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I am wondering why they use such small cogs on a bike. Problem with that is as you get down below about 15 teeth, you cannot get precise steps to the next gear. For example, you can go 15 to 14 or 15 to 13. 15 to 14 is about a 7% difference. 15 to 13 is about a 14% difference. Now lets suppose instead they had cogs twice the size such as 30 replaces the 15 gear. Now you have twice as many choices. You can go from 30 to 29 (about 3.5%), 30 to 28 (about 7%), 30 to 27 (10%), 30 to 26 (about 14%)...

Where this method really shines is when you get down to about 12,11,10 teeth. Then you are forced to space the next gear about 10% away. With my system you could space very high gears within 5% of each other. For example, the 2nd highest gear on a bike could have a 21T cog and the top gear could be 20T, representing only a 5% difference. Cadence would drop from say 100 RPMs to 95 RPMs which someone could likely power thru much easier than a 10% increase.

Also, I suspect the larger gears are marginally more efficient in power transfer and should help extend chain life.

So is this a practical idea and if so, why don't bike manufacturers offer it?

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    I'm assuming it's because to have the same gear ratios you'd then need front gear rings with twice as many teeth,too. This would make them too large to be practical. Also there's a substantially increased mass.
    – Lui
    Jan 22, 2016 at 1:14
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    But you said in this post you need big spacing so you can feel it bicycles.stackexchange.com/questions/36641/… Your 3:3:3 proposal you have spacing over 20% bicycles.stackexchange.com/questions/36661/… Or a 11 speed to a 3 speed 11:22.33 with 100% ratio jump bicycles.stackexchange.com/questions/36633/… Your daily ideas are all over the spectrum and use extra stuff. Manufacturers don't do it because your ideas are bad.
    – paparazzo
    Jan 22, 2016 at 2:07
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    Please stop spamming idle thoughts on here. If you are actually trying to fix your bike or build a new one by all means ask practical questions that can be answered. In the meantime I'm voting to close this and any other speculative questions from you.
    – Móż
    Jan 22, 2016 at 2:16
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    @Mσᶎ Uh, you answered two of the (crazy) speculative questions. You encouraged this.
    – paparazzo
    Jan 22, 2016 at 2:39
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    @Frisbee yes, and he's made me sorry that I did. I didn't realise he'd post 10 similar questions in just over a day.
    – Móż
    Jan 22, 2016 at 2:42

1 Answer 1

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Everything on a bike is a tradeoff between efficiency and weight and complexity and cost. If you imagine the simplest drive ratio which is 1:1 then the cog and chainwheel could be any size up to the rim size. and down to the mechanically smallest possible around the axle, perhaps 4 or 5 tooth?

Why not a really big chainring and cog on the simple bike? Because big weighs more. Plus you need some space under the chainring for ground clearance. Big also uses more materials to make, and even more space in to ship worldwide.

Why not a super small one, like the mountainbikes with compact chainrings? Because its expensive to make them so small, the forces exerted on smaller cogs and chainrings increase exponentially as size drops linearly. Historically such leverages would break components, its only in the last couple decades that materials engineering has make them possible. But they're still expensive.

Why do bike makers not offer weird-stuff? First, because the bulk of the buying public buy "average" bikes that look like normal bikes... and that's where the money is. I have a recollection that none of the big makers shows a profit out of their top-end groupsets, that they all cost more to design than they earn. Its only once the quantity of sales increases that there is a profit margin.

Why are there so few recumbents and tandems and other non-conventional bikes on the roads? Because they don't sell well. Only once a market segment reaches a "critical mass" does it self-perpetuate. Without volume its not worth tooling up a production line, and without the volume made, the price stays high, leaving the item in the Boutique class for rich buyers only.

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  • The whole weight saving thing on a bike is pointless since the rider is much heavier than the bike. I have a motorcycle and when I remove 10 or even 20 pounds I cannot even feel it. For example, a 5 gallon gas tank weighs about 30 pound more when full vs. almost empty and I cannot feel the difference so you want me to believe an extra 1 or 2 pounds on a bike makes a difference? Hogwash! Larger cogs in the back (double size) should work cuz there is tons of clearance and a simple doubler gear to magnify the front sprocket would work. This would make 5% gear changes possible for high gears.
    – David
    Jan 22, 2016 at 10:33
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    @david no its not hogwash at all. I changed from a 17 kg steel bike to an 11 kg aluminium bike and my hill climbing times dropped by 10-15%. Now I'm ~100 kilos dressed, so thats a change of ~3% for a 10% gain. As one of your other questions points out, there's only a couple hundred watts in the pushbike's motor so we have to use it wisely.
    – Criggie ♦
    Jan 22, 2016 at 19:42
  • I am not buying that weight savings thing and it is more like a 5% change in rider+bike combined weight in your case. They are 2 different bikes unless you only changed the frame. I have an aluminum frame BSO (Walmart bike) and it weighs in at close to 17kg. I make a 6.5 mile trip about once a week and can check my average speed. I suspect if I strapped a 6kg backpack on me vs. no backpack, everything else equal (fitness, stamina, traffic...), there would be little or no difference in my average speed. Something else changed on your aluminum bike u r not telling us. Different gears?
    – David
    Jan 22, 2016 at 23:40
  • @david comments aren't for extended discussion. Weight makes a difference. If you want to question that then post a new question, or use the search function bicycles.stackexchange.com/search?q=weight+of+bike and find questions/answers like bicycles.stackexchange.com/questions/29793 bicycles.stackexchange.com/questions/1549 bicycles.stackexchange.com/questions/7133 bicycles.stackexchange.com/questions/18155
    – Criggie ♦
    Jan 23, 2016 at 0:52

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