Weighed: F80 basic stripper configuration - 3,450 pounds

[CSBM5]

Originally Posted by swamp2

Incorrect. Rotating mass can always be seen as an equivalent non rotating mass. It is that simple. The effects are therefore constant across speed and act the same for acceleration and braking.

Inertia effects are not constant with respect to acceleration -- that is what I was referring to.

[myzmak]

Originally Posted by mkoesel

It isn't typically mentioned because this isn't a dating website so personal information is usually not brought into the discussion. And height is irrelevant since it can't be changed. So that can be left out completely (like you conveniently did with yours).

Experience tells me that the vast majority of BMW owners are not obese, so you are talking about diminishing returns. I'd venture to guess that most M3/M4 owners are significantly less than 100 lbs. from their ideal weight. For a random sampling, if you wish to satisfy your apparent curiosity, check out some of the ED picture threads. Not many fatties to my recollection.

More to the point, it doesn't matter. If I weigh 155 or 255 the car doesn't care and a heavier car will react the same to me either way vs a car that is 100 lbs lighter.

[bruce.augenstein@comcast.]

Originally Posted by CSBM5

Inertia effects are not constant with respect to acceleration -- that is what I was referring to.

You are sort of, kind of correct - but speed isn't the direct issue. It's gearing.

You have more rotational inertia in first gear than, say, second gear, because everything north of the transmission tail shaft has to pick up more revs per MPH gained - and per Sir Isaac, they don't like doing that, and thus resist.

[swamp2]

Originally Posted by CSBM5

Inertia effects are not constant with respect to acceleration -- that is what I was referring to.

And that is still incorrect.

Again, on a gear by gear basis, all rotating mass in a vehicle can be mathematically and precisely converted to a FIXED equivalent mass. So back to the wording of your original post it is incorrect to say that they are a FUNCTION OF ACCELERATION. At different acceleration levels in the SAME gear the effects of rotational inertia are again, constant, they are nothing but a constant additional non-rotating mass. This is not an approximation but a rigorous and precise mathematical/physics result. Yes overall as change gears ACROSS multiple gears in a WOT acceleration run the equivalent mass changes to different specific fixed values (as Bruce emphasized above). But it has nothing to due with acceleration (nor speed), only the component intertias, final drive ratio, individual gear ratio and wheel size, period.

[CSBM5]

Originally Posted by swamp2

And that is still incorrect.

Again, on a gear by gear basis, all rotating mass in a vehicle can be mathematically and precisely converted to a FIXED equivalent mass. So back to the wording of your original post it is incorrect to say that they are a FUNCTION OF ACCELERATION. At different acceleration levels in the SAME gear the effects of rotational inertia are again, constant, they are nothing but a constant additional non-rotating mass. This is not an approximation but a rigorous and precise mathematical/physics result. Yes overall as change gears ACROSS multiple gears in a WOT acceleration run the equivalent mass changes to different specific fixed values (as Bruce emphasized above). But it has nothing to due with acceleration (nor speed), only the component intertias, final drive ratio, individual gear ratio and wheel size, period.

I think we're saying the same thing? My point, however poorly it was presented, is that the effect of changing rotational inertia (such as lighter wheels with identical radius of gyration) is highest when acceleration is highest. If there is no acceleration, changing inertia has no effect. What you're telling me is this isn't true?

[bruce.augenstein@comcast.]

Originally Posted by CSBM5

I think we're saying the same thing? My point, however poorly it was presented, is that the effect of changing rotational inertia (such as lighter wheels with identical radius of gyration) is highest when acceleration is highest. If there is no acceleration, changing inertia has no effect. What you're telling me is this isn't true?

What swamp was explaining to you is that the effect of rotational inertia is constant, regardless of the rate of acceleration.

Said another way, if you can accelerate at a given rate at a given speed in a given gear, doubling the torque will give you exactly twice the acceleration. This would be true on the road, or on a (horrors!) chassis dyno.

[swamp2]

Originally Posted by CSBM5

I think we're saying the same thing?

No we are not.

Originally Posted by CSBM5

My point, however poorly it was presented, is that the effect of changing rotational inertia (such as lighter wheels with identical radius of gyration) is highest when acceleration is highest.

Very loosely there is some truth in this. However, it clouds the issues with an unneeded lack of precision and creates confusion. It is kind of like correlation vs. causation. An good example might be the following: High vehicle speeds are correlated with high engine oil temperatures. However, we clearly know that a high vehicle speed does not CAUSE high engine oil temperatures. High engine rpm is the root cause and in general high speeds and high rpms are correlated but in no way causal. Obviously stick to 1st gear and high rpms give relatively low overall speeds. Seems overly simplistic but it is almost identical to your fallacy.

So AGAIN, the effect varies by GEAR not by acceleration. Yes in general lower gears = more potential acceleration but it does not always equate precisely to more actual acceleration.

On a minor secondary point I think with regards to wheels a much more interesting case is that of wheels that are identical in weight but have different moments of inertia. Here, a vehicles with identical power to weight will accelerate slightly differently (in highly accurate, highly controlled tests, which are required due to practical differences that would be very small) due to the wheels different intertia. Same thing happens in the rest of the drivetrain. Probably no debate on that particular point.

Originally Posted by CSBM5

If there is no acceleration, changing inertia has no effect. What you're telling me is this isn't true?

I've not at all addressed the case of a vehicle not accelerating. However, you can not conclude that component inertia being irrelevant during steady state speed IMPLIES that effective mass varies with acceleration. That is simply completely false. Again, analogy to oil temperature case again...

In short, please have a look at my long-ish post on drive train inertia here. The math shortcuts to the heart of the issue and provides precision lacking in some of the language descriptions. Just ignore all of the details about the rough calculation of mass factors, focus on the explanations and equations. I do hope this will help provide some additional insight and clarity for you here.

[CSBM5]

Originally Posted by swamp2

Very loosely there is some truth in this. However, it clouds the issues with an unneeded lack of precision and creates confusion. It is kind of like correlation vs. causation.

Thanks for the details. It's been almost 40 years since dynamics class, so recall is in question here. I was stuck on a super simple image in my mind (note: I have to keep things simple) -- that of a wheel/tire assembly, in a lab class, attached to a bearing along with a device to measure torque. The experiment would accelerate the wheel from zero to a fixed velocity (say equating the tire turning at 60mph) using an acceleration profile typical of a car (highest acceleration at lowest speeds, falling with increasing velocity) -- again trying to be super simple, but definitely not a constant acceleration since that is not similar to a car.

I then had the thought of decreasing the moment of inertia of the wheel/tire assembly (say perhaps a 30% lighter wheel, whatever) and rerunning the experiment. I was assuming that I would be able to measure the greatest effect of this when acceleration is at its highest. The higher we crank acceleration of the assembly, the more "useful" the lighter assembly (scratch that, the one with the lower moment of inertia) becomes (assuming one cares to reduce the amount of energy input into the system through the process or said the other way, achieve more output for the same energy input).

Probably too simple of an example, and I may in fact be missing something in the above, but that was my simplistic thought. So before destroying Joe's thread any longer, am I correct in the above? (or has it been "that long" I need to start review...my specialty in grad school ended up being heat transfer/fluids and it's been ~25 years since I have worked directly in that field).

Thanks,
Chuck

[swamp2]

Originally Posted by CSBM5

Thanks for the details. It's been almost 40 years since dynamics class, so recall is in question here. I was stuck on a super simple image in my mind (note: I have to keep things simple) -- that of a wheel/tire assembly, in a lab class, attached to a bearing along with a device to measure torque. The experiment would accelerate the wheel from zero to a fixed velocity (say equating the tire turning at 60mph) using an acceleration profile typical of a car (highest acceleration at lowest speeds, falling with increasing velocity) -- again trying to be super simple, but definitely not a constant acceleration since that is not similar to a car.

I then had the thought of decreasing the moment of inertia of the wheel/tire assembly (say perhaps a 30% lighter wheel, whatever) and rerunning the experiment. I was assuming that I would be able to measure the greatest effect of this when acceleration is at its highest. The higher we crank acceleration of the assembly, the more "useful" the lighter assembly (scratch that, the one with the lower moment of inertia) becomes (assuming one cares to reduce the amount of energy input into the system through the process or said the other way, achieve more output for the same energy input).

Probably too simple of an example, and I may in fact be missing something in the above, but that was my simplistic thought. So before destroying Joe's thread any longer, am I correct in the above? (or has it been "that long" I need to start review...my specialty in grad school ended up being heat transfer/fluids and it's been ~25 years since I have worked directly in that field).

Thanks,
Chuck

Not at all the same case... Also depends though a bit on exactly what "effect" you measure.

This case is simply torque = moment of inertia x angular acceleration. If in the two cases of different I's with perhaps an identical applied torque then the new angular acceleration would simply be the initially measured angular acceleration multiplied by the (initial I/new I). Now that being said constant torque provides constant angular acceleration. Constant angular acceleration yields an angular velocity that rises with time (seems likely that angular velocity would be measured). As time progresses the gap between these two angular velocities grows. That could be the "widening" effect you are thinking of. A bit of a stretch, who knows... Cheers.

[e39>all]

This thread managed to get derailed into some rather complicated shhh .... stuff

[SakhirM4]

Originally Posted by e39>all

This thread managed to get derailed into some rather complicated shhh .... stuff

Yeah, I feel like we started off in Weight Watchers class and ended up in MIT.

[ciressiws]

Just join a club and drive it on the track in a driver school.
You will find out all the dynamics in a practical way.
And leave the silly drifting to adolescents.

[CSanto]

Originally Posted by CanAutM3

According to BMW, the 6MT to DCT weight difference is 89lb.

I think that was debunked here.
http://f80.bimmerpost.com/forums/sho...t=weigh&page=2

[CanAutM3]

Originally Posted by CSanto

I think that was debunked here.
http://f80.bimmerpost.com/forums/sho...t=weigh&page=2

Read the thread, and no it was not debunked at all.

In the meantime, I will stick in believing the BMW EU difference of 89lb

[rzm3]

Sorry to revive an old thread...

I finally had a chance to weigh my completely stock E92 stripper today on competition scales during an SCCA event...

E92 M3
19" wheels
DCT
Heated seats
1/4 tank gas
3,484 lb, or
3,460 lb dry

vs.

OP's F80
M-Adaptive Suspension
18" wheels
6 MT
3,432 lb dry (3,450-6 lb/gal*3 gal)

Delta: 28 lb

[Seagull]

Originally Posted by e39>all

This thread managed to get derailed into some rather complicated shhh .... stuff

"Mommy, I'm not having fun anymore"

:P

j/k

Very informative as usual on this forum!

[myzmak]

Originally Posted by rzm3

Sorry to revive an old thread...

I finally had a chance to weigh my completely stock E92 stripper today on competition scales during an SCCA event...

E92 M3
19" wheels
DCT
Heated seats
1/4 tank gas
3,484 lb, or
3,460 lb dry

vs.

OP's F80
M-Adaptive Suspension
18" wheels
6 MT
3,432 lb dry (3,450-6 lb/gal*3 gal)

Delta: 28 lb

Sorry, not to be difficult but was your car then not as configured in your signature? (Rear seat delete, racing seats, etc)

[rzm3]

Originally Posted by myzmak

Sorry, not to be difficult but was your car then not as configured in your signature? (Rear seat delete, racing seats, etc)

Correct. Gone but not forgotten!

[Rupes]

Originally Posted by rzm3

Sorry to revive an old thread...

I finally had a chance to weigh my completely stock E92 stripper today on competition scales during an SCCA event...

E92 M3
19" wheels
DCT
Heated seats
1/4 tank gas
3,484 lb, or
3,460 lb dry

vs.

OP's F80
M-Adaptive Suspension
18" wheels
6 MT
3,432 lb dry (3,450-6 lb/gal*3 gal)

Delta: 28 lb

Me too. My totally stock e92 M3 was weighed last week at a CAT certified scale at 3480. Had a quarter tank and left all floor mats, tire change kit, etc. In the car. Posted the results / slip in the e92 forums. I was surprised there wasn't as big of a weight delta as originally claimed by BMW.

[rzm3]

From karussell's M4:

6MT
YMB paint
Adaptive Suspension
USB bluetooth
1/4 tank gas

3,416 lb

Add 55 lb to convert to F82 DCT = 3,471 lb

vs. my stock E92 M3 3,484 lb

Where is the weight savings BMW??

[consolidated]

Originally Posted by rzm3

From karussell's M4:

6MT
YMB paint
Adaptive Suspension
USB bluetooth
1/4 tank gas

3,416 lb

Add 55 lb to convert to F82 DCT = 3,471 lb

vs. my stock E92 M3 3,484 lb

Where is the weight savings BMW??

Very light E92, nice. But it's not exactly stock as you state. Rear seats and bucket swap is a big weight savings. I've done it. My E90 M3 DCT weighed 3620# on reserve.

[gthal]

Originally Posted by myzmak

Sorry, not to be difficult but was your car then not as configured in your signature? (Rear seat delete, racing seats, etc)

Originally Posted by rzm3

Correct. Gone but not forgotten!

rzm3... doesn't that mean the delta is 28 lbs + the weight savings from the rear seat delete and racing seats? Meaning the delta is larger? Racing seats and rear delete must be worth 80-100lbs, no? Delta now 100+ lbs? What am I missing?