Fixed wheel physics question

[Trigger]

Just a disclaimer to start: I never paid any attention at school and I've never ridden one, so this may seem like a stupid question to some of you University Challenge types

Fixed wheel bikes, does the momentum actually help with power required to turn the cranks? for some reason I seem to have it in my mind that if you can't free-wheel then that forward momentum must reduce the amount of effort required on flat ground.

I'm probably wrong, so can someone explain it?

Ta.

[robgul]

I'm no scientist but ride fixed quite a lot - you certainly get a "flywheel effect" with the direct drive - and acceleration is better than with a geared/freewheel bike.

Going down steep hills is a pain as there is a tendency for your legs to be ripped out of your pelvis with the cadence ... and for uphills you need to anticipate and attack ... but again the flywheel effect helps.

Rob

[stewartpratt]

Energy isn't created or lost. The same net energy is required to move along at a given speed regardless of free or fixed. A fixed wheel doesn't give you free energy.

If the momentum of the bike is pushing your legs round for part of the stroke then that's acting to slow the bike and so you have to compensate for not lifting your leg on the dead upstroke by pushing more on the downstroke.

If you're spinning properly then of course there's no difference anyway...

[Brucey]

if it really helped very much, in a sort of physicsy-way then 'ordinary' bikes (i.e. with freewheels) would have to be intriniscally inefficient in some way, which they are not. If they were, then the freewheel would not be popular for racing...

Where the fixed gear does help is that it teaches you to pedal smoothly over a wide cadence range; freewheeling is not an option.

cheers

[Si]

The flywheel effect on a fixed does not help you at all going up a hill - discuss (copied from when I've had similar discussions before).

Perhaps we could think about it like this:

If the momentum in the rear wheel is pushing your feet through the low power section of the pedal revolution (as some people claim) then your feet must be applying a braking force to the pedals - this can do nothing but slow the bike down. If you had a freewheel then your feet would be left behind by the wheel in the top/bottom of the pedal stroke but would apply no braking force - they would then catch up and apply a forward force later in the revolution. Therefore, rather than going power-brake-power-brake-etc like on a fixed, you'd be going power-roll-power-roll-etc on the freewheel and the freewheel bike would climb faster than the fixed.

However, in reality, your feet would apply forward force throughout the pedal revolution (even if the amount of power diminishes at certain points) . In this case the rear wheel never pushes your feet around and any fly-wheel effect is exactly the same on a fixed or free wheel.

Where the fixed certainly does win out in climbing is:
- it's generally lighter so requires less effort.
- there is less energy lost in the drive train with it having a straight chain line and no jockey wheels to wrap around.
- with a fixed you know that you don't have a bail out gear so you have to give it full dog from the start so you put more power in and this leads to you getting fitter.
- the physiological effect #1 - people think that they have a flywheel assisting them so the climb seems easier.
- the physiological effect #2 - the need to beat non-fixed riders up the hill to demonstrate the correctness of the One True Way.

[[XAP]Bob]

Yep - the easy way to think about it is that a fixed is no different to a bike which isn't free-wheeling in most respects.

The fact that you can (and do if you can't keep up) brake the bike with your feet on the pedals does encourage a smoother pedalling action, and gives a good degree of confidence in the traction at the rear wheel, but is not more efficient than a single speed.

This is easily proved by a little thought:
- On a fixie the only thing that can resist forward motion is the feet on the pedals

- On a freewheel the same resistive force could be applied (think about a jammed freewheel), but the freewheel limits the resistive force to very small. The energy required to pull the ratchet pawls up and down is being taken from the forward motion of the bike, but is insignificant.

Hence - Freewheel more efficient than Fixed.

Notes:
- I've ignored the tiny additional mass at the hub for the single speed
- I've used single speed so the rest of the bike looks the same
- On a fixie you could drop the rear brake, saving even more grammes - or you could do something sensible and put a hub brake up front

[james01]

+1 Agree with XAP Bob. If you find that the fixie is turning your legs faster than they want (eg downhill) then you end up wasting energy resisting it. A freewheel allows you to switch off the power as you approach a stop. A freewheel was available on some 1950s Rover cars and was reckoned to be worth a couple of percent improvement in fuel consumption.

[andrew_s]

However, in reality, your feet would apply forward force throughout the pedal revolution (even if the amount of power diminishes at certain points) . In this case the rear wheel never pushes your feet around and any fly-wheel effect is exactly the same on a fixed or free wheel.

It does push your feet round a bit. If you ride gears after only riding fixed for a substantial period, you'll find your feet have a tendency to stop at cranks vertical beacuse you've got lazy and used to letting the bike push your feet past the dead spot.

Where the fixed certainly does win out in climbing is:
- it's generally lighter so requires less effort.
- there is less energy lost in the drive train with it having a straight chain line and no jockey wheels to wrap around.
- with a fixed you know that you don't have a bail out gear so you have to give it full dog from the start so you put more power in and this leads to you getting fitter.
- the physiological effect #1 - people think that they have a flywheel assisting them so the climb seems easier.
- the physiological effect #2 - the need to beat non-fixed riders up the hill to demonstrate the correctness of the One True Way.

- with gears you lose momentum at each change of gear

[Mick F]

- with gears you lose momentum at each change of gear

With hub gears, yes.
With derailleurs, not always by any means.

With a rear derailleur, you can still propel forwards during a gear change, in fact I seldom ease off unless I'm pushing very hard. Normal riding, rear changes do not lose momentum.

Front derailleurs OTOH can be changed without losing momentum, especially close ratio doubles. With wide ratio triples, it's more difficult to change sympathetically whilst still under power.

[crimble]

The way I understand it is this....

All bicycle wheels are flywheels, they store energy and release it. Energy is put into the flywheel in two ways:
1. Kinetic energy ie the energy that you put in through pedalling.
2. Potential energy energy that can be potentially released because of where you are. the fly wheel has lots of potential energy at the top of the hill [due to gravity] and none at the bottom. This is why you can free wheel down and not up.

A fixed gear bike is a bike with a feedback system linking the cranks and the wheel. The energy from the rear wheel is transmitted to the crankset and drives this round [take your feet off the pedals and they will still turn]. Assume a fixed gear bike travelling on a straight road at a uniform velocity. If the cyclist pedals faster this will in turn feed kinetic energy into the rear wheel and the bike will go faster. If the cyclist resists [works against] the forces transmitted through the chain and pedals slower this will be a negative energy input and reduce the total energy in the system and slow the bike down.

A freewheeled bike is a bike with no feedback system. the freewheel is a clutch. Whenever the forces transmitted through the cranks are less than the forces transmitted by the rear wheel the clutch disengages. This is why tired riders can experience gaps in their pedalling. If the cyclist on the flat road pedals slower than the speed required to maintain the rear wheel speed then the pedals will spin and the bike will start to slow. If they pedal faster they will increase the energy in the system and speed up.

Going uphill the advantage of the fixed wheel bike is that as long as the rider is pedalling within their capacity they are topping up the flywheel all the time even as it loses energy due to climbing [loss of potential energy]. The disadvantage with the freewheel is that on steep hills the rider can effectively disengage the clutch and so the flywheel system loses more energy than would occur with the fixed gear bike.

[[XAP]Bob]

On a freewheel you can't reduce the energy - that can't be a disadvantage.

Up hill (i.e. power needed all the way) then there is no difference between a single speed and a fixie (again ignoring the small additional hub mass of the freewheel). A freewheel allows you to stop pedalling - to change your pace, to bang down and recover. A fixie forces you to keep going, with a constant circular pedalling action.

There cannot be a loss of efficiency associated with a freewheel:
- Either it's engaged, and there is no difference between fixed and not
- Or it's freewheeling, and there is a lower power loss due to the freewheel than the pedals (else it would be driving the pedals)

There is an insignificant mass change.

[Trigger]

It's starting to get out of my comfort zone but I get the gist of it, thanks all.

The reason for the question is that I've pretty much always had a singlespeed bike knocking around for use of one reason or another, but over the last few years I haven't had one and was thinking of building another, if for nothing else than to get a break from the heavy plodding machine.

I had a thought about trying fixed instead of SS, seeing as I've never ridden one and all that, but I don't fancy my legs doing 200rpm down our considerable hills so I may stick with what I know.

[Ribblehead]

I think the flywheel effect helps when climbing hills.

Taken to the extreme, if I had a sufficiently large flywheel, I could pedal at the same level of exertion for an entire ride, while maintaining an almost constant speed.

In real life, the flywheel mass (combined bike and rider mass) is nowhere near enough to create this effect, but it still applies on a smaller scale. Having a fixed wheel helps to even out the speed difference between the 12/6 and 9/3 o'clock positions.

[stewartpratt]

If we're being pragmatic, fixed is harder. When you're applying power it makes no difference; when you're trying to rest your legs you will end up slowing the bike; when you're trying to descend quickly it will thrash your legs (and scare the bejesus out of you) and if you're braking through the drivetrain it will work your legs in new and uncomfortable ways. There's no situtation where it's mechanically easier going than a freewheel.

Also, it's somewhat more challenging than a freewheel when you get leg cramp or suffer a build-up of gas...

[Ribblehead]

I had a thought about trying fixed instead of SS, seeing as I've never ridden one and all that, but I don't fancy my legs doing 200rpm down our considerable hills so I may stick with what I know.

Just try it! Most people find it surprisingly addictive, and some never go back. If you're worried about high rpm just make sure you have front and rear brakes, although, with practice, you'll need the brakes less and less.