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Sizing dilemma: am I misunderstanding the bike or is Trek misunderstanding my dimensions?

curiousbynaturecuriousbynature Posts: 10
edited April 2019 in MTB buying advice
Hi all :)

newbie on this forum, and posted something similar on another forum as I was waiting to get my account activated here so apologies for that

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! :D


  • Ive had similar issues to you in the past with other brands of bike, im 5'11 with 33" inseam and longer arms. My last F/S was 19.5" and I struggled to get a comfortable position with changing stem and saddle position. My current one is 18.5" and feels much more natural and easier to handle, Ive also got a 17.5" HT which I put a slightly longer stem (80mm) and wider bars (747mm) and that feels good too.

    Id suggest the 18.5" then adjust the bar height and width - the stem already looks pretty short on the website pictures, so a few extra spacers to raise it a bit. It comes with 750mm handlebar - you can experiment with bar width by moving the components and grips inwards before you need to cut the bar but you could easily take it down to 700mm which will put you a bit more upright / less stretched out.
  • The RookieThe Rookie Posts: 27,748
    Height is a crappy way of determining bike size.

    The saddle height is pretty much infinitely adjustable, as you note the length is not.

    I have the opposite problem to you, short legs and a long torso and I'm always riding up a size to get one to fit my reach, in your case it wouldn't surprise me if the 17.5 would suite you better, that said a gentle pootle will always make you feel more stretched out compared to riding harder where the torque reaction from pedaling is always trying to push you upright.

    A simple fix is swapping the setback seatpost the Roscoe comes with for an inline, that will give you about 20mm which is commonly the ETT change between sizes, that has less effect on how the bike rides than changing the stem or bars although it doesn't change reach when out the saddle.
  • Thanks!

    For now I think I’ll stick to 17.5. While 18.5 with some mods could be the perfect one, I am a bit cautious about giving away one on which I’m comfortable (and yes may be a bit sketchier when descending fast) for one that may never get comfortable
  • Lemonenema: thanks for the suggested mods. All makes sense. I indeed thought about shorter stem, narrower bar, perhaps with more riser.
    But given your experience of fitting better on the smaller one, how come you suggest picking the larger one?

    The Rookie: fiddling with the seatpost is a bit like shifting the saddle foreword and backward... it affects my knee position a the pedals. So I would rather adjust the reach only with stem and handlebar... at the expense of altered handling. But thanks for the suggestion
  • steve_sordysteve_sordy Posts: 2,131
    @curiousbynature: As I'm sure you have discovered by now, bike shapes vary across manufacturers as well as sizes between bikes. You will just have to keep searching until you find a geometry that suits you (or close enough) and then make the mods.

    Like Rookie, I have the opposite problem to you. What works for me is to know what the reach and stack dimensions are that I can work with. I have successfully bought bikes without the chance of riding them first. A combination of saddle height and fore/aft changes, followed by raising/lowering the bar, and sometimes bar rotation as well have all made the essential difference. So far I have not had to make any changes to the stem and/or bar width.

    Don't give up and settle for something that isn't close enough; there will be a bike out there that suits you. :D
  • Thanks. Valuable advice. Unfortunately buying the bike through a work scheme so I’m limited to a certain budget and bike shop chain (that sell certain brands).
  • Yes I agree with you regarding knee pain / saddle position and height, thats why I didnt suggest it before. Saddle and seatpost for general riding should be set to the correct pedalling position rather than to compensate for frame size.

    I got the 17.5" frame for the HT because it was heavily discounted and I wanted it as a fun project so didnt mind a few compromises.
  • The RookieThe Rookie Posts: 27,748
    I've yet to see any rational explanation of how the vertical orientation between BB and sit bones affects knee pain, the legs don't know if your riding an MTB, a road bike (more upright) a Tri-bike (even more upright - some have 80+ degree seat tubes) or a recumbent (laid and perform the same circular motion of the feet via the same knee articulation, none of those create more or less knee pain. In my opinion it's an 'urban myth' spread without applying any logical thought.
  • steve_sordysteve_sordy Posts: 2,131
    Vertical orientation I will take to mean the height difference from top of saddle to pedal axis when in the lowest position. This dimension affects the angle that the knee joint is operating at peak pressure. We all know from personal experience that if you get the saddle too low, then it takes more effort to pedal up that hill. It's to do with the angle of the knee joint that allows for max leverage from the body. Also, if you get the saddle too high, your hip bones are rocking from side to side and you are unstable. The regular advice to get the Goldilocks position for efficient pedalling is to put the heel on the pedal when it's at the bottom of its stroke and to have the leg almost straight and definitely without the knee being locked out.

    Horizontal orientation I will take to mean the horizontal difference from the centre of the saddle to the pedal axis when in the forward position. I put my leading foot on the pedal in its normal position, with the pedal forward and the cranks horizontal. Drop a line over the forward face of the knee and the line should drop through the centre of the pedal axis. Again this is to allow for max pedalling efficiency. I'm not as fussy about this measurement. I will try to achieve it by moving the saddle back or forwards, but if that doesn't do it then as long as I'm within less than half an inch I'll go with it. I suffer from knee pain when I stress the joint, so I know it is an important dimension. But so far I have had greater success in changing the gearing to reduce my peak knee stress than by changing the crank length, or buying a layback seatpost.
    If I was an inch or more out, then the bike is the wrong one for me.

    But this is all for seated pedalling! As soon as you stand up or drop the saddle, then it no longer matters. But when you are climbing out of a steep hole or trying to get up that long hill the usual advice is to be seated so that your legs are not carrying your weight as well as pushing the pedals. Then, efficient set up can give your legs the best chance for max power (pain free hopefully). :)

    This is a useful guide to getting a good fit on your MTB: ... fit-29498/
  • The RookieThe Rookie Posts: 27,748
    Setting the saddle too low is one thing, it's reducing sit bone to BB distance and increasing the articulation angle and also the moment force on the knee, but if you keep the distance the same and just rotate the saddle about the BB it it has no effect at all - the inside of the knee doesn't care where vertical is, which is why the very different bikes I mentioned above all work fine.
  • steve_sordysteve_sordy Posts: 2,131
    The Rookie wrote:
    Setting the saddle too low is one thing, it's reducing sit bone to BB distance and increasing the articulation angle and also the moment force on the knee, but if you keep the distance the same and just rotate the saddle about the BB it it has no effect at all - the inside of the knee doesn't care where vertical is, which is why the very different bikes I mentioned above all work fine.

    I agree with all of that.
  • what did you decide on?
  • Thanks all for the input. The updated is coming a bit late but I thought I should share the experience so far, as maybe someone else will be in a similar dilemma. And that's just fair to pass on the good karma I received here!

    I ended up going for the 18.5 Trek Roscoe over the 17.5.

    To correct the fit/body position I replaced the stock stem (not sure how long it was, but not very long) with the shortest I could find, the Renthal Apex 31

    I went for a few nice long mtb rides. The bike really comes to life when in attack position and I have no criticism in terms of position when up on the pedals (unlike the 17.5 that felt a touch small in that position).
    However, on the long climbing parts or generally cycling to get to the trail, with the seat high and me actually sitting on it, I still felt a bit uncomfortable. After a few hours I would get sore upper traps. I found I wanted to rise up and sit upright, riding no hands at every occasion.

    So I replaced the stock handlebar (9 degrees backsweep, 15mm rise) for a Syntace Vector 7075 High20 with 12 degrees backsweep and 20mm rise. I wish I had found one with 30-40mm rise but Syntace doesn't seem to make it and the only other brand I found making 12 degrees backsweep handlebars, SQLab had their handbars actually first sweeping forward, then backward, hence not shortening the reach. That really makes sense for those who have the right setup but just need to change the angle of their wrist, but didn't solve my issue, so Syntace it was!
    I hope I won't have too much more to spend on parts as otherwise I might as well have gone for a much better bike! :lol:

    I haven't gone on a proper mtb ride since changing the handlebar. I cannot imagine this will negatively impact the great 'proper' mtb riding behaviour of the bike I already experienced when 'attacking'. I have gone for a long city ride though. It is definitely marginally more comfortable but I am still leaning quite a bit forward. I didn't feel as fatigued in the upper back as after the 4h long rides with the old bar, but I still found myself wanting to sit more upright and letting go of the handlebar at every occasion.

    In reality I don't know if:
    -this is something other people experience, and mtb are really made to 'attack' and will make you a bit uncomfortable on the pedalling part (but I would think that the more comfortable you are the faster you will get up that mountain!)
    -if this me. maybe I lack the flexibility, or the core strength (I do notice that if I maintain my torso through core muscles, as obvious as it sounds, it releases pressure on the arms)

    But as I am planning a 2 weeks trip on this bike this summer, I am a bit concerned it still loads the shoulders a bit too much (and to make things better I had a couple of shoulder surgeries in the past...). And yet I am wondering if what I am trying to do here is to transform a mountain bike into a hybrid bike... and still ask it to perform on the trails!
    But when I sat on a Trek Remedy 18.5 in the store, everything felt so upright and comfortable (probably courtesy of the longer fork).
    So is the quest for comfort legitimate or am I fighting a battle I cannot win? :?:
    Even with this discomfort, I struggle to think the 17.5 would have been right, as it felt a bit cramped as soon as I would get up on the pedals. When picking up some speed at least I feel secure on the 18.5.

    On another but related note, I also find myself struggling to manual. This is of course down to my beginner's lack of technique. But add to my lack of experience the Plus sized wheels at the frame on the large end of the spectrum for me and even when fully stretched over my rear wheel, I really need to push hard with my feet to get the front wheel going. Which usually isn't enough... and if it is, I pushed like such a maniac that I fall on my censored ! :lol:
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