Continuing our celebration of the Stacktennial, I thought it appropriate to write something to the ladies or, as we call them around here, The Womens. (It’s a private joke known only to a couple of hundred thousand Slowtwitchers.)
The problem is, of course, the constraints placed on bike design if you mandate that the bike be built with 700c wheels. I’ll end this with a wheel size anecdote, but, for now, let’s just stipulate that, at least for road race bikes, the wheel size is going to be 700c for all sizes. While the issues and problems are almost directly analogous when we talk tri bikes, today I’m writing in the context of road bikes, aka road race bikes, not tri bikes.
Bikes are built with 700c wheels down to 42cm in size, and the mechanical problem created by these larger wheels isn't getting the handlebars low enough, nor is it standover height. It’s getting the cockpit tight enough. The metric in play is front/center, which is the distance from the bottom bracket to the front wheel axle. Once this distance dips below 57.5cm you’re going to start to run into trouble. I’ve never seen a 700c-wheeled bike made with a front/center less than 56cm and with the curious exception of Cannondale, which seems less bothered by this, most companies are fain to make any bike with a front/center of less than 57.5cm.
The problem is shoe overlap, which is not a problem in the course of normal riding, but it is when the wheel is turned sharply. This often occurs at low speeds, when the rider needs to execute an abrupt turn of the wheel in order to balance. Imagine a rider taking off from a dead stop, trying to clip into the pedal before reaching sufficient speed. Imagine a rider climbing a difficult hill, ascending without sufficient speed to keep balanced. “Learn how to ride better, and with appropriate gearing, and shoe overlap is not a problem,” you might say, but this is cold comfort to one who’s found out about shoe overlap the hard way.
The CPSC (Consumer Product Safety Commission) agrees. But it does not mandate a specific minimum front/center. Instead it says that the distance between the pedal spindle and the front tire can never be less than 9cm. Some companies are more conservative than others, and every company makes its own decision about the minimum front/center number. For most companies, it’s 57.5cm. But this depends on what crank length is spec’d on these small bikes, and of course this assumes 700c wheels. You could go down to a front/center of 55cm if you used 650c wheels.
Companies have for decades engineered around shoe overlap. Terry Bicycles did this by placing a 24” wheel on the front. Tri bike companies do this through the use of dual 650c wheel bikes. Some forward thinking bike companies offer dual 650c road bikes, or have offered them, but, 650c road bikes are pretty much a dead market.
Most companies engineer around shoe overlap though short crank lengths on small bikes; and by increasingly steepening the seat angle and shallowing the head angle, as the bike gets smaller and smaller in size. Look at your favorite bike companies' geometry charts. It’s not uncommon to see road bikes in under-50cm sizes with seat angles of 74.5 and even 75 degrees (even 76 degrees). For some years, bike companies forlornly tried to pass off on their women customers the fiction that these steeper seat angles were a better match for their gender-specific morphology. But that has never been demonstrated. Give a 5’2” woman a choice of seat angle and she’s as likely to pick 73 degrees as is a 6’2” man. The only defensible reason for making a small size road bike in a steeper geometry is that a particular seat post and saddle — if used throughout a size run — has a fixed “setback.” Therefore, that given setback pushes the center of the saddle further behind the bottom bracket of a lower saddle height than it would a taller saddle height. But this is a minor amount, and might account for the difference of a degree of seat angle throughout an entire size run, 47cm to 62cm.
The chief reason for steepening the seat angle in smaller sized bikes is to “push” the top tube forward, which in turn pushes the head tube, fork and, as a result, the front wheel axle forward. This increases front/center. An elegant solution, yes, if a woman really wanted to ride at 75 degrees of seat angle. However, if she does not, she’ll slide the saddle rearward on its rails to achieve a “virtual” seat angle of 73 degrees, or as shallow as the bike can be adjusted. The problem: this lengthens the cockpit to an untenable degree. The bike maker has achieved nothing.
In the image above, on the left you see the traditional way of making a bike in a small size. Those who still consider top tube length the determiner of a bike’s length (for fit purposes) blithely point to the example at left of a bike that preserves its top tube length, hence preserving the rider’s preferred cockpit, while increasing the bike’s front/center and therefore overcoming a shoe overlap problem.
Here’s the issue: fit coordinates are immutable. If you want to ride with your saddle 6cm behind the bottom bracket, that’s what you’re damned well going to do. As you should! If you hike your leg over the bike and sit yourself down on a bike that has a seat angle steeper than you like, you’ll simply push the saddle back on the rails—or trade the seat post in for one with more setback—in order to achieve your desired setback. Once you do this, you’ve turned your 74.5° seat angle bike into a “virtual” 73° seat angle bike, and your cockpit is now too long. You’re too stretched out.
This is therefore not the right way to design a bike. Note that if you design a bike this way, the Reach of the bike increases (if you got to this party late, and you don't know what Reach is, refer to our intro in our celebration to the Stacktennial). Yes, you kept the top tube a specified length, but top tube length is not the determiner of a bike’s length. Reach is. This is the point of Reach: to tell the absolute truth, for fitting purposes, about a bike.
The idea is to design a bike to fit properly while preserving its good handling. Fitting well means it should not just have good stand-over height (that’s easy if you build the bike with compact geometry, such as Specialized does with its Ruby). It means also building the bike in a seat angle a woman prefers (which is the seat angle a man prefers, more or less) while keeping the cockpit short enough so that she’s not too stretched out. If you’re bound and determined to make this bike with 700c wheels, there are two ways to do it right.
First, shallow the head angle and add fork offset in a way so as to keep the bike’s trail close to constant. (This is what is done in the right hand example in the image above.) Let’s take a bike in a typical size as an example. If that bike is built with a 73 degree head angle, the fork might have 45mm of offset, giving that bike 56mm or so of trail. That bike is a fairly typical handler. If you want to keep that trail number constant, then you’ll have to add about 5.5mm of fork offset for every degree you shallow the head angle. A 72 degree head angle would require a fork with about 50mm of offset. A 71 degree head angle needs a fork with about 55mm of offset.
Note that you add offset when you shallow the head angle, and both these techniques push the front wheel in front of the bottom bracket. If you wanted to build a parallel 73 degree bike in 48cm, but you couldn’t because the front/center would be an unmanageably short 54.5cm, building this bike with 71 degrees of head angle and 55mm of fork offset gives you the 57cm of front/center you’ll need, and you can do this without lengthening the cockpit and without steepening the seat angle. The bike will fit properly, and it will still handle in a relatively sprightly fashion.
The second technique involves spec. If you place a 165mm crank on the bike you can engineer it with a shorter front/center. Then, place on the bike a handlebar in a geometry appropriate to the need. Handlebars are now made with as little as 70mm of “bar reach,” which is measured from the center of the handlebar bore (where the bar passes through the stem) to the center of the bar at its most forward protrusion, in the horizontal axis. (You'll see this displayed in an image above.)
Building the bike in this fashion conspires to give these shorter-statured riders the 700c wheel size they want (whether or not they should want this wheel size); with the seat angles they want to ride at; the cockpit distances they need; and with handling characteristics that make sense.
How will you know when bike companies are adopting this engineering motif? The signs are: seat angles will not shallow more than a degree throughout the size run, and will be no shallower than 73.5 degrees for the smallest sized bikes; head angles will shallow and fork offsets will increase accordingly, yielding trail numbers not to exceed 60mm; the Reach of these smallest bikes will be in the neighborhood of 360mm or less; and the bikes will be spec’d with short reach handlebars, not to exceed 80mm in bar reach, and with appropriately scaled crank lengths.
Would you know this sort of thinking, as expressed in bike design, when you see it? Consider as one example Cervelo’s R3 geometry. The entire size run is built with 700c wheels; and every size has a 73 degree seat angle, even size 48cm. While the 54cm size has a 73 degree head angle and a 43mm offset, the 48cm size has a 70.5 degree head angle and the fork has 53mm of offset.
Some bikes will conform to this motif, yet still employ (what I think is) an annoying habit of seat angles too steep. The Specialized women’s line (Amira and Ruby) features 70.5 degree head angles and 51mm fork offsets. So far, so good. However, they also have seat angles as steep as 76 degrees. That established, I’m less bothered by this because the top tubes are short on these bikes. If a rider moves the saddle back on the rails (which is very likely to happen) the bike is still going to fit well. Plus, Specialized specs a handlebar with 75mm of bar reach and 123mm of bar drop. All in all, Specialized did a really nice job with this bike.
Finally, let’s look at two examples of how a bike in a small size is made. Trek’s H1 geometry, available in bikes like its Madone, is built in a size 50cm with a Stack of 506mm. Pretty low. Cervelo’s R series bikes are built in size 48cm with a Stack of 505mm. So, here are two bikes that are, for fitting purposes, equally tall. The Trek has a head angle of 72.8° and a fork offset of 45mm. No problem. Very standard. However, with that very standard steering geometry the front/center of that bike would be too tight. There would be a shoe overlap problem. So, while this bike is built with a 73° seat angle in 55cm, it’s build with a 74.5° seat angle in 50cm. That allows this bike to keep to a front/center of 57.5cm.
The R series Cervelo in size 48cm keeps to 73° of seat angle. It has a very different steering geometry. It’s got a head angle of 70.5°, and a fork offset of 53mm. But the trail remains 57mm, about equal to that in Trek's H1 geometry. This kicks the front wheel axle out significantly, and the front/center is, you guessed it, 57.5cm.
Here’s the difference. The Reach of the 48cm Cervelo is 360mm, the reach of the 50cm H1 Trek geometry is 388mm. How long is that 388mm? Well, Trek’s more popular H2 geometry in size 56cm has a reach of 387mm.
Does this mean the H1 is a bad geometry? No. It’s a very good geometry for somebody—male or female—who’s very long in the torso and short in the legs and who needs that size bike. But that does not describe the fat of the bell curve. It should also be noted that H1 is probably Trek’s oldest geometry, and its newer geometries—H2 and H3—go a long way toward solving these problems. Let’s look at Trek’s H3 in size 44cm. Stack: 510mm; head angle: 70.3mm; fork offset: 53mm; Reach: 360mm.
I think this H3 geometry in this size might be a bit too shallow in the head angle—trail ends up at 66mm—but it’s a nice bike, and the thinking is in my view modern and correct.
What is keeping companies from adopting this motif? Some are adopting it, as noted above. Look for more to follow suit. Some of the bike companies to whom I’ve spoken are unacculturated to fork offsets that match these shallower head angles, even though they’re clearly indicated both for handling and for shoe clearance. So, although your flagship brands might not yet be on board, this should be the way of the future, and a push in the rear by your LBS will probably help things along.
Bottom line, women shorter than 5'6" regularly find that they need road bike Reaches to be less than 370mm, maybe 360mm, and even narrower. The Stack will vary depending on several factors, but it's not uncommon for some of these women's-specific geometries to have Stacks that are too tall versus that size's Reach. There are unfortunately too few fitters and bike sellers who really understand this. (We're working on it.)
About 650c in road race bikes, can I close by flying both the white flag in defeat, and the bird in defiance? I noted to Emma Pooley the odd quirk that some fast female time trialists started as triathletes. “No coincidence,” replied the then-reigning world time trial champion, when we talked at the Amgen Tour of California a couple of years ago. “Many of us did, and it’s because there’s no easy entre into bike racing for females. Many of us came through the door of triathlon because it is, as a sport, friendlier to women.”
I asked this as a preface to my next question. “Did Cervelo have to twist your arm to ride a dual 650c bike to your world championship victory?” “Convince me?!” she replied. “I’d choose these wheels in road racing as well, were I certain to receive spare wheel support during road races.”