Actual vs. Effective Tube Angle?
 

Greetings all,

Quick question regarding a measured seat tube angle. On the drawing below, note that the down tube and seat tube are made from one piece of bent tube. The effect of this is that the line of the seat tube is moved rearward. If one wanted to duplicate this geometry without the bent tube, when measuring the seat tube angle, should one use an imaginary line drawn through the center of the seat tube (red line) or should it be a line drawn from the intersection of top and seat tubes down through the center of the bottom bracket (blue line)?

http://i128.photobucket.com/albums/p...tTubeAngel.jpg

It would be the Blue Line. Though even that would diverge from the 69.5 degrees effective because as one raises and lowers the seat, the relative angle of the seat contact point will change. So long as the seat is sufficiently adjustable there shouldn't be a practical problem, but some seats are narrowly adjustable, and builders can occasionally forget the seatpost and seat when they make adjustments to the seat tube angle, even on normal frames. I'm a big believer, when departing from the norm, in having all the parts before the frame build starts. The frame is there to serve the rider's position with the parts intended. Seat tube angle isn't a deliverable, on a custom frame, it is the seat contact point relative to the BB. On this frame that will be a slight variable fore and aft depending on extension.

Oh, I have seen that kind of bend on bikes like utility bikes where the seat tube is a large tube, and sleeves a subtube that is the actual host of the seatpost. In that case it would be theoretically possible to orient the inner tube to be on a radial line with the BB. The same could be done with the example you have here, simply by overbending and then recurving the seat tube, so that the top of it is radial to the BB, on the correct angle. Not saying it needs to be so, just that it could be if you want too add the detail.

If you are doing this as a one-off, in theory, it is 7 times easier to bend back a tube than it is to nudge it forward, meaning it is easier to overbend by a degree, than underbend.

You should go with loopback stays, too.

forget the angle, you really have to design for the saddle location. Seat tube angle locates the saddle, but only loosely.

Thanks everyone for your replies.

Is that to say two bikes will ride the same - one with a shallow seat tube angle vs. another with a steeper seat tube angle and a set-back seat post - so long as the relationship between the seat and the bottom bracket is the same?

Not a builder here, but I doubt you could say that.

if all you change is the seat tube angle (and tt length and seat stays to match) and keep the bb to seat relationship the same, then yes, I would say that the differences would be imperceptible. You aren't changing the rider's position, center of gravity, or relationship of the rider's weight to the axles. What would change? It's not voodoo.

It may not be voodoo, but I think it's not as black and white as you would make it seem. What changes is the angle of the tube with respect to the ground and the weight that the tube bears. The conceivable effect is that there is a difference in the way that bumps and vibrations are transmitted to the rider. How that dynamic plays out is even less intuitive. One could imagine that a more vertical tube could produce a harsher ride. I have no idea whether that is true or not, but it doesn't on its face seem unreasonable. The next question is which tube are we talking about? Wheelbase being a constant, if you relax the angle of the seat tube, the seat stays become more vertical. Because they are connected to the axle, the seat stays seem more likely a culprit for transmitting road vibration to the rider than the seat tube, don't they? But the relaxed seat tube angle is more associated with a smoother ride. Whatever.

I was never one of those guys that had the misty water-colored vision of the "perfect" magic frame. On a recent trip to northern Europe, I had the opportunity to ride a lot of bikes. One of them stood out head and shoulders above the rest. The mind-bending thing about it was that it wasn't a bike with high end components and a handmade frame. It was a Sparta grandpa bike. It had a phenomenal ride. It felt good. I know that there are a lot of elements that contribute to the feel of a bicycle, and that this bike had many elements that I'm not used to. It was MUCH heavier than what I normally ride. It had 650B wheels, fat tires, a sprung saddle, etc.. While I don't ride bikes with those traits every day, I rode many of them while I was there. Yet this one bike stood out. Maybe the geometry is not the issue. Maybe it's the looped frame. I'm just asking questions to try and take the je ne sais quoi out of it, in an effort to shake the misty water-colored magic frame fantasy that has raised it's ugly head in my usually empirical mind since I rode an old opa fiets.

you are describing third order effects. So far down in the noise level as to be inconsequential. If you look around the internets for builders that will admit how they place their seat tubes, you will find that most admit they do it so that the seat post intersects the seat at an eye pleasing setback. The rest will obfuscate and throw misty rose petals at you.

As far as the old bike feeling more spritely than expected, I would guess that is due to the front end.

>Is that to say two bikes will ride the same - one with a shallow seat tube angle vs. another with a steeper seat tube angle and a set-back seat post - so long as the relationship between the seat and the bottom bracket is the same?

"Seat tube angle isn't a deliverable, on a custom frame, it is the seat contact point relative to the BB. On this frame that will be a slight variable fore and aft depending on extension. "

So having decided where these parts are supposed to be in space, you then build the correct support for them. So anything other than that support would be a compromise, but less about position, and more about getting stuff perfect. Simple things, like it's in the right place, but it should look nice, and have max adjustment, and the best structural support. Sorta irks me to see a seat post with set-back, on the back of the rails of a saddle, kinda thing. Sorta irks me to see a seat post with set-back on a frame with long stays , though I have brought that up, and it didn't seem to bother others.