So you want to build an electric hot rod the fast, easy and cheap way? Then hub motor hod rodding is for you. Inevitably, new electric bike owners decide they want more power and speed out of their hub motor bikes. Hub motor power is such an entrancing thrill because its so quiet and intimidatingly smooth. After you get some miles on the stock hub motor bike, and get comfortable on it, almost everyone wants to go a little faster, accelerate a little harder, and climb a little steeper.
If you are not passing spandex-clad road bikers easily, then its probably time for a hub job. If you decided it’s time to soup up and modify that hub motor of yours, you’re on a slippery slope to join a small but passionate group of ebike hot rodders that exist all around the world.
Check out our list of the 10 fastest ebikes ever built by an elite class of hotrodders.
Luckily it is super easy to squeeze extra horsepower out of almost any hub motor, but you will have to sacrifice a little range and reliability. And you will have to swap out your controller, and add or replace batteries. You are now being sucked into the wonderful world of guerrilla E-bike building. Most hub motor E-bikes are factory resticted to stay under federal law power-limits and minimize company liability. But there’s not much stopping you from becoming a guerrilla rider, and breaking those limits.
As it turns out almost all hub motors can handle more current than what they are rated for.
Welcome to the electricbike.com hub motor hotdrodding primer. If you want to know how to build an ebike the expensive, difficult, noisy and sexy way…you are in the wrong place: go to the Astro Motor Hot Rodder’s Section
Is hot rodding an E-bike illegal?
If you are in the USA, the federal power limit that you are allowed to run to a hub motor is 750 watts. This is mostly a manufacturer restriction. I have never heard of a cop pulling over an E-biker and put a watt meter on his motor to make sure he is legal. As of this writing, as long as you ride your hot rodded E-bike safely and in a sane manner, there will be little chance that you will ever be pulled over by a police officer. And remember, any ticket you do get on your E-bike should not effect your driving record.
If you want to be 100% legal and still want to run a hot rodded motor…then ride your hot rodded motor off road on private property only.
What is hot rodding a motor?
Hot rodding a motor is simply running more wattage through your motor via a souped up controller. More amps or more voltage (or both) will give you better torque and a higher top speed. In general if you want more speed you run more voltage. And if you want more torque you run more amps, but…in general you get more speed and more torque whenever you pump up the wattage to the motor. Calculating wattage to the motor is simple: voltage X amps = watts to motor.
So if you have a 50-volt battery pack and you run it at 50-amps, you will have a 2500-watt set up, that will get you up to 40-MPH and beyond without pedaling, depending on variables such as tires size and the type of wind in the motor (its “Kv”).
A good resource for hot rodders is the E bikes.ca simulator: ebikes.ca hub motor simulator, which will alow you to estimate speed and efficiency given a particular motor, tire size, voltage, and amperage. Play with this simulator for a while and you will start to understand, and you can then plan a configuration.
What does tire size have to with anything?
Hub motors spin your tire around, and depending on the diameter of your tire, that will determine the gearing of your set up. A big 29-inch tire is going to provide high gearing giving you higher top speed. A small 20-inch tire is going to give you low gearing and better torque and hill climbing. Always keep in mind that hub motors, like all electric motors, are more efficient and more reliable when they spin fast (high RPMs). The larger your tire, the more likely you are to smoke your motor when you hot-rod it, but you are going to have fun doing it because you will also have higher top speeds.
What are “motor winds”, and what does it have to do with anything?
In general, hub motors are either wound for torque, or wound for speed. It will act like a type of gearing that will decide if your hub motor is fast or its torquey. Motors wound for torque hold up a lot better when hot rodded, because in general, when you raise the voltage (which should raise the top-speed) and then the motor starts to bog down on a hill… the controller will start adding more amps to try and get the motor up to the higher speed you are commanding. The extra amps will mean more heat, and if the motor and controller get too hot…they will be fried.
If your system gets up to top speed rapidly, the controller will have only sent as many amps as needed to do that, and the “high amp” phase of your acceleration will be short.
The pic above shows two stators from the same model of RC motor. There is only so much room for “copper fill”, so the stator teeth can be wound with either many turns of small diameter wire, or fewer turns of fatter wire. A low turn count is a “fast” motor, and…many turns would make the motor spin slower when they both have the same volts applied. If you buy a MAC geared hub, they are listed by turn-count. A 6T is the fastest, and they also stock a 7T, 8T, 10T, and a 12T motor.
A fast 6T hub motor might get you up to 25-MPH in a 26-inch wheel when using an inexpensive 36V battery, but…it will bog down badly on a medium steepness hill. Lots of heat. If you wanted to reduce the hill-climbing heat, there are several things you could do.
Raising the voltage would add more horsepower (745-Watts = one-HP) so the motor would bog down less, but going from 36V to 48V would also raise the top-speed about 25%. At the higher voltage, a 6T going 31-MPH might be too fast for you, and you end up using “half-throttle”. Swapping to a higher turn-count motor (a slower wind) means you can use full-throttle, a higher voltage (for more power), and still maintain your desired top-speed.
The most common controllers support both a 36V battery and a 48V battery, the next bump up in controller power tops out at a 72V battery, and there are even controllers available that will support up to 100V.
What is the weakest point most likely to fail in hot rodded hub motor?
The weakest link inside your hubmotor is definitely the phase wires going into the motor. If you wish to run more amps to your motor than the manufacturer intended, one of the first steps is to replace the thin wires going into the motor with as thick of wire as you can manage to fit inside the axle. Many extreme hot rodders even bore out their axles to make way for thicker wire. No matter what thickness of wires fit through your axle, as soon as they are out of the axle, you can add much thicker wire to act as a heat-sink (this actually helps) between the axle outlet and the controller.
You can read our entire guide on Upgrading the Wires Going into your hub motor here.
Hot rodded controller:
The #1 ingredient you will need to be a hot rod is a juicy controller. You will need a controller that can handle high voltages and has beefy mosfets (FETs) to handle high amps. In general, the more FETs the more powerful your controller. A FET is a Field Effect Transistor, its an on-off switch that sends power to the motor phases. Most motors have three phases, so FETs come in sets of 3.
Basic controllers have 6 FETs. You can get larger controllers all the way up to 18 FETs if you are looking for freeway speed (which will need lots of amps to overcome wind-resistance). Also the size and quality of the mosfet will determine how much amperage it can handle. It is worth paying a little extra to get authentic 4110 FETs, and if heat still proves to be a problem, consider swapping in the cooler-running 3077s (3077’s top out at a 60V battery, but they run about 25% cooler than 4110’s). A 12-FET controller with 4110’s that can adjust its Low-Voltage-Cutoff (LVC) to support a 48V-72V battery is proving to be very popular.
Common generic Chinese FETs will produce 5-times the waste heat of an authentic 4110 or 3077 FET. As an example, when pumping 25A of power through an authentic 3077, it will convert only 1.8A of power into waste heat, and a 4110 would produce about 2.2A, due to their high-quality materials having a low resistance of only 2.8 and 3.7 mOhm’s.
The #1 resource for buying a hotrodded controller in the E-bike community is Lyen, a Chinese electronics expert who sits in his little apartment in San Francisco, brewing up hot rodded controllers every night at an affordable price: www.lyen.com. Contact him and tell him what controller you have, and how fast you want to go, and he will ship you a hot rodded controller in less than a week.
What about the battery?
Now we are getting into the thick of it. The biggest limitation for hotrodding an E-bike is the battery. This is where you are going to start spending money, time, and headaches.
If you want to upgrade the voltage you are putting into your hub motor via your controller, you will also have to upgrade the pack to an appropriate voltage of battery. You always want to make sure that the voltage you want to run is compatible with your controller. A 48-volt battery will need a 48-volt controller. However if you invested in a hot rod infineon controller as mentioned above, many of these are flexible, and allow an input voltage range, such as 24-72 volts.
In addition, there is an issue of battery amperage:
IF you are running the older heavier Sealed-Lead-Acid batteries (SLA), amperage is not much of an issue. These suckers can put out a lot of amperage, but they suck in every other possible way. Their high-amp capabilities only last for a short while, and they have a steady drop-off of amps due to the Peukert effect. You can look it up if you want, just understand that for every mile you ride, SLAs get weaker.
But…if you are using common Lithium-Ion…these batteries are limited on how much amperage they can put out. If you bought a stock E-bike with a lithium-ion battery, chances are it has a cheap low-amperage battery that will not be able to handle a hot rodded motor without either triggering the Battery Management System (BMS) and shutting itself off for its own protection, or drastically lowering your battery packs life.
Now you may be starting to understand the pain of the E-bike hot-rodder. Building a fast E-bike is not cheap, and more than likely you are going to have to upgrade your battery.
If you double the amount of battery you are using by adding an extra battery to your set up, you also double the amperage capacity of your pack. For example, lets imagine you have a 48 volt / 10-Ah size of battery, and it only has a 20-amp limit on what it can put out. When you add an extra battery pack and connect the two packs in parallel, not only do you double your E-bike’s range, you also double the amount of peak amperage the battery can put out to 40 amps. Note, that to achieve this the two batteries need to be wired together in Parallel.
Also consider buying high amperage packs. The two most popular high amperage lithium packs currently available are A123 lithium packs, and Hobby King LiPo packs. If you decide to use Hobby King, you will have to research endless-sphere on how to safely build and use an E-bike pack that’s made from LiPo.
edit: in 2014, we collected the information and published an article on some new batteries that put out high amps of current, but are NOT the risky LiPo chemistry.
Read more about lithium battery pack selection here
How fast can I get my bike to go?
A magic number that hot-rodders savor over, is that…for every 750 watts you crank through your hub motor you can expect one-horse power. If you can get 3-HP on a light weight bike frame you are doing pretty good. You will find that achieving 30-40mph is pretty easy. Above 30 mph your bike starts to suffer in efficiency. Above 40mph you have to be careful not to smoke your motor.
Play with the ebikes.ca Simulator to see what kind of speeds and efficiency you can achieve with different combinations of battery/motor/controller. 30mph is kind of a sweet spot where you have good efficiency, good reliability, good hill climbing ability, all at a safe and sane speed. 30mph is a good target zone for a first time hot-rodder and can be achieved with almost any hub motor.
Remember to do the wise thing and invest in good brakes, preferably hydraulic disc brakes and make sure your bike frame is strong enough to deal with the stress of high speed.
How much will it all cost?
Hot-rodding your ebike is fairly cheap, given the performance you are going to get. For under $1000 you can build a bike that will smoke a $13,000 Optibike, no question. If you already own a hub powered bike, and all you need to do is upgrade the controller and battery, you can do this for about $500 more or less depending on how radical you go. Its possible that for under $600 you could upgrade your hub motor bike to do 50-MPH and smoke its rear tire on take off. Alternatively you can spend thousands of dollars and buy a Hi-Power Cycle bike which is basically a hot rodded conversion bike which they are selling to the public with weak warranty support at a premium price.
For a DIY hot rod job you will need a high performance Lyen controller (around $100-$150 ) and then spend as much as you can afford on a high quality battery pack with as many amp hours as you can afford/carry. If you construct your pack from hobby king cells, then $400 will buy you a decent pack. Also I highly recommend you also purchase a Cycle Analyst to monitor your battery and motor temperatures. The Cycle Analyst can also cap the amps (adjustable amp limit) the controller puts out, to prevent you from accidentally overheating the motor and controller.
Remember, one downside of a fast bike is it wont be as efficient at high speeds as your old rig.
Hotrodding ; what can go wrong?
To start with you can start a fire. Lipo batteries running at high amps without a BMS can be a fire hazard. When building your own battery pack, fire should be your #1 concern.
#2 You can crash. Make sure if you are going to start hod rodding your hub motor that the rest of your bike is up to the task. For example dont skimp on cheap brakes for a bike that go over 40-MPH. Disc brakes and preferrably hydraulic disc brakes should be mandatory on a 40-MPH E-bike. Use torque arms to strengthen your drop outs. Do not use aluminum forks with a front hub motor, especially suspension forks. Use quality tires. Common sense applies…wear a helmet and gloves.
#3 You can damage your motor, controller or battery. You have to decide how much wattage you are going to run to your motor. The more wattage you run, the less reliable your hub motor and controller will become. The biggest problem you have to deal with when hotrodding a motor is over heating. Heat dissipation becomes a big issue.
Heat – electric hotrodding enemy #1
Figure out how to dissipate heat when your motor starts to get hot, and you are on your way to having a reliable hot rodded hub motor. Heat is your biggest problem.
- Use common sense when riding your hotrod hub motor, know when to push hard, and when to let off.
- Avoid running your hub motor up steep hills at low speeds.
- Run a smaller diameter tire to lower your gearing.
- Install a temp sensor and monitor temps via a guage.
- Buy the new Cycle Analyst V3 which has a temperature monitoring option, and will actually reduce power to your hub motor as you get close to over heating.
- Air cool your hub motor by drilling holes in the cover of your hub motor to help let the heat out.
- Oil cool your hub motor!
- Run thicker wiring into your hub motor.
- Install a 2nd hub motor in your other wheel to lessen the load on the first motor.
- Don’t be lazy…Pedal assist you hot rodded hub motor bikes up hills!
- Running your hub motor until it slows to near stop grinding up a super steep hill at full throttle.
- Running your brakes and motor at full throttle at the same time.
- Running your motor up a long hill for a long time at full throttle.
- Running your overvolted hub motor at full throttle for extended periods of time with no mind to heat build up.
- Running a geared hub motor over jumps at full throttle and not letting off the throttle when the motor hits the ground.
- Running too many watts for your particular hub motor
- Hooking a controller up wrong to the wrong hall wires, which is easy to do since not all controllers have the same color wires.
- Crashing your front wheel drive hub bike into a brick wall.
- Getting water stuck in your hub motor, and it can’t get out because your hub motor is sealed.
- Gunning your over-watted gear motor to full throttle with no mind to the thread of peanut butter gear.
A list of Good Hub Motors to Hot Rod
BMC/eZee Hub Motor– (read review here) Good to hot rod up to 2000 watts. Beyond that they become unreliable. You can run 2500 watts but you have to be really careful when you apply power that you don’t strip your gears or fry the motor. Because it is such a light motor, the BMC hub motor is a blast to hod rod, but a risky proposition given the general delicateness of the small motor and how expensive it is to repair or to replace.
MAC Motor – the MAC motor is almost the same motor as the BMC but a little less expensive. Recently MAC motor has started including thicker phase wires going into the motor, and you are even be able to buy one with a built-in temperature sensor which will really help you keep from overheating it.
Crystalyte 5X hub motor…the original – The original 5300 series of Crystalyte motors are monster heavy and capable of monster power when hot rodded. THe stealth bomber uses a Crystalyte 5304 running 72 volts at 50 amps for a total of 3600 watts, and can run this power reliably. The problem is that this motor weighs 25lbs, which is a LOT of unsprung weight to be in your rear wheel, and most of these huge motors are too wide to fit in a standard width bicycle frame. The Stealth Bomber is an example of a commercial bike which uses this awesome power plant.
The new smaller Crystalyte motors (3500 series) (read review here) are smaller in size and weight (15 pounds) than the 5300 series and are also a good candidate for hotrodding, but they do not handle as much power as the 5300 series motors. You can pump as much as 3000 watts into the smaller Crystalytes safely, but that should be for a temporary burst of speed, and for continuous power (or climbing a hill) you should try to stay below 2500 watts. The new Crystalyte motors are very similiar in design to the 9Cs, but with a 35mm wide stator, compared to the 9C’s 28mm.
9C hub motor. This is a favorite hub motor for hot-rodding because it is a widely available, cheap, and a competent motor. This is the most common high wattage hub motor, which is made in many factories in China. This motor is sold by mulitple dealers and ebay sellers for prices as low as $120 and can handle power levels of 3000 watts continuous and 5000 watt bursts much like the smaller Crystalyte mentioned above. Because of its cheap replacement cost it is a favorite platform for hot rodders. I have seen over-volted 9C set-ups hit speeds of 60-MPH. They have been run at up to 111V in order to get more power while limiting the heat from amps.
Bafang SWXH / Cute hub motors
These motors are the small “fit in the palm of your hand” motors that are found on E-bikes all over Europe. They are small geared motors that are made in huge numbers in Chinese factories, and therefore can be obtained for under $100. They are very small and lightweight, and their gears tend to be fragile, so in general they do not make good candidates to be hotrodded. However, hot rodders who are entranced by their stealth and light-weight nature, have pumped up to 1000 watts through these little motors with some success (they are designed for 250). It is recommended that you replace one of the 3 nylon gears in this motor with a metal gear to reduce the likelihood of stripping the gears. Also you have to be careful how you apply power (nice and steady) so you don’t turn your gears to peanut butter with a super-hard take off.
Magic Pie hub motor:
The magic pies are heavy and robust and make an OK candidate hub motors for hot rodding. Do not buy the magic pie with the controller built into the motor if you want to hot rod. Hub motors with internal controllers are terrible candidates for hot rods. Even a small amount of heat will be trapped inside, but the integral controllers work well at the stock recommended power levels.
Bionx Hub Motors –
Forget about it…just sell the thing and buy something else listed above. Bionx is among the hardest motors to hot rod because they have proprietary components. And they are overpriced, and have built in controllers which is useless to the hot rodder.
Will my hot rod hub bike be less reliable than a stock bike?
Absolutely! Same as if you hot rodded a car, don’t expect the same reliability from a hot rodded E-bike as you would from a stock e-bike. In general none of your components on your E-bike, the controller, motor, battery, and even your bicycle components such as tires and brakes will last as long. What did you expect? The good news is that when things fail you can replace them with higher quality and more robust components. Welcome to the wonderful world of the E-bike hot rodder…always buying, repairing, disassembling and upgrading!
What if you smoke your motor?
Unless you are running a BMC or a MAC motor, most hub motors are fairly inexpensive to replace. Find a good cheap source for your hub motor, and in the worst case…if you smoke it, it will only cost you a $100-$200 to replace it. Just replace it and go out and ride again. Failing, learning, rebuilding, and riding again are all part of the Hub Motor hot-rod culture. Post about your troubles on Endless-Sphere, and fellow hot rodders will help you through.
If you are super handy and motivated, you could take apart your motor and repair it. Most of the time when you fry a motor you are only frying either the phase or hall sensor wires going into the motor, and they can be replaced with thicker wire to make your new motor more robust. To do this you will have to take apart your hub motor, and learn about how it works. A nice process. Some extreme hot rodders have even rewound burned hub motors with thicker more robust copper cable. This is not an easy task and should be left to Chinese factory workers:
If you smoke your hall sensor wires, you can just invest in a sensorless controller, and run sensorless. Sensorless set ups are more reliable than sensored motors, because they can take more heat.
If you smoked the copper windings of your hub motor, you could possibly rewind your motor with thicker higher quality copper wire. But these tasks are reserved for more advanced E-bike builders. Stay tuned for part 2 of this series, a kingfish guide to rebuilding a hot rod hub motor.
Hub Motor Hod Rod Extreme:
- No one in the E-bike community has been a more extreme hot-rodder than liveforphyics/LFP of Santa Cruz California. This guy has hot-rodded E-bikes to a ridiculous extreme, some of his latest bikes going 80-MPH on a home built extended-frame BMX bike.
A few years back, liveforphysics was attempting to run more power through one of the new HS Crystalyte hub motors than anyone else; 40 thousand watts. (check out our review of the Crystalyte HS-35 here.)
Liveforphysics was preparing to run one of the new Crystalyte HS35’s in the Arizona Death-Race, a motorized bike race inhabited mostly by gasoline-powered bikes. Not happy with running his Crystalyte at any reasonable wattage number, liveforphysics modified his motor with thicker phase wires, hall wires, and drilled penis-shaped holes in the side covers to provide air cooling.
He hooked a gigantic controller and a home made Hobby King Nanotech battery pack to the bike capable of producing 40K watts.
He completed building his bike the night before the race and decided to test the monster E-bike on a motorcycle dyno and gave it a whirl. He managed to test the limits of the motor, and literally caught the motor on fire. Said liveforphysics: “I do tend to lean on things a little harder than most folks”. Be sure to see the end of this spectacular video which documents the experience:
Written by senior editor Eric, June 2012