Hey guys i'm running a 39/15 on a 172.5 campy crank, what gear ratio can i change to so i can go faster. if i change to a 49/18, would i be able to go faster, skid and would it be easier to pedal? Would a 49 it on my crank? What happens if you have longer chain inches? i'm having a little trouble finding the right gear ratio for myself, and understanding the chain inches and what it does. thank for any input.

Chain inches? Do you mean Gear Inches? Look at the stickies for "Gearing". Punch some numbers and go from there.

48/17(170cranks/27wheel)~~76.2 Gear inches

Use Sheldon Brown's calculator at https://www.sheldonbrown.com/gears/. I put in your current gear set and it says 20.9 mph at 100 rpm.

By the way, crank length has no affect on the speed calculation.

Bob

it doesnt change distance travelled per rotation of the cranks, but it does affect the rate and comfort of spin and is also a factor in pedal strike, especially on fixed conversions.

RABBIT...Super awesome gear calculator

I ride @ about 70 inches here in NewYork, and its a little slow, but I can really stop.

I would get rid of the 39 chainring, it'll be difficult to go much faster with it. Get at least a 42 or even better, a 47 or 49. Odd numbers get you more skid patches, generally.

I ride a 42-15 mostly. It's high enough for the flats but I can still climb the many hills around here. If you have fewer hills or they aren't too steep, go higher.

Changing cogs is cheaper and easier than changing chainrings and you the difference is more dramatic. Even a one tooth difference is quite noticeable on the cog.

Higher gearing is more difficult to skid with but I recommend having brakes on the bike anyways. You don't have to use them but they are awful nice just in case.

Sheldonbrown.com Read all of it. He's got big sections devoted to singlespeed and fixed gear and explains gear inches, gain ratios and just about everything you need to know.

It's not odd numbers you want, its prime numbers. For once all that stupid elementary school math comes in useful.

The gear you're running now will likely be the easiest to start off with and skid with, but not to go fast - there's a compromise you'll have to make.

I'd go ahead and do as said above and swap your chainring to a 42, or a 44/46 kind of range, that way you leave yourself open to a nice range of ratios by only needing to change the back cog in the future.

I run 46:15 on 23c tyres with 170 cranks and it's a pretty tall gear (80GI?), I started with a 17 on the back which was nice - I was riding 52:19 on my roadbike for a long time, which was around the same range. Use the gear calculators and figure out what you want to run, I guess. I'd go with 60-65GI, or if you're feeling adventurous 65-70GI to start off with, ride it for a bit while you get comfortable with the resistance/skid/skipping on that gear, and then see how you feel.

Agreed. I was just saying that you plug any crank length you wish into sheldon's calculator and it will not change the speed calculation if that's the output format you choose.

Bob

pedal faster.

troll?

no need to get a prime number chainring if you want skid patches, find the one that works for you, then spin the wheel around one or two links every once in a while.

Newbie.

39/15 = 2.6
48/18 = 2.666

2.6666/2.6 = 1.025, 2.5% faster
So if you're going 25kph at a good cadence now you'll only be going 25.6 with the new gearing.

43/15 will put you at 2.86666 which is a 10% improvement.

This is assuming you are spinning out your gear. If you're not then you need to find some hills to make your legs bigger.

learn how to spin faster
I need a higher gear too (SS), but since I'm waiting for money for a fixed gear hub I'm doing everything with 44x18. I used to ride 46x18 on my previous bicycle.
Only had this one for about a month, but today I went 40km/h on the flat, which is a 130rpm, where I used to spin out at 30km/h.

Okay.. what range of Gear Inches would be best for fast acceleration? I was thinking a 1:3 (48/16) ratio which is rated as 81 gear inches with a 27" diameter drive wheel would be pretty good.. no? That's high and it seems like people advise against being that high.

Can someone explain how it works? Higher Gear inches vs. Lower gear inches? I'm looking for acceleration, not necissarily top speed. Also if you can explain the difficulty of pedaling it will be with it also?

My impression on gearing:
if you have a 1:3 ratio 16t cog/48t chainring you pedal one revolution the cog rotates 3 times, thus the drivewheel rotates 3 times. So, if you had a 1:4 or 1:5 etc, one rotation = 4, 5 revolutions respectively. I would imagine that being harder to start from stop. Am I right? Also, I would imagine your top speed would be reduced...wait, would it be increased? If you have a 1:1 you would have to pedal once to move one revolution (not as far as 1:3) so, top speed would be greatly reduced and it would be easy to start pedaling. I dont know what that would do to acceleration though.

I live in orange county/ huntington beach california not many hills, mostly flat land etc. I want to be able to accelerate, skid, stop etc..

Can someone please explain this stuff to me!?

Thanks,
Ericson Calderon

if you want acceleration you need lower gear ratios

but what a low gear ratio is is very personal... I think most utility bicycles here have 46/18, so that's pretty safe
but if it's all flats 48/18 would still be okay

if you ride a lot those gearings will still be a bit too low

you have to be super strong to accelerate from a stop with 80 g/i. 65 g/i is more normal for super quick acceleration, 60 if you're weak.

70-75 will get you a higher speed at a given rpm. i'd suggest 60 if you're weak and 65-70 if you're in shape or strong.

this.

My legs are strong. So, higher gear inches = faster acceleration?


and a lower gear ratio means a smaller cog with a bigger crank or chain ring, right? People say "Three to one" when it's actually 1:3. So smaller gear ratio to a "three to one" would be a "four to one" 1:4, right? Because if you went by an actual 3:1 a 2:1 ratio would be smaller. Does anyone get this confused over ratios? Ugh. haha.

what speed are you accelerating to and from? lower gear inches is faster acceleration always- until you can't spin any faster.

no. the ratio doesn't care how big which one is, it cares about the difference between them. it's not 1:4 it's 1/4. for lower gear inches you make the chainring SMALLER or the cog larger. more gear inches is the opposite.

1:4 = 1/4 fyi.

Oh my god. Everyone is way over complicating this. Figure out your ratios. If you can't do that, you failed 8th grade math.

I mean, it's like a geared bike. When you make the chain go onto a smaller gear in the back, it gets harder to pedal but you can go faster. If you make it go to a bigger gear, it gets easier to pedal, but you can't go as fast. ugh.

Not everyone here has an 8th grade math level like you, buddy.

Speed and acceleration are two different things.

I used to run a 46x18 which is close to you 39x15. I would top out too easily and bounce around in my saddle with the 46x18 setup. Now I can go a little bit faster and skid/stop still without trying too hard with the 48x17.
Changing a cog is a lot easier and less expensive then changing chainrings. Try a 14t cog, maybe you will like that a little better.