frame alignment - DIY or LBS?
 

My '76 fuji track frame failed the "string" test by ~5mm - gap is 33mm right, and 28mm left. The bike rides okay, but when I look down at the front tire when tracking straight ahead, the back half of the tire isn't completely hidden by the down tube. I can see that I have to steer slightly (5mm?) left to keep the bike straight. Is this something I can mostly fix ala sheldon brown's "2x4" frame alignment method? Or should I give in and pay $$$ to the LBS? My front fork is straight and both wheels are dished correctly...

Cold-setting a rear w/ a 2x4 isn't that difficult. You didn't mention if the dropout spacing itself, but assuming it's the original (not spread), then you'll simply have to be patient and work one side at a time. I'd move the off side 1mm or so, then work on moving the other side to follow it. 5mm isn't that much, it should come with a little patience and thoughtful application of force.

thanks, I just re-read sheldon's method - it doesn't look too difficult but I will have to be careful. Rear spacing is 120mm. My plan is to move each side ~2mm and check spacing.

I went out and bought the Park tools for this and although they are expensive, I've cold set and realigned enough frames that I think it was worth it. You can certainly make your own without too much trouble, though.

Before you get all bendy on the frame, do the math.

1. Determine how far off you are from the correct spacing
2. Determine how far to move each side
3. Cold set it

1 is pretty easy. If the spacing should be 120 and it's 123 you are off by 3 mm.
2 Requires string or a gauge. If the dropouts centered 5 mm to the left of center then you need to move the left side in 4mm and the right side out 1mm.

So rather than just going from 123 to 120 and then trying to center it, which will usually involve more bending, just figure it out in advance and bend it once.

Hex wrenches make great measuring devices.

I strap a 2x4 to the rear triangle as a pivot to keep from putting too much stress on the brake and chainstay cross pieces.

And when you are done, don't forget to realign the dropouts!

okay... I went out and measured the spacing, the rear fork ends are 115mm apart. My rear hub spacing is 120mm. Using a string, the left rear side measures 28mm left of center (@seat tube), the right side is 33mm. So, if I increase (cold set) the left side 5mm, I will have ~120 spacing, and a ~33mm string gap both left and right of center, correct?

Yes. I would not hesitate to attempt this myself.

Don't forget that a 5mm difference in string measurement actually represents only 2.5mm misalignment!

But I need to cold set only the left side, the full 5mm outward, correct? Then, my rear spacing will increase from 115 to 120, and the left string gap will increase from 128 to 133, matching the right string gap at 133. Please stop me if that isn't right!

I did it, using a long peice of lumber - it's not that bad. Now my rear spacing is 120, and I got ~32-33 on each side using the string. I didn't re-align the dropouts, I figured the clamping to the hub would "align" them. I test rode it slowly and it tracks much straighter. Probably not perfect but it's better than it was. Tomorrow I will be able to tell more/better on my commute to work and back.

Clamping to the hub does align the dropouts, but it puts bending pressure on the axle. If the alignment is very bad, it can cause the axle to bend or even break. Check your dropout alignment.

The tire not being hidden by the downtube could indicate your fork is slightly bent as well.

I had the front fork alignment tweaked and trued w/ an alignment tool at my lbs several months ago. I will have the dropouts checked though, easy enough.

How do you align the dropouts? I used the article on Sheldon's site and it just says "there is no good home tool." This could spin into a new thread as I have several rear triangle questions and pictures.

I had a geared road bike dropouts aligned at my lbs, they used a simple tool and bent them straight pretty quick. Not sure if it's the same with track dropouts but I will find out soon.

well after this mornings commute I could tell it's still ~slightly~ off but way better than it was. I don't plan on tweaking it further though, it is barely noticable. It seems that njs rear spacing is 110mm, possibly the reason my bike was spaced ~115mm, after being ridden for years w/ a 120mm spaced hub?

http://www.parktool.com/repair/readhowto.asp?id=40

There are some many variables that come into play, wheel dish, fork alignent both lateraly and front to back as well as single single leg invlovement, fork end alignment with good blade alignment, rear triangle, rear dropout and it s possible to have the entire front triangle twisted.

IIRC modern pro track riders have bikes that naturaly track to the left to aid in turning.

Hows your wheel dish?

My bike tracks to the right, must be a rare, "reverse" track model! Wheel dish is very close, I built them and used a campy dishing tool front and rear. I checked the string thing again, it's about 2mm off now. So many variables as you said - I'm going to just ride it now.

Good tip. Better than my crude approach:D.


Can you explain this, or maybe post a pic? Do you mean using the end of the wrench for small distance measurements?

Slide a 3, 4, 5mm or whatever size allen key between the string and frame. Its and easy way to measure 1mm increments.

I wasn't skilled enough to bend once, I spread the frame (left side mostly) in 3 or 4 small increments - I was afraid of overbending so I worked up to my final result without going over.