ELECTRIC TRIKES AND MOUNTAIN BIKES:
SCORPION 'INTERCEPTOR' E-TRIKE ASSIST PROJECT
My wife said three things when I told her I wanted
to put electric motors on our new HP Velo Scorpion recumbent trikes
as stable comfy more old-age-appropriate ride-replacements for our
1. Don’t you dare trash my sweet ride.
2. Make it not scary for a non-tech person to
3. Make it pretty.
My own goals were a) to easily make the run from
town up to my house (~3000 vertical feet in 4.5 miles), b) run
efficiently at both low and high speed, and c) look cool.
I tested a BMC hub-motor and didn’t like the power
band, nor the gear knashing noise, and so chose RC mid-drive. I decided
to go for a MadMax Interceptor look, with its big timing-belt turbo
spinning up at the flick of a switch. A belt-reduction unit would be
user field serviceable, and that seemed a big plus. The main problem
with the HP Scorpion is that because of the full suspension and
complicated driveline, there is little usable space behind the seat to
locate a drive unit, as has been done nicely on a number of other trikes
(e.g. like FFR’s) that have squarish or extendable frames. Fortunately
Matt S (recumpence) offered to build me a couple of flexible and
beautifully-made 2 stage reduction units that I could modify to fit
under the front boom. The key idea was simply to rotate the boom
extension 180 degrees to put the symmetric (i.e. works the same CW or
CCW) Schlumpf high-speed pedal-drive on the left side. This allowed for
installation of a double freewheel and jackshaft in a quite tight area
(with a net clearance of like 1/8 inch or so), to drive the main
chain-line on the right after a couple of low-angle re-directs. Here
are some pictures of the build.
Design Sketch - Front bearing tube with freewheels
The main modification to the two stage davinci
drive was to replace the 2nd stage bearing tube with a new
one, holding a keyed 5/8 shaft (vs. unkeyed ½”). This allowed two ENO
freewheels to be squeezed side by side in the small area between the
pedal and the boom, linking to the 8mm drive pulley and the pedal chain.
Modified Recumpence davinci 2 stage drive with
Astroflight 3215 motor.
Black parts by Matt, silver parts and chain rollers
are my mods. Total weight with Astro 3215 motor = 11 pounds. My mods
(heavy ENO freewheels, bigger shafting, etc.) added ~3 pounds.
Top View Front Boom
My wife and I are short, so everything is tight.
But it all runs smooth with no scrapes or bindups, in the first few
hundred miles anyway. The motor unit is held by two super-neat
recumpence boom clamps, and by extending three large existing frame
bolts into custom attachments I put on the reducer body. It ain’t
Nice thing about the Schlumpf high speed drive is
that with motor at full speed, you can still pedal-assist at a
reasonable turnover rate (about 90rpm). The kick-shift for starts,
etc., is way nice.
The pulley ratios are 20-80 (5mm pitch x 1”wide),
14-50 (8mm pitch x 7/8”wide), and 16T with 9 speeds (11T – 34T) in the
After foolishly showing my wife some videos of
LiPo’s burning, I calmed her down a bit by installing quick release
battery mounts. Though under the seat (well, there was no better
place), a quarter turn of two huge wingnuts and a light pull gets the
thing off. I sure would hate to burn up these trikes (see her point #1
12S (~48V) 16Ah Lipo batteries
Battery installed. DB25 connector is for cell-log
and charging cable attachment.
Some specs: Fullup weight (with one 13 pound 12S,
16Ah battery) about 73 pounds. Min ground clearance: 4.5”. Top speed
(as geared above, on the flats), Astro3215 – 40mph, Astro3220 – 45mph.
I’m sure a different gearing would get you over 50, but holy crap, even
40 seems pretty scary to me! It takes about 7Ah to go the 9miles home
from downtown (4.5miles on the flats and then 4.5 miles up about 3000
Mountain Bike Builds (2D)
I had a lot of parts left over from adding
mid-drive motors to our two trikes. Timing belt pulleys, bearings,
shafts, freewheels, some cheap motors, HV80 controllers, etc. So I
decided to go back to our old mountain bikes, which were temporarily
abandoned in favor of the more stable and comfortable recumbent trikes,
and see what I could do to add 2-stage belt-reduction electric
crank-drive assist to them. Worked out surprisingly well, using the
cheap parts (like narrow plastic pulleys and Turnigy motors, perhaps
$300 total per drive). Hopefully we can attack the slickrock in
Moab this fall. Some Pix:
I had built a gas-assisted (30cc 4 stroker) mountain bike
in 1997. I used it for canyoneering (hide the bike at the end of a
long canyon, do the technical descent, motor-assist back to the car at the head
of the canyon after a long day). Deep sandy back-roads in Utah gave it
a workout, but it made it (after letting air out of the tires).
However, the engine was noisy and riding was quite awkward. So I
decided to remove the gas engine, and install a lighter/smaller/quieter
electric drive with a timing-belt 2 step (~18:1) plus chain crank (5:1) reduction system.
A 5mm pitch by 15mm wide first stage belt has
proved adequate (no skipping at all). Surprise to me: I haven’t
been able to break the cheap and lightweight plastic pulley.
Second stage required 8mm pitch to avoid skipping
on hard full-power up-shifts.
The big 10S 20Ah lipo battery I originally made for the bike is a
bit heavy, high, and far back for anything but mild roads. Kind of
kills the maneuverability.
A smaller battery (12S 8Ah) on the cross bar feels
My wife’s old mountain bike, similarly converted. This
second bike-build has the motor, gearing, and battery lower down and it rides a lot
Tiny Turnigy 6374 (max ~2hp) Motor and 8mm pitch 2nd stage belt. 48V 8Ah
battery below. Motor was later replaced with a quieter (less
whining) Astroflight 3210.
First stage (5mm pitch, 15mm wide HTD belt), HV80, and
Have to be careful with these. Easy to do
unanticipated wheelies. Can’t wait to hit the slickrock…….