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PostPosted: Thu Jul 12, 2018 11:36 am 
Dirt Disciple

Joined: Wed Apr 15, 2015 9:19 pm
Posts: 62
Depends what you want it for, I guess. I use 48x17 (74") for everything; it seems a good compromise. You can cruise at 30kph at a natural 85 cadence, spin out at about 50kph (around 140 cadence) and climb most things, albeit out of the saddle once it's above 10% (I did find the bridge ramps on the Lea Canal needed a run up or my legs would just stop, though). I use it for commuting, for RP laps (chatlaps and Z2/Z3 stuff - sub-7 on 74" is a bit frantic), for general getting around and for the occasional lower-paced Herts club ride if I feel like making my life difficult. I've tried 70" and just found it was too low a gear for general riding and hopeless for club use as even 40kph has you courier-spinning like a loon.

I'm on single-speed at the moment, but ran two brakes even when fixed - for safety and knee protection. If you're running fixed with no rear brake, you will need an uneven ratio - not just to save on tooth wear but also to give you multiple skid patches so your rear tyre doesn't wear too quickly.


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PostPosted: Thu Jul 12, 2018 3:45 pm 
Old School Grand Master

Joined: Mon Feb 11, 2008 9:42 am
Posts: 3114
I have a vague recollection of always using either 41 or 43 tooth rings to make it easier to mismatch the ratios.


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PostPosted: Thu Jul 12, 2018 5:17 pm 
rBoTM Winner
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Joined: Wed Jan 17, 2018 8:42 pm
Posts: 1022
Location: East Yorkshire
When you say miss match, do you mean odd/even numbers for the ring/sprocket or just as I thought was mentioned earlier, combinations that don't divide by 3? i.e. 52t & 16t as opposed to 48t and 16t :? :?:


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 Post subject: Re:
PostPosted: Thu Jul 12, 2018 7:19 pm 
Retro Guru
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Joined: Mon Sep 24, 2012 9:10 pm
Posts: 1046
Sheldon made a case for using even-toothed chainrings and sprockets on single-speed drivetrains, with the proviso that a tooth has to be identifiable on the chainring (and the sprocket?) which must always be surrounded by the outer links, never the inner links.
I'm not quite convinced by his reasoning.. and it would probably be a futile discipline anyway unless all the components were more or less factory-fresh at the time of committing oneself to it.

My ideal is for any given link to work it's way around all the teeth in turn. My insinct tells me that prime numbers is the way to achieve that. I haven't conducted any empirical experiments though. I've never had enough chainrings. Especially ones with uneven tooth count. My instinct could be dead wrong.

Take 17x51 for example. They both look like prime numbers. Neither of them are in the times tables. But 51 is 3x17, so in that respect it's just like 16x48. 51 turns out not to be a prime number.

combinations which have a common denominator might be the same thing in a milder form. 18x42 for example. Both those numbers are divisible by 6. Three sixes and seven sixes. That might set up a weird pulsation in your frame and wear your chainring into a heptagon, which is significantly closer to being a circle than triangles are, but still.

I don't lose any sleep at all over this stuff though... really! :)


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 Post subject: Re:
PostPosted: Thu Jul 12, 2018 7:56 pm 
Feature Bike
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Joined: Fri Aug 11, 2006 8:13 pm
Posts: 1233
Location: York-ish UK
Torqueless, I think your reasoning is genius! I've had single speeds, but never fixed; Will be going fixed now..

FWIW, I will be starting with 34 x 13: not both primes, but are Fibonacci numbers, and not far off the 70 gear inch rule of thumb (69! for 700c).

I expect pedalling Nirvana. :)

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PostPosted: Thu Jul 12, 2018 8:45 pm 
rBoTM Winner
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Joined: Wed Jan 17, 2018 8:42 pm
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Location: East Yorkshire
Ok, stupid question alert!
But please put this single speed/fixie virgin out his misery please.... 70”. I assume that’s the result of an equation involving cog teeth x chain ring?


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 Post subject: Re:
PostPosted: Thu Jul 12, 2018 8:57 pm 
Feature Bike
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Joined: Fri Aug 11, 2006 8:13 pm
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Location: York-ish UK
Gear inches are an ancient way of expressing how far you go each pedal, with a given gear/wheel size; not sure why it's stuck. There are other ways to express gear ratios.


Sheldon is your friend:

http://www.sheldonbrown.com/gear-calc.html


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 Post subject: Re: Re:
PostPosted: Thu Jul 12, 2018 9:23 pm 
Retro Guru
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Joined: Thu Jun 23, 2011 11:25 pm
Posts: 2060
Location: It's not easy being a dolphin.
doctor-bond wrote:
Torqueless, I think your reasoning is genius! I've had single speeds, but never fixed; Will be going fixed now..

FWIW, I will be starting with 34 x 13: not both primes, but are Fibonacci numbers, and not far off the 70 gear inch rule of thumb (69! for 700c).

I expect pedalling Nirvana. :)

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IMG_0189.JPG


Small sprockets and chainrings and chain length would wear quicker, but you get a lighter build. You can off-set this
with more substantial quality parts though or accept it's all cheap enough to replace more frequently.

I've fiddled a lot with SS on MTBs with vertical drop-outs and finding the magic gear ended up
with some weird combos to
get an acceptable gear.

Interesting about the 'prime' number thing, but can't help
but think it's not worth having a
hang-up about.

I don't really get Sheldons logic, but suffice to say rear wheels have punctures and lining chain links to a tooth sounds like a faff to 'get' even wear.


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 Post subject: Re:
PostPosted: Thu Jul 12, 2018 9:42 pm 
Retro Guru
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Joined: Mon Sep 24, 2012 9:10 pm
Posts: 1046
Quote:
Torqueless, I think your reasoning is genius! I've had single speeds, but never fixed; Will be going fixed now..

FWIW, I will be starting with 34 x 13: not both primes, but are Fibonacci numbers, and not far off the 70 gear inch rule of thumb (69! for 700c).

I expect pedalling Nirvana.


Well.. If I was a genius I could do the maths without having to mess around with chainrings.

My present 16x38 isn't fixed. I might go back to fixed when they fix the bloody roads, not on that ratio though..

I think you are on to something with the Fibonacci. Are 13 tooth fixed sprockets available? Can always go 21x55 if not!


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PostPosted: Fri Jul 13, 2018 4:59 am 
Dirt Disciple

Joined: Mon Mar 05, 2018 4:03 pm
Posts: 11
The gearing suggested sound like a good starting point but depending on your age and physical ability may need some changes.
Do not over gear. My maximum speed is about 22mph before I spin out but that allows me to get up all the hills I need to. You have a flywheel effect that does help you considerably on hills.
I found that a freewheel on my flip flop hub did not suit me. No bonus of the flywheel effect going up hills and with the low gear I spent a lot of time freewheeling down hills and my legs would stiffen up.
Fit front and back brakes to avoid having any knee trouble as braking with your legs can cause problems.
Interesting theory about the even and odd number of teeth but I understand you would need to mesh the drive train back in the same position when replacing a wheel after maintenance etc.
Clip in pedals keep your feet in place as it could be painful if your feet slipped off the pedals.
Really good fun and not to be missed, go for it and good luck


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