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Old 19th September 2008, 14:26   #16
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If you had read the wiki, you would have understood. Anyway, let me tell in simple exmple:

Suppose, when you pedal your cycle in a given gear at 40 rpm, assume the wheel will rotate at 100 rpm, let's say equivalent to 20 Kmph speed. Now you pedal at 80 rpm, the wheel will now rotate at 200 rpm causing the speed to double at 40Kmph.

To generalize, rpm of your pedal, the cycle's speed will be:

speed = rpm * 20 / 40

Assuming you can pedal at max 100 rpm even without any friction with the wheel rotating in the air, your top speed will be
100 * 20 / 40 = 50Kmph.

Again my assumption that you have much more strength than the resistance that will come at 50Kmph
What you're calculating is engine rpm vs vehicle speed. This will only tell you what speed you're doing at a given engine rpm and gear. This has got nothing to do with actual top speed of a vehicle.

To calculate the actual top speed of a car, you need to know the rpm and gear at which the car maxes out. To do this, you will obvoiusly have to take your car to it's limit.

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Old 19th September 2008, 14:33   #17
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Final drivingshaft (wheel) speed can be increased with a different gear ratio as I mentioned. If the Bugati had a fifth gear ratio of 6000:1 and differential ratio (from transmission to final drive shaft) of 1:1, the maximum speed it can do is to rotate its tyres only once per minute. What will be the use of that torque now?
LOL. so what you're saying is that if we give you a 500bhp/500nm Veyron, you can still make it hit 407kmph using only gearing and wheel size?

2ndly, using a 6000:1 5th gear and 1:1 final drive means the engine has to revv at 6000rpm to make the wheel rotate once. Thats again a mechanical limit, has nothing to do with aerodynamical top speed. A vehicle is said to achieve it's aerodynamical limit only when it does it without having to redline in that particular gear.

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Last edited by Shan2nu : 19th September 2008 at 14:43.
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Old 19th September 2008, 14:49   #18
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Ok, see if you cal calculate the actual top speed of this vehicle.
100bhp
125nm
weight 2000 kgs
5th gear - 0.774
Final Drive - 4.2
wheel size - 195/60 r14
engine redline - 8000rpm

Lemme know the result.

Shan2nu

Last edited by Shan2nu : 19th September 2008 at 14:51.
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Old 19th September 2008, 15:10   #19
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Originally Posted by Shan2nu View Post
Ok, see if you cal calculate the actual top speed of this vehicle.
100bhp
125nm
weight 2000 kgs
5th gear - 0.774
Final Drive - 4.2
wheel size - 195/60 r14
engine redline - 8000rpm

Lemme know the result.

Shan2nu
Humn!! 125Nm with 2 ton and that too with that gear ratio. Ok boss. You win

Anyways, please let me know if any of popular modern cars don't satisfy that formula. I give up here.
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Old 19th September 2008, 15:20   #20
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Humn!! 125Nm with 2 ton and that too with that gear ratio. Ok boss. You win

Anyways, please let me know if any of popular modern cars don't satisfy that formula. I give up here.
See, i understand what you're trying to say, but that formula will only help you to find out the top speed of a vehicle that is limited by it's gearing.

EG :
If a 5 speed car redlines it's 5th gear at a given rpm, you cannot make that car go any faster wrt power, torque, weight, aerodynamics. The only way you can increase it's top speed is by
1. Lower 5th gear ratio
2. Lower final drive ratio
3. Increase engine rpm
4. Bigger diameter wheels

But the thing is, there are very few cars that will achieve their top speed due to mechanical limits. All the others are limited by aerodynamics, friction etc. Finding out this top speed is almost impossible coz natural conditions are never constant.

I think the Accent crdi used to have a mechanical limit. You can use this formula on it and see what results you get.

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Old 19th September 2008, 15:35   #21
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Originally Posted by Shan2nu View Post
If a 5 speed car redlines it's 5th gear at a given rpm
There is nothing like a different rpm redlining for a fifth gear. It is just that opposing forces are stronger than the torque. Redlining is something that engine is not capable of reving beyond even without any load or there is hardly any torque beyond that rpm.

And engine at a given rpm produces the SAME TORQUE in ANY GEAR. The final drive shaft torque is what is different in different gears.
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Old 19th September 2008, 15:46   #22
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There is nothing like a different rpm redlining for a fifth gear.
Dude, every engine has it's own redline. I didn't say diff redline in every gear. I said redline at a given rpm (meaning this redline is the same for all the gears in that transmission).

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Redlining is something that engine is not capable of reving beyond even without any load or there is hardly any torque beyond that rpm.
Again there are diff types of redlines, engine rpm can be limited due to electronically cutting fuel supply, mechanical redline of the engine (which your talking about) and transmission redline (where the resistance produced by the transmission limits engine rpm).

It's stupid to even try and max out the mechanical limits of the engine and transmission and electronic cut off wont let you cross that rpm even if you want to.

This is why manufacturers choose to space out the gears such that top speed is achieved well below redline in top gear. And then have the electronic limiters to see that people don't push the engine beyond a specified rpm in lower gears.

Shan2nu

Last edited by Shan2nu : 19th September 2008 at 15:52.
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Old 19th September 2008, 15:57   #23
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It is possible to find out the theoretical top speed using formulas similar to given here
http://www.team-bhp.com/forum/techni...tml#post152768

HOWEVER, in real life, it may not be possible to achieve this due to several other limiting factors (like efficiency of mechanical parts, undulating road surface etc.)

Usually manufactures mention top speed of the car in manual although some cars can achieve higher speed than mentioned.
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Old 19th September 2008, 16:53   #24
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Originally Posted by Shan2nu View Post
I think the Accent crdi used to have a mechanical limit. You can use this formula on it and see what results you get.
I used Road Test: Hyundai Accent CRDi - Technical Specifications to get the details of the Accent CRDI. It does 35.51 Kmph per 1000 rpm in fifth gear.

So, at max rpm, i.e. 4000 rpm, it will reach 35.51*4 = 142Kmph, which I'm sure it is capable of doing on plane roads.

I suggest you try calculating again for most cars:

Top speed (in Kmph) = (Max rpm * 60 * tyre circumference in Km) /
(fifth or whatever gear ratio * final drive gear ratio)

where,
tyre circumference (in Km) = {(Section Width in mm * Aspect Ratio * 2) +
(Radius in inches * 25.4)} * PI / 1000 / 1000 / 100

I have not tried calculating though. I will definitely try when I get more time.

Last edited by opendro : 19th September 2008 at 16:59. Reason: Corrected typo diameter to circumference and also aspect ration divide by 100
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Old 19th September 2008, 17:18   #25
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In absense of friction, wind resistance etc., it would work, but in real life hardly any car can redline in 4th gear, let alone 5th gear!
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Old 19th September 2008, 18:29   #26
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Originally Posted by tsk1979 View Post
In absense of friction, wind resistance etc., it would work, but in real life hardly any car can redline in 4th gear, let alone 5th gear!
Again, quoting from the link Road Test: Hyundai Accent CRDi - Technical Specifications,
the max speed in 1st gear is 40Kmph, which would translate to 40*1000/8.08=4950rpm (as it says that in first gear, it moves 8.08 Kmph per 1000 rpm).

It also says that in fifth gear, it does 188Kmph. So, the rpm at that speed would be 188 * 1000 / 35.51 = 5294rpm.

So, it reaches more or less the same max rpm that it could reach in first gear.

I don't know what is the redline Hyundai has marked though.
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Old 19th September 2008, 19:51   #27
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sorry to butt in here, but then i have been grilled by all technical guys like shatanu and basak (and if they are questioning you, you better do you homework well hehehe).

you are absolutely right in giving your formula about top speed and the reference of wiki. but then you have made a wrong assupmption. do you mean to say that the fuel economy of a car will remain same with a payload of 50 kg and 500 kg?

what you have missed out is that when you go up in the gearing, the effective torque to the wheels keep on decreasing and he torque required to accelarate a vehicle depends on weight, wind and slope etc and torque required is proportional to square of speed. (ref to basak's formula in the link provided and graphs provided by me). but the torque available is rises, flattens out and then dips.

the car will accelerate as long as there is net positive torque available (NPTA) (as mentioned by you) but alas, in practical life, it is not so. that is why you find in most of the petrol engine cars, the top speed it hit in the 4th gear instead of 5th, because the NPTA is low (it is purposely done to improve on fuel economy). racing bikes and cars have more gears to keep the NPTA high (see the Yamaha - R-15).

yes, there are powerful cars which can hit the top speed (if not goverened), but then as you increase the top speed, so do the torque required increases.

even, the crdi, if having a huge payload and going against a strong wind and going up a steep slope will not be able to reach the top rpm as the torque required will be huge after certain speed and NPTA will be zero as you have rightly said.

PS: crdi redline is 4500 (most diesel car redline is 4000~4500 and petrol is 6000~7000)

Last edited by jat : 19th September 2008 at 19:52. Reason: redline rpm crdi
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Old 20th September 2008, 13:00   #28
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Jat I didn't say anything about fuel economy.
I did not ignore the torque altogether. You will see that if you read my post. But I made an assumption that most modern cars have enough torque to pull against all resistance on plane surface (not slope) even if it takes very long time as the resultant torque (engine torque minus resistance) would be less. I could be wrong, but then nobody is coming out with a specific modern vehicle that does not satisfy my formula. That is rediculous argument I would say. I don't claim to know how much is the minimum torque that is required to reach the top speed though.

Another thing. I think many people have assumed that the max RPM as the techometer redline or whatever. I mentioned in the formula that it is the max RPM the vehicle is capable of doing in first gear on the plane surface again.

And I have also not come across a vehicle (pardon my lack of experience with various cars) that travels faster in fourth gear than in fifth gear. Yes, that is again possible if the engine has very little torque and gear ratio is too small. But which modern car (among the ones we drive around) have that kind of engine and gear ratio.

Anyways, my intentions were:
1. To remove the notion that a powerful engine can do higher top speed (I remember how people were trying to calculate using torque when Verna's CRDI engine was plonked on Getz)
2. To give an easy way to calculate top speed without knowing any internal intricacies of gear ratio, torque, power etc. (putting my assumption about sufficient torque)

I could be wrong, but I somehow lost the value I used to hold about team bhp, because the arguments and criticisms are very sub standard and less substantiated or at least I felt that way in this thread. People don't try to re-read the original post before they bang back with their points, such as max rpm, red line blah blah. When it is okay to make a point and leave instead of carrying on the argument, I feel it does not give the kind of conclusion and direction to somebody who would seek the right answer through such forum.

Last edited by opendro : 20th September 2008 at 13:02.
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Old 20th September 2008, 14:38   #29
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I could be wrong, but I somehow lost the value I used to hold about team bhp, because the arguments and criticisms are very sub standard and less substantiated or at least I felt that way in this thread. People don't try to re-read the original post before they bang back with their points, such as max rpm, red line blah blah. When it is okay to make a point and leave instead of carrying on the argument, I feel it does not give the kind of conclusion and direction to somebody who would seek the right answer through such forum.
Dude, i gave you a simple test to calculate the top speed of this car :

100bhp
125nm
weight 2000 kgs
5th gear - 0.774
Final Drive - 4.2
wheel size - 195/60 r14
engine redline - 8000rpm


And what did you say " Humn!! 125Nm with 2 ton and that too with that gear ratio. Ok boss. You win "

If your formula was so accurate that you could cal the top speed of any car, why did you back out?

Like i've said, only as as long as a car redlines all it's gears, you will be able to calculate the electronically/mechanically limited top speed of that vehicle. Calculating aerodynamically limited top speed of a vehicle is not a joke, There are many variables like power curve, torque curve, gearing, vehicle weight, road surface, head/tail wind, coefficient of drag, frontal surface area, rolling resistance, mechanical friction, altitude and god knows how many more.....

U need to understand that.

Quote:
And I have also not come across a vehicle (pardon my lack of experience with various cars) that travels faster in fourth gear than in fifth gear. Yes, that is again possible if the engine has very little torque and gear ratio is too small. But which modern car (among the ones we drive around) have that kind of engine and gear ratio.
My car does. OHC VTEC, top speed comes in 4th gear not 5th. Heck, my car can't even redline 4th. But this doesn't mean that if i were to alter the final drive ratio from 4.058:1 to 5:1 i would be able to increase the top speed of my car, even though the top speed would now be achieved in 5th. The only thing that will change is the max speed in every gear that i readline and the rate of acc in those gears but not top speed.

Quote:
Another thing. I think many people have assumed that the max RPM as the techometer redline or whatever. I mentioned in the formula that it is the max RPM the vehicle is capable of doing in first gear on the plane surface again.
What r you saying man!!! U think a car redlining 4th or 5th has the same resistance it faces in 1st? U can't even imagine the amount of aerodynamic resistance it has to face while redlining those higher gears. Even if you revv beyond the recomended redline, there is no way your engine can go as far in 4th/5th as it does in 1st.

Theres no point getting angry about it, if bad goes to worse, this thread will get locked and your intentions will fade away with it.

Shan2nu

Last edited by Shan2nu : 20th September 2008 at 14:56.
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Old 21st September 2008, 11:14   #30
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Originally Posted by opendro View Post

And I have also not come across a vehicle (pardon my lack of experience with various cars) that travels faster in fourth gear than in fifth gear. Yes, that is again possible if the engine has very little torque and gear ratio is too small. But which modern car (among the ones we drive around) have that kind of engine and gear ratio.
Lots of vehicles like that.
Esteem, Indica, Getz P etc.,
Most petrols sold in India will not redline in 5th gear.
Some of them will take a lot of time to redline in 4th gear too.

Diesels due to their high torque can get probably the closest to redline, but if you look at peak torque, it tapers off near the redline, so you will hit something like a "wall"

Nowadays other factors also play the role. For example ECU limits etc., which electronically limit the rpm to a certain value in each gear
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