Tandem Tom

As I was going through Doug Fattic course during my build I found that the alignment process went quite well using the BB clamped to the post. I have heard mentioned using a method of the HT.
As a real newbie what would be the reason for this method over the other? Pro's & Con's?
Thanks!

unterhausen

the bb shell sees a lot of distortion due to brazing/welding. this can make the bb shell an unreliable reference. Holding it by the head tube and supporting the seat tube gets away from all that. If you are going to muscle it into alignment, holding it by the BB is pretty much required.

Tandem Tom

I think I get it. So what would the "support" for the HT look like?

8aaron8

Usually bench centers similar to this photo

Andrew R Stewart 

Here's a shot showing another way to hold a frame during alignment checking. The head tube is held in what's a form of bench centers (in my case made by Alex Meade) and the seat tube is held up with an height adjustable device (jack screw). When the HT and ST are of the same height off the surface then any twist of the main triangle or rear end can be measured. But note that this isn't how I hold the frame during any actual prodding. For that I mount the shell to the post, which is shown just below the DT and past the shell, ready to be used. This method take out of consideration the alignment of the shell from that of the tracking. By using a combination of shell on post and HT in bench centers all aspects of alignment can be had and with slight trickery the two methods can serve as double checks on most of these alignment aspects.

8aaron8's image is when the rear end only is being checked and also likely corrected as the main frame looks to be rigidly held at it's four junctions (thus making this shown set up useless for main frame alignment checking as the rigid surface and mounts will just flex any twist out which will just spring back when removed. One needs to go past the point of alignment to have the spring back be that aligned point). Andy

Tandem Tom

Thanks Andy!
The pic helps me understand the idea.

Andrew R Stewart 

Tom- Glad it explains the method. It did take a few minutes to search through my folders to find this, I need to get ghastly sick and be desk ridden so I have the time and boredom to organize my photo files better then I have Andy

unterhausen

Andy, how far off the table is the centerline of the bench center?

Andrew R Stewart 

Eric- About 162.+mm. Sorry I don't have the exact measurement. IIRC when I had Alex mead make this for me I speced a range which would result in the frame's centerline being around the same height off the surface as when the shell is mounted on the post. This way the various tools for aligning would fit with either frame holding method.

Alex hit the heights of the two ends of the through shaft within .001" of each other! I then shimmed the bases to get even closer. Andy

Tdotbikes

I like your indicator holder, did you adapt something to attach the indicator to the base?

unterhausen

That's a height gauge missing some parts. I have a height gauge to hold my rear dropout alignment gauge and a Starrett Surface Gauge to do what Andy is doing with his height gauge/indicator setup. The Starrett gauges are pretty cheap on ebay, and you can get a height gauge for decent prices too. Just make sure they have all the parts you need. For whatever reason, height gauges often don't have the pointer and people don't know they should be heavily discounted if they don't have that part. Although the dropout alignment gauges take the place of the pointer.

Andrew R Stewart 

Not sure if the indicator holder was from a height gage or not. There's no markings on it, as in measurement lines. The dial is bolted to a couple of self made Al links which are C clamped to the vertical element. I now have a couple of proper height gages complete with Vernier scales and carbide scriber tips. But I find I use this one most. Andy

Tdotbikes

Thanks for that, I might cobble together something similar. In the mean time, I plan on making a BB post similar to yours. I hate to hijack the thread, can I ask what are the dimensions of your bottom bracket post? Should I try to make the post so the BB face sits a specific distance from the flat surface?

unterhausen

I didn't bother worrying about the dimensions of my bb post.

Tdotbikes

How about the diameter of the face of the BB post? There is more than one size BB shell now a days.

unterhausen

I don't see that it's super important. I use the face as a reference. I only build with threaded BSA bb's, but I suppose if I had to, I could make an adapter ring. That's not important unless you plan on bending something into alignment.

Andrew R Stewart 

So here's my post's aprox. dimensions. Diameter is 99mm (a skim cut under 4"). The height is about 127mm. The top surface is reduced in diameter to about 58mm (just large enough to fit a 1 piece American BB) and the angle of the top edge is about 30*. The post was surface hardened then the top and bottom were ground flat. The through hole was threaded at the top and bottom to 3/4"x10tpi.

Overall this design is extremely solid and with one exception works really well. Since I wanted to have a large top surface (larger then the common 68mm threaded shell) there has been a need to place a spacer between the shell and the post's top surface infrequently to lift the chain stay above the top surface's edge.

My post was fabricated by a retired machinist back in the mid 1990s for about $250. IMO a very good deal. Andy

Tdotbikes

Thanks Andy, that is very helpful. There is machine drawing of BB post from the Paterek manual floating around that is tiny compared to yours. Most BB posts I have seen are not threaded on the top face. Most I have seen have a threaded shaft that is part of the post. Any reason you decided to use a large bolt instead?

Andrew R Stewart 

Using two bolts, one for the shell (the top one with a machined cap/washer on the top of the shell) and the other to secure the post to the table, just seemed easy to do. The shell tightens down really well on the post top. I see no need for an inside the shell sleeve as some designs have. Threaded shaft/stud sticking up or a bolt doesn't seem to me to be that different.

I agree that most posts don't have the mass and diameter that mine has. Possible overkill. Andy

Tdotbikes

I plan on making some mountain bike frames, T47 bottom brackets have a 2” outer diameter. I’m wondering if I will have chainstay clearance issues when using a 1.5” bottom bracket on a post with a 2” face?

Andrew R Stewart 

Depends on who close to the shell's edge/face the stays are attached at, any crimping and how the stays angle out from the shell. But the solution is simple. Make a spacer/washer to sit the shell's face on and thus lift the stays away/above the post's surface. Andy

Doug Fattic 

I think adding a washer to the top of a post is a great idea and not just because Andy suggesting it is one of my buddies. If a post is used enough eventually a groove will be worn where the shell contacts. A hardened washer is part of the solution. I use my alignment table as a tool where I will spot seat/down tubes to BB shells. It is much easier to clean flux off of a washer than a post attached to a table. In addition the washer can be sized to the shell being used.

Tdotbikes

Thanks for the replies everyone! I plan on starting my BB post project this week some time. I’ll create my own thread once I get started.

Andrew R Stewart 

Tdotbikes- I'm only 2 or 3 hours away in Rottenchester, NY. If you ever are in the area feel free to stop by and we can talk tools and builds. Andy

Tdotbikes

I’ll might take you up on that.
My wife and I just got from Niagara Falls, NY today. We love the US side of the falls, we find a new place in the park to explore every time we go. Thanks for the offer, I’ll let you know when are headed that direction again.