Gear / Crank Set up

Evenin' all,

Tonight i am trying to get my head around the relationships between the various items involved in the cranks and gears.

I understand that the centre of the gap between the two front chainrings should line up with the centre of the cassette. What is the margin of error with this measurement?

How do i decide on the length of the bottom bracket axle?

An example - am i right in saying that if i buy a Shimano 600ax BB, crankset, cassette/freewheel, front and rear mechs and fit them to any bike there should be no problems with its' operations?

If i want to use a mixture of parts eg. Shimano cranks, Campag BB, Suntour cassette/freewheel what would the outcome be?

Is the length of the BB axle governed by the crankset or the frame?


I am not sure if i have explained this well, but i would appreciate any insight into this topic.

Thanks all

Yup sure fire way to get everything working sweet is by having a matching groupset.

The chainset dictates the length of the bb generally speaking, there are exceptions of course.

A shimano crank would fit too far onto a Campag bb and potentially rub. This would make front mech adustment awkward and give a severe chain angle when on the smallest sprocket (you may get away with it but it may grind on the next cog up) , you could probably compensate by using a longer campag bb but what is the point? Your Suntour freewheel would not index with your shimano shifters although if you was using friction it would not be a problem.

Hypothetically speaking of course.

There are compatible parts that will run in harmony with others, look online and you'll find the answer, one example is Campagnolo 8 speed Ergo levers are supposed to work very well with 7 speed shimano gears. Never tried it but the logic on paper works and I'm sure people are happy to run that set up.

Generally speaking if you do not stick to the same group then sticking to the same manufacturer for your parts will aid in compatibility.

Tel,

Thanks again - you know too much!

So the length of the BB is dictated by the crankset?

I should say that my example of the various bits was only an example!

yep, the BB length depends on the chainset. Each make/model/variation of chainset will require a different length BB in order to get the same (or very nearly the same) chainline for road or MTB. This can depend on the age of the groupset and the rear frame spacing though - an old frame with 120mm dropouts and 5 or 6 speed will probably require a slightly different chainline than a modern 130mm frame with 9 or 10 speed. There's usually a few mm margin either way though, so unless you're way off, everything should work OK. Chainstay clearance is another thing to watch out for - not all frames have the same design of chainstays, and depending on any curve, indentation, length of chainstays, etc. you might need a different BB.

The easiest way is just to put everything together with parts you have and see what happens. Then you can decide if you need a different BB. if you're missing some parts, then it's more complicated but the best place to start would be with the correct BB length (or very close to it) for the chainset you have.

Foz,

Thanks for that, so starting with BB is the secret. Is Velobase an accurate source of data or should i be searching a manufacturers website for this info?

Curiously, Tel h pointed out some misnamed parts on it recently.


Thanks

yep, velobase is a pretty good starting point, but asking on here and googling should find you the answers too. unless it's a very strange or rare chainset, the answer should be easy to find.

If you are wanting to index everything, mixing and matching is tricky - I was running 8spd Shimano dt shifters with a 7spd Campagnolo cassette and it indexed perfectly (Campagnolo and Shimano rear mechs). but moving to Campagnolo Syncro shifters, it all went terribly wrong...

Front mechs rarely index well unless they are perfectly matched within a groupset (different cable pull/ reach between models and STi) but if using 'multi click' STI like early Campagnolo Ergo, you can use almost anything.

Friction is a cure all as you can run pretty much anything you can attach to a frame, all you have to do is match cassette to chain (9 with 9 etc).

Chainsets and BBs can be mixed and sometimes it works better for chainlines but as BBs tend to be fairly cheap and just about all sizes available, this isnt usually trouble unless your budget is really tight.

I replaced a Shimano 600 EX crankset with a 7402 Dura Ace and needed a shorter BB axle width as the 600 sat further inboard, as Tel says, just suck and see bearing in mind that a matching group is preferable in most cases. it's not difficult to source used parts that are a better match and then sell off the non matching items, either here or Ebay. If you are lucky, you can actually end up with a better group set at no extra cost.

Thanks everyone - i think i have some idea how this all works.

Bikenut, you mention the 600 EX sat further inboard that the DA - what was the result of this - gears not operating, rubbing etc?

How did you then decide the BB axle length? Guesstimate or by ordering the DA BB or equivalent?

When fitted the chainline was obvious wrong so the new BB was ordered with the right length to correct the error