Radial Truing/Roundness

I have some old wheels on an old bike that I'm constantly having to true. I'm new to truing and have just about gotten comfortable with lateral adjustments, and it's not a big deal to correct some side-to-side wobble anymore.

Anyway, when I was inspecting my front wheel today, I noticed a "low spot." It's quite minor but it's causing the brakes to rub a little bit there. All the stuff on vertical truing I've read deals with "high spots." That seems pretty straightforward, you tighten the spokes and it pulls the high spot back down into round. However with this low spot I don't know what to do. I read one thing that just said loosen the 2 spokes at the low spot but they're already looser than the surrounding spokes and doing that doesn't help.

I tried tightening the spokes at right angles to the spoke that leads to the low spot, and that helped very slightly. Well I've been playing with the thing for an hour and I've got the low spot from maybe 1.2-1.5 mm out of round to say 0.7-0.9mm

Anybody got any tips at this point? Replacing the wheel or even going to an LBS at all is not an option right now.

 

Well, if you insist...

Cut three wooden blocks, two to fit well against outer face of the rim and one to fit well against inner face of rim.
Unlace section of rim where the dip is.
put rim vertically on sturdy work bench near the edge.
Support rim outside the low spot on two of the blocks you've just cut. Put 3rd block as a saddle over the low spot.
Get a big c-clamp, set it to squeeze against the saddle shaped block.
measure height from work bench to rim.
Crank down on C-clamp, past your target measure.
release clamp tension, check measurement. Repeat if needed.

Put spokes back. True and tension.

 

Aluminum rims, that stay true with spokes under tension, can sometimes take a wack and then be deformed there. Even tension there is the key, the 'low spot' cannot be fixed. Bending that spot out weakens the rim to the point of possible failure. Replace the rim or just live with it.

 

they have an old tool for trying to 'pull out' a low spot, it has a round brace with a hook that pulls the rim into the brace using a screw mechanism... every time I have seen it used the results arent great, often the inside of the rim will dent before the low spot will move outward... old steel rims have a higher success rate with this this operatoin than lightweight aluminum ones.

perhaps just adjusting the brake pads a little will allow you to continue to use the wheel as it is...

if you're really desperate to try something or as a last ditch effort there is one other method I have seen done, leaving the 4? spokes around the low spot untensioned increase the tension of all the other spokes in the wheel gradually in an effort to pop that low section out... this required over tightening all the other spokes to a great degree with much lubrication, trepidation, and anticipation of utter failure. but if the rim at the low spot yeilds at all you may then be able to de-tension everything and build it back up with some improvement. this really is like rebuilding the wheel and takes quite a bit of time and maybe more experience than what you have tried in the past...

good luck

 

Thanks for replies guys. I just found out about this community bike shop that will let you pay $6 to use all their tools for an hour, so I'm gonna take my wheel there tonight and try some of these techniques on an actual truing stand. I managed to secure a new wheel from craigslist for $15, so if I can't fix this flat spot I've got a plan B.