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  1. #1
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    New question here. Sizing: is Trek misunderstanding my body shape or am I misunderstanding modern frames

    Hi all

    newbie on this forum. I just bought a new Trek Roscoe 6 after a few years away from cycling.
    I am wondering if I am misunderstanding the nature of this bike or if the Trek sizing chart is misunderstanding the nature of my short-torsoed body!

    So to start, here is what the Trek size chart says:
    • 17.5: 161-172cm/5'3.4-5'7.7" tall & 76-81cm/29.9"-31.9" inseam
    • 18.5: 170-179cm/5'6.9"-5'10.5" tall & 80-84cm/31.5"-33.1" inseam


    At 5"10/1m78, on paper I should be all the way at the top end of the 18.5 range (I even fall within the 19.5 range).

    And yet when I tried the 18.5 in store I felt a little bit overstretched. Nothing crazy but enough to notice that my shoulders were not completely relaxed and enough to notice my lower back (and if that's the case during a test ride, imagine after a day of riding). But it definitely felt stable at speed and pretty good overall.
    Perhaps it's because I have long legs (34inch/86cm inseam) and a pretty short torso (and -2cm ape index).

    On the 17.5 I felt right when sitting, no pain, comfy fit, but when I get up on the pedals I feel a touch more forward compared to the other one. Nothing crazy. But it felt a little bit less confidence inspiring when cornering at speed so I can only imagine once on a steep descent (sadly I was trying it in the city, couldn't test it in 'real life conditions' ).

    So my question is:
    • am I completely misinterpreting this bike, and should get the 'recommended' 18.5 size, swap the stem and maybe the handlebar, so I can make the most of the bike?
    • Or does it just have to do with my body dimensions, making their recommended size irrelevant, and the 17.5 is the right size since I feel stress-free on without making a single modification?



    My worry is of course that if I get the 18.5 and fiddling with stems and handlebars doesn't fix it, I'm left with a bike i hate, don't ride and end up reselling it.
    Or, on the other hand, that I get the 17.5, all good when sitting, but as soon as I get up on a fast trail I feel like i'm on some kind of kid bike ready to throw me over the handlebars at the first root! And then I completely miss out on the potential of a great bike!

    As if I was 'in between' the 17.5 and the 18.5 when on paper I am in between the 18.5 and the 19.5!! Or am I just an idiot, used to bikes that are too relaxed and should trust Trek and go for 18.5 to get the full potential of this bike
    But I also have a little voice inside telling me to just get whatever I am most comfortable on...(17.5) but I also know that comfortable is different when test riding it on city streets and when blasting down a trail...

    Going a little bit nuts with this dilemma lately

    Any input super welcome!

  2. #2
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    It's up to your preferences and riding. I'm 5'9 and 18.5 was good for me stock as it was delivered. I could go down a size for a more "sporty" ride.
    Do the math.

  3. #3
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    I am 6 foot and short torso-but a 34.4 bike inseam and a +1 ape-----the trek fuel (granted a different model)n 19.5 was to small in the climbing position----top tube just to short without any doubt Be careful lots of chatter here on trek sizing and more than a few bought to small. I would guess you are at least an 18.5 and could ride a 19.5---but a 17.5 seems significantly small .
    Honestly the parking lot test is not worth much--try to demo as only you can really decide

  4. #4
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    Have you been riding a while?
    Because your comments sound like the issue a lot of us have; where we've gotten so used to bikes that are way too small, that when one actually fits it seems weird. Plus stupid ideas like KOPS used to be conventional wisdom, which means having the seat way too far back.

    The dealer should swap stems or bars out for you, before you commit to buying one size or the other. They should also let you ride it more than just the parking lot. If they aren't willing to do both, find a dealer that is.
    Trek is not known for their bikes being "big for their size"

  5. #5
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    This bike has a 40mm variation in reach or Effective Top Tube length between the 2 sizes that you are looking at. That is a lot.
    Forget the Seat Tube length, all MTB's are short in this respect.
    Swapping stems out means either adding to the 17.5 or shortening up on the 18.5. Problem here is that modern bikes don't allow for much shortening.
    Because the front centre also lengthens by 40+mm the 18.5 will ride more solid and the 17.5 will be more agile.
    Match the bike to where you willl ride.
    I don't consider this bike to very 'geo forward' modern. The seat tube is very relaxed at 71 degrees and is much like an old school bike with a lengthened Top Tube. Quite a heavy bike.
    Any reason why you are fixed on this particular bike?
    I would look around a bit more and try some other options if you haven't already.

    Eric
    If I don't make an attempt, how will I know if it will work?

  6. #6
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    I can pretty much guarantee you need the 18.5.

    If you have long legs, you'll be at the top of the range. Cut 3 inches off your inseam, and you're still in the fit range for the 18.5. I'd go as far as saying you're closer to the 19.5 than the 17.5.

  7. #7
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    Hell if I know which size is right for you, but I do know that folks with short torsos like ourselves are more particular about fit because we have a smaller "sweet spot." Get used to threading the needle.
    2017 Diamondback Haanjo
    2016 Pivot 429T
    2011 Kona Unit

  8. #8
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    Make the dealer install a shorter stem and also move the grips inboard to mimic less wide bars. He should do that as a minimum before you buy. If he won't, he doesn't deserve to sell you a bike.

  9. #9
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    I didn't look up the geometry however if the other reply is correct and it has a 71 seat tube then it just may be a geometry that doesn't work.

  10. #10
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    I'm 5'11", have the same inseam but +4 ape index. I rode a 2017 Fuel Ex 8 for a year or so. I rode the 18.5" and I just couldn't deal with the slack seat tube angle. This is right before STA became the next big important # in geometry marketing. Anyway with the post extended I'd be really close to the rear axle. Pedals felt too far in front of me. Those in combination with he shorter chain stays made the bike a wheelie fest while climbing steep terrain. I could push the seat forward which helped a little.

    Given my reach and once my skill level matched and started to exceed the bikes capability I definitely wish I had gone bigger than 18.5". I would definitely fiddle with the 18.5... shorter stem (18.5" stem is like 60mm stock), handle bar roll. The bikes as they sit are just starting points to fit...

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