Quantify the efficiency gain from going fixed

Dan B

Disengaged member
I recently acquired an early 80s road bike with horizontal dropouts and some Suntour gears (stem-mounted shifters!) which I'm not really getting on with. When I replaced the gear cables I didn't tighten the bolt at the back up enough either, so for the past few days I have been riding in the smallest sprocket at the back. And I think it's time to fixify it.

Sheldon says 42x14 on 27" wheels is 81". If I'm happy with this gear on a singlespeed, what size of gear should I put on a fixed to give me the same level of effort - bearing in mind the "flywheel effect", straight chainline, the weight decrease from removing the mechs, shifters & sprockets, and all the rest of the gubbins generally talked about the increased efficiency of the fixed wheel bike

The answer that will make me happiest, by the way, is 85", as I have a cheap 700c wheel with a fixed 16T sprocket ready to be fitted, and I plan to reuse the 52T chainwheel. Does that sound doable?

I'm expecting also that replacing the platform pedals with SPDs might give me a bit more acceleration at the lights, which is the only real problem area right now ...
 

Rob3rt

Man or Moose!
Location
Manchester
I ride 85 inch and its fine, 20mph is very little effort, but don't expect to be skid stopping, it will buck you out of the seat.
 

BigSteev

Senior Member
52x16 is nearer 88". I think that'd be a bit harsh, even for the flatness of London. As a guide I used to run 42x16 and am now on 44x15 which is about 79" and I don't think I'd want to go any higher.
 

GrasB

Veteran
Location
Nr Cambridge
BigSteev means that you'll be putting your legs under a lot of pressure with a gear that big. My advice would be start with what gearing you've got & if you need to change sprockets do so, personally I like to keep my flat cruising speed at around 100rpm so tend to move to a 80-82" gear.
 
OP
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Dan B

Disengaged member
RecordAceFromNew said:
Incidentally if you run 700x20c it is just under 85" with 52x16. Obviously 20mm tyres are not to everyone's taste or for all circumstances, but they are quick. :ohmy:
Uh huh. Sheldon says 85.4 with 700x23, so I'm not sure where Steev is getting a higher answer
 

swee'pea99

Legendary Member
Blimey, you're talking high gearing there, it seems to me. I ride a 70" gear, and that's plenty high enough for me - and I'm no slouch. I actually tried a higher gear for a while - I went from a 42 at the front to a 48, but kept the 16T at the back, just to see how it felt, taking me from 69" to 79", give or take, and I just couldn't get on with it. Too high. Personally, in your situation, I'd stick with the 16T but go with the 42 at the front, giving a 70" - the definitive urban gear. But each to his own.
 

Rob3rt

Man or Moose!
Location
Manchester
bah, 85" is cake on the flat ;) Bit slow off the mark though. Only real downfall.

Im going to buy a gearing each side of my current ratio and see how I like them though.
 
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Dan B

Disengaged member
Well, I ended up on 48x16 (= 78") when I found that the existing crankset couldn't be reused as the big ring was not detachable from the spider. So I had to get a new crankset (and in the process realised I needed a new BB as well) and 48 seemed like it would bring me in at about the ratio I've been used to recently, bearing in mind that I downsized to 165mm cranks at the same time.

I've only ridden it around the block so far (only got it reassembled at lunchtime today and I am supposed to be working right now) but it seems quite comfortable.
 

Rob3rt

Man or Moose!
Location
Manchester
Flying Dodo said:
Hope you have fun.

However, I'd take issue with the thread title. I wouldn't say fixed is more efficient (other than the fact of having a shorter chain and therefore a tiny bit less friction. It's just it forces you to make more effort than using a geared bike. :wacko:
(Potentially) Less weight, shorter chain, perfect chain line, flywheel effect (and no dead spots on the pedal stroke as a consequence).

Less effort going uphill or on the flat than a geared bike with all conditions equal, i.e. identical environmental conditions and gear same ratio, this is why you find people often ride the same hill or stretch on a fixed bike in a ratio that is harder than that they usually ride it on a geared bike. Downhill is another issue altogether though :biggrin:
 
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Dan B

Disengaged member
Rob3rt said:
Less effort going uphill or on the flat than a geared bike with all conditions equal, i.e. identical environmental conditions and gear same ratio, this is why you find people often ride the same hill or stretch on a fixed bike in a ratio that is harder than that they usually ride it on a geared bike.
Well, this was actually the original question. A lot of people claim that, but so far nobody has been willing to put a number on "how much harder".

Pictures, for the curious, via my blog http://www.coruskate.net/Give_me_a_lever , and http://yfrog.com/4bkywbj is why I want to replace the handlebar foam
 

Rob3rt

Man or Moose!
Location
Manchester
It's hard to quantify though since there are many variables, for one example varying wheel weights meaning different rotating mass and varying assistance from the flywheel effect.
 
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