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Old 05-20-2012, 11:39 PM   #876
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Quote:
Originally Posted by Calamity Jesus View Post
You do it since you're the one making the claim. I'd like to see how increasing mass increases the force that the engine can produce.
It's not about changing the force that an engine can create, it's about the counter force created by increase friction and mas of a heavy vehicle. The force on the transmission is a function of both the engine torque, and the opposing torque created by the wheels resistance to movement.
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Old 05-21-2012, 09:02 AM   #877
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Originally Posted by LastResort View Post
It's not about changing the force that an engine can create, it's about the counter force created by increase friction and mas of a heavy vehicle. The force on the transmission is a function of both the engine torque, and the opposing torque created by the wheels resistance to movement.
exactly.
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Old 05-21-2012, 11:15 AM   #878
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Are you talking about steady state cruising?

I'm talking about WOT full accleration pulls.. there should be no difference in torque transferred through the transmission.. the car will just accelerate more because it's lighter.

The mass of the vehicle cannot put any more force into the transmission than the engine puts in. Period. The only thing the mass does is create an equal and opposite force.

more mass with a smaller acceleration = a given force = less mass with a larger acceleration


I think you guys are confusing the heat and friction produced due to extra throttle needed to maintain or reach a given speed which would be higher in a heavier vehicle due to more power being put through it. That kind of heat and friction in the WRX transmission is not what snaps gears during 2nd gear pulls in highly modded WRXs... it's the transmission's inability to take high torque loads. The maximum torque seen at the gears does not depend on the weight of the car.
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Old 05-21-2012, 11:26 AM   #879
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Originally Posted by Calamity Jesus View Post
more mass with a smaller acceleration = a given force = less mass with a larger acceleration
More mass created more inertia. More inertia means more resistance to velocity change.

try this... take a tissue and put on your first and punch through the air. Did the tissue tear...no because there was no resistance force to tear it. Now hold the tissue with your other hand and punch thorugh it. Did the force of you punch change at all? No, but since the tissue could not accelerate at the same rate as your had as before it tore. Its not the force that brakes things, it's resistance to that force.
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Old 05-21-2012, 01:33 PM   #880
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The ability to move a tissue vs a hand is power.. not force. A human is unable to apply the same force to a piece of loose tissue as a punched hand simply because the tissue would accelerate too fast for the hand to move with it.. and it's also a completely different scenario given that the tissue isn't attached to anything while the hand is attached to muscles which provide their own force.

Okay, lets start over. Lets say we have two carts. One weighs 50kg and the other weighs 100kg. Now, lets say we've got a pair of identical rocket motors that produce 100kg of thrust (this is force equal to the weight of 100kg, nothing more, it can also be expressed as ~1000 Newtons). If you put the carts down and light both of the motors at the same moment, the 50kg cart will accelerate at 2G while the 100kg cart will accelerate at 1G.

At 1G of acceleration the momentum of the 100kg cart pushes back against the motor with 100kg of force.
(f=ma) 1000Newtons = 100kg * 1G

At 2G of acceleration the momentum of the 50kg cart pushes back against the motor with 100kg of force.
(f=ma) 1000Newtons = 50kg * 2G

You see.. both carts push back an equal amount to the thrust provided. Half of the mass results in twice the acceleration with the same force applied.

Taking a 220lb-ft engine out of a 2,200lb WRX and putting it into a 1,800lb 818 doesn't reduce the amount of torque the engine produces... nor does it reduce the amount of force that the car pushes back against the motor with. The transmission is going to see the same forces at WOT regardless of what vehicle it is in.


Really, to suggest otherwise is to ignore Newton's 3rd Law of Motion.
The mutual forces of action and reaction between two bodies are equal, opposite and collinear.

Last edited by Calamity Jesus; 05-21-2012 at 01:38 PM.
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Old 05-21-2012, 02:55 PM   #881
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I understand what you are saying now. The force provided by the motor is constant. The increase in acceleration is the manifestation of the fact that the engine now has a proportional excess of force, as compared to the force required to move a wrx at the same rate of acceleration.
The only reliable way to increase the longevity of the transmission, is to limit the force provided by the motor so as to maintain the stock wrx rate of acceleration. (But why bother with the 818 if that is the case?)
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Old 05-21-2012, 03:12 PM   #882
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I don't think anyone will have problems with a mostly solid transmission in the 818 unless they want to build it into a big power track car.. but then, just like the WRX, they'll need a better gearset or figure out how to install a STI trans. I mean, a stage 2 WRX motor will make the 818 plenty fast for most and I doubt we'll see many broken transmissions from that.

This whole discussion is really just some mental circle jerking since there's not much else to talk about.
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Old 05-21-2012, 04:52 PM   #883
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Quote:
Originally Posted by Calamity Jesus View Post
The ability to move a tissue vs a hand is power.. not force. A human is unable to apply the same force to a piece of loose tissue as a punched hand simply because the tissue would accelerate too fast for the hand to move with it.. and it's also a completely different scenario given that the tissue isn't attached to anything while the hand is attached to muscles which provide their own force.

Okay, lets start over. Lets say we have two carts. One weighs 50kg and the other weighs 100kg. Now, lets say we've got a pair of identical rocket motors that produce 100kg of thrust (this is force equal to the weight of 100kg, nothing more, it can also be expressed as ~1000 Newtons). If you put the carts down and light both of the motors at the same moment, the 50kg cart will accelerate at 2G while the 100kg cart will accelerate at 1G.
You are correct, I was using the term force, when I should have been using Power, but was trying to convey the concept of intertia when talking about applying force and how mas does play a role. But your equation is still overly simplified.



So lets just take the standard solid cylindrical object of diameter D (radius R=D/2) and mass, M

Jaa=1/2 MR^2

Jaa =mass moment of inertia about axis aa (polar moment of inertia)


the greater the mass the greater required Jaa.

if you start talking about shaft drives:
Jtot = Jmotor armature + Jload

And once you start adding gearing:

Jtot = Jmotor armature + Jgear 1 + (N1/N2)2 [Jgear 2 + Jgear 3 + (N3/N4)2 {Jgear 4 + Jload}]

Yeah, I am working with knowledge I haven't touched in many years, but mass will play a part in the mechanical stress, even if the force is the same.
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Old 05-22-2012, 10:50 AM   #884
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F=MA is all fine and dandy when your are talking about an object in linear motion.... but all this transmission/engine/wheel stuff isn't in linear motion. its all in rotational motion... tau = I*alpha , tau = Torque, I = mass moment of inertia, and alpha = angular acceleration.... second law still applies to some extent, but doesn't really tell us anything, and it sure doesn't get us closer to knowing anything about the stresses in the transmission. this isn't a textbook problem that can be solved by a quick wiki search...

by the same logic (F=MA), a car driving a constant speed up a hill has no force on the transmission because it is not accelerating (velocity is constant).....
The problem is that in this case the sum of all forces are zero. the wheels provide a force equal to that of gravity times the mass of the vehicle, in this scenario a change in mass has a huge roll in the forces involved. so please explain how the mass isn't important???... people are forgetting about the importance of the opposing forces, we don't live in a vacuum here on earth....

also don't forget that drivetrain shock is probably the biggest killer of subaru transmissions, if you don't think drive train shock will be "generally" reduced in a lighter car, then i really don't understand where your heads at...

Last edited by StatGSR; 05-22-2012 at 11:33 AM.
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Old 05-22-2012, 11:46 AM   #885
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But to kill your transmission with drivetrain shock you'd have to have an engine capable of producing the torque needed to kill the transmission right? And that amount of torque does not change as a result of the mass of the car. How it's applied to the transmission and what parts must absorb/transmit the force from the engine do change on the mass of the car.

Is that correct?
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Old 05-22-2012, 12:37 PM   #886
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^ yup, as well as the mass of any rotating equipment.
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Old 05-22-2012, 04:46 PM   #887
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Quote:
Originally Posted by Calamity Jesus View Post


force = mass * acceleration


The engine generates the force.. that part doesn't change. The mass changes, so the acceleration changes. Force is what breaks transmissions.. not mass or acceleration.

I agree that it's simple physics. Very simple.
The force in the equation is that of the force to move the vehicle forward...I believe I worded as the force to break the gearset (ignoring wheelspin or shock loading).
Also you contradict your own statement

Quote:
Originally Posted by Airplay355 View Post
I think he's saying an engine's ability to produce force is constant, ie X engine produces Y force at Z point in it's power band. If you change the mass of the car, by removing parts/replacing with lighter parts, mass goes down and thus acceleration must go up since force is constant. If mass goes up, like when you drive with a carload of dead hookers, acceleration must go down.

Breaking transmissions comes from having an engine that can produce enough force to break the transmission. M and A don't affect how much force the engine produces.
Pretty much what I'm getting at.

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So how about we try to do apples to apples. Take a 300torque motor. Take a 3100 pound car and an 1800 pound car. Based on the final drive and speed of say 3rd gear at peak torque(300 at throwing a random number 4500rpm). Wouldn't the 1800 pound car be accelerating faster then the 3100 pound car? Couldn't you calculate a more accurate accel for the WRX vs the 818 then simply making them the same? Also another note, the WRX is not 1636KG, it's 3100 pounds(1406kg, 230kg difference).

Would love to hear what you have to say.
I'm trying to make a complex simplified, which is proving to be not simple

Yes, if both cars have the same torque curve, and one car is half the mass of the other car, then the half-mass car should have twice the acceleration...neglecting wheelspin, clutch slip, driveline losses, rolling resistance, drag, bird crap on the hood, dead hookers in trunk, etc


Quote:
Originally Posted by LastResort View Post
It's not about changing the force that an engine can create, it's about the counter force created by increase friction and mas of a heavy vehicle. The force on the transmission is a function of both the engine torque, and the opposing torque created by the wheels resistance to movement.
THANK YOU!!! my LastResort gets it

The opposing force is what I'm talking about.

NOTE: the use of mass and force in the upcoming part as I'm using pounds and it confuses many. Not lb-mass and lb-force. Force is mass * the 1G in this scenario.

I think the easiest example would be if a 200lb man (dead hanging = 1G...our engine) is using a wooden lever (fulcrum...our transmission) to move an item at the other end...To simplify, we'll say the fulcrum is a 1:1 ratio.

Let's say it's a 2" wooden dowel, to give some visual aid, and it can only handle 150lb-force. The wooden dowel is like our transmission and it is the path that our force is projected through.

Scenario 1)
We have on the other end a 100lb bag.

When the man goes to hang on one end of the dowel, the 100lb bag lifts. He's applied his 200lb-force to the stick and moved the bag. Because the resultant force (mass of bag and the same 1G of acceleration acting on it) is only 100lb-force, the stick only saw 100lb-force...though the man applied his 200lb force.

Scenario 2)
Now he has a 200lb rock on the other end.

Though the man can apply his 200lb-force force, the stick (transmission) is only capable of holding 150 lb-force. So the stick breaks. The stick could only handle 0.75 G acceleration of the man's 200lb-mass....the 150 lb-force rating.

I hope this can clarify some of what I was trying to project.

To those that understood the above, you can then change the ratio of the fulcrum, much like the gears in a transmission. The gears are a torque multiplier!

This is the reason why in semi-trucks that we actually have to derate the torque curve according to gear (well I don't do semi powertrain consulting anymore). The mass of the truck (loaded) is great enough to break axles, u-joints, diffs and turn driveshafts into pretzels...though when the truck is unloaded we can give it full torque
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Old 05-23-2012, 02:49 AM   #888
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I don't see why you would hold acceleration constant in your two scenarios since it will vary with the mass of the car.

What do you think of this:

The feet of our man are anchored to the ground and he is capable of applying a 200lb force at 1G and a 100lb force at 2G. If you ask him to lift the 100lb rock at 2G and then the 200lb rock at 1G, which scenario is more likely to break the stick?
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Old 05-23-2012, 11:43 AM   #889
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well in my man hanging on a stick scenario, it would be kind of hard to change gravity

but your update carries the same math.

I still don't understand why it is so hard for people to understand this
It's perhaps one of the most simplistic formulas and you can add what ever mass or acceleration you want.

In my scenario we could consider the WRX being the 200lb rock and pulling .75G while at the stick's limit. Then the 100 lb bag being the 818 and being able to pull 1.5G while at the stick's limit.

I still expect to hear of ring and pinion failures.
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Old 05-23-2012, 04:34 PM   #890
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So, we agree that the stick (aka transmission) will handle 200lb @ .75G and 100lb @ 1.5G, but you're saying that when the same engine is installed in either chassis, the 100lb rock will not see the proportionate increase in acceleration?
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Old 05-23-2012, 04:53 PM   #891
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It may see the same acceleration. The power is there to do it. I didn't say the trans wouldn't break. I said the acceleration would have to come up to make the same force exerted on the trans....hence all of these scenarios don't take into account numerous other scenarios.

I know when my old 94 WR'L' sedan was RWD, I could roll into second gear and break the rear end free with ease on 225 Azenis RT215's...at 220whp on a heart breaker Mustang (stock 05 STi's were making 205-210). I also grenaded two R160 diffs (4.11 and 4.44) and then went 3.9 R180 and snapped a DSS 'Stage 5 1000hp axle'
The 3.9 holding the power then lead to two gearset failures. It then went 6-spd and back to AWD.
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Old 05-23-2012, 05:11 PM   #892
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Quote:
Originally Posted by Homemade WRX View Post
It may see the same acceleration. The power is there to do it. I didn't say the trans wouldn't break. I said the acceleration would have to come up to make the same force exerted on the trans....hence all of these scenarios don't take into account numerous other scenarios.
That's my point. I am one of the few people (idiots or whatever name you want to use for me) over at the FFR forums that is skeptical that the transmission can handle significantly more power if its in an 1800lb chassis instead of a 3100lb chassis.

So many people have said something along the lines of "duh, of course less weight is easier to move, F=ma, mass is going down, so the force is less." And I have said, "well, if mass goes down, what happens to acceleration?"

For some reason, the instinct of most people seems to be that acceleration can be ignored somehow. Everyone acknowledges that the 300hp 818 will accelerate faster than a 300hp WRX, but many people seem to think that the corresponding increase in acceleration due to weight reduction will not also proportionately increase transmission stress at the same time.

One counter argument is that the engine has a finite limit of acceleration, which is true of course, as can be easily seen be depressing the clutch and reving the engine. But is going from a 3100lb chassis to an 1800lb chassis a big enough change to make all the rotating and reciprocating engine/drivetrain mass a significant factor in 818 acceleration? As chassis weight gets smaller, it plays a bigger role, but at these numbers, I don't think it means that the trans will stand up to huge increases in power over what it can take in a WRX.
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Old 05-23-2012, 05:41 PM   #893
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more than just the engine is the turbo as well. Same reason you don't see the same boost in first gear than second or second to third.

When I had my 4.44, I had to be in fourth gear to have the full 'girth' of the VF39's torque curve. Keep in mind that the torque curve is heavily dependent on the turbo.
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Old 05-23-2012, 05:44 PM   #894
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Yes, but now you're starting to get away from holding power constant. Remember, the question is how much more power, if any, will a transmission take in an 818 compared to a WRX.
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Old 05-23-2012, 05:59 PM   #895
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No, I'm just getting down the some details of the reality for the actual scenario where traction, drag, rolling resistance and torque curve aren't consistent...or constant
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Old 05-23-2012, 06:15 PM   #896
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There's little value in getting into the details if you are just bringing them up to say "I don't know, it's complicated" unless you're going to make the argument that we shouldn't even have the discussion.

Oh well, I've made my point as clear as I care to. I can't tell how much we agree or disagree, but it's just not important.
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Old 05-23-2012, 06:18 PM   #897
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I think the discussion is just that pointless is what I'm getting at. It's a bunch of e-racers arguing over a hypothetical thing that in the real world has so many variables, it's silly to try and argue.

I don't think the 5-spd gearset is going to sustain much more power than it does in a WRX when put into the 818 however for the same power, the car will be much quicker.
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Old 05-23-2012, 06:31 PM   #898
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No, I'm just getting down the some details of the reality for the actual scenario where traction, drag, rolling resistance and torque curve aren't consistent...or constant
^ this,

and like i said, F=MA has nothing to do with the transmission at all. the only F that can be used in that equation is the force that is exerted by(to) the tire to(by) the ground.

when you want to work that back to the transmission, then its all rotational motion, tau = I*alpha , tau = Torque, I = mass moment of inertia, and alpha = angular acceleration. Even then, you have know a little something about stresses and material strengths to determine anything useful about failure levels.

simply put Mass really only changes one thing the the transmission can concerned about, and that is the decreased friction force where the tire meets the road (friction is only decreased so long as contact patch is decreased. the amount of friction is based on both tire compound, road surface, temperature etc.). The tire is a "fuse", if its slips, it helps protect the transmission, but just because it's slipping doesn't mean that force isn't being applied back through the system, and can even increase the frequency of shock forces if there is any form of wheel hop....

i just don't think people are grasping how difficult an problem/equation this is. that's why we all just need to see what happens when people start messing around. honestly i figure it will be a new transmission problem such as the front diff that will want to give out first (same reason a rear diff wants to give out on a RWD conversion). its just too hard to say at this point, other parts may live longer, and in there place, something new will break....
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Old 05-23-2012, 07:03 PM   #899
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I just went to one of the simple online 1/4 mile calculators and put in some numbers. A theoretical 3600 lb 300 hp car (STI w/driver) would do the 1/4 mile in 13.335 seconds and trap at 102.2 mph for an acceleration of 3.44 m/s^2. A theoretical 2000 lb 300 hp car (818 w/driver) does it in 10.963 seconds at 124.33 mph for an acceleration of 5.05 m/s^2.

Comes out to 5617 N force on the 3600 lb car, and 4581 N on the 2000 lb car or about 18% less force experienced by the theoretical 818.

Again, I'm no engineer so please shoot holes in this if it's way off base, but I did stay at a Holiday Inn express last night.
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Old 05-24-2012, 12:26 AM   #900
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Holiday in Express, score!!

I'm curious what/how many variables they summarized and used in the formula for the calculation. It's obvious that they were taken into account. I'm assuming drag and rolling resistance are in there as they increase greatly (exponential for drag and I believe rolling resistance IIRC) with speed, while the power is still low...I just got in a from a few beers with friends, so excuse any errors

As for StatGSR, I completely agree. I was asked to come and comment on this and I tried my best to give a very diluted and simple answer and you can see how that even confused some. I actually have a minor in motorsports engineering where what we do is model car simulation for performance aspects and apply them to real world...so yeah, many things go into it and even with engineering, it is nothing but getting into the ballpark with accuracy dependent upon the variables/constraints on the model.

As for the diff failures, I do know that the front 'R160' ring and pinion gears are much beefier than the rears, so I suspect they won't be quite as fragile. Spider gears are another story. As for people making big power, I think the reduced weight and the given speed will keep the power down...traction or not.

My $.02,
Micah
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