Diamondback's Online Fit Calculator

When we poll you all on what vexes you most about your cycling, it’s contact point discomfort (saddle, mostly) and your position. We’ve developed and deployed solutions for both, but they require you physically visiting and availing yourselves of tools and expertise.

Those who patronize direct sellers of bikes (Canyon, Premier, Diamondback) may never experience the in-person solutions to which I allude. What Diamondback wanted was an online tool for folks who won’t rely on the in-person experience. I’m going to describe here the math I proposed for that company. It’s been deployed (with considerable coding skill) on the Andean Custom Studio, which is the cool configurator Diamondback uses to “build” your Andean.

The online calculator shows up after the decisions you make on spec and color, but you can navigate directly to that calculator. Below is a miniaturized (to fit this article’s column width) screenshot if part of the output when I subject myself to this prescriber.

The results of this endeavor will determine whether my gaze exceeded my grasp. My goal was to “prescribe” with high granularity your pads position (Pad Y and X, to pad-center) based solely on your overall height and your saddle height (from BB to the top of your saddle, 4cm behind the nose). Not the size of the frame. The exactly pad placement. Many of you on our reader forum helped me in this endeavor.

I’m going to walk you through the hows and whys of this process because I’m going to predict something about this predictor: It’s going to be a highly visited link on Diamondback’s website out of sheer curiosity, and (rightly or wrong) as a fit tool, whether or not a customer ends up buying a Diamondback bike.

Why overall height and saddle height?

Why did I choose only these values, and why these particular values? First, I wanted this to be easy. Second, the fewer values I chose the fewer the opportunities for an end user to make a mistake in measuring. As to why values, overall height needs no explaining, and saddle height gave me a glimpse into your morphologies. Imagine two people 6’2” tall (1880mm). The rider with a saddle height of 780mm is long in the torso; the rider with a saddle height of 830mm is long in the leg.

Why not just use inseam instead of saddle height? Because saddle height got me further down the road. It represents morphology plus pedaling action (whether you’re a toe pointer or heel dropper). And, because I now know your saddle height I can cross-reference your final pad position with what data I have on armrest elevation drop from the top of the saddle. Of course, this all assumes you can provide a good value for saddle height! If you input a bad number for saddle height everything the calculator spits out will be off. And, I mean tri bike saddle height, not the Lemond formula, and I mean saddle height at the seat angle representing the modern consensus (an orthodox saddle position today on tri bikes is considerably forward of what you’d ride on your road bike).

I asked a lot of Slowtwitchers to help me with this. You gave me your saddle and overall heights, and I graphed the Pad Y and X positions prescribed here versus the actual positions you ride. You can see where many of you end up in the chart above. There’s a lot on that chart! Each data point represents the Pad Y and X of about 100 Slowtwitchers (blue diamonds), and a bunch of pro athletes (red squares for men, green triangles for women). The arrows point to the positions the prescriber here generated for you based on your height and your saddle height and the numbers are Pad Y and X respectively generated by the math in this Diamondback prescriber.

Note that you fit into a band. The deep green line is a slope that is the mean. The lighter green line is the range of allowable positions this prescriber generates based on the differences in morphologies. All Slowtwitchers using the Diamondback fit calculator are going to fit inside the wider green band (except "Duckies" above, who was robbed of a torso at birth). Just notice that the blue dots are where you really ride, so either the calculator is telling you where you ought to ride, or it’s tell you to ride in a place you’re not capable of riding. Which is it? Good question. Let’s talk more about it.

The blue squares on the chart just above are the prescribed positions and they all sit in that narrow band I just wrote about. The green squares are where you really ride (if you participated in this exercise). The longer the line between those blue and green squares, the more the prescriber “blew it”. Or, the more your position isn’t orthodox and perhaps isn’t optimized.

Note these blue squares don’t sit on a line but in a band (that faint pink band). On the center of that band sit the prescriptions you’d get if there were no consideration given for morphology. Those at the top of the band are leggy; those at the bottom of the band are torsoey.

The prescriber adheres to a fairly rigorous orthodoxy-of-position. It expects you all to ride alike one another, as if you were all goose stepping in sync in the army of a petty dictator. The only variance you’re granted is for morphology; there is no consideration given for fitness, fatness, age, ardency, skills, or ambition. There is no “I just want to finish” button to click on this prescriber. Why? Because I haven’t found any data to suggest “finishers” tend to select “unambitious” positions.

What I have done – pay close attention to this – is give you the “pro” position (remember those red dots) plus 20mm of Pad Y. I added some pad height to the “pro” position. That’s your age group fudge factor. What this means is that if you truly want to race like a pro does, you want your position to be right where those red squares are representing the pro men (a couple of charts above), and you want to use this prescriber, use the numbers this prescriber outputs but choose the solution that allows the bars to be lowered by 20mm to 30mm.

Variances between prescribed versus actual

As noted, the longer the line between a prescribed position versus the rider’s actual position, the more our predictor and your position fail to match. What explains this? Could be several factors. I highlighted a forum user in the chart above with the greatest variance, with the user name LAI. His position is just below.

He’s been in the A2 wind tunnel, having his position dialed. He rides fast. But you can see why his position varied considerably from that generated by Diamondback’s prescriber. LAI rides very steep; he rides very low, and the steepness pushes the pads out, and the lowness brings his Pad Y down. I’m comfortable not producing a prescriber that prescribes LAI’s position, because I don’t think very many of you could or would want to ride his position, even if your overall and saddle heights are exactly LAI’s.

But is anything wrong with LAI’s position? Not for LAI.

Accordingly, what you see in the actual-versus-prescribed chart above is a lot of blue (prescribed) squares inside that narrow band of orthodoxy and a lot of green (actual) squares sitting all over the place. That's Slowtwitch for you! Do I want your green squares to be closer to the blue squares? Half the time yes. Half the time no. Depends on the rider.

Static versus dynamic fitting

And this is precisely why you should be going to Trent Nix, Anne Barnes, Mat Steinmetz, Ian Murray, Jon Blyer, JT Lyons, Matt Cole, Dean Sprague, or any of the truly good fitters around the country (I can give you the name of the right fitters in just about every U.S. region and every non-U.S. country)! This is why your default decision should be to locate the right fitter with the right equipment in his fit studio. You may end up in a position like LAI, described above, and here’s an image of him riding below. I’m not prepared to tell him to obey my prescriber and to change his position. Neither am I prepared to create math that tells you all to ride LAI’s position.

However, many of you just won’t go, or don’t have access, or lack the confidence and trust to patronize, to such a fitter. So here you go: Diamondback’s tri bike position prescriber. Just know that a proper analogy would be for me to prescribe your shoe size based on your overall height and the length of your ear, from its top to the bottom of its lobe. Better just to avail yourself to a direct measurement of your foot (and of your bike position).

Two parts to the fit process

What is the ambitious (and revolutionary, or foolhardy, depending on the critic) element to Diamondback’s prescriber? It’s that it engages directly on part-1 of the 2-part fit process.

Part-2 is the easiest, because it’s simply a math problem. Looking at Diamondback’s tool, it asks you if you already know your Pad Y and X. You would know this if you are confident in your position and you simply measure the rise and run from the BB to pad-center on your tri bike; or if you go to a fitter such as I list above.

Part-1 of the process gives you your Pad Y and X. There hasn’t been a good process for this. Some will quip that there still isn’t a good process for this. Nevertheless, this online tool is in my view significantly advanced over what came prior.

Saddle position? Adjust to suit

Note that this prescriber outputs pad position. Not saddle fore/aft! What do you do with the saddle? With this prescriber, this system, this exercise, you place the pads where the prescriber tells you to and then move the saddle fore/aft until it’s comfortable. If the math in this prescriber is anything close to good, your hips should end up where I want them to be, which is where the hips are versus the crankset) on the positions of the pro triathletes you see in the photo galleries on Slowtwitch.

Yes, there are variances, including among the pros. How do I factor in those variances? I don’t. I’m assuming you’ll ride pretty much the way the mean pro athlete rides, and I don’t find that pro athletes ride steeper than age groupers who are properly fitted, by a good fitter who owns the appropriate fit tooling. I only find that pros ride lower than age groupers, and have normalized accordingly by adding 20mm to pad heights when prescribing your position.

Should you buy a bike based on this prescriber’s recommendations?

No. Yes. It depends on the resources available to you, and your willingness to seek an alternate solution (i.e., to visit a good fitter). Would I use this prescriber? The prescriber works for me because I ride quite an orthodox tri position. The prescriber is going to miss the mark by only by so much, and probably not by a whole frame size (maybe by a stem length, however).

I wrote above that I believe this prescriber is vastly better, more accurate and more helpful than any online calculator for tri bikes. The others I’ve seen are either hopelessly bad or maddeningly imprecise. If you’re buying a superbike and you’re buying it online (or special order, such as a Trek Speed Concept via Project One) you need to make firm decisions on a proprietary stem. On where to miter the steerer. On the pedestals you’ll anticipate using. On how long the cables and housing should be using that stem and those pedestals. A direct or special order seller is going to send you what in the mail, if the only prescription given is frame size? What length will the cable housing be cut to? It seemed to me the only helpful prescriber needs to call its shot with precision, otherwise what’s the point?

The Andean’s prescriber will always give you 3 options, which may well include more than 1 frame size, and you must choose based on handling and adjustability. Revisiting my solution (above), I could be on an XL, but I would have very little room to move the pads back, or down. I prefer the size-L and I prefer the stem length that gives me pads in the center (neutral) position. With this solution I can move to a different stem size if need be, I can move the pads fore and aft, and I can move the bars higher or lower.

How will Diamondback build the bikes, now that it has this prescriber? I don’t know. My guess – and what I would counsel – is that they build the bike to match the solution you choose, adding a bit of extra cable and housing to accommodate an extra 20mm of height and 10mm of length (should you find the prescribed solution not quite high or long enough), with the steerer cut to allow an extra 20mm or 30mm of height (that much steerer sticking up above the stem). For those who buy an Andean off this prescriber, how the complete bike is assembled is a question you’ll want to ask.