TRACKMAN

Seeking a logical explanation for why a bicycle equipped with a fixed gear can gofaster than a bicycle with a freewheel .
both have same ratio 💋 keeping it simple 45x15. Not sure of the math pretty sure though there is a significant drop-off for freewheel regarding power transfer above 100 rpm of the crank.

Mikefule

Mainly psychology. If you slack off on a fixed, the bike lets you know that it's noticed. At the same ratio and the same cadence, the speed will be the same. However, there is no "flywheel effect' to remind you to maintain your cadence on a freewheel.

Speedway2

I never wondered why myself but ^^^^ this seems like a logical reason....

Broctoon

The only way it can is if the rider is pedaling faster. With the same gear ratio, the two bikes will go exactly the same speed at a given cadence, and that cadence will require the same effort in either case. The only time a difference comes into play is when the rider wants to coast, for example if he gets tired or descends a steep hill.


False. Power transfer on a fixed vs. free drivetrain is the same, at any cadence.

Sometimes I’ll ride my fixed gear bike on a particular route and end up with an elapsed time that is shorter than I’d get on my single speed over the same route (therefore the average speed is higher). This is because on the fixed gear I’m motivated to keep my legs moving fast; I can’t take a break by coasting.

On the other hand, over a route with a steep descent, I’ll be faster on the single speed because my legs are not holding me back; I can’t coast fast down the hill on my fixed gear, but I can on my single speed.

Besides on a descent steeper than my legs can keep up with, my average speed tends to be higher on the fixed gear, but there is no mechanical reason that it must be so. If I discipline myself to never coast, the elapsed times will be the same.

DiabloScott

In fact when NOT torquing the crank, the FW is coasting and overcoming a little friction. When the FG is "coasting", it's powering your legs to move around, so the FW bike is faster.

Broctoon

Very true.

TRACKMAN

Damn. Not one mechanical engineer in the bunch
thanks for soo much dribble
Have ridden track for ages,
I know if I am cranking a 45x15
​​​​​at 200 rpm doing 28-30 MPHA freewheel rig with equal gearthree to one ratio goes no where NEAR twenty eight mph. At any RPM

TejanoTrackie 

Too much ganja.

DiabloScott

Me: Licensed mechanical engineer in HI and CA.
You: Either completely wrong, or leaving out some kind of critical information, or poorly wording the question.

Gear, wheel size, and rpm are the only factors that go into the speed calculation... assuming no coasting or skidding.

200 rpm in a 3to1 gear is 47 mph. FG or FW

So you're definitely wrong there.
Professional ethics prevents me from stamping this calc, but I submit it for peer review.

Broctoon

TRACKMAN <— Ridden track “for ages,” and states “Not sure of the math pretty sure though there is a significant drop-off for freewheel regarding power transfer above 100 rpm of the crank.”

If you’re pretty sure this happens and just need help working out the math, I wonder what is your understanding of “math.”

Power transfer happens the same regardless of whether a cog is fixed to the hub directly or through a freewheel. There is no mystical additional factor coming into play. Legs turn crank, crank turns chain, chain turns rear hub via cog, power transfers to the ground via spokes, rim, and tire. That’s it.

79pmooney

I get just over 48 mph using my all-purpose 100 RPM in a 42-17 = 20 MPH. (Keep It Simple Stupid so I can do the math in my head on rides. I call 42-17 67" and 3:1 81". I don't do roll-outs. Only racing juniors need that kind of accuracy. So 47, 48. If it's not a time trial, who cares. But 47 vs 28? That's a little more than round-off error!)

Cooperator

Back to OP's question: A fixed gear can go uphill slightly faster than a SS because a cog+lockring is marginally lighter than a freewheel.
That is all.

ofajen

I think you may be asking the wrong question. Clearly, the same gearing and RPM produces the same speed and the same external (delivered to the pedals) power.

I think the question may be whether there is a significant advantage in terms of the internal work required to move your legs and feet up and down as you pedal.

I can show you experimental data from a few years back that documents this internal work for freewheeling drivetrains. The internal power requirement seems to be independent of the external power delivered to the pedals but strongly dependent (as we would expect) on pedal RPM. That is, we spend a lot more W/kg spinning our legs fast than we do spinning them slow. For example, at 110 RPM, the internal work runs about 1 W/kg, which is already significant and it is a sharply increasing function at this point.

What I can’t show is similar data for FG riding. Without such data, I’m open to the possibility that the ridiculous internal work required to pedal a SS at 160 or 200 rpm could be more than the internal work required to pedal the same speed on FG.

Your cardiovascular system has to meet the combined load of internal and external power needs, so it may be that there is an advantage in total work for FG at high RPM.

Otto

Broctoon

Um… what?

You sound like you know what you’re talking about, and yet it still all strikes me as a bunch of total nonsense. Maybe I don’t understand the question posed by the OP and addressed here.

When you’re coasting (bike rolling faster than crank is spinning, with a freewheel), this is a special case. Special case = physical effort vs. bike speed can’t be calculated the same way as with a fixed gear. Ditto when you’re on a fixed gear and descending a hill faster than your legs can keep up with, so the drivetrain is moving your legs rather than vise-versa.

Anytime you’re applying power through the pedals and crank, whether it’s one watt or 500, at any cadence and any gear ratio, there is no difference between a fixed gear and a freewheel. Internal power, external power, W/kg, blah blah blah. Doesn’t matter! Your legs cannot tell the difference between fixed and free. Neither can your heart and lungs. Of course there is a difference when coasting or trying to keep up with your fixed gear drivetrain… nobody will dispute this. But it’s absurd to suggest positive power delivery from your legs to the ground is different between a FG and SS with the same gear ratios.

79pmooney

I can see that there could be an advantage to fix gears at really high RPMs. Say it is so high you cannot keep up with he chain at pedal top and bottom, ie 12 'clock and 6 o'clock. The fix gear neatly pulls your feet across and there is almost no catch up to do to apply power again. But on a freewheel, you may have to close a real percentage of the pedaling circle simply for the pawls to catch up to the teeth.

Now, this difference depends on both chain slack and the number of engagement point in the freewheel. With enough engagement points and a slack enough chain, advantage goes tot the freewheel.

I have noticed I can get some power to my fix gear when I am going so fast downhill I wouldn't even think about trying to catch up to a freewheel. (For very short amounts of time!) I have never gotten scientific about this. What I have loosely documented is RPMs. (Loosely as in a car pulling up beside me, rolling down the window and telling my I'm doing 40, I've done this on the 42-17. 200 RPM. I've ridden a bunch faster many times but cars have rarely tried to pass and never rolled down the window. (They probably thought I was completely out of control.)

Diablo Scott knows one of those hills. One that I used to ride down at 45+ a year after I finished racing when I still had my spin. Joaquin Miller. Pretty close to the perfect high-speed fix gear descent. No car ever passed me.

ofajen

Please re-read. At no time would I ever suggest there is a relevant difference in power delivery to the ground at the same RPM and gearing. It’s impossible. But unless you have data I don’t have, neither of us knows whether the internal work done moving our legs up and down is identical in FG vs SS. If there is a difference in internal work, your CV system would definitely notice it.

Think of it this way. Take off the chain. Ride on a rear wheel trainer so you don’t fall over. It still takes work to spin the pedals and move your legs. While tiny at slow speeds, this work becomes significant at 90 or 110 rpm and even more so at higher speeds.

But is it the same for FG as SS?

Otto

ofajen

Here’s the cite:

https://kclpure.kcl.ac.uk/portal/fil...12500.full.pdf

Formenti’s group used the Velotron Racermate which may actually be sort of intermediate. It uses a freewheel but has a 55 lb flywheel.

The point remains, beyond the delivered power, we do internal work moving our legs up and down. That internal work is sharply affected by cadence, and we don’t yet know if that internal work is the same for FG vs SS at a particular cadence.

Otto

DiabloScott

What you're talking about is efficiency; in essence the difference between produced power and delivered power, and that's a valid topic to explore.

But the OP's question was not about power or efficiency of the "engine", it was about simple principles of gearing... and perhaps his misperception of how freewheels work.

ClydeClydeson

I get the sense that OP isn't convinced. HIs observations that the bike doesn't go as fast with the same gear ratio and pedal cadence is not rooted in reality, though. There is no mechanism for a FW equipped bike to lose any energy or speed per leg revolution compared to FG.

This is nonsense. Sorry, TRACKMAN .


There was a track racer at international competition some years ago (Olympics? American? Riding the kilo?) who had fitted a 2-cog freewheel and a derailleur (and a brake?), and this person was competitive, although I don't think they won.. OP's theory that there is a loss of power with freewheels would immediately make such a setup uncompetitive. I don't know if the rules have changed to specify that such drivetrains are not allowed, but I haven't seen or heard of anyone else trying it.

Broctoon

Thanks for the link. That is an interesting study, but it has nothing to do with differences between FG and SS drivetrains. I am not surprised that VO2 is affected by cadence; surely there are many other differences in a rider's performance across a range of cadences.

I think we're saying sort of the same thing, but I emphasize the phrase "when you're applying positive power through the crank" as the key qualifier to my claim that your body can't tell the difference between FG and SS. You're focusing on the fact that at very high cadence, there can be performance differences caused by unique characteristics of the two drivetrain types. We are both right in our respective assertions.

To say that "neither of us knows whether the internal work done moving our legs up and down is identical in FG vs SS" is ridiculous within the parameters I noted (legs able to keep up with the crank so positive power is being applied). We absolutely know there is no difference, just as we know that there is no difference in power delivery for a wheel painted black vs. one painted red. I mean, there might be a psychological effect when the rider knows what kind of drivetrain he's on, just as there might be when he knows what color his wheel is. The psychology of FG vs. SS is a separate topic that might make for an interesting discussion.

79pmooney made an interesting observation a few posts up. Some performance differences might appear when pedaling cadence is very close to drivetrain speed, i.e., your legs are almost able to keep up or just barely able to keep up with the bike. Certainly in that special case we'd want to look at delivered power separately from internal power. It's an important fact that riding a bike requires our legs, muscles, and joints to move in a very specific way (because the pedals must follow a circular path).

I'll go back to one point brought up in the original question of whether "there is a significant drop-off for freewheel regarding power transfer above 100 rpm of the crank." Worded in a less awkward way, I think the OP was asking, "Does a rider get less power to the ground on a SS bike than a FG bike at high cadence?" I believe that yes, there is probably less power reaching the ground any time the cadence is so high that the rider struggles to keep up. This is only because the FG forces the rider to try harder at keeping up. If the cadence gets so high that he cannot keep up no matter how hard he tries, then it's actually just the opposite: SS drive gets more power to the ground (zero) compared to FG (negative power). This seems ridiculously obvious. The point I was trying to make a few posts back (which perhaps is likewise so obvious that it didn't need to be stated and therefore caused some misunderstanding) is that SS and FG drivetrains are identical in their function anytime positive force is being applied. To ask whether one type of cog is more efficient at conveying applied power is like asking whether you'll ride faster when there's a leprechaun watching. That was really my point, that there's no mysterious, mechanical advantage or curse attached to either drivetrain type.

ofajen

Yeah, I think that there is probably no way for there to be any difference in internal work, either, at a given cadence. If it’s dictated by the dynamics of moving the weight of your legs around at a given speed, it will be the same at least averaged over more than one revolution, regardless of whether it’s FG or SS.

I do think cyclists should take note of this data on internal work. A key takeaway is that, other things being equal, a lower cadence at the same speed is energetically favored, because you do less internal work and deliver the same power to the ground with a lower total load on your CV system.

OTOH, there are other factors, such as needing to accelerate, available strength and impact of stress on joints that will favor higher cadences in many cases.

Otto

TejanoTrackie 

Something that hasn't been mentioned here is that some people are better at spinning high rpms than others. I was always a fast twitch spinner in sprints, both on the road as well as the track. I could crank it up to 200 rpm on rollers and over 140 rpm in sprints. I used a lower gear than many other racers, but still beat them because I could wind that gear much faster in a sprint. I remember one dude in particular who was the Illinois master's TT champion and finished second at nationals. I really had his number and he absolutely hated me. He would try to beat me in a gear that was at least 20 GI higher than I was using, and simply could not get on top of it as the speed increased, since wind resistance increases with square of the speed. No doubt he produced more torque than me, but power is proportional to the product of torque and rpm, and I was producing more power so I was faster. So it's like a small F1 engine turning 20000 rpm vs a big NASCAR V8 turning 8000 rpm, the F1 engine produces more horsepower even though it's much smaller.

Mikefule

There are so many possible variables.

Take a bike with a flip flop hub, with fixed on one side, free on the other, both cogs being the same size.

Whichever way round you fit the wheel, the weight of the bike will be identical. The frontal area will be identical. The aerodynamics may be infinitesimally different.

If the two cogs have the same number of teeth, then the conversion of rpm at the crank into speed along the road will be identical. For a given steady cadence, the bike will go at the same speed whether it is fixed or free.

If the rider is the same, and the conditions are the same, then it is easy to think there should be no difference in performance.

However, one important variable is the ratio chosen.

On fixed, if the gear is low enough to get up a steep hill, it may be too low to allow you to spin down the same hill. The same ratio on free would allow you to get up the steep hill and coast down, possibly faster than you could spin down on fixed.

However, if we assume a gear ratio that is "fair" for the expected conditions, giving no such bias towards free, then I think you would go faster on fixed for a number of reasons, but they are all to do with riding style.

Fixed is quite an intense experience. You have to pedal all the time and it encourages you to work hard, keeping the momentum on the ups and spinning for the hell of it on the downs. Many fixed riders try to avoid braking, so they ride for smoothness and rely on timing and coordination for junctions, traffic, and other hazards. The mindset is to keep rolling.

Freewheel can be a more relaxed experience. You need to use the brakes to adjust your speed, and it is tempting to feather them, or coast up to junctions, or even stop when you might keep rolling on fixed. On a gradual downhill, you may decide to coast comfortably rather than spinning hard. Freewheel single speed does not encourage that same determination to keep rolling. It feels more optional.

Therefore, on "average mixed riding" with no especially long or steep descents, I think most of us would maintain a higher average speed on fixed, but not for mechanical reasons.

ofajen

Internal work, with a bit more relevant detail.

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

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 Fornenti’s 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, I’d 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 it’s 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 all too familiar for FG riders I’m sure, but i find it helpful to have a more quantitative idea of the energy involved.

Otto

Broctoon

^ ^ ^ That is a good explanation.