Frame Geometry Question...

[SirJoey]

I'm working on my plans for a LWB bent. The boom tube will be parallel to the ground. I want my head tube angle to be set at the "standard" 72 degrees.

That said, I know that both ends of the down tube & the end of the boom tube where the 2 tubes meet, should all 3 be cut to the same angle, but I'm not sure what that angle should be. :o

Can anyone help?

http://img384.imageshack.us/img384/7131/sirjoeysigmedij1.gif

[AtomicZombie]

You will now need to know your inseam length and the position of the seat and cranks to get the other tubes. You could weld in the head tube, drop in the front forks + wheel, then prop up the main tube so you can sit on the frame and figure out the position of the down tube.

That's what I would do if I were me!

Brad

[SirJoey]

Okay, here's a crude sketch. Hope this clarifies what I'm trying to do.

I know there's a way to figure it out, other than trial & error with the grinder, but geometry is not my strong suit... :(

http://img381.imageshack.us/img381/4186/lwbsketchme8.jpg

http://img384.imageshack.us/img384/7131/sirjoeysigmedij1.gif

[AtomicZombie]

That is the opposite way I normally do things. I would prop up the main boom to the height you want and then prop up the head tube with all bearing hardware installed and fully inflated tire to the desired angle. From there you can clamp the down tube or even a bit of 2x2 wood and mark the angles using a marker or angle finder.

If you try to "math it out" ahead of time, it will most likely be less acurate. The "try it and see" method takes into acount things like head tube height, bearing cup clearances, tire inflation, etc.

By getting the angles from the actual frame parts, you simply can't go wrong.

Just a thought.

Brad

[SirJoey]

Thanx, Brad. That's prolly what I'll do.

I was just trying to figure out a way to make all 3 of those cuts end up being the same angle, kinda like they were on the DW, even though it had a different head tube angle & a sloping boom tube.

Unfortunately, I still can't use TrueSpace, & using PhotoShop as CAD, is a tough way to do it! :rolleyes:

http://img384.imageshack.us/img384/7131/sirjoeysigmedij1.gif

[SirJoey]

Wow, that's excellent, Papa! Yeah, the rakeless fork is the way I'm going, too.
That'll get me in the ballpark, & I can mock it up, per Brad's suggestion.
Outstanding, thanx! BTW, what would the trail be with this geometry, & a straight fork?

http://img384.imageshack.us/img384/7131/sirjoeysigmedij1.gif

[SirJoey]

Why no rake?

Cuz a straight fork is all I have to work with, which is the main reason I wanted to go with the "standard" 72 degree head angle. Then later, maybe I'll be able to locate a raked 20" fork, & all will be right with the world! ;)
In the meantime, trail still wouldn't be too outrageous.

Also, bottom side of boom, to ground clearance will be 12", instead of 10", so that prolly throws EVERYTHING off, right? :(

My bad! Sorry, I guess I should've thought to include that dimension. Don't worry about re-figuring everything, unless you enjoy this sort of thing...

Thanx!

http://img384.imageshack.us/img384/7131/sirjoeysigmedij1.gif

[AtomicZombie]

The unknown distance between the bottom of the head tube and the tire will effect the final angle in the drawing. It will depend on the length of the fork legs, crown length, as well as the cup thickness. I guess it depends on how parallel you want that main boom.

Brad

[SirJoey]

The unknown distance between the bottom of the head tube and the tire will effect the final angle in the drawing. It will depend on the length of the fork legs, crown length, as well as the cup thickness. I guess it depends on how parallel you want that main boom.

Yep, that's what I figured, thus the need to still mock it up, AZ style.
I'd really like my boom as perfectly parallel to the ground as possible, just for looks.
Or, if anything, slightly higher in the front would be okay, as long as it's not LOWER in the front!

http://img384.imageshack.us/img384/7131/sirjoeysigmedij1.gif

[SirJoey]

That's great, Papa!

That's exactly what I wanted, nice & tight between the down tube & tire!

Thanx a lot! :)

http://img384.imageshack.us/img384/7131/sirjoeysigmedij1.gif

[AtomicZombie]

Perfect!

Brad

[AtomicZombie]

Thanks for thinking of us!

I think it is great that you take time to offer advice, as you certainly have a lot of experience. I have no problem at all with drawings / mods / photos being posted here as long as they are not directly from plans.

It's all about the bike building community here, and think we have the best neighborhood on the Internet here!

Thanks.

Brad

[SirJoey]

Good call, Papa. And thanx again for the help. It is most appreciated! :)

http://img384.imageshack.us/img384/7131/sirjoeysigmedij1.gif

[gbbwolf]

I can understand your frustration.
Many inventor's get their idea's stolen everyday.
Then someone with more money and connection's in Industry, takes that idea and makes a fortune with it.

Not saying that bike builders will be making a fortune.

But they could make money selling plans on their own.

Me if I come up with an idea, I just share it.

Guess that is why I am broke, and poor.

I have had an idea for a fully retractable cover much like a convertible top for a motorcycle.

But it never got past the drawing board stage.

This being said, I now expect to see one on the market within six months.
Sad thing is I have had this idea since 1981 when I bought my first motorcycle and got rained on the first ride.

Nelson

[TheKid]

The local landfill is chock full of my drawings that wouldn't work practically, as well as discarded photos and parts that didn't pass the inspection test. In addition, the electric authority pulled in plenty from lights being on all hours of the day and night.
I fully understand your position Papa, and agree 100%.