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Old 05-01-12, 09:55 PM   #1
Heatherbikes
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advice on single speed crank that needs a 30t chainring.

I am building up a bike with an 8 speed sturmey archer hub which is a bit funny and requires a 30t chainring. White Industries and Paul's components make beautiful single speed cranks that can come with 30 t or around that. But a bit too expensive. I do not understand too much about the numbers that fly around, but what sort of crank set and sizing etc should I look for to work with 30t?
Thanks
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Old 05-02-12, 06:01 AM   #2
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Any road triple crank or older MTB crank will have a 74 mm bolt circle in the granny position and that will take a 30T chainring.
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Old 05-02-12, 06:12 AM   #3
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But a single crank that is 74 BCD? Wow, sounds specialized.

Any way you can instead use a 110 BCD single crank (plenty of inexpensive BMX ones, e.g. Origin8 $37) which goes down to 33 tooth chainrings? Maybe with a slightly bigger sprocket in back?
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Old 05-02-12, 06:40 AM   #4
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There are also the mid 90s era mtb cranks with a 5 bolt 94mm bcd. Those can get down into the 20s as far as tooth count. I think the newer, 4 bolt MTB cranks can also run rings that small. Of course these will be triple arms, but, aesthetics aside, you should be able to do what you're trying to do no problem.

Another option would to run threaded BB "euro" bmx cranks and get a 30t sprocket [instead of a chainring and cranks with a spyder].

What's the frame? And can you change the cog in the back to something bigger to use a bigger chainring/sprocket up front?

A little pricey, and definitely quality- but this site might give you and idea of the BCD's that you can get 30t on. Poke around- http://www.homebrewedcomponents.com/store.php
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Old 05-02-12, 04:35 PM   #5
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Originally Posted by IthaDan View Post
Another option would to run threaded BB "euro" bmx cranks and get a 30t sprocket [instead of a chainring and cranks with a spyder].

+1 to this....
And, I'd think twice about what I presume to be a Sturmey-Archer 8speed... I've read some real horror stories about the reliability, and having the direct-drive ratio as the lowest gear makes sense only on 20"-wheeled bikes.
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Old 05-02-12, 07:47 PM   #6
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Why not just use a mountain crankset with a 32 tooth chainring? Generally it's the torque that is the problem with internal geared hubs, so going up 2 teeth on the chainring shouldn't cause any problems. There are plenty of options for mountain cranksets out there that are a lot cheaper than White Industries or Paul's Components.
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Old 05-02-12, 08:09 PM   #7
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Sturmey Archer 8 speed is all overdrive ratios, 1:1 is the low one, in the hub.

so Bigger wheels need smaller chainrings , Truvative hammerscmidt http://www.sram.com/truvativ/product...th/term-id/297
comes in 22 or 24 t, then adds a 1.6X cable operated overdrive internal gear.
so then its a 35, or 38t + a half tooth worth
Swiss made Schlumpf speed drive is also 1.6X,
Schlumpf High speed drive is a 27T then the overdrive is 2.5X
or a huge one.. no cables.. ankle button in crank center shifts sideways.

then the BB shell just needs an edge chamfer for either .

Last edited by fietsbob; 05-03-12 at 12:19 PM.
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Old 05-03-12, 05:15 AM   #8
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Hammerschmidt requires ISCG mounts. The fsa patterson has a 28t ring with a 45t overdrive...but you cannot use it with coaster brakes(not sure which S-A 8speed you're looking at), you'll never push any of those gears in the overdrive position, and it costs like $300. I think a BMX 3piece crank with a euro bb would be your best bet, unless you haven't bought the rear hub yet, in which case an Alfine or a SRAM rear hub might do you better. (Or even just a 3- or 5-speed S-A hub)
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Old 05-03-12, 12:13 PM   #9
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They go better in folding bikes with small wheels..
instead of the 8 speed go with the 5 speed the direct gear is in the the middle.
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Old 05-03-12, 03:23 PM   #10
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is there an echo in here? =D
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Old 05-03-12, 06:38 PM   #11
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Velo-Orange has a crankset with a 50.4 BCD. http://store.velo-orange.com/index.p...bare-arms.html
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Old 05-03-12, 08:15 PM   #12
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No, I've already looked into the sturmey archer 8 speed hub, and people use it on road bikes etc with regular sized wheels, no problem. The only thing is that the front chainring has to be around 30t. And reliability is always a chance, some people have said their SA 8 speed hubs outperformed the shimano ones.
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Old 05-03-12, 08:17 PM   #13
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And how do I use a mountain bike or road crankset meant for 2 or 3 front chainrings if I am going single speed? What about the chainline? Oh I think I understand, but to clarify...
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Old 05-03-12, 08:29 PM   #14
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between the spindle length of the BB, which side of the main spyder (big or middle ring position) and where you put the cog on the freehub body with spacers, you have more than enough adjustment.

you will, however, need either shorter chainring bolts or washers to take up the space of the missing chainring (axle washers work great and are cheap and available in a gazillion widths).
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Old 05-03-12, 08:45 PM   #15
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You're not going singlespeed. You're going with an IGH. There are many different IGH varieties, with several different chainline possibilities. You haven't mentioned which S-A 8speed you intend to use, or on which frame, but there are at least 5 different versions of the S-A 8speed, with different chainline for each variation. (128mm OLD disc, 135mm OLD disc, 132mm OLD drum, 120mm OLD rim-brake, 135mm OLD rim brake.) You'll need to figure which best suits your intended frame/braking system, and decide which crankset you hope to use. Chainline on a IGH can generally be moved in or out a few mm by using a dished cog. These are, of course, considerations you'll want to handle before you order anything. Specs found here; click on the hub you think you want, then click on "specs": http://www.sturmey-archer.com/products/hubs

Ppl do use the S-A 8speeds on bikes with full size wheels, but doing so requires the use of a tiny chainring. Even then, the direct drive will be the lowest gear, so they'll tend to be insanely high-geared, unless you use a giant cog as well. (Up to 23t is available, which would give around a 115" gear.)

The conventional wisdom on IGH hubs is that you want to run > or = a 2:1 ratio direct drive, lest the torque destroy the hub. (Meaniong a 15t rear cog with a 30t up front.) In this case, thankfully, this isn't an issue due to all the gears being overdrive. S-A's site claims they used the direct drive as the low gear to allow for the use of a tiny chainring; to me, it seems more like this strategy has made a tiny chainring a necessity. Unless you're running a 20" wheeled folder or something else with a small rear wheel, which is why a lot of ppl recommend these for those bikes.

I hope this helps
-rob

PS- One more time: BMX crank with a 19mm spindle, euro BB, 30t chainwheel. OTOH, if you're running the rim brake 120mm OLD version, you could use a road triple with a 30t 74mm BCD 5 bolt ring on the granny position, which would help achieve the extremely narrow 39mm chainline. (Most track bikes use 42mm chainline.) Running a road triple with only the grannie should look... interesting. Check out Sheldon Brown's articles on chainline if you want to learn how to calculate chainline when converting a multispeed crank to single-ring use.
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