Single speed drivetrains have significant appeal. They’re simple, lightweight, and tough. They boil the riding experience down to its most basic form, eliminating the distractions associated with more complex drivetrains while putting the rider more closely in touch with the terrain. The trade off for this simplicity is the inability to change gear ratios (of course) which makes riding in varied terrain more challenging, particularly if any kind of loads are involved. Plus, riding a high gear at low cadence can put considerable strain on joints and connective tissue which can lead to injury over the long term.

At the other extreme are 3×9 27-speed drivetrains designed for mountain biking or loaded touring. These drivetrains are well-suited for riding in rugged, off-road conditions or carrying heavy loads long distances over mountain passes and across deserts. In other words, they provide a wide range of gears to suit those who ride in a wide variety of conditions. They’re so effective that they’ve become standard issue on many bikes, even bikes that may never see a single track or a mountain summit. The downside to these versatile drivetrains is the added complexity and cost, as well as the higher maintenance required to keep them running efficiently.

After riding a number of 8-speed and 9-speed IGH-equipped bikes for the past two years, I’ve found they cover my needs for city riding quite well as long as the overall range is sufficiently low. And on my bikes with double and triple cranks, I’ve found I spend nearly 100% of the time on a chainring in the 40-42 tooth range. This has me thinking that a 1×9 drivetrain with a single 42 tooth ring up front and a 11-34 cassette in the rear may be a perfect set-up for the type of utility riding that I do on a daily basis.

Consider the following:

  • The gear combos I use 99% of the time on my derailleur bike: 42 x 11/34 (700c)
  • The gear inches for that range: 33.4″ – 103.1″
  • The gear/cadence/speed relationship in the highest gear: 42×11 @ 80 rpm = 24.5 mph
  • The gear/cadence/speed relationship in the lowest gear: 42×34 @ 60 rpm = 6 mph

On a city bike to be used for commuting or running errands, I rarely top 20 mph, and I certainly have no need to spin out past 80 rpm at 25 mph, even on a downhill; beyond that I just coast.

On the low end, I find 33” (6 mph at 60 rpm) plenty low for loads up to 60-70 lbs on moderate hills, the typical max I experience. Of course, on a cargo bike in mountainous terrain, where the loads are greater and the hills are longer and steeper, lower gears are a must.

I’d say that I’m going to give a 1×9 drivetrain a try, but the fact is, I’ve effectively been using a 1×9 drivetrain for a number of years. You only have to look at my crank to see what I mean; the outer ring was replaced with a chainguard long ago, and the inner ring has zero wear on it. I do plan on replacing the triple with a single crank, if for no other reason than to make it official.

All calculations were made using the late Sheldon Brown’s excellent Gear Calculator.

58 Responses to “1×9”

  • Pete says:

    Having “jumped” this topic yesterday I guess I have nothing to add, except, why are your well-used bikes STILL cleaner than my brand-new bike?!

  • Alan says:


    “Having “jumped” this topic yesterday I guess I have nothing to add, except, why are your well-used bikes STILL cleaner than my brand-new bike?!”

    Ha! In a word… wax (and perhaps a sprinkling of obsessive behavior). :-)

    Please feel free to chime in again on the gearing discussion!


  • Ant says:

    I noticed the same thing on my cargo bike, large ring was only used downhill on long rides, and never used the small ring. After a couple weeks of 1×8, aside from the extra space on the bars and a little cleaner look from the side, I haven’t noticed not having the other rings at all. I have mine geared a bit lower, to deal with larger cargo loads, but it’s still plenty of range for anything around town.

  • geoff says:

    very interesting idea. I recently switched down my 52/42 chainrings for 34/46, so that i could do a little heavier and hillier pulling (gear plus double tandem trailer). i have a 12-30 freewheel in the back. But I’m realizing that the 46 is still too small when i’m just riding a flat stretch. I still want to have a granny gear, but i have a feeling that 36/48 or 38/50 might work better, especially if i upgrade to one of Rivendell’s 11-36 shimano cassettes. we’ll see, but this is expensive experimentation!

  • JonP says:

    I did the same thing with my commuter. After riding on the middle chainring 99.9% of the time, I ditched the shifter and cable for the front. I went from 24 speeds to 8 — and it had zero effect on my riding.

    Eventually, I wouldn’t mind swapping out the cranks and triple chainring, but it’s not worth the cost to me.

  • Alan says:


    The other plus is not having to trim the front derailleur when the chain is on either end of the cassette.

  • Aaron says:

    Another big plus to the 1×9: you can also use a short cage rear derailleur, even if it’s a 12-36 cassette on there. Less derailleur dangling, and snappier shifting!

  • Fergie says:

    I went 1×9 on my trail bike years ago with a 32 tooth ring (a standard middle ring for MTB setup) and an 11×34 cassette and it works great. Different conditions than I ride on my commute but in theory it should work fine for commuting and maybe even light touring. I’m currently thinking that my next commuter should take advantage of internal hubs and dynos, something along the lines of a Civia bryant belt alfine. That would work too..

  • Pete says:

    Since my old MTB has thumb shifters, ditching the front derailer freed up room on the bar for a bell I can ring without taking my hand off the left grip. Now, if I could just get the pedestrians to stop wearing headphones!

  • Bogdan says:

    I have gone through this change a few years ago. On a Specialized Crossroads with 700/45 Schwalbe Marathon tires I am using a 34 tooth chainring w/ 11/34 cassette. Sounds pretty granny but it works great. I am a somewhat fit commuter wanting more comfort and looking for wider tires. For those of you contemplating the chage , consider the added weight of possible fatter tires.
    Good luck to all!

  • Justin says:

    How is it that you can use a short cage rear derailleur with a 12-36? I always thought you had to go long cage if you had large (aka lower) gears back there than are typical on a road bike. Enlighten me!

  • Ryan says:

    Anybody have issues with the chain dropping off when getting rid of the front derailleur?

  • j. pierce says:

    I switched from my old MTB with a 3×6 to an 8 speed IGH – I’ve gotten more in shape lately and think it might be time to switch out the cog; I rarely need the lower gears- but the hills in Vermont make me think twice. Having only one bike for commuting, shopping and fitness means I coast a bit more, but that’s okay most of the time; but I wouldn’t mind having a higher gear when I’m out for fun. Maybe the upcoming 11 speed is in my future. I’m always surprised by the number of bikes I see with a pletora of gears where only one gets used – it seems a large number of bikes I see are being spun to death or standing pedaled (or pushed!) up the most moderate of inclines. I’ve seen quite a few used bikes for sale where it was evident that only one gear has been used for a long, long time.

  • Aaron says:

    j. pierce – Unless I’m wrong, most small-cage RD’s have a wrap capacity of something like 30-34 teeth. The difference between a 12 and a 36t cog is 24 teeth, which fits within that capacity. You’re removing the extremes of small gear/small ring and big gear/big ring so you just need to size the chain appropriately.

    On my bike I use a medium cage RD with an 11-32 cassette and 26-36-48 triple. You only really need the extra capacity of the long cage if you want the chain to fit those extreme combos like 11-26 and 32-48, which shouldn’t be used anyway. I will stretch the derailleur almost taught if I use 32-48, and with 11-26 the chain is sagging. But again, those combinations shouldn’t really be used anyway.

  • Aaron says:

    Oops, I meant Justin, not j. pierce!

  • Pete says:

    I thought that deraillers also had a “largest cog” rating that was about the cage actually fitting below the largest cog? I know you can exceed this by a few teeth, but most short cage road deraillers have a pretty low limit.

  • Nico says:

    If you go to a single chain ring, what will you use as a guard? I like that one that you on the outer chain ring now. Clean and simple. You could go to a double crank and get the same effect? Incidentally, a thorn got me yesterday. I purposely wanted to avoid the River Trail because of the flat I got there the other day. But, as we riding through midtown, my friend pulled over to adjust his seat. I slowly came in behind him. Then ‘pop’ air shot out to the side. Looked like an air hose had cleared the dirt away from my now flat tire. Upon changing the tire, I found the culprit, a 3 mm thorn. It’s a thorny world out there…

  • Androo says:

    As far as I know, the large cog-rating isn’t the product of the cage length, but rather the B-tension screw (I believe it regulates the spring tension, but has the effect of rotating the angle of the derailleur to accomplish the clearance).

    Shimano’s new ‘shadow’ MTB derailleurs can all clear large cogs, but I needed to do a little bit of finagling with my slightly older Deore in order to get it to clear my new 12-36 cassette. I actually removed the M4 screw and replaced it with a cap-head hex bolt and installed it from the other side, so that I got a lot more effective adjustment. Works like a charm.

    Incidentally, I am pretty glad my derailleur is a long-cage, though, because I need that much for my 12-36 x 50-39-30 set-up, which gives me an astonishingly useful 23-114 gear inches. It means I can still ride as fast as I want when it’s unloaded, but can easily tour with it if I want to.

  • Matt in Tacoma says:

    Over time, I took my Cross Check city bar commuter down to 1×9. 36t and Salsa ring guard up front and 12-32 in the back. Using a bar-end shifter on the downtube in indexed mode, I had the most crisp shifting of any setup I’ve tried. The 36t gave me enough low end for the steep hill climbs in Tacoma and plenty of high end for faster club rides. I highly recommend this setup.

  • Dwainedibbly says:

    If you can spin faster, you can set the bike up with a smaller chain ring & gain lower lows. 80 rpms isn’t fast at all. If I’m going 24+mph, I want to spin anyway, just so that it *feels* faster. :) Change that 80rpm to 100, and you can lower your gearing by 25%.

    I like the concept and have been contemplating it for a while. I have an NOS Vitus 929 in a bronze color that I’m considering building up as a 1×9, using 9 of 10 on 7 because these bonded frames shouldn’t be spread beyond their original 126mm spacing. (See Sheldon Brown’s comments on 8 of 9 on 7. It’s the same idea, and it works, just barely.) Some nice upright bars, a Dura-Ace 10-speed bar-end, and a vintage Nervar 50.4 BCD crank, a honey brown leather saddle & grips to match…. could be nice. I already have the bar-ends & the crankset. I wonder if I can find a 50.4 BCD outer chain ring in the appropriate size. Some hammered VO fenders would go nicely with the aluminum stays, etc. Hmmm….

  • Pete says:

    I meant to ask Aaron if he used the old “reverse B screw” trick too. It’s pretty popular with the large cog crowd!

  • eddie f says:

    think it depends on the terrain where you ride. here in the east bay of SF, i would die with no gears lower than 40/34. that might make for a fun grocery/go to coffe bike, but not a real “let’s go do 50 miles” bike. of course age and conditioning are really impt. factors too.

  • Alan says:


    I totally agree, Eddie. In fact, when I lived in Seattle, I routinely used a 24/32 to get up the steep hills in the city with a full load.


  • jamesmallon says:

    Had a singlespeed I wanted more gearing on, as I now have a child to tote, but I wanted to do this as cheaply as possible (keep the old wheel): only a six-speed freewheel allowed that, so I have 40-80″, which is plenty coming from singlespeed!

  • Alan says:


    “If you go to a single chain ring, what will you use as a guard? I like that one that you on the outer chain ring now. Clean and simple.”

    I’m not sure. I’d like to see if I can get my hands on one of the Civia Bryant chainguards, but those aren’t available as an aftermarket item at this point. We’ll see.

    “You could go to a double crank and get the same effect? “

    Sure. Or even leave the triple on there. :-)

    One of the advantages of going to a single crank is a reduced Q-factor, something that I’m looking forward to because of my bad knee. This is reason enough for me to make the switch.

    As far as set-up, the main thing is to make sure the front chainring is centered on the rear cassette so you have a good chainline; this minimizes wear on the components and reduces the chances of dropping the chain.

    What some people may not realize is that mountain bikers have been using 1×8 and 1×9 drivetrains successfully for years. There’s a load of information about these kinds of drivetrains on the various mountain bike forums.


    PS – Sorry to hear about your flat – it’s definitely that time of year in CA!

  • Dan says:

    Nine gears seems to be enough for me too.

    I have two bikes, a 1973 Peugeot PX-10 and a 2008 Rans Rocket, both bought new. The Peugeot is now set up with a 50 / 34 double in the front and a 14 – 28 6-speed freewheel in the back. That gives me eight usable and distinct gear ratios, from 32 to 95 inches. I go up and down several long and/or steep hills every day, and this setup gives me all the range I need, and I don’t have any gaps where I wish I had another gear. I did wish for another gear when I still had a 5-speed freewheel.

    The Rocket came with a 62 / 52 / 39 triple in the front (to go with the 20 inch wheels) and an 11 – 32 9-speed cassette in the back. This gives a range from 24 to 113 inches, which is waaay more than I need. On a regular basis, I only use eight gear ratios, all on the two large front sprockets and the six smaller rear sprockets.

  • Tobias Linder says:

    I’m for while using a 1×9 setup with my foldable bike and it really fulfills my needs in almost all situations. I found two bike parts that improve the 1×9 setup and that is

    the SRAM X9 rear redrailleur with short cage.It’s especially made for 1×9 setups and is thanks to the very short cage very precise, light and robust. It works perfectly with my 11-34 setup.

    the 10% ovalized EggRings with chain guard rings. the chain guard rings keep the chain nIcely in position without the front derailleur and the ovalized ring improves the flow of the pedaling especially when the gear is not optimal because only 9 are available.

    Here a photo of my setup:


  • aj says:

    I’ve equiped my c.1984 Univega Nuevo Sport with four panniers with a 1×6 (39 x 14/28). It meets 99% of my needs for commuting and shopping, and I only have a wee bit of strain when absolutely loaded or pulling a fully loaded Wike cargo trailer up steep hills. I love it!

    Human Powered Commuting

  • MohjhoRyder says:

    Hey Matt in Tacoma, I too have a Cross Check but with a 42 tooth front ring and a Salsa chain gaurd. It is a single speed now, but I am planning to build a rear wheel with a 9 speed to expand my riding experiance. The down tube shifter seems like a simple idea, I find the bar end shifters get in the way. Im just not sure which derailer to go with, so many choices!

  • Alan says:


    Most rear derailleurs today perform pretty well, even the economy models. You’ll just want to be sure to use a Shimano derailleur with your downtube shifters because the SRAM derailleurs have a 1:1 actuation ratio and require special shifters (you probably knew this already). Here’s a good deal on a very nice derailleur:


    The mid-cage would be great for a 1×9 set-up.


  • Phillip says:

    Ryan-In response to your question about the chain coming off without a front deraileur. On thursday’s post I chimed in about the 1×7 drivetrain on my rainbike. I mentioned that I was initially concerned about dropping the chain but that it never happened even once. After I was done posting I began to ponder why I’ve had no problem when others have. I think it’s probably because the single I’m running up front is one of my old track cranks. The sprocket has taller teeth and no shift ramps or pins. Anyone else have a similar experience?

  • Alan says:

    @Ryan, Phillip

    I think the chain dropping issue may get more press than it deserves due to the fact that 1×9 drivetrains are becoming so popular among mountain bikers. Most of the folks I’ve talked to running single chainrings on the road rarely, if ever, drop a chain.


  • Alistair Williamson says:

    To everyone: What might be the 1×9 Reference Components?

    I’ll consider the Alfine Internal Hub as an internal gear reference. With it’s gear rations of 0.53, 0.64, 0.75, 0.85, 1, 1.22, 1.42 and 1.62, my (perhaps simplistic) calcuation has a 21 sprocket giving and 11-34 equivalent. That’s handy.

    Now what’s a reference brand and range for a cassette, hub and rear derailer that are equivalent quality? (e.g. an SRAM X9 set). Is the 11-34 the refence gearing spread? 12-28?
    I think that’ll help me, and other get a feel for a difference in price and weight.

    Cheers, Alistair

    P.S. I’m assuming and an intenal gear set up is about $75 and 750 grams more than a refence 1×9 derailleur setup.

  • doug in seattle. says:

    My road bike is equipped with what I call “country gears.” A 46/34 up front and a seven speed 11-32 in the back. I almost never drop to the 34, but when I need it, I really need it! The 46/32 combo is fine for all but the steepest hills in Seattle, especially since the cargo capacity is limited to a relatively small saddlebag. This bike used to have a standard 53/39 up front, but I have not regretted the change one bit. The new setup is perfect for the cruising day-tours I do all the time.

    My touring bike is more conventional, with a 46/36/24 crank and 8sp 11/32 in back. On my recent Cascade bike ride, I discovered I don’t actually need the 46 chainring when loaded. I only used it on the most extreme, paved descents, like Chinook Pass. Otherwise, the 36/11 was plenty of gear for me.

    I guess I am a slow rider!

  • Tobias Linder says:

    My EGGring actually also has a extra tall teeth for mono-use so the chain stays nicely on the ring and the chain guard helps also to make the chain stay where it belongs.

  • randomray says:

    I can see how the 1×9 will work , as teen my bikes really were ten-speeds . Frankly I can see where a three speed would be the perfect choice . I can see in a flat to rolling hill area a nine speed would be more then enough especially with a 42 chain ring . If you’re older with bad knees and serious hills you will need a little more . I’m kinda confused that you think that multi chain rings are so complicated . At least you can easily fix them . I wonder how many people fix thier internally geared hubs on the road ? Not that I have anything against them as they are very reliable and protected from the outside . These are just some thoughts not criticism as I’m sure my 17 year old could probably do fine on a single speed around here .

  • patrick says:

    It seems a little odd that nobody is talking about efficiency. The more crooked your chain is, the less effort is transformed into speed. That’s why back in the day you could get a five speed with a single front chainring, but when you put more speeds in the back it makes sense to put a deraileur in the front.
    also, the more crooked your chain, the greater the possibility of a dropped chain.
    you could put a chainguard on both sides, but the loss of efficiency in the extremes is absurd.
    have to agree, though, it looks better to have one ring up front.
    good luck.

  • Alan says:


    Hi Patrick,

    The Human Power Journal did a fairly rigorous study of derailleur drivetrain efficiency (http://www.bhpc.org.uk/HParchive/PDF/hp50-2000.pdf) and found losses due to crossover to be negligible (<0.5%) when compared to other losses introduced by factors such as cog size and chain tension (chain tension was a biggie). From the study:

    “It was found that chain-line offset and chain lubrication have a negligible effect on efficiency under laboratory conditions. Calculations of frictional loss resulting from offset indicate that this loss should be small compared to those produced by other mechanisms. This was verified experimentally. Lubrication effects on chain efficiency were tested using three different chain lubricants under a variety of test configurations. No significant quantifiable effect of lubrication could be inferred from these tests.”

    This doesn’t address accelerated wear, which may be an issue, but I think we can be fairly sure the loss in efficiency from shifting across 9 cogs is not a big issue for non-competitive bicyclists, particularly if the chainring is centered on the cassette.


  • John says:

    Kudos to those who can get by with a single chainring. I live in a hilly area and I’ve got a touch of arthritis in my knees. I’ll stick with my triple chainring and its granny gears. It works for me, and keeps me riding.

  • Alan says:


    I hear you, John. If I lived in a hillier area, I doubt it would have even occurred to me to consider going to a single chainring. Gotta’ keep those knees healthy! :-)


  • Bob B says:

    The majority of my riding is on single chainring derailleur bikes. They are simpler, more affordable, have a more elegant look and you can mount a chainguard if you like. Most people who do the math will see just how wide of a range they can get with todays components, whether you’re talking about a 14-34 6/7-spd freewheel or new 12-36 9-spd cassette.

    Our local bike coop recently set up a 1×8 bike with a manually shiftable inside chainring. The owner needs to haul a cargo trailer once in a while and needed a lower gear range for those rides. Most people don’t even notice it unless you point it out to them.

  • charles says:

    My tour/all rounder bike has 44x32x22 & 12-32, 8-speed cogs giving me I think about a 18-100 inch range which works fine for my old fat body and bad knees. On my “commute” bicycle I am converting my triple to a 26×40 and 12-28 7- speed which should give me about a 25 low up to around 90 gear inches. I will use a chain guard in place of the outer “big” ring. I spent about a year riding a deraileurless 2 – speed bike with 51 & 65 gear inches and learned to spin at 135 rpm so tall gearing doesn’t impress me much. You can’t often use it on the flats and anything over 35 mph on descents is pointless in my opinion. My thought is that most cyclists ought to ride much lower gears at higher rpm.

  • Alan says:


    “Now what’s a reference brand and range for a cassette, hub and rear derailer that are equivalent quality?”

    Perhaps a Sugino RD2 Track crank, Shimano Deore XT hub, XT mid-cage derailleur, 11-34 or 12-36 cassette, and Dura-Ace bar-end shifter…


  • Ted S says:

    Great article! I converted to a 1 x 9 a couple years ago to a “tribute bike” that I built from a 1982 Specialized that belonged to a late buddy. It works great in Sacramento Valley terrain. Yes,the chain slipped off the front chainring more than I would care to admit. However, Paul (bike parts mfr) makes a simple front chainring mount (“chain keeper”) that stops this issue completely. I have had ZERO problems since I installed the Paul chain keeper.

  • patrick says:

    thanks for the info. im not particularly fond of their method (using a motor instead of an appropriately inefficient biped) but it was worth a read. Apparently different lubes have no effect on efficiency, but the bit about chain tension was really interesting. it makes one wonder why deraileurs don’t snug the chain tighter. and why can’t there be a spring that adjusts tension as the arms get extended.
    also. i love the idea of the manual front derail-er that BobB wrote of. if you hit the hilly section just pop the chain over with your shoe. a secret double.

  • Ryan says:

    I had a similar experience with my Triple, a year ago I noticed alot of slipping in the middle ring and when I got to looking at it the 42 was very worn down and the 52 and 32 still had lots of life. My current steel bike, A triple equipped Handsome Devil, will eventually morph into a 1×8 or 1×9 with some moustache bars and become the dedicated errand commuter bike. I have looked at Sheldon’s wonderful gear calculator and knew the range would be sufficient for my riding (Hilly West Seattle) but its nice to hear from others that it works well in practice too.

    Alan thanks as always for the wonderful shots and making me feel like I need to wax my Bike!


  • Alan says:


    The study got me wondering about derailleur tension as well. I have noticed that modern derailleurs seem to have much more tension than what I remember from decades ago, so perhaps the manufacturers are addressing this issue.


  • Alan says:


    I lived in West Seattle many moons ago. Climbed Admiral Way nearly every day for years on a 42/52 with a 11-23 cluster; that was when my knees were young and healthy. Nowadays I’d probably need a 24 up front… :-)


  • Alistair says:

    @Alan (suggesting reference gear for a 1×9)

    Perhaps a Sugino RD2 Track crank, Shimano Deore XT hub, XT mid-cage derailleur, 11-34 or 12-36 cassette, and Dura-Ace bar-end shifter…

    Thanks Alan,
    very useful, there are so many choices. At first glance that looks like a $60 and 600g less that the Alfine; a little less than I thought. I converted my current workhorse bike (an old Cannondale 400) to an 8 speed a few months back but my mind inclines towards an internal gear setup and space for a grandkid on the back.

    Cheers, Alistair

  • Doug R. says:

    I am thinking I need a Surly karate monkey or cross check setup with a 1×9 drive train, hhhmmm?

  • Sweet William says:

    Been thinking about this for a while – mainly because front derailleurs are still the devil’s plaything. And 11-33 is more range than most of my twin-ring set ups anyhow.

  • Ryan says:

    Alan climbing Admiral, from either direction, in a 42×23 combo makes my knees ache just thinking about it! I keep my 26×32 in reserve but its nice to know its there ;-)

    You might notice a few bike related changes if you come back to our little burg; “Sharrows” on California ave and Beach Drive, an actual bike lane on Fauntleroy from the Ferry to Alaska ave, Bike path along the shoreline from Spokane street to the west end of Alki beach, triple bike racks on all metro buses, etc. The Ice cream at Husky deli remains unchanged its still fabulous.


  • Fergie says:

    Alan, you mention reduced Q factor as a reason to go to a single ring up front. In my experience, single and double cranksets have the same or similar q factors – when compared to a triple crankset the Q factor of a single is lower but the limiting factor for road (110 BCD) cranksets as far as Q factor is concerned is chainstay clearance. If your knees hurt, it’s far more likely due to bad bike fit rather than a too high Q factor. Just sayin..

  • Fergie says:

    Another interesting option for those with cash to burn and a desire to drop the front derailleur entirely while preserving a large range is the Schlumpf two speed crankset:


  • Alan says:


    My bad knee is a result of too many years of abusing it on mountain bikes and motorcycles combined with the after-effects of breaking my leg in multiple locations when I was younger. A lower Q-factor along with good fit are only mitigation for these underlying, long-term issues. In regards to the specific cranks I’m dealing with (Sugino XD600 versus Sugino RD2 Track), the difference of 20mm (165mm versus 145mm QF) will be a welcome change.


  • ontario bacon says:

    Chains falling off single chain rings:

    I have been using a 1X5 drivetrain on my road bike for some time…I was worried about the chain falling for some time, but it has never happened. I think there are a number of factors; with only five sprockets on back, the chainline is always pretty good. And I don’t have a tight chain. I suspect that the type of rear derailleur makes a difference. Some provide more wrap around the sprocket than others. It depends on how far forward the top pulley is located: the further forward, the more the chain wraps.

    ( the area that I live in has very few hills, and the few we have aren’t long. five gears is more than enough. the lowest one is just there for moral support. )

    There may be different ways for a chain to come off. If you are bouncing the back end, the chain can ride up over the back sprocket, and suddenly the segment between the top of the sprocket and the chainring is slack. If the chainline is off, or if you are cranking the chainring hard at that exact time, that is one way the chain can derail.

    It happened at least twice during this year’s Tour de France. Once, to Schleck, when Contador rode away. And got criticized for not being “sporting” and waiting…kind of ironic when half the field is hopped up on performance-enhancing drugs. And it happened again on the last time trial, some guy came churning out of the gate, his chain slipped off, and he wiped out right away. He was all right though. It kind of made me feel a bit smug that mine never falls off like that.

    But then on old time-trial bikes, it was common to have just a single front chain ring and a few small sprockets in back. I wouldn’t know what they use now.

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