jimmuller 

I'll show some ignorance by asking the experts. Two numbers issues.

The first: I'm thinking of picking up a spare FD for the tandem. It must handle a triple, and a Simplex would be nice. There is a Simplex FD on ebay listed as "probably" for a triple (the inner plate looks big enough) and having clamp size 1-1/8". I can't tell how that translates to my tandem, an '82 Peugeot TH8. It measures to about 28.x, where x varies from 2 to, oh, I dunno', maybe 6, depending on how/where I measure it. So I measured my other bikes. The Raleigh and Centurion are bigger. The Moto is definitely smaller than those, and I have a nice Cyclone FD that is too loose on it, so it is clearly French. IIRC, the UO8 is small too. The trouble is, I'm thinking the various layers of paint are obscuring my readings.

Of course, none of those other bikes are pertinent. They have function FDs which work nicely. But 1-1/8" comes out to 28.575mm. I suppose that will fit the tandem's stoker ST, but I'm wondering if the seller's measurement is a bit sloppy. As sloppy as mine!

The second: I'm looking to replace the Centurion's small chainring. It's a Tri-color crank, 130mm BCD as computed from non-adjacent bolt spacing. I need a 39T, and a satin-finish Sugino would be fine. Ebay has a lot of them, but the spec that puzzles me is "Chain compatibility 9-speed". I don't know doodlysquat about new chain widths. My bikes have PC870s and work just fine. I can't imagine that a thinner ring would make much difference, but perhaps it will space differently from the big ring. Am I just be paranoid or is there something I'm missing and it really matters?

Thanks.

Pompiere

I grabbed a Sun Tour FD out of the parts bin to put on my TH8 and didn't even think about the tube size, because it bolted right on. So I guess it's close enough to 1 1/8 to not worry about.

I just went out and measured the TH8 and I got 28.0 mm. The two Japanese bikes that were handy were 28.6 mm, one of which is the bike the Sun Tour came from.

oldskoolwrench

Jim,

If the Seller is calling the Clamp Size 1⅛", there is a possibility that they are quoting the 31.8mm dimension commonly associated with that fractional measurement. If it's an old Simplex though, and other 'modern' DR clamps fit 'large' on your Peugeot, then I'd bet you need a 28.6mm clamp size.

9 speed cogs and chain rings are cut slightly narrower than 8 speed, so there might be a touch more play from an 8 speed chain on a 9 speed chain ring; just enough to cause a slight pedal lag or 'click' under pressure.

likebike23

I wouldn't worry too much about the clamp size, worst case it's too large. You can always shim it without issue. As for the chainrings, 9 speed rings are backwards compatible. Trying to get a 9 speed chain to engage some chainrings designed for 5 speeds can be problematic. Hope that helps.

RobbieTunes

+1

I think 28.about anything will work.


I've run 8-sp and 9-sp chains on 8-sp setups, with no issues.
I've swapped tons of BioPace rings out for 9sp rings, never had an issue.
I bought several sets of those Race Face 9/10 chainring sets, and they work fine with 8-sp.

jimmuller 

Thanks, all.

But...but...but...1-1/8 inches is closer to 28.6 (actually 28.575), and 31.5 is a twinge over 1-1/4. So I'm thinking you meant that. and the FD on ebay is actually meant for 28.6mm. One reason I asked the dumb question is I couldn't remember the nominal tube diameters. Okay, so that will probably work just fine.

[A curious thing is, I remember swapping out the Simplex on my UO8 long ago and discovering that a "standard" FD was loose, must have picked up another because what is on there now is not shimmed. Then when I did the Motobecane last year I didn't think about it until discovering the Cyclone I wanted to use was definitely too loose. So I grabbed a Vx from my parts bin and it fit fine. Now I'm wondering how I ended up with a spare FD with French/metric clamp size. Karma works in mysterious ways.]

As for the ring, well, the inscription doesn't say "one ring to rule them all" so it should do. Neither if these is a big purchase anyway.

Thanks again.

oldskoolwrench

Jim,

It's the same 'relationship' as Quill stems... why a 22.2mm stem is classified as 1", a 25.4mm stem is 1⅛", and a 28.6mm stem is a 1¼".

Same thing with F. DR clamps... 28.6mm is called 1", 31.8mm is 1⅛", and 34.9mm is called 1¼".

In the end, the DR and the chain ring should be just fine.

photogravity

@jimmuller I really don't know much of anything about the topic. Just buy the derailleur and shim it if you have to. Chain? Dunno about that either. I ride bikes with internal gear hubs. They use 1/2" x 1/8" chain.

Tell your "sweetie" that my better half and I said hello,.

jimmuller 

What the...? I realize I'm derailing my own derailleur thread here, but 2.54mm = 1" exactly, and 1-1/8" = 28.757mm exactly, and well, you get the idea.

likebike23

Check your math on the 1 1/8" conversion, looks like you may have transposed some numbers. Anyway, the quill stems are classified by the OD of the fork's steerer, not the OD of the quill. Front derailleurs are 1 1/8 (28.6mm), 1 1/4 (31.8mm), or 1 3/8" (34.9mm). They are sized to fit the OD of the seat tube.

repechage

1.125" = 28.575 mm or 28.6, the typical OD of seat and down tubes on C&V bikes save the French who used 28.0 OD.
Oh, and they use 26.0 top tubes too instead of 25.4... before paint on all mentioned.
The French are superior you know, so they understand these bicycle frames, just ask Jan Heine, he will advise on the planing abilities of Metric tubed bikes... Disclaimer, I own a half dozen French bikes and love them all, but they are kept in different arrondissements so they all think they are special.

The Cyclone does not fit well due to the clamp mechanics and design, the band cannot draw up high enough to provide enough force.
Most two bolt or hinged front derailleurs will tighten down on a French bike, with the Simplex and Huret most likely to fit native. Huret Jubilee front changers often fractured on British bikes unless you were Raleigh and ordered them to fit.

Jeff Wills

Huh??? I don't know where you get this.

Most newer front derailleurs have the clamp diameter cast on the inside face. It also appears on the inside of many braze-on adapters. What I've seen is 28.6mm, 31.8mm, and 34.9mm, which convert to 1 1/8", 1 1/4" and 1 3/8".

Shimano AD11 Front Derailleur Braze-On Adaptor 1-1/4" (31.8mm) - Harris Cyclery bicycle shop - West Newton, Massachusetts
Shimano AD15 Front Derailleur Braze-On Adaptor 1-3/8" (34.9mm) - Harris Cyclery bicycle shop - West Newton, Massachusetts

oldskoolwrench

You are absolutely correct; my egregious error.

jimmuller 

You are correct. (In fact I had already observed that 1-1/8" was "a twinge less than 28.6mm. ) Brain lock. That zombie that jumped me last night aftectd me semohow. i thimk.

Jeff Wills

It was either that or a lack of beer.

Mmmm.... beer...