Sunstar Ibike S03 electric bike kit £868

Turns your bike into a pedelec

BikeRadar score 4/5

Several companies offer electric bike conversion kits but Sunstar's S03 is the first we've come across that will turn your bike into a crank-drive pedelec. It’s a quality system that gives a lightweight, powerfully assisted and very 'bike like' ride.

Ride & handling: Smooth, powerful and not too far removed from the feel of a conventional bike

Most e-bike kits use a hub motor. The only other setup we've used that drives through the bottom bracket – the Gruber Assist kit, which we like a lot – requires you to press a button to activate the motor and simply drives the crank until you cut the power.

The S03, however, is a true pedelec system. You just set your power level and start pedalling. The motor's torque sensing mechanism then allows it to smoothly provide assistance in proportion to each push on the pedals.

Crank drive systems have one major advantage over hub motors: while wheel speed varies massively, your pedalling cadence remains more or less constant. This makes it easier for the motor to deliver smooth, efficient power across a range of gears.

Some hub motor setups are starting to rival the smooth efficiency of crank-drive systems, most notably the BionX system, though this weighs considerably more. Crank-drives have another ace up their sleeve, though: because the mechanism is mounted 'amidships' and low down, it helps maintain a conventional-bike-like ride quality and handling. The drawback is reduced ground clearance; we'd be wary of flying over really bumpy ground with the S03.

On high power (level 3 of three), the little Sunstar motor did amazing things on our moderately hilly test course, making it round faster than any e-bike we've tested there. Despite being fitted to a very unsporty women's hub gear town bike it managed to finish just above the more expensive and heavier BionX kit. In our separate steep hill climb test it came in just behind the BionX and Koga E-light, which is commendable for such a small motor. 

The lower two power settings also provided meaningful assistance. Although they're a step change down from level 3, they'll greatly extend the battery range. Sunstar say batteries will come in four sizes – 137Wh, 216Wh 264Wh and 432Wh – which our testing indicated would give approximate ranges of 12, 18, 22 and 36 miles respectively.

These figures are based on having level 2 on constantly. Using level 1 instead would greatly extend battery range, as would switching off power when not needed. This is easy to do using the thumb control, and pedalling without power assistance doesn't feel too bad. Some bikes with crank torque sensors feel terribly spongy when the motor is turned off but the Sunstar is perfectly practical for use on flat or downhill sections, even on our heavy test bike

The only slightly disappointing aspect of the kit was the high-pitched noise it emitted. This wasn't a surprise, as smaller gears tend to emit more and higher noise, and it's a small price to pay for the system's light weight and outstanding performance. Build quality and engineering are impressive, and while we’d like to see more weatherproofing on the wiring connection boxes, the S03 kept going through lashings of winter rain.

Fitting, pricing & availability: Light and relatively simple to fit but pricey when you take batteries into account

Sunstar are newcomers to the UK market but previous S01 and S02 versions of this kit have been available for a few years in their home country of Japan and in several European territories. The company are in the process of setting up a British dealer network which will provide trained mechanics to fit the kit to a bike of your choice, with a likely launch date of spring 2012.

Fitting involves removing the existing bottom bracket and inserting the Sunstar motor unit and torque arm fixing bracket. The battery will fit to one of three places, depending on what size is chosen – seatpost, down tube or rear rack. A cable run to the handlebars is needed for the on/off switch and power level control buttons.

Prices will start at around £1,100 including fitting, depending on battery choice and other options – for example, battery-motor connector cable length and whether you want a bash guard on the underneath of the motor (not featured on the review model). 

This might sound pricey when you consider you can get an off-the-peg Panasonic-powered pedelec for as little as £1,300. However, the non-electric bits on bikes of that nature are often on the budget side and the Sunstar system stands up well next to other top-end kits. 

Sunstar play on the fact that this kit is one of the lightest on the market, with a claimed combined motor and controller unit weight of just over 3kg. Add the wiring, connectors and handlebar control and, depending on battery size, you'll have a kit that weighs from 5kg up to around 6.5kg. While some variants of the Tongxin hub motor (also known as the Nano) might creep in under this weight, it's still very impressive and lighter than the vast majority of retrofit kits.

The S03 kit's main drawback is that it only comes with a one-year battery guarantee when two years is becoming the norm and certain companies offer the option of an extended warranty. Exact battery replacement cost wasn’t fixed at the time of writing but it's certainly going to be several hundred pounds, whatever size you go for.

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