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Gearing - Can Someone Explain It To Me Please

SecteurSecteur Posts: 1,971
edited June 2011 in Road beginners
Can someone please explain how it all works - 23/25/27, teeth, cogs, size - it's like a foreign language that I just dont understand.

My bike has 10 on the back and 3 on the front. Beyond that I have no idea what size they are or how many teeth, but I'd like to understand how different rings actually affect the ride, especially as I might get a Ribble as my next bike, and you have to choose the gear sizes.

Feel free to assume I know nothing! (or to just link to a useful web article, but I think the discussion on here is nicer)

Posts

  • rakerake Posts: 3,281
    is this a real question?

    50 tooth front 25 tooth rear the rear will turn twice as quickly because it has half the teeth.
    there are some free online gear calculators, put in the gear sizes and cadence(pedal rpm) and get the speed of the bike out.
  • AndyOgyAndyOgy Posts: 579
    It's one of those things that you just get a feel for after many years of riding. In the meantime, this is probably one of the best explanations of gears that I have ever read:

    http://southcoastbikes.co.uk/articles.asp?article=Gears


    And if you ever get a bike with numbers on the shifters, please ignore those numbers.
  • PigtailPigtail Posts: 424
    If your pedals were attached to the centre of your wheel (like on a penny farthing) everytime you turned your pedals your wheel would turn once.

    What gears do is vary that. So in a high gear, which has the biggest front ring and the smallest back ring you will go further with each pedal stroke, but you will have to push the pedals harder. With 48 teeth on the front and 12 on the back each revoliution of the pedals will turn the wheel 4 times. Ideal for good conditions - like downhill.

    In a low gear that has the smallest front ring, combined with the biggest back ring you will not go nearly so far with each pedal turn, but the pedals will be easier to push. 30 teeth on the front and 30 on the back will give you one turn of the wheel for each pedal turn - better for diificult conditions, like a headwind or going up a hill.
  • optimisticbikeroptimisticbiker Posts: 1,657
    lets assume it is a real question.

    the purpose of gears, just as in a car, is to match the road speed (wheel rotation) to your legs (pedal rotation). Your legs are most efficient at about 80 revs per minute (rpm) although everyone varies a bit. If the wheels were connected to the pedals directly then the wheels would turn at 80rpm and since each wheel revolution is approx 2.1m you'd be going at about 10km/hr. But any faster and you'd be struggling to keep up and any slower and you'd be finding it hard on your muscles. This is because the other thing gears do, apart from matching speed is provide 'mechanical advantage' - matching the effort of your legs to the effort needed to move the bike (just as a lever can help move a heavy object). Just as in a car the engine is most efficient and delivers most power and torque at a certain revs so your legs are most effective at a certain speed and the goal is to keep them turning at that speed (or cadence in the jargon)

    So for climbing a hill you need a low gear so that your feet are moving faster than or close to the same speed as the road wheels (like 1st gear in a car) and for speed on the flat a high gear (like 6th in the car) so your feet are turning much slower than the wheels.

    Now the gears on the front - the chainset - usually 2 (called a double or a compact - they have different sizes of gears) or 3 (a triple) are bigger - that is have more teeth - than those on the back (the cassette, which is made up, usually, of 8, 9 or 10 cogs). It is simple to work out the gear ratio - it is simply the number of teeth on the front chainring divided by the number of teeth on the rear cog. So if you biggest ring on the front has 50 teeth (as is typical) and your smallest cog on the back has 11 then your ratio is (50 divided by 11) or about 4.5, that means every turn of the pedals turns the rear wheel about 4.5 times so pedalling at 80rpm will result in the rear wheel turning at 360rpm and you'll be moving at about 46km/h.

    Different chainsets have different numbers of teeth. A triple will often be 53 teeth on the big ring, 42 in the middle and 34 on the smallest. A compact is often 50 on the big ring and 34 on the smaller.

    Typcially the cassette on the back has 8, 9 or 10 cogs (gears) ranging from 11 to 25 teeth (such as 11-12-13-14-15-17-19-21-23-25 on a 10speed). So with a triple front gear and an 11 - 25 cassette the ratios typically range from 4.5 (50/11) for going fast to 1.36 (34/25) for climbing hills. What is important though is the difference between each step of the ratios. To big a step and you'll have to keep changing gear to find the best one for the road, too small (close gearing) and you may find you can't climb a steep hill easily or spin too fast on a fast section.

    Generally when buyng a new bike you only need to decide if a triple or a compact chainset is what you want. A compact is lighter but you'll be changing gear more often than on a triple. The rear cassettes are pretty standard in the number of teeth on each cog, though you can swap the 25 for 27, 28 or even 32 for those really nasty hills.

    Hope this helps... happy to answer any more questions


    edit: must type faster :)
    Invacare Spectra Plus electric wheelchair, max speed 4mph :cry:
  • Paul057Paul057 Posts: 167
    I actually logged on tonight to ask the very same question for the same reason (thinking of buying a Ribble). Now i don't need to ask due to some really helpful explanations.
    Thanks for posting the question, and even bigger thanks for the really helpful responses.

    :D:D:D
  • SecteurSecteur Posts: 1,971
    Hi all - yes, it was an entirely serious question (remember once upon a time when you were new and knew nothing too!).

    Thanks for the outstanding answers!
  • deswellerdesweller Posts: 5,271
    Secteur wrote:
    Hi all - yes, it was an entirely serious question (remember once upon a time when you were new and knew nothing too!).

    Thanks for the outstanding answers!

    I thought most people learned about this stuff at school; pre GCSE IIRC...
    - - - - - - - - - -
    On Strava.{/url}
  • optimisticbikeroptimisticbiker Posts: 1,657
    desweller wrote:
    Secteur wrote:
    Hi all - yes, it was an entirely serious question (remember once upon a time when you were new and knew nothing too!).

    Thanks for the outstanding answers!

    I thought most people learned about this stuff at school; pre GCSE IIRC...
    Sadly not these days - gearing, mechanical advantage, torque transfer... once was basic O-level physics...now you'll be hard pushed to find it in an A-level paper... not that we're dumbing down.... (just don't get me started ok)
    Invacare Spectra Plus electric wheelchair, max speed 4mph :cry:
  • optimisticbikeroptimisticbiker Posts: 1,657
    AndyOgy wrote:
    It's one of those things that you just get a feel for after many years of riding. In the meantime, this is probably one of the best explanations of gears that I have ever read:

    http://southcoastbikes.co.uk/articles.asp?article=Gears


    And if you ever get a bike with numbers on the shifters, please ignore those numbers.

    A good article - though (being picky)
    Smaller chainrings are easier to pedal in, but less power is transferred to the rear wheel. Conversely the largest chainring provides more power to the rear wheel, but is harder to pedal in.

    is factually incorrect, he doesnt mean power but torque... power is a constant determined by the rider... (about 250w or 1/3HP for the average rider)
    Invacare Spectra Plus electric wheelchair, max speed 4mph :cry:
  • Paul057Paul057 Posts: 167
    Having read this thread i've been looking on the Ribble website and deciding on my gear set up.

    I've decided on a compact 43/50 chainset, and a 11-28 cassette

    This gives me a fairly wide range, although i do accept that it possibly means some wide gaps between the middle gears on the cassette.

    Just wanted to check that this isn't a particularly weird set up i'm going for?
  • suzybsuzyb Posts: 3,449
    desweller wrote:
    Secteur wrote:
    Hi all - yes, it was an entirely serious question (remember once upon a time when you were new and knew nothing too!).

    Thanks for the outstanding answers!

    I thought most people learned about this stuff at school; pre GCSE IIRC...
    Would think it depended on the subjects you took and the country you studied in. Studying standard grade physics I was never taught about gear ratios.

    Thanks for the explanations though, especially OptimisticBiker's comprehensive one.
  • EarlyGoEarlyGo Posts: 281
    Paul057,

    Your choice of a 50/43 chainset sounds a bit weird to me! IIRC most people go for a 50/39. The cassette sounds fine though! Hopefully someone with a bit more knowledge than me will also comment though.

    Regards, EarlyGo
  • Wacky RacerWacky Racer Posts: 638
    I have a masters in Mechanical Engineering and I went to university with some people who couldn't explain gearing and gear ratio. In fact I have even worked with so-called engineers who would struggle to put across a plausible understanding. Just making the point that even those you would expect to know often don't, let alone people who haven't studied such things, or they don't work in those fields.
    Ridley Orion
  • Paul057Paul057 Posts: 167
    Paul057 wrote:
    Having read this thread i've been looking on the Ribble website and deciding on my gear set up.

    I've decided on a compact 43/50 chainset, and a 11-28 cassette

    This gives me a fairly wide range, although i do accept that it possibly means some wide gaps between the middle gears on the cassette.

    Just wanted to check that this isn't a particularly weird set up i'm going for?

    Ha ha. I've just realised that my number dyslexia kicked in on that post - it was supposed to read 34/50 (not 43/50). Does that sound a bit more like it?
  • MountainMonsterMountainMonster Posts: 7,423
    Paul057 wrote:
    Paul057 wrote:
    Having read this thread i've been looking on the Ribble website and deciding on my gear set up.

    I've decided on a compact 43/50 chainset, and a 11-28 cassette

    This gives me a fairly wide range, although i do accept that it possibly means some wide gaps between the middle gears on the cassette.

    Just wanted to check that this isn't a particularly weird set up i'm going for?

    Ha ha. I've just realised that my number dyslexia kicked in on that post - it was supposed to read 34/50 (not 43/50). Does that sound a bit more like it?
    Sounds quite a bit better, much better for climbing some hills!
  • Paul057Paul057 Posts: 167
    Paul057 wrote:
    Paul057 wrote:
    Having read this thread i've been looking on the Ribble website and deciding on my gear set up.

    I've decided on a compact 43/50 chainset, and a 11-28 cassette

    This gives me a fairly wide range, although i do accept that it possibly means some wide gaps between the middle gears on the cassette.

    Just wanted to check that this isn't a particularly weird set up i'm going for?

    Ha ha. I've just realised that my number dyslexia kicked in on that post - it was supposed to read 34/50 (not 43/50). Does that sound a bit more like it?
    Sounds quite a bit better, much better for climbing some hills!

    ...without losing too much speed on the flat and downhill?
  • MountainMonsterMountainMonster Posts: 7,423
    The 50 chainset won't be the fastest on the flats, but won't be much slower. I have 53/11 as my highest gear, and can't even reach full potential speed with that anyways so it's all good!
  • DubaiNeilDubaiNeil Posts: 246
    Welcome to the wonderful world of Bicycle Parts Acquisition Syndrome, a lifelong journey on which you are just embarking...

    Depending on the terrain you intend to ride, a compact (front chain set 50/34) with 11-28 rear cassette will provide a HUGE range of gears, with unfortunately large jumps and (IME) these jumps are at exactly the wrong places.

    For reference, that is the sort of gearing I use for seriously mountainous stuff in France, where I really just need a two speed bike with 34-28 (climbing) and then 50-11 (descending)

    For more general riding, a 12-25 rear cassette would probably be more useful, giving a better spread of gears (without the large gaps), while still providing sufficient speed (50-12 @ 100rpm is 32.4mp/h)

    It is easy & quick to change the rear cassette, which is why I now have a collection of them!

    For riding where I live (flat as a flat thing & high speed group rides) I use a 23-11 cassette, for France I use a 11-28 and for other places (South Africa, UK etc) I use the 12-25.

    The last thing I would worry about would be speed of the "highest" gear until you have been riding for a while - particularly in groups, where the wind resistance is lowered. You will also probably find that your cadence will increase with training, so riding at 90rpm (or higher) becomes the "norm", with bursts higher when needed.

    Neil
  • Paul057Paul057 Posts: 167
    Thanks again to everyone on here for taking time to explain, it's very helpful for a newbie like me
  • People keep saying that a 50 ring on the front wont be fast enough on the flats, but if you have an 11 small tooth on the cassette then it's fine. at 80 rpm a 50/11 combo will actually go faster than a 53/12 by 1.3 kmp/h :D
  • rakerake Posts: 3,281
    we must have people who can maintain over 30 mph on the flat cos thats what youl be doing on a 12/50. 11 tooths are useless.
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