bcd on old suntour chainring

davek said:

have searched, but can't find any conclusive answers to this question,
so wondering if anyone knows...

My old Raleigh Royal (late 80s vintage) still has the original Suntour
chainset, with 52T & 42T rings. I want to change the inner chainring
for a 39T (well, it's cheaper than fitting a triple) but I don't know
the BCD for the current set-up. Can anyone help? I'm afraid I don't
know the specific model name of the current chainset.

I will try measuring the BCD myself, but I don't really trust myself
to be able to do it accurately. Will also keep trawling the web in
search of answers...

Don't know personally, but AASHTA:

" "C-C" is the distance between adjacent stack bolts. This is easier to
measure directly than the actual Bolt Circle Diameter. Multiplying this
dimension by 1.7 will give you the actual BCD for a 5 bolt chainring"

--

Dave Larrington - http://www.legslarry.beerdrinkers.co.uk/
World Domination?
Just find a world that's into that kind of thing, then chain to the
floor and walk up and down on it in high heels. (Mr. Sunshine)

Dave Larrington said:

" "C-C" is the distance between adjacent stack bolts. This is easier

to

Quoted message said:

measure directly than the actual Bolt Circle Diameter. Multiplying

this

Quoted message said:

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

Aha. Sounds like a plan. Thanks.

(btw, aashta - wtf?)

d.

davek said:

My old Raleigh Royal (late 80s vintage) still has the original
Suntour chainset, with 52T & 42T rings. I want to change the
inner chainring for a 39T (well, it's cheaper than fitting a triple)
but I don't know the BCD for the current set-up. Can anyone
help? I'm afraid I don't know the specific model name of the
current chainset.

Quoted message said:

I will try measuring the BCD myself, but I don't really trust myself
to be able to do it accurately. Will also keep trawling the web in
search of answers...

A ballpark measurement would help. Several bolt circles that are now
obsolete, some of which were quite close together, were in use in the
mid-late eighties. 84mm, 110mm, 116mm, 118mm, 122mm, 130mm, and 144mm are
all possibilities.

Measuring between the centres (or corresponding edges) of two non-adjacent
bolt holes and dividing by cos18 (=0.9510565) should get you close enough
to distinguish.

James Thomson

one other question: is it safe to assume the old bolts will fit the
holes in the new chainring?

James Thomson said:

Several bolt circles that are now
obsolete, some of which were quite close together, were in use in the
mid-late eighties.

Damn. This is what I feared would be the case.

Quoted message said:

Measuring between the centres (or corresponding edges) of two

non-adjacent

Quoted message said:

bolt holes and dividing by cos18 (=0.9510565) should get you close

enough

Quoted message said:

to distinguish.

Thanks. I shall try this in conjunction with Mr Larrington's method. If
the two answers are in the same ballpark, I shall consider it safe to
proceed...

d.

davek said:

James Thomson wrote:

Quoted message said:

Quoted message said:

Several bolt circles that are now obsolete, some of which
were quite close together, were in use in the mid-late eighties.

Quoted message said:

Damn. This is what I feared would be the case.

It's probably not going to be a problem. The only case that might cause
trouble is distinguishing between 116 and 118mm, 118 being the more common.
Some of the more obscure sizes might be hard to track down, but cross that
bridge if you come to it.

Quoted message said:

Thanks. I shall try this in conjunction with Mr Larrington's method. If
the two answers are in the same ballpark, I shall consider it safe to
proceed...

The two methods are equivalent geometrically (1.7 is 1/sin36) so you should
get the same answer.

Quoted message said:

one other question: is it safe to assume the old bolts will fit the
holes in the new chainring?

Almost certainly. Practically all chainrings use standard Campag-style
bolts.

James Thomson

davek said:

have searched, but can't find any conclusive answers to this question,
so wondering if anyone knows...

My old Raleigh Royal (late 80s vintage) still has the original Suntour
chainset, with 52T & 42T rings. I want to change the inner chainring
for a 39T (well, it's cheaper than fitting a triple) but I don't know
the BCD for the current set-up. Can anyone help? I'm afraid I don't
know the specific model name of the current chainset.

I will try measuring the BCD myself, but I don't really trust myself to
be able to do it accurately. Will also keep trawling the web in search
of answers...

If it's the type I'm thinking of, you'll need spacers as well. The old
chainring is stamped so it sits clear of the outer ring.

davek said:

one other question: is it safe to assume the old bolts will fit the
holes in the new chainring?

Bound to.

Use the table on this page working out the BCD:
http://www.sheldonbrown.com/gloss_bo-z.html

Have a butcher's at Settle Cycles' chain rings.

~PB

davek said:

(btw, aashta - wtf?)

As Always, Sheldon[1] Has The Answer

1 -Sheldon Brown <URL: http://www.sheldonbrown.com/harris/index.html>

--

Dave Larrington - http://www.legslarry.beerdrinkers.co.uk/
World Domination?
Just find a world that's into that kind of thing, then chain to the
floor and walk up and down on it in high heels. (Mr. Sunshine)

Zog The Undeniable said:

If it's the type I'm thinking of, you'll need spacers as well. The

old

Quoted message said:

chainring is stamped so it sits clear of the outer ring.

How do you mean stamped? Both chainrings on the current set-up are
completely flat, and the only markings are the size of each chainring.
d.

Will do, cheers.

d.

davek said:

have searched, but can't find any conclusive answers to this question,
so wondering if anyone knows...

My old Raleigh Royal (late 80s vintage) still has the original Suntour
chainset, with 52T & 42T rings. I want to change the inner chainring
for a 39T (well, it's cheaper than fitting a triple) but I don't know
the BCD for the current set-up. Can anyone help? I'm afraid I don't
know the specific model name of the current chainset.

I will try measuring the BCD myself, but I don't really trust myself to
be able to do it accurately. Will also keep trawling the web in search
of answers...

d.

My Raleigh Royal from 1986 had a Sugino Chainset with 110mm BCD rings,
36/50. The larger one is now fitted to the Deore chainset of the same
vintage which is on the bike.

"davek" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

Zog The Undeniable said:

If it's the type I'm thinking of, you'll need spacers as well. The

old

Quoted message said:

chainring is stamped so it sits clear of the outer ring.

How do you mean stamped? Both chainrings on the current set-up are
completely flat, and the only markings are the size of each chainring.

Sorry to put a possible damper on but where you propose to get said 39 ring
from? the only spares I have seen are either Shimano (normal and compact) or
Campag but ISTBC. Have you tried offering the Suntour ring up to either make
chainset in a shop/on a bike?

James Thomson said:

The two methods are equivalent geometrically (1.7 is 1/sin36) so you should
get the same answer.

144 it is, then. 🙂

Thank you!

d.

Dave Larrington said:

As Always, Sheldon[1] Has The Answer

Of course. I guessed what the S stood for, but I couldn't work out the rest.

d.

davek said:

144 it is, then. 🙂

Ah, now you have a problem.

There's no such thing as a 39t chainring for a 144mm bolt circle. It's
physically impossible to fit anything smaller than a 41, and 42 is the
smallest common size.

Sorry.

James Thomson

James Thomson said:

It's
physically impossible to fit anything smaller than a 41, and 42 is the
smallest common size.

Having now actually looked at the bike, this much is patently obvious.
Nnnnn!

Still, at least I now know how to measure theBCD, which is progress of a
sort.

d.

davek said:

James Thomson said:

It's
physically impossible to fit anything smaller than a 41, and 42 is the
smallest common size.

Having now actually looked at the bike, this much is patently obvious.
Nnnnn!

Hmmm. I've had two tourers with a 42 smallest chainring. The first was
fine because I didn't know it was wrong to pedal uphill in a high gear.
The second was fine because it was matched by a 34 cog on the back.

The latter solution might work for you.

--
Nick Kew

davek said:

You will note the obvious flaw in my plan to fit anything smaller than
42T. Doh!

Easy mistake to make, but at least you've now got a good excuse to get a
new chainset :-)

~PB