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Thread: Why 27.5?

  1. #1
    mtbr member
    Join Date
    Aug 2005

    Why 27.5?

    As I go further down the rabbit hole of gravel biking I see plenty of manufacturers are selling 27.5 inch specific gravel bikes/frames and I donít see why.

    We all know why 27.5 is sometimes preferable for mountain bikes but none of those advantages appear to apply to gravel riding.

    Unless youíre a shorter rider, 700c/29 inch wheels seem like the only choice for covering the varying surfaces of gravel grinding. So what I am missing about 27.5 for gravel?

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  2. #2
    mtbr member
    Reputation: SteveF's Avatar
    Join Date
    Mar 2004
    More rubber and air volume with no increase in overall wheel/tire diameter. That matters more in smaller frames as you surmise, but it can also effect stability, agility and handling in any bike. Once you get over 35mm wide tires I personally find that a 700c wheel/tire starts to feel a little ponderous. Some people appreciate the stability, rollover and momentum advantages that a larger overall diameter brings but I like a bike that's a little quicker to react to turning input but that still has a generous width and air volume for secure cornering, cushion and float in loose dirt, so 650bX42-52 suits me well. It really comes down to personal preference-it's nice to have options!

  3. #3
    Reputation: amish_matt's Avatar
    Join Date
    Aug 2006
    I'm probably going to throw a set of 27.5 wheels with 2.2" tires on my Checkpoint. For some rides (those that include singletrack), I'd like a little more tire than my 700x38. But I already have toe overlap, and that becomes more of an issue on singletrack, so I want to keep the diameter in check.

    If not for the toe overlap, I'd just run bigger 700c tires.

  4. #4
    mtbr member
    Join Date
    Dec 2016
    If you want to run big tyres, and by big I mean around 47mm+ 27.5 makes more sense on a gravel/adventure/big tyre drop bar/whatever bike.

    It's MUCH easier to design a frame to accept a wide 27.5 in the back. Most of the time gravel frames use 68mm BB shells and have clearance for a road compact crankset. This makes things complicated compared to MTBs when you want to cram a big tyre in there.

    Another big one, much less toe overlap. It's super annoying. Unlike MTBs gravel bikes have steep head angles and shorter top tubes. Throw on a big 622 tyre, maybe fenders too and the toe overlap can be deadly even on larger frames.

    Multi wheelsize compatibility. Generally speaking a 622 28-30mm tyre is about the same as a 650x47mm in terms of height. So if you want you can switch back and forth between a more offroad and a more road oriented setup without affecting BB height etc.

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