Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Question on Gears

  1. Sep 2, 2011 #1
    Why is it that most of the bicycles have 10-20 gears, wherein motor bikes have mostly up to 6 gears? Is there an engineering reason behind it?
     
  2. jcsd
  3. Sep 2, 2011 #2

    jtbell

    User Avatar

    Staff: Mentor

    The range of "pedal force" (or inversely, the pedaling rate) that a person can exert comfortably for a long period of time is rather small. When I was into bicycle touring years ago, I usually "spinned" the pedals at 90-100 rpm, for a typical road speed of 15 mph. I could do this basically all day, with occasional short rest stops, shifting into lower gears going uphill, and higher gears going downhill. If I had pedaled slower in a higher gear, the pedal force would have increased, and my knees wouldn't have lasted very long. If I had pedaled faster in a lower gear (again for the same road speed), I would have had trouble maintaining a smooth pedaling style, and lost efficiency because of that.
     
  4. Sep 2, 2011 #3

    BobG

    User Avatar
    Science Advisor
    Homework Helper

    One reason is probably that engines (even motorcycle engines) generate more power than a bicyclists legs can. It's hard for bicyclist to make a huge jump in gear ratio, so they need more intermediate steps in between. (The overall gear ratio depends on both the big front gears and the small rear gears. They're set to get a wide span from first to high gear. Some of the gear combinations in between essentially duplicate each other when it comes to the overall gear ratio, so having 20 gears doesn't mean you have 20 unique gear ratios.)
     
  5. Sep 2, 2011 #4
    The only reason I ever saw for gears on a bike was to start the spring on a low gear, and then end the fall on a high gear. Other than that purpose, I'm all about gearless (even when I've lived in hilly areas).
     
  6. Sep 3, 2011 #5
    My mountain bike has 18 gears. The purpose of that is to be able to boast about how many gears it has.
     
  7. Sep 3, 2011 #6

    Jonathan Scott

    User Avatar
    Gold Member

    The way the usual derailleur mechanism works (by shifting the chain between adjacent sprockets of slightly different size) makes it hard to cover a large enough range with a single set of rear sprockets (which typically have 6 or 7 but sometimes up to 10 or 11) while keeping the ratios comfortably close together. That made it necessary to have a coarser switching mechanism on the front sprockets (chain rings), effectively providing two or three sets of 6 or 7 gears covering overlapping ranges. That means that a bicycle with about 18 gears might in practice only have about 12 different gear ratios available.

    In my experience, combinations involving the chain crossing from an outside front sprocket to inside rear sprocket or vice versa do not work well, in that the chain runs less smoothly and keeps trying to jump to the next one, so in practice some of the gears are not very usable either, but it can generally be arranged that those are duplicated by ones in other sets.
     
  8. Sep 3, 2011 #7

    Astronuc

    User Avatar

    Staff: Mentor

    I always thought that more than 5 gears was overkill. In JHS and HS, I road a three speed that I converted to a fixed gear. At university, I road a 10 speed that I pretty much left in the highest gear ratio.

    Motors are geared to more or less optimize motor performance and vehicle speed. Also pedal bikes have gears ratios from 1:1 to 5:1, whereas motor bikes from from low to high.
     
  9. Sep 4, 2011 #8
    We humans have smaller rpm "sweet spots" than a WWII diesel. Gears make up for that, allowing us to pedal a comfortable, but narrow 80 to 110 rpm while travelling at a wide range of velocities.

    My bike has 27 gears (3 x 9).

    Where it really pays off is on the hills. Sometimes first gear is barely navigable going up a steep hill, while 27th can't keep up with gravity while travelling down a steep one.
     
  10. Sep 4, 2011 #9

    jtbell

    User Avatar

    Staff: Mentor

    There's a trade-off between the number of distinct gear ratios, and ease of shifting through them in sequence. If you have a large number of distinct gear ratios, you often have to "double-shift" (shift both front and rear gears) in order to go up or down a step.

    The bike that's been sitting in my garage for several years has 18 gears: three chainrings in front and six sprockets in the rear. I think I actually used only about 8 to 10 of the possible combinations. Most of the time I used the middle chainring, switching over to the large one for downhills and tailwinds. Some of the gears on those two chainrings are duplicated, which gave me a choice of when to shift over from one to the other. I used the small chainring only with the largest two sprockets, for climbing steep hills. I seldom double-shifted.
     
  11. Sep 4, 2011 #10
    Another thing to think about is the cost of implementing more gears: that is, it would require a MUCH more complex transmission system in a motorcycle.
     
  12. Sep 5, 2011 #11
    True, jtbell, there's a lot of overlap between the gearing. Using #1 front gear with #9 rear is working cross-odds and is quite noisy, so I usually use the front gears as follows:

    1 (small) - uphill only
    2 (medium) - 80% of all riding
    3 (large) - downhill only

    Then, I usually use only 3 or 4 gears for each. If I have to go beyond that, I switch to the next front gear.

    I used to think they could simply make do by using wider gearing in both the front and rear such that 10 gear would pick up where 9th left off, but in practice, there are too many times when you need small increments from one gear to the next. There are also times when you need to go back and forth between 9th and 10th, which would require one to move the rear derailler fully. Thus, staying in the middle front gear and hopping back and forth using the middle rear gears is oh so much more comfortable!
     
  13. Sep 5, 2011 #12

    jtbell

    User Avatar

    Staff: Mentor

    Yep, when you're in "cruising mode," small steps between gears let you adjust to slight uphill/downhill grades, or a slight breeze one direction or the other, without changing your perceived "effort."
     
  14. Sep 5, 2011 #13
    You're right, jtbell, although when I lived in Florida, the only hills I encountered were overpasses! Incremental gearing is required for countering shifting winds.

    Every ride has their own unique cruise RPM. Mine's 93. The only time I change it is if I need to increase the rpms in order to rapidly accelerate (traffic or up a short hill) or decrease it to cool down while heading downhill or downwind. If I need to cool down on the flats, I simply maintain my pedaling rpm while reducing speed.

    The editing feature appears not to be working, even though it's been less than 24 hrs, so here's a modified re-post, instead:

    I usually use the front gears as follows:

    1 (small) - uphill only, rear gears 1-5
    2 (medium) - 80% of all riding, rear gears 3-7
    3 (large) - downhill only, rear gears 5-9
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Question on Gears
  1. Metal Gear Solid (Replies: 7)

Loading...