XC Mountain Bike Sizing help

Would be greatly appreciated if anyone can help me out on choosing the correct size for my first real XC mountain bike.

I’m currently looking at BMC Two Stroke. Geometry seems to be newb friendly? (Please confirm lol)
I’m pretty sure I should get size small??

-Height 169CM (longer on the torso side)
-Total newb at mountain biking. Would like to deep my toe into mountain biking and eventually race local XC events
-Hardcore road cyclist. (ride 52cm Tarmac with quite an aggressive position. 4.5FTP on a good day)

Post a geometry chart for the bike. You could be a small or medium frame depending on how they size their bikes.

Capture 2

Thank for your help.

Based on the reach numbers i’d say a medium but you should see if you can try them. This video also does a pretty good explanation of how you want to size a bike.

Thanks for the response. The video is great but I’m just now more confused lol… seems to be the message of the video is don’t buy larger bikes… but you are recommending me a medium…??

I would love to try out some bikes but it’s just not possible with current situation.

Just to add more info:
My inseam is just slightly shorter for my height. ( just don’t have a way to measure accurately at the moment).
I would prefer more of an aggressive position.

Also the bike will be mainly used for climbing steady gradient and I won’t be tackling courses that are too technical…

Maybe I missed it, but is there a reason you aren’t checking with a local bike shop for sizing?

When possible, especially for new riders, getting you in person and on a bike (multiple sizes) is really handy. I get that some shops and general bike availability can be an issue, but I at least want to reinforce that you should take any chance you can with real people in person vs pure number crunching on line.

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Hey Chad,

My local shop is telling me that I’m right in between the size as well lol… some say I’m small some say I’m medium. Just wanted some second opinion from the forum.

So at the end I’m just following manufacturer’s guideline and the crank length.

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OK, good info that helps frame the option.

Not a concrete suggestion, but when people are on the bubble for sizing, my instinct is to choose the smaller size in most cases.


Do not look at road geo equalling MTB geo for a start. Very different design goals for a start. In MTB the rider needs to be more centered on the bike hence longer reaches and shorter stems.

As a half decent guide the Canyon sizing tool is pretty reasonable. If you were to use their calculator on the Stoic or even the Neuron then it will give you a frame size. The reason for these two bikes is more to do with the head angle being in the ballpark of 67 degrees. Scroll to the geo chart after being sized and take note of the reach, stack, and seat tube length. Then compare to the BMC geo.

As a longer torso person myself and on the cusp of different frame sizes I always size UP (and not just to contradict Chad). I will sometimes change out the stem by 10mm, but have switched out dropper posts to a lower stroke on all but one (I have 8 MTBs, and no it is not an addiction, Mary!) to accommodate my shorter legs.

With a longer torso you can accommodate the extra 20mm in reach between the two bike sizes. You can also move the saddle forward or use higher riser bars to help.

At the end of the day you do not sit on an MTB like you do on a road bike. You stand and move around, you hover over the seat even on flattish singletrack, etc, to allow the bike to carry speed over rough ground. This is where reach and stack become the most important so as to place you “in” the bike. By comparison a road bike places you ahead of the bike. Old MTB geo placed you “on” the bike - upright and behind the BB.

As for not wanting to do technical… you’ll build up to it. We’re not talking RedBull Rampage, but you’ll want to progress off green trails quickly. That is what makes MTB fun - the challenge of progression. And don’t let age be a barrier. I know plenty of 65+ year olds hitting the black trails where I ride.

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For XC I completely agree. Go with the small @NCSMD


I’ve tried using the Canyon sizing tool and it told me there was no size for me. :rofl:

I’m a short arms, short legs, long torso person. Shops often try to size me down, but like you, I often prefer to go up. In addition, I’m that weird person that really needs a 55 or 57 and not a 56 or 58. I also want jeans with a 31 inseam.

All that to say, that I generally agree you’re better off sizing down because you can buy a longer stem/seatpost if needed, but I personally would rather ride a short stem on a longer frame than a short frame with a long stem, especially on an mtb.


What would be the biggest difference if I go small or medium?

Handling and ride characteristic wise?

A small bike will be more nimble, turning corners easier, but you can find it hard to move around on it to get in the best position to weight the bike correctly, for instance end up too far over the front wheel (especially on downs) or too far off the back (especially on downs).

A larger bike will have a longer wheelbase which provides more stability. Space is opened up to move around the bike and your body weight will be less likely to be too far forward or too far rearward. On high pitched climbing the front wheel may tend to float, however this is more likely due to head angle than bike length. 65 degrees and under will tend to float, but 67 is pretty stable, and a slight slide forward on the saddle will usually alleviate that. On a shorter frame, you may find sliding forward pushes your knees into the handle bars.

A longer reach bike will ultimately provide more stability and inspire more confidence.

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169cm, or 5’ 6-1/2", I’d be going for a medium.


For the R.A.D am I being dim?

It’s basically the unknown of a right angled triangle isn’t it( so a2 + b2 = c2) no numbers come right for me on a canyon lux sizing

Height 183cm x 4.47 =818
M size -reach 430, stack 595 -RAD = 734
L size -reach 450, stack 609 -RAD = 757
XL size -reach 470, stack 632 -RAD = 787

Those are canyons sizing, on their website i sit on the cusp of Medium and Large. XL still comes short. I am working on the assumption that the reach is central, it doesn’t account for any rise/sweep of the bar.

What am I missing?

I’m another vote for medium. Size “up” and run the shorter stem and enjoy the added stability. For a new rider I can’t think of a negative here, especially an in between sizes rider. I always size up when I’m on the fence.

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After consulting with my LBS and the fitter, I decided to get medium with 170 crank. Really appreciate the help from everyone.


I think you will be happy with that decision. My general rule of thumb when between sizes - size down for a road bike and size up for MTB. Especially if you aren’t super skilled on the MTB. Bigger MTB bike is generally more forgiving and stable, particularly important with XC geometry. Anything that keeps me behind the bars on a mtb is a good thing.


Good choice.

I like to size up too, when between sizes. The smaller size will feel like yesteryear’s bike geometry. It’s just outdated and being “over” the bike wheel vs “in” the bike makes a world of difference in technical terrain.

Dirt roadies (aka groadies, gravel bikes) you can go either way, but a modern MTB with a long stem and trying to find a good dropper setback seatpost is just bad.


Good decision on sizing up. I’m a couple of centimeters shorter than you, but similarly have a longer torso. I was in the market for a Yeti SB100 (not really XC), but had the luxury of demoing a small and medium. The small felt more nimble, but the medium felt way more stable. Like others mentioned, there’s a lot more “room” on the larger size to move about the cockpit and rear end position, yet still feel “in” the bike. The smaller size felt smallest when I was climbing out of the saddle.