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I don't own a bicycle. Whenever I go cycling, I would need to rent one. The first thing that I always check is to see if the brakes are working properly.

Sometimes, I have no idea that the gear is faulty until I encounter a steep slope where I need to pedal hard. This may cause the gear to jump or slip if it is faulty. Other times, when I change the gear, the pedalling suddenly becomes inefficient, with constant grinding noises from the gears.

In the end, I try not to change the gears at all to avoid the above problems. I would like to know if there is a way to visually inspect the gear and tell the shop assistant that, "Hey, the bike is faulty!", instead of coming back halfway to do the exchange.

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2  
Its going to be difficult to inspect things like gears visually, but there's nothing to stop you spending the first five minutes of your rental just riding around the block to make sure everything checks out. – PeteH May 26 '13 at 7:36
    
@PeteH, the problem is the terrain is usually quite flat around the bike rental area to be able to test the gears properly. – Question Overflow May 27 '13 at 3:02
    
you should be able to click through lowest to highest without much of a whimper, I'm not sure you can do much more of a test than that – PeteH May 27 '13 at 8:33
    
Find a bike rental that rents good stuff, and continue to go there. The brake check is a wise idea for your own safety, but much more than that would be doing their job for them. If the rental bike is poor, simply tell them on return that's why you'll not be returning. Another option is buying your own bike and finding somewhere to store it, either in a lockup, or even at the bike rental place, for a fee. Folding bikes are possible, through they aren't as much fun as a big bike. – Criggie yesterday

If you're talking about a derailleur-style bike, a quick check is to suspend the rear wheel somehow (sometimes you can hang the nose of the seat on something, eg, or see if the rental place doesn't have a work stand) and crank the pedals with your hand while you shift through the gears. On a bike with indexed shifters the transitions should be smooth and you should hear no more than very slight "chatter" from the chain once the transition is complete.

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Also, avoid running the smallest chainring with the smallest cog/largest chainring with largest cog...this is called cross chaining and will often result in a lovely grinding sound as the chain drags along the front derailleur cage. – WTHarper May 26 '13 at 13:49
    
Is it possible to tell based on the amount of slack on the chain when switched to the lowest gear? I am more concern on the gear slipping when climbing up a steep slope. – Question Overflow May 27 '13 at 3:33
    
@QuestionOverflow - Place the chain on the smallest front sprocket and middle rear sprocket. Look at the derailer and chain. The derailer should not be curled up all the way (though close) and the chain should not be slack. – Daniel R Hicks May 27 '13 at 12:29
    
@QuestionOverflow - But slippage is often due to a worn chain/sprocket. Place the chain on the large front sprocket and pull out on a link near the front side. The chain should not pull away more than about 2/3rds of the height of a sprocket tooth. – Daniel R Hicks May 27 '13 at 12:32

In addition to improperly tuned derailleurs, drivetrain slippage may also be caused by a worn chain or cassette.

Worn Chain: One way to check whether the chain is worn is to use a chain wear indicator. They're cheap and easy to carry with you, but if you'd rather not buy one and if the rental place has a repair shop, you could ask to either borrow theirs or see if they could check the chain wear. You would use that tool by placing it in the chain like this: Chain wear indicator

If the indicator is able to be completed inserted into the chain (case #1), then the chain is worn and may be prone to slipping.

Worn Gears: To quickly check the gears (the chainrings, cassette and pulleys) for excessive wear, look for "shark fins". This is where the teeth of the gear has been worn to the point that the top of the tooth looks more pointed than flat, reminiscent of a shark's fin. This is a sprocket that is very worn:

Worn sprocket

If this is present on your rental bike, the chain will very likely slip under load (like when you're out of the saddle) - especially when combined with a worn chain.

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If OP can't store a bike of his own he's unlikely to store bike tools. – Criggie yesterday
    
True, not that chain wear indicators are large by any means, but that's why I suggested they ask the rental place if they have one he could borrow or that they could use to check. – altomnr 20 hours ago
    
And there's the difference between borrowing a bike from a volunteer coop or bike library, and renting a bike from a shop/rental company. As the customer, you're paying good money for a working bike, and if its not, stop renting from there. – Criggie 15 hours ago

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