Wheel building theory problem
 

Hi Everyone

I am building my first right now and have a cognitive gap Im hoping you can help close. I have read Jobsts book and browsed some of sheldons writing on wheel building and I understand that the three goals of wheel building are true, roundness, and tension. From the reading I have done, it sounds like Jobst' approach assumes that the wheel be built without the aid of a tensiometer, and so there is an interative process to slowly bring all three factors into line over the course of the build. I undertook my wheelbuilding with the use of a tensiometer, so I was able to tension my spokes equally within the accuracy of the gauge (I am using the Park Tools meter.) Now when I finally had all my spokes at the correct tension, I looked again at the wheel to see that it was neither true nor round. So now I am in the situation where I must sacrifice even tension to improve true and round. So from this perspective it looks like you can not have all three exactly correct. I suppose this has much to do with the initial shape of the rim you are building up. In my case I went a little fancy for my first wheel and chose an Araya 16b Gold tubular rim, that I would expect to have a very good initial shape (though I failed to test this before building the wheels.)

Is there some flaw in my reasoning, and now what would be the next step and what should my end conditions be? Do I sacrifice tension for true? These are track wheels to be ridden without brakes so the braking surface need not be great, but I cant imagine an untrue wheel being a great conclusion for this fancy build.

Thanks
Alex

ps. I'm sure this has been mentioned on this forum before, but the sheer volume of wheelbuilding threads makes it difficult to find it so I am asking again. Thanks for your patience.

Sacrifice tension for true. A reasonable goal is to have any given spoke within 20% of average wheel tension. This means average tension of the built wheel, not target tension. So if your target tension is 100 kgf (80 - 120 = +/- 20%) but 28 of your spokes end up at 115 kgf and 4 spokes are at 85 kgf, your average tension is 111 kgf and the 4 lightly tensioned spokes fall below that.

Park has a spreadsheet to help you out with it if you go that route.

Me, I do it simpler. I build within a 10% +/- variance of my target tension. This means that any given spoke won't be much more than 20% off from any other spoke, worst case scenario and almost certainly will be within 20% of the average tension.

I like to bring my spokes up to around the minimum of this window and start truing and rounding from there, working initially to only increase tension, not detensioning any spokes to start with.

Thanks Trevor, I'll try to hit the 10% margin on tension. I think Im uniformly low at this point too, so it sounds like im in good shape.

One thing you also need to pay attention to is dishing. Here is a great reference for wheelbuilding:

http://www.bbinstitute.com/manual.htm

scroll to the bottom of that page and download:

"Download Chapter 17: Wheel Truing and Repair"

I think you'll find this a much more useful reference for wheelbuilding than JB. Have patience and you'll be very happy. It's not easy at first to balance all of the "goals", so don't be afraid to back up and redo some steps. The more you build, the easier it gets. Good luck!

Thanks for the link cascade. Luckily this wheelset has a dual-fixed flip flop hub in the back so my dishing will have no offset.

When I build wheels, the first thing is make sure all threads are oiled. Then worry about is roundness, get that first, then get it straight side to side and finally work on tension. Each spoke is not going to be at perfect tension, just make sure the average is near the spec you're using.

The last thing I do is stress the wheel. I put the wheel on the ground and put one foot on the rim. Then I balance myself with the end of a table and with the other foot, I GENTLY apply pressure until I can just feel the rim start to give a little. Lots of pings and pops. Go around that side, then flip it over and do the same on the other side. Then I put it back in the truing stand and true it and stress again. Repeat until you can stress the wheel and it doesn't go out of true. I've had wheels that I haven't had to true in years using this method.

Hmm....interesting thread. Now I'm not so sure about spending $60 on a tensiometer. I've been tensioning my wheel builds by feel and never had a wheel fail....they've stayed true for long periods of time with very little tweeking in-between. I'm not so sure a tensiometer is going to do me any good.

Welcome to reality.


Hear, hear.

The oiled threads becomes more and more important with low spoke-count wheels. Those spokes will be at higher tension than high spoke-count wheels. This generates more friction on the threads and winds up the spoke more. Then when you stress-relieve the wheel, they unwind and throw the whole thing out of true. Then you gotta start all over again.. :(

I like to over-turn a nipple, then turn it back a bit to combat the twisting. Is that helpful or am I deluding myself? (Not enough experience to be able to tell either way.)

Hold the spoke between the fingers of your free hand to get a feel for spoke wind-up as you turn the wrench.

The tension ometer is handy for high end fussy low spoke wheels like the Lews Or if you have a cracked rim and the maker claims you tightrened to much.
Goes like this:

Mavic: Is that what you say? It's broken?
Chuck: Yeah. That's it, it's broken.
Mavic: You sure?
Chcuk: I'm positive.
Mavic: Maybe you twisted it to hard..
Chuck: I twisted it just right.
Mavic: How could you be so sure?
Chuck: [sighs] You will look in the manual, you will see that this particular model requires a range of 100 to 110 KgF. I routinely twisted the maximum allowable force.
Mavic: How could you be sure you used 110 KgF?
Chuck: Because I used a Park Tool factory certified tensionmeter. The kind used by Dt certified wheel builders. And The Pro road circuit
Mavic: Well, in that case, how can you be sure THAT's accurate?
Chuck: Because a split second before the tensiometer was applied to the spokes , it had been calibrated by top members of Park, the USCF and the UCI... to be dead on balls accurate! Here's the certificate of validation.
Mavic: Dead on balls accurate?
Chuck: It's an industry term.
Mavic: [tosses paper away] I guess the ****ing thing is broken.

A couple of ideas.

Every spoke affects the tension on every other spoke. Sneaking up on your target tension a little bit at a time will make it much easier to get even tension.

Once I get to the point that you are at, my method is to tighten and loosen opposing pairs of spokes an equal amount to get the wheel round and true.

Thanks for link!

Barnett's is a great reference - especially when you can get some of the juicy parts for free ;-) The wheelbuild chapter is a better guide than anything I have found. Excellent.

Ah , about that dishing... Ok, it may be equidistant with that hub, but you still have to check it and adjust it during any wheel build. It's not going to just come out "equal" by itself (unless you are really lucky).

Nah, it pretty much comes out "equal" by itself.

What happens if you have the dish out by say 5-10mm?

Then you've got a screwed up hub, unbelieveably badly drilled rims, or mismatched spokes.

The simple truth is, when you're building track wheels, equal tension and equal spoke lengths pretty much guarantee that your rim will also be equally centered.

Yeah, you can feel the spoke and get a sense of how much to unwind the nipple, but it's hard to be spot on. They'll always be 5-10 degrees of twist that you'll hear as little ticks and creaks when you ride the bike the first time. I also don't like to turn nipples back and forth too often, it wears out the threads, making the nipples easier to loosen later. What I'll do is make 5-10 passes around the wheel with tightening only to bring tension up to spec as well as truing at the same time. Then the final-pass for the right tension, I'll do the twist and untwist a bit to undo the preload. Then some minor truing and we're done. :)

No, actually I mean what will happen if you built a rear wheel with a cassette and got the dishing wrong for whatever reason. Will the bike handle weird or something? Will wear be faster?

If the dish is off, it might track a little funny. The more noticeable problem is the tire might touch the chain stays.

I would add that getting a good adjustment on the brakes becomes problematic. I'm not an engineer, but I'd venture a guess that brakes are designed for properly dished wheels.

With the dishing off, riding no hands in a straight line will require the bike be leaned over slightly. That's to put the contact-patch directly underneath the center-of-gravity. No problems unless the dish is really off, like 5mm or more. :(