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      02-25-2011, 06:33 PM   #287
bruce.augenstein@comcast.
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Quote:
Originally Posted by swamp2 View Post
I'm pretty sure the author of CarTest confirmed with me that he included drivetrain inertial effects. I never said it is not important. It clearly is, it translates along with the car and it also takes extra energy on top of that to spin up all rotating components. I just doubted certain details of the math relationships with you.

Other than knowing gear ratios there is obviously no way of knowing how large or what materials (i.e. densities) or sized all of the relevant drivetrain components are. I checked Gillespie, he simply substitutes the so called "mass factor" for not knowing the details about each cars drivetrain. The mass factor is an approximation and certainly does not take into account actual details of the driveline. The mass factor in 1st gear between the 335i and M3 does vary by about 18%. I'll try to check CarTest to see if this "mass factor" is used as part of its acceleration calculation by specifically comparing the M3 and 335i.
My comment was in regard to the 4000/8000 redline cars with equal power and a totally flat torque curve. The 8000 rpm car's acceleration would be significantly reduced because it had gearing that was twice as aggressive as the 4000 rpm car.

Your CarTech results are absolutely incorrect if they don't show this, and I expect you'll find something in that tool that allows for varying rotational inertia.
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      02-25-2011, 09:46 PM   #288
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Quote:
Originally Posted by bruce.augenstein@comcast. View Post
My comment was in regard to the 4000/8000 redline cars with equal power and a totally flat torque curve. The 8000 rpm car's acceleration would be significantly reduced because it had gearing that was twice as aggressive as the 4000 rpm car.

Your CarTech results are absolutely incorrect if they don't show this, and I expect you'll find something in that tool that allows for varying rotational inertia.
I understood your comment and again agree about the existence of an effect dependent on total drivetrain inertia (never have disagreed about the effect qualitatively). However without knowing the drivetrain inertia you can not say the gearing/rpm alone will decide the faster car. It is gearing+rpm+INERTIA. Look back at Gillespie, he substitutes a "mass factor" to account for drivetrain inertia and gearing. I'm almost certain the author of CarTest indicated he used both Gillespie and the references in Gillespie including the "mass factor". However, I guarantee there are no user accessible factors for drivetrain interia nor "mass factor". Are their access to such things in QuarterJr? Does the effect show up in QuarterJr?

The drivetrain inertia is pretty well unknowable without complete access to the engineering drawings or a full drivetrain disassembly followed by complex measurement exercise. AGAIN hence the "mass factor" is used in vehicle acceleration simulation. And again I'm pretty sure I can test CarTest to see if it uses mass factor. Stay tuned.
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      02-25-2011, 11:19 PM   #289
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Quote:
Originally Posted by swamp2 View Post
I understood your comment and again agree about the existence of an effect dependent on total drivetrain inertia (never have disagreed about the effect qualitatively). However without knowing the drivetrain inertia you can not say the gearing/rpm alone will decide the faster car.
Of course you can. With identical drive train rotational inertia for the two cars, the 4000 rpm car absolutely pulls away from the 8000 rpm car, because it only has to spin up the rotating parts half as quickly.

Quote:
Originally Posted by swamp2 View Post
It is gearing+rpm+INERTIA. Look back at Gillespie, he substitutes a "mass factor" to account for drivetrain inertia and gearing. I'm almost certain the author of CarTest indicated he used both Gillespie and the references in Gillespie including the "mass factor". However, I guarantee there are no user accessible factors for drivetrain interia nor "mass factor". Are their access to such things in QuarterJr? Does the effect show up in QuarterJr?
No. "Quarter" has input requirements for drivetrain inertia, but not Quarter Jr.

Quote:
Originally Posted by swamp2 View Post
The drivetrain inertia is pretty well unknowable without complete access to the engineering drawings or a full drivetrain disassembly followed by complex measurement exercise. AGAIN hence the "mass factor" is used in vehicle acceleration simulation. And again I'm pretty sure I can test CarTest to see if it uses mass factor. Stay tuned.
Since CarTest predicted identical acceleration between the two cars (which is absolutely incorrect), if there's no way to factor in rotational inertia, then the software has a problem.

These two cars absolutely will not have identical acceleration.
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      02-26-2011, 03:37 AM   #290
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Quote:
Originally Posted by bruce.augenstein@comcast. View Post
Of course you can. With identical drive train rotational inertia for the two cars, the 4000 rpm car absolutely pulls away from the 8000 rpm car, because it only has to spin up the rotating parts half as quickly.
NO, you can not. Patently false. Key statement - with identical drivetrain inertia - that means all of the rotational inertia of the entire drivetrain from flywheel to wheels sum to the same total inertia. If this is true then you can make some accurate a priori predictions. However for any two cars that is quite unlikely.

Quote:
Originally Posted by bruce.augenstein@comcast. View Post
Since CarTest predicted identical acceleration between the two cars (which is absolutely incorrect), if there's no way to factor in rotational inertia, then the software has a problem.
I never said that was the prediction from CarTest. In fact the predictions are close but not identical. The biggest difference is in 1st gear which makes sense. Also I did not use a totally flat torque curve for a quick investigation of this. Like I said, more investigation is required. Stop jumping to conclusions.

Quote:
Originally Posted by bruce.augenstein@comcast. View Post
No. "Quarter" has input requirements for drivetrain inertia, but not Quarter Jr.
And the predictions from Quarter Jr. then for this 4000/8000 rpm case are? Since that tool does not have control over rotational inertia is must be total crap.... I don't really believe that, but I'm playing your part for you.
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      02-26-2011, 07:17 AM   #291
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Quote:
Originally Posted by swamp2 View Post
It's not me who needs to do that math, its you who needs to read. I said 2% on for the selection of cars in mixja's post. I explicitly agreed with you about 5% for the 335i.



Yes, this is more or less correct. As long as you want to continue to obsess about engine torque (prior) and then torque to the wheel (later) and now finally torque to the wheels per weight (now). Once you simply graduate to power to weight you'll be getting there. Assuming the 335i makes 320 hp. The numbers are:

335i: 11.2 lb/hp
M3: 8.9 lb/hp

That is a 26% worse value for the 335i
. That is why when you choose the right gear and use the car the M3 stomps the 335i from and to any speed and in distance at a given time. If you like the flexiblity of good acceleration in a high gear at low speed and very low rpm that's fine, it is a preference.

You are still missing something about tire size as well. Why don't you simply put a set of RC sized tires on the rear of your car. Torque multiplication is HUGE. For essentially free you would have the highest wheel torque car on the planet. Even if traction was not an issue there is a reason why this does not magically make a car faster by the increased percentage of torque multiplication. This is exactly why you are also confused about FD ratio modifications as well. You must consider what happens not at an instant but ACROSS multiple gears.
I think now you have gone sideways. My argument was to suggest that the 335i makes for a great daily driver and will often outaccelerate the M3 short bursts on the street, where most driving is between 500 and about 4000 rpm and the M3 actually has a torque disadvantage. No one has ever suggested that stock 335i will out drag race stock M3. All the magazines have confirmed teh M3's dragstrip advantage many times.

Your diversion in to horsepower to weight ratios is not really relevant. We can all divide the cars' weights into their horsepowers and figure out in our heads that the M3 will win. But peak horsepowers are not what you see in daily driving from 500 to 4000 rpm. Again, no one disputes that stock for stock, the M3 wins the drag races, but the 335i will outsquirt it in daily driving around town in the lower rpm ranges and the mightly M3 will not often have the space to catch up. If you dragrace around town, you will win, but many speed contests are not what you see in your simulations or on TV, they are impromptu, in gear races (ideally with time to shift, but not always). This is where the 335i does really well due to those fast spooling little turbos that produce great low end torque.

I don't think I am missing anything about tire size. You confirmed my math, right? 5% advantage for 18s for the 335i over those on the M3. Do we all realize we could go to absurd extremes? Sure. You don't need to make a spreadsheet or simulation to illustrate that point. And I am increasing my gearing slightly by going to wider tires. Even if you did want to go in the direction of absurdity, you would have to do some work to put wheels smaller than 18s on the M3. And I like the 35 to 40 series range for daily driving due to comfort and wheel protection. Any slight change in gearing is a side effect rather than the goal for running wider wheel and changing tires.

I was not the one that brought up torque multiplication, claiming that it had to be factored in when determing what car would be faster and when. I understand it, having run 3.15, 3.23, 3.38 and 3.64 diffs on my 99M3. I have also played with the www.diffsonline.com calculator for gearing and rpm everytime I have changed diffs or tire sizes to determine the rpm changes in gears and at certain speeds. And I have compared all this to my dyno plots to determine the advantages and disadvantages (the 3.64 made my car accelerate faster in each gear than the 3.38, but it was less quick in the quarter mile -- across the gears -- because I then needed an extra shift into 5th gear). I actually do this stuff and test it out in the real world on my own car.

From what I have read, you went to a dragstrip once, a junior strip of only 1/8 mile, and despite having written countless posts and done countless spreadsheets and probably memorized your owner's manual, you could not get your launch control to work. I will speculate that you probably had not used it much if ever, since that would be abusive to the precious car you had been dreaming about for a year before it was released. You ran an 8.35 at 86.3, which you guesstimated would work out to 12.7 to 12.8 at 110-114. I plugged your actual 1/8th numbers into a calculator I found on the internet (since calculators and spreadsheets are foolproof) and got an estimated quarter mile time and trap speed of 13.0 at 107. Not impressive for an M3. I think you should drive more and simulate less. I am going to go work on one of my cars now.
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      02-26-2011, 09:41 AM   #292
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Quote:
Originally Posted by pbonsalb View Post
, where most driving is between 500 and about 4000 rpm
Is this for real? Are there real people driving at 500rpm?
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      02-26-2011, 10:06 AM   #293
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Quote:
Originally Posted by swamp2 View Post
NO, you can not. Patently false. Key statement - with identical drivetrain inertia - that means all of the rotational inertia of the entire drivetrain from flywheel to wheels sum to the same total inertia. If this is true then you can make some accurate a priori predictions. However for any two cars that is quite unlikely.
Time to rewind, as is often the case between us in our "discussions".

I was assuming identical drivetrain inertia between the two cars. With that the case, the 8000 rpm will accelerate more slowly than the 4000 rpm car.

Quote:
Originally Posted by swamp2 View Post
I never said that was the prediction from CarTest. In fact the predictions are close but not identical. The biggest difference is in 1st gear which makes sense. Also I did not use a totally flat torque curve for a quick investigation of this. Like I said, more investigation is required. Stop jumping to conclusions.
Deal.

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Originally Posted by swamp2 View Post
And the predictions from Quarter Jr. then for this 4000/8000 rpm case are? Since that tool does not have control over rotational inertia is must be total crap.... I don't really believe that, but I'm playing your part for you.
No way to model this in Quarter, Jr. You get to key in displacement, max power @ rpm, and the shift point. That's it for the engine. Forget torque or any type of power curve.
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      02-27-2011, 02:10 AM   #294
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Quote:
Originally Posted by pbonsalb View Post
I think now you have gone sideways. My argument was to suggest that the 335i makes for a great daily driver and will often outaccelerate the M3 short bursts on the street, where most driving is between 500 and about 4000 rpm and the M3 actually has a torque disadvantage.
No, no and no. Only with the qualifier that you choose a terrible gear in the M3 and a better gear in the 335i (or as mentioned earlier you have a highly modified 335i).

Quote:
Originally Posted by pbonsalb View Post
Your diversion in to horsepower to weight ratios is not really relevant.
It was clearly a process to get your "torque to crank" blinders off. It has almost worked.

Quote:
Originally Posted by pbonsalb View Post
We can all divide the cars' weights into their horsepowers and figure out in our heads that the M3 will win. But peak horsepowers are not what you see in daily driving from 500 to 4000 rpm. Again, no one disputes that stock for stock, the M3 wins the drag races, but the 335i will outsquirt it in daily driving around town in the lower rpm ranges and the mightly M3 will not often have the space to catch up. If you dragrace around town, you will win, but many speed contests are not what you see in your simulations or on TV, they are impromptu, in gear races (ideally with time to shift, but not always). This is where the 335i does really well due to those fast spooling little turbos that produce great low end torque.
Let's stop BSing and using these flowery imprecise descriptions. Please describe an everyday scenario EXACTLY as to starting speed, gear and ending speed for both cars. Show me the stock 335i besting the M3 and being such a better daily squirter... Wow, an inferior powered car can win a non drag race style "street race" when the M3 guy is caught in the totally wrong gear. What a revelation...

Quote:
Originally Posted by pbonsalb View Post
I don't think I am missing anything about tire size.
This also tied into your obsession with crank torque and your then graduation to wheel torque. If you can not describe precisely why a tiny remote control car size tire with HUGE wheel torques doesn't really "work" I don't think you still get it. You did not answer this question yet.

Quote:
Originally Posted by pbonsalb View Post
I understand it, having run 3.15, 3.23, 3.38 and 3.64 diffs on my 99M3. I have also played with the www.diffsonline.com calculator for gearing and rpm everytime I have changed diffs or tire sizes to determine the rpm changes in gears and at certain speeds. And I have compared all this to my dyno plots to determine the advantages and disadvantages (the 3.64 made my car accelerate faster in each gear than the 3.38, but it was less quick in the quarter mile -- across the gears -- because I then needed an extra shift into 5th gear). I actually do this stuff and test it out in the real world on my own car.
Apparently not. You can run a million different final drives at that web site and also run them in your car. When you believe that a final drive mod or changing wheel weights will produce more power that is a laughable misconception (you posted about this right here. This makes all of that wonderful "real world" experience of yours just about null in my book.

Quote:
Originally Posted by pbonsalb View Post
From what I have read, you went to a dragstrip once, a junior strip of only 1/8 mile, and despite having written countless posts and done countless spreadsheets and probably memorized your owner's manual, you could not get your launch control to work. I will speculate that you probably had not used it much if ever, since that would be abusive to the precious car you had been dreaming about for a year before it was released. You ran an 8.35 at 86.3, which you guesstimated would work out to 12.7 to 12.8 at 110-114. I plugged your actual 1/8th numbers into a calculator I found on the internet (since calculators and spreadsheets are foolproof) and got an estimated quarter mile time and trap speed of 13.0 at 107. Not impressive for an M3. I think you should drive more and simulate less. I am going to go work on one of my cars now.
OK even more OT... You are getting hosed down in this debate so you want to ridicule me about my drag racing skills... OK I'm game...

I am highly flattered that you care so much about my illustrious side career as a drag racer. And care enough to put on your detective cap and dig around on the forum to find the dirt on old swampy. If you really want to dig hard you can find a post where I posted my picture. Maybe then you can also call me ugly . Perhaps obviously I prefer using the car for what it was built for - canyons and road coarse style driving (and the occasional autoX or trackday) - as opposed to the drag strip. Heck, I even use it as a fun commuter and daily driver. Had my buddy in his giant and monstrously torquey Dodge Challenger SRT8 not egged me on I probably would have never gone to the strip in this car.

LC is not really designed for the drag strip. From my own use of LC prior I also knew a very good traction surface is ideal to prevent some pretty wild wheel spin (that part is obviously a non issue at a strip). Also the tranny must be suitably warm for it to be enabled. I knew that but also thought it still might be warm enough idling in the queue. It was not, no big deal. I got a killer launch on my 3rd or 4th run sans LC.

I suppose you can tell me what is so terrible about a sub 2 second 60' time in a totally stock M3 with street tires?

You can use all of the calculators you like, I'll stick to my estimates (calculations) in my post about what I would have ran the full 1/4. If I really cared all that much I would just head to the strip. If you have not validated that particular calculator you used nor know what method it is using to calculate then it is just as believable as about 90% of the crap on the internet. Ironically you even joked about not believing it. By the way I did get booted off the track as well - I was running too fast to not have a helmet as per track regulations...

I'm glad you worked on your car tonight. We were thinking the same thoughts. I put on my new big brake kit. There goes your theory about me never getting dirty as well.

I think you should simulate more and waste a lot less money on differentials, labor to install them and wasteful dynos for them. In seconds a simulator will tell you PRECISELY how a differential will perform in any car under almost any circumstance. Unfortunately most old school hot rodders like yourself without the knowledge nor capability to use one will continue to be stuck in the dark ages.

We can continue ad nauseam if you like, I'd rather get back on topic myself.
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      02-27-2011, 03:10 AM   #295
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I didn't go through all the pages. The engine specs are not everything. If you look at the test the M3 is also faster then a 335i in high gear pulls at low RPM!


These times are from Sport Auto, very known European magazine. The MT 335i times are the best of 2 which where available. The other one was significantly slower.

BMW M3:
80-100 km/h
MT (4./5./6./7. Gear) 2,3 s /2,8 s /3,8 s / - s
DCT (4./5./6./7. Gear) 2,1 s /2,6 s /3,1 s /4,2 s

80-120 km/h
MT (4./5./6./7. Gear) 4,7 s /5,6 s /7,2 s / - s
DCT (4./5./6./7. Gear) 4,4 s /5,4 s /6.1 s /8,1 s

80-140 km/h
MT (4./5./6./7. Gear) 7,3 s /8,6 s /10,8 s / - s
DCT (4./5./6./7. Gear) 6,6 s /8,2 s /9,3 s /12,0 s

80-160 km/h
MT (4./5./6./7. Gear) 9,9 s /11,8 s /14,7 s / - s
DCT (4./5./6./7. Gear) 8,9 s /11,0 s /12,8 s /16,3 s

80-180 km/h
MT (4./5./6./7. Gear) 12,7 s /15,3 s /19,1 s / - s
DCT (4./5./6./7. Gear) 11,4 s /14,2 s /16,8 s /20,9 s


BMW 335i Coupe N54
(DCT times for EU version with standard N54, so not 335is)
80-100 km/h
MT (4./5./6./7. Gear) 2,5 s /3,0 s /3,7 s / - s
DCT (4./5./6./7. Gear) 2,2 s / - s /3,3 s /3,8 s

80-120 km/h
MT (4./5./6./7. Gear) 5,1 s /6,2 s /7,3 s / - s
DCT (4./5./6./7. Gear) 4,5 s / -,- s /6,7 s /8,0 s

80-140 km/h
MT (4./5./6./7. Gear) 8,0 s /9,5 s /11,4 s / - s
DCT (4./5./6./7. Gear) 7,0 s / -,- s /10,2 s /12,4 s

80-160 km/h
MT (4./5./6./7. Gear) 11,0 s /13,3 s /16,1 s / - s
DCT (4./5./6./7. Gear) 9,6 s / -,- s /14,1 s /17,3 s

80-180 km/h
MT (4./5./6./7. Gear) 14,5 s /17,5 s /21,4 s / -,- s
DCT (4./5./6./7. Gear) 13,1 s / -,- s /18,3 s / -,- s

Last edited by marcel b; 02-27-2011 at 03:17 AM..
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      02-27-2011, 09:36 AM   #296
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Quote:
Originally Posted by swamp2 View Post
I always like to add this clarification to that comment:

For actual production cars with gears and FD ratios that are fairly well matched/optimized to the particular cars engine and redline this is true. However, gearing is very important in theory and the physics of car performance. A big change in a cars gearing after already "optimized" will result in a huge performance change. We are just lucky because cars with very different engines are already matched to their drivetrains. That is the only reason gearing appears transparent in car comparisons and why we can shortcut straight to power to weight ratio.
Without rancor, I hope you relaize that when I say (as I did) that power and weight are all you need to know at any given instant, your comment is not needed.
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      02-27-2011, 11:50 AM   #297
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Quote:
Originally Posted by bruce.augenstein@comcast. View Post
Without rancor, I hope you relaize that when I say (as I did) that power and weight are all you need to know at any given instant, your comment is not needed.
Then you're both wrong, or perhaps guilty of being too simple. Traction is very important as so is aerodynamics. Weight is less important as speeds increase, which is where the importance of aero takes over. BMW has actually done some interesting things with aero on the 1M to decrease turbulence around the wheels. It shows they're paying attention. Back to the "acceleration" quest, it's really so one dimensional it's a shame it gets such attention. I've enjoyed BMWs since the 70s and to me they've always been about cornering and torque at high rpms. Never the all-out quickest on the road, but always with the ability to drive very fast for a long period of time without bad things happening. The turbos are awful at high rpms so it goes against what I like about BMW motorcars. Hopefully BMW will do better with subsequent generations of turbo cars. And perhaps going against the grain, I don't think an engine needs to scream at 8,000+ rpms to be great. I like that they can but I don't visit 8,000 rpms with any great frequency day to day. But when I do, it's generally rewarding (unlike a 335i at redline).
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      02-27-2011, 02:33 PM   #298
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Quote:
Originally Posted by bruce.augenstein@comcast. View Post
Without rancor, I hope you relaize that when I say (as I did) that power and weight are all you need to know at any given instant, your comment is not needed.
I strongly disagree. The only reason we can say gearing "does not matter" is because the OEMs have already chosen "the right gearing". Also performance is much more than "at an instant".

I suppose you can also tell the enormous community of drivetrain engineers at all of the OEMs that for performance that gearing doesn't matter .
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      02-27-2011, 02:36 PM   #299
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Quote:
Originally Posted by marcel b View Post
I didn't go through all the pages. The engine specs are not everything. If you look at the test the M3 is also faster then a 335i in high gear pulls at low RPM!
Great information, very relevant to this 335i vs M3 debate I've got sucked into.

But wait, wait, what if the 335i guy is in second gear, modified, running high boost and race gas and the M3 guy is in 5th gear in S mode and does not downshift. The 335i will win by a mile. It is such better daily driver because it has so much torque at low rpm...
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      02-27-2011, 02:38 PM   #300
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Originally Posted by MVF4Rrider View Post
Then you're both wrong, or perhaps guilty of being too simple. Traction is very important as so is aerodynamics.
...
Good post. I think both Bruce and I know and appreciate what BMW M is known for. Driving pleasure in the twisties or on the track. However, despite not being any sort of drag racer myself, we both like to get into the details of straight line performance.
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      02-27-2011, 09:36 PM   #301
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Originally Posted by MVF4Rrider View Post
Then you're both wrong, or perhaps guilty of being too simple. Traction is very important as so is aerodynamics...
Agree - plus rotational inertia, drivetrain friction, rolling resistance...

Guilty of leaving out "everything else being equal,..."
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      02-27-2011, 10:05 PM   #302
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I strongly disagree. The only reason we can say gearing "does not matter" is because the OEMs have already chosen "the right gearing". Also performance is much more than "at an instant".

I suppose you can also tell the enormous community of drivetrain engineers at all of the OEMs that for performance that gearing doesn't matter .
Ohferchrissake. Do you not have a clue as to what "at any given instant" actually means?

Furthermore, OEM gearing has absolutely nothing to do with it.
My point was and is that power to weight wins, either for this instant, or over a longer stretch. As long as the two cars start side by side at the same speed, of course.

Doesn't matter if one car is at 6800 in whatever gear while the other car is at 8300 in another gear, as long as they're side by side and the other factors just mentioned by MVF4Rrider (and myself) are similar. The better power-to-weight car will begin to pull away immediately.

Over some distance like a quarter mile, the car that makes the best power to weight over that distance (and time span) will tend to win. Obviously, gearing and gear spacing now get important because they will change average power over that distance. Yeah, it's a simplification, but you get my drift.

You agree with this. You have always agreed with this - because it's clearly true. You have just been "discussing" this with pbonsalb, trying to help him see the light (which he probably never will). Why switch now?

Last edited by bruce.augenstein@comcast.; 02-27-2011 at 10:10 PM..
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      02-28-2011, 01:21 AM   #303
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Originally Posted by bruce.augenstein@comcast. View Post
Over some distance like a quarter mile, the car that makes the best power to weight over that distance (and time span) will tend to win. Obviously, gearing and gear spacing now get important because they will change average power over that distance. Yeah, it's a simplification, but you get my drift.
That was my point. Overall performance numbers although relatively insensitive to precise gearing choices, you can see some changes. Times to speed and trap speeds seem to be more sensitive than time to distance. Some key metrics like 60-130 can change quite a bit with gearing. And a string of those (varying the 60 and 130 numbers) are what make up the easy straights on a road coarse. I'm actually surprised how robust distance vs. time is to gearing changes.
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      02-28-2011, 02:59 PM   #304
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Originally Posted by swamp2 View Post
When you believe that a final drive mod or changing wheel weights will produce more power that is a laughable misconception (you posted about this right here.
Confused...how can changing the weight of a wheel NOT have an effect on power to the ground (everything else being equal including tire size)?
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      02-28-2011, 03:29 PM   #305
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Confused...how can changing the weight of a wheel NOT have an effect on power to the ground (everything else being equal including tire size)?
In short when you change a wheel weight (or any drivetrain inertia) you affect performance (acceleration) but you do not affect the power.

To understand this fully it pays to look at a simple case. Let's consider the case of a car moving in a straight line first. If you add a passenger to your car does it change the cars power output (crank or wheels)? No, however, it does change it's acceleration. This is simply changing one side or the other of Newton's Law, force = mass x acceleration, F = ma. Change the engine and make more output and you change the left side, change the mass and you change the right side. Both will affect acceleration.

Now when we add the effects of rotating bodies we simply realize that rotating bodies translate along with all of the sprung mass of the car and they spin up relative to the translating car. All changes to rotating masses or moments of inertia change the sides of the equations representing the mass (or intertia) and thus bodies resistance to forces or torques. They do not change the "source" the force or torque terms (and thus power as well).

However, if you use the "wrong" type of dyno (say Dynojet - inertial dyno) to measure a before and after where a wheel (or drivetrain component) changes its mass or inertia you will get a different number because an inertial dyno simply assumes the same inertial mass for every car. A hub (brake) dyno will not show a power gain from modified drivetrain masses or inertias.

These subtleties are certainly worth understanding in detail. Hope that helps.

P.S. I see you are in San Diego as well. Ever come to the San Diego e90post.com meets?
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      02-28-2011, 03:43 PM   #306
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Quote:
Originally Posted by swamp2 View Post
In short when you change a wheel weight (or any drivetrain inertia) you affect performance (acceleration) but you do not affect the power.

To understand this fully it pays to look at a simple case. Let's consider the case of a car moving in a straight line first. If you add a passenger to your car does it change the cars power output (crank or wheels)? No, however, it does change it's acceleration. This is simply changing one side or the other of Newton's Law, force = mass x acceleration, F = ma. Change the engine and make more output and you change the left side, change the mass and you change the right side. Both will affect acceleration.

Now when we add the effects of rotating bodies we simply realize that rotating bodies translate along with all of the sprung mass of the car and they spin up relative to the translating car. All changes to rotating masses or moments of inertia change the sides of the equations representing the mass (or intertia) and thus bodies resistance to forces or torques. They do not change the "source" the force or torque terms (and thus power as well).

However, if you use the "wrong" type of dyno (say Dynojet - inertial dyno) to measure a before and after where a wheel (or drivetrain component) changes its mass or inertia you will get a different number because an inertial dyno simply assumes the same inertial mass for every car. A hub (brake) dyno will not show a power gain from modified drivetrain masses or inertias.

These subtleties are certainly worth understanding in detail. Hope that helps.

P.S. I see you are in San Diego as well. Ever come to the San Diego e90post.com meets?
It does make sense and I understand the example of adding a passenger vs. being a lonely driver...but...if you change the mass of an object (wheel) that you want to rotate (we'll say decrease it) against a surface, then it becomes easier to rotate it (less force). So, doesn't it stand to reason that if less force is needed to rotate that object ,the translation of applying the same input power is that the object (wheel) will spin faster (increased acceleration)? If so, then if we draw a line on the ground, the power measured when the vehicle crosses over that line with the heavier rotating object (wheel) will be less than the power measured when the vehicle crosses over that line with the lighter object (wheel). Essentially, you are at a higher RPM when you cross the line with the lighter wheel (screw the object reference) then you were with the heavier wheel.

Are the e90 meets in Clairemont on Wednesdays? If so, I never know which Wednesday is a meet day (I believe it's every other Wednesday).
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      02-28-2011, 08:35 PM   #307
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Originally Posted by Dodge2Dub View Post
It does make sense and I understand the example of adding a passenger vs. being a lonely driver...but...if you change the mass of an object (wheel) that you want to rotate (we'll say decrease it) against a surface, then it becomes easier to rotate it (less force). So, doesn't it stand to reason that if less force is needed to rotate that object ,the translation of applying the same input power is that the object (wheel) will spin faster (increased acceleration)? If so, then if we draw a line on the ground, the power measured when the vehicle crosses over that line with the heavier rotating object (wheel) will be less than the power measured when the vehicle crosses over that line with the lighter object (wheel). Essentially, you are at a higher RPM when you cross the line with the lighter wheel (screw the object reference) then you were with the heavier wheel.
You're not adding/increasing power, you're freeing up some normally lost power (perhaps too little to notice in most cases). Keep in mind also where the wheel weight is located makes a difference to rotating mass efficiency. The outer most area is most important in terms of mass/lightness than the center. Switching too an overall lighter wheel helps (as it's probably well engineered and lighter in the rim area over stock). So does switching to a lighter tire. Tires can vary considerably in weight. PS2s are very light. Some alternatives can be several pounds heavier each. Something to consider when replacing tires. But anything toward reduced unsprung wight has many benefits (acceleration, braking, cornering). Ceramic brake discs only weigh 1/4 the weight of OEM. That's a huge reduction. Too expensive to add on but reasonable as an option extra for cars so equipped. Lighter wheels are hard to feel in a road car. On a motorcycle, carbon or magnesium (or forged aluminum) make a gigantic difference vs cast aluminum in terms of performance. Extremely noticeable.

As for reducing weight of engine parts such flywheels and clutches, there you would feel an appreciable difference in how fast the motor builds power. The downside is the power is not stored as easily due to less rotating mass and the power can be somewhat of a light switch, making it harder to apply power in bends. On a Ducati I once had, I replaced a 4 lb. flywheel with a 8 oz one. Power delivery was incredible, and so was the engine braking! Off throttle was like applying the brake. It was also difficult to keep the front wheel on the ground. I had also replaced the clutch with a considerably lighter one. So the rotating mass was incredibly affected.
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      02-28-2011, 09:59 PM   #308
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Quote:
Originally Posted by MVF4Rrider View Post
You're not adding/increasing power, you're freeing up some normally lost power (perhaps too little to notice in most cases). Keep in mind also where the wheel weight is located makes a difference to rotating mass efficiency. The outer most area is most important in terms of mass/lightness than the center. Switching too an overall lighter wheel helps (as it's probably well engineered and lighter in the rim area over stock). So does switching to a lighter tire. Tires can vary considerably in weight. PS2s are very light. Some alternatives can be several pounds heavier each. Something to consider when replacing tires. But anything toward reduced unsprung wight has many benefits (acceleration, braking, cornering). Ceramic brake discs only weigh 1/4 the weight of OEM. That's a huge reduction. Too expensive to add on but reasonable as an option extra for cars so equipped. Lighter wheels are hard to feel in a road car. On a motorcycle, carbon or magnesium (or forged aluminum) make a gigantic difference vs cast aluminum in terms of performance. Extremely noticeable.

As for reducing weight of engine parts such flywheels and clutches, there you would feel an appreciable difference in how fast the motor builds power. The downside is the power is not stored as easily due to less rotating mass and the power can be somewhat of a light switch, making it harder to apply power in bends. On a Ducati I once had, I replaced a 4 lb. flywheel with a 8 oz one. Power delivery was incredible, and so was the engine braking! Off throttle was like applying the brake. It was also difficult to keep the front wheel on the ground. I had also replaced the clutch with a considerably lighter one. So the rotating mass was incredibly affected.
You're misinterpreting what dodge said. He said that all else equal, lighter wheels will increase power to the ground. Not power at the crank, but power to the ground (i.e. after all drivetrain losses). Wheel/tire rotating mass is a obviously component of drivetrain loss, so what he said was correct.
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