The Batec Rambla handcycle is a Spanish built machine whose key to fame is that it has a stand which allows the machine to stay upright when disconnected from the user’s wheelchair. The connecting mechanism is so designed that the 20kg force necessary to make the connection is provided not by lifting or pushing but by the user’s weight tipping over backwards. These two features mean that this is an ideal machine for the independent user as no helper is required.
Batec offer a powered version of their machine but it is very powerful and its intended market seems to be as a cycling replacement. Since our machine is used as a walking substitute, we sought power assistance of a more moderate nature and had our machine electrified by electric bike guru Tony Castles. Details of that conversion are provided elsewhere. Having had some time to get accustomed to the machine with its power assistance a number of small but significant modifications seemed appropriate.
It was identified that with power assistance the likelihood of front wheel spin was quite high. This was somewhat exacerbated by the weight of the wheelchair bag that normally lives on the pusher handles at the rear of the chair. In discussion with Tony Castles we agreed to try fitting a Brompton bag frame instead of the small battery bag of the initial implementation. The latter is mounted on a Brompton luggage bracket on the front of the handcycle. The wheelchair bag would then be secured to the bag frame and become the battery bag as well as having its more usual function of carry all. The idea was that the extra weight over the front wheel would improve traction yet the overall weight of the machine would stay the same.
An important question when electrifying a handcycle is where to put the usually bulky and often valuable battery. Tony Castles’ low power conversions often use the ubiquitous Bosch power tool battery which is about the size of a small loaf. Tony has identified a small bar bag that he converts to hold the battery and mounts onto the Brompton luggage bracket.
At first sight, the obvious place to mount the luggage bracket is onto the rectangular plate down near the front wheel. The handcycle body is drilled M6 at 19mm by 121mm for the plate which appears to have no apparent function other than to cover the hole in the frame. That position has the advantage of being low down thus helping to keep the centre of gravity low. It also means that the weight of the battery is not going to make the handcycle unstable when it is unclipped from the wheelchair. As the saying goes, “What’s not to like [about that spot]?”
If the handcycle is to be a road runner that never makes sharp turns the answer is probably “Nothing”. However, for the ‘walking’ handcycle that weaves in and out of kamikaze pedestrians in town or round the aisles of the supermarket the answer is “Everything”. The reason is that when the steering is turned to the left, the chain tubes sweep over the plate to allow the chain to flow smoothly. Placing the luggage bracket on the plate prevents that from happening and effectively curtails sharp left turns.
What is needed is a solution that allows the chain tubes to continue making their movement but also allows the luggage bracket to be positioned low down near to the front wheel. The solution is to raise the plate and leave a space underneath into which the chain tubes can go. Idea 1 was to raise the plate on three pillars. That would provide most of the space required, although two would clearly be better. Unfortunately that puts a lot of load on the pillars which is probably not a good idea. Idea 2 was to use square section tube with one side removed. This is much stronger and the four original round headed hex socket screws can be used to secure the amended tube.
Experiment suggested that 50mm X 3mm square section tube, 135mm long was suitably large and strong enough for the job as well as being readily available. One side was cut off, the sharp edges filed smooth and then the four fixing holes were drilled M6.5 to allow for a degree of imprecision. The bracket was then offered up and test screwed into position. Testing the movement of the chain tubes in a left turn showed that the bracket provided virtually no obstruction. This is the point at which the new bracket should be removed and painted black.
The next stage was to address the Brompton luggage bracket. On the Brompton folding bike this sits on the head tube and sits vertically. However, mounting the Brompton bracket on the tube bracket un-modified raises a number of problems. Firstly, the Brompton luggage frame with wheelchair bag attached is quite large. With the bracket parallel to the stem, the top of the wheelchair bag snags against the crank handles. Secondly, with the Brompton bracket well off the vertical it becomes very likely that a bump in the road could be enough to throw the bag off the bracket.
The solution is to cut the base of the Brompton bracket at an angle of 75 degrees. It is not possible to cut it at a greater angle than this as it is important not to damage the internal structure that is used for bolting down the bracket. If the handcycle has already been converted, it may be necessary to disconnect the wiring to the Brompton luggage bracket. Do remember to mark which colour goes where on the bracket so that the correct polarity is achieved during re-assembly.
For maximum security it was decided that M6 X 30mm round head hex socket screws would be used to fix the Brompton bracket. That meant that the Brompton oblong washer needed to be drilled out to M6. In addition the holes in the Brompton bracket needed to be enlarged to M6 and moved towards the ‘uphill’ side of the bracket. About 2-3mm movement was sufficient.
The internal structural member of the Brompton bracket is normally parallel to the surface on which it is mounted. By cutting the base at 75 degrees that is no longer true and a spacer of about 9mm is required. An angled spacer could be cut from a block of nylon for an exact fit, or a ubiquitous 9mm tubular spacer could be used. The main idea is that the oblong Brompton washer is set up to be parallel to the surface on which the bracket is mounted.
The main bracket needs to be drilled for the Brompton luggage bracket fitting. Two holes are required, on the centre line 26mm apart with the lower hole about 20mm from the lower edge of the main bracket. It is advised that the round headed socket screws are fitted head down with the securing nyloc nuts inside the Brompton luggage bracket. This is to provide minimum obstruction for the two chain tubes. There are two possibilities for the drilling. One possibility is to drill the main bracket M6 and simply push the screws through the holes as required. The other option is to drill the holes M5.5 and tap the holes M6. The screws can then be screwed into place and will stay put whilst the Brompton luggage bracket is fitted. The latter is a bit of a fiddle to fit so we opted for the threaded hole approach.
Once the Brompton bracket is secured the job is effectively done. All that remains is to secure the four round head bolts holding the new bracket to the Batec. If the wiring from the Brompton luggage bracket has been removed to facilitate the cutting, remember to connect that back taking care to maintain the original polarity.
Finally, for those who like to take their 'FussPottery' to extremes, there is one final adaptation that could be made. The motorised wheel is 4mm wider at the axle than the un-motorised wheel. The Batec forks are manufactured wide enough for a motorised wheel and have a 4mm spacer for the manual version. Removing the spacer, moves the alignment of the chain idler mounted on the head of the forks by 4mm. This does mean that in manual mode gear changes are not quire as good as they should be.
The solution is to remove the bracket and elongate the two M6 fixing holes by 4mm. This can be done with a file (hard work) or with a slot drill in a milling machine. The bracket can then be refitted and slid across to achieve the original relative position of the idler to the sprockets.
(c) 2018 Cycling Otherwise