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Old 12-21-05, 01:25 PM   #1
jberenyi
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What should I do about a slight wobble on my chainring?

I own a brand new Felt F2C with a Truvativ Carbon crankset (their top of the line setup). I noticed while on the trainer that the large chain ring has a slight wobble that bugs me. So I proceeded to check just how much it was. Using a toothpick taped to the frame I checked the max wobble between it and the chain on the large ring. What I found was a .025 spacing. The smaller chain ring has a .015 spacing.

I called Truvativ and talked with them about it and they said 1mm (.039) max is acceptable but you can remove the crank arm and re-install it with grease and also try a new position on the spindle. I'd really like to get it better but am I being too fussy with this whole issue. Being an aerospace engineer I tend to be anal about tolerances...one of my downfalls I guess. I'd be open for suggestions from all you folks.
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Old 12-21-05, 03:07 PM   #2
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Thats the problem with trainers. You end up looking at your bike instead of the road.

I wouldn't worry about it, especially if a gradual wobble (i.e. the ring is flat, but not mounted perfectly flat) vs. a deformed bump (chain ring bent).

You get far more chain misalignment due to alignment between different rings/cogs than from a wobbly (within spec.) chainring.

Al

Last edited by noisebeam; 12-22-05 at 09:46 AM.
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Old 12-21-05, 04:01 PM   #3
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Being an aerospace engineer I tend to be anal about tolerances...one of my downfalls I guess. I'd be open for suggestions from all you folks.
Being a gunsmith, I'm anal about tolerances... where it counts. A 1mm wobble isn't pretty but it's absolutely harmless mechanically on a bicycle's chainring. It isn't an intake fan blade

Speaking of which, if you really want to see how perfect your bike is (or isn't), check the wheel alignment: 10 to 1 you'll have a surprise. A major European manufacturer told me their tolerance for wheel alignment is +-2cm sideways, even for very high end bikes, which very noticeable if you're looking for it. But of course, as long as you're not, you don't notice it and you end up enjoying and riding the bike instead of eyeballing it suspiciously in the garage and worrying.

Anyway, if you're that bothered by the wobble, you can try rotating the ring on the spider: with any luck, the ring isn't true either, and you'll find a position that'll cancel out the spider's own wobble.
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Old 12-21-05, 04:13 PM   #4
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....their tolerance for wheel alignment is +-2cm sideways...
I assume you mean +- 2 mm. +- Two centimeters would be +- 3/4 inch which would be pretty hard to ignore.
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Old 12-21-05, 04:28 PM   #5
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I assume you mean +- 2 mm. +- Two centimeters would be +- 3/4 inch which would be pretty hard to ignore.
Nope, I meant 2cm. It's indeed pretty hard to ignore if you know it's there, but otherwise it doesn't jump at you. On the worst offenders, if you're careful, watching the rear of the front tire against the top tube as you ride, you can see the front wheel riding a little out of line with the frame. You can also see it by removing both tires, holding a long square section of tubing against both rims (with the help of a friend) so that both wheels are in line, and seeing if the bar is in line with the frame or not. According to the manufacturer I talked to, it's actually quite rare to have perfectly good bikes, and the more badly aligned are found amongst carbon frame bikes because the carbon doesn't always cure evenly or something (I know nothing about it, just repeating what I was told).
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Old 12-22-05, 07:35 AM   #6
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Thanks for the replies everyone. I know I shouldn't rely on my trainer to tell me if everything is okay but lately the weather has been so terrible (snow, rain) I can't even ride the bike. Just for giggles I removed the crankset last night and will be taking it into the shop today where we have some high tech equipment to check alignment and flatness. I will post my findings tonight on what we found with this $340 carbon crankset.
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Old 12-22-05, 09:27 PM   #7
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As promised here is what I found upon inspection of the chainrings and spider arms. The chainrings were checked first by placing them on a granite slab that we use for inspection. The small ring was perfect but the larger ring was about .020 out of flat. This was due to the staking operation at the factory when they install the chain stop. Apparently they did not have enough support under the ring when staking the pin. This was corrected by bending the ring along the centerline of the pin. Next we checked the runout of the spider arms to the titanium shaft. This was done by using a V-block on a magnetic surface grinder slab and then rotated the shaft checking for runout with a .0005 test indicator. Except for one arm everything was within .005. One arm was out .020. This was corrected by hitting with a nylon hammer until it came in to about .002. Everything was re-assembled and checked on the bike. The chainset runs out maybe a couple of thousandths. I'm really excited now and happy with this crankset.
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Old 12-22-05, 09:51 PM   #8
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You should get rid of the bike. It is unsafe and you'll never be happy with it. You should place it out with the trash. BTW what's your address and when's trash day?
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Old 12-23-05, 05:49 AM   #9
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Originally Posted by jberenyi
The chainset runs out maybe a couple of thousandths. I'm really excited now and happy with this crankset.
Don't look at it when it's cold outside, the spider arms might constrict unevenly and throw the rings out of true by another thousandth
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Old 12-23-05, 08:05 AM   #10
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my truvativ crank does the same thing whilst on the trainer. I attribute it more to flex of the BB than the crank itself.. But it sure does look odd when pedaling and staring at the crank.

good luck-
Jim
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Old 12-23-05, 08:48 AM   #11
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Hrmm...I'm an aerospace engineer, and I'm not that anal.

Maybe because I work for the government, and everything's "close enough".
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