Skarper DiskDrive: From Regular Bike to Ebike with Just One Click

Conversion kits for ebikes is a tricky business. It is certainly attractive to adapt a bicycle that has been ridden for years to new personal needs with an additional e-drive. Or to participate in this technical development in a relatively affordable way. Often, however, the original bicycle has to be spoilt with dowdy components and numerous cables, or even complete wheels have to be replaced. That is, if your own bike is at all suitable for the respective conversion kit. A London-based company has now presented a solution that is also not compatible with every bicycle, but seems to be well done both technically and visually.

1. Parts and assembly
2. Technical specifications
3. How does the drive work?
4. Who are the potential target groups?
5. Does such a drive pay off?
6. Possible hurdles
7. What Skarper plans for the future
8. Skarper DiskDrive at a glance

1. Parts and assembly

Skarper has named its kit DiskDrive. It consists of only four parts:

• the drive unit
• a brake disc
• a mounting device to be attached to the chain stay
• a Bluetooth-enabled cadence sensor

From this list, you may already recognise one major limitation of this set. It requires that your ebike is equipped with disc brakes. If you only have rim brakes or other systems, you will be left empty-handed. All others can hope for a fairly simple retrofit. According to the manufacturer, three steps are all it takes:

• replacing the rear brake disc with the DiskDrive disc
• attaching the cadence sensor to the crank
• mounting the bracket to the chain stay on the non-drive side

Once this is done, the drive unit can be clicked onto and off the bike in seconds. The unit should weigh no more than three kilograms. Together with the 300 grams for the associated brake disc, this results in a total system weight of 3.3 kilograms. Unfortunately, Skarper does not give the exact dimensions on its website.

2. Technical specifications

At least there you will find all the relevant information regarding the actual performance of the DiskDrive. The drive has a continuous output of 250 watts, which is permitted in Europe. According to the current status, it will develop a torque of 50 Newton metres. That is a remarkable value. It puts Skarper on a par with rear hub motors such as Mahle’s X35+, FSA’s HM 1.0 or motors fitted to the latest models from Ampler and Cowboy. In addition, the 50 Newton metres directly applied to the rear hub should easily come close to the 80 Newton metres of a mid-mounted motor.

What makes DiskDrive impressive at first sight is the fact that motor and battery fit together in such a compact unit. Presumably, this was one of Skarper’s primary goals. The way there obviously required one or two compromises. One of these is the battery’s capacity of only 202 watt hours. With support up to the 25 km/h permitted over here, the manufacturer reckons with a maximum range of 60 kilometres. In addition to the many other parameters that have an influence on this, the bicycle on which the conversion kit is installed is also a variable in this case. Probably more can be achieved with a sporty city bike weighing only eleven kilograms and featuring 28-millimetre slicks than with a 15-kilogram Dutch bike.

Even if you reach 60 kilometres, you will either have to calculate your trips exactly in advance or realise that one battery charge might not be enough for a whole week when commuting to work. It’s good to know that the drive, and thus the battery, can be removed in the blink of an eye and recharged quite flexibly in pretty much any place with a power socket. Skarper estimates 2.5 hours for a complete charging cycle.

The DiskDrive even has its own carrying strap.

3. How does the drive work?

One of the big unknowns about DiskDrive at the moment is how it will feel while riding. Skarper has so far only produced prototypes and made them available to only a few journalists for short test rides. According to its own statement, the riding is supposed to be a smooth and at the same time powerful experience. Which manufacturer, however, would promise anything else from its drive? What is clear is that Skarper has developed its own algorithm called DynamicClimb. This is supposed to process data from several wireless sensors. The manufacturer does not say which sensors are meant apart from the cadence sensor. However, it is said that both the rider’s performance and the gradient of the road as well as the parameters of the drive unit are read thousands of times per second.

Between the lines, it can be seen that the drive will dispense with different levels of assistance. The whole thing is more likely to resemble an automatic shifting system that only provides more assistance when it is actually needed. This saves the battery and could feel pleasantly intuitive.

Presumably, the handling of the drive will be kept rather simple.

4. Who are potential target groups?

Although not all the crucial details are known yet, Skarper has been able to attract some attention with DiskDrive. Almost every well-known cycling magazine has picked up on the news. The same goes for several tech magazines. And this is probably not only due to the fact that Chris Hoy, one of the greatest British cycling legends, supports the project with money and his name, who has even been awarded the title of Sir by the Queen.

Multiple Olympic and world champion track cyclist Sir Chris Hoy is not only the brand ambassador for the project, but is also said to have had a hand in its development.

Rather, it could be due to the obvious advantage that the set brings with it: Theoretically, it turns a large part of the current range of conventional bicycles into potential ebikes. Why? Because disc brakes have been on the rise for years. They have long since found their way into lower price segments as well. There are certainly people who have found their favourite brand among bicycle manufacturers. But maybe it doesn’t build ebikes. Or at least a certain model only as an regular bike. Or someone doesn’t like the look of the part of the ebikes that rely on frames with voluminous dimensions. Or someone simply likes the idea of operating one and the same bike alternately with pure muscle power and then again being able to enjoy the luxury of an electric motor. All these people belong to the potential target group of the DiskDrive.

5. Does such a drive pay off?

When it comes to his own economic orientation, Uri Meirovich, Chief Operating Officer of Skarper, does not primarily look at the suppliers of other conversion kits. In an interview with the British bike magazine “Bikeradar”, he emphasised that the company’s aim is rather “to offer a viable alternative to expensive mid-drive and hub motor systems without the need to purchase a full-on electric bike.”

The Skarper team with COO Uri Merovich (5th from left), Chris Hoy (7th from left), CEO Ean Brown (5th from right) and chief developer Alastair Darwood (3rd from right).

An example shows what Skarper might be aiming for. For 1,000 euros, trekking bikes, mountain bikes, gravel bikes, road bikes and city bikes are available on the market, with components and parts that meet fairly high standards. Skarper currently estimates the price of a DiskDrive at 1,000 British pounds. According to the current exchange rate, this would be about 1,170 euros. If you now compare an ebike with a price of 2,200 euros with Skarper’s thought experiment, it becomes clear that in the vast majority of cases the ebike is technically worse equipped and is likely to be heavier. Moreover, it cannot be transformed into a regular bike in a few seconds, whenever you want. And back again. It is very possible that some people will like this theory.

6. Possible hurdles

In practice, however, there are legitimate concerns. The most obvious concerns the warranty. A regular bicycle is neither designed nor approved for operation with an e-motor. Even if the conversion appears to be problem-free, any manufacturer is likely to tell his customers that from that moment on they ride at their own risk.

Should a part not be able to withstand the additional loads, the cyclists will probably have to take full responsibility for the consequences. This leads us directly to the second important aspect: Does the continuous operation of a regular bicycle with a system like the DiskDrive pose a safety risk? Especially the rear wheel and the chain stay are in focus. Dr. Alastair Darwood, chief developer at Skarper, gives the all-clear in this regard. The force that acts on the brake mount, the brake disc and also the chain stay during braking far exceeds the one delivered by the DiskDrive, he told Bikeradar. The system works well within existing standards, he said.

7. What Skarper plans for the future

Only longer-term tests will show whether this is the case. These will probably be possible from next year. From the current prototype status, Skarper then wants to have made the step to series production and offer its system for sale in 2023. The price will be close to the 1,000 British pounds mentioned above. Internally, the company is already looking further ahead. It confirmed that it is working on a version of DiskDrive specifically for mountain bikes. The partner in this project is none other than Red Bull. This means that Skarper can be sure of a certain amount of media attention in the future.

8. Skarper DiskDrive at a glance

• Motor power: 250 W
• Speed: 25 km/h
• Torque: 50 Nm
• Battery: 202 Wh
• Charging time: 2.5 h
• Range: maximum 60 km
• Weight: 3 kg + 300 g for the disc

Pictures: Blue Sky IP Ltd.