bcd on old suntour chainring

[davek]

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.

 

[Dave Larrington]

davek wrote:
> 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)

 

[davek]

Dave Larrington wrote:
> " "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"
Aha. Sounds like a plan. Thanks.

(btw, aashta - wtf?)

d.

 

[James Thomson]

"davek" <[email protected]> wrote:

> 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...

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

 

[davek]

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

 

[davek]

James Thomson wrote:
> 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.

> 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.

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.

 

[James Thomson]

"davek" <[email protected]> wrote:

> James Thomson wrote:

> > 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.

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.

> 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.

> 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

 

[Zog The Undeniable]

davek wrote:

> 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.

 

[Pete Biggs]

davek wrote:
> 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

 

[Dave Larrington]

davek wrote:

> (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)

 

[davek]

Zog The Undeniable wrote:
> 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.

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.

 

[davek]

Will do, cheers.

d.

 

[MSeries]

davek wrote:
> 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.

 

[MartinM]

"davek" <[email protected]> wrote in message
news:[email protected]...
> Zog The Undeniable wrote:
> > 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.
>
> 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?

 

[davek]

James Thomson wrote:
> 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.

 

[davek]

Dave Larrington wrote:
> 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.

 

[James Thomson]

"davek" <[email protected]> wrote:

> 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

 

[davek]

James Thomson wrote:
> 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.

 

[Nick Kew]

davek wrote:

> James Thomson wrote:
>> 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

 

[Pete Biggs]

davek wrote:
> 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