This topic comes up occasionally and deserves special attention to prevent us from repeating ourselves. Much on this subject has been learned since the oldest known post was made on this forum back in April 2008. There is a lot of good useful experience out there, and some disinformation. The purpose of this FAQ is to demystify the concepts, point to relevant topics, and provide concise answers.
Disclaimer: This thread does not address All-Wheel Drives or dual-motor mid-drives, although these are certainly interesting topics that rightfully deserve their own FAQ.
What is 2WD?
Two wheels driven by two separate motors and providing forward motion. Used within the context of "bikes", these wheels may be attached to a bicycle or to a tricycle (recumbent or 3-Wheel bicycle with 2 rear wheels or bicycle with sidecar). Motor-driven cycles (other than motorcycles using ICE) may also use 2WD. Technically, the primary employment of 2WD is through the use of Direct Drive (DD) hub or Geared-Drive motors, although not exclusively: The two types may be used in combination. Brushed motors may also be used although they are less frequently discussed on this forum. In brief: The concept is simply about using two driven wheels to provide electric assist towards forward motion, whatever the technology.
What is required to create 2WD?
Two motors affixed to two sturdy wheels securely, two controllers – each attached to one motor, and at least one throttle. It is rare to find a single 2WD controller specified for "bike" use, therefore creating a 2WD bike is nearly always a custom-job requiring basic wiring & soldering skills. Each motor requires its own torque arm to secure the axle from spinning.
Do I need two throttles?
No; modifications can be made so as to drive both controllers with one throttle. However care must be given on how the mod is applied. Not all controllers are equal, and neither are throttles – therefore a bit of experimentation will be required to find the correct match for the application. Some folks enjoy having two independent throttles which works well for Front & Rear configurations.
Do I need two CAs?
No – although given with trepidation & reluctance: Having one attached to each controller will help to monitor activity and diagnose issues, especially if one unit malfunctions.
Will I be able to use Regen braking?
Yes. Regen can be applied to one, or with both controllers using careful modification. Note: 2WD-regen can be jarring and hard on the frame. Adjust the strength according to need.
What other features can be used with 2WD?
Any feature may be used. If both controllers are paired equally, then it is possible to use one dashboard device to affect both wheels. Typically, Cruise-Control and 3-Way Current Control may be unified with good results.
Do both motors require synchronization?
No, especially motors aligned Front to Rear. The motor attached to the steering wheel will turn at a different rate and therefore it is preferred to have both motors run independent of each other, hence employing two controllers.
Is 2WD more efficient than one-wheel drive?
Yes and No. A 2WD bike outfitted identically with a single-wheel drive bike (sans extra motor/controller) will outperform the single-wheel in acceleration off-the-line, in hill climbing, when facing cross or headwind, or otherwise pulling a heavy load. In these conditions, 2WD is more efficient at applying power to the ground by splitting the load between two controllers and two motors; neither will get as hot as a single controller/motor at the same power rating. 2WD bikes therefore run cooler and better under load farther and longer than a single-motor. In essence, a 2WD bike will have approximately twice the available torque and traction in all conditions.
However, 2WD bikes are not more energy-efficient. On flat level ground without any headwind (discounting wind resistance) a 2WD bike will consume more power in cruise mode because of the energy consumed to manage the second system, including overcoming resistance to freewheel. Likewise coasting downhill will cost more due to drag created by the extra driven wheel. This loss can be partly offset by employing dual-regen braking. Two motors will use up to twice the energy twice as fast over that of a single motor! Be prepared to double the battery capacity of your pack, or cut the estimated distance by at least 1/3 when using 2WD. The easiest way to visualize the difference between 2WD and single-wheel drive is to imagine the difference between a 2WD truck and the same model & engine as a 4WD: Which will have more power (should be the same), which will climb hills better (4x4), and which will get better gas mileage (2x4). There’s no free lunch and it’s the same physics with electric 2WD bikes.
Will 2WD experience contention?
Contention means two forces competing for control, and the answer is Yes, they will. Contention is most noticeable on flat level ground. There are however several ways to reduce contention to nearly imperceptible levels.
- Program the Front & Rear controllers to output slightly different power levels.
- Use different tire treads or types or sizes.
- Use different rim sizes.
In a Front-to-Rear alignment, which motor should have more power?
Traditional physics suggests dividing up to 70% of the total system power to the Rear motor. If programed evenly the Front motor will occasionally slip when accelerating off-the-line, especially on loose or slick surfaces. This can actually become a liability when cornering due to the loss of traction. Conversely, Regen should be set highest on the Front motor for the same reason why motorcycles have dual disc brakes forward and not aft.
On a 2WD bike, where is the best place to put the batteries?
In the Triangle; the center region of the bike. I have found that saddle bags over the triangle increases the weight distribution forward of center and helps prevent the Front wheel from spinning in rainy weather.
What is the best bike frame for 2WD?
Depends on the use. If money can afford, a full-suspension Downhill frame with rear horizontal dropouts is quite possibly the best.
How many torque arms do I need for 2WD?
At least one per wheel, although one on each side of each wheel is strongly preferred. It really depends upon how much power is being applied. Best to err on the side of caution and use two per driven wheel.
How fast can a 2WD bike go?
How fast do you want to go? Imagine that you have a single wheel drive and you push it as hard as it can go when the batteries are freshly topped off; write down that speed. Then run the freewheel test. Note the gap between the actual speed and the freewheel speed. With 2WD it is feasible to cut that gap in half using the same voltage ~ and possibly more with current adjustments. The motor winding has a large effect on how well the top-end is improved with lower wind motors benefitting the most.
What happens if one motor fails?
The beauty of having 2WD means you are not stranded! 2WD = Redundancy; two separate & independent systems. In most cases a single-wheel drive can get you home safely.
What are the most common problems with 2WD?
- One motor periodically cuts out under load:
- Check to ensure that the Battery Voltage does not sag below LVC when asking for WOT. It could be that the Battery cabling is too small, C-rating insufficient, Capacity (Ah) is too low, or pack is nearly depleted.
- Check to ensure the Throttle is provided enough voltage to drive both controllers. Good cabling should solve this.
- Motor makes horrible grinding noise and appears to lock-up:
- Phase and/or Signal wires are damaged. R&R.
- CA gets real flakey, powers-up, dies, resets, reports battery voltage falsely against the opposing well-functioning CA; may appear in addition with no throttle:
- Check the power connection to the controller; it is likely dirty or damaged. If it can be repaired, the CA should appear normal and in parity with its opposite. If the throttle is still not functional, cycle the power OFF, then ON again to reset the controller.
Is 2WD more fun and worth the trouble than a single-wheel?
You have to ask? Yes, many believe so; it’s a kick in the butt!
Related threads sorted newest to oldest:
This is by no means the complete list of topics on 2WD, though easy to find with a quick search.
- Front and Rear Hub Motors
2 wheel drive, the ultimate mtb ride?
Back in the Saddle: Going to California: 2011
Phoenix Rising: P1
2 wheel drive DH bike build (Delta/Wye Upgrade)
20KW 2WD having misfires (but can do a double burnout)
2 motors, 2 controllers, 1 throttle + 1 switch
2 brushed motors 1 controller with "Turbo" button
RLT Project #1 - 2 wheel drive. (Now startingMk2)
dual motor build
Opinions expressed are my own based upon direct experience. I thank the Community for providing knowledge which helped advance my own.
Go for it! KF