brians647

In my never-ending quest to learn about bikes, I've decided to build my own wheels.

I built up a rear PowerTap, no problem. However, the front build is not going so well. After building it up to within .5mm of lateral true, and even less then .5mm of vertical runout/roundness, I get a high frequency shimmy when braking hard. I did not have this problem with the wheels that were on there before.

My rear wheel, using the same brand rim, has no such problems. Any ideas as to what I could be missing?

The front is a Kinlin XR 300 rim, laced radially to a Dura Ace hub - 28 hole.

Thanks in advance for any ideas!

AndrewP

Was the previous front wheel radially laced? Also check for evenness of spoke tension around the wheel.

brians647

Hey Andrew. Yep, the previous wheel was radially laced as well. However, that's a damn good catch on the spoke tension issue.

When I originally built them up, the tensions were pushing a 20% divergence. Since the rear spokes were within 7-8% tension, I knew I had a problem. I backed out all the spokes, and re tightened the whole wheel. I think my tensions are at 15% or so, so not great, but within (from what I've read) tolerances.

Why would a tension imbalance cause that problem under braking? I can't make it all add up.

AndrewP

Differences in spoke tension could result in different rate of transfer of braking force from the rim to the bike, setting up a vibration. No real knowledge on the subject - just guessing. Also try cleaning the rim braking surface.

brians647

Yeah, I tried cleaning the rim surface, and that didn't solve it. I've also received the suggestion of toeing in the brake pads - haven't done that yet. I mean, they weren't toed before, why should I have to now? Maybe I'll change it anyway.

The problem with the spoke tension is that (as best I can tell) if I bring them into line, the rim goes out of whack, and vice versa. I'm not sure how to solve that problem.

Thanks for the input however, much appreciated!!!

waterrockets 

That's a tough one. Sometimes, too much tension can cause the rim to "draw in" a little bit near the spokes. Maybe cover the braking surfaces with a solid layer of chalk, then go get it to shimmy and see if there are any untouched portions of the braking surface.

What was your final tension range? (lowest tension spoke to highest tension spoke). You might experiment with taking it to even tension all the way around and seeing if the braking problem subsides. Sometimes it's better to give up some true for level tension -- I always favor the tension myself.

Lastly, you might file or block-sand the pads a bit to clean them up too.

brians647

Thanks for weighing in WR. Yeah, I've read some of your old posts where you say that you favor tension over truing. With that in mind, I backed down the higher ones and cranked up the lower ones to narrow the spread. Now, I think my wheel is too far out of true for comfort as well.

I'll try backing down all the tensions a bit and see if that "pinching in" isn't part of the culprit.

Thanks much!!

waterrockets 

Before you do that, try the braking with even tension!!!! These will be valuable data for diagnosing your shimmy.

brians647

Okay, I'll leave it as is and see how it feels. I'll try and post back later with results.

Thanks!!!

waterrockets 

You know, a radar chart of your tensions could be really helpful for you to see what's actually going on in the wheel. Fill this out with your even tension, then again after you bring it closer to true. You're likely to see stuff you're not visualizing yet: https://www.parktool.com/repair/howtos/TCC_version10.xls

brians647

Awesome, WR. I saw that chart for the PT build that you did and I thought it was a pretty neat little tool - but without a degree from MIT how was I gonna come up with that?

I'll give it a shot and see what it reveals.

AndrewP

To even tension, loosen the tightest one and increase tension in the adjacent spokes to the same side of the hub. Tighten the loosest spoke and tighten the adjacent ones to the same side of the hub. This should maintain the lateral trueness. If you are having steering shimmy, rather than a simple fore/aft vibration, your wheel may be dished which would turn the braking force into a steering torque.

douglas fir

Hey Guys, I bought one of morningstartools bike mounted dial indicators to ease rotor truing and have found that I was able to true the wheels with it also ! The neat thing is, as the instructions said, that a stronger wheel is built when every spoke's influence is seen- I rarely need to true them now and riding harder !

brians647

Hey Andrew, WR, et al...

Haven't been able to post an update until now due to a shortage of time to follow your advice.

Andrew, I actually backed out the tension before reading your above post; and, of course, your method makes sense and keeps things simple.

WR, I'm not sure how to post the tensions as you did in that PT build from the Park Tool sight, but my compilation shows how one side is higher almost the entire way around than the other, and the lower tension side is much more even. I wish I could post it so I could get some feedback.

I'll back the tension down and keep checking for dish to make sure that doesn't go away in the process. Thanks for your help!

waterrockets 

I did a screenshot, clipped the charts, saved with MSPaint, and posted to www.imageshack.us

DasProfezzional

I would be inclined to say that the problem is not in the wheel.

As for the truing aspects, I've found that tensionmeters can complicate things unnecessarily. If you've already got one and plan on using it, they can be helpful, just don't go thinking that your problems will disappear if every spoke's the same.

brians647

Well, you may be right, but I've been riding this past week with my Dura Ace wheel, and the problem is no longer noticeable.

I think that the truth lies somewhere between what you and WR are implying. Keep the tensions in a certain range, but don't live by those numbers alone. That I sort of get.

However, with that tension chart, it becomes very obvious that I'm chasing a high spot at the valve hole. I thought most times higher tensions were seen on the opposite side by the seam. Regardless, to get the wheel round I have to go berzerk on that tension reading, and then do my best to bring the rest up to par. The higher the tension, the better the roundness of the whole wheel gets.

Also, the right side tensions seem to be lower and more consistent than the left. How can that be?

Sorry if I'm asking too many unanswerable questions, I'm just hoping that the issue is with my building skills and not the rim. My skills I can improve a lot. The rim - not so much.

operator

Like any tool it's up to the person wielding it to have the know-how to use it correctly.

waterrockets 

It sounds to me like the problem is the rim unless the hub axle ends aren't spaced evenly. The question is, how big of a problem is the rim? What's the left-right tension difference?

DasProfezzional

Easy, killer. Know-how also means knowing when you're using a tool too much.