10-22-2013, 04:38 PM | #221 |
Major General
5459
Rep 7,037
Posts |
Important point, it's how special the car is when it's launched that counts. I don't think an IBM AT is very impressive now but when it was launched it was the tits. Is the S55 as special in today's market? Maybe not taken it's two predecessors.
|
Appreciate
0
|
10-22-2013, 04:55 PM | #222 | |||
General
21132
Rep 20,742
Posts
Drives: 2021 911 turbo
Join Date: Jan 2012
Location: Montreal
|
Quote:
Quote:
My understanding: The angular velocity of the wheels is directly proportional to the speed of the vehicle regardless of the selected gear. In other words, their equivalent mass is independent of the gearbox ratio. The same concept applies for all the rotating components downstream of the gearbox (and some inside the gearbox). Only the engine's (and part of the transmission's) angular acceleration relative to the car's linear acceleration varies as a function of the selected gear. The mass of the car does not need to be considered for optimal shift points because it does not vary with different gears, same applies to rotating components downstream of the gearbox. For a given torque generated at the gearbox output, the same proportion of that torque will be used to accelerate the downstream rotating components regardless of which gear is selected. Quote:
Last edited by CanAutM3; 10-22-2013 at 07:27 PM.. |
|||
Appreciate
0
|
10-22-2013, 07:24 PM | #223 | |
General
21132
Rep 20,742
Posts
Drives: 2021 911 turbo
Join Date: Jan 2012
Location: Montreal
|
Quote:
The fact that it promises such a broad power plateau and if the engineers are able to eliminate lag through some smart technical trick could well make the S55 a special engine for its time. |
|
Appreciate
0
|
10-22-2013, 07:30 PM | #224 |
Major General
5459
Rep 7,037
Posts |
I agree, it all comes down if it's revolutionary free from lag. If it is, then yes it will be very special indeed. I clearly remember being blown away by every M cars launched to date first by specifications and then in real performance. This time the specifications isn't really mind blowing with a typical FI power curve and for today's M cars moderate power. The real life performance remain to be felt. No lag, some lag or a lot of lag? We'll see.
|
Appreciate
0
|
10-22-2013, 08:09 PM | #225 | |
General
21132
Rep 20,742
Posts
Drives: 2021 911 turbo
Join Date: Jan 2012
Location: Montreal
|
Quote:
|
|
Appreciate
0
|
10-23-2013, 12:42 AM | #226 | ||
Banned
209
Rep 7,298
Posts |
Quote:
|
||
Appreciate
0
|
10-23-2013, 12:50 AM | #227 | ||||
Banned
209
Rep 7,298
Posts |
Quote:
Something wrong here. |
||||
Appreciate
0
|
10-23-2013, 06:40 AM | #228 |
General
21132
Rep 20,742
Posts
Drives: 2021 911 turbo
Join Date: Jan 2012
Location: Montreal
|
Those were theoretical gear ratios to optimize the power plateau. With the nearly instantaneous shifts provided by the DCT, more frequent shifts are not as inconvenient to optimize performance. When I calculated those ratios, I did not factor in the impact of the engine inertia on shift points. When I have some time to spare, I will recalculate them. It should not make a huge difference, especially in the higher gears. These ratios are unlikely to make it to the actual car.
FD 3.154 RPM km/h 1 4.310 7500 70 2 3.218 7500 94 3 2.403 7500 126 4 1.794 7500 169 5 1.340 7500 226 6 1.000 7500 303 7 0.747 1850 100 Last edited by CanAutM3; 10-23-2013 at 09:10 AM.. |
Appreciate
0
|
10-23-2013, 07:29 AM | #229 |
Private
3
Rep 76
Posts |
Wouldn't the auto rev-match have an on-off switch? 370Z has the auto rev-match and has a switch to turn it off.
|
Appreciate
0
|
10-23-2013, 10:37 AM | #230 | |
Banned
209
Rep 7,298
Posts |
Quote:
|
|
Appreciate
0
|
10-23-2013, 10:39 AM | #231 | |
Banned
209
Rep 7,298
Posts |
Quote:
|
|
Appreciate
0
|
10-23-2013, 11:09 AM | #232 | |
General
21132
Rep 20,742
Posts
Drives: 2021 911 turbo
Join Date: Jan 2012
Location: Montreal
|
Quote:
|
|
Appreciate
0
|
10-23-2013, 12:06 PM | #233 | |
Banned
209
Rep 7,298
Posts |
Quote:
|
|
Appreciate
0
|
10-23-2013, 01:09 PM | #234 |
General
21132
Rep 20,742
Posts
Drives: 2021 911 turbo
Join Date: Jan 2012
Location: Montreal
|
|
Appreciate
0
|
10-23-2013, 01:20 PM | #235 |
Banned
209
Rep 7,298
Posts |
should be good....one thing i guess that having torque down low is not having to downshift to 2nd on tracks. but i guess its even more throttle modulation or just power slides
|
Appreciate
0
|
10-24-2013, 02:42 AM | #236 | |
Lieutenant General
612
Rep 10,407
Posts |
Quote:
This is the total propulsive force accelerating a vehicle. The first term is the real basic one obvious to most, torque x combined gear ratio x combined transmission loss. The entire second term is the "inertial term", I (e,t,d,w) are moments of inertia of the engine, transmission, driveshafts and wheels. Nf is the final drive ratio, ax is the vehicle acceleration and r is the drive wheel radius. This of course is not F = ma for the entire vehicle this is just the propulsive force. This is equation 2-9b from Gillespie as referenced prior. This entire 2nd term is most often completely ignored, even I have been guilty of this in the past. Now let me try to show in a somewhat qualitative way how wheel inertia can affect shift point. The first term just gives us basically an upside down "lump" shape mirroring the shape of the engines torque curve (assume plotting vs. speed). Now since we agree that ideal shifting is based on when these propulsive force curves from different gears intersect it should start to be obvious already. Imagine the effect of the second term. How can it affect shift points (as you already agree it will) but then Iw can not? This term with an Iw changed from one value to another will drop the "lump" in an rpm dependent (i.e. speed dependent) fashion because it is explicitly acceleration dependent (ax term)! It will drop the curves in each gear by a different amount (clearly from the Ntf term). Thus how could it possibly be that the curves with two different Iw's will intersect at the exact same rpm? They won't, period. Yes it is a small correction but it is a real effect. I see why you feel it is a bit counter-intuitive but that's the reason for the precision of the equations! Thus to correct your reasoning above try to forget about which terms are gear dependent and focus on how they are acceleration dependent and thus implicitly speed dependent and imagine how the intersection of the curves will be affected. Unfortunately, CarTest does not seem to capture this subtlety and in fact neither does my huge complex spreadsheet that performs the same basic calculation. CarTest does capture the effect of wheel inertia on acceleration but does not capture its effect on shift points. It is also clearly captures the I (e, t and d) terms and of course the losses as well. My own spreadsheet uses a slightly different approach (really a "cheat" of sorts) of effective mass as outlined in Gillespie to include all inertial effects both on acceleration and on shift points. But due to the way that is implemented Iw can not be explicitly changed and see the effect directly. Fortunately these Iw corrections are small, both on the shift points, on the overall acceleration and from any change from this interaction (changing acceleration by changed shift point(s)). But again the total corrections are anything but small. Recall that the overall acceleration in 1st gear for a car like an M3 is about 20% lower (in 1st gear) when including the inertial term or not including it. These effects are quite substantial overall. It's like 750 lb, clearly a heck of a lot more than the component weigh! In higher gears the percent "correction" of the second term is closer to 5-10%. Hope this helps.
__________________
E92 M3 | Space Gray on Fox Red | M-DCT | CF Roof | RAC RG63 Wheels | Brembo 380mm BBK | | Vorsteiner Ti Exhaust | Matte Black Grilles/Side Gills/Rear Emblem/Mirrors | | Alekshop Back up Camera | GP Thunders | BMW Aluminum Pedals | Elite Angels | | XPEL Full Front Wrap | Hardwired V1 | Interior Xenon Light Kit | |
|
Appreciate
0
|
10-31-2013, 07:27 PM | #238 | |
General
21132
Rep 20,742
Posts
Drives: 2021 911 turbo
Join Date: Jan 2012
Location: Montreal
|
Quote:
However, acceleration is still part of the resulting simplified equation; and acceleration does vary with Iw. If it is the case, it also implies that optimal shift points will vary depending on the total mass of the car. So if you have 3 passengers, you might end up with different optimal shift points compared to when you are driving by yourself. I still find this a bit counter intuitive and will play with the math some more. I'll post my findings ...if any While I underestimated the impact of drivetrain inertia on optimal shift points, I still think that Cartest overestimates the impact by using the mass factor rather than the actual inertia. Last edited by CanAutM3; 11-06-2013 at 08:35 AM.. |
|
Appreciate
0
|
10-31-2013, 09:04 PM | #239 | |
Brigadier General
380
Rep 3,934
Posts |
Quote:
|
|
Appreciate
0
|
11-01-2013, 12:16 AM | #240 | |
Lieutenant
34
Rep 421
Posts |
Quote:
If you calculated F in gear 2 at an rpm in which F was higher than in gear 3 at the same velocity, it would all still be true with larger wheels. i.e. the propulsive force in a given gear would drop when increasing Iw but it would drop in all gears and by an amount which kept the optimal shift point constant. |
|
Appreciate
0
|
11-01-2013, 12:20 AM | #241 |
Private First Class
1
Rep 116
Posts |
CanAutM3, I read your PDF-file and found no flaws - which doesn't mean very much of course. So, if you correctly eliminated Iw, you don't have to include it again afterwards only because acceleration is still part of the equation, do you?
|
Appreciate
0
|
11-01-2013, 09:09 AM | #242 | |
General
21132
Rep 20,742
Posts
Drives: 2021 911 turbo
Join Date: Jan 2012
Location: Montreal
|
Quote:
But since vehicle acceleration is still part of the equation to be solved, it means Iw still has an impact. Albeit a very very small impact because the wheel inertia becomes burried in the overall equivalent mass of the car (equivalent mass = total mass + inertial effects). What I did not expect, is for the mass of the car to have an impact on optimal shift points. I am not yet fully convinced of this and will try to doodle some more with the math... |
|
Appreciate
0
|
Post Reply |
Bookmarks |
|
|