Custom electric surrey bike

[mikecbabay]

Hey Endless Sphere crew,
My name is Mike and I'm building a custom electric surrey bike for family use — something safe, comfortable, and capable of cruising at 25 mph on flat streets and gentle hills. I'm a welder in California, so fabrication is no issue. I’ve got a MIG setup at home and a solid plan, but I’d love second opinions from folks who’ve built multi-passenger rigs or worked with dual hub motor systems, fat tires, or hybrid frames.

Drive System – Dual Hub Motors​

• Motors: Dual hub motors sized for 5" fat tires on 100 mm rims
• Voltage: 60–72 V system
• Power: ~3 kW combined continuous, 4–6 kW peak for starts
• Controllers: Two matched FOC controllers, throttle split to both
• Torque arms: Dual per motor, gusseted dropouts (6–8 mm steel plates)
• Pedals: Optional crankset for backup/manual input (not primary drive)

Battery & Runtime Goals​

• Runtime target: 2–3 hours
• Battery: 60–72 V, 40–50 Ah (2.4–3.6 kWh)
• Mounting: Under center bench, low and central
• Charging: 5–10 A smart charger, 6–8 hour charge time
• BMS: 100–150 A continuous, with thermal and overcurrent protection

Tires, Brakes, and Steering​

• Tires: 24" or 26" × 5.0" fat tires, PSI 12–15 for comfort
• Rims/spokes: 100 mm double-wall, 12–13 ga spokes
• Brakes: Disc or drum, mechanical preferred; parking brake for loading
• Steering: Ackermann geometry, kingpins ~46–48" apart, reinforced arms

Questions for the Community​

• Best dual hub motor kits for 5" fat tires and 100 mm rims?
• Tips for dropout reinforcement and torque arm design at this scale?
• Controller setup: throttle split vs independent control — what’s worked best?
• Brake setup advice for a ~300 lb chassis + passengers?
• Any lessons learned from building wide-track, multi-passenger e-bikes?

Thanks in advance for any critique, lessons learned, and advice. Just want my family safe while riding.

 

[amberwolf]

Curious: Any reason specifically this is in the general discussion instead of one of the "build thread" forums? It will probably get more views / replies there, so I can move it if you want.

 

[amberwolf]

Regarding the topic itself:

If you haven't already, you may want to check your local / state "transportation code" to be sure a four wheeler is a legal bicycle, or that it fits whatever other category you can apply to it. I bring this up because there are places (like where I am) that bicycles cannot have more than three wheels in contact with the ground (so training wheels are excepted but quads of any kind are not). (the exception is specific pre-manufactured tour pedal cars for multiple passengers that there is a list of specific approved suppliers for, and these are also only allowed to be used in specific locations like parks).

Similarly, is the power level required legal for the class of vehicle it's being built as?

It would suck to spend all the effort and money to build this and then be unable to use it or to have it siezed by LEOs for a technicality of an extra wheel.

(that is why the SB Cruiser is a trike, not a quad, because it would be much more useful, stable, and safe as a quad for my purposes).

• Power: ~3 kW combined continuous, 4–6 kW peak for starts

Is that enough to provide the necessary torque for the total weight of vehicle, passengers, system, anything else bieng carried, to go up the worst case slope at the required speeds, without overheating the motors or controllers or straining the battery?

And to overcome the air resistance of the entire setup, which will liekly be much higher than a typical bicycle?

• Torque arms: Dual per motor, gusseted dropouts (6–8 mm steel plates)

I'd recommend looking up The Torque Arm Picture Thread to see the many variants of clamping / pinching dropouts.

Alternately, using motors with integrated torque arm solutions like the GMAC or GAA (Grin All Axle), or modern Heinzmann types.

• Tires: 24" or 26" × 5.0" fat tires, PSI 12–15 for comfort

I recommend the largest diameter you can go with for ride quality. Smaller makes a worse ride over imperfect surfaces, regardless of tire type. For me, after years of the SB Cruiser's front 26" and rear 20" (closer to 21"+ with the Shinkos on there) tell me that 26" is the minimum I would go with, and 29" is what I intend to use on the Mk2 on all three wheels.

• Rims/spokes: 100 mm double-wall, 12–13 ga spokes

Be certain that your rims are *designed* for that thick a spoke. If they are not, you will be applying spoke tension to them that is too high, and rim damage will occur (deformation and/or cracking at the nipple hole) that then loosens the spoke, and allows flexure of elbows, breaking spokes as well and potentialy causing wheel collapse.

Pedicabs that use bicycle rims and carry at least as much weight as you probably will but on only 3 wheels stil use thinner spokes of 14 or 15g.

Thicker is not better unless all the parts (hubs and rims) are designed for that.

• Brakes: Disc or drum, mechanical preferred; parking brake for loading

I use a single Avid BB7 MTN 200mm on the front of SB Cruiser (a heavy heavy-cargo trike at 20mph) and it is more than sufficient to skid the wheel easily with easy matching Avid SD lever operation by my arthritic hands. If I had them on the rear wheels (instead of variable regen) they would be able to skid those, too, and short of skidding would easily stop the trike very quickly.

So they would probably work for your purposes as well.

If you need better thermal mass for the rotors, you could probably double up (or more) the rotors, or make your own rotor, use a pitbike or MC rotor, etc, and split the caliper (they bolt together as halves) and add a spacer between the halves to clear the thicker rotor. I'veconsidered the expeiment but nver gotten round to it.

If you need even stronger braking and your wheels can handle the force, you can adapt MC rotors to your wheels, and adapt MC calipers to your frame--but all of these will liekly be hydraulic, not cable oeprated.

• Best dual hub motor kits for 5" fat tires and 100 mm rims?

Don't know of any kits like that.

• Controller setup: throttle split vs independent control — what’s worked best?

Single throttle works fine--just send the same signal to both sides. My SB Cruiser has worked that way for years, though I use cadence pedal control to vary the motor torque to vary the speed, rather than a hand throttle (it has one but it's notused much other thanto start from a stop in situations I cannot push hardenoughon the pedals.).

 

[Nix Liou]

Hey Endless Sphere crew,
My name is Mike and I'm building a custom electric surrey bike for family use — something safe, comfortable, and capable of cruising at 25 mph on flat streets and gentle hills. I'm a welder in California, so fabrication is no issue. I’ve got a MIG setup at home and a solid plan, but I’d love second opinions from folks who’ve built multi-passenger rigs or worked with dual hub motor systems, fat tires, or hybrid frames.

Drive System – Dual Hub Motors​

• Motors: Dual hub motors sized for 5" fat tires on 100 mm rims
• Voltage: 60–72 V system
• Power: ~3 kW combined continuous, 4–6 kW peak for starts
• Controllers: Two matched FOC controllers, throttle split to both
• Torque arms: Dual per motor, gusseted dropouts (6–8 mm steel plates)
• Pedals: Optional crankset for backup/manual input (not primary drive)

Battery & Runtime Goals​

• Runtime target: 2–3 hours
• Battery: 60–72 V, 40–50 Ah (2.4–3.6 kWh)
• Mounting: Under center bench, low and central
• Charging: 5–10 A smart charger, 6–8 hour charge time
• BMS: 100–150 A continuous, with thermal and overcurrent protection

Tires, Brakes, and Steering​

• Tires: 24" or 26" × 5.0" fat tires, PSI 12–15 for comfort
• Rims/spokes: 100 mm double-wall, 12–13 ga spokes
• Brakes: Disc or drum, mechanical preferred; parking brake for loading
• Steering: Ackermann geometry, kingpins ~46–48" apart, reinforced arms

Questions for the Community​

• Best dual hub motor kits for 5" fat tires and 100 mm rims?
• Tips for dropout reinforcement and torque arm design at this scale?
• Controller setup: throttle split vs independent control — what’s worked best?
• Brake setup advice for a ~300 lb chassis + passengers?
• Any lessons learned from building wide-track, multi-passenger e-bikes?

Thanks in advance for any critique, lessons learned, and advice. Just want my family safe while riding.

I don't know a ton about anything with more than 2 wheels here, but I have some questions about the power and battery plans.

With the run time target of 2-3 hours, and a battery around 3kwh, you will need to be only using 1kw continuous- I'm not sure if that would be enough for a larger vehicle, big tires, starts and stops, etc.

Of course if you want to run the full 3kW continuous, you would need 6-9kwh of battery capacity to get 2-3h of run time, which is a lot for sure.

It seems like you are planning on building the battery yourself? Have you built packs like this before? Only asking because this would definitely be a spicy first pack... Anyway, I would stick to 60v if you only want to go 25-30mph, at higher voltages most motors will just be wound to go faster than your target speed.

For the BMS I would look at ANT, JBD, or JK for options at this power level, and definitely a smart BMS.

For the cells, if you are doing 50Ah and 100A continuous discharge, there are plenty of options for cells capable of 2C discharge. EVE 50e and 58e, Samsung 50g, 50e, 58e, etc. Most modern cells can do 2C discharge.

If you plan to put it low down in the chassis, you should design it so that you can just have a single big slab of all the cells, it will keep the construction simpler by far. It sounds like you won't be too space constrained?


As for the brakes, is there a reason you would prefer mechanical brakes? I generally advise hydraulic brakes on any ebike, though mechanical can be acceptable. But with a heavy battery, multiple passengers, and sizeable motors, I would choose hydraulic disc brakes myself. You can get finned brake pads for better cooling, which may be prudent.

 

[Papa]

....you may want to check your local / state "transportation code" to be sure a four wheeler is a legal bicycle, or that it fits whatever other category you can apply to it.

This criteria needs to be established well in advanced.

I would not build dual motors - the higher initial build cost, far more complex, and not as efficient as a single motor and differential.

 

[ZeroEm]

The bigger motors have one gear. I'm a fan of more motors, spread the traction around. Don't think you will need 72V. I like 72V but rewound my motor to go slower. Find what is legal. Wanted a scooter but gave that up when required a DL. Will stick with trikes.

 

[Papa]

I'm a fan of more motors, spread the traction around

No justification for two motors here. OP mentioned California, "flat streets and gentle hills"... not off-road or harsh winters. Besides, a differential will more than accommodate 2wd if the need arises.

 

[ZeroEm]

No justification for two motors here. OP mentioned California, "flat streets and gentle hills"... not off-road or harsh winters. Besides, a differential will more than accommodate 2wd if the need arises.

My first build wanted dual motors, found that one was all that was needed. That did not drive away the voice in my head that told me that really need two or three for a trike.

 

[Risky Rob]

In California, an ebike is legally limited to 750 watts.