In this post, I will map out a procedure we employ to check frame alignment precisely. A piece of string does an ok, rule-of-thumb, job. A shop tool such as Park Tool's FAG 2 is better. But in the case as outlined below, this was not going to do it. In this particular case a cyclist returned a 7005 aluminium, triple-butted, roadbike frame via a dealer complaining that the rear brake mounting aperture was out of alignment. This indicated to him that the whole frame was out of alignment. The groupset was Shimano Tiagra with Tektro sidepull brakes. The issue seemed, superficially, to be one of insufficient toe-in. These types of brakes are not impossible to toe-in, but certainly not as easy to do as with higher-level groupsets (for example 105 and above; Rival and above). Initial diagnosis was that it may be simply a case of botched toe-in adjustment. To confirm this, we employed the following 6-step procedure.
Step 1 involves a quick check with the portable calibrator. This is simply fitted into the dropouts. It sits square in the dropouts giving, firstly, a chainstay measurement relative to the centerline.
Step 1 Initial callibration: chain stay
You can immediately see here that there is, at least, a one millimeter deviation from the centerline towards the right-hand chainstay. An appointment with the alignment jig is a certainty. But it is first necessary to check the seat stays. So it is rotated vertically to give a seatstay reading.
Step 2 Initial callibration: seat stay
Here we see that the alignment looks pretty much spot-on. This is important since it is this area that's at the center of the cyclist's objection to the frame. Although it passes the initial test, the skewed seat stay reading means that we must move on and see how the frame measures up on the exceedingly precise frame jig.
The Frame Jig: side view
The dropouts are secured with a quick-release into the rear mount. The BB shell sits in its mount. The headtube rod is secured in position. Any deviation outside of a 4mm tolerance will be immediately visible. The key instrument here is the chain stay/seat stay calibrator which rotates square to the jig vertically from the seat stays to the chain stays. This gives us the definitive measurements to 0.1mm.
Dropout mount. You can also see the callibrator arm extending forward to the BB shell
Hence, Step 3 in this process is checking the chain stay.
Chain stay check overview
Within the block, which slides forward, there is a slide rule which moves left or right from a zero degree centerpoint.
Chain stay measurement closeup
Zooming right in:
Centered exactly
At maximum forward extension, the right-hand bevelled end of the rule is moved towards the right chainstay. It makes contact at about 0.8mm, meaning there is a 0.8 skew. The tolerance is 3mm. However, in this situation anything over 1mm would have been clear evidence in support of a mis-aligned frame. It comes in under 1mm, so we dismiss the earlier reading given in the initial calibration.
Step 3 complete, we move on to Step 4, seat stay measurement. The arm is rotated vertically to engage the seat stays and give us the information we are seeking about the alignment of the aperture:
- Seat stay callibration overview
Here we see that the rule has less than 1mm degree of freedom to move. The rear triangle passes inspection.
It looks good
But this not quite the end of the story. Let's go on to Step 5 and check how the BB shell aligns with the rear triangle and the headtube i.e. the front triangle. Looking from the front towards the back in the following shot, we can see the BB shell mount. Basically, if the frame, once the dropouts have been secured in the rear mount, does not sit within the limits of the BB mount, the frame fails. Here it fits. So we are still looking good.
Sitting square in the BB shell mount
We finish off with Step 6: checking Head Tube alignment.
Step 6: Head Tube alignment
Sitting square
By following these six steps, a frame can be quickly and easily checked for alignment. It is accurate and reliable. The frame examined here passes all steps.
So what is the upshot of the story. We conclude that the brake shoe toe-in was probably the issue. Howerver the rea issue here is as much about perception and experience than anything else. If you feel there is something not quite right about your bike, especially if you are a dedicated cyclist, then it just doesn't darn well feel right. End of story. We ride. We get it. Hence, although the frame passed inspection, the cyclist got a new replacement frame anyway. And so far, so good.
Checking Alignment (Symmetry)
It is important that both fork ends be equally far from the centerline of the frame, otherwise the bicycle won't track properly. Ideally, the frame alignment should be made using a special fixture that clamps the frame by the sides of the bottom bracket shell, and takes those faces as the reference point for all other measurements. This requires special tooling beyond the reach of the home mechanic or small bike shop.Assuming the bike was built straight to begin with, alignment can be checked by comparison of the rear triangle with the front triangle. Park makes an adjustable gauge for this, the FAG-2, and this is what most bike shops would use.
The usual "home mechanic" technique, however requires nothing more than a piece of string and a ruler. This is a bit slower than using a purpose-built gauge, but gives basically good results, if you are reasonably careful.
The String Method:
- Tie a piece of string to one rear fork end so that the string runs along the outside of the fork end.
- Run the string around the front of the head tube and back to the other rear fork end.
- Pull the string taut, making sure that it's contacting the same part of each fork end.
- Use a ruler to measure the distance from the string to the side of the seat tube where they cross. Take this measurement on both sides, it should be the same on each side.
- If the spacing is as desired, and the string test is correct, you're almost done. If not, you'll need to go back to levering with the lumber to correct any error.
- If you find that you've over-corrected on one side or the other, the same basic approach can be used, except that you lay the bike down so that the stays that are too far out are facing down. Run the lumber under the bottom fork end, and, again, over the seat tube, then press down as before.
