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I was checking some bikes on Decathlon (a large sports orientated chain store) and I found this size guide for some of their BTwin bikes. On their chart [pdf] that maps rider measurements to bike frame size they have some overlap. So for some measurements, you can have two bike sizes that will fit you and the recommendation is:

  • Larger bike if one desires more comfort
  • Smaller bike if one focuses on performance

My question: Is there any reason for this recommendation? Size Guide

I dont know enough about bikes to realise if this question is too opinion-based, I hope not.

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Look at the angle of the guy's back, that's the key to why they say that. For performance, the back will be in a more horizontal position.

You need to be careful, though, how much faith you put into this sticker - if at all possible you should try both sizes and judge for yourself. Also, don't forget that different frames, with different geometries and tube angles, will also come into play. If you test the two sizes, and you still end up thinking that neither one fits you better than the other, at that point you could apply the smaller=performance/larger=comfort principle.

This advice is especially true for somewhere like Decathlon, which is a large enough store that a shop will probably have both sizes in stock (or could get hold of them if you ask).

I bought a hybrid bike from Decathlon many years ago, they had height versus size charts then, but I remember totally ignoring them and buying the size that they said was for someone a whole 15cm/6" taller than me, just based on how comfortable that particular size felt when I rode it. I'm not saying that's how it would be for you and for this bike, it's just how it was for me - it's quite an individual thing.

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  • Thank you very much. I dont trust the image a great deal. For a smaller bike, I'd expect the top tube to be shorter so the driver would be more vertical. Isn't that correct? – user3371583 Jan 1 '15 at 14:34
  • I think the image is good as a schematic. As regards the variance between sizes, I'd expect (but haven't checked) that all tubes would vary in length, keeping the proportions (angles) the same. – PeteH Jan 1 '15 at 14:44
  • Of course even with the exact same tubes a decent fit including handlebar and stem height and angle can make a big difference in back angle and therefore the comfort /performance tradeoff. – Chris H Jan 1 '15 at 17:14
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    I think the idea is that on a small frame the seat would have to be raised higher to get proper leg extension, making the seat higher relative to the bars, where as on the larger frame the seat would be lower relative to the bars. – Kibbee Jan 1 '15 at 22:20
  • @Kibbee That makes a lot of sense and explains why he's hunched like that. Thanks a lot. – user3371583 Jan 2 '15 at 11:13
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I second the suggestion to try the bike. My experience is as follows

I'd always thought that having a bad and inflexible back that I would prefer to have my road bike handle bars more or less at the same height as my saddle. So two road bikes later, set up in this way and i've not been comfortable. In fact I've found more comfort in road bikes that have handle bars significantly lower than the saddle or in city bikes where the handle bars are significantly higher than the saddle.

So big picture you'll need to try it.. and be open that you may not get it right the first time. I've spent loads of time playing with saddle position, handle bar position etc to find that perfect fit... I'm almost there... I think.

Some considerations for you. If you are intent on city riding.. cycling on bicycle lanes and start-stopping in traffic and looking out for pedestrians, being as upright as possible is much more enjoyable / safer

But if you're going for more or less uninterrupted speed and distance, a bike that gets you more hunched over will be more aerodynamic (especially over the 25 km/h) range

I'm almost 180cm and prefer my slightly undersized 54cm bike over a more 'appropriate' 56cm

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  • I've been there too - your sore back feels better with the forward position is because its a nice gentle stretch. Consider it a light massage for your spine while riding. – Criggie Sep 30 '15 at 0:30
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It would be GREAT if you could try both, as several people mentioned here. Or if the shop can provide bike-fitting with an expert, that would be excellent. In general theory is not always right when it comes to bike sizing, you have to try by yourself.

In all cases, here are some notes:

  • Assuming that it doesn't differ with you which 'mode' you want to get into, back flexibility is KEY to whether you could/could not go into performance mode. I personally have some lower back issues that hold me back from going to performance mode for a long period of time.

  • Bear in mind that your inseam & torso lengths are critical, it's not just a matter of how tall you are. High inseam means that you would probably raise the saddle and so your handlebar reach will be affected. A long torso means that you can have flexibility raising the saddle without pressuring your back as much.

I wish you best of luck in finding the right size of you. Let us know how it went off for you :)

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