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My compact double crank has undrilled inner pads at 74 bcd---why?

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My compact double crank has undrilled inner pads at 74 bcd---why?

Old 05-30-11, 12:59 PM
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My compact double crank has undrilled inner pads at 74 bcd---why?

I'm commuting on an '05 specialized sirrus with a compact double (50/34t) crank set by "RPM". No model number, but I think it's manufactured by Sugino.

Anyway, I've found that I rarely use the 50t ring. I coast downhill, and don't really care to go faster than 25 mph. Living near Seattle, I also wouldn't mind lower gearing for some of the steeper hills. So it's the usual story: Nobody makes a basic, good-value 110/74 compact double that could be used to achieve something like 42/26 or 40/24. Hence the usual solution of taking a touring triple (like the Sugino XD600) and running the inner two rings only.

But wait! Upon closer inspection, my RPM crank/spider has undrilled protrusions which appear to be at 74 bcd. Now I'm wondering,

1) Given the non-existence of 110/74 compact doubles, why did they bother putting the hunks of metal in the right place if they weren't going to use them?

and

2) Is it feasible to have these drilled and tapped so I can replace the 110mm inner ring with a 74mm inner ring? The undrilled studs protrude ~ 1mm past the seating plane of the inner 110mm ring, so they would also have to be ground slightly to switch to 74 bcd while preserving the existing ring spacing.
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Old 05-30-11, 01:07 PM
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That would be putting a granny on/defeats the purpose of a compact double. You would need a longer spindle....and the granny would have to be small enough to clear the stays. A smaller than 50 ring sounds more feasiable
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Old 05-30-11, 01:23 PM
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I thought the purpose of a compact double was to cover the desired gear range with two rings? Wouldn't running 42/26 be in the same spirit? Or maybe you misread this as proposing to add a third ring?

Anyway, there's at least 1 1/2" inches of clearance to the chain stays with the 34t ring. I don't think dropping to 26 or 24 would be a problem, clearance-wise. The FD also has plenty of travel.
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Old 05-30-11, 01:39 PM
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Originally Posted by mark03
I thought the purpose of a compact double was to cover the desired gear range with two rings? Wouldn't running 42/26 be in the same spirit? Or maybe you misread this as proposing to add a third ring?
Basically the same spider casting appears to be used for both triples and doubles. No doubt that you can drill and tap for a granny gear, find a machine shop to do the work to insure perfect centering (and also bring the gear you intend to use to the shop).

Brad

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Old 05-30-11, 02:39 PM
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[QUOTE=bradtx;12714447Basically the same spider casting appears to be used for both triples and doubles. No doubt that you can drill and tap for a granny gear, find a machine shop to do the work to insure perfect centering (and also bring the gear you intend to use to the shop).[/QUOTE]

I've done that just using hand tools. It's been probably 15 years but as I remember there were indentations to make starting the drill easier. I did have to get a longer BB spindle to make it work.
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Old 05-30-11, 04:11 PM
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Originally Posted by mark03
2) Is it feasible to have these drilled and tapped so I can replace the 110mm inner ring with a 74mm inner ring? The undrilled studs protrude ~ 1mm past the seating plane of the inner 110mm ring, so they would also have to be ground slightly to switch to 74 bcd while preserving the existing ring spacing.
Might I suggest selling your compact double (road) crankset - and buying a mountain crankset to replace it? If you get an old 58mm/94mm five arm crankset, you could make a 30/44 double using 94mm bcd rings. Alternately, there are lots of ways to set up a double using new mountain cranks.

If you want cranks like your current ones, then buy a Sugino triple and sell the double. The triples are a lot easier to find.

BTW, if your crank is machined like the Sugino double(s) I have, the 74mm pads have been shortened from the height they'd be on a triple.
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Old 05-30-11, 06:59 PM
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Originally Posted by Retro Grouch
I did have to get a longer BB spindle to make it work.
Why would you need a longer spindle? The 74 bcd ring will be in same plane as (that is, replace) the existing 110 bcd inner ring, correct?

I'm curious as to the pros/cons of a mountain bike crankset. Somewhere I thought I read that would have a higher Q. Maybe it would bother me, maybe not; it's another unknown.

Just putting a 74 bcd inner in place of the existing 110 bcd inner seems relatively foolproof, apart from the machining challenge.
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Old 05-30-11, 08:43 PM
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Originally Posted by mark03
Why would you need a longer spindle? The 74 bcd ring will be in same plane as (that is, replace) the existing 110 bcd inner ring, correct?
No, it would be one chainring spacing interval further inboard.

The 110 bcd compact double uses the middle and outboard positions of a triple. To switch a chainring to the 74 bcd position would mean that the inboard and middle positions of a triple are in use.
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Old 05-30-11, 09:02 PM
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Originally Posted by laura*
The 110 bcd compact double uses the middle and outboard positions of a triple. To switch a chainring to the 74 bcd position would mean that the inboard and middle positions of a triple are in use.
Aha, thanks for clarifying. I don't have a triple crank to look at, but measuring mine more carefully, the flat surface of the 74 bcd lugs is ~1.5 mm back from the mating surface of the "middle" 110 bcd ring. From this, I gather the 74 bcd rings must attach using spacers to make up the remainder of the nominal ring separation (less 1.5 mm)?

I am assuming that this extra 1.5mm would be removed from the 74 bcd lugs as part of this modification, and the granny ring would be installed without spacers, so it would indeed be in the same position as the present 110 bcd "middle." If I'm still missing something, and this isn't possible, then it may not worth the trouble...

It looks like there used to be mountain cranks (e.g. Ritchey Logic) with reasonably narrow Q, but they are hard to find now. The custom options (TA and White) are out of my price range.
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Old 05-30-11, 10:18 PM
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They're just using the same crank mold they use for a 110/74 triple. If you got them drilled you'd just have a regular triple crank -- setting it up as a double using inner/middle positions will give you either a messed up too-narrow chainline or too wide a Q-factor.

The main benefit of the "hypercompact double" over a touring triple is reduced Q-factor and/or better chainline, if these don't matter to you you might as well use a triple.

Current options for a hypercompact double are

(a) hunt down one of the older 94mm mountain bike crank that can take a 29t middle ring
(b) use a TA cyclotourist, or one of the current lookalikes like from Velo-orange or Electra
(c) if you have loads of money Sugino has an actual 110/74 double now. (you notice the difference between this and a 110/74 triple, is that the outer chainring sits on the inside of the spider arms so that the 74mm ring can nestle up, and the spider is positioned to give a narrow gap between the outer chainring and arms.)

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Old 05-31-11, 06:04 AM
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Originally Posted by mark03
...measuring mine more carefully, the flat surface of the 74 bcd lugs is ~1.5 mm back from the mating surface of the "middle" 110 bcd ring. From this, I gather the 74 bcd rings must attach using spacers to make up the remainder of the nominal ring separation (less 1.5 mm)?
No - the 74 bcd "lugs" would have been left taller if the casting was going to be sold as a triple.

Originally Posted by mark03
I am assuming that this extra 1.5mm would be removed from the 74 bcd lugs as part of this modification, and the granny ring would be installed without spacers, so it would indeed be in the same position as the present 110 bcd "middle." If I'm still missing something, and this isn't possible, then it may not worth the trouble...
And at that point, the 110 bcd part of the crank will prevent the chain from fitting onto the chainring you mount at 74 bcd.
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Old 05-31-11, 07:34 AM
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Get a new crankset with the gearing you want -- would be cheaper than having a machine shop drill out your current cranks.
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Old 05-31-11, 02:53 PM
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Originally Posted by mconlonx
Get a new crankset with the gearing you want -- would be cheaper than having a machine shop drill out your current cranks.
Well, it would be, if anyone manufactured a 110/74 double crank. (Ok, technically the Sugino OX801D counts, but at $500 not really practical.)

Guess I'm stuck with a repurposed triple. I'm sure it will work out ok. I may pick up some of those cheap 110/74 Dimension (Andel) cranks to experiment with.
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Old 05-31-11, 03:08 PM
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The Velo Orange Grand Cru Crank is what your looking for... When they have some in stock.

https://store.velo-orange.com/index.p...-crankset.html

This Polyvalent Cranks is good also.

https://store.velo-orange.com/index.p...-crankset.html
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Old 05-31-11, 05:14 PM
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Thanks for the suggestions. I was just looking at those last night. The Grand Cru is a bit over my price range. The Polyvalent crank is nice, but only the 46/30 rings are available and the cranks+chainguard aren't available separately (I asked). So, if I insist on my preferred gears (42/26), this too will be expensive.

It irks me enough that I'm willing to try hacking something together on my own. E.g., the cheap Dimension cranks, a TA 42t pinned/ramped middle ring, and a steel 26t inner. Am I nuts?

Measured Q on my current setup isn't amazing---somewhere around 160mm---and I'm ok with it, so maybe the 94/58 mtb crank idea has merit. But, I don't know which ones to look for.
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Old 05-31-11, 05:51 PM
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Originally Posted by laura*
No - the 74 bcd "lugs" would have been left taller if the casting was going to be sold as a triple.
Not necessarily. The old Sugino AT and AT look a likes were mounted on spacers for the inner ring. I used to seek them out for use with Mountain Tamer quad adapters and Avid micro adapters. The lugs had to be flush with the inner face of the crank or these kinds of adapters wouldn't work. In fact, cheap cranks had the lugs while the more expensive cranks used spacers.

mark03, another way to go would be with a wide range rear cassette and a new rear derailer. Easier and possibly cheaper than a new crank.
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