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Finding a bike that fits is crucial to enjoying the sport of mountain biking. A well-fitting bike will handle better on the trail, and of course be more comfortable for longer rides.

When I first shopped for a mountain bike, I looked at just the standover height, which is exactly what the name implies. As long as I had enough room for the boys, I was good. However, I am 5′ 8″ with a stubby 29″ inseam, a fairly long torso and normal length arms. Since I was using only standover height as my “fit” guide, I would generally end up with a bike that was too small in other measurements, and thereforeuncomfortableto ride and unstable on the trail. The trick is to take all of the bike measurements into account, rather than focusing on any one. Also, each builder uses different frame dimensions, so bikes of the same nominal size, but from different brands, will fit differently.

Illustration credit:Kona Bikes

When looking at a mountain bike geometry chart, there are three measurements that affect how the bike will fit:

  • Seat tube length
  • Top tube length
  • Standover height

The seat tube length can be measured a couple of ways, but in either case it is usually given as the nominal size of the bike. For example, a 16″ could be a small, an 18″ a medium, and so forth.

Top tube length is the distance measured from the steerer to the intersection with the seat tube. With modern bikes employing fancy curved top tubes, some makers list both the actual and the effective, or straight line measurement for this. If only one length is listed, it is usually the effective length.

Standover height is obviously just the height from the top tube down to the ground.

For getting a good fit on a bike, in very basic terms, we are looking for a bike that allows good leg extension while pedaling, anaggressive, but comfortable reach to the handlebars, and enough room to not hurt yourself if you come off the pedalsunexpectedly.

Illustration credit:Bikes Direct

Seat Tube

In my opinion the top tube is thecriticalmeasurement, but most sizing starts with the seat tube length. When pedaling, your leg should extendnot quite all the way at the bottom of the pedal travel, but also not squish into your gut on the upswing. For mountain biking, you also want to leave some room to be able to lower the seat for technical downhill sections, so go with a size that allows a decent range of seat post adjustment. If you have only a couple inches of seatpost showing, go a size smaller. If you have a 400mm seatpost extended all the way to the warning line, go a size bigger.

Effective Top Tube

The top tube length will affect your reach from the saddle to the bars. Mountain biking benefits from an ‘aggressive’ position, which means you will be slightly stretched out when seated. This position allows for optimum counter effort to your pedaling force from pulling on the handlebars. If your top tube length is too long, you’ll go past the sweet spot forefficient power transfer, and most likely suffer from lower back pain and have difficulty steering the bike where you want it to go. Too short and you will find yourself going over the handlebars … a lot. Once you get a top tube length that is close, you can fine tune the fit with a longer or shorter stem. Just like the seatpost though, if you find yourself needing anextremely short or long stem, go up or down a frame size.

Standover

I’ve been informed by girl riders that this isn’texclusivelya boy problem. The general rule of thumb is to be able to stand flat-footed and have an inch or so of space between you and the top tube. However …. If you have a really short inseam, and you have the other two measurements where you want them, you can make an exception on this one … just don’t come off the bike. 😀

In all seriousness though, if you are having trouble with standover, check out trail or all-mountain bikes. Due to the way these bikes are intended to be ridden, they tend to have curved top tubes specifically designed to allow more standover room. XC bikes tend to have higher, straighter top tubes resulting in a significantly taller standover height.

 

Photo Credit: Giant Bicycles

Homework

With these three measurements in mind, go to your local bike shop and ride as many different bikes around the parking lot as they will let you get away with. Later, look up the geometry specs on the manufacturer’s web site and make a note of the measurements of the bikes that felt good, as well as the ones that didn’t. You will quickly start to notice what dimensions work together, and you will also weed out the bikes that will never fit you well, regardless of how crazy you get with seatposts and stems.

Once you have beenthroughthis exercise, you should be able to check out bikes on paper and have a good idea going in whether the bike will fit you or not. It can save a lot of time and frustration, and you will be a smarter shopper riding a bike you are comfortable and confident on.

See Also
By Corey Maddocks
 
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# Comments

  • mtbgreg1

    Great article maddslacker! I don’t really think about top tube length all that much… thanks for bringing it to my attention! I’ll have to consider it more in the future. But like you said, often you can fine-tune it with a longer or shorter stem.

    “The top tube length will affect your reach from the saddle to the bars. Mountain biking benefits from an ‘aggressive’ position, which means you will be slightly stretched out when seated.”

    I kind of disagree with this statement. I think it depends on the style of riding. XC, then yes. But once you get into aggressive trail riding, all mountain riding, freeride, and downhill, having a rather upright posture on the bike will allow you to handle technical sections more easily and get air with better confidence. The slightly stretched out feeling can definitely help on the climbs, but not so much on the descents.

    All depends on what you’re riding and how you’re riding it! 🙂

  • dgaddis

    Quick note on top tube lengths. The ‘effective top tube’ (ETT) length can be defined several ways, so it’s important to know how each manufacturer is doing it. For example, in your first diagram pictured (where they call it top tube horizontal) they are measuring from the center of top of the headtube horizontally till they intersect the imaginary extension of the seat tube centerline. See how they continued the seat tube centerline up past the tube? On the second diagram however, they’re measuring to the center of the top of the seat tube, not it’s extension. The difference between those two types of measurements can easily be the difference between two different frame sizes, so you have to pay attention.

    One method of frame sizing that a few manufacturers are listing is ‘stack and reach’, where the stack is the vertical measurement from the BB to the top center of the headtube and reach horizontal distance from the BB to the top center of the headtube. Seat tube length and angle are mostly unimportant IMO. Saddle height and fore/aft position is easily adjustable, whereas changing reach is much more difficult (and expensive) ’cause there’s only so many stem and bar combinations.

  • dgaddis

    One last note on top tube lengths. Not only are they measured differenty by each manufacturer, but they’re called different names. Horizontal top tube, actual top tube, effective top tube, virtual top tube, etc. Main thing is to look at the drawing and see where they’re actually measuring to and from.

    Oh, and nice article madd!

  • maddslacker

    @mtbgreg, yes, all mountain and freeride bikes tend to have a shorter top tube, however, too short will still cause endos. ALL mountain bikes still have an aggressive stance as compared to say, a touring bike or a flat-bar road bike. People new to MTB may not realize this and end up with a mountain bike that it too upright, and then wonder why they fly like superman all the time. 😀

    @dgaddis, yeah it’s a mine field trying to compare specs, but as long as you don’t get too wrapped up in the minutia, you can still read the charts and get a general feel for how a bike will fit you before you throw a leg over it.

    I should have mentioned that this can come in really handy when you are travelling to a riding destination and need to reserve a rental bike before you get there.

  • burnedthetoast

    Nice article – I’ve never looked at the numbers much, mostly going by feel. I like the idea of looking up the numbers on frames I know fit me – seems a very practical way to weed things out in the new-bike-selection process I’m going through right now!

  • AK_Dan

    Nice write up man. I’ll be linking to this one for sure, thanks.

  • trek7k

    I’m a big fan of the sloping top tube on my Tallboy 29er. Not only does it decrease standover but it’s also great when I’m tired – no need to lift my leg as high to get back on the bike. 🙂

  • skibum

    The myriad of ways mtb geometry is measured and is not standardized just reinforces the need to try before you buy. Many times, I have seen/heard questions like “Should I get a Santa Cruz Blur LT or a Yeti 575?” When I encounter this, I know the questioner hasn’t thrown a leg over either, but rather has become enamored with some reviews he/she has read. Both are great bikes but feel completely different; for any given category, SCs tend to have relatively short top tubes and Yetis lie at the long end of the spectrum. Both are great bikes, but not many riders will have the best possible experience on both. Yetis fit me perfectly, but that doesn’t mean I’ll ever tell anyone “Buy the Yeti.” And top tube is only one part of the “stretch” equation–stem length is equally important, and also has a big impact on steering characteristics, another variable that many riders should be concerned with. And then there’s layback seat posts or even just where you position a standard seat on the rails. It takes at least a few test rides to figure out what works best for each individual.

  • minkeyman

    I agree with trek7k 🙂 my Tanuki helps me out there as well. Good article! I think I may try out a shorter stem now. My lower back hurts on some rides but it’s only when I have a pack on. And my new pack alleviates most of it.

  • maddslacker

    @Trek7k, I thought my younger daughter was the only one who mounter/dismounted that way! The rest of us swing a leg over the back wheel… 😀

  • Jared13

    Nice article, maddslacker!

    I’m looking forward to part 2 (and more!)

  • JSatch

    mtbgreg1: “All depends on what you’re riding and how you’re riding it! ”

    very nice article for a frame sizing intro.

    a measurement i think is critical is the head tube angle (ha). xc bikes, like you’re describing here, are up around 69-70 degrees. as per mtbgreg1, a more aggressive ride will have a shorter top tube for a more upright position with a slacker head tube angle. yes, you are more prone to go otb with short top tube, assuming your ha is the same. however, slacken the ha and you’re less likely to go otb, and will feel way more comfortable/confident on jumps and steeper descents. dh bikes run shorter top tubes yet 65degrees or under ha. for me, i like approximately 67degree head tube angle (and about 6.5″ travel). i’m not a goater by any means, but that ha still lets me climb up for the fun down. to each his own.

    skibums description of the sc vs yeti is right on. sc lt has a shorter top tube than the yeti, and a slacker ha to match.

    chainstay lengths also figure into your style of riding. for example slopestyle cs are relatively short whereas dh bikes are longer for more stability (longer wheelbase).

    in the end, it’s all magic in the numbers to fit your body and riding style.

  • JSatch

    @maddslacker-

    my bad. i noticed the “Part 1” after posting.

    looking forward to the next articles and will try to not jump the gun. again.

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