Friday, 29 May 2015

Bike Sizing: Stack And Reach

When it comes to bike size and fit, the industry has suddenly started talking a new language. And high time too. 

I am flying out to Oregon in a few weeks and for the first time intend to cycle while I'm there. Details are still to be confirmed but one option I've been looking into is hiring ('renting' for the Americans) a road bicycle from a bike shop. But my bitter experience has taught me that stock sizes provided by mainstream manufacturers simply do not fit me. Nonetheless, I have indulged in two short periods of daydreaming about renting a bicycle utterly different from my own, just for the sake of it. 

As it happens, the only two models of bicycle available for me to hire are
  • the Raleigh Capri (year/model and precise build not known)

the 2015 Raleigh Capri Carbon 1 (not available in the UK)

  • a 2013 Jamis Xenith Comp  

the 2015 Jamis Xenith Comp (as the 2013 archive data is virtually illegible)

The Raleigh is available for me to hire in a size 50cm, the Xenith in a 51cm (as the sizing and measurement methods vary by manufacturer). While size numbers usually relate to seat tube length, the crucial question for me is, how long is the reach? Typically for this size, the effective top tube will be 53-54cm, which is much too long for me even with the tallest headtube, an uncut steering tube and fitting the shortest high rise stem known to man. (Don't ask me how I know this. Okay, do ask -- but it'll all get covered in my Surly Cross Check review, when I get around to writing that!)

It was a matter of 15 seconds to determine that there was no possible way I could ride the Raleigh or the Jamis in any size. 

However, perusing their respective geometry charts revealed something very intriguing. Alongside data for seat tube angle, head tube angle, seat tube length, top tube length, chainstay length, standover, etc etc were two new 'dimensions':  

stack  and  reach

'Reach' of course I know quite a lot about, but I knew immediately from the figures (e.g. 370mm for the Jamis) that this could not be 'reach' as I know it, i.e. the measurement of the top tube, effective or otherwise, plus the length of the stem to the back of the handlebars. 

As for 'stack', I've been using that word myself for some time to refer to the height of the front end of my bikes, from bottom of the head tube to the top of the stem where it sits on top of all the spacers. But with figures like 523mm for the Jamis, again it was obvious that the manufacturers were using a totally different definition than I was. 

So I ran a Google search and found a few images illustrating what bike manufacturers mean by 'stack' and 'reach', such as this one: 

Stack is the vertical height of the frame, from the centre of the bottom bracket up to the top of the head tube. 

Reach is the horizontal distance between the centre of the bottom bracket to, again, the top of the head tube. 

The result:  a square -- or, to be more accurate in the world of bicycles -- a rectangle. 

I'll admit, with my obsession with fit and geometry, this came as something of a revelation -- and a very welcome one. Finally, we have measurements that are intrinsic to the frame without reference to angles, without danger of 'dilution' by how a bike is built up, i.e. by choices of seat post, stem, handlebars. 

I then found a bike fitter's website with an article on determining the right size of bicycle. After explanations of the aspects and variables, there is a subheading titled "So What Is Your size" with a table: 

Suddenly, in a tangible, visible sense, my problems with the industry's approach to sizing became obvious. I am 5' 4" (164cm) tall. My inseam (or pubic bone height if you insist on getting both personal and technical) is 32", which happens to be exactly half my height. Unusual? I don't know. But let's look at that table. According to that, my height means I should ride a size 50cm frame. My inseam however puts me on a 54cm frame.  

This was beyond interesting, it was becoming downright intriguing. I dug out the Bike CAD drawing of my custom Enigma Etape. Its seat post measures 510mm, which by industry standard makes it a size "51cm" or "XS" as Enigma would normally categorise a stock geometry frame (although that seems relative only to men, to be honest; you'd hope that "XS" in a women specific design would be considerably smaller than 51). 

The Enigma's reach is 365mm, which isn't too far out of the ballpark compared to standard-sized frames. But its stack? A whopping 593. In mainstream bike frame sizing, that's a size 58cm! Compare that with the (pitiful) 523mm stack on the Jamis Xenith in size 51cm. 

(Funny story here: I had never heard of the Xenith until the Oregon bike shop told me that's what they had. Later I went to have a look at the geometry and spec but even while on the phone during that first call, I found myself saying "yeah, that might work... if the steerer's uncut and you can put 60-70mm's worth of spacers on top"!) 

Before I go any further, here are the respective geometry charts for the Raleigh and the Jamis, and then the "frame blueprint" for my Enigma. (Geometry geeks, feel free to pause here and re-join the discussion in a few hours. Or days. No, seriously, I know how you feel!)

Raleigh Capri Carbon 1
Annoyingly, the chart does NOT specify (a) the effective top tube length,
which for the 50cm will be longer than 515mm), (b) the stack, or (c) the reach.
Not particularly helpful.
As you can see, Raleigh's whole approach to sizing is to ask
how tall you are and that's the end of it. 

 2015 Jamis Xenith
For the 51cm size, the effective top tube length pushes 53cm.
Of more relevance is the stack (517mm) and the reach (371mm). 

My custom Enigma Etape
(Oh the joys of custom!)
Seat tube length: 510mm all well and good.
Effective top tube length: 526mm -- only a smidgen shorter than the Jamis.
Reach:  an unexceptional 526mm (only slightly shorter than usual for a "51").
Stack:  a sky scraping 593mm.
(THIS is why my bicycle looks so "odd" to casual observers.)

So while I'd been thinking of the reach in terms of the length of my body plus arms (with an attempt to factor in variables such as flexibility, handlebar height, how much do I want to lean forward, etc.), it seems we have additional data to consider that may be more objective. Could this make comparisons of the sizes and potential fit criteria across lots of different brands and models easier, more meaningful and potentially much more helpful in making purchase decisions? (Or bike hire decisions, as the case may be.) Even if the answer is "yes", does adding stack and reach to the list of figures to compare between bikes add an additional layer of complexity to the process? Hmmmm. 

Then, reading the July 2015 issue of Cycling Active magazine, I noticed something new in their group test of "sportive" bikes: 

Firstly - an express interest in geometry (although the reviews themselves did not actually address it)

And then this info box, which essentially aims to boil down the huge unwieldy study of frame geometry into one simple matrix:  Stack-to-Reach-Ratio 

Bearing in mind the arbitrary classifications (>1.5 = comfortable; <1.4 = racy), no wonder the reviewers couldn't detect any real difference between the bikes in their group test in terms of comfort. The Merida Ride has an STR ratio of 1.61 (stack = 611mm; reach = 378mm). The Lapierre Sensium 200CP scored 1.51 (stack = 578mm; reach = 383mm). The Tifosi Scalare Ultegra (described as having a frame that is "pretty compact, even by today's standards") has a counter-intuitive STR ratio of 1.38 (stack = 540mm; reach = 395mm, the very antithesis of "compact"!)  In the final analysis, the Tifosi won the group test, quite surprising given the stated purpose was to find which one of the three was "most comfortable" over longer distances. The reviewer admitted to preferring the Tifosi's more stretched-out feel and frankly, lighter weight, better groupset and marginally better (of a bad lot) wheelset. Hardly a meaningful review with regard to geometry, except....

Cycling journalists are using a new data matrix,
christened the Stack-to-Reach ratio or "STR". 

So, getting back to my "real world bicycles" -- 
  • the Jamis has a STR ratio of 1.396 (pretty racy)
  • the Raleigh has an STR ratio of 1.41 (still pretty aggressive)
  • my custom Enigma as my point of reference has an amazing STR ratio of 1.625 (which actually isn't all that surprising once you've digested all the facets of its geometry). 
As a side note, the Surly Cross Check that I ride most days has an STR of 1.357, the most aggressive of the lot! This demonstrates the vital role played by its uncut steering tube and approximately 80mm of spacers. (Gulp.)

If confirmation were needed that this new STR calculation isn't just one cycle journalist's conceit, an article specifically on the industry's non-standard approach to sizing appears later on in this issue of Cycling Active, with STR mentioned as a tool that cyclists themselves can use to compare different bicycle frames. 

So there you have it. It's all in the STR!

Post Script 6th June 2015

Today I tried out Sigma Sport's new "Fit & Find" online sizing calculator. The only measurements requested are height and inseam (in millimeters). I put mine in: 1640 and 820 respectively. I got an "warning" message:

I clicked "Auto Calculate" to see what the recommendation would be:

The auto-correct changed my inseam to 8020.

I tried it again, this time clicking Find bikes" to proceed anyway with the correct inseam of 820. Here is the result:

I could not make this up if I wanted to. 


  1. It's easy to see how/why people spend decades become experts in fit. Like you, I've spent entirely too much time learning about as much of it as I can (and still always learning), and I can even add that some builders/manufacturers measure reach differently than the image you've pulled. For example, I find most "stock frame" makers use the method you've indicated, but some I've come across measure from the center of the seat tube to the center of the handlebar stem... and they still label it as "reach," which is entirely confusing. The first time I realized this was trying to compare two different makers and I realized that the numbers were so far off (as in 200+mm) that I knew it had to be a different measurement they were using for a similar sized bike.

    Anyway, I agree that the stack-to-reach ratio is far more effective in determining the proper size for a rider than the seat tube or top tube length. I have also found that seat tube angle and head tube angle make a big difference in my personal comfort or ability to ride a particular bike.

    Also, I'm about 1/2 cm (0.2 in) shorter than you, but my PBH is approximately 2 in (5 cm) less. I think your proportions are more "typical female" in my experience, meaning that you have longer legs and a shorter torso (too many drawing/painting the figure classes!!). I think it's also why many females struggle with stock geometry bikes as they are built more to typical male specifications.

    I am finding more and more that I can ride some stock geometry frames, assuming that I understand the right measurements, which are not necessarily the ones many shops/websites want us to use. It's so frustrating to have things seem so complicated and then realize that perhaps the wrong method has simply been used all along.

    So, what is your plan for riding in Oregon? Have you picked a better option or is there another location you can rent from that might have a better fit for you? I would think, depending on the city you're visiting, there could be quite a bit of possibility?

  2. "measure from the center of the seat tube to the center of the handlebar stem... and they still label it as "reach" -- yes, that's definitely the definition understood by most of us for quite a long time. It seems a new measurement has emerged but they're recycling the name. Not ideal. I guess the old definition refers to the rider's reach, the new one to the bike frame's reach. What I like is that, armed with that figure and knowing what seat tube and head tube angles you prefer for comfort, suddenly you can rule out a lot of bikes quickly and definitely sight unseen, no wasting time with test rides etc.

    Your penultimate paragraph - YES!! I've just commented on blog post from today as you know, speaking from a very similar place.

    The plan... hmm. Well, I could take the Cross Check. I bought a sturdy travel case for it. But United Airlines want to charge me $200 each way.

    Bike shops in Eugene seem to be assume that anyone wanting to hire a road bike has flown in for a competitive event and need a RACING bike. Road bikes i.e. drop bar bikes for simply riding around (transport, exercise, sightseeing) are just not on their radar, for renting at least. They certainly SELL the right kind of bikes -- I really wish the one offering me the Jamis would just rent me one of the Surly Stragglers on their shop floor instead!!

    The other option I've been pursuing is to borrow a bike. I don't know any local cyclists "in real life" as I only took up cycling after I moved away. The cycling blogosphere is coming to my rescue and I've got a good lead on a 1980s road bike that would work very well indeed, except that it's got a single chainring and 7-speed cassette. Need to think about that one, as there's quite a climb between where my parents live in Roseburg and where one of my sisters lives several miles out of town. My hope was to be able to ride out to her house early each morning and get a lift back with her in the car when she drives into town. May have to give up that idea and just ride along the river each day to clear my head. Multi-use path though so won't be able to ride hard. Oh well.

    1. For $400 to ship your CC, you could probably purchase something second-hand in Eugene that would work. Holy moly! That's quite a bit, but not surprising given that many airlines are charging quite a bit just to check a bag these days. Of course, I understand that it probably wouldn't fit as well as your Cross-Check either, or would require a lot of fiddling, which is likely not what you'd want to spend your time doing while visiting family/friends.

      We went on a vacation one time and decided to buy used bikes off of Craigslist in an effort to avoid shipping bikes out and I have to say that was a really, really horrible experience for me. I think I rode the bike all of 50 feet, after which I refused to touch it because it just felt wrong. We ended up selling the two bikes to some guys who were working on a house next door to where we were staying. They were ecstatic - and I was glad to have learned a lesson about trying to just make something work.

      I know there's a shop in Bend that sells second hand, repurposed bikes that you may like, but that's at least a couple hour drive from where you're headed (If I'm remembering correctly... I know I drove over mountains to get to Eugene from Bend). I have little knowledge of the Eugene area... I've visited the city just once and it's been many years (something like 20 years... wow - didn't realize it had been that long!). Anyway, it's a shame that the shop isn't open to renting the Straggler as you've stated. At least you'd know what you're getting with that and it wouldn't be a big surprise.

      I hope you're able to work something out so that you can take advantage of riding while you're in Oregon.

    2. Last time I rented a bike on holiday was in Ireland - it was a hybrid and just horrible. Still better than walking or renting a car though (which I had done for previous visits to the same area).

      I think the bike loan is going to work out. It's a 1985 Trek 420 in what today would be a size 50cm but geometry more "square" e.g. ETT to ST, rather than longer reach as modern bikes seem to be. And it's got a quill stem. I will take my own saddle on its own seatpost, as my saddle is tricky to dial in and I don't want to mess up Lesli's set up for herself. Will just mark the quill stem height to make sure I put it back for Lesli when I'm done. So yeah, own saddle, pedals, lights, helmet and lock. I've been route checking too and it looks like the Trek's gearing will be sufficient for my little engine to haul it up the local hills. For a week with family, I will be travelling very light so all my bike stuff will fit into one suitcase, which I *think* qualifies as cabin bag not check in. It certainly fits in overhead cabin compartments, just need to check its dimensions against United's limits. So hopefully all set.

  3. All very interesting, and I'm quite certain not one of my bicycles is the correct STR for me. I'll get around to figuring that all out someday.

    1. As ever, I'm not sure there is a "correct" STR for any given rider. But I think it does give an indication of whether a bike will need only minor tweaks to be made comfortable or whether it really is outside the limits of rideable. As you no doubt already know from your own experience, the range of what's acceptable can be very broad. :)

  4. Thank you for this article. Very intriguing read for a rider who has learnt the hard way not to trust S, M or L.

    But surely it can't be just STR? A medium or large or XL frame could have the same ratio, but for a rider to make up his/her mind on what size to get, the individual numbers of stack and reach will also be important?

    1. You're absolutely right: STR has nothing to do with size. It's a ratio, so tells you what the relationship is between other points of data. For example, a bicycle designed for racing (perhaps time trialling) will have a very similar STR across its entire size range.

      I don't think STR will help you find the right size. But using it as another point of data will help avoid a situation where you look at, for example, reach (only) and think "oh that's my size, then", only to find that the proportions or position is all wrong for you. Sadly that discovery is often made only after we've bought the bike.

      Those of us with particular issues will no doubt always look first to see what a bike's reach is (or its stack, if that's been more of a problem) as a pointer towards SIZE, and then look to the other measurements and data for that size to see if it will work. STR, as an additional piece of data, can help with that.

      What follows from this, of course, is -- you can significantly "change" the STR in real-world terms according to the way you build up a bicycle. The frame's STR doesn't change but the fit does. I've written about that here.

      I'm excited that more and more manufacturers are providing this information.

  5. I have exactly the same problem as you. Long legged and short torso. On my custom mountain bike my stack is very high and reach is low. It is typically the opposite in modern mountain bikes these days with more emphasis on 'long/low' so the reaches are very high, but stacks are much lower than my custom frame.

  6. This post is so informative and makes a very nice image on the topic in my mind. It is the first time I visit your blog, but I was extremely impressed. Keep posting as I am gonna come to read it everyday!carbon fiber wheels bike

  7. A very informative article. Out of curiosity, I checked the STR on my 20 year old traditional framed bike. It had an STR of 1.30! No wonder I felt so stretched out on it, compared to a newer compact framed bike that is supposedly of the same size.

  8. Sorry, but your mention about your Enigma stack is incorrect. I could calculate it exactly, but lets do it simple: 526 (efective top tube lenght) - 280/2 (half of saddle) - 50 (minimum setback) = 336mm maximum. Top tube lenght minus half of saddle minus minimum setback give you relative bottom bracket position.

    1. I am not sure I understand your comment.

      In this post, all stack figures are for the frame alone, as provided by the builder/manufacturer, without reference to eventual build such as size/length of saddle, layback, number of spacers etc.

      Relative bottom bracket position isn't mentioned at all here, but of cours it is in the follow up post, where I took actual measurements of two complete bikes. If I erred in those calculations, then I made the same mistake on both bikes, and the point is how they compare to each other so it is hardly relevant if I used a different method of calculation that what you have outlined above.

  9. Nice article. Great subject and well written, thanks

  10. This article help me. Thank your for sharing.


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