Working from home and having sold my car, I needed a means of carrying some cargo. Yes, there are panniers and more common rear racks, but they are only good up to a point. I needed more capacity for odd shaped things.
First round was to make a cargo bike using an old steel framed hybrid bike from the ‘90’s that had been lying around since I went recumbent. Hacking and chopping it was extended forwards with a 20” front wheel, and a 60 x 40cm cargo platform in front of the handlebars. It worked well, very well actually, except I missed the recumbent comfort…
Some riders rely heavily on their trikes for daily tasks, and while they manage OK, the tadpole trike platform has stability limitations when loaded about the rear wheel. Delta trikes are better although harder to find, and the load platform is smallish. Loaded stability is good though. (HP Velotechnik have introduced a dedicated cargo delta)
Thoughts turned to quadricycles (let’s call them quads). Great stability, larger load platform, but not readily available, pretty much stuck with a modification of some kind. Most of the modifications replace the rear wheel with a rigid bolt on axle assembly, and while they work, they apply significant torsional load on the frame when used on uneven ground.
We chose our Greenspeed GT20 trike as the recipient for this project – the folding frame meant we could separate the rear wheel and subframe and replace it with a new rear frame, axle, wheels, and cargo platform.
The design is based around using the Bafang BBS mid drive, an internal geared hub for the transmission, and a tilting rear axle assembly to eliminate the torsional load on the frame. 20” wheels and a 600x600mm cargo platform rounds out the specification.
We committed time in the workshop and soon had a “proof of concept” vehicle. Not terribly refined in its details, but incorporating the features we wanted. Initially fitted with a 6 speed cassette and no e-kit, we put it in service and see what breaks, what works, and what could be improved.
Initial rides are very encouraging. Most significant is a dramatic increase in lateral stability – not because of the 4 wheels, but instead because of the huge change in rear roll centre as a result of pivoting the rear axle nearly 250mm above ground level. Or to put it another way, it has the same effect as lowering the seat height from 300mm down to 200mm, a mere 40mm higher than the GS Aero seat, and all without changing the actual height! The tilting rear axle works brilliantly, crossing kerbs, speed bumps etc at an angle is a non-event, and only driving one wheel (on this PoC model) is unnoticeable.
Size wise, this completed quad is still shorter than a standard Greenspeed GT26 trike although it looks much bigger due to the rear platform width. Speaking of the load platform, our goal was to carry 40kg of stuff. The platform has been tested to 80kg with no problem (through the very unscientific means of giving each other rides on the platform…). Next stage was fitting a crate of some sort to take the shopping bags. A box was duly fitted, and the quad used some more.
Overall it was very practical and versatile, but a limitation of one drive wheel was apparent on steep grades – wheel spin was an issue when lightly loaded.
Back to the workshop for more hacking and chopping and modifying to two-wheel drive. A simple (!) drive system was devised using two common BMX freewheels. The parts needed a fair bit of machining (cutting a 24 thread per inch thread using a metric lathe…) but the end result worked effectively.
The double freewheel mimics a differential, but is different in that it sends the drive torque to the wheel with the most grip, unlike a standard differential that sends the drive to the wheel with the least grip. It is very effective. The problems were in the number of parts to be made, and the assembly was very fiddly and time consuming for a supposedly simple item. Keeping the driveshafts in line also proved problematic.
Even so, this version proved good enough to attract someone interested enough to take a chance and order a quad. Stand and Deliver time.
This time the donor trike chosen was a Greenspeed Magnum. Several reasons, the client had already had a Magnum and liked it, the Magnum offered a slightly bigger and tougher design to start with, and it came standard with heavy duty wheels and tyres, and disc brakes. The proposed quad also needed to have single hand brakes and Enviolo hub transmission.
The proof-of-concept layout was kept, dimensions slightly adjusted to match the wheelbase of a Magnum BW, load tray and track adjusted to match the front wheel track and ensure a 600mm internal width tray.
The major re-design centered around the rear axle and freewheels. The end result is a central drive unit incorporating spragg clutches. These simple bearing-like objects allow free rotation in one direction, but lock solid and transmit drive when rotated in the opposite direction.
Machining the housings was more straightforward, and the end result is a robust but easily serviced bearing housing. Rated torque is 170Nm per axle, or 340Nm total. Heaps, given that the e-kit is rated for 85Nm.
Construction proceeded apace, proper bearings and bushes were substituted for our original quick and dirty bolt together, but the actual shape and construction remained the same. Along the way we discovered that the frame mounts for the Enviolo hub were so rigid that we couldn’t “spring” the frame enough to fit the hub so the mating surfaces were machined for width and flatness. All the parts were machined in house to suit the design.
The bare chassis rolled out into the daylight mid-September 2024 for initial road testing with no issues arising. The load platform was made and fitted, and loaded testing was done. 45kg of pool salt and chlorine was barely noticeable, and that was mostly in the braking department.
Speaking of braking, a friction lever park brake applying both front brakes was designed and installed.
Final configuration for this quad is:
- Bafang mid drive,
- 17Ah battery,
- 46T chainring driving an Enviolo CVT hub with 21T sprocket, output from the Enviolo is a 30/34T drive to the rear axle.
- 20”x 1.75” tyres all round. Rear wheel track is 10mm narrower than the front, even though it looks much wider!
- No idea what the weight is, after all it’s a cargo quad with e-assist, battery, and a sizeable load capacity. Not something to lift up onto the roof racks!
But the big question, how does it ride? Well. Really well. You don’t notice the extra weight and width, and it feels much more stable than a standard Magnum (that’s really very stable to start with).
In fact, it’s so good, I think I’m going to have to make me another one.
