Want to build road legal bike switchable to something for offroad

[The Maestro]

Hi,

I'm interested in building a road legal bike offering good acceleration up to the 15 mph limit, but also make it derestricable for offroad use, in this mode it should be able to get the best speed possible out of the 250 watt motor.
I'd like to base it on a lightish alloy road bike not a mountainbike.

In essence it would be like the Wisper 905SE but hopefully with better topspeed and based on a bike with 28" or 700C wheels rather than moutainbike style 26" wheels.

I've searched the forums for ideas but am struggling to pull all the information together (I'm thick). Could anyone recommend the kit options/suppliers to do this?

Thanks,

The Maestro

 

[ElephantsGerald]

Sounds like you're looking for a Mk I Torq...

 

[john]

There are certainly hub motors which will run to 20-30 mph (unloaded) at 36V. If you then want to restrict it to be legal, the only way I know of doing this is to limit the throttle to effectively limit the voltage. This can be done simply with a resistor. However, this will also affect the low speed performance as it will likely limit the torque.

Kit suppliers are few and far between. You may have to look outside the UK for the right motor.

 

[Tiberius]

Look on www.ebikes.ca Homepage of the revolution for the Cycle Analyst.

I believe this has an option for overriding the throttle at a given speed or current. So if you program it correctly and connect up the override it will limit the bike to 15 mph. Then for off road use, disconnect the override.

Nick

 

[The Maestro]

Aaargh everyone keeps telling me I want a Mk 1 Torq, and they are right but I can't find one! I'd probably rather build my own though so I'm familiar with it and can fix it if it goes wrong.

I've read a lot about kits/motors but am not sure which is the best way to go, e.g.

Heinzmann - expensive, I believe it can't be derestricted
Cyclone - 180 Watt motor is only legal one and its meant to be weak
Puma - Lots of people seem to recommend this but I can't find much info. let alone a supplier.
Crystalyte - Ditto

I'm not quite sure that I understand the legal position either. If you have a 500 Watt motor but restrict it to only use 250 is that legal or does it have to have a motor rated at 250 watts? I'd rather not get into legally dubious ground on this but I do want a switch to derestrict it for certain situations and just to see the full potential.

 

[Tiberius]

Meastro,

I think our last posts crossed. The 250 W motor limit is a bit of daftness. Electric motor power ratings are not like petrol engines. The power (either in or out) depends not only on the motor but on the voltage you give it and the rpm. So what is the difference between a "500W" motor only given enough electricity to run at 250W and a "250W" motor that hasn't been given twice the power?

I think the actual spec is 250W continuous rating, which is a little more clear. But one manufacturer's continuous rating may not be the same as another's.

Its a grey area and I don't think its been tested in court. Best advice is to be reasonable and don't draw attention to yourself so as not to be the test case.

BTW. I have a Torq 1 and am currently collecting the bits to build something similar to what you propose.

Nick

 

[pwylie]

I have ordered a Heinzmann system from Kinetics in Glasgow.

I am hopeful it will arrive in the next week or so and will post a full review in due course.

I have ordered the Pedelec kit - front wheel, 9.6Ah Li-Ion battery, 1122 motor (53Nm, 33A, Steel gears) hopefully 15% climbing, 15mph + on a 28” wheel although I don't really know what those things mean. I think it might go a wee bit faster than 25 km per hour but we will see. The kit weighs about 8kg & will be fitted to my 12kg hybrid.

I will watch with interest what you go for. If there had been a bionx supplier nearby I would have gone for a bionx Pl-350 as a retro fit. I did not want to buy a new bike as I really like the one I have.

The more info that is available on the alternatives that are out there the better.

 

[Jeremy]

Nick is right, the 250 watt limit is a bit of definition daftness.

I've been trying to get to grips with the DfT over the similar power definition for an electric motorcycle I'm building. In my case, to keep it in the moped category, the power limit is 4kW.

The DfT have confirmed that provided that the actual power delivered to the motor cannot exceed the 4kW limit, then the actual motor rating doesn't matter.

I'm using a motor with a rating that is dependent on battery voltage, but can deliver in excess of 10kW. The controller will be limited, so that the power delivered to the motor cannot exceed the 4kW limit.

I think that using the Cycle Analyst speed limiter would be fine. In practice, there is no easy way to measure the average power into the motor, at least as far as the law goes, but there is an easy way to check the maximum speed. I'm sure that just using a speed limiter would be OK in practice, particularly if the bike doesn't look obviously OTT in terms of it's appearance.

Jeremy

 

[frank9755]

Jeremy, you'll know this better than me: I can't imagine it being a problem on a discreet implementation, but wouldn't the switch make it, strictly speaking, illegal?

 

[Jeremy]

I don't think it would, Frank, as electric bikes aren't subject to vehicle type approval (yet). As long as the bike is actually legal when ridden on the road (with the switch in the "on road" position) then it would be complying with the law. In some ways this is analogous to the road speed limits. Most cars are easily capable of breaking most speed limits, the onus is on the driver to ensure that his vehicle doesn't exceed the limits.

If electric bikes ever become subject to something like ECWVTA, then the dodge with the switch may well be illegal, although even then there may well be the opportunity to approve a bike with such a system. Other specialist vehicles are type approved with features which, if activated, are not road legal.

Something as simple as a throttle limiter may well work OK to restrict an otherwise illegally powerful electric bike. A resistive potential divider across the throttle voltage signal, with a switch to over ride it, would probably work well as a restriction system on an illegally powerful motor. Such a system could be set to only allow a percentage of full throttle power when in restricted mode.

Jeremy

 

[The Maestro]

BTW. I have a Torq 1 and am currently collecting the bits to build something similar to what you propose.

Nick

What motors etc. are you considering?