't have tools to measure dropout alignment. However, I will tell you, 1st time I got a tool to measure and align dropout hangers, I went across all my bikes. I found 1 in 3 were out of spec or near it. Only 1 in 10 was bad enough to induce noteworthy drivetrain chatter --- it happens more than I think people realize.
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Old 12-07-20, 09:45 PM
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I received a rather nice Superbe equipped bike with both small cogs worn out. The rear shifter had a buggered convex spacer and the lever wouldn't hold its position. After I fixed that, I noticed the cog issue. As mentioned above, the PO left it on the small ring and only shifted the rear. Cross chaining murders cogs.
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Old 12-08-20, 02:21 AM
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I went through the same thing a couple of years ago on a Miyata I had then. Back in when I first bought the bike it came with a lesser set of wheels, so I upgraded it and hung the original wheels on the garage wall. After thousands of miles and about 38 years, (the last ten were it hanging on the wall in the basement), I decided to dig it out and get it back into riding shape. At first I just pumped up the tires, lubed the chain and cables and took it for a ride but the wheels needed lots of help.

The first thing I did was dig out the original wheelset since the wheels on it were 'well worn' and likely the reason I quit riding it almost a decade ago. I tore down the freewheel and gave it some new bearings and fresh lube. I noticed right off the bat that a few cogs were worn more on one side and that the front chain rings were worn the same way, but nothing that should give me any immediate concern. I mounted up the freewheel, back on the original wheel it came off of 38 years ago and took it for a ride, or at least attempted to. The chain jumped/skipped in the smallest cog and the two largest cogs. I hadn't changed the chain, only cleaned everything. I checked chainline, derailleur adjustment, but all was fine. I even tried reversing the chain but it still jumped. I swapped in a good used chain, no deal, I tried a new chain, no deal. I pulled the freewheel thinking that maybe somehow I flipped a couple cogs over or something but I had marked each cog and they were in the same orientation as when I took it apart.

Finally I gave up and swapped out the freewheel for an old clunker off the shelf and the skipping was gone. The bike had been parked because the rims were cracking around the spoke nipples, not for a chain issue. The only thing I did was clean the freewheel, chain, and front sprockets.

The other day, I needed a freewheel for another bike, and the one that skipped was the only one handy, and it was clean, without realizing it till I saw this post, I had reused the problem freewheel and it didn't skip on the bike I put it on, (a cheap old mountain bike). The mountain bike likely has smaller front chain rings and a different chainline but it didn't skip, even under my 280 lbs.


I think sometimes its just a matter of things being 'worn in' together and making a change often causes problems.

I had a chain skipping issue on an older bike but only on the larger two freewheel cogs, that started when I had installed a new chain. I had broke a few links and decided to just buy a fresh chain. It was fine for a few days but started to skip after about 30 or so miles. I dug through my box of old freewheels, found one that was similar and swapped it over and all was fine. That was on a 40 year old Nishiki that's been my beater bike for 30 years.

With the swapped freewheel, although I didn't have any skipping issues I had lots of chain noise but I had gone from a Suntour to a Shimano freewheel. I eventually dug out and cleaned up an older Suntour and installed that one with good results and lost all the noise.
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Old 12-08-20, 05:15 AM
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Originally Posted by cudak888 View Post
Here in flat lands Florida, not so. The "fear of gears" thing is particularly prevalent, as someone can get by treating their bike as a singlespeed. Small-small is often what I see out there when bikes are being ridden for simple commuting or leisure.



I did mention the reduction in the number of teeth carrying the load in the original post, but that's a very interesting point about link articulation. I'd say the link articulation and chainwrap is also affected by how far or close the upper jockey wheel sits from the smallest cog. Helps to get one more tooth engaged.

-Kurt
That's not the articulation that wears the rollers - it's the articulation under load, as the chain enters the pitch circle and leaves it. The location of the jockey wheel has no effect on this, as the chain always enters the pitch circle at the tangent.
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Old 12-08-20, 09:02 AM
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Originally Posted by oneclick View Post
I think that's not the most likely case. One would expect the usage of the gears to be somewhat normal in distribution, even given the bi- or trimodal distortion caused by double or triple chainwheels, and so the smallest and largest cogs would thus get less use than those in the middle. Never the less, the smallest cogs do seem to wear the fastest of a set, and the largest the slowest.

I think it much more the case that

a) smaller cogs have fewer teeth to share the load, and the wear; and
b) the link articulation over a smaller cog is greater, so wear occurs over a greater range of pivot motion.
The wear patterns you describe are typical of bicycles owned by experienced cyclists or those equipped with indexed shifting systems. However, bicycles with friction shifting that were owned by novices are a completely different matter.

Derailleur equipped bicycles were relatively rare in the USA until the early 1970s bicycle boom. Then, suddenly, owning a lightweight, derailleur equipped bicycle was the "in thing". People wanted to participate in the latest trend but most had been raised on single speed roadster style bicycles, 2 speed bicycles with "kickback" hubs or 3 speed bicycles with internally geared hubs. Ten gears, derailleurs and two infinitely variable shift levers were intimidating to large numbers of casual, recreational cyclists,

These "derailleur novices" bought primarily entry level models. While some learned how to use the derailleurs, large numbers didn't, despite the fact that many shops had functioning displays set up, so store personnel could demonstrate how derailleurs worked. I was a mechanic (and later manager) of an LBS during the early 1970s bicycle boom and the phenomenon was so prevalent that it was coined DAS (Derailleur Anxiety Syndrome). Large numbers of bicycles left shops to be infrequently or never shifted.

Friction shifting, derailleur equipped bicycles were basically the equivalent of an automobile with a standard transmission. Almost everybody had the ability to learn how use one but many were intimidated by the prospect and never did. It was the main reason why Shimano went on a ten year quest to develop and perfect the semi-automatic bicycle transmission we call indexing. Shimano knew that if they could simplify the shifting operation, that they could increase sales and dominate the entry level segment, which is the largest portion of the market.

When Shimano introduced SIS for the 1985 model year, they were still 2nd to SunTour in derailleur sales. In 1986 they trickled down SIS to mid-range (New 600EX) and entry (Light Action) models. The sales success of SIS at these levels contributed to a mini-boom in the industry during 1986-1987 and further shifting enhancements with Hyperglide and STI led to a near Shimano monopoly of the entry and mid-range markets by the mid-1990s. Indexing effectively eliminated the issue of bicycles being ridden in one gear.

A couple of decades ago, when the C&V used bicycle market started picking up steam, it was easy to find original owner, boom era, entry level models like the Peugeot UO8, Raleigh Grand Prix or Nishiki Olympic. The odds of finding one with significant wear only on the small cog was quite high. These bicycles have largely disappeared or passed through several hands and have had the freewheels and chains replaced. The used bicycle market has also skewed into a more modern era, where indexing dominates. Both trends have significantly reduced the prospects of finding a bicycle that was only ridden in the small cog. However, if the bicycle is entry level and friction shifting. with a meshing issue only in the small cog after installing a new chain, there's a good probability that the previous owner rode primarily or always in the small cog. It was a common issue.

Last edited by T-Mar; 12-08-20 at 09:08 AM. Reason: typographical errors
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Old 12-08-20, 09:26 AM
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repechage
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Originally Posted by T-Mar View Post
My first suspect would be small cog wear. It was fairly common for new riders to suffer from derailleur anxiety. They would find a gear they liked and leave it there. This was usually the small cog and small chainring combination. Consequently, the small cog would receive a high amount of wear and when you replaced the chain, it often wouldn't mesh properly with the worn small cog but would work fine with the others. This is most common with entry level, boom era bicycles but I still see it on used,1980s models.
but you always go faster in the small cog!
I bought a bike that the ONLY cog worn was the 14t cog.
hey, itís 5th gear... half way up or down ... unless one references a gear chart.
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