Author Topic: Gear choice for fixies  (Read 3121 times)

Gear choice for fixies
« on: August 18, 2018, 08:44:15 pm »
One question that is often asked on forums by those new to fixed is "what chainring and sprocket should I choose?".  On low-budget fixie conversions, you'll probably be re-using a chainring (since they're more expensive than sprockets of the same quality) so this simplifies things a little.

A good starting point for road use, in terms of gear inches, is 70".  This equates to 48 x 18 (most track chainsets have 48T as default), 42 x 16, 52 x 20 or 53 x 20 on a 700c wheel.  These feel subtly different; the bigger chainrings are smoother and less sensitive to your chainring being not-quite-circular, but there is a small weight penalty, mainly from the additional chain links.  They all work well enough.

70" may seem quite high if you normally spin derailleur gears at 100rpm or more, and you won't be able to really spin this gear on the flat unless you are very powerful.  However, the important thing with a fixie is to choose a gear such that you're not spinning out on the downhills, and let the flats and the climbs look after themselves.  Fixies climb well even when they might seem to be hopelessly overgeared.

For velodrome use 48 x 15 is typical for a 250m velodrome or larger.  For the very small 1/7 km track at Calshot, 48 x 16 (or equivalent - 48 x 16 happens to be a dreadful choice, as I'll explain below) is recommended.

Some people ride velodrome-sized gears on the road, and it works fine for them.  They'll find downhills easier and climbs a bit harder.

For time trials the sky is the limit.  Traditionslly, TTs were ridden on a medium gear, which was 72" on a 27" wheel, or the usual 48 x 18.  These days, short TTs are more like weightlifting on bikes and gears of up to 125" (56 x 12) are not unheard of.  It's about having a gear high enough that you can get power down at all times, even on downhill sections.  You don't want your feet to be merely following the pedals in a race..

When choosing chainrings and sprockets, avoid those that are a direct multiple, such as 48 x 16 (48/16 = 3).  The power strokes will always be on the same teeth at the poor little sprocket and it will wear unevenly and quickly.  Ideally the chainring teeth/sprocket teeth should be an irrational number (hence choosing chainrings with prime-numbered teeth is often recommended).  Even on a 48 x 15 setup, a used sprocket will show an uneven wear pattern repeating every third tooth (both 48 and 15 are divisible by 3).  None of this happens on bikes with derailleur gears because the chain jumps and slips during gearchanges and the chainring and sprocket are not in lockstep with each other.

If you do skip-stops (not that I know anyone who does), the above is even more important, as it also evens out the wear on the rear tyre.

Try to avoid sprockets smaller than 15T as they are less efficient due to the tight radius of the chain.  Sometimes you have no choice, e.g. on an MTB conversion where a chainring larger than 40T would hit the chainstay.

Finally, choose good sprockets as cheaper ones can be noisy, soft or have poor threads which can strip your hub.  EAI are probably best, and are quiet out of the box, but Surly and Shimano Dura-Ace are also good.  The Shimano ones do need a fair bit of running in until they're quiet.
Never tell me the odds.

LittleWheelsandBig

  • Whimsy Rider
Re: Gear choice for fixies
« Reply #1 on: August 18, 2018, 08:51:25 pm »
Good advice.

The wisdom of the ancients for road fixed was 66-72". I find that the start-stop of city commuting can be tough on my knees when using higher gears.
Wheel meet again, don't know where, don't know when...

Re: Gear choice for fixies
« Reply #2 on: August 18, 2018, 09:00:53 pm »
I Audax and commute on 48*16.  Never really thought about the effect on sprocket wear but I do swap chains, chainrings and sprockets quite often and don’t skimp on component costs.

TT bike has a 14t sprocket on the disk now as well.

Turns out I’m doing it all wrong.

zigzag

  • unfuckwithable
Re: Gear choice for fixies
« Reply #3 on: August 18, 2018, 09:11:29 pm »
When choosing chainrings and sprockets, avoid those that are a direct multiple, such as 48 x 16 (48/16 = 3).  The power strokes will always be on the same teeth at the poor little sprocket and it will wear unevenly and quickly.  Ideally the chainring teeth/sprocket teeth should be an irrational number (hence choosing chainrings with prime-numbered teeth is often recommended).  Even on a 48 x 15 setup, a used sprocket will show an uneven wear pattern repeating every third tooth (both 48 and 15 are divisible by 3).  None of this happens on bikes with derailleur gears because the chain jumps and slips during gearchanges and the chainring and sprocket are not in lockstep with each other.

just being pedantic and adding that the above multipliers are irrelevant on ss bikes as well (i.e. no derailleurs) as the rear sprocket changes it's rotation position randomly every time the bike coasts. i am happily running 48x16 which is a great ratio for not too hilly terrain.

edit: as pointed out by lwab the relevant position of the sprocket doesn't change when coasting (i was thinking about the tyre's load/skid patches which do change), but for some reason i have not experienced irregular wear on the teeth at all. maybe there are differences at microscopic level, but do they really matter?..

LittleWheelsandBig

  • Whimsy Rider
Re: Gear choice for fixies
« Reply #4 on: August 18, 2018, 09:30:19 pm »
A SS sprocket doesn't change its position relative to the chainring when coasting, so the concentrated wear problem still exists. A geared bike changes relative positions of chainring and sprocket every time the rider changes gears.

That said, I've not found concentrated wear to be a major problem but YMMV. Perhaps I have flats often enough to mitigate the effects.
Wheel meet again, don't know where, don't know when...

Re: Gear choice for fixies
« Reply #5 on: August 18, 2018, 09:36:21 pm »
Quote
70" may seem quite high if you normally spin derailleur gears at 100rpm or more, and you won't be able to really spin this gear on the flat unless you are very powerful.  However, the important thing with a fixie is to choose a gear such that you're not spinning out on the downhills, and let the flats and the climbs look after themselves.  Fixies climb well even when they might seem to be hopelessly overgeared.

Doesn't it depend on how fast you can ride? I would say choose the gear that lets you pedal with your choice of effort on the flat with no wind, and let the up and down hills sort themselves out.

Quote
For velodrome use 48 x 15 is typical for a 250m velodrome or larger.  For the very small 1/7 km track at Calshot, 48 x 16 (or equivalent - 48 x 16 happens to be a dreadful choice, as I'll explain below) is recommended.

So you ride faster on a smaller track and slower on the bigger track?

Quote
When choosing chainrings and sprockets, avoid those that are a direct multiple, such as 48 x 16 (48/16 = 3).  The power strokes will always be on the same teeth at the poor little sprocket and it will wear unevenly and quickly.  Ideally the chainring teeth/sprocket teeth should be an irrational number (hence choosing chainrings with prime-numbered teeth is often recommended).  Even on a 48 x 15 setup, a used sprocket will show an uneven wear pattern repeating every third tooth (both 48 and 15 are divisible by 3).  None of this happens on bikes with derailleur gears because the chain jumps and slips during gearchanges and the chainring and sprocket are not in lockstep with each other.

Err...you could just take the wheel out and put it back in etc, I wouldn't let it affect my choice of gear.


Quote
Finally, choose good sprockets as cheaper ones can be noisy, soft or have poor threads which can strip your hub.  EAI are probably best, and are quiet out of the box, but Surly and Shimano Dura-Ace are also good.  The Shimano ones do need a fair bit of running in until they're quiet.

Shimano Dura-Ace sprockets are good quality and are relatively cheap but don't do bigger than 16 t, otherwise there wouldn't be any need for EAI or other brands.

Re: Gear choice for fixies
« Reply #6 on: August 18, 2018, 09:54:13 pm »
My favourite fixed wheel gear is 44 x I8, gives me a gear in the mid sixties, very good for Warwickshire's rolling countryside. If I'm going well I might go up to 46 x 18 and a gear in the high sixties.

LittleWheelsandBig

  • Whimsy Rider
Re: Gear choice for fixies
« Reply #7 on: August 18, 2018, 10:01:57 pm »
hubner, 48 x 16 is a lower gear (slower) than 48 x 15. Very tight tracks have noticeably higher G-forces (rolling resistance) and need a lower gear to accelerate out of the bends, particularly with less distance between the last bend and finish line. The biggest tracks are outdoors with slower surfaces and adverse wind slowing riders. Indoor tracks around 225-300m tend to be the fastest and need the highest gears. I understand that world-class trackies are running over 100" nowadays. I never got above 95" on the track, except for motorpacing, but often used 90-94" as I was more comfortable with a couple of extra gear inches than most on the velodrome.
Wheel meet again, don't know where, don't know when...

Re: Gear choice for fixies
« Reply #8 on: August 18, 2018, 10:03:09 pm »
The wisdom of the ancients for road fixed was 66-72". I find that the start-stop of city commuting can be tough on my knees when using higher gears.
I was advised 63-66" by my club in the 1970s. This was in the context of riding through the winter, then adding a few inches for the season-starter medium gear 72" race.

There again, I'm not going that fast...

LittleWheelsandBig

  • Whimsy Rider
Re: Gear choice for fixies
« Reply #9 on: August 18, 2018, 10:35:47 pm »
I was in sub-tropical Oz, so riding through the winter there was a little different to doing it in Britain. Significantly colder weather tends towards lower gears.
Wheel meet again, don't know where, don't know when...

'breff

  • M' back!
  • Head Banger.
Re: Gear choice for fixies
« Reply #10 on: August 19, 2018, 12:40:02 am »
My Reid has 46-16 and the calcumalatoriser I looked at, Sheldon's, puts that at around 76". I am a slow rider and can't spin due to Neurological "Stuff" but I find it a bit low for me. I need to drag the rear brake on, even slight, downhills. Would I be better with bigger at the front or smaller at the rear?
(It's a Flip-flop hub so the freewheel side is okay at that ratio)

Re: Gear choice for fixies
« Reply #11 on: August 19, 2018, 12:22:49 pm »
Any chainring/sprocket combination where the highest common factor is 3 or less is ok for evening out sprocket and tyre wear (which occurs through scrubbing on the road even if you don't skid) even for real up-and-down pedal mashers.

Singlespeeds are immune to the tyre scrub problem because sprocket and tyre are not fixed in relation to one another,  but 48 x 16 fixed will give you very uneven tyre wear if you ride out of the saddle much. (Added "fixed" to last sentence for clarity)
Never tell me the odds.

Re: Gear choice for fixies
« Reply #12 on: August 19, 2018, 12:50:19 pm »
highest common factor


"greatest common divisor"
"Yes please" said Squirrel "biscuits are our favourite things."

Re: Gear choice for fixies
« Reply #13 on: August 19, 2018, 03:39:29 pm »
highest common factor


"greatest common divisor"
"O" level maths was a long time ago  ;D
Never tell me the odds.

Re: Gear choice for fixies
« Reply #14 on: August 26, 2018, 04:10:05 pm »
I'm TTing on a fixie that I originally built for commuting running 42x16. It's currently got a 52x16 on it - the sprocket is pretty worn (and the 42 tooth chainring is pretty knackered), but the 2 unused sprockets I have (16 and 15) are both 1/8, and the chain (and chainrings) I have are currently 3/32. The PCD is 122mm.

That PCD is an obsolete Stronglight one - there are some NOS rings about but they are all 3/32 and £30 (and they aren't going to be around for ever). Am I better off just getting a track crankset (or a road crank with a sensible pcd) and going all 1/8th?

'breff

  • M' back!
  • Head Banger.
Re: Gear choice for fixies
« Reply #15 on: August 27, 2018, 03:12:18 pm »
I got it wrong! The Reid is a 46X18. Had a few more rides and, on reflection, I can cope with that ratio for the time being. When I do change, Rogerski says 48X16 ain't good, Would 46X16 be okay?

bludger

  • Randonneur and bargain hunter
Re: Gear choice for fixies
« Reply #16 on: August 27, 2018, 03:27:00 pm »
Mine's 44/16 which I'm very happy with. It may be a little high for optimum on the flat but I'm a bit of a grinder on climbs anyway and I like not having my legs flailing around on the descent.
Bikepacking bargain basement: reviews of high value kit great for the tourer, bikepacker and randonneur on a budget

https://yacf.co.uk/forum/index.php?topic=109048.msg2312359#msg2312359

'breff

  • M' back!
  • Head Banger.
Re: Gear choice for fixies
« Reply #17 on: August 27, 2018, 03:33:42 pm »
Cheers, bludger. Neurological Damage means I have no "Fast Twitch" muscle co-ordination, can't spin at all. 46/16 feels a little low for me but Acceptable.

Re: Gear choice for fixies
« Reply #18 on: August 27, 2018, 05:16:19 pm »
47/16 is very good.
Never tell me the odds.

Re: Gear choice for fixies
« Reply #19 on: August 31, 2018, 11:42:18 am »
A question , is there any theoretical advantage and or actual advantage to riding larger chainring / larger sprocket , as in 51/19 etc...
Boots an Spurs

Re: Gear choice for fixies
« Reply #20 on: August 31, 2018, 11:55:27 am »
I've always ridden 43x17, except when I haven't; never competed on fixed.

Currently it's on 43x18 for the sole purpose of making friends laugh as they follow me downhill.

woollypigs

  • Mr Peli
    • woollypigs
Re: Gear choice for fixies
« Reply #21 on: August 31, 2018, 12:57:18 pm »
94" is where it's at :)

Re: Gear choice for fixies
« Reply #22 on: August 31, 2018, 05:40:01 pm »
94" is where it's at :)

'It' might be there, but I'm not.

LEE

  • "Shut Up Jens" - Legs.
Re: Gear choice for fixies
« Reply #23 on: August 31, 2018, 05:47:50 pm »
My heavy winter bike is 64"

My light summer bike is 72"

I'd be fairly happy anywhere between the two but it depends on the bike and your individual ability to spin a low gear and/or grind up hills in a bigger gear.

You can't have a perfect single speed gear ratio but that's the whole point, you don't waste time thinking about gears, you just get on and spin/grind the one you have.
Some people say I'm self-obsessed but that's enough about them.

Re: Gear choice for fixies
« Reply #24 on: August 31, 2018, 09:03:07 pm »
94" is where it's at :)

That’s my dual carriageway race gear.   Takes a little time to wind it up.