Question on measuring frame alignment
 

With all this recent talk of people building up Peugeots, I decided to rebuild my AO-8 back to a drop bar 10/12 speed as it had been for years. After using it as the test mule for an IGH build, all the important bits were transferred to my Trek IGH build and the frame has sat forlorn in the basement for about a year.

Anyway, one thing I had probably done badly when I was building the 3 speed was setting the rear triangle, because the spacing on a Sturmey Archer hub is considerably narrower than a normal freewheel/freehub setup. So first thing was to set spacing back to where I could fit the normal rear wheel back into place.

This I did but am having trouble with geometry or my own sense of space, because when I measure alignment with strings set up as below:

http://i197.photobucket.com/albums/a...ps761bc4b1.jpg

...I run into something strange:

The two lines that cross high (from seat stay to fork and back crossing with dropouts to head tube and back) seem to align perfectly. The cross at the exact same spot side to side, and they are equidistant from the seat tube. This I interpret as meaning frame is not tweaked and front/rear alignment is good (dropout to front axle).

However, the line running from fork to rear dropouts is NOT equidistant from the seat tube. It's out by about 2mm (ie. its about 2mm closer on the non-drive side than the drive side to the seat tube). This I interpret as meaning the seat tube and the stays are not parallel...but that contradicts my conclusion from measurement #1 .

Can anyone suggest either a) what is my logic flaw (please be kind!), or b) where do you think I should be adjusting the frame?

Leave the fork out of any calculations when using string.

A Park ***-2 and FFG-2 would be much better than string.


Evidently certain Park tools are banned. It is a frame alignment tool, very handy.

i'm impressed with your work ... and that beautiful frame.

but if the whole length of the wheelbase (from front fork to rear drops) is only 2mm different on one side, this means it's really off by only 1mm, right?

so i would just focus on the geometry of the rear triangle (from seat tube to drops) and get it to within a mm difference.

Regarding your string from fork tips to DOs, how do you know the steering is centered exactly? As you swing the steering back and forth the fork tips move laterally, so if the steering isn't exactly centered the fork tips won't be centered on the frame. The effect will be bigger for the strings to the DOs than for the strings to the seatstays. This is because at the point where you would measure them (i.e. the distance to the ST) the non-moving attachment point to the seatstays is so close. That is the reason for jiangshi's comment above - leave the steering out of it. Just worry about the main diamond vs. the rear triangles. The fork will still be just as straight (or not) as it always was.

Thanks all for comments so far.
[MENTION=346401]jiangshi[/MENTION]: I'd love the park tool frame alignment jig but I don't do enough of this - string is almost free and I'm cheap.
[MENTION=232813]eschlwc[/MENTION] and [MENTION=190941]jimmuller[/MENTION]: I included the forks to be sure overall alignment was good, but if you are saying measure only on the frame then I guess I am good since the seat stay to fork and head tube to rear DO strings are both equidistant from the seat tube. The 2mm of "outage" is from the fork to the DOs. (Also Jim, I eyeballed the front fork squareness by staring (a lot) at where the two strings leave the fork dropouts, and carefully balanced the tension and angle of the strings on both sides - not perfect but pretty good....).

Any other measurements I should be taking?

I confess I've never done any of this, but since you have the thing rigged up, measure the distance from each fork tip to its corresponding rear DO. If the steering isn't dead center you should expect to see different lengths, one fork tip moving forward, the other backwards. But do you really want to know even more stuff that might convince you there is a problem? 'FIwuzyou I'd leave well enough alone! :D But seriously, the rear triangles are the only things you should worry about for now, especially since that is the only thing you would have messed up with your previous tweaking.

Get a well-trued and dished front wheel. Install it in the front fork and note if the tread is centered under the brake mounting hole and if the side clearance tire to fork blade is the same on both sides. Then reverse the wheel and try it again. If the wheel centering and clearances were the same both ways you don't have any fork issues. You can also string through the caliper hole and the two fork ends, then measure carefully to make sure the blades are effectively the same length. This can also shed light on whether a blade is bent farther forward than the other. If it all looks balance with these tests, the fork is most likely good.

Ultimately you want the bike center of gravity to be in the same plane as the line between the two contact patches, when the front and back wheels are in the plane of the frame and incidentally in the same plane.

You need one string, from head tube to rear drops. The strings attached to the fork are a waste of time since the fork is not stable to the frame.

This feature article on preparing a frame set might prove helpful. Hope it does.

I believe this is right. 1mm is too little to worry about.

The cause of the misalignment is merely an academic question. Are the head tube and seat tube parallel to each other on the vertical plane?

+1 Do this. Keeping it simple is the way to go.