Theoretical question.
Discussion
300bhp/ton said:
I was meaning a much simpler situation.
300lb ft @ 2000rpm with 1:1 gear ration and 3.42:1 final drive
vs
380lb ft @ 2000rpm with 1:1 gear ration and 3.42:1 final drive
In this scenario the car with the higher flywheel torque will still be making more wheel torque.
I fully agree with what you are saying though. Torque multiplication is highly affective, hence why off roaders have a low ratio gear transfer box.
The only down side to torque multiplication is the trade off in wheel rpms. You can make more torque, but less rpms, so HP remains fairly constant gear on gear.
So you didn't mean "regardless of the torque multiplication" then.300lb ft @ 2000rpm with 1:1 gear ration and 3.42:1 final drive
vs
380lb ft @ 2000rpm with 1:1 gear ration and 3.42:1 final drive
In this scenario the car with the higher flywheel torque will still be making more wheel torque.
I fully agree with what you are saying though. Torque multiplication is highly affective, hence why off roaders have a low ratio gear transfer box.
The only down side to torque multiplication is the trade off in wheel rpms. You can make more torque, but less rpms, so HP remains fairly constant gear on gear.
You meant "if the torque multiplication is the same". Which is kind of the exact opposite.
The Black Flash said:
So you didn't mean "regardless of the torque multiplication" then.
You meant "if the torque multiplication is the same". Which is kind of the exact opposite.
Not really.You meant "if the torque multiplication is the same". Which is kind of the exact opposite.
As most of the time, a car that makes more flywheel torque will also make more wheel torque also.
Of course there are exceptions, such as comparing different engine/fuel types. Or choosing extreme examples such as a 9000rpm screamer with ultra short final drive compared to a 4000rpm TD with ultra tall final drive.
The key to making power is having enough torque combined with the ability to make it at higher rpms.
300bhp/ton said:
Not really.
As most of the time, a car that makes more flywheel torque will also make more wheel torque also.
Of course there are exceptions, such as comparing different engine/fuel types. Or choosing extreme examples such as a 9000rpm screamer with ultra short final drive compared to a 4000rpm TD with ultra tall final drive.
The key to making power is having enough torque combined with the ability to make it at higher rpms.
All besides the point. You said:As most of the time, a car that makes more flywheel torque will also make more wheel torque also.
Of course there are exceptions, such as comparing different engine/fuel types. Or choosing extreme examples such as a 9000rpm screamer with ultra short final drive compared to a 4000rpm TD with ultra tall final drive.
The key to making power is having enough torque combined with the ability to make it at higher rpms.
300bhp/ton said:
But the more flywheel torque the more wheel torque regardless of the torque multiplication
Which is just wrong.300bhp/ton said:
Not really.
As most of the time, a car that makes more flywheel torque will also make more wheel torque also.
Of course there are exceptions, such as comparing different engine/fuel types. Or choosing extreme examples such as a 9000rpm screamer with ultra short final drive compared to a 4000rpm TD with ultra tall final drive.
The key to making power is having enough torque combined with the ability to make it at higher rpms.
Oh, Oh, Race!As most of the time, a car that makes more flywheel torque will also make more wheel torque also.
Of course there are exceptions, such as comparing different engine/fuel types. Or choosing extreme examples such as a 9000rpm screamer with ultra short final drive compared to a 4000rpm TD with ultra tall final drive.
The key to making power is having enough torque combined with the ability to make it at higher rpms.
We'll buy two identical BMW 328i's.
We'll have a rolling start. You can choose what speed we start at, presumably you'll want to be at 3,000rpm. Peak flywheel torque for that engine.
Since what gear you are in doesn't matter to you, you will be in 5th gear.
I will match your speed, but be in a gear of my choosing, probably 3rd, I'll be doing about 5,000rpm at that point.
We will then both floor the throttle. My engine will be making much less torque then yours as I have foolishly choosen to put it way past it's torque peak :-( . Since you are making more flywheel torque, (and we know from what you have said that the car making more flywheel torque will also make more wheel torque) you will scream off into the distance and I will lose.
When do you want to get together and give it a go?
Edited by varsas on Wednesday 4th May 13:51
The Black Flash said:
Which is just wrong.
I think you are taking it too literally. If I wasn't clear enough to begin - then sorry. But I've since added more info to what I meant. Which bit of it don't you agree with?I never said ignoring rpms, I just failed to specify that I meant EQUAL rpms and gearing.
varsas said:
Oh, Oh, Race!
We'll buy two identical BMW 328i's.
We'll have a rolling start. You can choose what speed we start at, presumably you'll want to be at 3,000rpm. Peak flywheel torque for that engine.
Since what gear you are in doesn't matter to you, you will be in 5th gear.
I will match your speed, but be in a gear of my choosing, probably 3rd, I'll be doing about 5,000rpm at that point.
We will then both floor the throttle. My engine will be making much less torque then yours as I have foolishly choosen to put it way past it's torque peak :-( . Since you are making more flywheel torque, (and we know from what you have said that the car making more flywheel torque will also make more wheel torque) you will scream off into the distance and I will lose.
When do you want to get together and give it a go?
No idea what you are on. But its ok. We'll buy two identical BMW 328i's.
We'll have a rolling start. You can choose what speed we start at, presumably you'll want to be at 3,000rpm. Peak flywheel torque for that engine.
Since what gear you are in doesn't matter to you, you will be in 5th gear.
I will match your speed, but be in a gear of my choosing, probably 3rd, I'll be doing about 5,000rpm at that point.
We will then both floor the throttle. My engine will be making much less torque then yours as I have foolishly choosen to put it way past it's torque peak :-( . Since you are making more flywheel torque, (and we know from what you have said that the car making more flywheel torque will also make more wheel torque) you will scream off into the distance and I will lose.
When do you want to get together and give it a go?
Edited by varsas on Wednesday 4th May 13:51
BTW - this has zilch to do with what I said.
If you want to race then do so. But lets both be at 5000rpm in 3rd. I'll take a 328 and you have a 318..... are you really certain that flywheel torque won't have a bearing on it? I'll even use 4th if you prefer....
doogz said:
You did say regardless of the torque multiplication. Since a gearbox is a torque multiplier, it wasn't an obvious assumption to make, that you meant equal gearing.
I like this thread. Threads like this are why i like PH. Can we please not turn it into a petty grammar match, or slagging match?
I admit I may have written it badly, but I didn't mean regardless of the torque multiplication or else it would have been a self defeating statement I like this thread. Threads like this are why i like PH. Can we please not turn it into a petty grammar match, or slagging match?
Alfanatic said:
I don't think that's true. Gear multiplication typically has a bigger influence on torque at the wheels than engine efficiency.
In, for example, the case of an engine that produces a torque peak of 100nm at 6000rpm, and the torque then drops off to 80nm on the way to the redline at 7000rpm: If you stay in, say, 2nd, you'll have a faster rate of acceleration at 6000rpm than you would at 7000rpm. The same holds true if you stay in 3rd gear, and all the others. I don't dispute this at all - for a vehicle with only one gear.
But lets say 2nd gear is a ratio of 2:1 and 3rd is 1.4:1. Final drive ratio and wheel size doesn't change so we can assume that's all 1:1.
at peak torque in 2nd, the wheels will be transferring 200nm to the road (100nm*2).
At max revs in 2nd, the wheels will be transferring 160nm to the road (80nm*2).
At peak torque in 3rd, the wheels will be transferring 140nm to the road (100nm*1.4).
So, at max revs in 2nd, you are still getting more torque at the wheels than at max torque in third. Thus, while acceleration will tail off from 6000rpm to 7000rpm in second, it will tail off even more as soon as you change to 3rd, no matter what revs you make the change at. Hence, for best acceleration, stay in the lowest gear as long as you can.
The gear ratios and torque numbers I have used and the torque figures are made up, but this is what the RX8 chart above is showing, I have seen similar results for a Honda Civic, and I have heard that there are few, if any, road cars that buck this trend. You'd need a very large drop off in torque between peak and redline, or very close gear ratios, to buck the trend.
So far this is the one reply I have read that seems to make the most sense and I understand it In, for example, the case of an engine that produces a torque peak of 100nm at 6000rpm, and the torque then drops off to 80nm on the way to the redline at 7000rpm: If you stay in, say, 2nd, you'll have a faster rate of acceleration at 6000rpm than you would at 7000rpm. The same holds true if you stay in 3rd gear, and all the others. I don't dispute this at all - for a vehicle with only one gear.
But lets say 2nd gear is a ratio of 2:1 and 3rd is 1.4:1. Final drive ratio and wheel size doesn't change so we can assume that's all 1:1.
at peak torque in 2nd, the wheels will be transferring 200nm to the road (100nm*2).
At max revs in 2nd, the wheels will be transferring 160nm to the road (80nm*2).
At peak torque in 3rd, the wheels will be transferring 140nm to the road (100nm*1.4).
So, at max revs in 2nd, you are still getting more torque at the wheels than at max torque in third. Thus, while acceleration will tail off from 6000rpm to 7000rpm in second, it will tail off even more as soon as you change to 3rd, no matter what revs you make the change at. Hence, for best acceleration, stay in the lowest gear as long as you can.
The gear ratios and torque numbers I have used and the torque figures are made up, but this is what the RX8 chart above is showing, I have seen similar results for a Honda Civic, and I have heard that there are few, if any, road cars that buck this trend. You'd need a very large drop off in torque between peak and redline, or very close gear ratios, to buck the trend.
Also I second what gareth.e said, sometimes I have no idea who is right or wrong but thanks all the same
alock said:
blank said:
doogz said:
A power curve would be the best way to figure this out.
If you know the rev drop between 2 gears, you want the section of the curve, where you draw a horizontal line between the curve at your specified engine speeds, and have the least area under this new curve you've made.
I think.
This is exactly right.If you know the rev drop between 2 gears, you want the section of the curve, where you draw a horizontal line between the curve at your specified engine speeds, and have the least area under this new curve you've made.
I think.
http://www.parkers.co.uk/advice/forum/topic.asp?TO...
In his 'change at 4500rpm' example, he states that the gear change is too late because he has a power drop of ~10% (140bhp to 125bhp). To avoid this power loss, he selects a 30% higher gear.
Extreme example of the flawed logic. Does you car produce a better rate of acceleration at peak power in 6th gear or at 70% of peak power in 1st?
Edited by alock on Tuesday 3rd May 22:22
It's mathematical fact.
My car obviously produces more acceleration at 70% of peak power in 1st, because at peak power in 6th virtually all the power would be being used to move the car at ~140 mph and there wouldn't be much left for acceleration.
Calculate the wheel torque if you like. But there is no need, because more power = more wheel torque (for same road speed).
blank said:
It's mathematical fact.
My car obviously produces more acceleration at 70% of peak power in 1st, because at peak power in 6th virtually all the power would be being used to move the car at ~140 mph and there wouldn't be much left for acceleration.
Calculate the wheel torque if you like. But there is no need, because more power = more wheel torque (for same road speed).
Gearing allows torque multiplication, so you get more more wheel torque but less rpms. The HP will still be the same though for a given rpm.
blank said:
It's mathematical fact.
My car obviously produces more acceleration at 70% of peak power in 1st, because at peak power in 6th virtually all the power would be being used to move the car at ~140 mph and there wouldn't be much left for acceleration.
Calculate the wheel torque if you like. But there is no need, because more power = more wheel torque (for same road speed).
Acceleration, according to Newton, is force / mass. The mass is constant. The force is the tyre pushing at the road. Torque is rotational force applied to the wheel, which in turn provides an equivalent linear force to the road via the contact patch. The torque acting on the wheel has been multiplied by the gearing. If you work out the accelerative force available off the torque chart provided, the Parkers example happens to have gotten the change point just about right, but only because he uses 4th to 5th, a ratio difference of 1.32. It's a fluke. Lower gears will have higher ratio differences and would need a change further up the rev scale to provide maximum possible acceleration.
I'll put a chart together based on those torque curves and whatever gear ratios I can dig up on the internet for a GT TDI and post it up for the boffins here on PH to examine and correct.
Righty, here's what I came up with:
Anything grey is information available about the Golf on the internet. I used the torque charts from the Parkers Forum example to get estimates of engine torque at 500rpm intervals, so my torque figures are probably not exact but they should be close enough to give an accurate overview.
The gear ratios come from a GT TDI DSG, but that gave 20mph/1000rpm in 4th and 26.4mph/1000rpm in 5th, which matches the figures for the same gears provided on the Parkers Forum, so I have made the assumption that all the ratios are the same as would be found on the GT TDI that provided the torque graph.
Example, at 3500rpm, the golf produces about 275nm of torque at the flywheel.
Anything in that horrible brick red colour is torque multiplied by gearing. This is torque at the diff input, not torque at the wheels, but since final drive ratio and wheel size are constants, it doesn't matter.
Example, at 3500rpm, the gearbox sends about 952nm of torque to the final drive.
Columns in blue indicate what RPM you'd find yourself in if you changed up at that point.
Example, at 3500rpm in 1st, changing to 2nd will drop the rpm to 2074rpm.
Analysis:
Sticking with 3500rpm for starters, you will see that, in 1st, you have 952nm torque. If you change to 2nd, the rpm drops to 2074rpm, and torque will drop to 656.
At 3500rpm in 2nd, torque is now at 564nm. Change to 3rd, rpm drops to 2220rpm, torque is somewhere between 416 and 442nm, etc.
So, at 4500rpm in 1st, which is about the max the chart goes to, you have 692nm at the diff. The absolute maximum torque you can supply in 2nd is 697, at 2500rpm, but if you change at 4500, you'll be just past that peak in 2nd. If you change at 4000, then you will get the meat of the torque in 2nd, but you would have still been at over 700nm in 1st when you changed gear.
From 2nd to 3rd, it's a slightly different story, you can change at 4000rpm and will see a slight increase in torque supplied to the wheels, so it makes sense then to change into 3rd ju-u-u-st before 4000rpm
From 3rd to 4th, again the best point looks to be a bit below 4000rpm, and from 5th to 6th, I'd say it's at about 3500rpm.
Based on this, I'd say, firstly, that I now stand corrected and I am now aware of a car that bucks the trend of max revs in any gear always giving more wheel torque than any point in the next gear up, and secondly that I disagree with the Parkers forum advice and await an expert's judgement of my methods.
Anything grey is information available about the Golf on the internet. I used the torque charts from the Parkers Forum example to get estimates of engine torque at 500rpm intervals, so my torque figures are probably not exact but they should be close enough to give an accurate overview.
The gear ratios come from a GT TDI DSG, but that gave 20mph/1000rpm in 4th and 26.4mph/1000rpm in 5th, which matches the figures for the same gears provided on the Parkers Forum, so I have made the assumption that all the ratios are the same as would be found on the GT TDI that provided the torque graph.
Example, at 3500rpm, the golf produces about 275nm of torque at the flywheel.
Anything in that horrible brick red colour is torque multiplied by gearing. This is torque at the diff input, not torque at the wheels, but since final drive ratio and wheel size are constants, it doesn't matter.
Example, at 3500rpm, the gearbox sends about 952nm of torque to the final drive.
Columns in blue indicate what RPM you'd find yourself in if you changed up at that point.
Example, at 3500rpm in 1st, changing to 2nd will drop the rpm to 2074rpm.
Analysis:
Sticking with 3500rpm for starters, you will see that, in 1st, you have 952nm torque. If you change to 2nd, the rpm drops to 2074rpm, and torque will drop to 656.
At 3500rpm in 2nd, torque is now at 564nm. Change to 3rd, rpm drops to 2220rpm, torque is somewhere between 416 and 442nm, etc.
So, at 4500rpm in 1st, which is about the max the chart goes to, you have 692nm at the diff. The absolute maximum torque you can supply in 2nd is 697, at 2500rpm, but if you change at 4500, you'll be just past that peak in 2nd. If you change at 4000, then you will get the meat of the torque in 2nd, but you would have still been at over 700nm in 1st when you changed gear.
From 2nd to 3rd, it's a slightly different story, you can change at 4000rpm and will see a slight increase in torque supplied to the wheels, so it makes sense then to change into 3rd ju-u-u-st before 4000rpm
From 3rd to 4th, again the best point looks to be a bit below 4000rpm, and from 5th to 6th, I'd say it's at about 3500rpm.
Based on this, I'd say, firstly, that I now stand corrected and I am now aware of a car that bucks the trend of max revs in any gear always giving more wheel torque than any point in the next gear up, and secondly that I disagree with the Parkers forum advice and await an expert's judgement of my methods.
Alfanatic said:
Based on this, I'd say, firstly, that I now stand corrected and I am now aware of a car that bucks the trend of max revs in any gear always giving more wheel torque than any point in the next gear up, and secondly that I disagree with the Parkers forum advice and await an expert's judgement of my methods.
How much of an "expert" do you want?Someone with an automotive engineering degree?
Someone with two?
Someone who tests cars for a living?
Trust me, the stuff on Parker's is 100% correct - do the maths. It doesn't need to "take account of gearing" because using the power figure already does that due to the constant relationship between power, torque and speed.
Alfanatic said:
the Parkers example happens to have gotten the change point just about right, but only because he uses 4th to 5th, a ratio difference of 1.32. It's a fluke. Lower gears will have higher ratio differences and would need a change further up the rev scale to provide maximum possible acceleration.
Absolutely correct. Unless the gear ratios are a perfectly geometric progression, the optimum shift points are different for each gear, but the way of working it out is the same.300bhp/ton said:
varsas said:
Oh, Oh, Race!
We'll buy two identical BMW 328i's.
We'll have a rolling start. You can choose what speed we start at, presumably you'll want to be at 3,000rpm. Peak flywheel torque for that engine.
Since what gear you are in doesn't matter to you, you will be in 5th gear.
I will match your speed, but be in a gear of my choosing, probably 3rd, I'll be doing about 5,000rpm at that point.
We will then both floor the throttle. My engine will be making much less torque then yours as I have foolishly choosen to put it way past it's torque peak :-( . Since you are making more flywheel torque, (and we know from what you have said that the car making more flywheel torque will also make more wheel torque) you will scream off into the distance and I will lose.
When do you want to get together and give it a go?
No idea what you are on. But its ok. We'll buy two identical BMW 328i's.
We'll have a rolling start. You can choose what speed we start at, presumably you'll want to be at 3,000rpm. Peak flywheel torque for that engine.
Since what gear you are in doesn't matter to you, you will be in 5th gear.
I will match your speed, but be in a gear of my choosing, probably 3rd, I'll be doing about 5,000rpm at that point.
We will then both floor the throttle. My engine will be making much less torque then yours as I have foolishly choosen to put it way past it's torque peak :-( . Since you are making more flywheel torque, (and we know from what you have said that the car making more flywheel torque will also make more wheel torque) you will scream off into the distance and I will lose.
When do you want to get together and give it a go?
Edited by varsas on Wednesday 4th May 13:51
BTW - this has zilch to do with what I said.
If you want to race then do so. But lets both be at 5000rpm in 3rd. I'll take a 328 and you have a 318..... are you really certain that flywheel torque won't have a bearing on it? I'll even use 4th if you prefer....
I do agree, flywheel torque will make a difference.
I also assert that it's wheel torque that actually makes the difference.
Flywheel torque is less important to wheel torque then being in the correct gear (the graph at the top of the page shows this). In fact, you want to be near peak power.
My 'race' shows this. You would have more flywheel torque, but I would have more wheel torque, because I was in the better gear, hence I would win.
I was showing that something you had not considered (see below, but you also did say even more explicitly that torque multiplication isn't important) actually does have a large effect. I see your point with me being in a 318, but I never said flywheel torque makes no difference. Anyway, you do now seem to realise what gear you are in is important, which was the point!
;-)
300bhp/ton said:
Not really.
As most of the time, a car that makes more flywheel torque will also make more wheel torque also.
As most of the time, a car that makes more flywheel torque will also make more wheel torque also.
Edited by varsas on Thursday 5th May 08:58
varsas said:
...
lol thanks Edited by varsas on Thursday 5th May 08:58
I see where you are coming from, and I guess I wasn't clear. But I certainly didn't mean what is now being implied.
I really was thinking of keeping it simple....
5.7 V8 vs n/a 1.0 litre
No matter what you do with the gearing on the little engine, it'll never make the same wheel torque as the big engine.
If you have more torque to begin with, you also have more to multiply. That was all I meant by it.
blank said:
Alfanatic said:
Based on this, I'd say, firstly, that I now stand corrected and I am now aware of a car that bucks the trend of max revs in any gear always giving more wheel torque than any point in the next gear up, and secondly that I disagree with the Parkers forum advice and await an expert's judgement of my methods.
How much of an "expert" do you want?Someone with an automotive engineering degree?
Someone with two?
Someone who tests cars for a living?
Trust me, the stuff on Parker's is 100% correct - do the maths. It doesn't need to "take account of gearing" because using the power figure already does that due to the constant relationship between power, torque and speed.
I know there's a constant relationship between power, torque and speed, but since torque at the wheels is dependant on gearing, power must be too, since there's a constant relationship between them.
I'm more than happy to accept that Parkers is right, but you'll need to be a bit more convincing, like showing me how the relationship works with an equation or two, and how the gearing is already included, if that's not too much trouble?
So, instead of saying "do the maths", you could show us the maths that proves you're right. Or you could just bang your head against a wall again like you did last time someone disagreed with you. Your choice, I'd preferred it if you chose to enlighten me though. I don't mind being proven wrong.
So how about it? What's the maths that supports Parkers?
EDIT: If it helps any, my maths is telling me that it would be power at the wheels that matters, which depends on torque and wheel rpm - or road speed, but if you change gear, the wheel rpm and road speed don't change at that instant, but if the torque goes down, you now must have less power at the wheels, so acceleration will reduce.
This may be flawed reasoning, but it is why I'm struggling to accept what you are saying at face value. I'm not an expert, and I'd like an expert to explain this to me. As would, I am sure, the OP.
Edited by Alfanatic on Thursday 5th May 09:46
300bhp/ton said:
varsas said:
...
lol thanks Edited by varsas on Thursday 5th May 08:58
I see where you are coming from, and I guess I wasn't clear. But I certainly didn't mean what is now being implied.
I really was thinking of keeping it simple....
5.7 V8 vs n/a 1.0 litre
No matter what you do with the gearing on the little engine, it'll never make the same wheel torque as the big engine.
If you have more torque to begin with, you also have more to multiply. That was all I meant by it.
Alfanatic said:
I don't mind, as long as they provide some constructive feedback on the table I posted and explain where it goes wrong, if it is wrong.
I know there's a constant relationship between power, torque and speed, but since torque at the wheels is dependant on gearing, power must be too, since there's a constant relationship between them.
I'm more than happy to accept that Parkers is right, but you'll need to be a bit more convincing, like showing me how the relationship works with an equation or two, and how the gearing is already included, if that's not too much trouble?
So, instead of saying "do the maths", you could show us the maths that proves you're right. Or you could just bang your head against a wall again like you did last time someone disagreed with you. Your choice, I'd preferred it if you chose to enlighten me though. I don't mind being proven wrong.
So how about it? What's the maths that supports Parkers?
EDIT: If it helps any, my maths is telling me that it would be power at the wheels that matters, which depends on torque and wheel rpm - or road speed, but if you change gear, the wheel rpm and road speed don't change at that instant, but if the torque goes down, you now must have less power at the wheels, so acceleration will reduce.
This may be flawed reasoning, but it is why I'm struggling to accept what you are saying at face value. I'm not an expert, and I'd like an expert to explain this to me. As would, I am sure, the OP.
Power at wheels is equal to power at engine (ignoring transmission loss which will be near enough a set percentage anyway) due to conservation of energy.I know there's a constant relationship between power, torque and speed, but since torque at the wheels is dependant on gearing, power must be too, since there's a constant relationship between them.
I'm more than happy to accept that Parkers is right, but you'll need to be a bit more convincing, like showing me how the relationship works with an equation or two, and how the gearing is already included, if that's not too much trouble?
So, instead of saying "do the maths", you could show us the maths that proves you're right. Or you could just bang your head against a wall again like you did last time someone disagreed with you. Your choice, I'd preferred it if you chose to enlighten me though. I don't mind being proven wrong.
So how about it? What's the maths that supports Parkers?
EDIT: If it helps any, my maths is telling me that it would be power at the wheels that matters, which depends on torque and wheel rpm - or road speed, but if you change gear, the wheel rpm and road speed don't change at that instant, but if the torque goes down, you now must have less power at the wheels, so acceleration will reduce.
This may be flawed reasoning, but it is why I'm struggling to accept what you are saying at face value. I'm not an expert, and I'd like an expert to explain this to me. As would, I am sure, the OP.
Edited by Alfanatic on Thursday 5th May 09:46
There is a link to the maths earlier in this thread or a link at the start of the Parkers thread. If they don't explain I can do so later, but I'm at work at the moment!
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