Proper gain ratio
 

Fellas,

I am building my first fixed/single and the decision for either seting depends on your advise. I live in a very hilly place and I am planning to set my bicycle either 5.5, 5.1 or 4.8 gain ratios. The 5.5 cames out of 42 - 15 fixed gear, the 5.1 from 42 - 16 single speed and finally, the 4.8 comes out of a 42 - 18 single speed. Now, I am not afraid of climbs, but I am afraid of descends and I think that a 5.5 gain ratio is going to make my life dificult on a descend because is too slow. On the other hand I can use the fixed configuration on flat places confortably. For the single speed configurations, I can always rest on descends and let the bicycle go. So, please guys, what is the best combination for my needs?

Thanks

work out the biggest gear you can get up your biggest hill with and work from there?

fsnl
sparky

ummmmmmmm... i think you're backwards on this. The higher the gain ratio, the slower the pedals will be spinning at a given speed.

i commute a very hilly 16 miles every day on a 5.5 gain ratio (49x18). getting my spin back up after the largest hills takes a minute. i don't remember the last time the pedals got ahead of me on a descent.

This is why freewheels and gears are so commonly used.

Yes you are right, but sers gave me the answer. What I thought is that the bicycle is going to "drive me" foward and not the other way arround. So if I have a fixed gear I have to slow the bicycle on descends if I have a small gain ratio. But, according sers, the 5.5 mark on the gain ratio is also enough for descends.

come on now...

I don't "think" in gain ratio, and prefer gear inches.

However, I will go to Sheldon Browns Gear Calculator and make a comparison:

http://sheldonbrown.com/gears/

I ride with a 53t chainring and with various cogs: 17t, 18t, and 19t.

53 X 17 = 81.9 gear inches, or a 6.1 gain ratio

53 X 18 = 77.4 gear inches and a 5.8 gain ratio

53 X 19 = 73.3 gear inches and a 5.5 gain ratio

53 X 20 = 69.6 gear inches and a 5.2 gain ratio

From my experience, in my riding environment, 73.3/5.5 gives the best all around uphill and downhill performance.
I really like the feeling of downhill control I have at 73.3/5.5.

However, I commute about 30 miles round trip per day, and I consider the extra speed I get at 77.4/5.8 a fair gain for the downhill control I loose.

I have ridden comfortably at 81.9/6.1, but I have almost no downhill control.
I went back to 77.4/5.8 NOT for the uphill ease but for the downhill control.

If I lived in an urban area with denser traffic and more hills, I would go with 69.6/5.2.
Regardless of ratio I have no difficulities going uphill.
The higher ratios affect the downhill portion much more than the uphill.

If I had to live with just one ratio for the rest of my life, I would probably go with 73.3/5.5 for the all around performance, and especially the downhill control.

Follow the link above.
Make sure to enter your correct tire and crank size.
Enter the chainring you expect to use (I have four different chain rings) and the various cogs above and below what you think you might use (I also have four different cogs, all EAI).

If you want to go fast downhill with a 5.5, pull up on the pedals with your feet and put more weight on the saddle, and your spin will increase dramatically with less or no "bobbing."
For myself, I find it more fun to go downhill in control than to go fast.
I save fast for uphill. :)

Thanks Cox


I am a men with a matematic background so I like to use tols that take into consideration the main elements in a system. The relation between wheel radius and crank length is very important, so I like the gain ration.

Cox, after your detailed explanations I am going to set the gain ratio of my bicycle on 5.5. Where I live I cannot ride flat ground, only up or down hill. If I go up when I leave home, I have a climb of 9 km with a slope of 2 digit degrees. In some bits of the climb the slope is pretty nasty. If I go down when I leave home, the slope is more friendly falling in the single digit degrees and stay like this for 20 km. So the 5.5 gain ratio is a good compromise

PS: the gain ratio is an index without units, so you use it in inches and I use it in meters ;) :D

Whatever gain ratio you choose..also keep in mind that fiding fixed on hills will make you stronger and will allow you to develop faster cadence abilities. So if you start out with something that seems too hard, it may seem like the right gear later. Or if you start out with something that feels just right, you may eventually want something higher.
I started riding fixed with a gain ratio of 5.1 and that seemed about right...but after awhile it just seemed too low and I moved up to a ratio of 5.7 which seems like the gear that I'll probably stick with.

Note: Cheap chainrings are easier to find than cheap fixed gear cogs.

With that said, big gears are better to have on descents. Now if all you did was descend and never had to climb it would be good to one big fixed gear. But then your ultimate destination would have to be hell, and that doesn't work for most people.

But what goes down must come up. Small gears on a descent aaaaaarrrrrrrrr aaaaaaa bbbbbiiiiittttt unnnnnnnerving. I tried going fast down a hill with 39-15 once and I'm not doing it again.

Brakes are good.

Gain ratios are really only a guide. 40-15 with 175mm cranks has the same gain ratio as 39-15 with 170mm cranks. But I don't think the ride is the same. The thing is that with shorter cranks, you can spin faster. But that doesn't mean you will spin faster.

For example here's my current setup: 40-15/16 fixed/freewheel with 175mm cranks. Sometimes, I feel like it's a bit too low. But get this. On my road bike my sweetest gear is 39-15 and on that bike I have 175mm cranks. That's a really low gain ratio.

Just start collecting chainrings and use what feels best. Then make yourself some windchimes out of all the old gears you have laying around.