BB shell angles help

[NBC]

hi all,

i'd like to use the geometry and fit that i've got on a current bicycle i own, so i've measured and drawn up the frame design on paper, however.....

the bottom bracket shell angles i've obtained seem to be a long way from most standard road shells

i need to obtain an angle of 59.5 degrees between the seat tube and chanistays, and an angle of 55 degrees between the seat tube and down tube

i'd like to take some advice from all you experienced builders out there, to see what's really possible !

if it is too hard to work a shell into those angles, then i'm left with either a fillet brazed shell, or creating a lug ?

thanks in advance

nathan

((sorry forgot to add, this is for OS tubes, and oval shaped chainstays))

[suhacycles]

May I ask, what kind of BB drop is that? What size frame and wheels?

[NBC]

700c x 35 tyres and wheels, frame size is about 54, but a reaaaaaaaaaaaaly low bb height, like 260mm !

[Eric Estlund]

All three (manipulating a standard shell, fillet, or from-scratch lug making) are all doable with those angles. If one manipulating the shell doesn't seem like it would work for you, but you feel comfortable fillet brazing, that seems like a pretty straight way to go.

[NBC]

i'd really like to manipulate a lugged shell, but i'm unsure how to go that far with one, so do you have any advice on that ?
thanks
nathan

[Andrew R Stewart]

I'd really like to manipulate a lugged shell, but I'm unsure how to go that far with one, so do you have any advice on that ?
thanks
nathan

Nathan- Some of the answer is up to you're skills, no, all of the answer. Getting a degree per pair of sockets is pretty easy with out much manipulation and often still being able to use a silver filler. The next degree or two is starting to require significant bending, some grinding and some hammering. The gaps left inside are probably best for brass to fill. Beyond this, maybe 3 degrees, amount of angle drift is hard and will also mean much after joining filing and major manipulation before. Some might cut a wedge from the socket to reangle it.

BBparticularlyudifficultfecult with the extra sockets and the narrow material between the main frame tubes. If your build process has the ST brazed into the shell first make sure you do all the socket manipulations first. Any "almost enough but not quite" angles will be a problem during brazing. The tight spots will hinder flow. Any big gaps will be a challenge to fill and need a lot of filing to make look nice.

This is why fillet brazing is so nice. Andy

[NBC]

i'm thinking it might be easier to make a lug for this !

i use brass anyway as my finances can't run to silver !!

if i use one of those cast shells with the chainstay sockets only, i could make a lug easier i'm guessing ?

do you braze the piece of socket tubing to the shell, then cut the interior hole ? or do you cut the hole to pass the socket tubing through to the inside and then braze ?

thanks

nathan

[Will Neide]

i'm thinking it might be easier to make a lug for this !

i use brass anyway as my finances can't run to silver !!

if i use one of those cast shells with the chainstay sockets only, i could make a lug easier i'm guessing ?

do you braze the piece of socket tubing to the shell, then cut the interior hole ? or do you cut the hole to pass the socket tubing through to the inside and then braze ?

thanks

nathan

Any reason why you aren't fillet brazing this?

[Andrew R Stewart]

I always like to match the joint style across the frame. A fillet frame would use no socketed joints if at all possible. So If I were the OP and I wanted to use lugs elsewhere i would try hard to use a lugged shell. Maybe to the extent of making one. Andy.

[NBC]

mr stewart is correct, the rest of the frame is to use lugs, i want to keep the continuity throughout !

as much as i love the look of fillet brazing, it's a skill that i feel is going to take a long while to master before i'd feel happy in using for a complete frames, plus i'm a sucker for lug lining !

do any of you guys weld lugs rather than braze them together and use silver ?

thanks

nathan

[Doug Fattic]

Nathan, it is possible for you to change the bottom bracket angle a few degrees from a standard 59 to 55. Start by grinding the bottom of the sockets that are keeping the tube angles apart. That of course would be the lower part of the holes most opposite of each other. It is possible to use a half round file by laying it flat on the socket and lifting the handle slightly and showing it who is boss. I find it much easier to use a die grinder with a grinding wheel just a little smaller than the socket size. Most common grinding bits have a 1/4" shaft. They might even work in a drill but they don't spin as fast as a die grinders' to be as effective.

The other resistance is going to come from the material right on the top of the shell between the seat and down tube. This can be stretched by bending with a couple of tough tubes or round bars. Grind and bend both areas until the seat and down tube match your desired angle.

You will find it easiest to braze the seat tube as a separate unit into the bb shell first. This is way many builders do to insure that the foundation of the frame starts out life right. I recommend brazing 2 separate spots (at different times) to hold both position and alignment before brazing the entire unit together. The first one (I call a position spot) should be on the inside point of the shell between the two tubes. This is to hold the depth and rotation of the seat tube into the shell. Next before brazing the 2nd spot (which I call the alignment spot), I not only hold the seat tube 90 degrees to the bb, I push the seat tube towards the down tube to take up any slack and make the angle more favorable to your frame design. Now the 2nd spot can be placed on the entire webbing above the window most bb shells have to hold this position. After the spots have cooled, the alignment can be checked and corrected before brazing everything together.

I begin the actual braze below the back window (and below the alignment spot). Once I see silver coming through the inside of the shell I switch to doing the front (starting on the centerlines compromises alignment the least) and finally finish on the sides. It is important to look inside the shell before turing off the flame to make sure silver is all the way down all the way around.

After the unit has cooled it can be cold set back into alignment (if the seat tube has moved a mm or 2). In addition any silver or tube sticking out into the down tube socket can be ground away so the DT can slide in without resistance. And finally the down tube/seat tube angle can be checked to see if it is actually 55º. If it isn't, more grinding on the base of the down tube socket can make the angle be what you want it to be.

Good luck,
Doug Fattic
Niles, Michigan

[NBC]

doug,

thanks for that info, but does grinding not create a weak spot in the shell ?

sorry for my lack of knowledge

nathan

[Doug Fattic]

Nathan, there is enough material in a typical bottom bracket shell to grind a bit at the bottom of each socket to change the angle between them a few degrees. You are only going to be grinding up from the bottom tip of each socket 4 to 6 mm. The wall sockets thicken some near the base. Your are not going to take off that much. By the way it isn't a framebuilder's tradition for pros to haze rookies with bad advice as a initiation rite :).

You will probably have to beat in the webbing above the windows some so they maintain contact with the tubes (as a result of the angle change) before they can be brazed.

[NBC]

doug, sorry if i caused a misunderstanding, just never done it before and don't want to end up making a mistake that hurts someone !

are there any tips on bending oval chainstay ports, or might 7/8" ROR chainstays be a better bet for this ?

thanks

nathan

[Doug Fattic]

It certainly is a lot easier to make a round chainstay fit into a round hole. Many oval chainstays take some manipulation to get them to fit – both in the socket as well as on the stay itself. Adding to your difficulty will be getting the angle to change as well. As a teacher of framebuilding classes, I always encourage simplicity because difficult things almost always lead to disaster. It is a rare student that already has all the skills to do something like you are wanting to do by themselves. One way to increase the width between the stays is to increase the length of them. It is always useful to have the wheels and tires on hand so you can see what works.

[suhacycles]

Nathan - I'm curious, is this for a CX frame but with a road-like BB height?

[Will Neide]

http://www.velocipedesalon.com/forum...les-30528.html


I found this thread helpful in changing angles.

[NBC]

thanks doug, your help is invaluable, if i had the time to hop over the pond for a course, i think i wouldn't have to think twice about it !

as i haven't ordered the parts yet, i've got plenty of tubing / lug choices to make, i'm thinking i might pick round chainstays for this one

if i use bars to bend the sockets, do i order solid, or the thickest gauge steel tube i can get ?

also with ROR chainstays, what's the best way to align the oval portions of the stays ?

in reply to suhacycles, yes it's basically going to be a chunky tyred road bike for winter, and the occasional light tour / weekend away, so using a 400mm a-c carbon disc cyclocross fork, hence the rather small angles we're talking about here !

thanks wilco, those pictures are really helpful too !

thanks to all who have posted, i feel now it's the time to practice, then practice a little more !!

nathan