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I'm new to bicycling (I have a 7 speed HEAVY beach cruiser I use for 10-20 mile rides). I am looking into used road bikes now. I checked several places to measure what size frame I should get for a road bike. Most recommend 56cm for me but some suggest a 58cm might be fine.

Inside Leg: 31.75" (80.6cm) Height: 5'9" (175.2cm)

I'm looking to buy used and a listing I'm highly considering was wrong and is 58cm instead of 56cm (it's a Cannondale SAECO). Should I stop pursuing this bike and wait for a 56cm frame or am I overthinking this?

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    Ultimately, you're going to want to try the bikes out. Each manufacturer has their own sizing schemes and they're not necessarily 100% cross-comparable. – MaplePanda Jul 22 at 23:10
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    2 cm is really not a lot. You're better off getting a leg over candidate bikes and seeing what you find comfortable now. – Criggie Jul 22 at 23:23
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    Medium/54cm or med-large/56cm is probably more realistic for someone your height – Argenti Apparatus Jul 22 at 23:50
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    I'm 6'1" and 56 is the sweet spot for me, I'd do 54-52 if I were you – whatsisname Jul 23 at 5:03
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    I am 174 cm tall and have to small (S) size bikes - e.g. marked 52cm, effective seattube length 53.5cm, I followed the manufacturer's recommendations. 58 cm seems too big. – Vladimir F Jul 23 at 5:54
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You have a few things to consider. First a Cannondale 56cm geometry is not identical to a Giant, Specialized or likely any other brand. Each brands designer has a specific goal in mind when they design a frame. So you can make a generalized statement that you may usually ride a 56cm it is not a hard and fast rule. One thing to consider is with the advent of compact frame designs the dimensions become virtual. This means that measuring the frame will yield a number other than 56cm. If the frame isn't marked for size you have to verify manufacturers dimensions to determine size or ask the owner how tall they are. The most important thing to remember is not size but fit. The most critical is stand over height. If you can't comfortably straddle the frame it's too big. It also has to be comfortable, if you haven't ridden a drop bar bike before it will likely feel awkward for the first few miles. I would test ride a few 56s but don't overlook a 54 or two for test rides. Similar to buying a house or car try not to fall in love with the first one you ride.

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    I suspect that 58cm is going to be way too large here. – jwh20 Jul 23 at 16:04
  • Better a frame 2cm under the ideal than 2cm above. – Carel Jul 24 at 12:09
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You'll really want to look at the specific geometry of the bike, rather than the seat-tube size. As stated in the other answers, standover height is important, and should be shorter than your inseam by 1-2cm. Besides that, Stack and Reach are probably the second most important measurements to consider.

If you can't try on a bike in person for pandemic reasons, there are many ways of calculating your "ideal" stack and reach by measuring various parts of your body and punching them into online calculators. I'll leave a couple of links below.

https://www.competitivecyclist.com/Store/catalog/fitCalculatorBike.jsp https://www.bikeexchange.com/promo/bike-size-calculator

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    There are websites that allow you to compare bikes geometry visually like 99spokes.com. Can't imagine buying a new bike and not comparing its geometry with my previous bikes. This helps me to consciously ignore the manufacturer's sizing recommendations in some cases to achieve the desired fit. – Sapphire64 Jul 24 at 8:59
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As many have pointed out in comments and answers, bike sizing is not an absolute but a “fit window,” where it is possible more than one frame size could work for a particular rider. That said, given you dimensions, the fact my height and inseam are within a cm of yours, and the fact I have trialed risen a large number of brands and sizes I can say with a high degree of confidence that you will not be happy with a 58cm Cannondale road frame.

Their bikes tend to fit low and long, the 58 will be too long with out resorting to extreme measures (e.g., ultra short stem). It might work as a flat bar road bike, as you typically want a longer top tube with a flat bar setup - but given that you are a beginner I doubt you want to undertake that costly change.

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