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Fixed gear physics question

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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)

Fixed gear physics question

Old 04-27-22, 05:52 AM
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gravelroad
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Fixed gear physics question

I'm a complete newbie and recently I learned about the existence of fixied gear bicycles. Even tough they are not for me yet since I'm not that fit and I live in a quite hilly area these bikes fascinate me alot. The fact that these bikes don't have a freewheel mechanism and you have an eccentric contraction of your leg muscles is very interesting to me. Has anybody an idea how the physics of these bikes work? When you try to pedal backwards to slow down, is the kinetic energy of your weight divided into smaller portions by the gearwheel and therefore you become slower? Do I understand that correct =D Sounds a bit nerdy but I have to experience and understand that. Must be very unique to ride such a bicycle. Would be fantastic if somebody can explain that to me
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Old 04-27-22, 09:33 AM
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I think it's a lot more fun to experience it than to calculate it, but there are members here who will have a nerdgasm at the opportunity to flex. Stay tuned.



Last edited by Rolla; 04-27-22 at 09:37 AM.
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Old 04-27-22, 10:52 AM
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79pmooney
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Fix gears are really simple. To get a feel for what's going on, ride any kid's tricycle. That's a fix gear with power to the front wheel, not the back and two rear wheels. Now ride a bigger tricycle. Just another fix gear with a larger gear. When the front wheel gets to the size of the biggest "ordinary" or high wheeler of 140 years ago, you have a fix gear with "normal" gearing for flat ground. All the chain and sprockets of modern bikes do is allow the same effect as the large highwheeler wheel using much more convenient (and safer) wheel sizes. Ability to slow using your legs is exactly the same as the highwheeler or tricycle.

I ride fix gears but don't spend the time analyzing where the inertia is going. Nor do I do more than gently slow the bike with my legs. Brakes do that far better and save my knees for driving me and the bike forward. There's no magic to fix gears. For the first 30 years, nearly every bike made was a fix gear. All the early speed and distance records were set on fix gears. Fix gear was never mentioned because in those days, they were just bicycles. When gears showed up, now that was new and different.
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Old 04-27-22, 12:01 PM
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DiabloScott
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Originally Posted by gravelroad View Post
1. The fact that these bikes don't have a freewheel mechanism and you have an eccentric contraction of your leg muscles is very interesting to me.

2. Has anybody an idea how the physics of these bikes work?

3. When you try to pedal backwards to slow down, is the kinetic energy of your weight divided into smaller portions by the gearwheel and therefore you become slower?

4. Must be very unique to ride such a bicycle. Would be fantastic if somebody can explain that to me
1. Your leg muscles work exactly the same when you're pedaling normally on a FG and a derailleur bike. I'm not sure why you're calling that eccentric contraction.
2. The physics of these bikes is exactly the same when you're pedaling normally.
3. You don't pedal backwards, you simply resist the motion of the crank, which is still moving forward. You can stand and let the motion of the bike move your weight up and down, and you can learn to STOP the crank and skid, and you can have regular brakes too. You can learn to pedal backwards and ride in reverse, but that's sort of a circus trick rather than a ride.
4. Here's how to think of this - the crank and the rear wheel are FIXED together - although they don't turn at the same RPM. A typical gear would have one crank revolution = three wheel revolutions or so.
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Old 04-27-22, 02:06 PM
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Zdawg
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Get on a fixed gear ASAP

You really just have to get on top of one =))

It's such a funny feeling. You'll either hate it or love it.
Don't worry about fitness or hills for that matter. Check out Mash SF 2007 or Macaframa 2008 for inspiration.
I say get a fixie as soon as you find one, either a conversion or a track bike and ride slowly, primarily on flat roads with traffic you are quite familiar with (typical suburbia).

You won't need brakes if you're sensible (don't go bombing any hills yet, leave that for later) and keep to the suggestions above. I started with none and enjoyed it better because that was part of the challenge.

Overall, it's a type of bike I think everyone should own. Great fun to get into tricks as well!
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Old 04-27-22, 04:55 PM
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Originally Posted by gravelroad View Post
I'm a complete newbie and recently I learned about the existence of fixied gear bicycles. Even tough they are not for me yet since I'm not that fit and I live in a quite hilly area these bikes fascinate me alot. The fact that these bikes don't have a freewheel mechanism and you have an eccentric contraction of your leg muscles is very interesting to me. Has anybody an idea how the physics of these bikes work? When you try to pedal backwards to slow down, is the kinetic energy of your weight divided into smaller portions by the gearwheel and therefore you become slower? Do I understand that correct =D Sounds a bit nerdy but I have to experience and understand that. Must be very unique to ride such a bicycle. Would be fantastic if somebody can explain that to me
Apply the appropriate amount of pressure on pedals move forward.
​​​or backwords.
not rocket since.
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Old 04-29-22, 02:53 PM
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ofajen
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To avoid getting too excited, Iím recycling a post I made about a year ago that explains how FG and SS are different when the rider is not pedaling hard enough to push the bike forward.

For FG and SS, both internal and external work are identical when you are maintaining the same speed in the same gear.

External work is what we normally think of, the work done to push the pedals against all the forces that resist the forward motion of the bike. The internal work is what is needed just to move a riderís legs and feet and the cranks and pedals as a function of pedaling rate.

Here is the cite of the most useful paper on quantifying the amount of internal work
https://kclpure.kcl.ac.uk/portal/fil...12500.full.pdf

If you stop doing any external work of pushing the pedals and just spin at exactly the speed you are going, you are still doing the internal work and it will be the same for both.

Where it gets different is when you slack off below doing the internal work of moving the pedals at the current speed.

On a freewheel, you can take the work all the way to zero, your feet stop and you coast. In fact, any level of work less than the current internal work makes no difference to the motion of the bike and it is just a matter of how much you are coasting.

On a fixed gear, you can take the work to zero but the pedals continue to turn and the bike will use its kinetic energy to do the equivalent of the internal work and keep your feet moving at the same speed. Of course, this has a bit of a braking effect.

For example, it takes me 180 W of external work to sustain 18 mph on gravel, which requires 90 rpm on my 67.5 inch gear.

Using Formentiís data, I can estimate about 0.7 W/kg at 90 rpm times my weight of 80kg or about 56 W of internal work.

So if my wheel had a fixed gear and I stopped pedaling and let the bike turn my feet, Iíd be initially pulling 56 W from the bike. Of course, this quickly lessens as the bike slows because the internal work depends on cadence.

With a fixed gear, the level of effort always matters to the motion of the bike, whether itís just a little bit less than the internal work, all the way down to zero or if you start adding resistance to brake even faster.

This behavior is familiar to all FG riders Iím sure, and here we have a more quantitative idea of the energy involved.
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Old 04-30-22, 07:19 PM
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SorryÖ a bit sloppy in the previous post. Of course Iím talking about instantaneous power (energy per unit time expressed in Watts) and not work (in units of energy expressed in Joules).

Otto
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