What is the correct chainline?

Lataxe

I have just bought a Boardman Urban MTB, partly on the good review it has on this website. It is indeed a fine steed but there is one problem, which is that the chainline seems to be incorrect. I am seeking the comments of you lads familiar with modern BBs, which now seem to come in 78 varities rather than just the square-tapered BBs I am familiar with.

On every other bike I ever had (and I've built a few from separate components in my 40 years pedalling them all) the lateral chainline was arranged so the centre-line of the chainset aligned with with the centre line of the block. On this Boardman urban MTB the chainset is an FSA Comet, 42/27 with a "SRAM external bearing" BB.

I noticed that the chainline looked way out when I had a first ride on it. The chain is severely misaligned when running from either chainring to the 4 largets sprockets. An engineer's straight edge reveals that the large chainring lines up with the 2nd-smallest sprocket and the small chainring ring aligns with the 3rd smallest sprocket - a long way from a centre-to-centre alignment.

In addition, the left crank passes closer to the chainstay than does the right crank by around 7mm. My heel just catches the left chainstay but misses the right chainstay, when I pedal.

The chain will drop on to the small chainring with no problem but is reluctant to climb back up to the large chainring if the chain is in one of the larger 4 sprockets at the back.

All this suggests that the factory have either put in the wrong BB or have put the spacers in incorrectly. Would you agree (and that the chainline should be centre-of-chainset to centre-of-block)?

There has been no joy asking the British purveyor of the machine (who you all know too well) for a fix or even any advice. They kept the bike a week then said, "Can't change it as Head office says it will null the warrantly if we change a factory-installed part". I will retain my feelings about this "service" for the moment.....

Finally, is it easy to swap the spacers about on these type of BB, to shift the chainset to the left a bit; or does it require special tools? There are two 2.5mm spacers on the chainset side and one spacer on the other side. Can I safely move them so that there are three spacers on the left side and none on the right (thus moving the chainset 5mm to the left)? The chainrings would still clear the right hand chainstay. I would adjust the throw of the front changer.

Thanks for any advice you can give.

Lataxe

supersonic

Can you measure the chainline? Centre of seattube to centrepoint between the two chainrings.

nicklouse

Please chek your parts as IIRC the crankset ou mention is not compatible with the cups you mention.

The Rookie

This one?
http://www.halfords.com/webapp/wcs/stor ... 65499#tab2
FSA Comet, no mention of BB, why do you think it's an external BB?

Simon

Lataxe

SS,

I'll measure the seat tube centre to mid chainring distance tomorrow, when the daylight returns.

NL & TB,

I was taking the BB name from the bikeradar review "full specification" list - which may be incorrect it seems. I'll put the magnifying glass on what I can see of the BB tomorrow.

I do recall that it has a hollow axle that I can stick my index finger down (on the chainring side) with the non-chainring side having some form of allen key bolt in it. The cups are knurled and sit outboard those washers I mentioned, which abutt the frame's BB.

If the BB is an FSA one intended for the Comet chainset, is it possible to remove it and shift it over (via washer-swapping) without special tools? I presume I'll need an allen key of the correct dimensions.....?

Do you think the chainset and block centre-lines should align and that I would be right to shift the chainset over in the bike's BB so that this becomes the case?

Thanks for your answers so far.

Lataxe

supersonic

Theoretcially the middle is the best, but is not always the case. Some good info here:

http://sheldonbrown.com/chainline.html

Swapping the spacers to three at one side may not allow the cups to screw far enough into the frame.

The Rookie

From looking at the bike in Halfords I think it's a BB30 on the latest version of the Urban MTB.

Simon

Lataxe

SS and Co,

I measured the chainlineon of the Boardman Urban MTB with its Comet double chainset and got 50mm. This seemed at least 5mm too wide according to the Sheldon Brown website page concerning chainlines (thanks for the url, btw). However......

The BB for the Comet chainset is a MegaExo BB7000. When I examined the FSA specification sheet for this VERSION OF THE cOMET CHAINSET, on their website, the chainline is specified at 49.5mm. Also, the washer configuration of the BB (2 x 2.5mm on the right and 1 X 2.5mm on the left) on my bike is correct according to the FSA installation guide. So it seems the chainset is installed on the Boardman "to spec".

This is puzzling. I also have a Specialized tricross (10sp block, like the Boardman) with an FSA Gossamer double chainset and similar BB. The chainline for that is specified at 43.5mm and the chainset and block are centrally aligned, as I would expect.

Why, I wonder, is the specified chainline for the Comet (double) chainset 6mm wider than that for the Gossamer (double) chainset?

* This 49.5mm chainline has the effect of misaligning the centre of the chainset with reference to the centre of the block by 6mm - a couple of cogs in effect.

* The distance between the crank (at the pedal end) and chainstay is 13mm on the right and 7mm on the left. So I am pedalling with my left foot 6mm closer to the bike than my right foot.

I am wondering - is the Comet meant for bikes with a wider rear drop-out width (which many mountain bikes have, I understand)? Perhaps the Boardman has a roadbike rear dropout width?

Anyroadup, I now find myself wanting to shift the chainst in by 5mm to get a centralised chainline, using those washers; but reluctant because of the FSA spec and installation advice.

Anyone got any thoughts?

I notice I also need a special spanner to undo the BB7000 cups. Dang!

Lataxe the confused.

nicklouse

50mm is quite normal and is the most common one now on MTBs.

PS dont try comparing MTB and Road as the specs and design are totally different.

you can move spacers about but you will be moving your contact points to the left.

Lataxe

NL,

If this Boardman Urban MTB was a pure mountain bike with a triple chainset, I could understand why the chainline might be best at 50mm. As the Sheldon Brown website explains, an MTB will normally spend most time in the small and middle ring, so the chainline is best centred with the block between those two chainrings. The big chainring is then more outboard but is only rarely used.

Ole Sheldon mentions the reverse arrangement with a triple chainset on a road or touring bike, where it is the inner ring of the triple that is the "rare" user, so the chainline is centred between the outer and middle ring.

But.... this Boardman "Urban" is meant as a sort of cross between a hybrid road machine with an MTB capability thrown in via it's geometry. It comes with slick tyres rather than knobblies, after all; and although the big chainring is but 42, the small sprocket is 11, so one may whizz along at 25mph no problem. Also, it has a double not a triple chainset.

Anyway, although I can understand that a pure MTB rider might prefer to centre the chainline up the block towards the lower gears, in fact this Boardman/Comet does the opposite! The chainline is centred towards the higher gears.

So my problem is now reduced to:

Can I shift the Comet chainset inboard 5mm by removing the 2 X 2.5mm washers on te right hand side of the MegaExo BB? The chainrings will still miss the chainstay (by 8mm rather than 13mm), the cranks will be central to the bike rather than sticking out to the right more than to the left and I will have my preferred chainline.

The issue is: will the RH BB cup screw far enough into the BB shell threads without those washers there? Not to mention the fact that to change the BB myself I will apparently need an FSA cup-removal tool and a torque wrench!

Cuh!

I have now emailed FSA concerning my little problem. I wonder if they will respond (many organisatipons seem to lose such emails in a cyber blackhole).

Lataxe, almost ready to moan about new-fangled modern thangs. :-)

The Rookie

Some bikes have assymetric chain stays, so suggest you measure from the inside of each crank arm to the centreline of the bike on each side, if it is offset by 2.5mm or more then swapping a spacer over shouldn't cause any issues.

Simon