I love low trail handling. But I’m a big guy. Proportionally, aesthetically, I think 700c wheels make the most sense for a personal bike. To this point I’ve only ever had 26" or 650b wheels on low trail bikes. My first question is whether anyone knows or thinks that low trail handling can be adequately achieved at the 42-44mm tire size. I’m not interested in 700x32 for this bike and most of what I’ve found on this suggests going no larger than that size.
To this end I am trying to suss out which lugs I can use to design a 700c low trail all-road single top tube mixte. I’ve always thought in terms of the HT/TT angle but this obviously won’t work in this situation. I’ve noticed that 60 deg HT/DT lugs are often paired with 73 deg TT lugs. I will want a sharper HT angle in this instance. My working idea at this point is to use a 58 deg bottom lug, with a 58/64/7 lugged bottom bracket.
What I’m most curious about is a desired axle-to-crown for the fork. This bike will be fendered, so I’ll be wanting a 20-30mm or so gap between the tire and fender. If I make the fork “too long” it will be more difficult to achieve low trail, right? So I want it to be as short as possible while making a perfect fender line possible. My research leads me to believe that I want a 390-400mm axle-to-crown. Does that sound right to you?
Just fillet braze it, unless you’re married to lugs for some particular reason. Lugs are great but for anything unusual… much less so.
If a lugged crown 390mm will give you plenty of fender room, yes. For a unicrown or segmented you might want to be closer to 400mm, but it will depend on the height of the crown itself.
Low trail is a state of mind There’s no perfect or ideal low trail geometry, despite what Jan Heine says. As a big guy, larger tires make sense. They’ll add a bit of extra weight. If you want the feeling of a smaller tire, reduce the trail a bit. How much? That’s for you to decide.
You’ll likely have to make your own lugs for the HT to get the angles you need. It’s not too hard, but you’ll need to know how to fillet braze. Look up how to make a bilam or bilaminate lug. Guys like Chapman, Coast, and Weigle probably have some examples out there.
You fork length depends on many factors. What brakes do you plan to use? What lugged fork crown do you plan to use? What’s the largest tire you plan to use?
This will be my first frame ever. So not only do I love and prefer the look of lugs, but as an autodidact it is the method I am learning. It’ll be a lugged crown and I basically have it down to the Pacenti PBP or the Herse.
Haven’t finally decided between canti and centerpull, but I’m leaning centerpull for this particular bike. This will be my first ever frame so I’m not thinking custom lugs are in my future quite yet!
I’ve got it down to two crowns, the Pacenti PBP or the Herse. I am designing around 42mm tires but it wouldn’t hurt my feelings to fit a 48 under there. Slicks only
To build on what others have said, this is usually a game of threading the needle. I’ve built two frames for 700 x 45 tires ( or 35’s and fenders) and I’ve tried to keep the AC as low as possible because of the bottom lug angle. I also like a 70-75mm BB drop so that works against me in that equation.
I’ve gotten away with using a flat crown fork (thinner than a sloping crown) and a 58 degree bottom lug. With a 45mm and fender your AC will be 400-410mm.
That’s fair. Just don’t be scared of fillet, it’s quite easy so long as you’re not concerned about making instagram-worthy perfection. I used to teach people to fillet braze well enough to do a frame in about 30 minutes. Some youtube and some bronze and flux and a bit of practice and you’re there.
The RH centerpulls are designed for a 42mm tire with fender clearance. If you stick a 48mm in there you’ll lose about 3mm clearance. That’s probably fine.
I don’t design for lugs so I won’t comment, but some general advice:
I always make the same mistake of trying to one-shot projects. Try breaking your project into smaller milestones, especially if it is your first frame. You will make progress faster that way.
On the next frame, try a custom fork, or brazing, etc…
Right now I think the custom geometry, construction techniques, and custom fork are all coupled together. If one of those does not work out, it puts the whole project at risk. You won’t learn anything from a pile of tubes that you can’t ride!
Are you using bikeCAD or hand drafting your design? If you post some drawings, it would help us answer some questions.
Thanks! For what it’s worth I am designing within the limits of the lugs. It’ll be fully lugged and I have the set picked out. As for the fork, I know it’ll be a challenge, but a) I want to learn and am not afraid of failing, b) I have a fork on hand that could work (though it’s a 650b fork), and c) If I were to purchase a known fork I’d definitely go for the Soma here
I plan to make a full scale drawing rather than shell out for (and learn) bikecad. Paul Brodie has a detailed video showing how to do it. I am also drawing quite closely on bikes I already have that fit me ideally. I could go into more detail on everything if that would be helpful. I am here precisely because I need to learn, not because I have it all figured out.
Ok, this is what I’m planning:
Bottom Lug - 31.8 x 31.8, 58 deg
Bottom Bracket - 31.8 x 28.6, 58/64/7 deg
Top Lug - 31.8 x 28.6, 108 deg
Mid-Seat tube Lug - 28.6 x 28.6, 71.2 deg
Hoping to keep the down tube as long as possible. Roughly 700mm c-c with a 680mm tube.
I need the full drawing to come up with the precise top tube length, but I’m assuming less than or equal to 650mm.
There’s no perfect or ideal low trail geometry, despite what Jan Heine says. As a big guy, larger tires make sense. They’ll add a bit of extra weight. If you want the feeling of a smaller tire, reduce the trail a bit. How much? That’s for you to decide.