Retracking a MTB Frame
 

I've retracked many frames in my lifetime. The bicycles that I usually work on are older machines where the seat tube and down tube are the same diameter. Therefore it is easy to take an accurate line across the seat tube and down tube.

However, on the MTB that I currently have in my workshop the seat tube is narrower than the down tube which makes it impossible to take an accurate line using my usual method.

Can anybody offer me any advice on how I might be able to retrack this frame accurately?

Since you have a method that works well with tubes the same diameter, consider putting a shim with a thickness of half the difference in diameters against the seat tube and doing the rest as you usually do.

Retrack? What's that?

I took it to mean realign the frame to get the two wheels tracking in the same line.

OK, that's a reasonable interpretation. Thanks.

assuming the down tube is larger than the seat tube, put a piece of tape the same width of the seat tube on the center of the inside edge of the down tube. would that work?

Same way you'd do it with a frame that had the same size tubes. I have a SS/FG where the head tube is bigger than the top, down & seat tubes, which are all the same size. Anyway, the middle of the head tube should align with the middle of the seat tube, provided the seat tube is welded in the middle of the bottom bracket. Without the wheels on a line should be able to be drawn from the bottom of the middle of the head tube to the rear of the bike. The dropouts of the fork & rear stays are 90* from that line and equidistant from the line on each side. With the wheels on the bike if all of that is aligned and the wheels are laterally & radially true the bike tracks a straight line.

|------|----------|---|

rear dropouts => seat tube/bottom bracket => Head tube => fork dropouts

trued wheels on that line and the bike steers & rolls in a straight line.

Edit: This is also the way you could do it for a BMX frame.

I think the shim is the easiest to understand, but all suggestions are much appreciated.

Thanks everyone.

There is no reason of which I am aware that would call for a different approach with differing seat/head tube diameters, and you do not have to locate the center of either tube. The classic method is to run a string from one dropout around the head tube and back to the other dropout. The distance between the string and the head tube is measured on each side to determine centering of the rear triangle. If the seat tube is larger the distance will just be smaller on each side, but they should still be equal.