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Singlespeed & Fixed Gear "I still feel that variable gears are only for people over forty-five. Isn't it better to triumph by the strength of your muscles than by the artifice of a derailer? We are getting soft...As for me, give me a fixed gear!"-- Henri Desgrange (31 January 1865 - 16 August 1940)

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Old 04-24-06, 06:41 PM   #1
HereNT
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Interesting - gear ratios

So I was cleaning my rear wheel and adjusting my cones before work today, and decided to flip over to the 14t cog, because, hey - it's windy, and I'm bored...

Anyways, I also moved my computer to where I could see it and was looking at the cadence/speed while I was riding in on the Greenway. Oddly, it didn't seem like I was going any faster on the track bike than I go when I'm on the beater commuter with the fatty tires. My cadence was down about 10 RPM since the gearing was higher, I think that had a lot to do with it. I checked out the specs on Sheldon's calculator when I got home, and this is what I found:

Fuji Touring Conversion:
700x32 w/ 171mm cranks, 42x16
Gain Ratio : 5.3
Speed @ 100 RPM : 21.1
Gear Inches : 70.9

Bianchi Track:
700x23 w/ 175mm cranks, 44x14
Gain Ratio : 6.0
Speed @ 90 RMP : 22.1
Gear Inches : 82.6

I'm actually wondering if I should put a larger tire on the rear of the Fuji. I have a 15t cog that might be good to put on there, too. It seems like with the larger tire, you get more distance and speed, but it doesn't feel like a larger gear. Even though that bike is 5-6lbs heavier, it's easier to get it moving, and easier to maintain the speed.

It also has me wondering what the point of the 23c wheels and tight geometry actually are. I mean, sure, it's a pretty looking bike, but if there isn't really much of an actual gain in speed... What's the point? A harsher ride and more likelyhood of pinch flats?

I'll have to do some more tests to be sure, because the weather conditions were different each time, but I have the feeling that I'm going to find that my beater is faster than my nice bike...

And, yeah, I don't know what the point of this post was. Just bored and thinking about it a little too much, I guess.
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Old 04-24-06, 06:49 PM   #2
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I understand what you are saying, and the numbers are interesting, but couldn't this be a result of your head f'ing with you. Maybe your cadence dropped, not because the gear was that much taller, but because it realised you were going your normal speed and there was no reason to work harder. My bet is with the fat tires on the beater adding resistance, your legs aren't working that much harder on the pista with the tall gear. Your mind has just found its comfort speed.
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Old 04-24-06, 06:55 PM   #3
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x2 ... it happened to me
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Old 04-24-06, 06:55 PM   #4
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Quote:
Originally Posted by jmgorman
I understand what you are saying, and the numbers are interesting, but couldn't this be a result of your head f'ing with you. Maybe your cadence dropped, not because the gear was that much taller, but because it realised you were going your normal speed and there was no reason to work harder. My bet is with the fat tires on the beater adding resistance, your legs aren't working that much harder on the pista with the tall gear. Your mind has just found its comfort speed.
IT'S NOT A PISTA!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Sorry.

The tires are actually at around the same PSI, 110 on the beater and 115 on the Bianchi (1987 lugged steel, BTW) so I doubt drag is an issue. I was actually trying to spin faster on the Bianchi, and was able to get it up to 24mph, but I couldn't hold it. I also never was able to get to feeling that I was actually spinning, even at the same cadence. For some reason, getting the upstroke worked in was hard, too...
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Old 04-24-06, 07:11 PM   #5
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Quote:
It also has me wondering what the point of the 23c wheels
The point is that at 40kph, 23mm vs 25mm has a much bigger aerodynamic difference. Of course if you aren't doing cat 1 races everyday, this then becomes irrelevant as the ride is much harsher and tires wear out that much faster.
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Old 04-24-06, 07:21 PM   #6
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Quote:
Originally Posted by operator
The point is that at 40kph, 23mm vs 25mm has a much bigger aerodynamic difference.
Narrow tires are usually used for decreased rolling resistance, not aerodynamic benefit.
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Old 04-24-06, 08:40 PM   #7
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Unless you are riding on glass roads, smaller tires probably mean MORE rolling resistance. The main purpose is for aerodynamics. Kthx.
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Old 04-24-06, 08:49 PM   #8
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Originally Posted by operator
Unless you are riding on glass roads, smaller tires probably mean MORE rolling resistance. The main purpose is for aerodynamics. Kthx.
Ummm, you are wrong. More rolling resistance on smaller tires? A smaller (diameter) tire means a smaller patch in which rubber is touching the tarmac, thus less rolling resistance and also less rotational weight the 'smaller' a tire gets. Before you get all "the main purpose is..." you should check your facts. Physics.
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Old 04-24-06, 08:54 PM   #9
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Originally Posted by HereNT
IT'S NOT A PISTA!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Don't be sorry, that was my mistake - I won't make it again. I'm sorry.
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Old 04-24-06, 08:57 PM   #10
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...Kthx.
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Old 04-24-06, 08:57 PM   #11
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Someone should also point out that unless the OP has a pursuit/TT bike the geometry of track bikes has more to do with responsiveness then aerodynamics.
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Old 04-24-06, 10:30 PM   #12
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Quote:
Originally Posted by ChicagoDave
Ummm, you are wrong. More rolling resistance on smaller tires? A smaller (diameter) tire means a smaller patch in which rubber is touching the tarmac, thus less rolling resistance and also less rotational weight the 'smaller' a tire gets. Before you get all "the main purpose is..." you should check your facts. Physics.
Bicycling magazine did a test way way back... with results showing that sometimes wider tires have less rolling resistance than skinnier tires.

Now before anyone jumps down my throat about this, the main thing was that often skinny tires have less rolling resistance than wider tires, and sometimes the other way around. Mainly the factors playing a role are rider weight and tire inflation. Both influence tire deflection and how much of the tire actually touches the road. Also... some tires suck, and others are good.

One thing that can't be argued is that skinnier tires weigh less because there is less material. Thus less rotational weight, and less rotational weight is an important factor if you wanna go fast. Sure, skinny tires they might also be more aerodynamic... but waxing your eyebrows is more aerodynamic too...

I did a quick google search and found this quote from Jobst Brandt (a well respected bike engineer). "Tire casing stress varies linearly with tire cross section at constant inflation pressure...." in short smaller tires can be pumped higher than bigger ones. And higher PSI = less rolling resistance.

Here's a link to and older discussion on the topic
http://yarchive.net/bike/rolling_resistance.html
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Old 04-24-06, 11:06 PM   #13
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The argument goes like this: Wider tires can have less rolling resistance because they are less affected by smaller imperfections in the pavement. On a totally smooth surface, with all other things being equal, a smaller tire will have less resistance.
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