Originally Posted by
dschad
Wow, thanks for this. A lot of interesting stuff to get through....some comments below.
This is one of the key questions I'm trying to understand - is there/what is the "key" number. May ways to get to the same trail/flop via HTA and rake. So is that the key numbers (trail/flop?). Looking at some key players in the touring bikes, the HTA/Offset/Trail/Flops are in a fairly tight range (trail flop calculated from
http://yojimg.net/bike/web_tools/trailcalc.php):
Rake and trail have the biggest effect on how a bike feels. It's actually trail. Rake is how to get the trail number you want. How much trail is appropriate for a given use is why there are so many different manufacturers out there duking it out with each other with all manner of design. Different uses will tend to cluster together.
There really isn't key number. Flop is a product of head tube angle and wheel diameter. Mostly it's useful for estimating a bikes desire to keep going the way that it's going in a turn. It's the rise/fall of the front end of the bike when you turn the bars. Really high flop bikes like modern 62 degree downhill bikes foe example have really wide handlebars to give the rider enough leverage to counter the force of gravity (lift the front of the bike/counter the flop) when attempting to turn the bars. It's not good or bad. It just is.
I really wouldn't pay "flop" too much mind. If you were to draw a parabola graphing a really floppy bike for stability, the shape would be very pointy. Perfect for riding in a straight line, but it would want to quickly stuff itself in to a corner. A road racing bike would be almost hemispherical. Consistant, predictable. A shopping cart, naturally, would be a flat line since there isn't any head tube angle at all.
That last option is your suggested hta/trail which gives a FC of 611, which is 2mm less than the current design. So the question is really - is that 70/58 a good number for a touring bike? Interestingly, the LHT 46cm with 26s is 70HTA, but they use the same fork offset for all, so the trail/flop is out of the norm.
I would say "Yes" Someone else may say "No" The 58 trail number may feel a bit light when riding unloaded or only rear loaded. But with front low-riders largely undoing the lightness, it'd ride more neutral. The 70hta would offer straight line stability. The flop would lend itself to wider handlebars or even flat bars for an upright posture.
Ok. I was wrong about that. I was going from memory.
I've been playing with this (
https://www.bikegeocalc.com) which is really fast for this type of fooling around:
The dotted is my existing setup (with short stem), the colored is the design I'm fiddling with. I am trying to match the BB position and handle bar position (note picture actually has 26 wheels, which end up very close toe overlap to the current bike).
This is partly why I want to have a significant amount of stem to work with - so I can get flexibility in my component selection. My bars are 75mm reach with 102mm drop. In theory I could even reverse my 40mm stem and still have my hoods in front, but then things are getting really funny looking.
I think it's a useful tool but BikeCad starts with your own personal body dimensions then maps the bike to fit. I would defer to Andrew R Stewart from up thread. He's the actual hobbiest builder here.
The Voyageur has 17.75s, so pretty long. Those are what are in my design, although I have heard people complain that long makes things sluggish, but your point about weight distribution is a good thing to consider. The overall weight distribution was also a concern with the FC seeming to get so short.
So much of the short chain stay trend (?) is the faster
feeling bike tends to get the sale. So manufacturers stuff the rear wheel forward to make the bike feel responsive. The weight of the rider being directly over the wheel also helps in steep climbs with low traction. It save the rider from body position skills. The trade off is the bikes propensity to tip over backwards with rear loading or really steep climbs weak riders would rather walk instead. A racing bikes also need to stuff themselves into smaller spaces behind the other riders for aero, draft, cornering position, etc...A touring bike has none of those concerns. Cadillacs and Rolls Royce's are not sports cars. The extra weight of a 6 inch longer frame is completely irrelevant. But the mild manners is something that has to be experienced to be appreciated. At almost 18 inches, your current bike is already at where modern gravel bikes are returning to.
"Boost"? What next, a sloping top tube?! 1x drivetrain?
60's may be too much, and I haven't really gotten to the thickness dimension yet, but I thinking that 700x40s and 559x54 would be good upper ends. I think beyond that the frame starts to get beyond my (hoped) esthetic parameters. But I don't know.
Thanks again for the great food for thought.
This is why it's fun to play with ideas on paper first
Good luck! 😉