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mechanic question
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i have just finished building up my first bike, and in doing so put myself through bike mechanic school. my question has multiparts.

Gearing: is 39X13 (on a 53/39 front and 12-25 rear) considered a crossover gear. one LBS guy says yes. the reason for this question is this, when i'm in the aforementioned gear my chain rubs the side of the 53 chainring making an irritating noise. at first i thought it was the chain rubbing the front derailleur but after adjusting the F/D i determined it was in fact the chain rubbing the chainring. so my next step was to move the entire rear cassette closer to the wheel with a thinner spacer. after some research it seems that i have the thinnest spacer available. my next thought was that i could take the current spacer to a machine shop and have them shave it down a tiny bit and make it thinner. but, if 39X13 is indeed a crossover gear, i just won't bother. i hope this makes sense.

if it makes any difference i'm riding a 58 soloist team, w/ ultegra.

thanks for any and all input. this is my first post here, been hanging around for quite some time reading...
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Re: mechanic question [goldmund] [ In reply to ]
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If anything you want to move your rear wheel - away - from the BB shell. This will lengthen the chain and decrease the angle that the chain crosses the chain rings at. Another thing to remember - there is no real reason to go big ring to big cog, or little ring to little cog...you will rub somewhere.

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Re: mechanic question [goldmund] [ In reply to ]
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I sense reverse logic here. 'If it is a crossover gear, I won't bother adjusting the alignment.' But whether something is a crossover gear depends on the alignment.
If you sight down the two chainrings and wind up centred on the rear cogset, then it's probably not a crossover gear for you. If it lines up on one of the bigger cogs in the set, then it probably is a crossover gear.
On the other hand, if you're out of line that way, you may want to go big-to-big, if the derailleur can handle it. :-)
Personally, I'd try to line up the middle of the front with the middle of the back, to avoid the derailleur extremes of big-to-big and little-to-little.
But the whole question is one of judgment. There is nothing evil about an extreme crossover, assuming there's no obvious rub, except that components wear more quickly (and/or auto-shifting takes place).
If you opt to try to move your cogset in toward the spokes, make sure it's not too far, or you'll get your chain and/or derailleur in the spokes.
(Come to think of it, maybe some of the chains have engineered limits for angle of deflection. I don't know. I try to minimise the crossover whenever I can anyway...)
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Re: mechanic question [goldmund] [ In reply to ]
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Try a larger outer chainring. I like to use the 11 and 12 and 13 on my bike and with the original 53/39 set up, it was not possible. I dug up an older non-ramped, non-pinned Dura Ace 8 spd outer ring and got a 42t inner. Now I can run in the 42x11 silently.

The noise you are hearing can also be atributed to a "wide" chain. Rohloff, SRAM, and Campy 9speed chains are a tad wider than ShimaNO. What kind of chain do you have now?

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Re: mechanic question [pedaller] [ In reply to ]
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yes, i guess you are right about the reverse logic. but i am trying to solve the alignment problem by moving the rear cassette in toward the spokes, as this is the only way i am aware of to change the alignment. i will look at the current alignment and see how i am lined up.

thanks for posting.
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Re: mechanic question [SuperDave] [ In reply to ]
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i have a shimano chain on right now.
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Re: mechanic question [goldmund] [ In reply to ]
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You shouldn't be running the 39:13. Apart from your chain rubbing your big ring, you are flexing the chain in a plain where it is not strong. Put the chain in the 39:13, and look at the chain from the back. While the chain is build to move from gear to gear, too much riding in a gear selection like small ring to small cog or big ring to big cog will reduce the life of your chain. Once you get down to say your 15 or 17 (on a 12-25), shift to your big ring for added speed. That should keep the chain relatively straight

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Re: mechanic question [goldmund] [ In reply to ]
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I could be way off, but I though crossover gears could be determinted by figuring the gear inches of a particular combination. Chainring/rear cog
39/13 = 3
53/18 = 2.944
53/17 = 3.12
This would show that you could get roughly the same gear inches using the 53/17 or 18, which would be a much more efficient chainline and your drivetrain would thank you.
someone let me know if I'm wrong on this.
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Re: mechanic question [goldmund] [ In reply to ]
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I don't understand why anyone would worry about the small chainring. Triathletes aren't supposed to use them. 55 up front with a 12-21 should work fine for everyone. They are just there to trick the roadies into letting us ride with them. :p

jaretj

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Re: mechanic question [Barry S.] [ In reply to ]
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barry, i think you are on the right track. this was something i left out of my original post, and have since started to educate myself a little more on the topic. i do believe that the calculation of gear inches can determine which gear combinations are considered "crossover gears".

thanks for the post.
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Re: mechanic question [goldmund] [ In reply to ]
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You wrote: "i do believe that the calculation of gear inches can determine which
gear combinations are considered 'crossover gears'."

(Just to clear up a possible misunderstanding:) There is nothing
ineherent in the number of gear-inches to indicate a crossover gear.
The gear-inches help you find a gear on the other chainring, similar
in pedal effort and cadence to the crossover gear that will wear your
equipment. That way you have a straighter chainline (less wear) and a
gear ratio that's still pretty close to what you wanted.
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Re: mechanic question [goldmund] [ In reply to ]
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I will do my best to answer your questions, or at least those that I think I remotely know.

--Whether a gear is a ‘crossover’ gear or not is not that important. Also, there are differing ideas of what the term even means. What is important is that you have chain/chainring rub and you would like to have as many usable gear combinations as possible.

--No surprise that your chain rubs on the big ring when you are in a 39 x 13 gear with your 9 speed set up on a Cervelo. I have a Cervelo P2 and the same thing happens to me with my 8 speed set up. Here is why…

--One cause of your problem is your use of ‘extreme’ gear combinations. Extreme gear combos are the use of the big chainring with the big cogs or the use of the small chainring with the small cogs. In your case, that would be the 39 x 12 and the 53 x 25. When you use those combos and others close to those, the chain has to take a fairly extreme angle from the chainring to a cog that is not directly behind the chainring, but to a cog well off to the right or to the left of the chainring, as the case may be. In other words, the chain doesn’t run straight. This can cause numerous problems, one being that, in the big ring/small cog combos, the chain will rub on the big chainring. Cervelo (and a few other bikes) make this a little worse because they have very short chainstays which make the chain take a more extreme angle in these gear combos than it would on a bike with longer chainstays.

--the chain rub problem doesn’t occur in the big ring/big cog combos because the small chainring is too small to get in the way (it ‘hides’ behind the big ring). But the chain angle is still quite severe and (as in all the extreme gear combos) the drivetrain wear is greater and the shifting less smooth.

--You could in theory solve your chain rub problem by doing just what you say, moving the entire rear cassette closer to the rear wheel, but there are problems with this. First, the chain angle would improve in the small chainring/small cog combos, but would become more extreme in the big chainring/big cog combos. That means even more drivetrain wear. Next, the only way you could get the cassette closer to the rear wheel would be to grind down part of the cassette itself, as there is no spacer between the cassette and the hub (in a normal Shimano set up). You could in theory also solve the chain rub problem by getting front chainrings that are more equal in size, but then you will vastly reduce your usable gearing range. You could also elimmate the small chainring entirely, but if you have any hills or mountains in your training or races, that would be a very poor choice.

--The solution: avoid the use of your extreme gear combos—for you, the 39 x 12 and the 53 x 25. And because you have a bike with short chainstays and 9 cogs, also avoid using your 39 x 13 as well. You lose only a few gear combinations and that solution tends to be the best compromise for a situation many riders are in on their race bikes.

I hope that helps with understanding the whole mess. Good luck.
Last edited by: Greg/ORD: Oct 2, 03 22:37
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Re: mechanic question [Greg/ORD] [ In reply to ]
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[reply]I will do my best to answer your questions, or at least those that I think I remotely know.

--Whether a gear is a ‘crossover’ gear or not is not that important. Also, there are differing ideas of what the term even means. What is important is that you have chain/chainring rub and you would like to have as many usable gear combinations as possible.

-- yes, and i agree.



--One cause of your problem is your use of ‘extreme’ gear combinations. Extreme gear combos are the use of the big chainring with the big cogs or the use of the small chainring with the small cogs. In your case, that would be the 39 x 12 and the 53 x 25. When you use those combos and others close to those, the chain has to take a fairly extreme angle from the chainring to a cog that is not directly behind the chainring, but to a cog well off to the right or to the left of the chainring, as the case may be. In other words, the chain doesn’t run straight. This can cause numerous problems, one being that, in the big ring/small cog combos, the chain will rub on the big chainring. Cervelo (and a few other bikes) make this a little worse because they have very short chainstays which make the chain take a more extreme angle in these gear combos than it would on a bike with longer chainstays.

-- the issue of the shorter chainstay makes sense.


--You could in theory solve your chain rub problem by doing just what you say, moving the entire rear cassette closer to the rear wheel, but there are problems with this. First, the chain angle would improve in the small chainring/small cog combos, but would become more extreme in the big chainring/big cog combos. That means even more drivetrain wear. Next, the only way you could get the cassette closer to the rear wheel would be to grind down part of the cassette itself, as there is no spacer between the cassette and the hub (in a normal Shimano set up). You could in theory also solve the chain rub problem by getting front chainrings that are more equal in size, but then you will vastly reduce your usable gearing range. You could also elimmate the small chainring entirely, but if you have any hills or mountains in your training or races, that would be a very poor choice.

-- in my set up i do have a spacer between the cassette and the hub, which is what my initital proposal was...to grind down that spacer and move the cassette in toward the wheel. i mean, i am talking about a minute amount. 1 mm or less.

--The solution: avoid the use of your extreme gear combos—for you, the 39 x 12 and the 53 x 25. And because you have a bike with short chainstays and 9 cogs, also avoid using your 39 x 13 as well. You lose only a few gear combinations and that solution tends to be the best compromise for a situation many riders are in on their race bikes.

-- this is probably what i will end up doing.

I hope that helps with understanding the whole mess. Good luck.[/reply]

thanks for your post. you cleared up quite a bit for me...thanks again.[i][/i][b][/b]
Last edited by: goldmund: Oct 3, 03 10:14
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Re: mechanic question [goldmund] [ In reply to ]
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now you got me curious....

i never knew shimano was using spacers behind any of their 9-speed cassettes. i wonder how you can do it, because even without a spacer, there is simply no additional space that I can see on most shimano 8/9-speed hubs to put a spacer.

so exactly what kind of cassette, rear hub, rear wheel do you have? who put the wheel together? who put the spacer there? who decided that it was needed? what is it there for?





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Re: mechanic question [Greg/ORD] [ In reply to ]
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the wheel is a mavic ksyrium elite, with mavic hub. the cassette is an ultegra 12-25. i put the cassette on the free hub, i bought the wheel completely built up. when i got the wheel, i took the flimsy black spoke protector off and put on the cassette. when i tightened the cassette down it was still a little loose. so i put on a spacer (probably 2-3mm) and tightened down the cassette and everything was good. then i started riding and i got the chain - chainring rub.
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