Why to use low end Torque motors

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Why to use low end Torque motors

Postby Baboon » Sat Apr 26, 2008 1:29 pm

Torque is a matter of leverage, acceleration of mass & the motors resistance to the mass slowing down.
For example, a motor with a long stroke has more leverage because of the crankshafts offset. The distance from the crank center to the big end is the amount of leverage. The bigger this offset, (the stroke) the more torque a motor will have.
This is because when you increase the offset it also has to be counter balanced to make sure the motor is balanced & runs smooth. This also means that a larger stroke offset will have increased mass, (weight) compared with a short stroke motor.
This increased mass rotating at a larger diameter than a short stroke crank has more inertia, the mass at a larger diameter has more centrifugal force & inertia which means its hard to slow it down. Now combine this with a heavy flywheel & you end up with a crankshaft assembly that is hard to stop.
This would be the combined mass of the crank, flywheel, con rods & pistons. But, because this rotating assembly is heavy, it also takes longer to accelerate. That is why a short stroke motor, ( large bore & short stroke) picks up its revs very quickly.
But a short stroke motor is also very easy to slow down, it does not have the leverage of the long stroke crank & the weight in the rotating assembly.
Now the higher you can rev a motor the more fuel you will consume over that given time. This equals more power.
So a short stroke revy motor will consume more fuel at high revs & produce more power than a motor of the same capacity with a very long stroke & small pistons.
Look at an F1 car, its 3 litres in capacity, very short stroke & revs in excess of 18,000 rpm. BUT, some drivers have problems on race starts because the motors have very little low rpm torque.
This means they have to rev the motor to increase power & torque to increase the rotating mass to a higher rpm, thus more leverage to get the car moving.
You will find that super short stroke revy motors have a very narrow usable power band. That is, the motor is really only useable within a certain rev range. Like a two-stroke motor that has a narrow power band, outside of this rev range it wants to stall.
This brings us to the relationship between horsepower & torque.
Horsepower is a product of RPM & torque combined. The higher the RPM the higher the inertia of the rotating mass, so the higher the RPM the harder it is to stop or slow down the motor.
So look at the two types of motor. A long stroke motor of High Mass that only revs to a low rpm can have high torque at low RPM.
A short stroke motor that revs to high RPM can have high torque because the motor is consuming a large amount of fuel at the high revs & producing power.
So we can see that a short stroke revy motor generally will not have any torque at low rpm, it does not have the mass or leverage to keep it going.

A long stoke motor will take longer to build up its revs & it will not rev as high but it will have more torque at low revs because the mass is greater & harder to slow down.
As far as motorcycle motors are concerned look at a KLR600 Kawasaki, it’s a single cylinder motor with a big bore & a long stroke.
Compare this to a Kawasaki ZZR600, a four cylinder motor of small bores & short stroke. The KLR600 will take longer to pick up its revs & will only rev to around 7,000 rpm. The ZZR600 will pick up its revs quickly & rev out to an easy 12,000-RPM. The KLR600 is good for about 50 hp, the ZZR600 around 100hp.
But the KLR600 has a broad spread of torque from low rpm right up to its 7,000 rpm. You do not have to keep working up & down the gearbox to keep the vehicle moving.
The ZZR600 has high power & torque, but only when its revving high & consuming all of that fuel.
If the revs drop, so does the torque. The KLR600 you can dump the clutch at an idle, the motors rotating mass is keeping the momentum moving, so it’s hard to stall.
The ZZR600 has to be revved to take off, it has little rotating mass so you need to increase the RPM to increase the torque.
Now both motors are great, but for different uses.
The ZZR600 has twice the power, but to use that power it has to be revved. Offroad driving is different to on road driving.
On the street traction is easy to come by, so the ZZR600 can be used right across its rev range.
But take that motor off road into a boggy situation & the load on the motor increases. To take off you need the torque to get the weight of the vehicle moving, so you give the ZZR a good rev so it won't stall, but that makes the rear wheels spin faster & you start digging a trench.
If you have very wide tyres & lots of floatation you may get out of that trench & get moving.
The KLR will take off at low rpm without spinning the wheels, you are getting positive traction & you start to accelerate, you are moving through the boggy sand fast because the torque is hooking up & the lack of rpm is preventing the wheels from spinning.
As soon as you get some speed up you can roll on more revs as you have some momentum now. The ZZR is still stuck in its trench, it can't climb out of it because as soon as you back off the revs to stop the wheels spinning the motor wants to stall & you slow down. The ZZR600 would be more usable with lower gearing, this will increase the torque as the lower gearing is increasing the leverage. But its power delivery is still at a high rpm & the power band is narrow, so you have to work the gears up & down hard to keep it in this power range, you are thrashing the motor hard.

What is great for the street is not always great for offroad, two totally different situations that require different solutions.

Horsepower = TORQUE x RPM divided by 5252
TORQUE = Horsepower x 5252 divided by RPM
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