How many teeth are there in the front and rear gears of a 15 and 21 speed bicycle. I assume there is a standard number.
There isn’t. BTW, most good bikes are now 18 or 27-speed, with 9 gears in the back and 2 or 3 in the front.
Your typical road bike will have a 53 and 39-tooth gear in the front and the back will be 11-12-13-14-15-16-17-19-21, but that might easily be 12-13-14-15-17-19-21-23-26 instead. You can put lots of different sprockets in the back. As I get older, the back sprockets get bigger. I have the 12…26 on now.
Mountain bikes usually have a 48-tooth gear as the largest front gear, and the rear sprocket has a broader range of teeth than a road bike-- often up to a 28 or even 32 tooth gear.
All depends on what you wnat (or need).
As someone who only owns an ancient Schwinn 5-speed, purely for exercise around the subdivision, I have a hard time seeing the need for 27 speeds. Unless maybe you’re a professional bike racer — a Lance Armstrong sort of racer, I mean. Even then, the choice just seems excessive. It would be like having a 27-way light bulb, or a manual car transmission with 27 gears.
No? Okay, I’m not really into biking that much, so I realize I don’t know what I’m talking about. But could you tell me why there’s any demand for 18 or more speeds? (And, if I went into a bike shop tomorrow, would I even be able to buy a 5- or 10-speed if I wanted one?)
Bikes have 10, or 18, or 27 (or whatever) different gear combinations, but not that many different gear ratios (=speeds). Some of the possible gear combinations are not used because they are excessively stressful on the geartrain and/or they duplicate the ratios in other combinations.
On a 10-speed, for example, a set-up with 6 different ratios allows a straightforward shift pattern, and a set-up with 8 different ratios requires some more convoluted shifting. No one tries to use the other two combinations because they involve excessive chain deflection. Having unusabel/unused combinations is inherent in the design and is just accepted and lived with. This is just the nature of the beast.
The only way to get more usable “speeds” is to use more gears, typically with more unusable/unused combinations. So a 27-speed bike may only have, I’m guessing here, 15-18 different ratios. But if you want that many speeds, you can’t get them with an 18-speed bike.
For efficiency when travelling on level ground, you want lots of gears spaced closely together, so that you can fine tune the rate your legs are pedaling. For riding up and down hills, you want gears that are very far apart, so that you can shift down on an uphill and go very slowly, and shift up on a downhill and go very quickly.
These two goals lead to the kind of gearing we are talking about here. Back in the old days of 5 or 6 speed rear gear clusters, you would often have to shift up two gears in the back and down one gear in the front to get the next gear ratio. Now, with 9 or even 10 rear gear clusters, you can just shift on the rear cluster to get the next gear ratio.
On my 18 speed road bike, I typically use the lower 5 gears in the rear with the front small chainring, and the higher 7 rear gears with the front big chain ring. This means I have about 12 usable gear ratios, and these overlap some. It would be possible for me to design a 10 or 12 speed bike that could cover the same range with nicely spaced gears, but it would be harder for me to shift.
See here for a chart of gear sizes and some explanation of why it is helpful to have more gears: http://www.motorlessmotion.com/gearchart.html
27 speeds are common on mtn bikes, where you’re often climbing pretty steep hills. But Lance would need fewer gears than you or I, since he’s a lot fitter and could climb the same hills in a higher gear. If you think about it, the weaker you are the **more **gears you need.
And **Gary **is right. Look at the gear rations I have on my road bike:
Big Chain Ring (53/12-13-14-15-17-19-21-23-26): 4.42, 4.08, 3.76, 3.53, 3.12, 2.79, 2.52, 2.30, 2.04
Sm Chain Ring (39/12-13-14-15-17-19-21-23-26): 3.25, 3.00, 2.79, 2.60, 2.29, 2.05, 1.86, 1.70, 1.50
There’s a big overlap in gear rations between the two chainrings. Also, you wouldn’t use 53-26 or 39-12 due to chain stress anyway.
Generally, pros are going to want tightly spaced gear ratios over a restricted range, while slobs like us want a broad range of gears and don’t care too much if the gears are tightly spaced or not. You want to lowest gear you’ll need to get up the steepest hill you climb.
If all you do is ride on absolutely flat terrain, 3-5 speeds is enough.
Suppose I only ever ride on downhill terrain? Then one speed would suffice.
Thanks to everyone for the education.
I think that manufacturers are adding more rear gear cogs because they can. Improvements in metals and techniques mean you can squeeze more cogs into the same space, while still having a strong wheel and chain.
Also, more people are buying bikes that need gearing. When I got my first “real” 10-speed in 1978, most people weren’t using geared bikes for anything but tootling around town. The gearing made these bikes look like racing bikes, but they weren’t, really. I learned how to do gearing (and even calculate equivalent wheel sizes) through a wonderful magazine called “Bicycling”, which started out at about the same time.
Althought you might think that 27 gears is ridiculous, they help in doing high-level athletic cycling. You want to keep your pedal rate at about 90 rpm, usually. Studies show that this is the optimum rate for endurance and speed. To do so, you need to be able to adjust your gearing to the terrain. The more gears you have, the more likely you can pedal at 90 no matter where you are.
I now have what’s called a “triple”, which is a bicycle with the full 27 cogs, 3 in front and 9 in back. This allows me to drop to a tiny 30-24, which i can spin at 90 rpm while moving at about 3 mph! Very handy for the steep winding hills one finds in the SF Bay Area.
Some racers go with a 42-52 front and a 9-10-11-12-13-14-15 in the back, which gives them lots of speed at the cost of needing lots of power. They eschew a triple because of weight and lack of need.
Can you find a 3, or 5, or other speed? Well, you can get a normal 9-speed cog in back and have a single cog in front, if you want. I have never heard of a 3 or 5-speed rear cog, but I suppose it’s possible. I once read that the great Race Across America rider Lon Haldeman uses a 3-speed cog.
The old-style “in-the-hub” Sturmey-Archer gears seem to have disappeared, but they’ve been replace with fancy (and expensive) 7-speed in-hub gear systems. These require a special hub, and may weigh a bit more than rear derailleur plus cogs.
Manufacturers have searched for improvements on the derailleur and cog system, but none has yet matched its relative simplicity and light weight. \
Strangely enough, single-speed bikes are now the vogue! These have one rear and one front gear, and no shifting! Velodrome racing bikes have always been this way; in fact, they don’t even have a freewheel so you can’t coast! To distinguish non-freewheel single-gearing from freewheel, the former are now called “fixed gear” and the latter “single-speed”. Single-speed are obviously much easier to use.
The great people at Rivendell Bikes in Walnut Creek, CA are proponents of a simpler approach to bicycling; they have lots of the old-style stuff.
By the way, derailleur gearing was invented AFAIK for the Tour de France. Before derailleurs, racers either used single-speed or an arrangement with one gear size on one side of the rear wheel, and a different size on the other. To switch gears, you had to take off the wheel! The second gear was mostly for climbing. The first derailleurs used a rod-actuated system that you had to reach back to activate.
In comparison, my newest road bike has indexed shifting built into the brake levers. Without moving my hands, I can shift through all 27 gears. My cyclocomputer can even show me graphically which gear I’m in! :eek: