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I need to replace the chainset on my commuter bike and I'm thinking of a change.

I've currently got Shimano: 28/38/48 front, 11-32 (8 cogs) back.

My daily cycle (north to central London and back) is 10km in a straight line each way. I never change out of top gear on the front (3) and really only use between 3/4 and 3/8. I generally cruise in 3/7. When the wind is with me (most days one direction) I'll be in 3/8 wanting a faster gear.

Whilst 99% of my cycling is this flat route, I don't want a bike that can't go up hills when I need to :)

According to the chap in the shop I have plenty of room for more cogs on the back.

He thinks I should go for one cog on the front and more range on the back.

To me, this doesn't seem sensible due to the angle of the chain. Is it not better to have more cogs at the front so when in top gear the chain runs in a straighter line? Because of this, I still feel I should have three on the front.

I've been loving this gear calculator: https://www.gear-calculator.com/?GR=DERS&KB=36,42,52&RZ=11,13,15,18,21,24,28,34,40,48&UF=2135&TF=90&SL=1.9&UN=KMH&DV=teeth

And based on this, I think I probably want 52 teeth at the front / 11 on the back which would give me an extra 10% top speed compared to now.

But the big front cogs (sorry, poor terminology throughout this post!) seem to mostly come in twos eg the Shimano 52/36.

And I being silly looking for three on the front? Should I just go for 52/36 front with 11-34 back?

Thought greatly appreciated!

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  • 5
    Typically, in the front, we say chainring. Cog typically refers to the rear. We do know what you mean, though.
    – Weiwen Ng
    Commented May 6 at 18:49
  • @WeiwenNg Sprocket. Cogs interact without chain. Cogs inside Sturmey.
    – Noise
    Commented May 6 at 21:58
  • 3
    Thanks, some people do say sprocket. Plenty of people say cogs, however.
    – Weiwen Ng
    Commented May 6 at 22:22
  • Shuriken perhaps?
    – Jamie G
    Commented May 7 at 10:08
  • Hi Jamie, If you are going to replace it with a compact together with new crank sets, make that the crank uses the same size standard as your current BB spindle, otherwise you may have issues with the chain line.
    – MindDBike
    Commented May 8 at 10:05

2 Answers 2

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There is quite alot to unpack here.

Single chainring (The cog at the front), (Often called 1x, meaning "1 times" rear cogs vs 2x or 3x) has been around for a long time in the MTB world and taken over, but the user benefits are largely limited to novices and riders who struggle with front and rear gears. The push for 1x came from frame designers and reduces assembly costs. Net is it is an advancement over 3x and 2x, but on an existing bike, not so much. I would not, especially if your goal is higher gearing for flat roads.

Another thing to consider is we get a lot of questions on this site asking how to get taller gears so the person can go faster. Unless you are pedaling at a cadence of 90-100rpm, the cheapest and most efficient way to go faster is to learn to pedal faster.

Next consideration is cost. Usually, it is not cost effective to make big changes to a bike that is not a newish bike of high quality. Given your bike is 8 speed, it is probably quite old. You may be better to consider selling it and using the funds from the sale and what you would need to spend to buy another bike

If you are going to go ahead and make changes to the bike, it does get a little complicated. There are limits on the maximum size chainring the front derailleur can cope with, as well as the maximum the read derailer will cope. You can usually get away with exceeding these a little (especially with Shimano who are quite conservative). You need to ensure the larger chain ring fits the frame without rubbing.

If all that can be made to work, a 54/36 front would be a good choice. For the cassette much would depend on the state of the existing rear cassette, if it is worn and need changing, then I might consider 11-34, but you could go to 52/34 front and get a similar low gear

Edit: In response to the comment asking for clarification on chain angles, chain angles are not a problem. Some people believed chains angles were a problem back in the day, and a few still cling to past beliefs. Technology has moved on, chains are more supple, lubes much better, the increase wear of 'cross chaining' might be measurable in a lab under ideal conditions, but where the rubber meets road, the additional wear is marginal. Chains are a consumable, they wear out, and are more likely to wear form poor lube or lack of cleaning than cross chaining.

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  • Subject to frame clearance, there a cranksets like 30/39/50, which would help a little
    – Chris H
    Commented May 7 at 9:44
  • Thanks @mattnz really appreciate the reply. I'm going for "I'm going to fix my bike, might as well aim to get it a touch closer to how I want it". It is indeed an old bike: v brakes, child seat, falling apart saddle... the list goes on, but... all the more fun for overtaking shiny lycra clad cyclists with their clip-in peddles and clickity-clickity cassettes :) And yeah, I enjoy cycling more when peddling slowly but going quickly, so it's enjoyment over logic I guess.
    – Jamie G
    Commented May 7 at 10:07
  • I think I probably do need to replace the rear cogs due to wear. I will check on the clearance I've got for a bigger chainring - good thought. I wasn't totally clear on your 1x vs 2x vs 3x. I think you were saying that I should not go 1x, but your recommended 2x. Is that just because 3x is harder to come by, or was there other reasoning too?
    – Jamie G
    Commented May 7 at 10:07
  • How important is cost? You may be able to get a 52 tooth in a 104BCD chainring that would bolt onto you existing crank (is it 4 or 5 bolt). 3x on speed is probably better as going 2x you probably get big steps between gears to too small total range. In 10speed+ world, go 2x.
    – mattnz
    Commented May 7 at 21:04
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    No, I am not against 1x at all. I am against people saying that old bikes benefit from an upgrade to 1x without qualification. Specifically, I will ask "what problem does the OP have that 1x solves? "
    – mattnz
    Commented May 8 at 0:34
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With many, but far from all, cranksets you change change individual chainrings, keeping your triple. Fitting a bigger one to replace the 48, the front derailleur will have to move up a little.

So to figure out your options, you first have 2 things to check:

  • Are the chainrings bolted together or riveted? Chainring bolts don't look like bolts at first glance.
  • How is the front derailleur fitted? Many clamp on with a band, some are brazed on and can't easily move. If band-on, check that there's nothing in the way. I wanted to change mine to a different model for greater tyre clearance, but would have have to file off a bottle cage boss.

If the chainrings are riveted, you need to change the whole crankset, and if doing that, you need to make sure the new larger inner chainring(s) don't touch the frame. However you could go for a double crankset or even a triple but remove the inner ring. Just be sure to set your low limit screw so you don't accidentally drop the chain.

If they're bolted, you can replace just the outermost, being sure to match the number of bolts and their BCD (bolt circle diameter, AKA PCD for pitch circle diameter) and re-position the FD. Just going from 48 to 50 you might get away with the FD position, going to 52 it's unlikely.

Now I'm a fairly low cadence rider myself, so I'm hesitant to say this, but at a typical 80rpm, your numbers imply that you're not dropping below 30km/h and reaching 45 km/h. I haven't ridden much in London (and I'm normally 200km into the ride when I do) but cars can't generally go that fast at typical commuting times so you must be flying past them. If you're not, that suggests a very low cadence and you might work on spinning faster.

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  • Thanks Chris, just had a proper look. I've got this: ebay.co.uk/itm/334156443328 Seem like bolts to me, but also doesn't seem like something that expects to be upgraded! I'm up for it if you think it'd work though. I'm unsure how to know what would be compatible though. The derailleur is a clamp - looks easy to move.
    – Jamie G
    Commented May 9 at 22:10
  • PS. It's bus lane all the way, so very start/stop due to traffic lights, but easy to get up to speed with higher cadence, then cruise contentedly at 35-40 km/h in top gear for a minute until the next lights. Repeat.
    – Jamie G
    Commented May 9 at 22:14
  • You might want to have a look at WolfTooth's BCD guide and compare to what you've got
    – Chris H
    Commented May 10 at 7:32
  • Great, will measure! Thanks
    – Jamie G
    Commented May 11 at 10:32

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