To John

 

Personally I would prefer that they keep trying to improve on the designs, I just like us moving forward on a wide front. I realise the implications of all the extra cost of this, but we are still in early days, and so I would like to see even more unusual designs be tried rather just everyone reverting to 1 design.

 

Fully agreed, but economics rule.

 

The Twist has done it quite well without a major adaption. Is the power too low on a Twist, and that's Ive not seen a thread of a broken hub on a Twist?

 

There have been quite a few. Many of the 4 speed Nexus hubs failed so the SRAM that you have replaced it, and quite a few of the 3 speed as well.

 

I dont understand this. My Twist applies power from the first push on the pedal, and it should be possible to have a speed sensor that cuts out assistance completely at 25kph without phase down shouldnt it?

 

Yes but you don't cycle at zero mph. You cycle from 5 mph upwards to the phase down point, and thats an 8 mph band. It's the only motor drive area where gears could be used, and it's too narrow for that to be useful. Cars go from 0 to 30 mph on one gear for example, and they are comparable in this respect. The 13 mph power phase down you have is an absolute requirement in European law, so designing it out isn't an option.

 

I dont understand this one Flecc. Surely if you are using you power efficiently with the correct gearing its always better? Surely its simply the physics of the gradient vs the mass and the force applied divided by the turning circle

 

It's difficult to deal with this since I've already explained it and the points just aren't being understood for some reason. Yes, the theory is fine, but are you seriously suggesting it's necessary to change gear four or five times between 6 and 12 mph? Of course you aren't, or I hope you aren't, for you'd always be changing gear and never driving the thing!

 

Lets take an example. At 8 mph on the peak power and slowing down on a hill, we move away from the power peak. At 7 mph we are still within 5% of that peak level of power, but if we use a gear hub to bring the peak power to the 7 mph point, that adds a 15% efficiency loss, so we are at least 10% worse off by using the hub. In other words, until the power falloff away from the peak is sufficient to exceed the gear inefficiency loss, gears won't be worth using. Because modern Hall effect motors have such a broad and fairly flat top torque/power curve, you have to move a long way off the peak for the loss to be serious. But since we only have to accomodate an 8 mph wide band, how far can we move off it? Obviously not very far. Therefore, as I've said, only two gears can be accommodated with any efficiency in that narrow band, one with the peak at 6 mph for good hill climbing in the 4 to 8 mph region, and one with the peak at 10 mph to handle the 8 to 12 mph region before phase down commences. But since the two gear system adds all the disadvantages from complexity I've already expressed, and the power curve on Hall effect motors is so wide, it's still not worth bothering for any reason, environmental or practical.

 

I think there is something I am missing here. Does all this relate to power bands in some way, and not motor efficiency? If we are saying that the newer hub motors are more powerful in most conditions, then is that because they simply dont have a power band in certain areas?? This doesn't make sense to me. The Torq for example allows you to apply power immediately doesnt it?

 

Because a wheel hub Hall motor covers the load requirements at all speeds, it's made a bit more powerful than needed to ensure the power at the ends of the user band and off the peak is sufficient. Therefore it has a built in inefficiency. But that surplus built in power makes it faster on hills, in headwinds, in acceleration and even on the flat when its anywhere near the peak power sections of it's power curve. That's the payback which offsets the supposed inefficiency. I'm forever preaching this. Theoretical efficiency in isolation is worthless, the work done has to be included in the equation, and when that's included, the supposed inefficiency largely disappears, cancelled by the superior performance of the more powerful motor.

 

If that wasn't true, only the tinyest and weakest of everything would be efficient, clearly a nonsense.

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Edited June 29, 200718 yr by flecc

Thanks flecc

 

Sorry to interrupt the flow of the thread John, just one other quick thing re brakes, hope you don't mind :

 

If V-brakes cannot be fitted, is there another alternative to discs i.e. can a roller brake or the like be fitted in place of the disc, if necessary/wanted? Or does that depend on the bike setup?

 

Yes any brake is ok, the hub determines these, so it means a wheel build with a suitable hub. Roller brakes are mounted on a short spline edge of the hub, but the shoe type brake drums are integral to those hubs. Both use a torque arm and a clip around a frame tube to prevent rotation.

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@ Coops - never a problem mate

 

@ Flecc - thanks for your 'perseverance' on this. I have noted you may have mentioned this once or twice before Its almost there, the sponge is beginning to absorb some liquid...but can I please clarify just a couple of little points please..

 

15% efficiency loss using the gears...is that right? Is it that much using a chain drive and Hub gearing? I had thought that the maximum power loss through chain and gearing was about 5% as that is why we ruled out shaft drive cranks that lose about 8%??

 

Changing gears between 6 and 12mph....erm, yes I might. I understand what you are saying there of course, but I thought that it was good practise to try and maintain an average cadence throughout the gears, and to have a sufficient gear range to do this (Ive been trying to teach myself this )?

 

Hub motor a bit more powerful than needed....Ah-ha, thats what I now understand. So that they have thier 'excess power' in the main asist zone for where most would need it (or want it) e.g. 11-13 mph in the Torq?

 

Thanks

 

John

@ Flecc - thanks for your 'perseverance' on this.

 

No problem John

 

15% efficiency loss using the gears...is that right? Is it that much using a chain drive and Hub gearing? I had thought that the maximum power loss through chain and gearing was about 5% as that is why we ruled out shaft drive cranks that lose about 8%??

 

Derailleur gears can reach the high 90s in efficiency, but are often lower. The highest efficiency claim for a hub gear ever was on the SRAM 3 speed at 92%. But that's a bit of a con, since it included the middle gear in the tests which is direct drive, so not geared at all! Hub gear efficiency is generally about 85% overall, but a bit worse in the lower gears, so you see I was being generous.

 

Changing gears between 6 and 12mph....erm, yes I might. I understand what you are saying there of course, but I thought that it was good practise to try and maintain an average cadence throughout the gears, and to have a sufficient gear range to do this (Ive been trying to teach myself this )?

 

Yes, but again that's you, the rider. I'm speaking of the motor, and it's pointless that using lots of gears between 6 and 12 mph. After all, with each gear change the drive is lost. That cannot be efficient! Routing the motor through the hub introduces the hub gear inefficiency, and also exposes the motor to the whims of the rider changing gears at moments inappropriate to it. My guess is that the average rider will achieve less motor efficiency than if the motor is just left to drive continuously across it's powerband, since rider cadence requirements and power constant are not in any way the same as a motor's power curve. If a motor is to be geared, it's best it has it's own gears.

 

Hub motor a bit more powerful than needed....Ah-ha, thats what I now understand. So that they have thier 'excess power' in the main asist zone for where most would need it (or want it) e.g. 11-13 mph in the Torq?

 

That's it John, high for the Torq as you indicated, for speed and iffy hill climbing, but at about 8 mph for best all round 5 to 15 mph performance on most legal e-bikes.

 

A lot of this didn't apply for older brush hub motors and other oddities when the Twist was introduced, but as said before, things are changing.

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Edited June 29, 200718 yr by flecc

I've learned quite a bit in the last couple weeks.

 

So...if cost were not an option, how close are the current hub drives to being the best current technology can come up with? In other words, are they already 80% there, 90, or more in your best guess?

 

So...if cost were not an option, how close are the current hub drives to being the best current technology can come up with? In other words, are they already 80% there, 90, or more in your best guess?

 

The law of diminishing returns applies here, as the efficiency rises, so each gain becomes ever smaller proportionally, so there's very little more at present that could make a dramatic difference electrically. Improvements in the metalurgy could bring us fractionally more efficient windings, but it would need a breakthrough in superconductive materials to make a big difference here.

 

Small mechanical gains in transmitting the power through to the wheel may still be found, but overall there's nothing on the horizon that would permit 90% to be reached, although marketing and advertising departments reach and surpass it without any problems.

 

Designers will get more clever in the way the available power is used in a bike though, so the overall package will show apparent advances to the user, even though they will be perceived rather than real. That doesn't matter of course, as long as the customer feels a gain, it's real. This is already happening.

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Edited June 30, 200718 yr by flecc

I agree Flecc

 

The law of diminishing returns applies here, as the efficiency rises, so each gain becomes ever smaller proportionally, so there's very little more at present that could make a dramatic difference electrically. Improvements in the metalurgy could bring us fractionally more efficient windings, but it would need a breakthrough in superconductive materials to make a big difference here.

 

Small mechanical gains in transmitting the power through to the wheel may still be found, but overall there's nothing on the horizon that would permit 90% to be reached, although marketing and advertising departments reach and surpass it without any problems.

 

Designers will get more clever in the way the available power is used in a bike though, so the overall package will show apparent advances to the user, even though they will be perceived rather than real. That doesn't matter of course, as long as the customer feels a gain, it's real. This is already happening.

.

 

That's what it needs Flecc, some better and better marketing; give Saatchi & Saatchi a bell.

Peter

But that's the familiar history Peter, it's the last paragraph that holds what's happening now in the smoke and mirrors department.

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