What torque do I need ?

[Palmthree]

Hello,

I'm looking for a commuter bike in Europe ; my knees are hurting and I don't want to be tired going to work !

I've read a lot about ebikes (and still do) but I can't really understand what torque do I need, especially as more powerful motors are usually found on much more expensive bikes (and I don't want to pay too much for a ebike since my area is pretty...dangerous !). I've tested a good ebike from 2015, equiped with a 48Nm Bosch motor and it was not really good to climb a 12% hill.

I'm looking for resources that can clearly explain what I need, since I can't test bikes on my daily commute.
Thanks !

 

[Brian-s]

I'm looking for resources that can clearly explain what I need, since I can't test bikes on my daily commute.
Thanks !

I hear Google is a good resource . And you might be able to test a bike for your commute. Find a local dealer/ bike shop. See if they will rent you a e-bike. And there you can try it for a couple hours. See how your knees like it.

 

[Deacon Blues]

Many of the new ebike motors put out 85/90+ Nm of torque, which is more than enough to power you up a steeper climb, when using their higher power levels.

 

[Slaphappygamer]

Youtube is also a great source. When watching the reviews, look at the rider body type. Sure the bike can carry the reviewer up that crazy hill, but does he weigh more than you? Also, were they wearing a backpack or have panniers? These are the other things I watched for. I found a guy reviewing the bike I bought, he appears to be heavier than me and the bike handled the hill well. Watch for these little things.

 

[dodgeman]

I agree with Deacon Blues, you probably want tour torque up in that 85 nm range. My Trek has a Bosc 50nm motor and on really steep hills you are working pretty hard. Throttles seem to be a love/hate thing on this forum but you might want one.

 

[Palmthree]

I hear Google is a good resource . And you might be able to test a bike for your commute. Find a local dealer/ bike shop. See if they will rent you a e-bike. And there you can try it for a couple hours. See how your knees like it.

Hello !
I've tried the search motors but I don't know why, I mostly find awful explanations/strange translations, copied/pasted content.
I can't rent any bike near my location.

Many of the new ebike motors put out 85/90+ Nm of torque, which is more than enough to power you up a steeper climb, when using their higher power levels.

Youtube is also a great source. When watching the reviews, look at the rider body type. Sure the bike can carry the reviewer up that crazy hill, but does he weigh more than you? Also, were they wearing a backpack or have panniers? These are the other things I watched for. I found a guy reviewing the bike I bought, he appears to be heavier than me and the bike handled the hill well. Watch for these little things.

Me + my luggage + a child = 100kg. I can't find any resources explaining what weight a torque can really carry (especially with hills). Something like :
80kg - 5%hiill - 50Nm = 14kph (for example).

I agree with Deacon Blues, you probably want tour torque up in that 85 nm range. My Trek has a Bosc 50nm motor and on really steep hills you are working pretty hard. Throttles seem to be a love/hate thing on this forum but you might want one.

I really can't work pretty hard. I don't want to go really fast in these hills : for example, I have a 15% hill as soon as I leave my house (400m - 60m). So, I guess I have to look at bikes with at least 70Nm (could be a compromise !).

 

[indianajo]

With about 80 nm I can start 145 kg up a 15% hill. Torque has to do with what you can start, although the spec tends to be peak torque at whatever rpm it is at maximum.
Speed up that hill would be predicted by power. In Europe except Switzerland, you are limited to 250 or 350 W.. I found a 1000 w motor took me for several years up 75 hills up to 15%. The clutch recently failed on that motor. 350 W motor made about 3 trips before the motor burnt the windings on a 6% grade. States limit bicycle power to 750 watts these days. I just installed a 500 w motor that made 1/6 of one trip before it burnt a phase wire on a 4% grade. I guess the state wants me to buy a $60000 fossil fuel pickup truck to haul my groceries out to summer camp. Burn those dinosaurs, roast the planet! Who needs polar bears? Only, computer controlled motor vehicles inevitably get check engine disease at 10-13 years and become useless. Check engine light comes on, limp home mode burns twice normal amount of fuel, nobody can repair it. At 2000 miles/year I would have to scrap such truck at 24000 miles. My brother just sold off a Chevrolet diesel, would not even start, 3 trips to dealer would correct the problem only for 1 or 2 starts.

 

[PCeBiker]

I think that you need a mid drive ebike.
A mid drive uses the gears on the bike to amplify torque. You'll be slower in low gear, but the motors rpm will be more efficient and effective.

Hub drives need to have sufficient speed to give maximum torque.

Mid drives are more expensive though, and they wear on the chain and sprockets harder than a hub drive, but that shouldn't be too bad with a 250-350 Watt motor.

 

[Stefan Mikes]

A 90 Nm mid-drive motor will conquer any hill.

 

[Grueso]

Hello !
I've tried the search motors but I don't know why, I mostly find awful explanations/strange translations, copied/pasted content.
I can't rent any bike near my location.


Me + my luggage + a child = 100kg. I can't find any resources explaining what weight a torque can really carry (especially with hills). Something like :
80kg - 5%hiill - 50Nm = 14kph (for example).

The torque computation is very straightforward: the tractive effort that pushes the bike uphill has to be at least as large as the weight force pushing you downhill.
In your example: 100kg (you+bike) *10N/kg (earth gravity)*0.05(5% hill) = 50 N tractive force.
To compute the torque needed at the rear wheel, multiply with the radius of your wheel. Let's assume it's a 700c wheel, so the radius is about 0.34m, so the torque needed is 17 Nm. For a 10% hill you need 34 Nm, for 20% about 70Nm. (% gradient is the tangent of the incline angle, the weight force pushing you downhill is going with the sine, but up to 20% the difference is negligible.)
For a hub motor, the torque at the wheel is the same as the motor torque (or gear output torque, for a geared hub motor). For a mid motor you have to multiply with the chain gear ratio. Motor torque = wheel torque * chain ring size / sprocket size, for example 17 Nm*38T/21T= 31 Nm needed from the motor.

To compute the speed with which you can climb the hill, divide the motor power by the tractive force needed, so 250W/50N = 5 m/s, or 18 km/h. This works as long as you are going slow enough that air drag does not play a role, up to about 15 km/h or so.
This is all based on the basic relations

• torque [Nm] = force[N] * lever arm [m] (or radius of a wheel),
• work = force * distance,
• power = work/time = force*speed,

and metric units make it all really simple.

 

[PCeBiker]

You can usually install a smaller front chain ring on a mid drive ebike if you have really steep hills or heavy loads to carry.
The only trade-off is a slower top end speed.

You can also install a new rear cluster/free wheel on the back as well to increase torque further, but that step gets more complicated and expensive.

 

[mfgrep]

My 70Nm mid drive seems powerful enough to do anything with minimal rider assistance when desired. 90Nm was an option to me but It was more expensive and seemed unnecessary for my needs. 90Nm is VERY powerful for a mid drive.

My other bike is a 35Nm mid drive which requires significant rider input for hills.

Hope this helps

 

[indianajo]

I think that you need a mid drive ebike.
A mid drive uses the gears on the bike to amplify torque. You'll be slower in low gear, but the motors rpm will be more efficient and effective.

HaHaHaHaHaHaHa! Unless you have a 46 or bigger rear sprocket. Those are extremely rare. Most mid drives have a 46 tooth front sprocket. Yes per post 11, you can change the mid-drive front sprocket and void your warrenty.
Mid drives cool themselves better than geared hub drives. Can be used on long slow slogs uphill. Direct hub drives are inefficient at slow speeds, burn way too many watthours compared to a geared hub. But can be cooled by ferro-fluid. I had a direct drive rear hub for about 3 months. Hated all that weight on the back, and cranking it unpowered was like dragging an anchor.

 

[PCeBiker]

Well,.. if @Palmthree is restricted to 250 or 350 Watts, and needs to deal with 12% grades, then he's probably better off with a mid drive?

Perhaps with the 25 kph speed limit in Europe, there is a geared hub ebike available that is geared for that speed limit (with either the planetary gear ratio or the number of windings) that has enough torque to deal with 12% grades without working the rider too hard or burning up the motor?

 

[Grueso]

The 250W limit is for 'continuous output power'. From what I have read, most 'name-brand' motors listed for Europe as '250W' can put out 2-3 times as much.

 

[PedalUma]

Gearing. For a cargo application in hill country you will want a large range of gears and a generally smaller chainring. Perhaps start with a 36 ring to an 11-50 cassette. 9-speed chains are thicker than 10,11, and 12-speed chains and the cogs can be more robust. Some brands, even most, will over estimate power. Specialized under lists its power output. It is not about the torque at the motor but at the wheel. Wheel diameter is also an important factor. Smaller wheels have effectively lower gearing.

 

[PCeBiker]

Me + my luggage + a child = 100kg.

I have a 15% hill as soon as I leave my house (400m - 60m).

Don't forget about quality/powerful brakes.
You'll need lots of continuous stopping power with 15% hills and a 100kg load.

 

[Palmthree]

To compute the speed with which you can climb the hill, divide the motor power by the tractive force needed, so 250W/50N = 5 m/s, or 18 km/h. This works as long as you are going slow enough that air drag does not play a role, up to about 15 km/h or so.

This means that speed is not linked to torque, right? The torque is used to "cancel the tractive force" (or a part of) ?
Thanks for the lesson ; it was really clear. I can compute more easily my needs.
What happens when the road is theoretically flat and the hill = 0 ?

Perhaps with the 25 kph speed limit in Europe, there is a geared hub ebike available that is geared for that speed limit (with either the planetary gear ratio or the number of windings) that has enough torque to deal with 12% grades without working the rider too hard or burning up the motor?

I don't really know ; to be honest, in Europe, all ebikes seem very similar to me and these technical characteristics are not often seen.

Don't forget about quality/powerful brakes.

Yes, I don't know how to choose them for the moment. They are almost all "hydraulic" but it doesn't mean a thing to me.

Gearing. For a cargo application in hill country you will want a large range of gears and a generally smaller chainring. Perhaps start with a 36 ring to an 11-50 cassette. 9-speed chains are thicker than 10,11, and 12-speed chains and the cogs can be more robust. Some brands, even most, will over estimate power. Specialized under lists its power output. It is not about the torque at the motor but at the wheel. Wheel diameter is also an important factor. Smaller wheels have effectively lower gearing.

You're right ; I shouldn't rely only on torque.

 

[Grueso]

This means that speed is not linked to torque, right? The torque is used to "cancel the tractive force" (or a part of) ?

Right.
Torque is force times lever arm (Newton times meter), and the lever arm is given by your rear wheel (assuming that is the driven wheel) radius.
The tractive force is the force shoving your bike forward; if that is up an incline (hill), then it needs to be at least as large as the force of gravity pushing you back down the hill. If your tractive force is exactly equal to the downhill gravity force, then you can't accelerate. If you are standing still, you'll keep standing still.

Speed comes into play when you compute the power you need to move uphill. Speed and torque multiplied determine power. Since we already computed the force from the torque, it's easiest to use force times speed is power. You could also take torque times 2*pi times revolutions per second, which gets you from the force times radius to force times circumference (=distance per revolution), and from there to speed by dividing by the time it takes for one rotation.

Thanks for the lesson ; it was really clear. I can compute more easily my needs.
What happens when the road is theoretically flat and the hill = 0 ?

If the road is flat, all you have to overcome is rolling resistance and wind resistance. At that point it's better to use the motor simulator at ebikes.ca or the cycling power computer, since it is no longer a good assumption that you are going slow and that rolling resistance and air drag can be neglected compared to gravity.

 

[PedalUma]

Don't forget about quality/powerful brakes.
You'll need lots of continuous stopping power with 15% hills and a 100kg load.

Torque is a bit like talking about the length of a lever. It also applies to brakes. Road bikes are lightweight and 'seek' to count grams. They will often have 160mm rotors. A good cargo bike is about hauling stuff, including kids, and not about cutting grams. They will often have four piston brake calipers with giant pads and have 203mm rotors. I rode a bike today with a 700c rotor because it has good quality mountain pads on the rim. When properly adjusted and in dry conditions it can lock up and skid. Stopping is more important than going and everything should be in balance and be proportional. Heavier bikes require everything to be heavier. It is somewhat like, you can't just drop a highspeed processor into a low speed computer. It all works together. In most cases less is more. Getting there is the hard part. Think of a racing sail boat.