power transfer efficiencies

Good Morning All

I am posting these questions in several groups. It is to be hoped that this is not a breach of group etiquette.

I stumbled across a copy of a book called, Pedal Power The History Of Bicycles. In the modern age chapter there is a statement, “Hub gears, generally with three speeds, are reliable and long lasting. The sealed mechanism only needs occasional oiling and adjusting. The drawback is a loss of efficiency, with only about 50 percent of a cyclist’s pedaling power being converted to movement of the back wheel.â€￾ This seem a high price to pay for a .75 under and 1.33 over, the statement was accompanied by a close up color picture of a Sturmey-Archer hub gear. Is this true, and if true, true for all internally geared hubs?

Finally, does anyone know or know of some resources for rated power transfer efficiencies: i.e. chain and cogs, straight cut gears, helix gears, planetary gears, v-belts, gates belts, internally geared hubs, NuVinci hubs……….

Thanks for any and all help -grant

Hi John,

I think there'd have to be something seriously wrong with a hub gear that was less than 70% efficient, at any useful power level...... in any case, 2nd gear is usually direct-drive in a 3 speed.

There are various efficiency tables in chapter 9 of "Bicycling Science" 3rd ed.

The only sensible figure I've seen for the NuVinci is 92%, at best.

Gates Polychain is claimed to be at least as efficient as a new chain drive. (96-98%).

Some links:
http://www.rohloff.de/en/technical/efficiency/index.html
http://www.rohloff.de/en/technical/speedhub/efficiency/index.html
http://www.bhpc.org.uk/HParchive/PDF/hp52-2001.pdf


Miles

Good question.

I know timing belts are around 98%.

Here's some numbers on a geared hub, but I don't know where they got their numbers from:
http://www.rohloff.de/en/technical/efficiency/index.html

Here's another reference, again not exactly guaranteed accurate:

From: http://en.wikipedia.org/wiki/Bicycle_gearing


Derailleur gears
External gearing utilizes derailleurs, which can be placed on both the front chainring and on the rear cluster or cassette, to push the chain to either side, derailing it from one sprocket to a neighboring sprocket. The sides of the sprockets may be sculpted to help catch the chain, pulling it up onto their teeth to change gears. There may be 1 to 3 chainrings, and 5 to 10 sprockets on the cassette or freewheel. Derailleur type mechanisms of a typical mid-range product (of the sort used by serious amateurs) achieve between 88% and 99% mechanical efficiency at 100W. In derailleur mechanisms the highest efficiency is achieved by the larger cogs. Efficiency generally decreases with smaller cog and chainwheel sizes.[1] Derailleur efficiency is also compromised with cross-chaining, or running large-ring to large-cog or small-ring to small-cog. This also results in increased wear because of the lateral deflection of the chain.


[edit] Internal (hub)
Main article: Hub gear
Internal hub gearing works by planetary, or epicyclic, gearing, in which the outer case of the hub gear unit turns at a different speed relative to the rear axle depending on which gear is selected. Rear hub gears may offer 2, 3, 4, 5, 6, 7, 8, 9, 12, or 14 speeds. Bottom bracket fittings offer a choice of 2 speeds, and are generally foot-operated. Internal hub gears are immune to adverse weather conditions that affect derailleurs, and often last longer and require less maintenance. However, they may be heavier and/or more expensive, and often do not offer the same range or number of gears. Internal hub gearing still predominates in some regions, particularly on utility bikes, whereas in other regions, such as the USA, external derailleur systems predominate. In a typical hub gear mechanism the mechanical efficiency will be between 82% and 92% depending on the ratio selected. Which ratios are best and worst depends on the specific model of hub gear.

Check me if I'm wrong, but isn't there a law of physics that says enery can't be produced or lost (something like that).

If a gear hub were operating at 50% efficiency, then where does the other 50% of the energy go? Heat? Sound? Kenetic energy? The hub would have to explode.

For example, I read that cars are 20% efficient. But notice that they heat-up to hundreds of degrees, and need huge radiators to dissapate the heat.

I imagine that gear hubs are about 95% efficient, and fixed gears must be 99.9% efficient (all losses in chain).

If we are losing energy, I want to know where its going.

Ref. the links that I posted, hub gears have efficiencies between 76% and 96% If you were putting out 200 watts going up a hill, @ 76% efficiency, that's 48 watts of heat to dissipate. With plenty of airflow, the hub isn't going to get that hot.... You, on the other hand, @ 25% efficiency, would be sweating like mad....

Beagle123 said:

Check me if I'm wrong, but isn't there a law of physics that says enery can't be produced or lost (something like that).

If a gear hub were operating at 50% efficiency, then where does the other 50% of the energy go? Heat? Sound? Kenetic energy? The hub would have to explode.

Click to expand...

Why?

Some parts of energy loss is dissipated as a heat from winding.
The other parts produced by friction is dissipated as a sound and heat again.
It is continuous process so it cant explode.

I have a 750 watt motor on my gear hub its rated load is about 500 watts, if it was losing 30% of that to heat, that's 150 watts. That's like 2 1/2 60 watt light bulbs. That would make it hot.

I'm going to feel it next time I ride it to see if its warm.

Beagle123 said:

I have a 750 watt motor on my gear hub its rated load is about 500 watts, if it was losing 30% of that to heat, that's 150 watts. That's like 2 1/2 60 watt light bulbs. That would make it hot.

Click to expand...

Yes, but how warm? How long would you be using it at 500 watts? A hub gear is a substantial chunk of metal......

Beagle123 said:

I have a 750 watt motor on my gear hub its rated load is about 500 watts, if it was losing 30% of that to heat, that's 150 watts. That's like 2 1/2 60 watt light bulbs. That would make it hot.

I'm going to feel it next time I ride it to see if its warm.

Click to expand...

If you lose 30% so the heat is 750*0.30=225W.
It is equal to 3 bulbs 75 watt each.

But it is only at max load equal 500 W, usually the load is much less than that.

Hey, Piotrek, we coincide in time and thought

eP said:

Some parts of energy loss is dissipated as a heat from winding.
The other parts produced by friction is dissipated as a sound and heat again.
It is continuous process so it cant explode.

Click to expand...

Friction is heat. And, it only takes a tiny tiny tiny amount of energy to produce sound. The only realistic way the energy is lost is from heat. And I don't think they're producing heat.

Think about it: the chain is pulling the sprocket causing the wheel to turn. IF the wheel is only receiving 350 watts of the energy, where is the other 150 watts going?

The only valid answer to this must account for the lost energy.

Also, use common sense: There gear hubs are made to be powered by humans. Do you think these "serious" cyclists would tolerate losing all that energy?

Beagle123 said:

Also, use common sense: There gear hubs are made to be powered by humans. Do you think these "serious" cyclists would tolerate losing all that energy?[/size][/color]

Click to expand...

No, if you were a "serious" cyclist you wouldn't use a 76% efficient hub gear. Try reading the Human Power link that I posted....... http://www.bhpc.org.uk/HParchive/PDF/hp52-2001.pdf

Beagle123 said:

eP said:

Some parts of energy loss is dissipated as a heat from winding.
The other parts produced by friction is dissipated as a sound and heat again.
It is continuous process so it cant explode.

Click to expand...

Friction is heat. And, it only takes a tiny tiny tiny amount of energy to produce sound. The only realistic way the energy is lost is from heat. And I don't think they're producing heat.

Think about it: the chain is pulling the sprocket causing the wheel to turn. IF the wheel is only receiving 350 watts of the energy, where is the other 150 watts going?

The only valid answer to this must account for the lost energy.

Click to expand...

120 watt as a heat from motor's case, 29 watt a a heat from the chain and gears, less than 1 watt as a sound and the rest as a energy for wearing ( materials particles disallocations, miniature breaks (dents) etc. )

Beagle123 said:

Also, use common sense: There gear hubs are made to be powered by humans. Do you think these "serious" cyclists would tolerate losing all that energy?

Click to expand...

If you have a freewheel only small parts of yours energy is dissipated at motor's freewheel.
But if you have a motor hub, idlle power is very important for you if you powered your bike by yourself most of the time.

I think the total loss is fairly small & a near constant because the amount of friction is relatively fixed. So when it gets expressed as a percentage relative to a higher level power input it works out to be a higher percentage efficiency at the higher watts.

I know someone is going to point out that friction isn't really fixed but I'm only speaking generally here. Yes it does go up with speed which would be the case at higher power in, but it doesn't increase geometrically the way wind resistance does. The friction increase is at a very slight, very shallow linear rate.

Tests with bicycle power would have to include the cyclical nature of pedaling which places peaks and valleys in the powertrain torque. These peaks and valleys would likely worsen efficiency in a geared hub.

The smooth and continuous power delivery that an electric motor provides might actually get slightly better efficiency. Most geared hubs list their efficiency in the range of about 95% plus or minus 2%.

And the BRAND of the hub matters... the Rohloff has excellent efficiency, while others will sure to have much worse.

The Sturmer Archer hub has also introduced some new ideas over the "old style" hubs in that now first gear is 1:1 and all the others are higher geared than that. This means that when torque applied is highest the gearing is direct. As the torque reduces with increasing gearing the mechanical friction increases, so they have matched things better. Also the shifter is external, which is just plain cool and not related to anything but design.

So in the "real world" with a more modern hub things might not be as bad as one might think...

You can definitely tell if a gear hub is losing significant energy by heating. Feel the temp before and after a hard 10min or so ride. The hub has plenty of air cooling, but when you stop, the heat inside will take a while to get out, so you would notice a temperature rise after stopping.

If you don't notice a significant temperature difference, then you can assume the losses are fairly insignificant compared to other losses.

I know the tires on my Vego heat up significantly when riding, even though they get plenty of air cooling and are pumped up to 60psi. Lots 'o watts wasted there.