Cycling UK Forum

Can anyone please help with a chainring compatability issue. Is it possible to put 9 speed MTB chainrings onto older 7/8 speed cranks without causing front deraullier problems (shifting incorrectly across 3 chainrings).
Any advice would be gratefully received

Retro', there are two issues here.The first is BCD distance - the distance from chainring bolt to next chainring bolt, centre to centre, i.e. BOLT CIRCLE DIAMETER. You need to measure the BCD on your current chainrings and then ensure that the new/replacement rings have the same BCD. An easy and accurate way to check is to use a metric marked' steel ruler. Though a tape measure will do.
If it's for an MTB, the BCD will likely be 74.6 from memory (though it could be 76.4!). Any decent shop will have replacement rings in stock. (small plug here for my fave' shop, SPA CYCLES, always very helpful).
The second issue is simpler to resolve. Basically if the new rings are much smaller or larger in total diameter than the old ones,then you may have to slide the front changer up or down the seat tube to effect accurate changes over the ring.Just undo the allen bolt clamp and slide',then adjust the changer cable. Providing you are still using using the old crankset, then with a bit of tinkering, the changer should still work O.K.
Best of luck.

This diagram may help illustrate the infomation you need as mentioned by 'appolo'



PCD is pitch circle diameter
BCD is bolt circle dimension (distance from centre to centre of adjacent bolt holes)

Once you have the information needed you may find the web site of Chicken & Sons useful, as they supply TA rings that will fit most of the popular chainsets

Paul Smith
www.bikeplus.co.uk

retrobloke wrote:Can anyone please help with a chainring compatability issue. Is it possible to put 9 speed MTB chainrings onto older 7/8 speed cranks without causing front deraullier problems (shifting incorrectly across 3 chainrings).
Any advice would be gratefully received

Thanks for info. The other thing I was thinking of was the spider arm thickness where the chainrings attach to, or has that always been the same for 7, 8 and 9 speed chainsets, so the only change of thickness would be the chainrings and by how much the front deraullier moves across the chainrings when pressing the shifter to align with each one of the chainrings.

Paul Smith SRCC wrote:This diagram may help illustrate the infomation you need as mentioned by 'appolo'



PCD is pitch circle diameter
BCD is bolt circle dimension (distance from centre to centre of adjacent bolt holes)

This is liable to cause confusion.
BCD normally means "bolt circle diameter", and is just another name for PCD. BCD is more common, on a quick poll of my bookmarks.
If the distance between adjacent bolts is specified, abbreviations won't be used, it will say (except for Bike+).

Some sites give BCD, some give PCD, some give a mix that seems to depend on chainring manufacturer (eg 135mm BCD Miche and 135mm PCD TA on the same page), and some don't say at all, just saying 4-arm or 5-arm.

Sorry, your'e wrong. Paul Smith is right. I didn't mention PCD initially simply so as not to confuse things.But BCD and PCD (pitch circle diameter) are distinctly different things.
BCD = distance from centre of bolt circle to next nearest bolt circle
PCD = distance going across centre of crankset from any bolt centre to another bolt centre.
Though normally when ordering replacement rings, it is sufficient to quote BCD size, hence why I didn't cloud the issue with this info' before!

(Though saying whether it's for a four or five arm crank spider also helps!).

As regards the thickness of the crank spider, if the changer worked before, it will do so again. The replacement rings won't be that much different if at all.

Hope this helps.

Actually both Apollo and andrew_s have made very valid points regarding my post; It is indeed confusing as often information differs.

To upate my post the TA distributor has a new website (you will see the link in the above post above no longer works) which is http://www.chickencycles.co.uk , the older website had details of how they measured their TA chainrings, (their new one hasn't got the same information yet) I had simply duplicated the same information on our website so that we were the same as theirs (duplicated as in litterally, I copied and pasted the informatiom from their website to the description of BCD in my diagram), in an attempt to be consistant with the supplier of our chainrings. It is also another reason why I use a diagram so that even if you don't agree with what our supplier and as such us are refering to as BCD (and as you can see from the picture from Sheldon Brown below you have good reason to) you can at least see what I am refering to.

When customers ring us for advice I always ask for the number of chainring bolts and measurement between the nearest two bolt centres. Whatever you call the measurements just make sure they match up with who you are buying them from, which is why like Chicken and Sons I listed my stock the same way they do, personally I prefer what I had listed on our web site before I amended it to match our supplier which was:



Sheldon Brown lists BCD as Bolt Circle Diameter so as far as he is concerned he has called BCD what I have called PCD



To quote SB, "Measuring BCD: This 110 mm BCD ring measures 64.7 mm center-to-center.

Multiplying this dimension by 1.7 will give you the actual BCD for a 5 bolt chainring"

He quites PCD as: Chiefly British variant of B.C.D, "P.C.D." stands for "Pitch Circle Diameter" but "B.C.D." ("Bolt Circle Diameter") is preferable, because the term "pitch circle" has a different meaning when speaking of sprockets.

(The primary, more correct meaning of "pitch circle " is the circle running through the centers of the chain rollers of a chain engaged with the sprocket.)

Paul Smith
http://www.bikeplus.co.uk (edit I now work at http://www.candncycles.co.uk)

appolo wrote:Sorry, your'e wrong. Paul Smith is right. I didn't mention PCD initially simply so as not to confuse things.But BCD and PCD (pitch circle diameter) are distinctly different things.
BCD = distance from centre of bolt circle to next nearest bolt circle
PCD = distance going across centre of crankset from any bolt centre to another bolt centre.
Though normally when ordering replacement rings, it is sufficient to quote BCD size, hence why I didn't cloud the issue with this info' before!

(Though saying whether it's for a four or five arm crank spider also helps!).

As regards the thickness of the crank spider, if the changer worked before, it will do so again. The replacement rings won't be that much different if at all.

Hope this helps.

Thanks everyone, for technical info

And thanks from me too.

Knowledge is a wonderful thing.

Mick F wrote:Knowledge is a wonderful thing.

Okay then Mick, here's an exercise:
From a little bit of geometry you can find that the constant relating the two numbers under discussion is equal to cosine of 54 degrees.
( 1/cos54 = 1.7 near enough for our purposes)

If that's all a bit too esoteric, then:
Stronglight helpfully print a little table of common sizes on their packets, whereas TA printed a sort of gauge you can hold the ring up to as well.

retrobloke wrote:Can anyone please help with a chainring compatability issue. Is it possible to put 9 speed MTB chainrings onto older 7/8 speed cranks without causing front deraullier problems (shifting incorrectly across 3 chainrings).
Any advice would be gratefully received

YES! It is possible, I have done it on three bikes.
John.

ddddddd wrote: TA printed a sort of gauge

TA one is printed on the back of their chain ring packaging (edited as image from original post no longer showed)

Paul Smith

Paul Smith SRCC wrote:Sheldon Brown lists BCD as Bolt Circle Diameter so as far as he is concerned he is called BCD what I have called PCD

He quites PCD as: Chiefly British variant of B.C.D, "P.C.D." stands for "Pitch Circle Diameter" but "B.C.D." ("Bolt Circle Diameter") is preferable, because the term "pitch circle" has a different meaning when speaking of sprockets.

(The primary, more correct meaning of "pitch circle " is the circle running through the centers of the chain rollers of a chain engaged with the sprocket.)

I don't think the venerated SB is quite right about the Britishness of "Pitch Circle Diameter", except in so far as we British are perhaps more inclined to stick to correct engineering terminology. So far as I know it, having seen this term on numerous technical drawings, pcd is an internationally recognised term for a virtual circle passing through the centres (or other significant points) of any feature of an item that is repeated in a regular circular pattern, be it sprocket teeth, bolts, rivets, or whatever.

"Pitch" moreover, can be used for any feature that is equally spaced, not necessarily in a circle but also in a straight line. A screw-thread has a certain pitch (e.g. 1mm in the case of an M6 thread), as does a chain (12.7mm in the case of a bike chain). All such uses of this term are equally correct.

So there's two features of a chainring that have a "pcd": the teeth and the attachment bolts. To avoid confusion in the abscence of a diagram, one should therefore speak of the "bolt pitch circle diameter". In everyday cycling parlance this has obviously become shortened to bolt circle diameter or simply bcd.

All well and good, until you realise that "bolt centre distance" also abbreviates to bcd!

stumbled upon this, found it useful and so this is an unashamed bump to bring it your attention.

this is the end of the public service announcement.

came across this thread and can't really see how BCD could mean the distance between adjacent bolts (if it is short for Bolt circle DIAMETER) as there is no diameter involved in that measurement. Far more likely as CJ and Paul mentioned to mean bolt circle distance/ dimension.. if it's meant to be that measurement.
BCD as in Bolt circle diameter to me means exactly that ie the diameter measured from the centre of the crank/bb to the bolt circle. Because of this I believe Sheldon is correct in his thinking as would be expected despite what many say .
Appolo is also incorrect it seems when he states that : "PCD = distance going across centre of crankset from any bolt centre to another bolt centre. "
if you have a 5 bolt spider for example you cannot draw straight line through the centre of the crank/bb between 2 bolt holes as they don't line up !
He also confuses things by saying the bc diameter is the bc distance ..!