How light can ebikes be made? (for a reasonable cost!)

[coops]

There seems to be quite some range in weight in ebikes of 26-28" wheelsize available (now or soon): Torq/F-series around 24-25kg, Wisper 905se 21kg (with a larger battery) and Kalkhoff Agattu 20.8kg. I realise some of the weight difference may be due to rear carriers etc., and the Agattu is a 26V Panasonic drive pedelec with lightweight Li-battery, but that doesn't appear to account for all the difference.

So my question is, what are the main factors which account for the difference in bike weights, do you think, e.g. frame build, wheels, components, - especially larger wheelsize bikes of 24-28" - and how light can an ebike of moderate/high power be safely and reliably made (within a realistic budget!), do you think?

My Aluminium framed MTB, for instance, is 15kg and the agattu weight, minus motor & battery etc., appears to be similar, the Wisper even lighter... and a friend of mine intends to try to convert a Halfords Boardman 28" wheel bike (about 9 or 10kg!) to an electric...

Apologies if this has been covered before, but I don't recollect it has.

(I recall flecc saying that it should be possible to make a sub-20kg 24V ebike, and though I can't recall the Twist's weight, the Agattu seems to be getting close, and for a reasonable cost too )

Stuart.

 

[Ian]

Hello Stuart,

mainstream alloy bikes got down to about 14kgs a long time ago and have stayed there, expensive specialised bikes have continued to get lighter but as has been seen from some of the spectacular failures that occur in races such as the tour they are only just strong enough to do the job with very little margin for an overweight rider or heavy load. I would say that the 14Kg of a conventional mainstream bike is probably as light as it is economically possible to go without compromising the strength needed for general purpose use and abuse, add the 5 or 6 kilos the lightest electrical parts will weigh and we're back at 20kg. While it is certainly possible to go lighter than that I think such a bike would have too many compromises to make it a mass market product.

 

[flecc]

Some more exotic alloys and techniques come in to get these full size bikes below around 22 kilos. Wisper use them on the 905 series, and I've no doubt Kalkhoff are doing the same,

Then there's pushing the luck on how thin things can be and still be strong enough, plus the diameter versus tube wall thickness decisions. A brittle metal can be formed into a larger diameter tube while still retaining dent resistance, but with the risk of fracture or compression failure increasing with each respective step in the brittleness, wall thickness and diameter.

After that the more exotic materials come in, carbon fibre, kevlar, titanium etc., all increasing the cost.

But best of all is the design cunning, making the optimum use of each scrap of material, whatever that is, and that usually means a return to the fundamental diamond frame basis as the start point. That's made more difficult on an e-bike due to weight distribution and battery placement requirements.

P.S. Just seen Ian's post. Snap, the same conclusion for a 20 kilo e-bike bike and luck pushing after that.
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[coops]

Thanks guys, that makes a lot of sense now.

Just a follow-up query, do you think battery placement could be improved to reduce frame weight requirements (including 'in-frame' batteries, though they would seem to have a limited size) or is the current 'norm' (behind seat tube) ok? It certainly seems to work well in terms of weight distribution and simplicity.

Stuart.

 

[flecc]

The problem on many e-bikes with the behind seat tube battery is the angle of that tube. To keep the wheelbase short, the tube angles forward at the base to compress the fame dimensions, so the commonly used suspension seatposts don't work properly. They need the tube to be as vertical as possible, and my 11 stones can't overcome the side friction to operate the spring on most of them.

We need trolley-bikes, a boom on the carrier picking up from overhead wires with a tiny recharging battery to get us across the gaps at junctions and allow an overtake with retracted boom.

The current should be free to all to encourage e-vehicle use, and our bikes would go like a bomb with plenty of current and no battery weight. Wouldn't matter how much they weighed either.
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[coops]

That would seem to solve all battery/weight issues in one go, flecc! I've never heard of that idea - could it actually be realised do you think?

 

[JohnInStockie]

....and maybe a bumper at the front

 

[flecc]

That would seem to solve all battery/weight issues in one go, flecc! I've never heard of that idea - could it actually be realised do you think?

Anything's possible Stuart, given the will. With a moderate voltage system and a clearly defined road lane banned to other vehicles, the boom arms with deep V engagement slots and auto guided into line by sensors, it could work. Could even have covered narrow "bubble car" three wheelers for the suit brigade.
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[Ian]

How about linear induction motors built into the road surface, then all we'd need would be an aluminium plate on the bike, no need to carry a motor even. The down side would be that anyone with a pocket full of coins could unexpectedly be propelled down the road at high speed.

On a serious note such a similar system could be used to induce a charging current in a coil on the bike, a sort of "wireless" charging system that could be used on the move, it could work for larger EV's as well.
Again it has a down down side for people with coins in their pockets, or worse a chain around their neck.. the fire risk!

 

[flecc]

Yes, I had considered that first option Ian, but thought the old fashioned low powered way might be best. I could be wrong, but I suspect the induction system is likely to be very wasteful of energy, although it's in use on some bus services, in Switzerland I think.

Your wireless energy transmission suggestion is very much up to the minute, though I think waste could again be a problem. I also think there might be a big backlash on the health issue with wireless energy transmission, still not enough known about that.
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