Discussion
I was thinking of putting this in the dumb question section . But i was doing some work on my old landy over the weekend and had to tighten some bolts to 70nm. Easy enough with one hand with a torque wrench with a 2 foot handle. Then it struck me. How come this amount of force can accelerate 300kg of me and my bike faster than a scolded cat? If I were able to run alongside with a torque wrench applying that amount of pressure I cannot imagine it would accelerate that quickly. Or have I just got it really wrong in my head.?
Don't forget the effect of the gearing, though,
Assuming your 70NM is at 10,000rpm and the bike does 100km/h at 10,000rpm in 1st gear, that's ~1,700 metres per minute for 10,000 revolutions of the crank.
Rolling circumference of the tyre is 2 metres for a 190/55 R17
So your wheel is only doing 1,700/2 = 850 revolutions for 10,000 revolutions of the crank.
So you can multiply your 70NM by ~12 in that particular scenario.
(Unless I'm also confusing matters, which is possible after a few pints this afternoon
)
Assuming your 70NM is at 10,000rpm and the bike does 100km/h at 10,000rpm in 1st gear, that's ~1,700 metres per minute for 10,000 revolutions of the crank.
Rolling circumference of the tyre is 2 metres for a 190/55 R17
So your wheel is only doing 1,700/2 = 850 revolutions for 10,000 revolutions of the crank.
So you can multiply your 70NM by ~12 in that particular scenario.
(Unless I'm also confusing matters, which is possible after a few pints this afternoon
)Acceleration is a function of power, not torque. Power to weight if we want to be more accurate.
You can probably generate more torque at the crank of your pushbike than a lot of motorbikes produce.
The reason the motorbike accelerates more is that it can put its torque out at 5-10,000 rpm while a pedal bike only does it at around 60-100rpm so produces far less power.
This means that you can use a gearbox to reduce the speed of rotation by a large factor which increases the torque at the rear spindle by a lot.
You can probably generate more torque at the crank of your pushbike than a lot of motorbikes produce.
The reason the motorbike accelerates more is that it can put its torque out at 5-10,000 rpm while a pedal bike only does it at around 60-100rpm so produces far less power.
This means that you can use a gearbox to reduce the speed of rotation by a large factor which increases the torque at the rear spindle by a lot.
SchillingTwo said:
Acceleration is a function of power, not torque. Power to weight if we want to be more accurate.
You can probably generate more torque at the crank of your pushbike than a lot of motorbikes produce.
The reason the motorbike accelerates more is that it can put its torque out at 5-10,000 rpm while a pedal bike only does it at around 60-100rpm so produces far less power.
This means that you can use a gearbox to reduce the speed of rotation by a large factor which increases the torque at the rear spindle by a lot.
What he said ^ You can probably generate more torque at the crank of your pushbike than a lot of motorbikes produce.
The reason the motorbike accelerates more is that it can put its torque out at 5-10,000 rpm while a pedal bike only does it at around 60-100rpm so produces far less power.
This means that you can use a gearbox to reduce the speed of rotation by a large factor which increases the torque at the rear spindle by a lot.
You can buy cordless impact wrenches capable of supplying 1000nm of torque. That's nearly as much as a Bugatti veyron makes. The thing is it delivers it's torque very slowly and hence doesn't make much power. An engine making 300nm of torque at 12000 rpm will be loads faster than one making 900nm of torque at 2000rpm
My first reading of the original post was pretty dumb, I read it as if you were asking how a fastner tightened to 70nm could transmit enough power to move you at speed, and I was about to answer that very few fastners actually transmit load, they just hold the parts together with enough friction, or hold them in place while splines or keys transmit the actual load. Then i read it again and realised.
Yes, it would be something to see a man able to turn a torque wrench by hand at 9000 rpm while putting 70nm of welly into it.
Yes, it would be something to see a man able to turn a torque wrench by hand at 9000 rpm while putting 70nm of welly into it.
Kind of answered, but torque without movement means nothing i.e.there s no power. Hang a 25lb weight off a 4 foot wrench on a nut and it s subject to 100ft/ b. But assume the nut is stuck, and so nothing happens. Leave it overnight and still there s no power being produced or expended.
So you need torque PLUS rotation to make power. Let s make our own units up of PHs as simply torque x rpm. So at 1 rpm our 100ft/lb makes 100PHs. But now assume we make the same torque but at 1000rpm, now we re making 100,000PHs. So knowing torque without rpm is pretty meaningless. Now in the early days of steam, trying to sell steam engines to pump out mines to replace horses, saying you had a machine that makes 100,000PHs was pretty meaningless. So it was decided (by Stevenson? I can t recall) if you divide by 5252 you get a reasonable approximation of what power a horse can make over the course of a shift. So our 100,000PHs becomes 19hp. BTW a horse is around 13hp at full tilt, but it can t keep that up over a full day, and 5252 was purposely pessimistic so as not to over-sell the steam engine and get a grumpy customer.
Now let s get on to gearing. Gearboxes can be known as torque converters, and rightly so. In our example if we gear down our 1000rpm to 100rpm, then the output torque is now 1000ft/lb. Same power though as 1000x100/5252 is the same as 100x1000/5252.
Take a real world example now - let s say we have a bike making 100ft/lb at 5000rpm at the crank. So 95hp. At 50mph the back wheel is maybe rotating at 700rpm. So with that gearing there s around 750ft/lb at the back wheel at that speed but it s the same power (excluding losses).
More useless musings of why torque without RPM is meaningless for power. Imagine a water wheel 10 feet in diameter, with half a ton of water (say 1000lbs) 5 feet from the shaft. So it s making 5000ft/lb of torque. Wow, sounds enough to win races. But it s only rotating at 5rpm. So it s only making 4.7hp, same as a Honda 50!. Now we have to gear it up maybe to run a machine at say 1000rpm. Now we only have 25ft/lb to play with but the same power.
So you need torque PLUS rotation to make power. Let s make our own units up of PHs as simply torque x rpm. So at 1 rpm our 100ft/lb makes 100PHs. But now assume we make the same torque but at 1000rpm, now we re making 100,000PHs. So knowing torque without rpm is pretty meaningless. Now in the early days of steam, trying to sell steam engines to pump out mines to replace horses, saying you had a machine that makes 100,000PHs was pretty meaningless. So it was decided (by Stevenson? I can t recall) if you divide by 5252 you get a reasonable approximation of what power a horse can make over the course of a shift. So our 100,000PHs becomes 19hp. BTW a horse is around 13hp at full tilt, but it can t keep that up over a full day, and 5252 was purposely pessimistic so as not to over-sell the steam engine and get a grumpy customer.
Now let s get on to gearing. Gearboxes can be known as torque converters, and rightly so. In our example if we gear down our 1000rpm to 100rpm, then the output torque is now 1000ft/lb. Same power though as 1000x100/5252 is the same as 100x1000/5252.
Take a real world example now - let s say we have a bike making 100ft/lb at 5000rpm at the crank. So 95hp. At 50mph the back wheel is maybe rotating at 700rpm. So with that gearing there s around 750ft/lb at the back wheel at that speed but it s the same power (excluding losses).
More useless musings of why torque without RPM is meaningless for power. Imagine a water wheel 10 feet in diameter, with half a ton of water (say 1000lbs) 5 feet from the shaft. So it s making 5000ft/lb of torque. Wow, sounds enough to win races. But it s only rotating at 5rpm. So it s only making 4.7hp, same as a Honda 50!. Now we have to gear it up maybe to run a machine at say 1000rpm. Now we only have 25ft/lb to play with but the same power.
Edited by tvrolet on Sunday 25th October 20:22
Edited by tvrolet on Sunday 25th October 20:25
Work is force times distance
Power is work / time, so, Power is force x distance / time, = force times speed.
Power is also torque x angular speed.
Your torque at the rear wheel (which is engine torque x gear ratio, give or take friction etc) creates a force at the tyre contact
Torque is force x radius, so force is torque / tyre radius.
Force is mass x acceleration Ignoring friction and stuff.
Force to accelerate at 1g is therefore the weight of the bike.
Power is work / time, so, Power is force x distance / time, = force times speed.
Power is also torque x angular speed.
Your torque at the rear wheel (which is engine torque x gear ratio, give or take friction etc) creates a force at the tyre contact
Torque is force x radius, so force is torque / tyre radius.
Force is mass x acceleration Ignoring friction and stuff.
Force to accelerate at 1g is therefore the weight of the bike.
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