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Bicycle Mechanics Broken bottom bracket? Tacoed wheel? If you're having problems with your bicycle, or just need help fixing a flat, drop in here for the latest on bicycle mechanics & bicycle maintenance.

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Old 05-15-08, 12:44 AM   #1
mark9950
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A hump in a rim

What causes a hump in a rim(the kind that is up and down)Is my rim shot?Is there a flat spot in my rim?How if possible can I get the hump out?

As an amateur wheelbuilder I haven't mastered getting humps out of rims yet,if at all possible.This hump is a big one.
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Old 05-15-08, 02:04 AM   #2
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This sounds like a flat spot.
Is it an old rim thats maybe hit a hole in the road.

If so I take the spokes from the place thats flat spotted and fit a car jack, the Diamond shaped one. In between the rim and hub. Then wind the handle so it pushes the flat spot out.
You may need blocks of wood. And remove the disc or cogs from the wheel.

Ive heard theres a proper bike tool that does a simmilar job
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Old 05-15-08, 04:04 AM   #3
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This sounds like a flat spot.
Is it an old rim thats maybe hit a hole in the road.

If so I take the spokes from the place thats flat spotted and fit a car jack, the Diamond shaped one. In between the rim and hub. Then wind the handle so it pushes the flat spot out.
You may need blocks of wood. And remove the disc or cogs from the wheel.

Ive heard theres a proper bike tool that does a simmilar job
This will work? really?
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Old 05-15-08, 05:13 AM   #4
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You true a wheel radially the same way you true it laterally, by adjusting spoke tension. (I would avoid the car jack suggestion).

As usual, Sheldon explains the process well: http://sheldonbrown.com/wheelbuild.html#tensioning

Bob
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Old 05-15-08, 06:33 AM   #5
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You true a wheel radially the same way you true it laterally, by adjusting spoke tension. (I would avoid the car jack suggestion).
Adjusting spoke tension will only get you so far. If the rim by itself is too far out of round/plane there's no way of getting it both resonably true and evenly tensioned at the same time.

The car jack suggestion sounds intriguing. Don't know if it's precise enough, (it'll be a heck of a strain on the spokes closest to the jack) but it is certainly something I'd consider for a flat spot.

Last edited by dabac; 05-15-08 at 06:50 AM.
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Old 05-15-08, 06:34 AM   #6
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You true a wheel radially the same way you true it laterally, by adjusting spoke tension. (I would avoid the car jack suggestion).

As usual, Sheldon explains the process well: http://sheldonbrown.com/wheelbuild.html#tensioning

Bob
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Old 05-15-08, 10:36 AM   #7
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Adjusting spoke tension will only get you so far. If the rim by itself is too far out of round/plane there's no way of getting it both resonably true and evenly tensioned at the same time.

The car jack suggestion sounds intriguing. Don't know if it's precise enough, (it'll be a heck of a strain on the spokes closest to the jack) but it is certainly something I'd consider for a flat spot.
If the rim is that far gone, it's irreparable and ready for the scrap heap.

Bob
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Old 05-15-08, 04:02 PM   #8
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Originally Posted by dabac View Post
Adjusting spoke tension will only get you so far. If the rim by itself is too far out of round/plane there's no way of getting it both resonably true and evenly tensioned at the same time.

The car jack suggestion sounds intriguing. Don't know if it's precise enough, (it'll be a heck of a strain on the spokes closest to the jack) but it is certainly something I'd consider for a flat spot.
As others have said, if the rim is that far out of round, it's scrap. However, adjusting spoke tension is the only way to adjust for trueness and roundness. There is just no other way of doing it.

mark9950: To take out a hump or a dip in a wheel, you have to work the wheel as a whole system. Think of the wheel as being a balloon. Poke it in one place and it will bulge in another. Part of the problem is knowing where to poke and how much.

Start with the definitions. A 'hump' is a high spot in the wheel where the rim moves down towards the truing stand. A dip is a low spot in the wheel where the rim moves up and away from the truing stand. Both are fixed in a similar way...by adjusting tension...but in the opposite direction. A hump is the result of too low a tension on a set of spokes. A dip is the result of too high tension on a set of spokes.

Using a hump as an example, find the middle of the hump. This is only an example, so don't treat each hump and dip the same way. There's a lot of 'feel' that goes into doing this properly Try and find the starting and ending point as well but those are harder to identify. Tighten the middle spoke a half turn, for example (this would be a huge high spot). The spoke on either side, turn about 1/4th of a turn. The spokes on either side of those, turn 1/8th, and so on, until you reach the ends of the hump. Spin the wheel and see if something else develops like another hump or a dip and adjust accordingly. For a dip, you do the same but you loosen the spokes.

This is all easier, as is truing, in a new build when the spoke tension is low. Get all the humps and dips and wobbles out before tensioning and life is a whole lot easier.
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Old 05-16-08, 04:11 AM   #9
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.., adjusting spoke tension is the only way to adjust for trueness and roundness. There is just no other way of doing it.
For final adjustment you're right, but the deformations that can be dealt with by spoke work only are fairly small. A badly tacoed wheel may be brought back to service quite evenly tensioned instead of scrapped if the rim is unlaced and bent back into plane manually before it's relaced and trued traditionally.

A hopeless hump(and badly peaking spoke tension) at the rim joint was dealt with speedily with the aid of a few contour cut wooden blocks and a fairly large mallet.

How much weaker and less durable will they be? I don't know, they haven't broken yet. An even if they would break it's not like I have a reference set available to compare them with.
Would I use them for loaded touring? Preferably not the previously tacoed one, but the other - sure.

It all depends on where you set your standards I suppose.
If I needed 100% of the strength of the wheels, and had to make repairs as rationally and reliably as possible, then I wouldn't bother with unlacing and cold-setting rims (well, maybe the rim joint hump treatment at 1st build).
But then I've never had a wheel fail while JRA, I enjoy the tinkering, and I think it's handy to keep a few beater/utility kind of bikes in running order at minimum cost.
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Old 05-16-08, 09:11 AM   #10
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For final adjustment you're right, but the deformations that can be dealt with by spoke work only are fairly small. A badly tacoed wheel may be brought back to service quite evenly tensioned instead of scrapped if the rim is unlaced and bent back into plane manually before it's relaced and trued traditionally.

A hopeless hump(and badly peaking spoke tension) at the rim joint was dealt with speedily with the aid of a few contour cut wooden blocks and a fairly large mallet.

How much weaker and less durable will they be? I don't know, they haven't broken yet. An even if they would break it's not like I have a reference set available to compare them with.
Would I use them for loaded touring? Preferably not the previously tacoed one, but the other - sure.

It all depends on where you set your standards I suppose.
If I needed 100% of the strength of the wheels, and had to make repairs as rationally and reliably as possible, then I wouldn't bother with unlacing and cold-setting rims (well, maybe the rim joint hump treatment at 1st build).
But then I've never had a wheel fail while JRA, I enjoy the tinkering, and I think it's handy to keep a few beater/utility kind of bikes in running order at minimum cost.
A badly tacoed wheel is a severely compromised wheel. Aluminum doesn't like to be stressed as severely as that and I wouldn't consider rebuilding a wheel with that kind of rim. No properly built wheel should every taco except in the a crash and then it should be replaced as soon as possible.

Nor would I consider using any kind of hammer on a rim except in an extreme emergency...like in the middle of nowhere with no other alternative. And then I'd dispose of the rim when I returned to civilization.

Saving a few pennies by returning bent wheels to service just isn't worth the cost of the dental bill.
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Old 05-16-08, 10:36 AM   #11
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A badly tacoed wheel is a severely compromised wheel. Aluminum doesn't like to be stressed as severely as that and I wouldn't consider rebuilding a wheel with that kind of rim. No properly built wheel should every taco except in the a crash and then it should be replaced as soon as possible.

Nor would I consider using any kind of hammer on a rim except in an extreme emergency...like in the middle of nowhere with no other alternative. And then I'd dispose of the rim when I returned to civilization.

Saving a few pennies by returning bent wheels to service just isn't worth the cost of the dental bill
.
+1. You DON'T cold-set aluminum.

Bob
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Old 05-16-08, 12:26 PM   #12
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These suggestions of hammering, cold setting and using a jack are the "Duct Tape" solutions of wheel work. Just because you have made them work doesn't necessarily mean they are practices to be recommended. Aluminum is NOT forgiving when it has been stressed and forced into a workable configuration. It will prematurely fail most likely. Since wheels are your contact with the road and a major safety factor, you are just begging for Murphy to kick you in the head. I will parrot the sentiment of others here. A wheel that is drastically bent radially or laterally IS scrap from a safety standpoint and should not be rebuilt. The few dollars that you save should at least be sent to the emergency room as a down payment on your upcoming stupid tax. YMMV.
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Old 05-16-08, 10:47 PM   #13
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+1. You DON'T cold-set aluminum.
Unless you're setting a boobie-trap out for a bike thief

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Old 05-16-08, 11:09 PM   #14
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It seems to be a flat spot,and the hump is what isnt a flat spot.

This is what the streets of south chicago and nortwest indiana does to bicycles after about 2500 miles.

If it done this to my rim,I could imagine what it would do to one of those expensive racing bikes.Or could those racing bikes with those thin tire hold up better?My rims are aluminum and use those 2.125 fat tires(absorb the shock?).
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