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Old 04-26-2012, 06:53 PM   #126 (permalink)
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i stand corrected.
jk
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Old 04-26-2012, 10:12 PM   #127 (permalink)
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I see now where you guy's are getting confused. You have failed to realize that momentum, that can be referred to as kinetic energy, grows exponentially with speed and is related directly to Mass.

Meanwhile, traction does not.

Double the speed, say from 50kph to 100kph, and you quadruple the kinetic energy.

The mass of the vehicle is related to kinetic energy;

Kinetic Energy (Joules) = ½ x mass x velocity2

The greater the mass the greater the kinetic energy, thus a heavier car will require a longer braking distance.

Momentum is also a force that has Vector. In other words it has both magnitude and direction. This explains why vehicles that have received an engine swap with a dramatically heavier engine and no suspension/brake upgrades will also handle like crap. And I'm not even going to start on about weight transfer and suspension loading dynamics under hard braking.

I'll tell you what, if you still have doubts about momentum, instead of this endless argument, why don't you place a link to a reputable website/organization (But please, not another forum where everyone's 2c is valid) that refutes my statement on momentum and it's relationship to braking distance? If you can prove me wrong, I will accept it......Deal?
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Old 04-27-2012, 01:17 AM   #128 (permalink)
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I see now where you guy's are getting confused. You have failed to realize that momentum, that can be referred to as kinetic energy, grows exponentially with speed and is related directly to Mass.

Meanwhile, traction does not.

Double the speed, say from 50kph to 100kph, and you quadruple the kinetic energy.

The mass of the vehicle is related to kinetic energy;

Kinetic Energy (Joules) = ½ x mass x velocity2

The greater the mass the greater the kinetic energy, thus a heavier car will require a longer braking distance.

Momentum is also a force that has Vector. In other words it has both magnitude and direction. This explains why vehicles that have received an engine swap with a dramatically heavier engine and no suspension/brake upgrades will also handle like crap. And I'm not even going to start on about weight transfer and suspension loading dynamics under hard braking.

I'll tell you what, if you still have doubts about momentum, instead of this endless argument, why don't you place a link to a reputable website/organization (But please, not another forum where everyone's 2c is valid) that refutes my statement on momentum and it's relationship to braking distance? If you can prove me wrong, I will accept it......Deal?
I think it's time for you to stop.
Momentum and kinetic energy are not the same, they don't have the same units or the same relationship with velocity.

That was just your first paragraph.

The people you are arguing against have a better understanding of physics, engineering mechanics and this braking topic and work in related fields. You have been proven wrong already, but you need to get the physics right to see your errors. This is the vicious circle you are in.

Take a look at the kinetic energy equation. You will notice kinetic energy is proportional to mass. Traction is also proportional to mass.
If you want to argue that kinetic energy and stopping distance are proportional to velocity squared, then you should re-read post 120.
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Originally Posted by Dougal in post 120 wrote
And all these physics models say the same thing, as long as your brakes can keep up then stopping distance is independent of weight.
If your brakes can't keep up, then stopping distance is proportional to total weight. Your stopping distance is always proportional to the square of your speed.

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Old 04-27-2012, 01:46 AM   #129 (permalink)
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Old 04-27-2012, 06:45 PM   #130 (permalink)
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I see now where you guy's are getting confused. You have failed to realize that momentum, that can be referred to as kinetic energy, grows exponentially with speed and is related directly to Mass.

Meanwhile, traction does not.

Double the speed, say from 50kph to 100kph, and you quadruple the kinetic energy.

The mass of the vehicle is related to kinetic energy;

Kinetic Energy (Joules) = ½ x mass x velocity2

The greater the mass the greater the kinetic energy, thus a heavier car will require a longer braking distance.

Momentum is also a force that has Vector. In other words it has both magnitude and direction. This explains why vehicles that have received an engine swap with a dramatically heavier engine and no suspension/brake upgrades will also handle like crap. And I'm not even going to start on about weight transfer and suspension loading dynamics under hard braking.

I'll tell you what, if you still have doubts about momentum, instead of this endless argument, why don't you place a link to a reputable website/organization (But please, not another forum where everyone's 2c is valid) that refutes my statement on momentum and it's relationship to braking distance? If you can prove me wrong, I will accept it......Deal?
Dude, you use momentum and energy like a drunk uses a lightpole - for support not illumination.

As far I could tell this argument started about whether a relatively small increase in weight/mass affected braking.

You can not win that because of a poor understanding and maths - you were simply wrong with your conclusions.

Now you are venturing into further into la la land, please don't start on the other stuff
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Old 04-27-2012, 09:09 PM   #131 (permalink)
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Dude, you use momentum and energy like a drunk uses a lightpole - for support not illumination.

As far I could tell this argument started about whether a relatively small increase in weight/mass affected braking.

You can not win that because of a poor understanding and maths - you were simply wrong with your conclusions.

Now you are venturing into further into la la land, please don't start on the other stuff
John, I wouldnt have picked you for the "dude" type...and I do like the drunk/lightpole analogy.

I getting the better braking part, Im sure there is a tipping point there somewhere and this would be based around the tyres used and standard brakes in good working order. I do feel that other perfromance areas are effected. Some may be better, like traction on hill climb? but others will be worse.....
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Old 04-28-2012, 08:31 AM   #132 (permalink)
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Listen Guy's, I am fully prepared to accept that I could be wrong. It certainly wouldn't be the first time, or likely the last. Certainly my understanding is not at the same level as an automotive engineer specializing in braking systems. I am a mechanic. But I really don't see where your argument for traction over momentum has any validity at speed?

And I'm going need a bit more than a condescending "dude, shut up, were smarter than you". Coming from folks that believe that locking up the brakes improves stopping distance I'm will take that self affirmation with a pinch of salt. You will need to prove it. No one has posted a link to a respected, 3rd party to disprove what I'm saying? If it is so obvious then a 30 sec Google search should turn it up?

So far all you have done is nit pick my posts and make vague generalizations and provisos (post 120) to purposefully avoid contradicting your first few posts. And if I am "leaning" on momentum then you are leaning on traction just as much. Please enlighten me just exactly how traction increases exponentially to the same degree as momentum? Outside, in the real world?

I would concede if two P38's, with one having a stock 4.0L V8 and the other a swapped in XXX brand diesel, brakes and suspension in perfect condition with new tyres on a perfect road surface under controlled conditions. At low speeds (30kph or less) there would probably be little difference in stopping distance between the two. But as soon as you get them up to greater speeds the heavier vehicle would take longer to stop.

You say that speed effects braking distance, and obviously it's true, but then you don't recognise that weight also has an adverse effect? Then you ignore other vehicle dynamics like weight transfer etc.

Now, take the vehicles out of the laboratory and into the real world (where you might find a P38 Range Rover) and you have a 10+ year old SUV, standard brakes (lets be generous and say in good condition) good tyres and standard suspension travelling at 80kph on a real world street. Traction is now variable in the extreme, were as the momentum is constant.

Last week I had a GM 14 bolt in the bed of my truck, it was harder to stop from 80kph than it was empty. The brakes and tyres are good. Why? How can this be? I am trying to explain what we have all experienced from wheelbarrows, bikes to 4wd's. Out in the world, it's harder to stop stuff that is heavy.
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Old 04-28-2012, 04:49 PM   #133 (permalink)
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Lets start with momentum.

P = Mass*velocity.
Units are Newton-Seconds:
http://www.physicsclassroom.com/clas...ntum/u4l1a.cfm

There is no exponential quantity anywhere in there. It is linear with mass and linear with speed.

Make sense now?

No-one has claimed that locked up wheels stop the shortest. We have consistently said that you need the ability to lock wheels or your brakes aren't good enough.
Stop making shit up and read what has been written.

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Old 04-28-2012, 06:00 PM   #134 (permalink)
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So where is the extra braking traction coming from to counteract the momentum at speed?
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Old 04-28-2012, 06:05 PM   #135 (permalink)
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In your case your truck could do with better brakes for the heavier load. When you have enough braking power to lock wheels up loaded, then you'll be able to find the same braking distaces loaded or unloaded by controlling the lockup.
Well, it must have been someone else that posted that then?
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Old 04-28-2012, 07:06 PM   #136 (permalink)
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So where is the extra braking traction coming from to counteract the momentum at speed?
I think what they are saying is that the extra 100kg sitting over the front axle is making better traction. I also think what they are saying is that the stock brakes in good working order in a Rover can be locked up unloaded or loaded. So adding only 100kg isnt going to take it past that point. Yes you have to control the peddle pressure, thus the braking force with your foot/brain, but if done right and right on the limit, you should be able to pull up the heavier deisel engined Rover same or bit better due to the added weight thus traction. Now if your stock Rover was already on the limit of traction braking force then added weight would push it over.

So yes there is more mass moving, but now more traction to help stop said mass.

have I got that right
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Old 04-28-2012, 07:24 PM   #137 (permalink)
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http://www.newyorkdefensivedriving.c...ample.html?p=6

http://www.procarcare.com/includes/c...pdistance.html

http://en.wikipedia.org/wiki/Vehicle_metrics

http://w3.shorecrest.org/~Lisa_Peck/...y_crashes.html

http://www.sdt.com.au/safedrive-dire...NGDISTANCE.htm

Dougal. There are many websites that seem to contradict you? They all list weight as a factor in increased braking distances?

Am I really missing something here? I'm done arguing, that is an honest question because during my search I also came across one or two sites (though nowhere near as many and mostly on forums) that indicated that the weight of the vehicle did indeed cancel out the momentum.
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Old 04-28-2012, 07:26 PM   #138 (permalink)
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I think what they are saying is that the extra 100kg sitting over the front axle is making better traction. I also think what they are saying is that the stock brakes in good working order in a Rover can be locked up unloaded or loaded. So adding only 100kg isnt going to take it past that point. Yes you have to control the peddle pressure, thus the braking force with your foot/brain, but if done right and right on the limit, you should be able to pull up the heavier deisel engined Rover same or bit better due to the added weight thus traction. Now if your stock Rover was already on the limit of traction braking force then added weight would push it over.

So yes there is more mass moving, but now more traction to help stop said mass.

have I got that right
Well, I guess that makes sense to me.
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Old 04-28-2012, 09:30 PM   #139 (permalink)
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I think what they are saying is that the extra 100kg sitting over the front axle is making better traction. I also think what they are saying is that the stock brakes in good working order in a Rover can be locked up unloaded or loaded. So adding only 100kg isnt going to take it past that point. Yes you have to control the peddle pressure, thus the braking force with your foot/brain, but if done right and right on the limit, you should be able to pull up the heavier deisel engined Rover same or bit better due to the added weight thus traction. Now if your stock Rover was already on the limit of traction braking force then added weight would push it over.

So yes there is more mass moving, but now more traction to help stop said mass.

have I got that right
Yes thank you. That's exactly it.
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Old 04-29-2012, 01:57 AM   #140 (permalink)
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It becomes simpler if you look at it in terms of energy conversion.

A vehicle traveling at any speed has an amount of potential energy equal to that required to accelerate it from rest to that velocity.
Obviously the heavier an object, the more energy is required to make it move.
To slow down again the objects kinetic energy needs to be reduced.

In a motor vehicle you have several sources of kinetic energy loss, be it aerodynamic resistance (friction through the air) or frictional loss in bearings and tyres, which is why a vehicle will eventually come to a halt if you push it on a flat surface and let go, and why you need the engine to keep it moving, albeit with a lot less output when running at a stable cruising speed.

Anyways, energy loss due to friction is a conversion of kinetic energy to heat, which is the same principal with brakes, albeit you have some control over this one.
The harder you press the pedal, the quicker things heat up, the faster the kinetic energy is converted, and the faster you slow down, obviously as you press the pedal the brakes will warm up as they generate heat, and more heat will be generated if your vehicle is heavier as it has more potential energy, so the brakes have to work that bit harder.

Obviously if the brakes get hot then they lose efficiancy as they can't convert the potential energy as quickly, which is why you work up to a bigger heat sink (disc) or make the ones you have more efficient at cooling, another energy transition as the disc heats the air around it, and in turn it's heat energy is lost.

Traction also comes into it, which is the frictional limit of the tyre upon the road surface. It's true that the more weight you put on top of something the more friction there will be (lay your hand flat and move it accross the desk, nice and easy? Now push down hard and move it and it becomes more difficult) but there are limits to the available friction between two surfaces (wet your hand, and pull it accross the desk again) which is why braking distances in the wet or in mud are much increased, or you're more likely to lock wheels.

On the subject of locking wheels, you're still experiencing a buildup of heat due to friction, but this time it's between the tyre and the Tarmac, and a tyre isn't as efficient at generating/radiating heat as your brake setup, which is why you eventually come to a halt, but it takes longer.

Does that clarify things any?
If you'd like some further reading then I suggest looking up Momentum and Kinetic Energy on Wikipedia.

If none of that, nor Wikipedia make sense to you, then cars work by witchcraft. If your religion doesn't permit it, then I suggest you arrange to have your local pastor excise it, or have it cleansed by fire.
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Old 04-29-2012, 02:39 PM   #141 (permalink)
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I understand that perfectly and I think elements of that post is what we have ALL been trying to say (though rather less eloquent and coherent) in one way or another over the last six pages.

I think the main issue, for me at least, was at what point does the extra traction that comes from increasing the weight become overcome by the increased momentum of the vehicle because of that increase in weight?

Could this be a measurable, detectable difference in real world situation at real world speeds with a P38 RR designed for V8 that has been converted to a heavier diesel engine?
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Old 04-29-2012, 08:06 PM   #142 (permalink)
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I think the main issue, for me at least, was at what point does the extra traction that comes from increasing the weight become overcome by the increased momentum of the vehicle because of that increase in weight?
Never. Both increase and decrease at exactly the same rate. Which is driven by vehicle mass.
There is no point where one overcomes the other.

The only difference between theory and real world here, is a misunderstanding of the theory.
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Old 04-29-2012, 11:01 PM   #143 (permalink)
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The only difference between theory and real world here, is a misunderstanding of the theory.
Yeah, upon review, I have come to realize (finally) the math works in your favour. And I am honest enough to admit I was wrong in that regard. Traction and momentum are indeed forces that do cancel each other out on paper. I understand that now.

I think however the theory would only be valid in practice under severely controlled conditions that would be near impossible to duplicate in a real world situations.

What of the variables in available traction involved in real world P38 RR, driving outside on the street that are beyond the theory?

Wouldn't momentum at any given speed be same as it is on paper? But braking traction be limited by all kinds of additional factors such as tyres, road surface, weather, weight transfer, brake COF, brake contamination, suspension condition etc.?

I'm not trying to be pedantic, I'm just trying to get my head around exactly why I have experienced so many times that it really does take longer to stop a heavier truck?
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Old 04-30-2012, 12:12 AM   #144 (permalink)
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Yeah, upon review, I have come to realize (finally) the math works in your favour. And I am honest enough to admit I was wrong in that regard. Traction and momentum are indeed forces that do cancel each other out on paper. I understand that now.
Excellent.

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I think however the theory would only be valid in practice under severely controlled conditions that would be near impossible to duplicate in a real world situations.

What of the variables in available traction involved in real world P38 RR, driving outside on the street that are beyond the theory?

Wouldn't momentum at any given speed be same as it is on paper? But braking traction be limited by all kinds of additional factors such as tyres, road surface, weather, weight transfer, brake COF, brake contamination, suspension condition etc.?

I'm not trying to be pedantic, I'm just trying to get my head around exactly why I have experienced so many times that it really does take longer to stop a heavier truck?
It doesn't matter what the conditions are, as long as they exist for both cases.

If you want to test stopping distance loaded vs unloaded on tarmac. That's fine.
If you want to test stopping distance loaded vs unloaded on gravel, that's also fine and the distance will be greater than tarmac. But both loaded and unloaded distances will still be within the limits of accuracy of your testing methods if the brakes don't suck.

If you want to test stopping unloaded on tarmac compared to loaded on gravel then it's a waste of time even reading about it.

Most truck brakes in the past weren't capable of lockup, which is why loaded trucks used to take longer to stop than unloaded. But now trucks and trailers have phenomenal braking power and multi-channel abs. They can stop at the limit of traction in most circumstances.
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Old 04-30-2012, 09:29 AM   #145 (permalink)
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Perhaps then, my confusion is due in part to perception?

Increasing the weight has a bearing on the adhesive limit of the tyres, allowing the driver/abs to apply more braking force. This is what you are saying, correct?

Since I don't drive around braking as hard as I possibly can every time I want to stop, the distance required to come to a halt is increased because 100% of the additional braking force afforded by the extra weight is not utilized under normal braking conditions but momentum is the same?

Could that explain why I have experienced increased braking distances between loaded and unloaded.
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Old 04-30-2012, 12:31 PM   #146 (permalink)
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Increasing the weight has a bearing on the adhesive limit of the tyres, allowing the driver/abs to apply more braking force. This is what you are saying, correct?
Correct

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Since I don't drive around braking as hard as I possibly can every time I want to stop, the distance required to come to a halt is increased because 100% of the additional braking force afforded by the extra weight is not utilized under normal braking conditions but momentum is the same?
Sounds about right, most sane people don't regularly brake at the absolute limit of traction
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Old 04-30-2012, 04:27 PM   #147 (permalink)
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Perhaps then, my confusion is due in part to perception?

Increasing the weight has a bearing on the adhesive limit of the tyres, allowing the driver/abs to apply more braking force. This is what you are saying, correct?
Yes

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Since I don't drive around braking as hard as I possibly can every time I want to stop, the distance required to come to a halt is increased because 100% of the additional braking force afforded by the extra weight is not utilized under normal braking conditions but momentum is the same?

Could that explain why I have experienced increased braking distances between loaded and unloaded.
Try pushing the pedal harder next time you have a load on.
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Old 04-30-2012, 07:25 PM   #148 (permalink)
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When Issac Newton sat under an apple tree, he would no doubt have observed:
  1. heavy apples hit harder than light apples when they fell on his head
  2. heavy apples stopped in the same distance as light apples when they fell upon his head or the ground
  3. and etc.

He came up with several laws of motion (still relevant today) including:
a body in motion maintains that motion unless acted upon by an external force
  1. force = mass x acceleration
  2. every action has an equal and opposite reaction
  3. and etc.

From the above he would conclude:
  1. given the same velocity as the apples strike the ground, and the same stopping distance, then acceleration is identical
  2. for the same acceleration, then the external force acting upon heavy apples is greater than that acting on light apples

For the argument in this thread, I will state the question in the following way:
If the mass of a particular 4 wheel vehicle is increased, can it stop from the same velocity in the same distance, i.e. at the same acceleration?
Assume the same conditions apply and that the vehicle brakes are capable of exerting enough torque at the wheels to cause lock-up with the increased mass, and the brake bias is such that all wheels lock-up at the same time.
And for the sake of reducing unnecessary (for this argument) complication, ignore drag such as wind, and road slope, etc.

For the same acceleration, then the external force required to stop the vehicle is greater when its mass is increased. And the value of the external force must be directly proportional to the respective masses.

Now the value of this external force acting upon the vehicle is the sum of the friction forces between each tire/tyre and the road.
The friction force between tires/tyres and road increases as the vehicle brakes are applied harder up until the limit when slip occurs (wheel lock-up).
Therefore, if the mass of the vehicle is increased and braking distance or acceleration is the same, then we need to show that friction force can increase directly proportional to the mass.

Now the limit of the friction force, when slip occurs at each tire/tyre is the product of:
the friction factor between the tire/tyre and road, and
the normal reaction between each tire/tyre and the road

The friction factor can be assumed to be the same when the mass is increased (for reasonable increases in mass).

The normal reaction between each of the 4 tires/tyres and the road changes when the mass is increased. When we sum the normal reactions at all 4 tires/tyres, this will equal the total vehicle mass x gravity (i.e. weight). If the height of the centre of mass changes, it will only affect the distribution between front and rear tires/tyres, but not the sum of all 4, which will always equal the total weight.

It should have become apparent now that the sum of the limiting friction forces has increased directly in proportion to the increase in mass. Therefore at the limit of braking the stopping distance or acceleration will be the same for both of the vehicle masses.

There are other ways to skin this cat, but none that will change the result. Change of momentum, change in kinetic energy, change in potential energy up or down a slope, work done by braking force, etc. all can be introduced to the braking problem and do have relevance if wanting to know the stopping distance or acceleration, but the above argument implies that stopping distance, acceleration, and because every other factor, except mass is identical for both cases, these are not necessary.
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Old 04-30-2012, 07:26 PM   #149 (permalink)
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Thanks for bearing with me and putting up with my shit (especially Dougal). It is very difficult to re-evaluate concepts that one has taken as fact for such a long time.

I was "taught" that braking distances were always adversely affected by weight during the formal theory portion of my auto apprenticeship many years ago. It would seem that while elements of this are not exactly wrong, it is far from the whole story and requires a much deeper understanding.
Because I had personally experienced added weight causing increased braking distance's and the reasons for it was (I thought) obvious, I never really questioned that conviction before this discussion. It took some time, about six pages of arguing, for me to realize I was in error.

Who say's old dog's can't learn new tricks!
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Old 05-29-2012, 08:53 PM   #150 (permalink)
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Join Date: Jun 2011
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Location: Savannah, GA
Posts: 16
Back to the OP's question...

With paradigm and his P38 6.2 swap that makes about 3 of us in the last 12 months that are very happy with our 6.2 conversions (the 2nd being Eric aka REDROVER over on ExPo with a 6.2 in his RRC -- he's covered something like 15k miles on an unholy trans oil and gasoline combo.)

Glad to see Bridger and Dougal (Laurel & Hardy?) are still around poo-poo'ing the NA 6.2L while offering such viable options as the "BMW diesel". LOL. Where can a regular American guy on a budget find one of these, again? I love how you point at the freakin' BMW 530d as an example. J-code 6.2 crate motors go for $900-$1200, or about what you'd pay for a serpentine belt on one of those Bimmer motors.

At least the Isuzu 4B motor actually exists in this universe -- but they're far more expensive and harder to find in good condition (in the U.S.) than the 6.2.

Bottom line on the 6.2: It's cheap, it fits, and it works. If you're on a budget, Eric over on ExPo has been sending bellhousing adapter templates to people. If you have some coin, and want a professional kit, Mark's 4x4 of Australia will send you a kit with everything you need (requires R380). The motor's not light (~750lbs) but it sits on OME XHD springs just fine. It's not any heavier than similar medium-duty NA diesels and it's a LOT quieter than the Cummins. Don't listen to Laurel & Hardy when they tell you this thing weighs 900lbs. That's nonsense.

Paradigm's reporting 24-26mpgs, which seem a little high to me. I'm getting 19-23 mpg but I'm also running an ARB and CT expo rack and my IP is turned up for a little more juice. He probably just has his IP dialed in a little bit more than I do -- a lot of the Chevy/GMC guys report high 20s with 3.73s so it's definitely possible.

The best option for a rock rig? Absolutely not. For anything else it's a very affordable, reliable, and viable conversion. The parts will interchange with about 500 different GMC and Chevy motors and the J-code will run on virtually anything combustible.
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