hansel

building first fixie track bike. i have the choice of a 28,38,48,52t main gear. and im assuming you can take any gear from the rear cluster and take it off? if that is true than i have alot of choices for that. what cog gear will fit each of those main gear sizes? what is the most used? witch is the fastest?

devilshaircut

Gearing on bikes is just like gearing in cars. Lower gears = better acceleration, more RPM, lower top speed; higher gears = poorer acceleration, fewer RPM, higher top speed.

On a fixed gear bike, there are 2 gears which determine your gear ratio (as above).

Your rear wheel has a cog (held on with a lockring). Your cranks have a chainring (bolted to the cranks, typically).

For riding in the streets, you want a middling gear ratio. Something low enough that you can climb hills without mashing but high enough that you can speed through traffic without spinning out. An example of that would be something like 46:16. That is, you would have a chainring with a 46 tooth count, and a cog with a 16 tooth count. These ratios can be expressed as gear inches (different ratios can still yield the same performance). 46:16 will give you 77 gear inches which is normal for the streets.

steveymcdubs

or be insane like devils and have a 973927973/1 ratio

devilshaircut

Indeed, I rode that ratio 60 miles yesterday.

hansel

yeah i get you. i was thinking about like a 48:16
what is the normal size for full blown track bikes? im assuming a very small cog?

what size are you running devils?

devilshaircut

48:16 might not be ideal if you plan to skid. 48:16 has 1 skid patch, meaning your rear tire may wear down fast.

I run 49:14, but it has been well-established in this forum that this ratio is not advisable. IMO you can take the following advice though: 60-70 gear inches is typically considered the low (easy to do skids or tricks); 70-80 is considered middling (bar crawling or group rides); 80-90 is considered high (track racing). It is possible, I suppose, to do anything with any of those ratios. But that is a general guideline of what is typically considered reasonable.

EDIT: To answer your question about track bikes, my instructor at the track advised 84-86 GI for the track. I've also been told by people there that something like 88 GI would be considered a sprinter's gear and 84 GI would be considered an endurance gear. 78 GI might be something like a warmup gear.

hansel

im on that gear calculator with the skid pad converter. there are no hills when i do ride it around town. but i do plan to track. i was going to throw a front clamp break on it a put a flip flop rear. than just flip it when i go to the track. im pretty sure i can keep up a good tempo/speed. this may sound ******** but can any size main, and cog mesh? i think i can go something bigger than that.

devilshaircut

If you look at a table like this ...



... you can see that any of the chainring tooth counts in the center will be the most flexible in terms of attainable gear ratios. For example, if you get a 46t chainring, you can get a 14t cog for 88 GI which would be considered relatively high, or you could get a 17t cog for 73 GI which would be considered relatively low. These cogs in these sizes are likely widely available at your LBS ... and in several other sizes no doubt. My LBS (that does not specialize in fixed gears) sells cogs anywhere from 13t to 22t as far as I know, so you should have no trouble finding a cog for a 46t chainring. Any chainring with roughly the same number of teeth will be equally flexible in terms of gear ratio. Some people like a 47t chainring because it is a prime number, making it ideal for skidding.

hansel

i see thanks alot for he help!

adriano 

id suggest erring on the side of low gearing, a larger cog or smaller chain ring.

dsh

I agree with Adriano. I started off with a 16T rear cog and had a choice between 52T and 40T chainring (as these were the two available on the 10-speed I was converting).

I tried the 52-16 first, 88GI, and while I could get around and conquer most hills, I was tiring out really quickly and had quite a hard time stopping.
I swapped the 52T for the 40T and have been cruising along a lot more easily on the 67.5GI. These are kinda opposite ends of the spectrum, and I spin out on even mild (sustained) downhills with the 67.5, but I find it's much more manageable overall, and I keep a much better tempo when cruising on flats and rolling hills.

I think a low-to-mid-70s would probably be perfect, and that's what I'm aiming for when I get a new cog - maybe a 40-14, or something similar.

icelemmings

I'm running a 40-15 right now. It's nice not having to mash down as much, though, I'm just using this to commute.

This has probably been discussed to death: what is the benefit of riding either 32-12 or 40-15 (the GI are the same for both options). I'll take my road bike out tomorrow evening and compare two similar GI ratios, but for now, any quick answers?

Johnny Nemo

Smaller chain ring and bigger cog. Stay around <70 for a good all around gear. And, the gears aren't fast, the rider is.

dsh

I would guess - and just common sense speaking - that more teeth (40-15 over 32-12) distributes wear more, and so will last a bit longer.

Also common sense, the difference in lifespan will be pretty negligible.

Conversely, smaller cogs/chainrings will be a couple grams lighter. Kinda reaching there, but yeah.

I'm not sure if there are more inherent efficiency gains between the two.

Sinsemilla

I am doing a 46 / 14 and its a kick in the saddle but fun and fast!

bratton

go straight for 53-11. You'll never regret it.

hansel

bratton, is that a serious statement? it seems everybody is running such lower gearing than that. i want to get a gear thats good for track, and will be somewhat competitive(im 18) and something to travel around town, but fast. so i guess you can say im looking for a fast gearing. there are no hills on the velo/town, so i think those wont really be a problem. and if i do take it on a path i think it will be a crash course on a great leg workout. im running strong now, and training for a 1/2 iron man. but want something different and i think a track bike is what i need,

what about like a 48:15

glacialreign

seriously... experiment yourself. start at around 70ish gear inches and work out if you a lower or higher gear from there. i don't think anyone on the internet can pick one for you.

devilshaircut

I hope your FG is for training and not for using in the half iron man.

Anyhow, you can pretty much err either way on either the cog or the chainring. As you can tell in the table I posted above, each component has such wide tooth count availability that with the right complementing component, each has a wide array of available gear inches.

hansel

i think my legs would literally fall off, and my knee caps would explode if i used that =) yeah, im going to set up my road bike and test some gears out. ill let you guys know the results.

j3ffr3y

If I were you, I'd start out ~70gi and then see if you are spinning out or want a higher cadence. I ride 48:18 which is 72gi and I find that perfect for me.

hansel

running a 48:18 wouldn't that give you a very short skid?

muckymucky

48x16 or 48x17 (if you skid)

devilshaircut

Gear ratio does not relate to skid length. Skid length is determined by how much you unweight your back wheel. Gear ratio only relates to how easy it is to initiate a skid. The lower the gear, the easier it will be to resist the rotation of the cranks.

Triple8Sol

On my first conversion, I started with 42:18. It was great for hills and learning to skid. Also for practicing getting in/out of the toeclips. Definitely sucked trying to keep up with friends though, so I went to 46:18, which I've stuck with on my new build.