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Old 10-30-11, 09:45 AM   #1
asmac
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What is "compact" crankset?

I'm hoping to build up a Salsa Vaya frame with a non-drop handlebar and mountain components. Salsa specifies that the frame "Fits any road compact double or triple" crankset.
Can someone explain what that implies? Specifically, what does "compact" mean and how does a "road" crankset differ from a "mtn" crankset?
I'd prefer to have 11-34 (or 11-36) on the rear and 44/32/22 on the front.
Thnx!
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Old 10-30-11, 10:00 AM   #2
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Originally Posted by asmac View Post
I'm hoping to build up a Salsa Vaya frame with a non-drop handlebar and mountain components. Salsa specifies that the frame "Fits any road compact double or triple" crankset.
Can someone explain what that implies? Specifically, what does "compact" mean and how does a "road" crankset differ from a "mtn" crankset?
I'd prefer to have 11-34 (or 11-36) on the rear and 44/32/22 on the front.
Thnx!
Quick, not so technical answer:

Road is usually narrower than mountain. Rear axle spacing is 130 road, 135 mountain. Hence road cranks are narrower than mountain, so chain line is more straight, i.e. it aligns more naturally with the width of the rear hub/cog.

Compact means a double that acts like a triple because the difference betwen in the inner and outer chainring is a big one, like 16 teeth diff vs. 10 each jump on a triple.

I thought a Salsa Vaya has a 135 rear, check you specs. If so, then assuming you use an all mountain drivetrain, with specs that match you bike, you are fine.

Last edited by FrenchFit; 10-30-11 at 10:09 AM.
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Old 10-30-11, 10:08 AM   #3
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Road cranks typically have two chainrings mounted on a 130 mm (or 135 mm) bolt circle diameter (BCD) with 5 bolts and the most common gearing is 53T/39T. A road triple has the same outer BCD and a 74 mm BCD for the inner (granny) chainring and are usually geared 52/42/30 or 50/39/30. The smallest 130mm BCD chainring possible is 38T and the smallest 74 mm BCD chainring is 24T

"Compact" road cranks have a 110 mm BCD and are most commonly geared 50/34 and the smallest chainring that will fit this BCD is 33T.

Current MTB cranksets have smaller BCDs for both the outer two and granny chainrings and commonly use only 4 fastening bolts. Typical chainring sets are the 44/32/22 you mentioned.

Older MTB cranks had 110/74 mm BCDs and were geared something like 46/34/24 but these are uncommon now. Some "Trekking" (i.e. touring) cranks still use these BCDs and similar gearing but often with a 48T large chainring.
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Old 10-30-11, 10:17 AM   #4
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Quote:
Originally Posted by asmac View Post
I'm hoping to build up a Salsa Vaya frame with a non-drop handlebar and mountain components. Salsa specifies that the frame "Fits any road compact double or triple" crankset.
Can someone explain what that implies? Specifically, what does "compact" mean and how does a "road" crankset differ from a "mtn" crankset?
I'd prefer to have 11-34 (or 11-36) on the rear and 44/32/22 on the front.
Thnx!
Most of the differences between road cranksets and mountain bike cranks boil down to the size of the chainring bolt circle or BCD for bolt circle diameter. There are all kinds of BCDs but they seem to have settled into 3 categories: Road, mountain and compact road.

A 'road' crank usually has a BCD of 130 mm. This limits the chainrings to 39 teeth. If the crank has a triple, the smallest ring can be limited to either 24 teeth or 30 teeth depending on the inner BCD. Road cranks usually use 52 tooth outer rings with 42, 40 or 39 tooth inner rings. They may also use a 50 tooth outer with a 39 tooth inner although this is less common. Most road cranks are also 5 armed.

Most mountain bike cranks currently available use a 4 armed spider with a 104 mm outer BCD and a 64mm inner BCD. This means that you can go much smaller on the middle ring than a road crank. It also means you can fit a 22 tooth inner ring on the crankset so that you have lower gears.

Compact doubles split the difference. They use a 110 mm BCD. You can put a 50 tooth outer on the crank and a 34 tooth inner (you could go lower on the inner). The idea is to have a wide range of gears but not have to put up with the derision of your fellow roadies for having a third chain ring.

A mountain bike crank can work just fine on a road bike. A 44/11 is a pretty small gear and you reach maximum RPM quickly. I'd probably look for a Shimano Trekking crank which uses a 48 tooth outer with a 36 or 34 middle ring which gives you a higher gear for cruising. Since the crank is a 4 armed mountain bike crank with a 104/64 BCD, you can change the inner ring to from the 26 it comes with to a 22. This gives you an astounding range.
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Old 10-30-11, 10:51 AM   #5
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Originally Posted by asmac View Post
I'm hoping to build up a Salsa Vaya frame with a non-drop handlebar and mountain components. Salsa specifies that the frame "Fits any road compact double or triple" crankset.
Can someone explain what that implies? Specifically, what does "compact" mean and how does a "road" crankset differ from a "mtn" crankset?
I'd prefer to have 11-34 (or 11-36) on the rear and 44/32/22 on the front.
Thnx!
Second thought, have you given much thought to your rear wheel? The Vaya is a 26" bike in the smaller frame, 700c in the bigger, correct? So what rear wheel/hub are you using, and how is it going to be spaced out? Have you thought about brakes, and dishing out the rear, and what type of cassette you are going to use on what hub? Perhaps you should think from the rear wheel forward, make sure your components will match.
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Old 10-30-11, 10:58 AM   #6
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Sun Tour introduced the Micro drive crankset for MTB's all cogs were smaller ,
ratios remained similar. 11:44,= 12:48,= 13:52.. 1:4
Shimano took over the design.
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Old 10-30-11, 05:03 PM   #7
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Thanks, all. My head is spinning...
You are of course correct, the Vaya specifies 135mm for the rear spacing and the size I'm looking at will have 700c wheels.
I was planning to use the Shimano XT hub and disc brakes if those are considerations.
I asked Salsa about using a Shimano XT triple crankset and was told "the only issue that comes up is the spindle length on thecrankset and that can be taken up by using the spacers that come with thebottom bracket. There are good instructions with the BB to show you where toinstall the spacers to keep the proper spacing and take up any slack." It sounds to me that I would be OK but don't really get what they were talking about.
Any further light that can be shed would be appreciated. Even just a thumbs up or down! I guess there's a reason to let the pros do the specifying but that isn't much fun!
Your humble noob...

Last edited by asmac; 10-30-11 at 05:09 PM.
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Old 10-31-11, 06:45 AM   #8
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Originally Posted by asmac View Post
Thanks, all. My head is spinning...
You are of course correct, the Vaya specifies 135mm for the rear spacing and the size I'm looking at will have 700c wheels.
I was planning to use the Shimano XT hub and disc brakes if those are considerations.
I asked Salsa about using a Shimano XT triple crankset and was told "the only issue that comes up is the spindle length on thecrankset and that can be taken up by using the spacers that come with thebottom bracket. There are good instructions with the BB to show you where toinstall the spacers to keep the proper spacing and take up any slack." It sounds to me that I would be OK but don't really get what they were talking about.
Any further light that can be shed would be appreciated. Even just a thumbs up or down! I guess there's a reason to let the pros do the specifying but that isn't much fun

]Your humble noob...
The rear hub spacing is a red herring. It really doesn't have much to do with the selection of a crank set. I'll assume a new external bottom bracket XT crank. The spindle on the crank...the part that connects the arms...is made for a bottom bracket the is 73mm wide, which is rather rare. Most bikes have 68mm wide bottom brackets. To make the 73mm spindle fit, you put in spacers behind the bearing cups. You can put them in behind the drive side or the nondrive side. Sometimes (often) you put them on both. The usual configuration is one on the nondrive side and 2 on the drive side. The number and thickness you put on the drive side depends on where your chainline needs to be to play nice with the rear cassette. People make a big fuss about chainline but it's really a nebulous thing. As long as the middle chainwheel of a triple crankset just sort of lines up with the middle cog on the cassette you are going to be okay. It's not really all that precise.

The XT will work just fine with the Vaya and it's not as hard to install as you think. The Shimano external bearing cranks are about as idiot proof as you can make a part on a bicycle be.
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Old 10-31-11, 05:28 PM   #9
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Confidence is rising

Thanks, again. It's becoming clear... idiot-proof is good!
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