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Thread: McLaren F1 transmission

  1. #1
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    McLaren F1 transmission

    Hi

    You can calculate a car's top speed in each gear this way:

    redline (rpm) / gear ratio / differential ratio x wheel circumference (mm) / 16667

    and the result will be in km/h. I made a huge Excel chart with these data (among others) for many cars, and it nicely showed each cars' top speed in each of their gears. But not with the McLaren F1. I used these data:

    redline: electronically limited at 7500 rpm,
    gear ratio in 6th: 0.93,
    differential ratio: 2.37,
    driven wheel (rear): 315/45-17.

    This way the circumference of the rear wheel is ( 17x25.4 + 315x2x0.45 ) x pi = 2247 mm. This means 7500/0.93/2.37 x 2247 / 16667 = 459 km/h top speed in 6th gear. Now some quotes from Peter Dron about the McLaren's record braking:

    "Wallace hit the rev limiter in sixth on his first run. It is normally set at 7500rpm (about 380 km/h / 236mph with tyre growth)."

    "Andy came in and said, 'I want to confirm this - I think I'm on the limiter in sixth.' We checked the data, and he was."

    There were many minor hiccups with other cars as well in my Excel chart, but this is by far the biggest.

    Is there any other set of gears in the Macca next to the gearbox and the differential?

  2. #2
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    AS aero drag increases exponentially with speed then you cant' rely on the top gear to determien top speed and there are other "losses" in the vehicle. Your calculation is a reasonable estimate of a theoritical top speed
    "A woman without curves is like a road without bends, you might get to your destination quicker but the ride is boring as hell'

  3. #3
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    Nice work! I guess these cars do reach top speed as per limiter in 6th, as opposed to say a 7-g tronic equiped benz that hits 155mph (yes, elec. limiter) in 5th, 6th and 7th.

    I would have thought the downforce generated in an F1 car was such that beyond the ratio in 6th little extra top speed could be found? maybe im wrong.
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  4. #4
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    F = Cw * rho * A * deltaV^2 / 2

    is the formula for calculating aerodynamic resistance from my head. Might be of help.

  5. #5
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    As others have said, you can't ignore drag.

    I ran the numbers in mathematica and ended up with 382.6 km/h, which is pretty close. That's ignoring rolling resistance and assuming that the engine is operating at peak torque. You could try repeating this with whatever the torque is at 7500 rpm and see if you get closer.

    You could make a spreadsheet that does the same thing but you'd have to find out the drag coefficient and frontal area for every car you want, which might be tricky.




    Hope this helps!
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  6. #6
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    We usually just calculate Cw and A with a formula. Not at home though, have to look it up..

  7. #7
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    No guys, you are wrong. Drag does not count here. There's fixed ratio between engine rpm and the driven wheel's rpm (in a given gear). It is impossible to change this ratio, as that way something would brake in the transmission. So in 6th gear engine rpm and wheel rpm is always 0.93x2.37 = 2.204. It's "compulsory". So for example the tachometer works as a sort of "percentage speedometer": If rpm is 3000 and speed is 100 km/h in a certain car in a certain gear, then at 6000 rpm speed has to be 200. You have to use the clutch under the speed that belongs to base rpm to prevent engine stalling. So drag only determines that the car can or cannot reach a certain speed, which speed dictates a compulsory rpm in each gear. If it's over redline, the car cannot be in that gear at that speed. If it's under base rpm, the car cannot be in that gear at that speed. If the car doesn't have enough thrust at that rpm in that gear, it can't climb to that speed in that gear. Watch the picture: you can only travel along the lines (it belongs to another car).

    I say it this way: if you would make an F1 to levitate in the air, switch it to 6th and rev it up to redline, the rear wheel would turn at an rpm which would dictate 459 km/h (without centrifugal growth of the tire). If now you put the car onto the ground, it would start accelerating up to that speed. But as you said drag would come in and it would stuck at a lower speed than 459. But at this point, engine rpm MUST be go down from 7500 as wheel rpm is lower. It must, as the engine rpm - wheel rpm ratio is constant in each gear. (You can change this ratio with the 6 different gears.) So with the above data Andy Wallace couldn't be at 7500 rpm at 380 km/h in the 6th gear.

    So my question still is: are there any more sets of gears in the McLaren F1?
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    Last edited by Kultag; 05-15-2008 at 08:58 AM. Reason: clarifying

  8. #8
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    Did you take into account frictions and losses through the entire driveline ? They also count up.

  9. #9
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    I don't know enough about this but maybe because of the slip ratio of the rear tires?

    To generete thrust the wheels must operate with a slip-ratio because the rubber deforms elastically. In this case the slip ratio is (459-390)/390=0,177. But this seems to high though...

    EDIT: But the question is not about the physics: rolling resistance, aero drag torque etc. It simple doesn't add up. The car was measured at about 390 km/h when it revved 7800 rpm. Not 459 km/h. If you use the ratios for final drive and 6th gear the car should show 6627 rpm at 390 km/h with zero slip, right?
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    Last edited by Knuto; 05-15-2008 at 09:31 AM.
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  10. #10
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    Quote Originally Posted by Knuto View Post
    I don't know enough about this but maybe because of the slip ratio of the rear tires?

    To generete thrust the wheels must operate with a slip-ratio because the rubber deforms elastically. In this case the slip ratio is (459-390)/390=0,177. But this seems to high though...

    EDIT: But the question is not about the physics: rolling resistance, aero drag torque etc. It simple doesn't add up. The car was measured at about 390 km/h when it revved 7800 rpm. Not 459 km/h. If you use the ratios for final drive and 6th gear the car should show 6627 rpm at 390 km/h with zero slip, right?
    You're getting it.

    I just read that the F1 has a transverse gearbox so there IS another set of gears. I think this is what I'm looking for, it should not be 1:1. This thing shows that there's not much sense in giving only the gearbox and the diff ratio. It also leads to question other cars' data. I don't thrust them very much now.
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  11. #11
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    ah, see what you're saying now, Kultag. Sorry I misread your proposition first time.

    As said slip is possible. The tyres and even the clutch will be slipping as the aero drag increases. Does raise an interesting puzzle tho'
    "A woman without curves is like a road without bends, you might get to your destination quicker but the ride is boring as hell'

  12. #12
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    Quote Originally Posted by Matra et Alpine View Post
    The tyres and even the clutch will be slipping as the aero drag increases.
    I don't think so tyre slip and clutch slip could be this high. That would make all other cars' calculations wrong as well. In fact they are a thing of beauty, they show so nicely each car's character, and regarding their top speed and else, they are certainly OK.

    It's a pity car makers' employees don't come to these forums to answer these questions. Or F1 owners...

  13. #13
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    I think I've read the F1 needs clutch replacement every 5000km. So it might be some slip. But I noticed the 5th gear reaches 382 km/h and 1st 137 km/h. It seems like one parameter is wrong
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  14. #14
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    Like you said, the data McLarem provided must be wrong, or somehow another type of gearbox was used...
    "Religious belief is the “path of least resistance”, says Boyer, while disbelief requires effort."

  15. #15
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    Quote Originally Posted by Kultag View Post
    I don't think so tyre slip and clutch slip could be this high. That would make all other cars' calculations wrong as well
    Remeber what was said LONG time ago .... drag increases exponentially.
    So there are always limiting points ... so for example a car with static downforce will likely have high drag at the higher speeds and so the forces at the contact patch are MUCH higher and hence slip will occur.

    Cars like the Veyron and Porsche CGT have dynamic aero to be abel to reduce the drag and adjust the downforce to optimise across wider range of speeds and so limit the lip potentially.

    I tihnk if you do the math on all those you'll find them significatn -- tho' I concur that quic mental estimate finds it difficult to see >10% that you're witnessing.
    "A woman without curves is like a road without bends, you might get to your destination quicker but the ride is boring as hell'

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