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Discussion Starter · #1 · (Edited)
G'day fellow bikers,

Well, as you can probably guess by the Subject, I want to know what the chain line should be for my bike and also what length of BB I should be using.

I started this discussion with Spécialitiés TA of France back in August.

I was recommended a 122 mm BB by them for their Carmina Crank arms using a Triple chain ring spider.

I received a reply 5 days ago from the sales person at Spécialitiés, which said in reference to a response she received from their engineer "He told me that for a mountain bike with a 50mm chainline the length for the axle is 122mm and he doesn't understand why you would need a 119mm axle."

However, you can see from the first 2 photos the difference in spacing between the arms and the chain stays.

Pipe Cylinder Material property Still life photography

Concrete Household hardware


At the time I started this discussion with Spécialitiés I needed to replace the BB on my old rigid bike and I thought they were on holidays. To confirm that a 122 mm Bottom Bracket was correct for my old rigid mountain bike, I contacted Peter White (http://www.peterwhitecycles.com/index.html, from Hillsborough, New Hampshire, who is a representative for Spécialitiés in the USA. Peter informed me that I should be using a 119 mm Bottom Bracket with the Carmina Crank arms and Triple Chain-rings which prompted me to ask him about the setup I have for the AMP and which, I think, probably explains why I have been having problems with the shifting with the chain sometimes falling off the smallest chain-ring at the front and also coming off the biggest chain-ring at the front as the chain-line is incorrect. If I remember correctly, Peter said that even though one of my bikes has a 73 mm BB and the other a 68 mm BB the width of the BB should be 119 for a Triple chain-ring configuration of the Carmina Cranks.

http://www.peterwhitecycles.com/carmina.asp

However, based on the above page, he could have said what he said if I happened to have told him that I thought the seat tube diameter is 1.25". If the seat tube diameter is 1 3/8" then maybe Spécialitiés is correct (I need some calipers). Why the difference in spacing, though, as you can see in the above photos?

I have the second version of the Carmina Cranks. The reason I chose the Carmina Cranks was because it was the cheapest solution I could find that would allow me to buy the left and right cranks separately in different lengths. My left leg is shorter than the right.

2 days ago I took the bike out for its first test ride since I completed the maintenance of the suspension.

During part of the ride, I had the chain on the Middle chainring and the largest cog. The rear derailleur has the L screw set to the maximum so as to prevent the chain from falling between the largest cog and spokes. However, the chain was wanting to fall down onto the 2nd largest cog sufficiently enough to be a bother; probably dependent on the movement of the suspension and how much force I was putting on the pedals. The following 3 photos I have attached I hope give a good indication of the difference in vertical plane between the Middle chainring and the largest cog.

Product Metal Pipe Office equipment Steel

Bicycle wheel rim Bicycle accessory Bicycle part Bicycle drivetrain part Spoke

Iron Bicycle accessory Bicycle Metal Bicycle tire


I believe I should be able to use the above combination of gears without this "skipping" problem. The only problem I can recall having with my previous Crank, BB and gearing setup was some ghosting in the higher gears due to suspension movement. Also, with the chain on the Middle chainring and the largest cog, if you rotate the cranks backwards the chain will fall off the Middle chainring onto the Inner chainring.

I was previously using low profile cranks from Profile Racing with a 107 mm BB, 24/36/48 chainrings, a 12-32 8 speed cogset and a shimano chain.

Before I got the Carmina Cranks, I installed new Race Face chainrings (24/36/48), an 11-34 9 speed cogset and a KMC X9 SL Chain. With this setup I did not expereince the "skipping" mentioned above.

With the Carmina Cranks I have a 122 mm BB from Spécialitiés, the Race Face chainrings, the 11-34 9 speed cogset and the KMC X9 SL Chain.

If my chainline should be 48 mm, instead of 50 mm as suggested by Spécialitiés, then would changing to a 119 mm BB bring the Middle chainring and largest cog more into line and prevent the "skipping" problem?

My contact at AMP Research cannot recall a chainline being specified for their bikes. This was in the 2nd half of the 90s. He says:

"I know we used 68 x 110 with Shimano compact crank sets (LX / XT) and we used 68 x 112.5 with XTR (which are also width adjustable)."

"I remember working in the shop (granted this was 12 years ago) and I would start with what I thought was right and adjust the length from there to get as close to the frame as I could to minimize chain suck (so yes I would have multiple BB lying there while I did it), and in theory get the best chain line I could, but I never worked off set measurements or any set guidelines, so it seems you may know more than I do."

Looking forward to all sensible feedback.

Thank you, Michael
Hospedaje Los Jardines & Sacred Valley Mountain Bike Tours
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Okay first....

the difference in distance from chain stay to crank arm end can be explained by one crank arm being shorter than the other. Spindles are symetrical so the same amount of spindle is portruding from both sides of the BB. Provided of course that you have the correct width bb installed. Anyway, the longer crank arm will appear closer to the stay on that side and the shorter arm will appear further away. This is due to the chain stays tappering toward the BB housing. The stays go from a 135mm spacing at the axle in the rear to a 73 or 68mm housing at the BB. So the longer crank arm will always appear to be closer (it actually is) to the stay because the end of the crank arm falls further back toward the rear, i.e. further back along the tapper.

As for chainline. Correct chain line depends totally on the age of the bike frame. Older MTBs up to about 03 or 04 (depending on the manufacturer) were designed with an intended chainline of 47.5mm. It is rare that you find a bike older than 05 designed with a 50mm chainline. And that additional 2.5mm of chainline can certainly cause the problems that you are experiencing. What I would suggest is that you measure your current chainline and go from there. To measure chainline you measure from the center of the seat tube to the inside edge of the middle chain ring. Seat tube diameter is imaterial. Finding your current chainline measurement is essential to the next step. If indeed you do find that your chain line is 50mm with your current set up, then I'd suggest that most likely your two far out for these older bikes, and would likely need something in the 47.5 to 48mm range to straighten things out. What you'll need to do is move the crank between 2 and 2.5mm inboard. That's easily accomplished with a little math. If you are running a 122mm spindle and getting 50mm, you'll need to shorten that 122mm spindle 5mm to 117mm to get a 47.5mm chainline or 118mm to get a 48mm chainline. Keep in mind that bb spindles are symetrical so you have shorten the spindle by twice the amount to achieve what you want to do on one side. So if you want to bring the crank inboard 1mm you'd have to have a 2mm shorter spindle as the spindle is centered in the bb and housing (or should be) and a 1mm shorter spindle would only move the cranks .5 mm per side.

The way to visually check your chain line is to shift the front onto the middle chain ring, and the rear to the middle cog on the cassette. Then stand at the rear of the bike and observe the chain. An ideal chainline will give you a straight chain bettween the two. Don't use the frame or the chain stay for reference, focus on the chain itself and follow it the rear to the front. It is vears to the right as you get closer to the crank then you crank is too far out. The ideal is a nice straight line from the middle cog to the middle chain ring.

I got that kind of out of order though. What I would suggest is, first visually check the chainline as described above. If you have a noticable swerve to the outside when it the middle/middle gear combo, you likely have a problem. If it isn't that noticeable, then measure. You don't necessarily need a caliper. A good machinests rule will get you close enough. If you are at more than 47.5 or 48mm, then this is likely your problem. The symptoms you describe certainly indicate that your chainline is too far out. From there reduce your chain line through a shorter BB spindle to get it as close to 47.5 or 48mm as you can. Exact matches aren't always possible. It depends on the bb spindle length you are using, and the available sizes that the bb manufacturer offers.

So, observe, measure, calculate, and then get the right spindle length. From there it's a matter of try it and see. Just keep an eye on your crank arms. There were some older frames and designs out there that were run with wider chainlines due to crank arm/frame clearance concerns. For those you just had to suffer with it. But if both crank arms clear the stays and it solves your shifting issues, your good to go.

Oh and as a side note: Make sure you stick with the same bb manufacturer if you decide to change spindle lengths. There are combinations of bbs and cranks that, even though you use the same spindle lengths, they will render different chainlines. I.e., using a Shimano 122mm bb with a Sugino crank won't necessarily render the same chainline as it would with say an LX crank. It's usually a function of the crank set, but it can happen when switching bb brands as well. Not that common, but possible. So stick with the bb brand that you are using now. It keeps things simpler and ensures the results you are looking for.

Good Dirt
 

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Discussion Starter · #3 ·
Hi Squash,

Thanks for your detailed reply!

The Right crank is the longer (180 mm) of the 2. The Left is 177.5 mm. So, if the Right one was also 177.5 mm then the gap on the right hand side would be just that bit greater.

Are spindles of Odd numbered length also symmetrical?

Looking from back to front, I can see the chain is pointing towards the right when the chain is on the Middle chain-ring and Middle cog.

I took some measurements and the middle of the middle cog is 22.5 mm inside the dropout which makes it 45 mm from the centre of the bike. The inside edge of the Middle chain-ring is 50 mm from the centre as suggested by the data from Spécialitiés and if you measure to the middle of this chain-ring the measurement is 52 mm which puts the Middle chain-ring and Middle cog out of alignment by 7 mm.

My bike was one of the first AMP B4 frames produced when I bought it in July 1995.

The Cranks came with 6.2 mm spacers for the Inner chain-ring but because of the problems I had with the chain dropping off the Inner chain-ring I added another spacer of 1 mm which helped a little. The gap I currently have between the Inner chain-ring and the chain-stay bracket pivot area is 2 mm. Under 122 mm, Spécialitiés has Bottom Brackets of 119 mm and 116 mm. Given this data it would appear that the BB of 119 mm would be the only option from Spécialitiés because the 116 mm BB would place the Inner chain-ring against the chain-stay bracket pivot area if I removed the 1 mm spacer. It would appear that a 118 mm BB would be the best option in case 0.5mm of clearance is not sufficient for the Inner chain-ring with a 117 mm BB.

Hasta pronto, Michael
 

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Squash said:
...As for chainline. Correct chain line depends totally on the age of the bike frame. Older MTBs up to about 03 or 04 (depending on the manufacturer) were designed with an intended chainline of 47.5mm. It is rare that you find a bike older than 05 designed with a 50mm chainline. And that additional 2.5mm of chainline can certainly cause the problems that you are experiencing...
The frame design should not make a bit of difference in the shifting quality. The cassette and hub specs did not change when the crankset chainlines moved outboard. The difference in the chainline allowed for more rear tire and stay/chainring clearance and nothing more.
The newer frames and 50mm chainline still increase the chain angle to the larger cogs as the cassette has not moved.
 

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"Are spindles of Odd numbered length also symmetrical?"

Yes they are. It'll just be a fraction on each side, i.e. a 118mm spindle would have would have 59mm from center to the end of each spindle, a 119 would have 59.5mm center to end.

"The Cranks came with 6.2 mm spacers for the Inner chain-ring but because of the problems I had with the chain dropping off the Inner chain-ring I added another spacer of 1 mm which helped a little. The gap I currently have between the Inner chain-ring and the chain-stay bracket pivot area is 2 mm. Under 122 mm, Spécialitiés has Bottom Brackets of 119 mm and 116 mm. Given this data it would appear that the BB of 119 mm would be the only option from Spécialitiés because the 116 mm BB would place the Inner chain-ring against the chain-stay bracket pivot area if I removed the 1 mm spacer. It would appear that a 118 mm BB would be the best option in case 0.5mm of clearance is not sufficient for the Inner chain-ring with a 117 mm BB."

What I would do in this case is drop the 1mm spacers and then go to a 119 or a 118mm bb. Your choice on that one. You have the bike right there and should be able to determine what will work better. Things to keep in mind, as you've noted, are inner chainring clerance and crankarm/chainstay clearance. As it stands right now your crank chain line is too far out. That's the biggest cause of your problems currently from what you've described.

Good Dirt
 

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Discussion Starter · #6 ·
If the Spindles are symmetrical then why is the gap, between the chain stay and crank arm, on the right side greater than on the left side? I find that to be strange!

It appears that Tange Seiki may make a Titanium Bottom Bracket in 118 mm as can be seen at the following page (http://www.tangeseiki.com/tangeseiki_news/bottom-bracket.php?page=1&class=4a&pic=200902051145430.png). I sent an email to Quality Bicycle Products yesterday but haven't received a reply. Does your shop get parts through QBP?

Thanks, Michael
 

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mtbvfr said:
If the Spindles are symmetrical then why is the gap, between the chain stay and crank arm, on the right side greater than on the left side? I find that to be strange!
Make sure you have the right BB shell width. For example if your frame has a 73mm BB shell and the BB is a 68mm the drive side will remain correct, but the non-drive will end up 5mm short.
 
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