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  1. #1
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    Frame Sizing: Going from 26" MTB to Fatbike

    Seeking advice on frame sizing. I currently have a Giant XTC 2 which is a large frame (chart below) and was playing around with Salsa Mukluk geometry chart this morning and it seems that my Giant XTC 2 falls between their medium and large frames. I also tried using Sheldon Brown (RIP) revisionist theory of bicycle sizing but that gives much the same result.





    (1) Are 26" MTB frame sizes a good basis for working out a fatbike frame size?
    (2) Assuming yes should I go larger or smaller if say looking at the Mukluk? Interestingly with Surly frames such as the ECR or Pugsley my XTC pretty much matches the ECR/Pugsley large frame.


    The bike will be used mainly for fatbike loaded touring and also at times fitted out with 29er wheel set for more dirt road/black top touring so I am thinking being able to maybe stretch out (will be using Jones H-Bar Loop bars) would be helpful.





    Thoughts?
    Andrew

  2. #2
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    If you're comfortable with the bike you have, get one with a similar fit. If you're going to be riding in snow a lot, then you will need one with a fair bid of standover height as you will sink into snow when you put a foot down,

    My Mountain bikes are all 17" frames with 23" effective top tubes. I went for a Surley Pugsley 16" as the fit was almost identical. It was the right choice for me. I'm 5" 7" with a 29.5" inseam.

    Best to test ride a few models if you can, before purchasing.
    Nothing ventured, nothing gained ...

    Latitude: 57 45' Aberdeenshire, Scotland.

  3. #3
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    Sometimes bikes with the same ETT length may have different angles, which messes up actual BB and handlebar locations in relation to each other. Sheldon Brown had good points in the article, but it's more complicated than necessary.

    Thankfully things have changed and bike sizing is more clear than ever: you figure out a stack and reach and that's it for any given bike type. (Naturally a mountain bike will have a longer reach, because we use shorter stems and bars that sweep back compared to roadies - but for the same type of cockpit configuration the same stack and reach will work from one bike to another.)

    Salsa are smart enough to let you know the stack and reach of their bikes. What you need to do is measure those numbers of your current bike and you'll know which size is right for you.

    First you prop up the bike upright with the front wheel against a straight wall. Then measure a few things:
    - BB height from the ground.
    - BB distance from the wall.
    - Front axle height from the ground. (Same as front axle distance from the wall.)
    - Middle of headtube top (as seen from the side) height from the ground.
    - Middle of headtube top (as seen from the side) distance from the wall.

    Then calculate stack and reach as follows:

    Headtube top height minus BB height is your stack.
    BB distance from the wall minus headtube top distance from wall is your reach.
    Last edited by Saul Lumikko; 1 Week Ago at 01:17 AM. Reason: Fixed incorrect info

  4. #4
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    Quote Originally Posted by Robopotomus View Post
    If you're comfortable with the bike you have, get one with a similar fit. If you're going to be riding in snow a lot, then you will need one with a fair bid of standover height as you will sink into snow when you put a foot down,
    I don't have a similar fit with the Mukluk hence this post ... no snow here so at least don't have that concern.

    Thanks
    Andrew

  5. #5
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    Quote Originally Posted by Saul Lumikko View Post
    Salsa are smart enough to let you know the stack and reach of their bikes. What you need to do is measure those numbers of your current bike and you'll know which size is right for you.

    First you prop up the bike upright with the front wheel against a straight wall. Then measure a few things:
    - BB height from the ground.
    - Front axle height from the ground. (Same as front axle distance from the wall.)
    - Middle of headtube top (as seen from the side) height from the ground.
    - Middle of headtube top (as seen from the side) distance from the wall.

    Then calculate stack and reach as follows:

    Headtube top height minus BB height is your stack.
    Headtube top distance from wall minus front axle distance from the wall (or ground) is your reach.
    Thank you very much. I will have to steal my XTC back from #1 Son and do the measurements.

    Hopefully it comes in much closer and makes a decision easy

    Andrew

  6. #6
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    I've ridden large frames before, and both my large sized fat bikes have fit fine.
    Riding.....

  7. #7
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    I have managed to steal by XTC back from my son and did the measurements - not 100% accurate but within a few mm. My numbers are in red.

    Quote Originally Posted by Saul Lumikko View Post
    First you prop up the bike upright with the front wheel against a straight wall. Then measure a few things:
    - BB height from the ground.
    330 mm
    - Front axle height from the ground. (Same as front axle distance from the wall.)
    335 mm
    - Middle of headtube top (as seen from the side) height from the ground.
    920 mm
    - Middle of headtube top (as seen from the side) distance from the wall.
    555 mm

    Then calculate stack and reach as follows:

    Headtube top height minus BB height is your stack.

    920 mm - 330 mm = 590 mm

    Headtube top distance from wall minus front axle distance from the wall (or ground) is your reach.

    I think this should read BB distance from wall minus HT distance from wall. Does that sound right? Assuming yes my reach is

    1015 mm - 555 mm = 460 mm
    So assuming I have it right I have a stack of 590 mm and reach of 460 mm which of course does not fit nicely with anything that Salsa has My top tube length is 610 mm.

    Given my TT length of 610 versus the Mutluk medium of 605 mm, the stack of the Mutluk at 603 mm versus 590 mm and reach of 421 mm versus my 460 mm I am guessing the Mutluk medium is probably the best option.

    I could also then I assume run a shorter stem as suggested by Transition Bikes.

    Sound reasonable?

    Thanks
    Andrew

  8. #8
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    Excellent! You didn't blindly follow my (incorrectly worded) advice, but did exactly what I had thought. Reach indeed is BB distance from wall minus headtube (top) distance from wall. I edited my original post so it's fixed now.

    What stem length are you running currently? A 460 mm reach sounds quite long so the bike might be designed for a short stem. Any spacers below the stem? They add to your effective stack and reduce cockpit reach.

    For example, if your current bike has a 590 mm stack and 460 mm reach, but you have 3 cm spacers below the stem, you could pick a new frame with a 621 mm stack and 435 mm reach, install a 1 cm longer stem slammed (no spacers) and the fit would be very close.

    Stack and reach is just the starting point for cockpit position: spacers, stem and bar finish the work. If a new frame has the same stack and reach as before, naturally you'll use the same amount of spacers, same stem... same everything. With a taller stack you'll use fewer spacers, with a longer reach the stem should be shortened accordingly etc. and adjusting one thing affects the other.

    There are calculators out there to figure out how much any number of spacers, stem angles and so on affect the bar position - which finally is the most important part.

  9. #9
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    Thanks Saul. Have had to put this aside for the past week but just got a chance to get back to take a look at my current mountain bike.

    Interestingly (and surprising to me) the stem is 110 mm which is longer than I thought it would be.

    Spacers below the stem amount to 30 mm or 3 centimetres.

    I do hope to make the purchasing decision this weekend, so need to decide medium or large frame. Ugh.

    Andrew

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