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Clear guide on bike sizing?

  • 24-01-2022 7:35pm
    Registered Users Posts: 2,192 ✭✭✭

    Can anyone point me in the direction of a guide for sizing a bike? Everywhere I look there seems to be different advice, and most of it is conflicting.

    I figure that the key factors that will drive everything will be your height, inside leg and arm length. So how do these tie into the geometry of a bike? Should I be checking the seat tube length with my inside leg and top tube with my height and arm span?

    Also I assume that the same guide for a mountain bike or hybrid wouldn't apply to say a road or gravel bike, am I right? Like with a mountain bike you don't have drop bars so surely you can't apply the same rules.

    It seems like a minefield out there and it feels like any bike salesman will sell you whatever bike is in stock so it's hard to get their unbiased assessment.


  • Moderators, Category Moderators, Arts Moderators, Sports Moderators Posts: 49,231 CMod ✭✭✭✭magicbastarder

    Always look for stack and reach measurements on frames or bikes. They're not subject to the vagaries of what one manufacture would consider as small ior medium, and are thus a much better way of comparing between brands.

  • Registered Users Posts: 2,285 ✭✭✭Mercian Pro

    Stack and reach are great if you already know what figures suit you and your type of riding but AFAIK, there is no published method of working out what numbers you should be looking for from your body measurements. That's where bike fitters come in and, having analysed your measurements, flexibility and riding intentions, will provide you with your ideal stack and reach dimensions. As MB says, armed with these you can then shop around ignoring irrelevant numbers like 54 or 56 and letters like M or M/L and, having bought the bike of your dreams, go back to the fitter for some fine-tuning of saddle height and stem length.

  • Registered Users Posts: 2,192 ✭✭✭VonLuck

    Thanks, but unfortunately that only helps in comparing bikes to each other. Need to figure out what dimensions work for me to get a starting point!

    I'm only getting my first 'proper' bike so I don't want to spend a chunk of money on a professional fitter before even buying a bike. Is that the norm though? There must be some sort of starting point before you go down the more expensive route, like for a novice?

  • Registered Users Posts: 583 ✭✭✭ARX

    Bike manufacturers usually have size guides on their websites - they'll recommend a size based on your inseam and height. But that doesn't really help you - for example, Giant might recommend a TCR in size M based on your dimensions, but that doesn't mean you'll be comfortable on a TCR, which is a racing bike with a low front end that's lower and further from the saddle than (say) a Giant Contend, which is similar in appearance but will put you in a more 'upright' position.

    Really you'd need to sit on a few bikes (easier said than done, I know) to see what sort of style suits you. If it's your first proper bike, a racing bike might not suit you - on the other hand, if you are very flexible it might suit you perfectly.

    A good bike shop would be able to advise you on what would work for you. I don't think a good bike shop would sell you anything they have in stock just to get rid of it, because they have no problems getting rid of stock at the moment - there is a shortage of bikes globally. That's not to say that a bike shop wouldn't send you away on a bike that's the wrong size. From what I understand, people tend to buy bikes too large for them.