Most people who decide that they want to buy a kit and assemble it onto an existing bicycle frame? They have to figure out two major decisions. Will the kit I want…fit onto the frame I want? And also…where the hell will I attach the battery pack? This article will try to help you consider some options, to see what will work.
Whats the problem?
Here’s a very common triangle bag below, and it is fitted to an equally common “hardtail” frame. This is a frequently seen style and shape of frame, and it allows a triangle-shaped battery to be easily mounted. Some frames will even allow a triangle battery pack as large as 20-Ah (or more). If you don’t mind a soft-sided containment, many new ebikers go this route…
[If you have a soft-side bag (like I do) I highly recommend that you cut out two hard panels that match its triangle shape, and slip them into the bag-sides to protect the pack from being damaged if someone runs into you]
Below is an example of a very popular hard-case. This is the “dolphin” style of case, and…although it does not hold as many cells as a large soft-sided triangle pack, it is usually large enough for most builders. Because of that, it has also become very popular these days. [the “Killer Whale” hard case is even larger than this, if you have enough room]
A dolphin hard-case battery pack, attached to the stock threaded bottle mounts.
Below is an example of an expensive and very capable “downhill” (DH) bicycle frame from the Foes company. You can immediately see that when someone wants a full-suspension frame, the rear shock is often placed in such a way that it takes up some of the available space in the “frame triangle”, so a battery pack cannot go there (or at least, only a small pack). The central “triangle” of the frame is almost always the best location for the weight of the battery pack…in order to keep the “feel” of the bike as balanced and nimble as possible.
A DH frame is usually a very good candidate for a high-performance off-road ebike (and also a high-performance street ebike). Now…if you have a low-speed and low-power ebike, you have many bike frame options, however…once you decide you want higher-than-normal speeds? A full-suspension frame is a huge benefit.
It is not just to make the ride more comfortable. If you have a solid fork (or even a really cheap suspension fork), it will be possible to easily lose control when you hit an unexpected pot-hole at higher speeds. Higher-quality suspension systems soak-up these road irregularities and allow the rider to maintain more control in a sudden and difficult situation.
Once you make a commitment to using a full-suspension frame, a desirable DH frame can be identified by several characteristics (compared to a more conventional “mountain bike”). They have a much more slack head-tube angle (because they are intended for speeding down a steep single-track run). They almost always have a significant dual disc-brake system, and…their front and rear suspension parts usually have a much longer stroke than common MTB’s.
There is ONE downside to DH frames. Since the only time they are intended to be pedaled is during the steep uphill section of the ride?…their chainrings are often small. That fact by itself isn’t an issue (a bigger chainring is only about $40), but…the frames often have their chainstay sections so close to the chainring, that the owner does not have the option of swapping to a larger chainring if they want.
For a non-suspension street ebike, the Manhattan Smoothie and the Electra Townie can easily fit an enormous chainring, if you want. Some ebikers swap-in chainrings as big as 52T-60T, so they can pedal along with their ebike when it is at its top-speed.
Under the downtube
The DH frame in the pic below is a wonderful example of the type. The low top-tube reduces the possibility of hitting your crotch if your feet slip off the pedals, and…the high Bottom Bracket (BB) allows it to crawl more easily over obstacles.
The rear shock (of course) intrudes into the optimum location for the battery pack, so…in the picture below, the owner mounted the battery hardcase UNDER the downtube. This is an option that is rarely used, but…if your ebike has a “long stroke” fork, and a slack head tube angle, this is actually a great place to locate the pack. (DH frames are typically 65-degrees or less, an MTB is often steeper, between 67 to 70 degrees. 90-degrees is straight up and down)
[note: if you try this, I advise you to add additional straps around the pack, since the stock slide-in cradle was not designed to hold it like this when you hit a hard bump ]
Of course, you don’t want the front tire to hit the battery case if you hit a hard bump, but…it should be fairly easy to calculate how far up the wheel will travel at its maximum stroke, in order to see how large of a battery pack you can fit. The pic at the top of this article is actually an electric “dirt bike” (it has pegs instead of pedals), but…I saved that picture because it is such a great example of locating a triangle battery pack under the downtube.
The pic below is Ben’s “Duty Cycle” 2WD Paratrooper frame (it folds). Because it doesn’t have a conventional downtube, this means that Ben could make a very large and unusually-shaped battery pack, and hang it from the top-tube. This may not appeal to everyone, but this 2WD ebike is one of my favorite top-ten builds of all time.
Here’s a Riese & Muller from Germany, in the pic below. All factory ebikes seem to have a minimum-size battery as the stock option (where the “shark pack” originated). However, instead of having a larger battery pack as an option, R&M has a dual pack option. There is a way to electrically isolate the two packs, but for the home-builder using a simple paralleling wire-harness…you MUST charge and discharge both packs at the same time.
If you plug in a low battery to a full battery, they will try to rapidly equalize (when both are connected to a simple paralleling wire-harness), and that rapid equalization can destroy a battery pack, and worse, they might even catch on fire.
You might use a three-pole switch, which limits the battery pack that is being used, so that it has to pass through a neutral center position when switching between two battery packs. This way you can run on one battery…and when it gets low, you switch over to the other.
Some frames have a compound-curved downtube, so it intrudes into the space between the front tire and the downtube. If a frame like that also has a rear shock in the middle of the frame triangle?…The battery mount options are very limited.
Matt wanted to build up a Norco Aline, and he decided that the best compromise was to mount a battery box just in front of the handlebars. If you have a short wheelbase and a high-powered system, then you will likely be very familiar with the tendency of a hot rod to “wheelie”. Moving the weight of the battery to the very front of the bike can help to soften that characteristic, when you hit the throttle.
Mini pack in a seat tool-bag
Tiny battery packs are easy to hide and mount on just about any frame. I have seen the Luna “Mighty Mini” fitted in a larger than average “tool bag” that hangs behind the bicycle seat. If you do want to use a very small pack, make sure it can provide the current you need. A stock 30A BBSHD mid drive can use a Mighty Mini cube that uses the 30Q cell. At 15A per cell in a 2P configuration, it can provide occasional 30A peaks without getting hot. Here is an article that my friend Karl wrote about this option
Many of the riders who have bought a Mighty Mini will only use it as a “back-up” battery. Just in case they are having so much fun on the large main pack that they run out of juice, and they need some compact reserve to get up the hills on the way home. These tiny packs are only 6-Ah, so when using as a pedal-assist on an efficient mid drive, you should still be able to squeeze out 12 miles. However, if you are only using the hand-throttle (no pedaling), and it is uphill all the way home? There would be no way to get 12 miles from it.
If fitting any battery onto your favorite frame is difficult, you could put one Mighty Mini in an under-seat toolbag, and a second Mighty Mini in a backpack. Unlike a full-sized E-Bike battery pack, these are only 3.3 lbs, so putting one in a backpack would not be a hassle at all…
Here’s a pic of my friend, Karl. He has several fatbikes that he has put various mid drives on, and most of the time…he puts the battery in a backpack. I know this may not be for everyone. The bike below clearly has enough frame space for a centrally mounted battery pack, but…whether you do it by choice, or do as a compromise for a favorite frame that doesn’t have any frame space…backpack batteries remain an option that can work with any frame.
For me? Riding an electric fatbike through the country is a great way to let go of all the stress after work…I highly recommend it! I hope this pic of Karl inspires you as much as it inspires me.
Flip the rear shock?
In the pic below, look closely at the shock absorber inside the frame triangle, where the front of it is almost hitting the battery case. The front of this shock is fatter than the rear, and this builder was barely able to fit a slim-line “shark” pack on top of the downtube. ‘Some types’ of shock mounts coupled with ‘some types’ of shocks…they might allow the builder to flip them front-to-rear, so the thin part of the shock would then be the part closest to the battery case.
Some similar frames would not even allow a battery case that is this thin to fit, and this tip might really help you. However…even in the pic below, this builder’s bike might have been able to fit a larger battery case if the shock had been flipped around.
One of the other tips above is to mount a battery pack under the downtube. On this red full-suspension ebike shown, I believe the builder could fit another shark pack under the downtube, to go along with the shark pack that is shown inside the frame triangle, for a dual pack set-up.
Don’t give up!
I am very concerned about 3,000-lb cars trying to kill me. Some by accident, and maybe a few on purpose. Either way, I want as much power and speed as I can afford. I ride safely and try to always blend-in (I have never been pulled over on my ebike by the police), but…whenever the discussion turns to having a large high-watt battery pack, and fitting to to a full-suspension frame (so it can safely handle higher speeds)? It’s a tough fit.
But…if you really want that, you just need to do some research and planning, and then decide which compromises you are willing to make, so you can have those features that are important to you.
Have fun, and ride safe.
Written by Ron/spinningmagnets, February 2017