# Touring - what is the difference between gear inches and a gear step?

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fatigoworld
02-24-08, 12:38 AM
can someone please explain the difference? i see that they both relate to the changes between gears but when i use the calculator they are not equal...

example:

42 front ring

14 to 16 = 10.1 gear inches , gear step 14.3%

25 to 32 = 10.1 gear inches, gear step 28.0%

so the gear inches between these gears are equal and the gear steps are very different. which of the 2 translates into actual feeling of difference in your legs?

SweetLou
02-24-08, 01:43 AM
The gear inches you have listed is the gain in gear inches from one cog to the next. This means that for every revolution of the crank, you will go an extra (or fewer depending on which way you are shifting) 10.1 inches than the previous cog.

The gear step is the percentage of the increase over the the lower. So, 32/25=1.28. The 32 tooth cog is 128% of of the 25 tooth cog. Or put another way it is a 28% increase.

fatigoworld
02-24-08, 01:56 AM
ok, so i see how the numbers are achieved. but im not sure i understand fully, which one will give me a more accurate reading of how this will translate to the actual feeling of the gear shift.

basically which one should i pay attention to if i want to space my gears out evenly so i would feel the same exact difference in every shift?

example.

all of the gears 13 inches apart.

all of the gears 15% apart.

which one would be more accurate to achieve even shifting?

fatigoworld
02-24-08, 02:03 AM
ok so i understand now how the numbers are achieved but im not sure i understand how it translates to exactly what differences i would feel in each shift.

lets say i wanted all of my gears spaced out evenly with the same exact difference in gear change between each shift...

which one would i choose?

1. 13 gear inches between every gear

2. 15% gear step between every gear

this would be in relation to the feeling in my legs only and not to shifting function.

TheBrick
02-24-08, 06:19 AM
IF you go with gear inches you will be having to increase the force by a constant amount for each gear change to go the same speed.

If you go for the gear step you will have to increase the force by the % of the gear step each time.

So if we take the lower end of the ratios with say 20 g.i as our lowest gear (I am just using nice numbers here) adding 13 g.i is clearly going to be 33 g.i adding 15% is going to be 23 gi.

Take is at hte other end and we shall have say 110 gi, adding 13 g.i will step you up to 123 g.i adding 15% will take you up to 126.5 gi. So a larger increase.

I think 13 g.i step soulds too big anywhere in a range and a 15% increase near the top is too much. I don't think you can say use this or that method. At diffrent points in your range you will want tighter and narrower diffrences between your gears.

Go play with Sheldons gear calulator.

Personally I think you are over thinking this. As long as you have a low gear you are happy with and a gear up top that you will not spinn out on too easly on a medium down hill there will be some in the middle to suite you for everything else. You will never beable to have a purfect ratio for every situation. It is possible to spend too much time swapping between gears trying to find the ratio wasting energy changing gear all the time than actually getting on with the job of pedalling.

rm -rf
02-24-08, 09:36 AM
You want to use percentage changes between gear inches, not gear inches directly.

Gear inches is related to the old high wheel bikes. The biggest wheel would be hardest
to pedal and would go fastest. So gear inches converts the size of your rear wheel multiplied by the ratio of the front chainring and rear cog into an equivalent size high wheel. (They were typically 45-60 inches in diameter-limited by leg length). It's used now to make a quick comparison between different combinations of gears; 50-20 is very similar to a 34-14 for instance. Adding 10 gear inches to a 30 inch would be a big jump, adding 10 to a 100 inch would be much less of a change.

I like to use Sheldon's gear calculator (http://www.sheldonbrown.com/gears/) and set it to mph at 80 rpm or 90 rpm. Then I can plug in different cog sizes and see how much the mph changes. Or I use the percentage change from one cog to the next. I like around 6% to 7% changes from one gear to the next, to get the pedaling effort exactly right. But, you might want a really large range, so you will have to have larger percentage gaps.

I currently have 50-34 chainrings with a 13 14 15 16 17 18 19 21 23 26 cassette. I lose the top end gear 50-12 (which is 29.3 mph at 90 rpm), but add the 18 cog. This way, at 90 rpm, 50-19 is 18.5 mph, 50-18 is 19.5 mph, and 50-17 is 20.7 mph. I'm in this 18-20 mph speed range a lot, so I can get exactly the right gear here. I've spun up the 50-13 to 34 mph on a downhill (about 115 rpm), any faster than that and I'm coasting anyway.

SweetLou
02-24-08, 09:50 AM
One other thing you didn't mention is air resistance. This will have a huge factor in determining the "feel" of how hard you need to pedal. Sheldon (pbuh) has an article about this that I recently read. Back in the day of 5 speeds, people thought that an 11 tooth cog was too small because they couldn't use it. The reason why was because they couldn't get enough speed in the 14 tooth cog to make the jump. Now that we have more gears, we can more easily build speed to get to the small cog.

Air resistance is is a cubed factor, meaning if you double your speed, the force needed to overcome air resistance will be 8 times as much. This is basically the reason why most cassettes have closer ranges in the small cogs and bigger jumps in the larger cogs. Something like: 11,12, 13, 15, 17, 19, 22, 25, 28,32. Since you will be going much slower in the 32 tooth cog, the larger jump to the 28 will "feel" like the smaller jump from 17 to 15. I didn't do the math to see if it is accurate, but the idea is there.

In conclusion, you might not want to evenly space out the gears, but to get ones that work for you.

fatigoworld
02-24-08, 10:47 AM
hey, thanks for all the help. the reason i am over thinking this is because i am running a single chainring front. i am going to run 33 to 102 inches as a range but am going to be picky about which gears i have inbetween. so from what you guys said, i most likely will choose these by percentage with the percentages being larger on the larger rings and then try it out, see how it feels...

Speedo
02-24-08, 11:31 AM
I've seen some cassettes that have moderate spacing between the cogs except for the largest cog, which is really big. I guess the idea is that you get a fine granularity in the gears that you commonly ride, and a really low low to get you over big humps.

Speedo

SweetLou
02-24-08, 12:34 PM
Do you have access to other bikes? If you do, you can figure out which 7 gears get you the range you want and test it in the area you plan on riding the bike. This could help you narrow down the choices. It won't be perfect unless the bikes are identical, but I think it would give you a good starting point.

fatigoworld
02-24-08, 12:49 PM
Do you have access to other bikes? If you do, you can figure out which 7 gears get you the range you want and test it in the area you plan on riding the bike. This could help you narrow down the choices. It won't be perfect unless the bikes are identical, but I think it would give you a good starting point.

yeah, ill try that out...

NoReg
02-24-08, 03:40 PM
Two gears with radically different lowest gears can have the same percent change between gears. So for instance, a lot of tourist like a low gear like 20 inches for the worst hills. A 13% increase would make the next gear up 22.6 inches. If your lowest gear were 40 inches a 13% increase would take you to`45.2. The latter bike would be twice as fast in it's lowest gears at a constant pedal rotation, obviously a huge difference in gearing you need a way of expressing. You basically couldn't tell the whether a bike had low gears or high gears just by studying the

On the other hand you could have two bikes with a lowest gear of 20 inches but one is a 24 speed and increases only 13% to 22.6 in it's next lowest gear. The other might be a 5 speed and increase 26% to 25.2. With all the spacings the same, it will be a lot harder to find confortable gears that aren't too fast or slow with the 5 speed than the 24, and that is what the percent increase indicates. In the real world you can play a lot with gear percent increases by changing sprockets, while something like a Rohloff has fixed intervals that may or may not suit you. Most tourists are only concerend with the broadest range from top to bottom so they don't get caught out. But if you know the situation well enough you can tighten the percent spacings and gain efficiency in certain circumstances.

Gear inches is a measure of the distance a bike would go for each rotation, except that it does not multiply out the Pi. So a 30 gear inch gear is like a 30 inch diameter wheel in direct drive. To find out the distance travelled per rotation you have to multiply by 3.14. But I guess wiser heads prevailed and realized nothing much was served by the extra computation and the larger number to toss around. These days with computers we can have it.

Gear inches do net out between different bike wheel diameters. In other words, a 20 inch gear is the same mathematically whether the bike has a 20", 26", 27", etc... drive wheel. You might notice differences in resistance based on the characteristics of the various wheels, but they nominally will cover the same distance if the gearing has the same gear inch number, per pedal rotation. And that is where a gear ratio comes into it. It takes a higher gear ratio to create the same gear inch number for a smaller wheel when compared to the same gear inch set-up on a larger wheel. Though personally I would only worry about gear ratio if stupid part combinations where starting to rear their heads.