Project Sleipnir - The Requirements

Part 2 of 7 in Project Sleipnir

Quite simply, I don't fit my bike. 

At its root, getting a proper fit is pretty simple. There are only three places where your body touches a bike:

1. Feet
2. Butt
3. Hands 

Feet

If you think about it, spinning a crank on a bike is a lot like climbing a flight of stairs. You essentially take a step up, then another, then another. But cranks (just like stairs) are only made in a size that fits the majority of people. But what that means for somebody like me is that if you want to go up quickly, that you have to take all these tiny little bitty steps really fast. Or you can do what most of us taller folk do, you can go two steps at a time. It's easier on the body and faster at the same time. 

This same basic principle should apply to spinning cranks too. Longer legs can and should move further per step than shorter legs. Unfortunately due to the nature of mass-marketing things to people they bless us with making 2 standard sizes of cranks: 170mm and 175mm. If you're lucky enough to be 'normal' then these sizes are great. 5'8"-10" are perfect. But this means if you're 5' tall you have to push and pull much further than your body would really like. And if you're 6'6" tall you have to make these movements that are much smaller than you should make.

But then the question is "How much bigger is best?" 

There's not a perfectly solid answer on that, but there are a couple of interesting pieces out there to help find that answer:

Zinn's Custom Cranks - 205mm

Custom Cranks from Germany - 199mm

Both of these (and many other sources) indicate something in the 20-21% of inseam is the ideal length. And when it comes to proportions I'll trust a percentage or ratio any day over somebody saying, that a 5'9" and 6'6" person need the same size crank. Similarly if you look at the size difference between the tall person and the normal sized person you'll see that the tall person is about 113% (78/69 = 1.13) the size of the normal guy. Apply that math to the normal cranks (175mm * 1.13 = 198mm), and you'll find the tall guy would be better off with something in the 200mm range. 

That number is surprisingly close to what I needed based off of various crank calculators. And now we have the first requirement:

Requirement #1 - Bike must fit 200mm cranks

That requirement actually poses a whole host of issues which we'll work out later.

Butt

One part of fitting properly on a bike is making sure your knees are not too far in front of the cranks. Doing that would put a lot of extra strain on a hyper-flexed knee joint and that will lead to lots of problems later. But to make that work on somebody with a long femur that means you have to push them back...way back.

There's a couple of problems with that. If you go back further your handlebars are now too far in front of you. Additionally because the person with long legs will also need the seat up really high and the angle of the seat tube you can end up with the butt's contact points being almost directly above the rear wheel. 

Being so far back actually changes the handling of the bike. Even on a flat your weight distribution is greatly changed from the optimum that's been honed over the past 170 years of the bicycle. But things get much worse on a slope where you actually end up pulling wheelies, which can get nerve wracking.

I made a simple diagram to show this below 

Figure 1: As the seat-tube gets longer and seat has to slide back the relative position of the rider's weight in relation to the rear wheel is adversely altered.

Therefore we now have the basis for the second requirement:

Requirement #2 - The position of my butt must remain the same relative distance from the rear hub as on a normally-sized bike

Hands

The final fundamental piece of fitting properly on a bike is where are your hands relative to the rest of you. Since the fundamental center of a bike is the axis on which the crank rotates, the best way of determining where your hands go is based on how high up your seat goes and then how far forward your arms and torso go.

The whole premise is summed up well by Lennard Zinn in this column on Stack and Reach

Figure 2: Stack and Reach for Frame and Handlebars

How this relates to the dynamics of riding that bike:

1. If you stretch too far up and forward your shoulders and neck will ache terribly. 
2. If you go too far low and forward and not only will your shoulders ache, but you'll also cut off blood-flow to your baby-making fun factory. 
3. If you go too far up and back your handling is worse and it's hard to go fast.
4. If you go too far low and back your knees hit the handlebars...which means you don't go anywhere.

There's actually a narrow range of what works to make you both fast and comfortable. Unfortunately this is one of the top areas where I currently suffer. Because the seat is so far back I have to stretch too far for the handlebars. Because I'm tall and my current bike isn't, I have to go low...making man-bits suffer and shoulders ache with creaking fire. Since that is unacceptable I've had to compromise a lot on bike handling and get up and back to the point of the near-ridiculous. This also puts me even further back on the bike exaggerating the problems seen in Figure 1.

The final requirement is actually pretty simple and the gist of the whole project:

Requirement #3 - The bike needs to fit my body

Requirements Conclusion
Looking around there wasn't a single bike or frame available for purchase that met all 3 of my requirements.

I actually almost lucked out with the KHS Flite 747 as it is the only production bike that meets Requirement #1. And based on the measurements it likely meets Requirements #3. Sadly Requirement #2 was sacrificed at the alter of mass-marketability. And another problem with this bike is that nobody has one of these in stock. They're pretty much a buy-and-pray-it-works bike. Combine that with a few features weren't what I was really was hoping for and it's off the list.

Fortunately for me I actually did find a bike that fit my body much like I wanted it to. The Specialized Roubaix in a size 64. Unfortunately although only the 3rd requirement was met and Requirements #1 & #2 were completely ignored. Still good news though as I was able to find some basis for what kind of fit my body needed on a bike. Plus with a $4000 sticker price, no used ones for sale ever, and the fact that the components were not really my preferences it would be a big bill for something that wasn't quite what I'm looking for.

Therefore since nothing exists on the market that meets my requirements that means that I would need to have my next bike custom built.

...and if you're going to custom build a bike you may as well unleash your imagination a bit and see what you can make.