The confusing world of bike sizing

The confusing world of bike sizing

When you go to buy a bike, you could be forgiven for thinking that bike manufacturers all size their bikes in a similar way and that as the size goes up, so the bike gets larger. Sadly, this couldn’t be further from the truth for many manufacturers.

Historically, bikes were sized a bit like clothing – Small, Medium, Large and so on. Some folks  actually gave (and in fact still do give) the sizes numbers, but exactly what that number correlates to can vary – some makers use the height of the bike, others use the length. It is, in simple terms, a bit of a mess.

Dan Empfield, the father of the F.I.S.T. protocol of bike fitting, proposed a method of bike sizing based around Stack and Reach.

What is stack and growth?Start by thinking of a bike and now overlay a rectangle on top of it. Place the bottom left corner of that rectangle at the bottom bracket (that’s the bit the pedals rotate around). You should now be visualising something a bit like this (see left):

Point 1 is our starting point. The Stack of a frame is the distance from point 1 to point 2 while the Reach of the frame is the distance from point 2 to point 3, where point 3 is the centre of the headtube.

Since both the height of the bike and the length of the bike are important in finding a frame that fits you, this makes a far better method of sizing a bike than simply an arbitrary figure for either the height of the seat tube or the length of the top tube.

Given that a large percentage of manufacturers have now started listing Stack and Reach for their bikes, you would imagine that as a bike’s size increases, its Stack and Reach should grow proportionally. If this happened, then when we plot a graph of Stack against Reach, we should see a nice straight line from one size to the next. It would certainly seem a logical conclusion. Unfortunately, and quite surprisingly, this is not the case for many manufacturers and that makes choosing the right size bike quite a minefield.

Here is a graph showing the sizing of bikes from four major manufacturers, Cervélo, Cannondale, Specialized and Time.

Stack and Reach visualOf these four, the Cervélo R3 does indeed show a perfectly straight line, with Stack growing proportionally to Reach throughout the range.

However, take a look at the Cannondale. This is a hugely respected bike ridden to many race victories and is widely loved by reviewers. Turn up at a sportive and it will be one of the most common bikes you see. But its sizing is completely non-linear. It starts off being long and low, but then in the middle, at what they call Size 54, it suddenly becomes very tall with no change to the Reach – it actually becomes a more ‘endurance geometry’ bike. Then to add even more confusion, the next size up, the “56” gets shorter in Stack but very much longer in Reach, reverting to the ‘racy’ geometry of the smaller sizes. In fact, if you looked at the sizes 56, 58 and 60 and compared them to the sizes of 48, 50 and 52, the angle of the line shows you that at the low end the bikes get longer more than they get tall, but at the bigger sizes they get taller without growing much in length And quite what the Size 63 is hoping to achieve is a mystery – it grows nearly 30mm in height, but actually gets shorter in length, giving a very ‘endurance orientated’ position.

You can follow this process for both the Secialized Venge and the Time Skylon too. You can see that the Time bike actually does a reasonable job of growing proportionally through the range with the exception of the third bike in the size scale, the size S, which suddenly gets shorter than the XS but grows a chunk in height.

This graph does tell us one other thing too as alluded to above – we can easily compare the style of a bike. For example, the Specialized Venge is quite a long bike with a lot of Reach, but a low Stack – it features what reviewers might loosely call an “aggressive, racy position”. Equally, the Cervélo R3 is much shorter in Reach but taller in Stack. It has what a reviewer, or indeed the industry at large, might refer to as “Endurance geometry” where you sit more upright and less stretched forward.

Of course, by changing the stem, the handlebars, the height of the saddle and indeed how far forward or back the saddle is, you can adjust the ‘fit’ of the bike, however the underlying geometry of the frame is always there and if you need an R3 style bike to be comfortable, but buy a Venge style bike ‘cause it looks good, you’ll either be uncomfortable or end up with a bike that looks like Frankenstein’s monster – neither of which would be considered a desirable outcome.

Having a bike fit before buying a bike will allow you to firstly find the style of bike that suits you Pythagoras and bike sizesand secondly find the right bike size within a manufacturer’s range that is going to give you the best fit.  If you don’t you’re taking pot luck that you’ll get something that fits you properly.

Manufacturers may say that making bikes grow in size proportionally is not easy. Unfortunately for them, Pythagoras, yes, him of the many hours of frustrating maths lessons, would disagree. Look again at the image of the bike with the rectangle overlaid. If you join the dots of those three lines, you have a right-angle triangle with the line that runs from point 1 to point 3 being the hypotenuse (dotted in red). Since we know the following:  a2 + b2 = c2

all they need to do is grow the length of the hypotenuse (c2) and the Stack and Reach will grow in proportion.
The growth of a bikeWhat does all this mean for the industry at large and for manufacturer’s bike ranges? Firstly, it shows how easy it would actually be for manufacturers to make proportionally sized bikes by following simple maths. Secondly, with most bike builders these days moving towards two ranges – a ‘race range’ and an ‘endurance range’ – The Specialized Venge and Roubaix, or the Cervélo R and S series, it would be very simple, using a visual graph, to create two distinct geometries. Logic says that a race range should be longer and lower to offer an aggressive, aero position, while the endurance range should be taller and shorter to give a more upright position better suited to consumers spending long hours in the saddle riding sportives. If this is the goal, then you would hope to aim for a graph that looks like the one here (left), showing the Cervélo S5 and R3. The graph shows very clearly that the blue dots (S5) have a longer Reach relative to Stack, compared to the green dots (R3) which are taller with Cervelo R3 vs Cervelo S5less reach. Until such time as that happens, the industry will be beset with awkwardly sized bikes and consumers riding bikes not suited to their morphology or goals.

If all this seems like a complete minefield to you, don’t worry, you’re not alone! Fortunately, we can help. Get in touch or drop in and we can arrange a time for a bike fit on our awesome GURU Dynamic Fitting machine. This way, you don’t need to know about bike sizes – you simply tell us where you are most comfortable and we’ll tell you what bikes will allow you to achieve that position best. Could it be any easier? :-)