Transmission loss - 'Constant ' or Percentage?
Discussion
Hi.
As the title says basically. FWD is usually quoted at around 15% loss, RWD around 18% (more because of longitudal location I believe?) and 4wd 28% or whatever.. but why is it a percentage and not a constant?
Theoretically would it not take the same amount of work of to power gearbox/diff/driveshafts etc in the same car regardless as to whether the engine was 100bhp or 200bhp?
I''ve seen brief discussions about it in the past but cant remember the outcome..
Cheers, Greg.
As the title says basically. FWD is usually quoted at around 15% loss, RWD around 18% (more because of longitudal location I believe?) and 4wd 28% or whatever.. but why is it a percentage and not a constant?
Theoretically would it not take the same amount of work of to power gearbox/diff/driveshafts etc in the same car regardless as to whether the engine was 100bhp or 200bhp?
I''ve seen brief discussions about it in the past but cant remember the outcome..
Cheers, Greg.
The drag of the transmission remains more or less constant in a given gear. The amount of power absorbed in overcoming said drag varies with the speed it's being turned at. The amount of power given out by the engine also varies with the speed it's turning at. So the transmission drag absorbs a more or less constant proportion of the engine power.
Greatly oversimplified because I can't be arsed
Greatly oversimplified because I can't be arsed
Go search the Holden forum lol...
They keep quoting guesstimated flywheel figures, then wonder why their wheel figures are so low.....when their flywheel guesstimations are so high. Then ask, wow, 150bhp is a lot to lose through the drivetrain. ummm ok.
No, its because their guess of drivetrain losses is completely incorrect, based in silly percentages, or numbers plucked from thin air.
Unless you are using an engine dyno, dont waste your time with flywheel figures. They are mostly BS, and far too easily manipulated.
Use wheel figures if you are on a RR, there is more chance of them being consistent.
This article from pumaracing.co.uk has an excellent description of transmission losses, there's all sort of other usefull stuff on that site too.
Here is the way I would look at it.
There is no fixed percentage loss. What you need to understand is that a gear pair's efficiency is a function of how much meshing takes place - so a helical cut will be less efficient than a straight cut.
For the bearings there will be some torque required to turn them. Now remember that in a constant mesh transmission ALL of the gears with the exception of the direct gears are on bearings, and not necessarily needle of taper roller bearings. So power is sapped here.
Then you have drag in the oil, as you turn the input shaft and get the countershaft to rotate the oil is flung, but there is still a torque loss.
As you increase input shaft torque and the torque reaction through the case starts distorting the meshing, you may get reduced efficiency in the gear pairs.
Remember that in a RWD transmission for a non-direct gear generally two gear pairs are used, so efficiency is reduced.
The net efficiency has often been measured at around 96% to 98%, compared to about 92 / 93 for a good automatic and 85% for a CVT.
A lot of people get very confused when it comes to lowering engine inertia and say it liberates BHP. That is nonsense. But in the same way that if you have a body lightened, using F=ma, if F is the same, and m is lowered, a will increase. If you look at inertia and then make your engine reciprocating and rotating components featherweight it will have the same effect as delivering more BHP, though the output is still the same.
There is no fixed percentage loss. What you need to understand is that a gear pair's efficiency is a function of how much meshing takes place - so a helical cut will be less efficient than a straight cut.
For the bearings there will be some torque required to turn them. Now remember that in a constant mesh transmission ALL of the gears with the exception of the direct gears are on bearings, and not necessarily needle of taper roller bearings. So power is sapped here.
Then you have drag in the oil, as you turn the input shaft and get the countershaft to rotate the oil is flung, but there is still a torque loss.
As you increase input shaft torque and the torque reaction through the case starts distorting the meshing, you may get reduced efficiency in the gear pairs.
Remember that in a RWD transmission for a non-direct gear generally two gear pairs are used, so efficiency is reduced.
The net efficiency has often been measured at around 96% to 98%, compared to about 92 / 93 for a good automatic and 85% for a CVT.
A lot of people get very confused when it comes to lowering engine inertia and say it liberates BHP. That is nonsense. But in the same way that if you have a body lightened, using F=ma, if F is the same, and m is lowered, a will increase. If you look at inertia and then make your engine reciprocating and rotating components featherweight it will have the same effect as delivering more BHP, though the output is still the same.
GavinPearson said:
A lot of people get very confused when it comes to lowering engine inertia and say it liberates BHP. That is nonsense.
It isn't really nonesense though. It's describing a very real and useful effect (i.e. power delivered to the prop shaft under acceleration is increased), just one that you won't see if all your power measurements are taken under steady state conditions.
It's a double benefit too, because not only are you reducing the inertia of the drivetrain, you're also reducing the inertia of the vehicle. This can be pretty important when considering such things as cast iron and aluminium clutches.
Massive weight savings available (perhaps at the expense of longevity).
Massive weight savings available (perhaps at the expense of longevity).
dilbert said:
It's a double benefit too, because not only are you reducing the inertia of the drivetrain, you're also reducing the inertia of the vehicle. This can be pretty important when considering such things as cast iron and aluminium clutches.
Massive weight savings available (perhaps at the expense of longevity).
Massive weight savings available (perhaps at the expense of longevity).
surely we're talking only a few lbs max difference between regular and lightweight flywheels? Unless we are talking all out racer, there are easier ways to save this weight.
Mave said:
dilbert said:
It's a double benefit too, because not only are you reducing the inertia of the drivetrain, you're also reducing the inertia of the vehicle. This can be pretty important when considering such things as cast iron and aluminium clutches.
Massive weight savings available (perhaps at the expense of longevity).
Massive weight savings available (perhaps at the expense of longevity).
surely we're talking only a few lbs max difference between regular and lightweight flywheels? Unless we are talking all out racer, there are easier ways to save this weight.
I don't know about you, but my iron clutch weighs quite a bit more than a pair of electric seat motors!
dilbert said:
Mave said:
dilbert said:
It's a double benefit too, because not only are you reducing the inertia of the drivetrain, you're also reducing the inertia of the vehicle. This can be pretty important when considering such things as cast iron and aluminium clutches.
Massive weight savings available (perhaps at the expense of longevity).
Massive weight savings available (perhaps at the expense of longevity).
surely we're talking only a few lbs max difference between regular and lightweight flywheels? Unless we are talking all out racer, there are easier ways to save this weight.
I don't know about you, but my iron clutch weighs quite a bit more than a pair of electric seat motors!
But you're not going to discard the clutch are you? My point was, although there is the secondary benefit of reducing weight, it is a very small effect compared to the primary benefit of reducing the flywheel / clutch inertia. That puma racing link highlights this very point; in the example given, the secondary benefit is only an additional 3% on the primary benefit.
Round and round then!
But it's certainly a double whammy, which is all I said. Reducing the mass of of the clutch, and flywheel will certainly have a major effect associated with it's rotational velocity.
1/2.m.v^2 where the v^2 bit relates merely to the instantaneous velocity of an arbitary point on the assembly. To put this in perspective, some numbers;
A 14" flywheel has a circumference of approximately 1 meter. Not all of the material in the flywheel/clutch assembly will travel that distance in one revolution. Material at the centre of the flywheel will have zero angular velocity. The "average circumference" of the flywheel is that at half the diameter, and is about half a meter.
At At 7500 rpm, the flywheel will rotate 125 times in one second. A point on the periphery of the flywheel will have an instantaneous (linear) angular velocity of 125 m/s at 7500rpm. On average the particles in the flywheel will experience an instantaneous (linear) angular velocity of 62.5 m/s.
Neither of these figures is strictly correct, but I dont see the point of doing the math properly here. Overall the 125 m/s is too high, and the 62.5 is too low, because the energy is related by the square to the absolute instantaneous angular velocity. Irrespective the difference in mere instantaneous velocity is not as great as one might imagine;
As a result of the linear movement of the vehicle given an arbitary top speed of 150mph probably at the 7500rpm, will result in a linear velocity of 67 m/s.
My statement was merely that it is a double whammy. It's actually more in favour of my statemnt than I would have expected for the terms laid out for me. I said what I said on the basis of the idea that most people strip out ancilliary equipment first, to gain performance.
I stand by what I said. There are big benefits in respect of both linear and angular momentum.
As for the transmission losses, they're very complex. If you just assume friction, then it's constant, for a given coefficient of friction.
But it's certainly a double whammy, which is all I said. Reducing the mass of of the clutch, and flywheel will certainly have a major effect associated with it's rotational velocity.
1/2.m.v^2 where the v^2 bit relates merely to the instantaneous velocity of an arbitary point on the assembly. To put this in perspective, some numbers;
A 14" flywheel has a circumference of approximately 1 meter. Not all of the material in the flywheel/clutch assembly will travel that distance in one revolution. Material at the centre of the flywheel will have zero angular velocity. The "average circumference" of the flywheel is that at half the diameter, and is about half a meter.
At At 7500 rpm, the flywheel will rotate 125 times in one second. A point on the periphery of the flywheel will have an instantaneous (linear) angular velocity of 125 m/s at 7500rpm. On average the particles in the flywheel will experience an instantaneous (linear) angular velocity of 62.5 m/s.
Neither of these figures is strictly correct, but I dont see the point of doing the math properly here. Overall the 125 m/s is too high, and the 62.5 is too low, because the energy is related by the square to the absolute instantaneous angular velocity. Irrespective the difference in mere instantaneous velocity is not as great as one might imagine;
As a result of the linear movement of the vehicle given an arbitary top speed of 150mph probably at the 7500rpm, will result in a linear velocity of 67 m/s.
My statement was merely that it is a double whammy. It's actually more in favour of my statemnt than I would have expected for the terms laid out for me. I said what I said on the basis of the idea that most people strip out ancilliary equipment first, to gain performance.
I stand by what I said. There are big benefits in respect of both linear and angular momentum.
As for the transmission losses, they're very complex. If you just assume friction, then it's constant, for a given coefficient of friction.
Edited by dilbert on Friday 29th December 00:11
Round and round and round then, whatever that's supposed to mean
There are many modifications which have second order benefits. In some cases these may be significant and worth considering when deciding to do the modification. In some cases they are not. IMHO the secondary benefits in this case are low enough to be considered noise in making the decision whether to go for the mod.
Incidentally, I understand the physics thanks, but the 150mph speed of the car has absolutely nothing to do with working out the secondary benefit. It doesn't matter whether you're doing 15mph or 150mph, the % reduction in mass (and so the % improvement in acceleration) is the same.
There are many modifications which have second order benefits. In some cases these may be significant and worth considering when deciding to do the modification. In some cases they are not. IMHO the secondary benefits in this case are low enough to be considered noise in making the decision whether to go for the mod.
Incidentally, I understand the physics thanks, but the 150mph speed of the car has absolutely nothing to do with working out the secondary benefit. It doesn't matter whether you're doing 15mph or 150mph, the % reduction in mass (and so the % improvement in acceleration) is the same.
Mave said:
Round and round and round then, whatever that's supposed to mean
There are many modifications which have second order benefits. In some cases these may be significant and worth considering when deciding to do the modification. In some cases they are not. IMHO the secondary benefits in this case are low enough to be considered noise in making the decision whether to go for the mod.
Incidentally, I understand the physics thanks, but the 150mph speed of the car has absolutely nothing to do with working out the secondary benefit. It doesn't matter whether you're doing 15mph or 150mph, the % reduction in mass (and so the % improvement in acceleration) is the same.
There are many modifications which have second order benefits. In some cases these may be significant and worth considering when deciding to do the modification. In some cases they are not. IMHO the secondary benefits in this case are low enough to be considered noise in making the decision whether to go for the mod.
Incidentally, I understand the physics thanks, but the 150mph speed of the car has absolutely nothing to do with working out the secondary benefit. It doesn't matter whether you're doing 15mph or 150mph, the % reduction in mass (and so the % improvement in acceleration) is the same.
O.K.....
So there are easier ways to shed the mass that you speak of. But there aren't many that are more effective at improving performance. Particularly because of the double whammy.
On that basis, it may be that it is easier to use a lighter clutch and flywheel over a stock one. For a given increase in performance, you'd have to remove more mass in static items, than in the clutch and flywheel. You've got the physics, so you can do the math.
If you get a chance, let us know what you find.
dilbert said:
For a given increase in performance, you'd have to remove more mass in static items, than in the clutch and flywheel. You've got the physics, so you can do the math.
It's possible to quantify the potential performance gains. Look at it this way: if you rev the engine in neutral under full throttle, it takes a second or so to spool up through the rev range. under those conditions, the entire power output of the engine is being absorbed by the inertia of the engine. Couple it to the driven wheels and it will spin up rather slower, perhaps three or four times slower in first gear, which implies that something of the order of a fifth of the power output is being absorbed by the engine's inertia. A normal heavy flywheel contributes a very large part of the total inertia, so there is plenty of scope to reduce the inertia and quite substantial effective power gains to be had from it in some cases.
Sorry, I cannot agree with your comments, because what you say is in pure Engineering terms factually incorrect.
If you have a 100 bhp engine and the car weighs 1000 kg and can do 100 mph (implying there is a certain CdA that restricts top speed), halving the weight will improve acceleration times in all conditions, however, the top speed will be exactly the same. Decreasing the engine inertia will improve accelertion but the top speed will be.... exactly the same. Because power is... exactly the same.
By the same token if the power is increased to 200 bhp and as drag is roughly proportional to speed squared, the top speed will now be roughly 141 mph. You could then add 1000 kg of luggage, passengers and lead weights, but the top speed would still be 141 mph, and acceleration would be comparable to a the 100 bhp 1000 kg model.
Bottom line - reducing engine inertia and vehicle mass give excellent benefits in terms of acceleration and probable chassis benefits too. But if you want more speed, you need more power, in pure steady state terms.
If you have a 100 bhp engine and the car weighs 1000 kg and can do 100 mph (implying there is a certain CdA that restricts top speed), halving the weight will improve acceleration times in all conditions, however, the top speed will be exactly the same. Decreasing the engine inertia will improve accelertion but the top speed will be.... exactly the same. Because power is... exactly the same.
By the same token if the power is increased to 200 bhp and as drag is roughly proportional to speed squared, the top speed will now be roughly 141 mph. You could then add 1000 kg of luggage, passengers and lead weights, but the top speed would still be 141 mph, and acceleration would be comparable to a the 100 bhp 1000 kg model.
Bottom line - reducing engine inertia and vehicle mass give excellent benefits in terms of acceleration and probable chassis benefits too. But if you want more speed, you need more power, in pure steady state terms.
I'm not sure that's quite right either. Also I'm afraid I'm not going to be able to follow this, because I'll be away next week.
What V8S might have said, is that you get an improvement in torque, for a given RPM, by reducing the rotating mass of the drivetrain. This must be because of F=MA. If you reduce the mass then you need less force for a given acceleration. By reducing the mass less is absorbed as rotational energy in the flywheel, and more is available for the linear acceleration of the car.
Since power is a function of torque and rotational speed of the engine, you can say that you get a power increase. It's not an increase in the power of the engine, but it is an increase in the power available at the wheels.
Along with friction, presumably this power difference, is one of the key contributions to the difference in flywheel and axle power ratings.
What V8S might have said, is that you get an improvement in torque, for a given RPM, by reducing the rotating mass of the drivetrain. This must be because of F=MA. If you reduce the mass then you need less force for a given acceleration. By reducing the mass less is absorbed as rotational energy in the flywheel, and more is available for the linear acceleration of the car.
Since power is a function of torque and rotational speed of the engine, you can say that you get a power increase. It's not an increase in the power of the engine, but it is an increase in the power available at the wheels.
Along with friction, presumably this power difference, is one of the key contributions to the difference in flywheel and axle power ratings.
Edited by dilbert on Saturday 30th December 09:55
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