3

I have a 52/15 gear ratio and I find it sort of difficult to pedal.

I'm wondering what I'd need to switch the chainring/cog sizes to that would allow for me to not remove any chain links.

I'm assuming I'd just make up for the difference, for example: if I switched the chainring to 48 the cog would need to be 19 in order to match??

I don't want a drastic difference because I like the control I have when going down hills.

With the current setup the wheel sits in the drops, with the chain is completely tight, and leaves about 5/8" of room I could move the wheel back to (if I were to change the sizes and needed to use the extra real estate)

The area I ride is mostly flat with some moderate hills (none i'd consider steep) I have a 40 tooth chainring lying around that I could use if that'd be a good fit.

I'm new to swapping out ratios and I'm going to order the parts online, so i want to make sure it works.

  • 3
    Could you expand on the reason for not changing the chain's length ? Have you had a bad experience or do you not have access to the right tools ? – Criggie Mar 11 at 10:44
  • 1
    Also - there's an answer which would only work for a singlespeed. Can you please confirm if this bike is fixed gear (where cranks spin with the wheel at all times) or singlespeed (ie you can coast without pedalling) – Criggie Mar 12 at 10:18
  • There are calculators that will help you find "magic" ratios that work for fixed gear conversion bikes with limited options for tensioning the chain. eehouse.org/fixin/fixmeup.php is a good one. – Fred the Magic Wonder Dog Mar 16 at 17:20
6

Different from the other answer, I read the question as OP wanting to change to lower gear ratio without changing chain length. 52/15 is an extremely high ratio.

For all practical purposes, if you want to keep the chain length same, the sum of numbers of teeth in cog and ring should stay the same.

The changing geometry does affect the result a bit, but the difference it will make is less than a half link's length. The exact math is a bit hairy, since you have to take into account that the part of chain that wraps around cog is not actually part of circle, but N-gon that has its angles at chain pins.

My recommendation would be to bite the bullet and get a chain tool and not try to optimize cog sizes. You will need one anyway later, and if the old cogs and chain were worn, it's best to replace the chain at the same time. A normal sized chainring will give you more options to play with ratios later, and prime numbers 41, 43 and 47 would let you use any cog without thinking about skid patches.

With lower ratio you might need to take the skid points into account.

| improve this answer | |
  • Half a link's length divided by two should be easily compensated by the available movement range in the rear fork end, unless it's close to either extremum already. – Kaz Mar 11 at 12:24
  • Thank you for pointing out that. The question mentions "about 5/8" of room" – ojs Mar 11 at 13:12
3

why dont you want to change the chain? you need to get used to fact, that each change of cog is also a change of chain/chain length.

just order 18 or 19 tooth cog and new chain and you will end with 52/18=2,9 or 52/19=2,7 much more comfortable ratio for day to day travels

just for comparision, 3,5 ratio is extremly hard/heavy ratio. even not all track cyclist use such ratio during competitions. with that ratio you can easly go 60km/h

| improve this answer | |
1

Yes, to the first approximation you need to keep the total number of teeth the same in order to use the same chain on the same base.

Think of it this way: to keep the same distance between the crankshaft (bottom bracket) and the rear axle, by definition you need the same number of 'free' (disengaged) links; or more accurately, the same number of links between the vertical planes containing these axes. (There will be a minor difference due to different slope of the chain, and in your case we can expect the base to stretch a tiny bit). Then, for the same chain the same number of links must be 'engaged', i.e. be on the front half of the chainring plus on the rear half of the sprocket. This will hold when the sum of the teeth is the same.

Now with respect to the actual numbers, I don't have much experience with the true fixed gears, but I think 52/15 is a very hard ratio (assuming a road wheel size). For me, this is in the range of 40+ km/h. 48/19 looks more reasonable for a city, but it may be quite a significant change for you. On this basis, your existing 40 is almost out of question. However, as already mentioned, for the fixed gear you also need to take skid points into account when choosing the ratio.

| improve this answer | |
1

I do a lot of single speed and fixed gear cycling, and if the geography in your area is relatively flat, that 40 t ring you have might be perfect with a cog between 15 and 18 t, depending on how strong your riding is or how much potential speed you might want to achieve.

If you plan on a long ride or maintaining a particular average speed, your ratio will be dictated in part by your ideal cadence. Its good to keep your ring as small as possible and work with the cog size as a variable. I also have a 40 t ring, and with that ring I swap between a 16, 17 and 18 t cog depending on how much climbing I foresee in my ride and/or how fast I am planning on riding, and I try to keep my cadence on flats at around 80 rpm which for me is a comfy cadence.

Consequently, I have three different chains for each combo, but its worth it because I use each one about a third of the time and they all end up lasting three times longer before I replace them.

You don't need to get too caught up in the cadence, but I recommend swapping the 52 t ring with the 40, and cut the chain to fit. If that 15 t cog is too big or small, step to a cog 1 tooth bigger, or 1 tooth smaller, and cut a new chain to fit if the original chain ends up being too small.

If you start with the 15 t and you suspect that you may end up wanting a smaller cog like a 14 t, scoot the wheel as close as you can to the inside of the fork end when you fit the chain, and you will likely have enough room in the fork end to scoot the wheel back a smidge when you fit the 14 tooth, and you will probably be able to use that one chain with both the 40/15 and 40/14 sets.

The only limitation there is if you have a rear rim brake it might require adjustment whenever you swap cogs. I hope that all made sense.

| improve this answer | |
  • 2
    consider an edit to introduce paragraphs to this answer. It would be much more readable if the wall of text was separated into sections – Swifty Mar 12 at 8:05
0

52:15 on any size of wheel is the same as 3.47 ratio, meaning 3.47 wheel rotations per rotation of the crank.

To achieve the same ratio with a 48 tooth chainring, a 14 tooth cog gives you 3.43 so near-enough the same. If you don't want to remove 1 or 2 full chain links, then the axle will move back around 2 inches, or 1 inch. Given that you require space behind the wheel nuts for chain tensioning, this is not possible.

To maintain the ratios, if the chainring shrinks then the cog has to shrink too. If you went 48:19 as per your suggestion, that's a ratio of 2.53, so would be too "easy" compared to what you're used to. 40:19 would be 2.11, which would be easier again (with an even lower cap on top speed)

52:15 combination gives you 15 skid patches, which is good. Going to 48:14 is only 7 skid patches, which is marginal. Swapping in your 40 tooth chainring with the 16 tooth cog gives 2 skid patches, which is awful.

I can't see any gains or improvements from your change, other than a very slightly lighter bike from a smaller chainring, at the cost of fewer cog teeth engaged in the chain.

Have an explore around the calculator at https://www.bikecalc.com/skid_patch_calculator and https://www.bikecalc.com/gear_ratios before you start disassembling your bike.

| improve this answer | |
  • 3
    The question is not about keeping the ratio: on the contrary, the OP wants to reduce the ratio, but keep the same chain. The mathematical problem is whether having the same total number of teeth is sufficient for it. – Zeus Mar 11 at 8:07
  • @zeus fair call - I am reading "I don't want a drastic difference because I like the control I have when going down hills" to mean keep the overall ratio the same. Could be I'm missing something - what intent do you read from that sentence ? Is OP talking solely about the chain's overall length? – Criggie Mar 11 at 10:42
  • 2
    I read it as if they didn't want to change to 40:27 which would be quite extreme in other direction. – ojs Mar 11 at 12:19
  • 1
    To somewhat offset the lighter bike, a smaller chainring and cog will produce slightly greater drivetrain resistance. Admittedly, that difference is insignificant to most riders, unless they're attempting the hour record. However, the same could be said of the weight difference. (This doesn't really address the OP's concern, I just couldn't help myself.) – Weiwen Ng Mar 11 at 19:28
-1

There are also an array of chain tensioners out there, if you really don't want to cut the chain.

| improve this answer | |
  • 1
    Would you care to explain why chain tensioner works with fixed gear? – ojs Mar 12 at 8:54
  • Yes, but the question is specific to fixed gear, and chain tensioners do not allow pedal braking, because there cannot be a lot of slack in a fixed gear transmission. – Criggie Mar 12 at 8:55
  • Oh, good point. I haven't ever used a tensioner, I wasn't thinking when I posted that answer, but it's obvious that wouldn't work unless he uses a freewheel. Sorry. – bradly Mar 15 at 15:41

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.