Join Date: May 2007
Bikes: KUUPAS, Simpson VR
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A larger clamp dia means a larger dia seat tube which resultingly places the der further away from the centerline of the bike (which is the point from which chainline is measured). This moves the arc of the FR der further away from this point as well. That means that the spindle has to place the crankset at a point in space that is the optimal position relative to the travel of the FR der. So...clamp dia matters when considering the performance of the FR der. (NOTE:This is also the method Campagnolo uses in their design specs given to frame factories. These specs ARE DIFFERENT than Shimano. So, using a Shimano clamp and a Campagnolo FR Der can, in some instances, place the der in a position that significantly degrades shifting performance.)
To achieve optimal chainline, the relation of the crankset to the cassette then comes into play. Campy has a 43mm chainline (FSA uses 45 which is why the shifting on FSA cranks in a Campy set up always sucks). Frame factories know this. So when they build a bike that they know will have Campy on it, they adjust the frame build accordingly.
In the aftermarket, when consumers start swapping parts and Frankenbiking (I would like to officially introduce this word as a verb into contemporary lexicology) they encounter shifting wierdness and have to shim BBs, cut up beer cans and make sacrifices to the shifting Gods because what they are doing is running counter to what was engineered in the first place.
When I was a shop manager I approached these kinds of issues in the same way and from the same perspective. After working with frame factories and dealing with these kinds of issues every day I now see them from a different angle.
You question is entirely correct when asked from the position of a consumer or someone in the aftermarket who is having to adapt to the situation as it is given. From that perspective I would probably approach it in the same way. I guess, from my perspective, I approach these kinds of questions knowing how these things were engineered and what considerations were given at the time.
Ultimately, we're both going to arrive at pretty much the same conclusions but we're going to approach it from a slightly different angle. Given a bike, a BB, a couple of wrenches and enough Doritos to choke a horse, we'd both end up encountering the same problem in the end.
Relative to my previous post more important issues remain regarding compatibility of components and the outlay of ducets. One solution sidesteps the chainline issue entirely. I strongly recommend looking at a compact crank. If the desired gear inches required can be thus achieved, I feel this would be the most prudent course of action.
That's about enough BS for anyone. I'm going to bed.
Last edited by Bob Dopolina; 06-29-07 at 09:38 AM.