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Old 10-05-06, 08:32 PM   #1
Adiankur
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understanding cranks

I dont quite understand cranks and cassettes and how they relate to climbing, top end speed and other such things. Is there a thread that gives some pretty sold explanation or can someone sum up for me?
I was looking at mtb cranks and noticed a crank with 48/38/28 with a cassette of 13/34 7 speed, while a more expensive model had 42/34/24 with 11-32 8 speed. What does that really mean?

appreciate help
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Old 10-05-06, 08:43 PM   #2
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well of course any sacrifice in weight will help the rotational intertia and keeping the crank going, but without higher level material that runs a risk of sacrificing rigidity, which will help deliver more pedaling force through the chain rather than being used to bend the cranks.

higher rigidity often means decreased impact and stress strength however, because the lack of flex causes the 'breaking point' of the material to be lower... of course expensive, exotic materials can give you a better trade off, but all materials have certain physical properties... so you're not going to get the strength of steel with the weight savings of carbon... carbon is extremely rigid but not very strong for impacts...

basically it's a very complex set of physical properties that must balance off eachother.


the cassettes on the other hand are like gears in a car, if you want to get up a mountain(overcome gravity) you'll need a high gear when matched to the power the engine can put out... in a top speed situation you'll want a very low gear ratio so that one pedal will get you going a great distance.

for MTB riding it really all depends on your strength, and rigidity may not be so important as you'll want strength instead.
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Old 10-05-06, 08:48 PM   #3
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Quote:
Originally Posted by cc700
well of course any sacrifice in weight will help the rotational intertia and keeping the crank going, but without higher level material that runs a risk of sacrificing rigidity, which will help deliver more pedaling force through the chain rather than being used to bend the cranks.

higher rigidity often means decreased impact and stress strength however, because the lack of flex causes the 'breaking point' of the material to be lower... of course expensive, exotic materials can give you a better trade off, but all materials have certain physical properties... so you're not going to get the strength of steel with the weight savings of carbon... carbon is extremely rigid but not very strong for impacts...

basically it's a very complex set of physical properties that must balance off eachother.


the cassettes on the other hand are like gears in a car, if you want to get up a mountain(overcome gravity) you'll need a high gear when matched to the power the engine can put out... in a top speed situation you'll want a very low gear ratio so that one pedal will get you going a great distance.

for MTB riding it really all depends on your strength, and rigidity may not be so important as you'll want strength instead.

Yikes!

I just put myself in the OP's newbie position and read your post.

Are you a theoretical engineer, by chance?
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Old 10-05-06, 08:53 PM   #4
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Addressing the OP's question (in a bit more practical way):

What are gear-inches?
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Old 10-05-06, 09:31 PM   #5
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Quote:
I was looking at mtb cranks and noticed a crank with 48/38/28 with a cassette of 13/34 7 speed, while a more expensive model had 42/34/24 with 11-32 8 speed. What does that really mean?
What does this mean? The second crankset will give you a higher high gear and a lower low gear. The second crankset with 8 cogs will give you more "in-between" on gear difficulty because it has more cogs.

The two cranksets listed above give nearly identical gear range.

Quote:
well of course any sacrifice ....
Very nice long explanation to a different question than the OP asked.
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Old 10-06-06, 05:20 AM   #6
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You can compare different chainring/cog combinations by using tooth ratios but on a bike the wheel size also plays a part, hence our preference for "gear inches". Two different combos having the same gear inches will feel the same, you will travel the same distance for the same effort.

There is a fashion in the MTB world (compared to traditional touring bikes) to use smaller chainrings and cogs to achieve the same ratio. This results in less weight and better ground clearance at the chainring BUT it has fewer teeth handling the same stress so the wear rate is higher. There is some evidence to show that smaller systems also have higher friction but it is not worth worrying about.

In practical term, a 28/38/48 is good for hostel touring and general commuting/riding in hilly areas. The 24/24/42 is more suitable for heaviliy loaded touring on unsurfaced roads and is generally preferred on specialist touring bikes.
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Old 10-06-06, 08:54 AM   #7
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So, since im commuting to work on paved roads that are quite hilly, and not heavily loaded, I would be better suited with the gearing I dont have. I have the 42/34/24. I probably wont know the difference, as I havent spent more than 20 minutes on a bike with the other setup, and it had knobbies on, while Im running the slickasaurus, 1.5's.

OK, got it, im going to do a gear inch table for my bike and see what comes up, and then observe my shifting patterns in relation to it to see how I feel when going into different combinations.

Once im done there, ill have to see if I will need any changes down the road, which will put me back at knowing absolutely nothing again.

Last edited by Adiankur; 10-06-06 at 09:28 AM.
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Old 10-06-06, 09:35 AM   #8
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Between those two cranksets honestly, there is none. It's a difference of < 2 gear inches in the top end and in the low end. Hilly or not, you can climb a vertical wall with your current crankset.

The only deficiency I really see is that you may not have enough gears for going fast on a flat road. Hills - no problem.
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Old 10-06-06, 10:02 AM   #9
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I top out on my MTB at about 25 mph on flat road right now, but thats mainly because my legs arent used to going that hard for very long I think. Maybe, if I can sustain my spin a bit longer, I can go faster. I havent maxed out my gears, at that speed, but not sure what my bike is capable of in higher gears. Im pretty sure I must be close to the max my el-cheapo bike can handle. Ill start looking into buying a touring bike in the spring.

The above speed doesnt take into account, any possible errors I made when setting up my computer. for all I know, I could be going 7mph.
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