The Technical Wizards at Transition Bikes are on a mission to educate riders about proper mountain bike fit and function. This "Get Dialed" article will help you decipher those pesky millimeters and angles and how they translate to your experience on the trail.

Bike geometry can be complicated. It's nearly impossible to know everything about how a bike will ride based on looking at these numbers alone (not to mention these numbers tell you nothing about how the suspension itself will function). But we hear it being done all too often - and to be fair, we do it too. Problems arise when riders focus on just a few of these numbers instead of how they all relate to one another.

There are no right answers here. We're opinionated about this stuff and we'll let you know it. In the end, there are preferences and our hope is that this article provides the grounds for a more informed discussion of bike geometry. We're going to keep it pretty basic, and some of you might find a lot of the topic a little too 101. But we frequently come across pretty educated riders who haven't fully considered some of this stuff. Even in the basics you might come across a few new ideas. We hope it makes you think more about all of the details in your bikes geometry - we think about it a lot.

We'll start with a topic that we see people confused about all the time....

Effective Top Tube Versus Reach

Effective Top Tube (ETT) is a traditional way to measure a bike's length. It measures a horizontal line, parallel to the ground, from the center of the head tube to the intersection with the seat tube or seat post. This number is useful for identifying how long a bike will feel while in a seated position.

Reach is newer way to measure a bike's length. It uses the same horizontal line as the Effective Top Tube, but instead of measuring from the head tube to the seat post, it measures from the head tube to a vertical line that runs perpendicular through the center of the bottom bracket shell to the ground. This measurement identifies how long a bike will feel while standing.

So Reach measures your cockpit anytime you're standing and Effective Top Tube measures your cockpit only while seated with your saddle at a specific height. They're both telling numbers and it's worth considering them together, but we put a lot of emphasis on Reach these days. Any time we're riding aggressively, we're out of the saddle, so it follows that this number is going to have a greater overall impact on the "handling" characteristics that define the modern trail or all mountain rider.

One of the most common problems we see is over-emphasis on Effective Top Tube length. As a whole, bikes tend to be getting longer front ends these days. But that can be done with a longer ETT, a longer Reach, or a combination of both. Riders who look only at Effective Top Tube without considering Reach may be selling themselves short, literally. That's because seat angle has a huge effect on ETT, but zero effect on Reach. Watch what happens when we make the seat angle slacker:

The Effective Top Tube is now longer, but the Reach has remained the same. Or to look at that in another way, it's not hard to imagine a scenario where the Effective Top Tube of 'Bike A' is the same as 'Bike B', but the Reach of 'Bike A' is actually shorter than 'Bike B' because 'Bike A' has a slacker seat angle. This happens; a lot. In these cases, Bike A may fit great while seated, but while standing, the cockpit length is too short and the rider doesn't have enough room to move around or find the bike's sweet spot.

The slack seat angle that afforded the rider adequate seated top tube length does nothing to help the rider move around on the bike while in a standing position. It has other effects as well. The rider's seated weight will be further over the bike's rear axle, making the rider work harder to keep the front wheel on the ground while climbing and harder to effectively transfer power to the cranks. The taller the rider/seat post, the more dramatic this effect will be because the seat itself keeps moving rearward as the saddle height is raised. So rather than slacken the seat angle to increase a bike's Effective Top Tube, why not increase the Reach? Proper position while seated and standing.

Another problem with ETT as a whole is that, because it's influenced by seat angle, the actual feel that a listed ETT provides will vary from one rider to the next depending on how high they run their saddle, even on the same frame. Reach is more constant. Consider a rider who is 6' tall with two bikes; both bikes have a listed ETT of 610mm, but they have different seat angles. ETT is always measured in a parallel line to the ground starting at the top of the head tube. Now let's assume this rider's actual seat height is 5" above where the ETT is measured. The bike with a slacker seat angle will actually feel longer while seated than the bike with the steeper seat angle, even though they have the same Effective Top Tube on paper. So saying that you prefer a bike to have a particular ETT isn't really considering all the necessary information.

Longer Reach also means that the measurement from the BB to the front axle will be longer.

Let's assume that you take a frame and increase the Reach by 25mm. The distance from the BB to the front axle will grow as a result and you now have a bike with a longer wheelbase. You can now shorten the bike's chainstay length to get the wheelbase back to where we started. Same overall footprint, just with more bike out in front and less in the back. Same stability at speed, but a lot easier to push the rear end around in corners and get the front wheel off the ground when you want to. Decreasing the chainstay without increasing the Reach would shorten the wheelbase, potentially sacrificing stability at speed. And a slack seat angle combined with the shorter chainstay is going to shift rider weight even further over the rear axle.

With a longer Reach dimension, riders can run a shorter stem while still having enough room in the cockpit, whether seated or standing. Short stems provide stability, which is a good thing when you're hanging on for dear life down a high-speed singletrack descent or pointing it through a technical section of trail. When climbing, your tire is less likely to get knocked off line by a root or ledge in the trail. We're going to make a blanket statement here that's intended to offend plenty of people... anyone who doesn't consider themselves a purely XC rider shouldn't be running a stem over 60mm on a mountain bike. That's being generous too. Really we think that max stem length should be closer to 55mm, maybe even 50mm, but we'll leave some wiggle room in there.

We see people defying this rule regularly - probably because their bike's Reach is too short so they put on a longer stem to provide enough room to move around. Or because their bike came spec'ed with a 90mm stem out of the box and they never changed it.


You can visit www.transitionbikes.com to check out their new 2015 range of bikes that utilize the concepts discussed in this tech feature.

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39 comments
  • hammer

    7/11/2017 3:31 PM

    I measured the down tube on both bikes, and they are in fact discrepant; however, I spoke with the manufacturer, and the rep assured me that down tube length is nothing to investigate as a possible source of my discomfort while standing on the smaller bike. It seems odd though that the two bikes (the way I have them configured) are nearly identical, except for down tube length. The bike with shorter DT length = terrible weight distribution in standing position. The bike with longer down tube length = engaged core/weight on legs in standing position.

  • fartymarty

    7/11/2017 12:56 PM

    hammer - you are right that the front washing out is to do with weight on the front wheel (or lack of). It could also be that you have a fork that isn't supple enough or a front tyre that doesn't have enough grip or you are running it too hard.

    Maybe the first thing to do is drop the bars as far as you comfortably can. Then maybe change the front tyre or run it softer if you can get away with it - im 95kg and run my DHR2 at 23psi on a Flow3).

    The reach on the 18.5 is 427mm which seems ok so its a matter of playing with the other bits until you get something that works.

  • fartymarty

    7/11/2017 8:25 AM

    hammer - One other thing I thought of...

    With a short bike and long stem when you are descending you have to move your weight back (butt over the back tyre) to stop you going over the bars. Moving your weight back maybe the cause of your back pain.

    With a long bike and short stem you can remain in the middle of the bike as you are less likely to go over the bars. It means you can stand of the pedals and remain neutral rather than hanging your butt over the back wheel.

    If the longer bike keeps the back pain away I would be riding that one. You can sort any issues with climbing by getting the bars lower to transfer more weight onto the front wheel.

  • fartymarty

    7/11/2017 6:56 AM

    hammer - why does the 65mm stem compromise the bikes performance? As it is an XC bike my guess is the front lifting when climbing? How wide are your bars and what rise do they have? Do you have the stem as low as possible?

    I am guessing you have the same butt to bar on each bike.

    I run a 50mm stem on my 29HT with 10mm rise bars and it climbs ok. I have it set up more for down rather than up. I went uo to a 140mm fork and had to increase the CS to keep the front down.

    Have you demoed other bikes with longer reach to see if this helps identify the problem?

    Also how tall are you and what is your unseam?

  • hammer

    7/11/2017 9:01 AM

    Yep, I forgot a critical part of the equation: bar rise. One of the main limitations for me on the 18.5 is cornering confidence. On the 17.5, I have great confidence in the turns, while on the 18.5, I get real timid, as I can feel the front wheel wanting to get out from under me (less so since I started scrutinizing my technique but still to what I believe is an excessive degree). I run the same 27 mm riser bar (720 width) on both bikes, and I know that 27mm is probably quite a drastic rise on a XC bike. The stock bars resulted in neck and back discomfort on BOTH bikes, and I read that these symptoms often stem from bars being too low; thus, I experimented with the riser bar. I know that bar height can make the front wheel light, and my present bar rise might explain why the front tire wants to wash out on the 18.5; however, here's the tricky part: I still don't understand which discrepant variable(s) might account for back and neck pain on the 17.5, given the fact that my data gathering employed the same riser bar on both bikes. Riser bar + 17.5 = wrecked back and neck. Same riser bar + 18.5 = engaged core and little to no back/neck discomfort. The stack between the two bikes is different by 1mm, and I played with spacers to offset this difference on the smaller bike, but achieved no desired results.

    My height = 5' 8.5" shoes off

    Inseam = 31.5" shoes off

  • hammer

    7/11/2017 6:30 AM

    NIce article. I've read every comment here, and I have a riddle that may relate to General's and Big Bird's assertions re: downtube length. Here goes: I've got BOTH a Superfly 17.5 AND a Superfly 18.5. I've set them up so that the reach is exactly the same on both bikes. The 17.5 (with stock stem) wrecks my back and neck, and I believe this back and neck discomfort results, at least in part, from the standing position; when I stand on this bike, I can't seem to get my body situated so that my legs support my weight. Rather, I can feel the strain on my back almost instantly. A few rides, and I'm done for a week or more and my posture is wrecked for a day or two. On the other hand, when I ride the 18.5 in the standing position, my legs support my weight, and I can feel that my core is far more engaged on this bike. The problem? I have to run a 65mm stem (25mm shorter than stock) on the 18.5 to achieve a manageable reach to the bars, which wildly compromises the bike's performance. So, I hypothesize that some variable/measurement discrepancy between these two bikes might elucidate why I can't seem to ride the 17.5, even though the reach is perfect right out of the box. If I'm understanding General's and Big Bird's comments, could DT length shed some light on my dilemma? If so, what can I glean from this understanding so as to, moving forward, find a bike that both performs like it's designed to AND allows my legs to support my weight in the standing position? I know this article is geared towards trail-oriented geos, while the Superfly is XC, but I thought this discussion might still be a good place to discover how I might find a different bike (I'm thinking of switching to a trail bike) that works for me.

  • fartymarty

    11/16/2016 6:24 AM

    Axa, The wheelbase gives you a good start. A bike with an 1100mm WB is never going to have a slack head angle (im talking low 62-64 degrees) otherwise everything else is going to be wrong. Whereas a 1250mm wheelbase is going to give you some room to move.

    If your bike is too short it's hard to make it fit better whereas if its longer its easy. I'm 6'1" and my current 29 hardtail has a 1170mm WB (441mm R) and it is probably 2" too short as the front is too light when climbing. Yes I could increase the stem length from 35mm to 80mm but then it is going to handle badly when riding downhill. If I had a longer frame I could use a shorter stem, move the seat forward, offset HS etc to get a better fit.

    It's worth reading some of Chris Porter's (Mojo) thoughts on geometry.

  • Axa

    11/14/2016 10:20 AM

    Since the sum (of all dimensions ) does not say anything about Head Angle, Reach or Rear-to-Center, I can't see how it can say much about fit at all.
    Find the type of bike you want (often defined by HA, RC, material and components) Then you choose the fit/size by Reach.
    What else do you need to know?
    I'm sure I've forgotten something. But wheelbase is to me equally important to the graphic design which I let to the metrosexuals and alike to play with..

  • fartymarty

    11/14/2016 4:11 AM

    Does anyone else use WB as a starting dimension?

    My logic is that the wheelbase is the sum of all the other dimensions. If the wheelbase is too short its likely the cockpit is cramped, head angle is too steep or chainstays are too short and the F/R balance is out.

    The next dimension I look at is the ETT as you spend most of your time seated.

    Also another key dimension is centre of seat (seatpost centreline) at XC seat height to centre of bars. As everyones seat height varies you cant compare this one on paper. Given your front and rear centre are balanced it removes seat angle from the equation.

    I do see the point of reach but it is just another number to compare and not the defining number.

  • gotdurt

    5/29/2016 10:45 AM

    I'm about 5'-10" (on a good day) with normal proportions, and accustomed to medium frames with around 410-420mm reach, and using a 50mm stem... I'm looking at a new frame, also medium (18"), that is 440mm (ETT at 616!)... that scares me, as it's a huge jump. In my history of riding mtb (about 28 years), with length comes stability, but at the expense of maneuverability, especially on tighter, more technical trails. As for fun, I've always enjoyed smaller frames more. The "small" (16") version of this frame is 420mm; on the long end of the bikes I've been riding the last couple of years. I'm leaning toward the small, but the word "small" is screwing with my head... can I be content with 440mm reach for a do-everything bike?

  • Axa

    5/19/2016 1:42 PM

    Is there any list/forum discussion covering Reach numbers on different manufacturer of trail/AM/enduro bikes ?

  • bturman

    5/19/2016 1:49 PM

    Not that I'm aware of. Are you trying to determine what reach value would be good for you, or simply to compare several bikes?

  • Axa

    5/19/2016 10:28 PM

    A bit of both, but mainly to compare different bikes/manufacturers to see how differently they space the Reach measure and how wide the "norm" is..

  • bturman

    5/20/2016 10:25 AM

    In general most Medium bikes are settling into the 420-435mm reach range, while Larges are 440-460mm. Bikes that haven't seen a redesign in the past 2-3 years are typically shorter than that.

    At 5'10" tall with longish arms, my preferred reach is in the 430-440mm range paired with a 50mm stem.

  • Varaxis

    8/13/2015 9:10 AM

    "Short stems provide stability" how is this true? I believed that they eased/quickened up steering, useful for slack bikes that fought steering inputs (and flopped more readily if you steered too much). If you didn't widen your bars when shortening your stem, it shifted weight back a little more and allowed you to get further back.

    Is part 2 out? Wondering if it ever mentions that as reach and stack height increase, flickability decreases. For example, who goes for the long frame option (when they can fit on small) and increases travel (to 140+) on their DJ bikes and prefers it that way for flickability (ease of throwing it around)?

  • dirtmagnet500

    2/7/2015 9:23 PM

    This new concept of long front center geo is simular to the Gary Fisher genesis geometry circa 2001.The idea was to move the bottom bracket to the rear. So if you move the bottom bracket 1" to the rear then your 17" chainstay just became 18". This pont has been missed here, you can not just make the chainstay short to get the wheelbase back to good again. Look around most 160mm bikes have 29" sized wheelbases (and effective18"chainstays).
    Just like Gary Fisher suggested, you will need a short stem to get your riding position to the rear as much as possible
    (65mm stem-25mm=45mm stem) wow thats the same distance that the BB was moved rearward, go figure....
    The way I size a bikes top tube is from the bottom of the headtube via a horizonal line to the seat tube (or just above it), This removes the head tube length variable and also the saddle height variable at the same time. i like the idea of measuring the down tube to understand BB position. I thought of hanging a plum bob from the seat to see how far the BB has been tucked backwards. The longer reach numers are part TT length and part BB rearward. In both cases the wheelbase gets longer, now you need 31" bars to control it all.

  • jan.czugalinski

    1/31/2015 3:17 PM

    SOooooooo Mondrakers forward geometry is the best thing since slice bread then

  • dberndt

    1/18/2015 11:22 PM

    How does this relate to ideas like KOPS (knee over pedal spindle) which I'm not saying is correct.... But it seems to me if you give a rider a really slack seat tube he'll likely just move his seat forward so that pedaling feels right in a seated position. If you give the same rider a bike with a longer reach but a steeper seat angle, the seat will probably be adjusted backwards by the rider to make seated pedaling feel correct again.

    So although you can make the bike longer with a longer reach and then try to fudge the numbers by changing the seat post angle, at the end of the day isn't the rider in most cases just going to do whatever is required with seat position on the rails/setback post to achieve a static horizontal distance from seat to BB? So the rider hasn't moved. His seated position horizontally is still primarily related to the Bottom bracket, and length/cockpit issues will be taken care of with bar/stem combos or frame sizing...

    The above assumes we're talking about a rider who cares about seated pedaling and isn't spending every minute of every ride standing up/climbing/descending/getting rowdy and as such being very dynamic on the bike.

  • chicha_glisha

    1/17/2015 12:15 PM

    I have an idea about how to measure a bike fit which also considers handlebar width and stem length. I believe whole concept is understandable and contained into this image. You can easily measure area of triagnle my splitting it in two right triangles (one shoud be splitted via ETT axis an another by reach axis + stem length) and then measuring ETT or reach + stem length and those "arm outstretch values" for both seated and standing situation via Pythagorean Theorem and then multipying it by two.

    Maybe it could help people people properly determine handlebar width and/or stem length that fits them ... or I just got bored and got nothing more important to do but creating another measurement mindf*ck

  • jon123

    1/16/2015 3:07 PM

    Brilliant. Superbly explained.
    And I wholeheartedly agree about stem length -- though I'd even say 50mm and 55mm is too long. Going to a 35mm or 40mm stem is a game changer, IMO

  • ripdogg1

    1/17/2015 7:36 AM

    My Covert came spec'd with a 60mm stem so I switched it out for a 45mm stem. I had to switch back because the cockpit felt way too short. I have it a good long try too. My Coverts reach is about 30mm shorter than the Patrol's, though.

  • peguinpower

    1/16/2015 2:57 PM

    Great article! However, Ive come to a different conclusion. I still find ETT as a more important number that REACH for trail riding. I size and fit my bikes for riding over 2 hours. That means seated riding. As much as I enjoy riding gnar as the next AM guy, and I DO, I dont care as much about leaning here or there a bit to get comfortable while standing on the pedals. But when I pedal to the top of the trail, which I DO, I need a good fitting cockpit to avoid back problems. My saddle position will change marginally due to seat tube angle as my saddle is always 32.7 inches from the pedal axle. Any difference can be chased with adjusting the saddle, use of an offset seatpost, slightly longer or shorter stem and rotating the handlebars. As for REACH, like I said, I can lean forward or back with my whole body. That probably gives me at least 4 inches of flexibility. Reach is still important, specially so for a DH bike, but not as important as ETT on a trail bike.

  • Kenneth_Davidsson

    1/16/2015 12:14 PM

    Top article, bring on the next

  • erwinmruiz

    1/16/2015 8:41 AM

    excellent article

  • onenerdykid*

    1/16/2015 8:34 AM

    Great article. As a bike-fitter in my previous job, I have been trying to get people to look more holistically at their geometry figures, with an emphasis on reach. The more this gets known, the better in my opinion.