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I've bought a road bike for my wife. The bike size is 45cm or XXS. The problem is, the frame is high for my wife. Can I change the rim from 700c to 650 to reduce the problem? For info, my wife height is 155cm and her inseam is 68cm.

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    What kind of brakes are on the bike? If they are rim brakes it would be very difficult to change rim diameters. You could change diameters if the bike uses disk brakes, but it would probably be cheaper to just buy a different bike. Oct 11 '20 at 16:08
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    According to a sizing chart I found after some quick search, this size seems perfectly right for your wife's height and inseam. One must consider however that she won't be able to put both feet on the ground while being seated in the saddle which is normal on a road-bike. I sense that the problem resides in that fact.
    – Carel
    Oct 11 '20 at 16:17
  • If your bike has caliper brakes, a switch from 700c to 650b is usually achievable by switching to long reach brake calipers such as Tektro R556. However a switch to the significantly smaller 650c wheel size is generally impossible unless you are using disc brakes.
    – Andrew
    Oct 11 '20 at 19:32
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    Changing to a smaller diameter wheel such as 650b is a possibility if the brakes are changed but you must also bear in mind that the bottom bracket height will also be lower and therefore there will be an increased risk of the pedals contacting the ground when cornering.
    – JoeK
    Oct 11 '20 at 20:47
  • If this is a new bike see if you can return it and resume your search for a bike that fits. Some manufactures make road bikes in XS and XXS sizes that have 650B rather than 700C wheels, which helps achieve the smaller geometry. Oct 11 '20 at 22:38
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Changing to a smaller wheel may cause other problems, such as needing brakes with a longer reach (that won't work as well), or the bottom bracket is lowered, so the pedals may ground around corners.

This bike is super small already - the fundamental problem is, as you have guessed, the wheel size, which governs the dimensions of the frame.

I would be looking at a bike with 26" wheels, or even 24" - they generally didn't make road bikes with these wheels, so you might need to adapt a mountain bike or a kids bike. It is quite possible to put road bars and gears on these frames.

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When you say "the frame is too high", what exactly does that mean? Can your wife stand over the top tube with at least a few centimeters of clearance? If so, then the bike is probably the right size. Many new riders expect to be able to put a foot flat on the ground while seated on the bike, but that requires a very low (and inefficient) seat position.

Also, check what shoes she's wearing when she tests the bike. Running shoes have much thicker soles than flip-flops. She should test the bike wearing whatever shoes she would normally go for a long ride in. I've seen cases where the bike didn't fit because the rider was wearing thin-soled shoes that day, instead of the running shoes she usually wore when riding.

I suggest you bring her and the bike to your local bike shop and get some fitting advice. Ideally that would have happened before purchasing the bike.

Changing wheel size is not really an option. As posted in Mikkel's answer, the frame is designed for a particular wheel size and you don't want to mess with that.

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