XC Mountain Bike Sizing - The Important Measurements

I looked through past threads and didn’t see anything that touched on this. I want to know what the crucial measurements are for xc mountain bikes.

I am looking at going from a Kona Honzo (Trail) to an XC bike. I found a nice used Specialized Epic, however, its a size larger (XL) than my Honzo. I am looking at the geometry charts for both bikes and I’m not sure which measurements are the most important with mountain bikes. I am 6’1" (~185 cm) and typically ride a Giant M/L frame on the road/CX. In some other research I did I see people saying stack and reach are the most important on MTB.

Here’s a comparison of the bikes geometry (L Honzo, XL Epic)

  1. Stack 642, 636
  2. Reach 475, 480
  3. Top Tube 647, 662
  4. Seat Tube 470, 520
  5. Standover 750, 847
  6. Front Center 769, 753

Taking into account these are different styles of bikes, do you think these compare well? Are there other measurements I want to look at? I don’t want to waste my time driving 1.5 hours both ways to find out it doesn’t come to to fitting.

Take a look at the seat tube angle of both bikes too, effective top tube measurement is pointless with out STA. When people size their bikes on stack and reach alone they’re basically ignoring a pretty important fact; we spend most of the time on our bikes in the seated and pedaling position, not the attack position.

If I’m sizing a XC bike, I’d prioritize ETT, and STA - just to get a starting point. Then I compare stack, reach, stand over… before that I’ve already looked at head angle, and BB drop, those two variables alone can be deal breakers before going any further.

  • Pretty sure that you have a typo on the Epic Standover. Hard to picture 100+mm [4"] difference there.

What year are these numbers? I’m not sure it’s current.


Reach is important out of the saddle, because that’s where you are, feet near bottom bracket to hands on handlebar, but if you’re mostly pedaling it’s less of an issue as the saddle is farther back. You need to account for your flexibility and your hip. The long reach numbers come from steeper seat angles, and marginally longer top tubes compared to a decade ago.

I think you should measure your bike and go from there.

I also think those numbers are a bit off. Or are you talking about the Epic hardtail? Hardtails, don’t think it’s worth the money, to be honest.

I think that is 637 on Honzo, to 662 on Epic HT.

Here’s an example.

Honzo 76 STA, 637 ETT, 475 reach.
Epic HT 74 STA, 662 ETT, 480 reach.

The Epic is going to be a bit longer for you. Unless you’re running something like a 90mm stem on the Honzo (which I really doubt given you ride ML Giant), or you find the Honzo too small, I’d guess you don’t have much to give there.

For me, the Epic is a touch too big, but I’m 5’11" and I ride a 56 or L MTBs usually. But at 6’1" I’m not sure why you are on the same size as me. Are you all legs?

@mcneese.chad The numbers are for a 2019 Honzo and a 2020? Epic. One is a trail bike the other an xc bike so I think that has something to do with it.

@iamholland To be honest, I have no idea why I’m on a large either. It was my first mountain bike and I just bought based of their sizing chart and knowing that I liked smaller road bikes. My honzo feels fine, however, I have no idea what a XC bike is supposed to be like.

Here are the bikes:


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Please watch these videos:


Hi, I’m thinking about buying an epic and trade my epic evo but I have some dubs on the size.
I’m 183cm riding at 77,2 (saddle) but it looks like I got slightly short arms. That’s why a bike fitter fitted me on a 54 super six evo 2021 road bike with a 11cm stem. Also the epic evo fits me in L but I didn’t have any bike fitting on it.
Watching the two videos posted about the RAD and the reach for the mtb, I don’t know if it is worth buying an epic M. It’s 2,5 shorter reach.
I measured a RAD of 80,5 cm.
Any suggestions? Are these measure so important for Xc bikes?

Try them both. And remember that you can always get a longer stem. If you have a fitter you trust, work with them! Many fitters can simulate the fit of any bike if they have the dimensions. I (personally) prefer the smaller of two frames if it fits. I don’t understand the appeal of going big. Unless you are a teenager, it’s not like you are going to grow into it!! They all have the same size wheels and gears…

Thanks for these!
I have the stepladders to do the RAD measurement, but the pencil-on-the-wall method is so much easier and faster!

And this gives me some for issues to clear up - ones that I didn’t know I had, but explain the challenges I’ve had with certain kinds of bike handling!