belt drive vs chain drive efficiency

[Brucey]

some interesting results here;

http://www.friction-facts.com/media/wysiwyg/Gates_Carbon_Belt_Drive_rev.pdf

These results indicate that, at highish continuous powers, the belt drive could be more efficient than a chain, if only the preload could be held down to a reasonable level.

However the required preload is determined by the maximum tension in the top run of the belt; if the power input goes up, so must a fixed preload if the bottom run is not to go slack and cause the belt to skip.

So does anyone know of a spring preloaded tensioner that can hold enough tension in the bottom run at all times, without incurring the drag of a very high preload?

A caveat; it is far from certain that for real riders the losses would be similar to those under test; if the belt is at all stretchy, the pulsy pedalling torque may generate additional losses that are not seen in continuous drive/steady torque applications.

In addition unlike the test rig, real bicycles are not always fitted with the best bearings; if you look at how the driver is supported inside many hubs, it is clear that high drive tensions can generate additional losses in IGH bearings that might otherwise not be seen. Again this may suggest that a (preloaded) belt driven IGH could be less efficient than a chain driven one, to a greater extent than simple tests might suggest. However I've not seen test data that has been generated with this idea in mind.

cheers

[[XAP]Bob]

Surely an upward sprung roller would be sufficient in the case of a gates style belt.

Don't need to engage teeth at all...

[[XAP]Bob]

A caveat; it is far from certain that for real riders the losses would be similar to those under test; if the belt is at all stretchy, the pulsy pedalling torque may generate additional losses that are not seen in continuous drive/steady torque applications.

Heavily dependent on the hysteresis curve, but yes. We know steel is pretty good at returning the energy

[Keezx]

So does anyone know of a spring preloaded tensioner that can hold enough tension in the bottom run at all times, without incurring the drag of a very high preload?

cheers

Derailleur model with 2 pulley's .
Enhanced teeth and belt form (rectangular instead of triangular)

I doubt if it will ever be as efficient as a well lubricated chain.
Advantage will only be less maintenance.

[[XAP]Bob]

I'd suggest that the efficiency question is already answered above...

A roller holding the belt in place as it contacts the rear sprocket would allow the system to run with a lower preload, and since it appears (from a skim of the above) that it is the preload which reduces the efficiency (by a really tiny amount) this could be a better approach.

[Brucey]

I was thinking of something like Moulton's snubber, but spring loaded. Gross belt adjustment would still be by moving the wheel or the BB, but the tensioner would take up the last 1/4" or so of slack and maintain tension on the lower run.

I have always worried about high preloads on belts and chain drives because (as well as the 'imperfect bearing' issue) bicycle quality sprockets and chainrings are not perfectly round. The fatigue damage that can accrue from a high preload might well be greater than that arising from the road or the normal (pedalling induced) transmission fatigue loadings.

cheers

[[XAP]Bob]

Like this...

Should solve the issue of jumping - which is currently solved by preloading (which brings associated problems)

I'm not sure why the snubber, maybe with an additional roller bearing up alongside the chainstay, isn't a more common solution.

[bobc]

It's not uncommon Bob, rohloff, for example, specify snubber fitment, though this may have something to do with their IGH's general "gearing down" and consequent small rear sprocket.
Incidentally the rohloff docs say the snubber should not touch the belt in normal operation, it's just there to prevent the teeth riding up & resulting skip (from memory they ask for 0.5mm clearance)
It would seem to make most sense for a tensioner to ride on the back of the belt, and this also increases wrap.
The belt manufacturers generally say 6 teeth should be in mesh to get proper performance and strength.
If you buy a gates carbondrive belt, it comes with lots of warnings about twisting it or bending it sharply - so a jockey should have plenty of radius. The snubber pictured above is fine as a snubber but could not be safely used as a tensioner (or a combined snubber/tensioner) IMHO.
My old bike always worked excellently without a snubber, I think if you can manage without, it is best to do just that... (a part that isn't fitted never breaks or wastes any power...)

[Redvee]

Has there been any comparisons on the efficiency/drag of a chain driven SS set-up over a derailleur geared set-up?

[Brucey]

If you look at the way friction facts test chains etc it is without jockey pulleys. They have also tested jockey pulleys separately. The only missing ingredient is arguably the losses that are incurred when running offset chainlines.

cheers