Hub Motor Dilemma

[Tfisher309]

My 1964 Sears Allstate is needing a hub motor. With virtually no suspension or brakes I am not looking to break any speed records. My target is 40mph cruising, 45 max..

The problem I'm facing is motor rpm. The 16 inch wheels, which I want to keep, will require a 625 rpm motor (if i did the math right). I am not finding this. The closest match I've found is a qsmotor 3000w 205 50h v3. This motor will spin fast enough, but is way more motor than I want to put in the bike. I'm thinking a 2000w motor will be sufficient, if I can find one that will rev high enough.

I looked into putting a 1000w motor in each wheel. Again I ran into the rpm problem. I guess the motors I am looking at are used with larger wheels and lower speed goals.

You guys have bailed me out in the past. Any ideas will be appreciated.

Attached is a picture of the bike as I bought it. For $325 it came with a title and a Hodaka Wombat engine graphed in. I'm weak and couldn't say no.

 

[AGS]

I’m going to fit a QS205 V3 in a Puch Maxi with a 19” rear wheel. I think it will be an ideal motor in your frame as well. I’m not going to go bonkers on the controller and going to use a Sabvoton 72100. I’m also looking for a cruising speed of around 40mph.

What are you going to use as a rear brake? I’m planning on using a bicycle 203mm rear disk with a hydraulic caliper and rely on electronic braking for real stopping power.

 

[amberwolf]

My 1964 Sears Allstate is needing a hub motor. With virtually no suspension or brakes I am not looking to break any speed records. My target is 40mph cruising, 45 max..

The problem I'm facing is motor rpm. The 16 inch wheels, which I want to keep, will require a 625 rpm motor (if i did the math right). I am not finding this.

At what voltage?

A motor is rated for a certain RPM at a certain voltage, at no load. It will run around 20% slower when loaded depending on the situaiton and system, so you need higher RPM by that much to be sure of reaching your speed goal. You also need higher RPM than that by some significant amount if you want good torque all the way up to max speed. You also need the battery voltage to be high enough even when the battery is just above empty to maintain that speed if your goal is to always have that speed available.

So, if you find a "48v" motor at 500rpm, then to get 625RPM (unloaded) you'd need 625 / 500 x 48 = voltage to run that motor at that rpm. Then 1.25x that to get that RPM when loaded. Then say, another 1.25 to 1.5x that to get good torque all the way up to the max speed goal.

So 625/500 = 1.25. 1.25 x 48 = 60v. That gets you that RPM unloaded

60v x 1.25 = 75. That gets you that RPM when loaded.

75v x 1.25 = 93.75. That gets you started on torque all the way up to max speed goal (might need higher voltage).

The closest match I've found is a qsmotor 3000w 205 50h v3. This motor will spin fast enough, but is way more motor than I want to put in the bike. I'm thinking a 2000w motor will be sufficient, if I can find one that will rev high enough.

I looked into putting a 1000w motor in each wheel. Again I ran into the rpm problem. I guess the motors I am looking at are used with larger wheels and lower speed goals.

Two motors plus two controllers plus mounting hardware (you'd have to modify the fork tips as well as the rear swingarm dropouts) would probably weigh what the QS205 would, and be more complicated and possibly more costly (and probably not as well-built for torque).

You can go to ebikes.ca motor simulator to build a custom simulation for each version (cromotor is close to a 205, Grin all axle is similar to various 1kw motors, and there is a 2WD option), to play with variatiions to see which one best suits your needs. Actual results with real parts will be different by however much the simulation inputs differ from your real parts.

 

[Tfisher309]

Since I am needing rpms I assume the motor will be 72v. For a brake I will use whatever disc will bolt to the hub I buy.

After hours of looking it seems the 3000w qs 205 50H V3 is the only hub motor that checks the critical boxes. Plus it's 150mm dropout works.

The drawbacks are weight, price, and it really is more motor than I need.

Here is a question. If I use the 3000w motor with a small controller (maybe 100a) will I still have the full rpm range I need without rotating the earth every time I accelerate?

 

[From-A-To-B]

Since I am needing rpms I assume the motor will be 72v. For a brake I will use whatever disc will bolt to the hub I buy.

After hours of looking it seems the 3000w qs 205 50H V3 is the only hub motor that checks the critical boxes. Plus it's 150mm dropout works.

The drawbacks are weight, price, and it really is more motor than I need.

Here is a question. If I use the 3000w motor with a small controller (maybe 100a) will I still have the full rpm range I need without rotating the earth every time I accelerate?

A motor will reach its top rpm, unloaded, with just a few amps. My guess is that 100A is plenty to get you moving down the road. 100A x 72V is 7200W, which should be plenty for cruising at 40mph.

Voltage and motor kv dictate rpm, amps get you to that rpm level. If you run out of amps (or need more of them), you will fail to reach the top speed and adding more amps will get you going faster. But I don’t think that 100A is anemic for a bike that wants to go 40mph. My ebike tops out at 32mph and will maintain that speed with 700W.

(I’m a bicycle fella and have virtually no e-motorcycle/moped experience.)

 

[neptronix]

These are 16" as in, an effective 20" in bike terms. ( motorcycle and bike wheel measurements are different )
A single 3T leafbike '1500w' motor would do really well on this size wheel, and would be capable of >2kw continuous, and is 16lbs.

I used said motor in a 26" wheel ( big continuous power disadvantage ), and with some air cooling, i could do 40mph continuously, 45mph for miles at a time, without it overheating.

You should be able to do the same, maybe more power, without air cooling, thanks to the higher RPM of your motor.

But.. you have chain drive on that bike already. You could get even better power to weight ratio, maybe a bit better efficiency by using a chain drive motor. The downside is that it would involve some fabrication.

 

[Tfisher309]

Good to know about the differences in rim sizes. I looked up the leaf bike motor and the dropout spacing wouldn't work, I need 150mm.

I have been talking with Buchanan Spokes and they tell me there is no good way to lace a big hub motor to my old 16 inch steel rims. Too steep of angle and not enough material in rim to change angle with a stepped drill bit. Spokes would be bowed.

Qs makes a 2000w v3 mid-drive motor with gear reduction, but it is very wide and would stick out far to the right in my narrow frame.

Thanks for the info and ideas. Something here is going to work.

 

[neptronix]

I believe leafbike makes a 150mm and/or 170mm variant, but haven't bought a motor there for almost a decade.

A 20" bike rim might work considering this motorcycle/moped looks super light. The smaller diameter the rim, the stronger it is, and this in your favor here.

QS makes some very heavy and large motors, you could definitely find smaller and lighter, and have a little extra battery room as a result.

 

[Eastwood]

2000w rated hub motor is not enough for that Moto build. I think the QS205 would be a perfect fit since you’re not looking for Crazy performance etc. The QS205 50H V3 can be laced into a 16” motor rim. That’s if you’re trying to keep the stock wheel size. If you did end up going with a 16” rear, would give you more torque, but less TopSpeed. With your target speed I would say 3T or 4T at 72v.

Also, are you sure you only want a max discharge of 100a, Seems a little low for a Moto build. 150a would suit you well

Edit: Are you going to restore the bike or just convert as is? If you are restoring it, you could consider getting new forks that would work with disc brakes.

 

[j bjork]

2000w rated hub motor is not enough for that Moto build. I think the QS205 would be a perfect fit since you’re not looking for Crazy performance etc. The QS205 50H V3 can be laced into a 16” motor rim. That’s if you’re trying to keep the stock wheel size. If you did end up going with a 16” rear, would give you more torque, but less TopSpeed. With your target speed I would say 3T or 4T at 72v.

Also, are you sure you only want a max discharge of 100a, Seems a little low for a Moto build. 150a would suit you well

Agreed, and why going smaller? In that range the stuff is still cheap anyway.
A 4t would do good with a controller with about 500 phase amps, I think a 3t would need about 680 phase amps for the same torque. That would probably mean a little more expensive controller, and more power at higher speed.