Ben is from Devon, in the UK. His custom off-road mid-drive build shows that he has an incredible set of skills. The clever jackshaft tensioning and motor heat-sinking on this build are really impressive.
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Specialized Big Hit
Downhill (DH) bicycles are popular for making a high-performance ebike, whether it is for off-road, or the street. They are designed a little differently than the more common “mountain bikes” in ways that may not be obvious at first.
There is a recreational trend of “single track” riding where you pedal your bike up a long and steep trail, and then turn around and roll downhill very fast. Due to the need for these frames to survive hard hits when traveling downwards at a steep angle, the head tube angle is slightly more slack. This fact makes DH frames more stable at high speeds when traveling on a street.
This also means the area where the head tube joins the main frame triangle is heavily reinforced, and that is another reason these DH frames are prime candidates when building up a powerful hot-rod ebike.
The Specialized Big Hit. These are so well-designed and ruggedly built that even after they are ten years old, the frame is still worthy of upgrading. Older versions can sometimes be found at very affordable prices.
Two more reasons DH frames are great for powerful ebikes are that the head tubes are typically are the 1-1/8 inch format, and that means they accept a wide variety of suspension forks, everything from affordable to wild. Plus…they often have significant brakes right from the factory, instead of needing to upgrade the brakes for them to be safe at hot-rod power levels.
The Lightning Rods mid drive motor, in the middle of the Big Hit frame.
Ben wanted to use the Lightning Rods mid drive motor, but he had some bold ideas about how he wanted to customize its mounting. As you will see just below, he did not take the easy path.
Bens custom motor mounting bracket
Ben wanted to make the motor mount out of thick aluminum that was TIG-welded into the aluminum Big Hit frame. In this way, the mount and frame became a heat-sponge, to help prevent the motor from overheating when using continuous high amps. By doing this, he could get lots of power from a smaller and lighter motor than a builder would normally spec for Ben’s power levels.
The Lightning Rods “small block” motor has already proven itself to be an unusually powerful motor for its size. It is an inrunner, so its shell inherently sheds heat well. The high reduction between the motor and the bottom bracket makes this system produce a impressive amount of wheel-torque from a reasonable voltage, rather than requiring a high-voltage battery in order to get acceptable performance.
This motor can be run at high voltages (it’s been successfully run up to 100V), but high voltage is not a requirement for great performance if you add a jackshaft and then spin the motor three times as fast.
Here, the frame has been notched to fit the motor mount.
In the pic above, Ben is calculating the best location for the jackshaft. By having two stages, the motor can run at a much higher RPM, relative to the road speed, and that magnifies the power density of the system, and also makes it more efficient.
The Tesla car company ‘could’ have used a larger motor that was simpler because it might not have needed cooling elements added. However, simulations showed that by adding a way to remove heat from the motor, they could get the amount of performance they wanted from a smaller and lighter motor, which improved packaging and also the power-to-weight ratio of the entire car.
The heat-shedding cast aluminum wheels of the 1925 Bugatti Grand Prix race car, with large 4-wheel brakes. The engine is an overhead-cam straight-8 cylinder.
The 1925 Bugatti Type 35 Grand Prix race car burst onto it’s first race season with distinctive cast aluminum wheels, and this happened at a time when the normal method was to use wire-spokes to make the wheels as light as possible. Aluminum was quite expensive and exotic before WWII, but it absorbs and sheds heat quite well.
The race cars of the day used drum brakes (many only had brakes on the rear, to save weight and make the front suspension more responsive), and drivers had to begin braking very early when approaching a sharp turn, to avoid getting the brakes too hot. The Bugatti’s could approach turns just faster enough to pass the competition, and then they could apply the brakes quite hard without fear of the brakes getting too hot. Once around the turns and onto the straightaways, the mass of wheel aluminum could then begin shedding the heat that it had absorbed, before they reached the next turn.
By Ben using the aluminum frame to absorb and shed motor-heat from high amp-spikes, this compact and light motor can produce much more power than it has normally been surviving. Ben noted that the performance of this motor is outstanding at 18S and only 40A.
A nominal 3.6V per cell times 18 cells in series is 65V, and also, it’s 74V when fully-charged to 4.1V per cell. 65V X 40A = 2600W. That doesn’t sound huge, but the reduction is over 30:1, so that 2600W is converted into an enormous amount of wheel-torque.
The finished frame, after welding-in the custom aluminum pieces.
One of the reasons I moved Ben’s project to the front of the list of custom ebikes we wanted to write about is that we wanted to feature his clever solution to tensioning the secondary chains that runs between the bottom bracket and the jackshaft.
The Lightning Rods system uses a jackshaft which incorporates some components from bicycle bottom brackets, and Ben adapted what is called an “eccentric” into it. If you look closely, you can see that the hollow central threaded mount is off-center from the outer edge of the cylindrical shell, which is rotatable.
Eccentric bottom brackets were developed because a small percentage of bicyclists wanted to convert a frame that had vertical rear drop-outs into a single-speed, and to then have the chain properly tensioned without the clutter of adding an idler wheel. Rotating the eccentric BB moves the pedal spindle forward to tension the drive-chain.
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Battery box decisions
I am not a fan of LiPo batteries for a daily driver ebike. I know that they are likely to remain popular for racing ebikes, but recently…ebike battery packs made from name-brand 18650 cells have improved their performance capabilities dramatically.
Ben uses a sophisticated and reliable balancing charger, and he is very familiar with the quirks of LiPo pouch cells. He chose Multistar sub-packs from Hobby King for this project, and their squared shape simplified the battery box construction.
Now that Ben has a riding season under his belt, his winter project is converting the battery pack to 100 of the 18650 cells in a 20S / 5P configuration.
Hobby King Multistar LiPo batteries, in a wooden mock-up to sort out the size and shape of the aluminum battery box. He is using three of the 6S / 15-Ah bricks in series to make an 18S / 15-Ah pack.
Laying out the aluminum sections before welding
The final form is taking shape here. I really like where Ben has located the battery pack on this frame.
Here is the red on/off switch that Ben installed. With the paint removed from the frame, you can see the beefy head-tube reinforcements and welds. It may be aluminum, but this frame is very strong.
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Fat tire, thin tire
With the Big Hit having a well-respected suspension system, you might think fat tires were not necessary, but Ben decided he wanted them. Because of the extra trouble he went to in order to have them, I am certain that he’s tried them before, and liked them a lot.
This pic shows the tale of two wheels, the old and the new. The tire on the right is a 24 X 4.0
Ben has fantastic fabrication skills, so when he pondered this project and chose an off-road mid drive ebike…with fat tires!…he decided he would rather build one up just the way he wanted, rather than shop for a compromise to just buy.
Dry-fitting the 4.0-inch wide tire to see where it rubs on the swingarm. Alert viewers will notice that Ben is using the very German Rohloff 14-speed IGH.
Ben was able to source a Manitou Dorado fork that accepted the tires he wanted in its stock form, but he knew the swingarm would be a challenge.
In the pic below, Ben has braced the pivot points so the swingarm geometry won’t be changed during the radical modification process. Then, he cut away the sections which would rub against the 4-inch wide tires’ diameter that he had selected. Last, he cut and shaped some reinforcements to beef-up the parts of the swingarm that had been compromised.
Heavily modifying the upper swingarm.
The 4-link swingarms, after the modifications were finished
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The final stage
In the pic below, you can see a close-up of the eccentric bottom bracket. By rotating it, the jackshaft center can be moved closer or farther away from the bottom bracket. This allows the builder to tension the secondary chain without adding a idler to it.
The eccentric bottom bracket re-purposed as a drive-chain tensioner.
The black color looks awesome on this frame.
On the left you can see the adjustable-position roller for properly tensioning the primary belt. The high RPM primary half of the drive runs much quieter with a belt, compared to using a chain.
The custom cover that Ben made to protect his leg from the primary belt is very professional looking.
Here, you can see the #219 second-stage chain that runs from the jackshaft to the bottom bracket. The very strong kart sprockets are 68T to 12T, for a 5.6:1 reduction
If you like this build, you might also like Simon’s Specialized Supercharged
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Written by Ron/spinningmagnets, November 2016
