Seat Posts and SetBack II

(another variation of this appeared in an earlier Reader, but…tough! This is far from a duplicate, with one new super duper insight.)

The problem with most small frames is too steep seat tube angles. The designers do that because a steep seat tube angle lets them use a short top tube (which they know you’re going to focus on) while still maintaining a certain front-center. That’s the distance from the center of the crank to the center of the front wheel, and in some circles the maker wants a certain minimum there. Saying that puts me on the edge of a related tangent that would get me way off track, but let me just say that the practice of making small bikes with steep seat tube angles is based on assumptions that I don’t buy. I mean, I understand what the manufacturer or designer is thinking, but I think they’re thinking’s off, or incomplete.

Ninety-five in 100 bikes 53cm and smaller has a steep seat tube angle, by which I mean 74 degrees or more. Fine. But when you combine that either with a seat post that has no offset, or a saddle (usually a women’s saddle) with a short usable parallel section of rails that’s biased toward the rear of the saddle, then it’s impossible to get a good position on the bike. You sit on your seat and the pedal’s too close to you. The saddle’s location itself is too far forward, which puts the forward pedal too close to you. Then when you pedal, the downstroke pulls you forward more and puts more weight on your hands.

The pedaling action is too unrecumbent-like. Here’s a way to visualize it in extreme, because visualizing in subtlety is harder to do. This is that new insight.

Imagine Hussein Bolt or Marion Jones or Bob Hayes on the starting blocks. They’re track sprinters. The guy fires the go-gun, they fire the first stroke and lunge forward. They go forward because their knee is ahead of their foot. ‘Fit were the other way around, they’d go backward.

Your pedals are the starting blocks, and if you’re too far forward on the bike, it is the exact bike-riding equivalent of putting the starting blocks behind your knees. When you push down, you’re pushing down and back too much.

Hold on. I know how the pedals move, in a circle and all. After 3:00 they move backward, no way to stop that. But what I’m saying is that a more rearward saddle puts the crank more forward and hugely tends to lessen that effect. A recumbent does it in the extreme, which is one reason why recumbent riders don’t scooch forward and feel weight on their hands. I know there are other reasons, but the point is, pedaling pushes them back. That’s why they need those funky seats with back rests.

The recumbent pedaling position isn’t the goal or ideal, but it illustrates (in a rearward extreme way) the same thing the track-sprinters taking off at the crack-o-the-gun does.

On a bike, the difference between a 72-degree seat tube angle and a 74-degree one doesn’t sound like much. But for every 1-degree difference and 55cm traveled, the fore-aft difference is 1cm.

Don’t be intimidated by the metric system or the math. All that means is, if you draw two 21.6-inch lines from the same point in space, and one is 1-degree different from the other, then the far ends of the lines will be 4/10ths of an inch apart.

Man, hold on! The top of your saddle is way more than 55cm/21.6-inches from the bottom bracket (representing the “point in space), and 74 minus 72 is 2, and so that 4/10ths of an inch easily grows to an 1 ½ inches. That itself is significant, but it doesn’t stop there. The difference in seat post offsets (where the clamp is relative to the center of the shaft) ranges from nothing to 45mm (1 ¾-inches). Add that to the 1 ½-inch difference from seat tube angles, and you’re up to 3 ¼-inches, and you’re still not finished. Seats vary a ton in how far back you can shove ‘em. The King of all seats was the Selle An-Atomica, designed by Tom Milton, who died of a heart attack last year on a double-century. It turns out that he put too much rearward shovability into the saddles, and heavyweights would shove them back too far and bend the rails. But the Selle-A saddles still set the record. On the other extreme, many women’s saddle, and even a few Brooks men’s saddles, don’t let you push ‘em back far enough even for Joe-Likes-To-Pedal-Forward.

It’s frankly too much trouble to measure the range of saddles. I’m not going to buy $1,000 worth of saddles and even then wind up with an incomplete survey and a bunch of saddles we don’t want. But the range in saddle rail clampability is at least an inch, and that’s not even including the off-the-charts Selle-A saddles.

As the leg gets longer the saddle gets higher and the seat tube angle difference increases. In small bikes, though, most start with such seat tube angle deficits that they need the right combo of saddles and seat posts to even stand a chance. Small bikes, as a rule, are the worst-designed bikes in the world. There are some other reasons for this. But the seat tube angle is a biggie.

This is supposed to be a useful, informative story, not a bummer, and certainly not a sales pitch for our bikes. But imagine if you felt this way, and you were designing small bikes. Would you blindly copy the mistakes others have made, just so you’d have a lot of company? Me neither!