Max speed design

[white911]

Originally Posted by clived

Firstly was that speedometer indicated 175, or a scientifically measured 175, say laser timing beams across the road? Even a 3 or 4% over-read (which I'd say is pretty likely at those speeds) sees that car actually doing somewhat less than 170. And there is a MASSIVE difference in the power needed to get a car to 170 to 186 - it is very far indeed from linear. .

Definitely a true statement!! My understanding the equations here is that wind resistance increases as the square of the speed increase. (Twice as fast = Twice as much drag). Power to overcome drag increases as the cube of the increase. (Twice as fast requires eight times as much power). If you assumed top speed of 170 with 300 horse power. (186/170) cubed times 300 should yield approximate power required for 186. If equations are correct.

[pumaknight]

Originally Posted by white911

Definitely a true statement!! My understanding the equations here is that wind resistance increases as the square of the speed increase. (Twice as fast = Twice as much drag). Power to overcome drag increases as the cube of the increase. (Twice as fast requires eight times as much power). If you assumed top speed of 170 with 300 horse power. (186/170) cubed times 300 should yield approximate power required for 186. If equations are correct.

Statements like that make you realise how seriously powerful a formula one car is - Go Lewis

[natedog_1959]

F1 cars aren't really that powerful. I would bet a Nascar makes more total HP. F1 cars win the lb/hp battle. 600-800 hp in a 1400 lb car with amazing traction control is crazy. Imagine how fast the turbo F1 cars were when they were pushing 1000-1400 hp in a similarly sized car with no traction control. Nate

[moff]

Originally Posted by mikeo

There is a 150mph speed limiter on the 135i becuase BMW AG lawyers say so.

The limiter is set at 155mph and not 150mph in Europe.

This is purely an agreement amongst some select manufacturers and not always the case. There isn't a law either in Germany or Europe prescribing the electronic speed limiter. The Mercedes SLR is not unrestricted, the Audi R8 is unrestricted too.

Porsche generally do not limit the top speeds of there cars, but they do restrict the Cayenne Turbo S purely due to the tyres.

[pumaknight]

Originally Posted by natedog_1959

F1 cars aren't really that powerful. I would bet a Nascar makes more total HP. F1 cars win the lb/hp battle. 600-800 hp in a 1400 lb car with amazing traction control is crazy. Imagine how fast the turbo F1 cars were when they were pushing 1000-1400 hp in a similarly sized car with no traction control. Nate

I was thinking along the lines of an "F1 car can do 200mph but still go round corners at almost insane speeds!" type power.

That is quite something to behold :biggrin:

[murellus]

Good rule of thumb. A sportscar (avg drag coeff) requires 500hp to reach 200mph

[john970]

Originally Posted by clived

Firstly was that speedometer indicated 175, or a scientifically measured 175, say laser timing beams across the road? Even a 3 or 4% over-read (which I'd say is pretty likely at those speeds) sees that car actually doing somewhat less than 170. And there is a MASSIVE difference in the power needed to get a car to 170 to 186 - it is very far indeed from linear. I've had experience of high power cars trying to reach their Vmax over a 2 mile runway. The lower power cars often reach pretty much their top speed in that space, whilst the higher power cars are still, very very slowly, gaining speed. But if you take a look at this list of recent results, to see the (typically very modified) cars that reach 186, you'd have to wonder if that speed is really within the reach of a 135i, chipped or otherwise. http://www.vmax15000bhp.com/results.htm I'd suggest not. These speeds are all laser trap verified, not readings from the speedo.

It was verified via GPS (Gamin nuvi I believe). Speedos are way inaccurate at those speeds as you mention.

[john970]

Originally Posted by white911

Definitely a true statement!! My understanding the equations here is that wind resistance increases as the square of the speed increase. (Twice as fast = Twice as much drag). Power to overcome drag increases as the cube of the increase. (Twice as fast requires eight times as much power). If you assumed top speed of 170 with 300 horse power. (186/170) cubed times 300 should yield approximate power required for 186. If equations are correct.

drag increases as the square of speed - i.e. twice as fast = 4x as much drag. The power curve isn't so easy, as generally you are overcoming more powertrain loss, more power loss to engine cooling, etc.

[john970]

Did you catch this?

Etlprws Porsche 996 Turbo S Cab 189mph

That car is a CONVERTIBLE. Crazy people out there.

[white911]

Originally Posted by john970

drag increases as the square of speed - i.e. twice as fast = 4x as much drag. The power curve isn't so easy, as generally you are overcoming more powertrain loss, more power loss to engine cooling, etc.

Correct. but, at the speeds we are talking about over 90% of the power is used to overcome drag.

[john970]

Originally Posted by white911

Correct. but, at the speeds we are talking about over 90% of the power is used to overcome drag.

Would love to see the math on that one.

[AwesomeBMW]

Originally Posted by john970

Would love to see the math on that one.

I can't verify the statement is true, but I can tell you how to figure it out:biggrin:

Wow, I'm really not in a helpful mood right now

[1ToBeOn2Wheels]

Thanks moff! I was hoping someone from across the pond would chime in and set us straight. :thumbup:

[aesthetect]

i did the calculations before for a chevy equinox (it was for a hybrid electric vehicle design competition so 0-60 time and vehicle weight were the design variables). i dont have all the values needed to do it for a 1er but really if you change the mass, Cd and frontal area its close enough. these are the values of the forces acting on that vehicle at 140mph (where drag is at peak)..

mass: 1536kg
inertial mass factor: 1.05 (this keeps us from having to do sloppy dynamics)
Cd: 0.33
Af: 1.86 m^2
Tire rolling resistant coefficient (this should be more, this value is for low rr tires found on hybrid project cars etc) = 0.01


rolling resistance force (mass*gravity*Crr) = 150.7 N
aerodynamic drag force (what weve been talking about) = 1442.7 N
inertial force (m*Imf*acceleration) is a bit trickier because we have to account for rate of acceleration. i used the times from the C&D test
fitted to the simplified nonlinear relationship v=Vm(t/tm)^z
where Vm=60mph and tm=4.7s(as tested) approximated the z value to be around 0.5 (gives a 25.6 0-140 time). the average acceleration during the last second using that model is about 2.76 m/s^2. thus..
inertial force = 4451.3 N

this sounds unreasonably huge inertial force but assuming a wheel radius of 0.34m this comes out to a required torque of 361lb/ft. which is a bit much if we can theoretically get to 150mph, but i dont think i need to tell you my model isnt perfect.

so, what have we learned?
if the car is going at constant velocity, yes, 90% of the forces acting on the car are from aerodynamic drag (fuckin spot on, actually). but if we are trying to accelerate in any way, that takes a hell of a lot more effort than anything else involved.



i really should have spent that 30 minutes on my thesis.

[pumaknight]

Originally Posted by aesthetect

i did the calculations before for a chevy equinox (it was for a hybrid electric vehicle design competition so 0-60 time and vehicle weight were the design variables). i dont have all the values needed to do it for a 1er but really if you change the mass, Cd and frontal area its close enough. these are the values of the forces acting on that vehicle at 140mph (where drag is at peak)..

mass: 1536kg
inertial mass factor: 1.05 (this keeps us from having to do sloppy dynamics)
Cd: 0.33
Af: 1.86 m^2
Tire rolling resistant coefficient (this should be more, this value is for low rr tires found on hybrid project cars etc) = 0.01


rolling resistance force (mass*gravity*Crr) = 150.7 N
aerodynamic drag force (what weve been talking about) = 1442.7 N
inertial force (m*Imf*acceleration) is a bit trickier because we have to account for rate of acceleration. i used the times from the C&D test
fitted to the simplified nonlinear relationship v=Vm(t/tm)^z
where Vm=60mph and tm=4.7s(as tested) approximated the z value to be around 0.5 (gives a 25.6 0-140 time). the average acceleration during the last second using that model is about 2.76 m/s^2. thus..
inertial force = 4451.3 N

this sounds unreasonably huge inertial force but assuming a wheel radius of 0.34m this comes out to a required torque of 361lb/ft. which is a bit much if we can theoretically get to 150mph, but i dont think i need to tell you my model isnt perfect.

so, what have we learned?
if the car is going at constant velocity, yes, 90% of the forces acting on the car are from aerodynamic drag (fuckin spot on, actually). but if we are trying to accelerate in any way, that takes a hell of a lot more effort than anything else involved.



i really should have spent that 30 minutes on my thesis.

And I should have spent the 30 minutes it took me to read it sleeping

[white911]

Originally Posted by aesthetect

so, what have we learned?
if the car is going at constant velocity, yes, 90% of the forces acting on the car are from aerodynamic drag (fuckin spot on, actually). but if we are trying to accelerate in any way, that takes a hell of a lot more effort than anything else involved.



i really should have spent that 30 minutes on my thesis.

Excellent!! Thanks for all the work you put into this. Using actual values is very helpful. The additional power for those 16 MPH is significantly more then the aero calculation yields.

It also helps to explain why I run into an aero wall before later model 911s. Up to 120 I can gap them; but, over 130 they have closed the gap, and by 140 they are gone. On the other hand, maybe it doesn't; but, another three day at VIR in two weeks I should be able to check out real world application.

[AwesomeBMW]

Originally Posted by aesthetect

i did the calculations before for a chevy equinox (it was for a hybrid electric vehicle design competition so 0-60 time and vehicle weight were the design variables). i dont have all the values needed to do it for a 1er but really if you change the mass, Cd and frontal area its close enough. these are the values of the forces acting on that vehicle at 140mph (where drag is at peak)..

mass: 1536kg
inertial mass factor: 1.05 (this keeps us from having to do sloppy dynamics)
Cd: 0.33
Af: 1.86 m^2
Tire rolling resistant coefficient (this should be more, this value is for low rr tires found on hybrid project cars etc) = 0.01


rolling resistance force (mass*gravity*Crr) = 150.7 N
aerodynamic drag force (what weve been talking about) = 1442.7 N
inertial force (m*Imf*acceleration) is a bit trickier because we have to account for rate of acceleration. i used the times from the C&D test
fitted to the simplified nonlinear relationship v=Vm(t/tm)^z
where Vm=60mph and tm=4.7s(as tested) approximated the z value to be around 0.5 (gives a 25.6 0-140 time). the average acceleration during the last second using that model is about 2.76 m/s^2. thus..
inertial force = 4451.3 N

this sounds unreasonably huge inertial force but assuming a wheel radius of 0.34m this comes out to a required torque of 361lb/ft. which is a bit much if we can theoretically get to 150mph, but i dont think i need to tell you my model isnt perfect.

so, what have we learned?
if the car is going at constant velocity, yes, 90% of the forces acting on the car are from aerodynamic drag (fuckin spot on, actually). but if we are trying to accelerate in any way, that takes a hell of a lot more effort than anything else involved.



i really should have spent that 30 minutes on my thesis.

I find no flaw with your work. I was just too lazy to do it all myself:biggrin: Now get to work on your thesis!

[aesthetect]

Originally Posted by AwesomeBMW

I find no flaw with your work.

thanks?

[john970]

Originally Posted by aesthetect

thanks?

[john970]

You need almost no power for acceleration provided it is non-zero. As a practical limit, you'd need acceleration to reach top speed within some time limit, like 2-3 minutes or so, but that number above seemed crazy high.

[aesthetect]

im not really followin you although that sounded too high to me too. double checked when i did it and all but really thats the simplest calculation in there, F=ma...

[Numb3rs]

Originally Posted by pumaknight

I was thinking along the lines of an "F1 car can do 200mph but still go round corners at almost insane speeds!" type power.

That is quite something to behold :biggrin:

Formula 1 :headbang: ftw!

Less than 2 weeks away baby!