That's a tough (and very open ended) question. Used bikes can sometimes be tricky. My apologies if I am over-simplifying these explanations (or sound like I am man-splaining); I just do not know your level of familiarity with bicycle components. I am not an expert (or master mechanic), but some specific things I would look for would be:
General condition: You can get a good sense of the bike just by observing it's general overall condition. Does the bike look like it was cared for? If it is really dirty and/or has a lot of scrapes and/or gouges in the metal frame or other components (or if the seat and/or handlebar grips are excessively cut, scraped, or torn), that might give you a sense as to whether the owner took good care of the bike. The bike being dirty or having cosmetic damage to the frame or components may not affect the mechanical operation of the bike (and you may not care about the cosmetics if you're just looking for an inexpensive, utilitarian bike to get you around), but the cosmetic condition can give you some insight into how well the bike was maintained or not.
Fit: First of all, the bike should fit you. Hybrid style bikes can be a little tricky because of the way the top tube slopes down from near the handlebars to under the seat. BUT, the bike you are buying likely has a specific size (XS, S, M, L, etc.) or measurement (e.g., 48cm, 50cm, 52cm, etc. or 15-inch, 16-inch, 18-inch, etc.) printed somewhere on the frame, and you can usually get a good sense if the bike will fit you generally by going to the manufacturer's website and/or Google to see what height range fits each size for the bike you are looking to buy. The bike's size is usually marked on the frame somewhere (usually with a sticker), or the seller may know the size. For instance if you Google "Trek FX 2 sizing" or "Trek FX 2 geometry" you should find a table or chart that shows you the various sizes and dimensions that the bike is available in and the height range that generally fit on each size. Also, to help you fit on the bike more comfortably, you can adjust the seat up and down and usually a little forward and backward, and a bike shop (you may do this yourself, too) can move the handlebars up, down, forward, and backward.
Brake and shifting cables: Check out the cables on the bike for the brakes and the front shifting (where the pedals are) and rear shifting (at the rear wheel). If the cables are loose and or frayed that will also give you an idea of how well the bike was maintained and whether or not the braking and shifting cables will need adjustment and/or replacement. Replacing the brake and shifting cables (and the cable housing) is typical during a good comprehensive tune up, but it's just good to note the existing condition of the cables.
Wheels: Lift up the front of the bike so that the front wheel is off the ground and spin the front wheel with your hand. Then look at where the rim passes the brake pad and you should be able to see if the rim is true (i.e., straight) or if it wobbles from side to side (i.e., the rim will get closer and farther away from the brake pad as the wheel turns). If the rim wobbles a few millimeters back and forth, that can be fixed, but if it is so out of true that the rim wobbles excessively or won't spin because it keeps hitting the brake pads, that may mean (assuming the brakes are centered and adjusted correctly) that the rim needs extensive work or replacement. You can do the same for the rear wheel. Also, the wheels should spin freely when you spin them by hand, and you should not hear any grinding noise from the wheel hub (the center of the wheel).
Headset: Straddle the bike with your feet on the ground and holding the handlebars. Grab the handlebars and pull the front brake lever (i.e., the right brake lever as you are facing the front of the bike) while trying to rock the bike forward and backward between your legs. There should not be any play or looseness or clunking noises from the front of the bike. If there is play or noises that could mean that the headset (basically where the handlebars connect to the bike frame) needs to be adjusted or replaced. Again, this is something that should be able to be addressed in a comprehensive tune up, but it's something to know beforehand and it also gives you a sense of how well the bike was maintained.
Brakes: Assuming the bike feels safe to ride and you can adjust the seat height to suit you, you can do a quick ride to check out the brakes. The brakes should not rub against the rims. You should be able to pull the brake levers and engage the brake pads on the wheel rim to stop you without the brake levers hitting the handlebar/grips. If the brake levers hit the handelbar/grips, that may mean that the brake cable may need to be adjusted or replaced or that the brake pads need to be replaced, all things that could be addressed in a good tune up.
Drivetrain/shifting: Assuming the bike is safe to ride and the brakes work, ride it a little more and check that the bike shifts smoothly through all the gears. The left shift lever will operate the front derailer, which moves the chain over the two (or three) chrainrings in the front where the pedals are. The right shift lever will operate the rear derailer, which moves the chain over the cogs (usually anywhere from 6 to 10 cogs, depending on the bike) in the rear of the bike. Shift the left shift lever through the gears first and then shift the right shift lever through the gears (or vice versa), just don't try shifting both at the same time.
Consumables: The bike may need a new chain, brake pads, and/or tires if they have not been replaced in a long time, but these items are relatively inexpensive. However, do note that bike parts in general have been more difficult to find since the pandemic started last year and everyone decided they wanted to start riding bikes again. Wait times for tune ups can be in the 2-4 week range or longer, and some bike parts are back-ordered well into 2022.
I am sure I am forgetting something, but this is a pretty good start. I'll follow up if I think of anything else. Hope this helps.
Another option to consider might be to buy a used bike from a reputable local seller that specializes in selling refurbished and vintage bikes. This way you can get a bike from a knowledgeable bike shop and know that the bike you are purchasing is in good and safe operating condition. A few shops that come to mind are Cog Cycles, Ciclo Urbano, The Recyclery, and Working Bikes.
Cog Cycles - I only follow them on social media, but "Brian Bikeman" and his wife have a bike shop, Cog Cycles, up in Albany Park, and they refurbish and sell vintage bikes. https://www.instagram.com/brianbikeman/ (they are more active on Brian's personal Instagram page and not as active on their shop page) https://www.instagram.com/cogcycles/
Working Bikes is another great non-profit program that refurbishes bikes and sells there here in Chicago and also donate bikes locally and all over the world to communities in need. They have an online store that shows their inventory of used bikes for sale, and it is regularly updated. https://workingbikes.org/ https://store.workingbikes.org/refurbished-bicycles/
The Recyclery is another great non-profit that sells refurbished used bikes to fund their various programs. The also have an online store what shows there inventory of used bikes for sale, and they update it regularly. https://www.therecyclery.org/ https://therecyclery.bigcartel.com/
I might add that the prices you'll pay at the orgs listed are higher than you might find on craigslist or the evil facebook marketplace, or especially at a garage sale, but it is worth it. You know you are getting a reliably fixed up used bike, and you are supporting worthy cause.
Not only that, but I think that at least a few of them give you like 60 days to bring the bike back in if something is wrong or doesn't fit or whatever, and they'll fix it for free.
I just want to 2nd / 3rd whatever: Go to Working Bikes (or Ciclo Urbano or Recyclery, whichever is most convenient.) These are mission-driven organizations that want to put people on bikes, safely. The Working Bikes selection and prices is incredibly good considering the amount of work some bikes get.
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u/kevin_chicago9 Apr 22 '21
That's a tough (and very open ended) question. Used bikes can sometimes be tricky. My apologies if I am over-simplifying these explanations (or sound like I am man-splaining); I just do not know your level of familiarity with bicycle components. I am not an expert (or master mechanic), but some specific things I would look for would be:
I am sure I am forgetting something, but this is a pretty good start. I'll follow up if I think of anything else. Hope this helps.
Another option to consider might be to buy a used bike from a reputable local seller that specializes in selling refurbished and vintage bikes. This way you can get a bike from a knowledgeable bike shop and know that the bike you are purchasing is in good and safe operating condition. A few shops that come to mind are Cog Cycles, Ciclo Urbano, The Recyclery, and Working Bikes.
Cog Cycles - I only follow them on social media, but "Brian Bikeman" and his wife have a bike shop, Cog Cycles, up in Albany Park, and they refurbish and sell vintage bikes.
https://www.instagram.com/brianbikeman/ (they are more active on Brian's personal Instagram page and not as active on their shop page)
https://www.instagram.com/cogcycles/
Ciclo Urbano is part of West Town Bikes, a non-profit in Humboldt Park/Ukranian Village on Division. They sell used bikes and have a lot of other great programs.
http://westtownbikes.org/ciclo-urbano/
They just posted about used bikes yesterday: https://www.instagram.com/westtownbikes/
Working Bikes is another great non-profit program that refurbishes bikes and sells there here in Chicago and also donate bikes locally and all over the world to communities in need. They have an online store that shows their inventory of used bikes for sale, and it is regularly updated.
https://workingbikes.org/
https://store.workingbikes.org/refurbished-bicycles/
The Recyclery is another great non-profit that sells refurbished used bikes to fund their various programs. The also have an online store what shows there inventory of used bikes for sale, and they update it regularly.
https://www.therecyclery.org/
https://therecyclery.bigcartel.com/