Captain_Slow

Yeah...I wasn't sure about the HP of tractors. My understanding was they were all relatively low. My own experience comes from what are considered the fastest "road tractors" on Earth. Both of my Unimogs were pretty quick (not acceleration...but top speed of 60 mph). My 404 had 90 HP. Needed a desk calendar to measure 0-60. The 416 had a "high output" 125 HP OM352 diesel engine. AWESOME motor. With heavy chain and straps I could drag 30-40 ft sections of 10 to 15 inch diameter trees in a bundle behind the 416. The Mog barely noticed. Very low gearing and portal axle design distributes the torque and thus the force to the reduction hubs instead of loading up the ring and pinion in the diff (which remains rather small in size compared to typical truck axles).

Wisconsin Joe

Well, (and off we go on tq/hp) diesels have very low hp ratings. Because they run at low RPMs. But they have a lot of torque.

The truck I drive "only" has 400 hp. But is putting out almost 1500 ft-lbs of tq.

http://www.internationaltrucks.com/t.../cummins_isx15

As was noted, tq is "force" - a static measure. Simply how much it can turn.
Hp is "power" - a dynamic measure. Force over time. As in how much work it can do.
As a quick example, a high torque engine could turn a pump that moves a lot of water per stroke. But if it turns very slowly, it would take a while to fill a swimming pool.
A smaller, faster engine would not be able to move as much water per revolution, but if it can go a lot faster, it would be able to fill a pool quicker than the other one.

And weight has a lot to do with fuel consumption. Believe me, I drive for a living, and fuel economy is a big part of my job. Light loads take less fuel than heavy ones. Period.

The "uphill is countered by downhill" argument simply doesn't work in the real world.

And the "Once you are up to speed, weight doesn't factor" isn't true either. A heavier load will produce significantly greater rolling resistance. Bearings have friction. The heavier the bearing is loaded, the more that friction will increase. Tires have rolling resistance. Heavier loads will increase the contact patch, increasing resistance.

mark kibort

Derailing here a little. yes, as I have always said, its a trade off of how fast you want to do the work. assuming same HP tractor vs car, both can do that same rate of doing work. And yes, if you have some time to burn, you can make a 5hp engine produce all sorts of HUGE force and torque. it will just take a long time to do the job.

Its all about power ratings. if you don't need the speed, then yes, you can get away with a very low power rating and produce a ton of torque. of course if something has more power... even like a F1 engine, and has 1000hp, it can be geared down to do some spectacular work too. (and produce 100s of times more torque too! the only reason why you don't see F1 engines in a Cummings driven 18wheeler, is that it might only last the one climb up the mountain, where the desiel will climb those hills for many years with out any wear or fuss. 1500rpm, vs 15,000rpm.

lets not get off track here. this is not a power / torque debate. power will indicate the amount of torque or force you can provide at any vehicle speed.
if you don't need the power, then you can do the work at that rate with a smaller ,less powerful engine because of gearing.

yes trucks might have the same HP as fast cars as your 400hp truck has, but its RPM range is much lower, and that's the difference. yes, low rpm, high torque.... our porsches are much higher RPM (about 2x) and lower engine torque. But the torque at the wheels will be the same at any given vehicle speed if the same HP is required.

Strange analogy with the pool pump. if you had a pump that absorbed the engine HP the same, both engines would fill the pool the same... same rate of work, same HP... that's HP by definition. large low reving engine, displacing a lot of water with one stroke, , would fill the pool the same as a lower displacement pump, running at 2x the speed. same HP engines, one high torque, the other low torque.

Now, your weight objection? really??? common, I've said that the weight adding to the rolling friction is known. what we are talking about here, is if that weight on the wheels vs in the car makes any difference, and it doesn't. AND the amount of weight we were talking about is in discussion here. we are talking 40lbs on all 4 tires/wheels. it makes "little" difference for constant speed, if that 40lbs is in the car or spinning. Please read my post if you are going to comment. I've said this a dozen times so far.

The point here is that the rolling weight is no different than the carried weight, and its SUCH a small part of the overall weight that its not a measurable difference. the main difference in fuel economy is going to be due to the rolling friction difference due to rubber compound, tread design and width, and aero drag If its wider.

Do YOU think that adding a passenger to your truck is going to effect your fuel economy?? good!! I'm glad we got that cleared up. (I'm not saying adding weight doesn't effect fuel economy... I'm saying that adding a small amount of weight will not effect fuel economy that much.

We are talking 1% weight increase here. Crazy to think that that much weight increase could effect gas mileage by , as was said on this thread, up to 20%)

And as far as acceleration effects, its like if that extra weight was sitting in the car, times 2 if its tire weight and times 1.5 if its on the rim. But, that is the effect.

yardpro

The uphill is countered by downhill argument doesn't always fly.

Here at the class we are at sea level.
Whenever I go inland on a trip I am always going up a slight grade... For hours.

Yes, when I return it is opposite. But I can tell you from having driven the same thing over and over again I will always get better mileage on the return trip.

worf928

Actually it never flys. See perpetual motion machine of the third kind.

MK has a history of arguments - going back to mailing list days - that eventually come down to violations of the Laws of Thermodynamics.

Arguing with MK is like being caught in quicksand. I'm only here - in this thread - to save as many of you as I can. Stop posting and grab this lifeline while there's still time. And don't come back here...

Pfc. Parts

Dave, you clearly underestimate the entertainment (and educational) value of Mark's contributions to this list. For example, I've been under the impression the advantages of larger diameter wheels is in their reduced unsprung weight, not their effect on fuel economy or acceleration. This comes from an old cheat sheet book on "Tuning a High Performance Suspension", and my interest in the subject came from trying to improve autocross times on a 914 many decades ago. I happen to still hold the opinion that replacing heavy rubber with light metal provides certain advantages. Perhaps it's passed into the realm of religion for me

You have to enjoy the banter I think. I, for one, enjoy Mark's opinions even if I don't always agree. He makes me think differently about the questions.

yardpro

LOL... i am very familiar with these discussions... i rather refer to them as discussions as arguments are a bad thing discussions are a good thing.

I have a minor in physics ( although it was 21 years ago).... so i definitely understand his "violations" although they are not thermodynamic, they are more in the line of kinetic energy....

There is no such thing as a free lunch... heavier wheels WILL cost you more... period..

no one here has discussed the cost f stopping the spinning mass...

I agree that a heavier wheel will have a flywheel effect in that it will be momentarily harder t slow down as it is storing energy with its rotating mass...BUT whatever energy is stores has to be generated in the first place...

again there is no free lunch...


now lets talk about larger wheels being MUCH more difficult t stop... the added brake wear and reduced performance.

because we live on a planet and have gravity is takes more energy to move more mass... period end of discussion...

To say that the 1% weight increase does not matter....LOL... tell that to a race team...

I actually think that these entertaining discussions are good fr the board occasionally....
they get people thinking and talking.....

mark kibort

my comment about conservation of energy based on going up and going down, was not to say that it counteracted, but was to say that it helped. yes, there is no free lunch and Ive NEVER meant to say there was with anything ive said hear on this thread.

so,with that said, what do you mean with your comment here? of course climbing a hill going inland will be worse gas mileage than coming home. going up to Tahoe from sea level I get 15mpg and coming back I get 20. the point of what I was saying is that my average around 17mph which is close to what I see by going both up and down . But ill be the first to agree that going up and down loses the efficiency of the engine which gets better gas mileage at lower power settings. we are diverging from the point here on the topic

[QUOTE=worf928;11839664] Actually it never flys. SeeURL="http://en.wikipedia.org/wiki/Perpetual_motion"]perpetual motion machine of the third kind.[/URL

MK has a history of arguments - going back to mailing list days - that eventually come down to violations of the Laws of Thermodynamics.

Arguing with MK is like being caught in quicksand. I'm only here - in this thread - to save as many of you as I can. Stop posting and grab this [URL="https://rennlist.com/forums/928-forum/295176-official-random-928-picture-thread-post-a-new-928-pic-or-staydon't come back here....[/QUOTE]

QUOTE].

You need to read my post a little more carefully. If you got the impression that going uphill and then downhill counteracts each other, you got the wrong impression. first of all , people always mock what they don't understand. I don't break any laws of thermodynamics Ive always been proud of bringing true to the myths on this list.
It seems that you grab a sentence and then extrapolate how it breaks any laws of physics. Ive said things like, " it doesn't make much difference to have 1% more weight spinning or otherwise on the car " This doesn't mean it doesn't make ANY difference, it just means that for our discussion it doesn't make much of a difference... especially in areas of increasing rolling friction.
THE REAL BREAKERS OF THE LAWs OF PHYSICS are those that think that a 1% loss of weight or gain, can have any effect measurable on gas mileage. OR that they can get 5mph faster down a 1/4mile straight. (in other discussions)
If I'm breaking any laws in my discussion, please let me know.. but to this point, ive tried very carefully to equate rolling inertia with transitional inertia
and have given the equations to do so on your own.... instead of talking about it, you mock it and say 'wrong" with no intelligent response of your own. how can you ever learn anything, or think correctly, with such a closed mind?

I remember, in the early days of the list, when folks like you accused me of breaking thermodynamic laws with the eRAM.... it was really funny... they said that I was claiming that a 1-5hp draw on the alternator would counteract the 5-10hp gains claimied and proven on the dyno. they forgot their education of how basic turbo theory worked...... yes, you can have a 1hp draw on an alternator, and it can provide energy for a compressor (electric) to compress air and fuel to provide many times that power in net gain..... anyone that understands the concepts has no problem with the discussion, those that don't, use the catch all for nay sayers , as "its a perpetual motion device" The point is, it worked and worked for all the right reasons. many couldn't grasp the basic concepts involved... maybe you were one of them???

Thanks Scott!

So, you tell me if you agree with what ive said here. a few have tried to derail by focusing on a misinterpretation of some points. I think your impression is correct . yes, lightening wheels and tires is always a good thing for performance. (unsprung weight helps with steering response/feel and body as well as the ability for the shocks to keep the wheel on the pavement)
heavier wheels can fight lock up as one advantage, especially in an autocross.
The confusion comes in the area of that somehow the weight on the wheel for acceleration or MPG is somehow disconnected from the inertial of the car itself..... its not. Ill explain more in my next post to yardpro.
all the anecdotal evidence never gets questioned, BUT when I put some formulas, equations behind it, im the bad guy? funny! That is entertainment. remember, the entire premise here on this thread is that someone thinks that heavier wheels hurt gas mileage and acceleration. Both are true, but no where near the extent that was talked about here.
How much?? again, acceleration is hurt to the same degree as if that weight was sitting in the car times 2. MPG? that is only effected as if that weight was in the car..... rolling or not, its just weight on the car. Do you think 40lbs in the car will effect (measurably WORF928!!) MPG??????

yardpro

i for one would like t see some equations i would love to brush up.

you don't always go down the same inline you came up.......

mark kibort

Great Steven, so you have a minor in physics. This means what I will explain here will make a lot of sense to you . Again, its been a while for me too, so chime in if I recall something incorrectly. and yes, to this point, I don't think we have been dabbling much in the area of thermodynamics, as worf928 said.

Now, there is a reason no one has talked about what it costs to stop the spinning mass. first of all, it has no bearing on our discussion here. it doesn't effect gas mileage. (stopping the spinning mass), but has the same effect as it does as if that extra weight was in the car. Yes, there is no free lunch, but as I explained, the extra spinning weight has the same effect on the entire system as if that weight was sitting in the car... The difference is that weight in the car would be 1.5 times greater if we are talking wheels , and 2 times if we are talking tires. (17-18" rim sizes).

what this means, Is that that spinning mass needs to be brought up to speed.. once it is up to speed, and there is no change of speed, the spinning mass has no bearing on gas mileage, just as if the weight was in the car.
acceleration? Yes, that spinning mass has the KE difference at speed as if 1.5x that weight was in the car. To get that spinning weight up to speed you need energy, and that energy is the same is if that weight was in the car, but 1.5 times that weight. make sense?

Now, don't confuse the spinning weight as being any different than if that weight was in the car.... the entire system is the inertia!!! both spinning and rolling.. they are tied together. to stop the car or brake system doesn't know what the wheel and tires weigh? it only knows it has an inertia it has to fight and slow.. that inertia is the combination of the rolling mass/inertia and translational (not rolling) inertia. So, when that 40lbs is on the rims, the brakes see a 3500lb car with an extra 60lbs in the car.

The larger rims are NOT "much more difficult to stop " as you say. The larger rims will have the effect as if 1.5x their weight is in the car.... Now, let me ask you .... if you put 60lbs in the car, is it harder to stop?? well, technically, it is, but how much harder???? yes, you got it.. not even measurably harder! Now, put 2-3 passengers in the car and you might notice a difference. as far as fuel economy, that spinning weight doesn't matter that much (notice I said, "not that much"). why ? because when you are cruising, that weight spinning or not , is just sitting on the tires, as with the rest of the car's weight. the difference in MPG will be due to rolling friction with that extra weight, and it doesn't cost any more to climb hills with it, than if it wasn't spinning (i.e. sitting in the car).
sure, it takes more energy to accelerate it to cruise speed, but we are talking about a 1% change in weight in the car, even spinning... not enough to change much.

Then, you go on to make a point about race teams being concerned about 1% weight? sure, that effective 60lbs (40lbs of wheel weight) will cost time, in handling and acceleration. this discussion is not talking about race cars. we are talking about fuel efficiency.

again, and Worf, or anyone else can debate these points:

wheel weight change has a negligible effect on MPG because generally during fuel efficiency tests, speeds are not changing. and if the speeds do change, the effects are the same if that weight was sitting in the car (x1.5 or 2 if its a tire)

if we care about acceleration, the weight of a wheel is worth the same as if that weight was sitting in the car x 1.5 or 2, as I mentioned above

the only thing that will effect MPG to a noticeable value will be the rolling friction due to different tread designs and width of the tire, and aero drag do to a wider tire.

it is not reasonable to think that a 1% change in overall weight of the car , during cruise tests, will effect gas mileage by 20% ..(the example of the 40mpg car loosing 8mpg with heavier wheels)
again

climbing hills at constant speed with rolling weight vs weight sitting in the car has the same effect. as long as speed doesn't change, that mass (spinning or not) has the same effect on power and thus MPG to keep the car at speed.

Sure, glad you asked... Guess, Worf928 needs this the most.

wheels have a KE rotational and KE translational . the total KE of the wheel will be the sum of both.

KE(rotation)=1/2Iw^2
so
KE (linear)= 1/2M(w x R)^2

(w = radians/sec , R= radius)
RPM = w/2pi

you substitute (w x R) for velocity ( Linear KE =1/2Mv^2) above, and equate the rotational portion of the wheels KE, with the linear KE.


you solve for M and you get the formula for a rule of thumb:

M= I/R^2

assuming a 17" wheel x 7" width and the added 1lb was located .5" from the outer edge of the wheel

this is why adding that same lb on the tire, will be 2x that of what was added to the rim.... AND, if that rim had that 1lb more evenly added to the rim, the effects would be a lot less!!!!!

for the 17" rim, I get M=.47

This means any spinning weight on the wheel is less than 1.5 x as if that weight was sitting in the car for all comparisons.
Now, for constant speed.............. that weight doesn't have any different effect, as if it was sitting in the car.

So, adding 1lb of weight to the edge of the wheel is the same as if 1.47lbs is sitting in the car. (again, if it is evenly distributed over the rim, it would be a lot less)

Worf928..... Dave, Do you see how YOU will be breaking Newtons 2nd law by thinking that the spinning wheel can effect MPG any differently than if it was in the car???? (as long as the speed doesn't change... and if it does, that increase mass effect during velocity changes is like that weight was sitting in the car x 1.5 or possibly 2)
why don't you focus on the real physics criminals here!!!!

attachment below shows the real culprit of why rolling mass has more KE than linear moving mass. It has to go faster to keep up with the linear mass.
Its total KE is the linear KE plus the rotating KE.

Jon,
check my work below.

The interesting, but simple reason why rolling mass is near the same as linear mass, is that in the equations, you can clearly see that the end result is how fast the mass is traveling. if you look at the outer edge of a tire, and look at the weight there, and calculate its velocity, its the same KE as if it was traveling that fast and sitting in the car. So, worst case, its always a max of 2 times the weight, as if it was sitting in the car. But, because the weight is at the smaller radius of the rim, the factor is 1.5x as if it was sitting in the car. AND that's if the weight was strictly added to the edge of the rim....... for example... Like adding a wider barrel for a larger tire.. However, in the context of this discussion, the rim might be assumed to be the same proportion of weight distr. so jus the weight is higher . What this means, is that the heavier rim, might only have the effect of 20% more weight as if it was sitting in the car.

So, next time someone says they lost time on the track accelerating due to a 5lb heavier wheel, you can say, "hey, that's really only the same effect as if that weight plus (20lbs) an extra 4 lbs for all 4 wheels was sitting in the car!" And the main point of the discussion, is that that extra weight, would have very little effect on MPG, because if you don't change speeds, the extra inertial effect of the extra rotating mass has the same effect as if it was sitting in the car.

For those of you looking at their physics books and trying to figure out if im right....

say we have a 17" wheel (.21m radius)
we added 2.2lbs to it (1Kg)
its spinning at 1000rpm (16.6rev/sec or 104radians/sec)
vehicle speed is 77mph because the overall diameter is 26" (34.4m/s)

Moment of Inertia = MR^2 = 1 x .21^2 = .044
Linear KE = 1/2Mv^2
rotational KE = 1/2 I w^2

linear KE would be 1/2 x 1 x 34.4^2 = 591 J
rotational KE would be 1/2 .044 x 104^2 = 238 J

this means that for ever 2.2lbs or 1Kg we put on the wheel edge, there would be only 45% more KE needed to accelerate it vs it siting in the car .... this is because the car is moving faster than the wheel at 17" is spinning. And if that weight was more evenly distributed in the rim, then it might only be 20%!!!!!

Now, if you put the weight at the tire tread, so that the radius of the weight is the same as the tire, the speed of both are the same.... guess what????? the KE is the same too.... this is why heavier tires will be like if 2x that weight was sitting in the car. BUT again, this is only effecting acceleration, not MPG..... if you don't change speed, that weight is just like it was sitting in the car , and the effect of 20-30-40lbs in the car, isn't going to effect gas mileage enough to notice.

Worf928 Dave...... Who's breaking laws of themo/physics, here?

Pfc. Parts

I don't think it's easy to disagree with basic physics and as you say the masses differ very little from the perspective of rolling weight. I've always believed the design decision centers on unsprung weight and doing your best to tune it correctly for the mass of the car and the capabilities of the suspension. From a pure weight perspective, it's my understanding that using larger diameter light metal wheels and lower profile (lighter) tires is a method to reduce unsprung weight.

In applications where reduced unsprung weight is beneficial, larger diameter alloy wheels are a useful tool.

Captain_Slow

For some reason this thread is reminding me of this cartoon...

mark kibort

Your the physics instructor.. What say you??

Captain_Slow

Someone mentioned perpetual motion. Actually, I really enjoy this discussion. Getting kinda heavy..but the important thing is that a lot of different contributors have added good insight, which is important because looking at the issues from several perspectives is constructive. What I like most is it's a "How stuff works" focused discussion and the formulas only enter the conversation AFTER conceptual discussion. This is how it should be.

You mentioned that actual mass distribution in the wheel is not entirely concentrated at the outside of the wheel with the recent large diameter, open design, with low profile tires. I agree....So now I'm wondering where the "wheel as a system" boundary is. Often this is an arbitrary decision an investigator makes to isolate components to work up free body diagrams on just a part of a larger system. What is included in the system is the first decision. This caused me to consider that the brake rotors are rotating with the wheel and could be considered part of the wheel, effectively distributing the mass of the (wheel+rotor) system more evenly relative to the axis of rotation. While rotational inertia has to go up with modern wheel designs, the effect is not as large as a hollow cylinder would create, and other benefits (less unsprung mass and lower overall mass) offset the increase in rotational inertia. Now...back to my MMs & OPG

mark kibort

Someone did hint at perpetual motion, but my comment on that was only to hint that some of the energy is returned , if you lose fuel efficiency by climbing a hill, because you do have to return. there are no free lunches, and climbing and coasting still has loses ... even in space, there is some drag, small, but drag on moving objects.

the wheel is mostly centered weighted. in fact, my outers of the rim, barrel and edge, are only about 5lbs, while the centers are 15lbs. I wanted to stay worst case scenario as to not burst, many of the "inertia kills performance" folks bubble. adding a same size heavier rim, is usually uniform in the weight disty. even going up in rim size, the weight at the edge that moves out, is very light compared to the centers. But, even if all of the weight added was to outside of the rim, the effect KE'wise would be as if that weight was 50% higher In the car. at the tire, at near 26", it would be double. still not enough to sway the scales into blaming the weight to a loss in noticeable performance, acceleration wise, and even less fuel economy wise.
sure there is rotor weight. in fact, I just changed out stock GTS rotors for two piece racing rotors of a larger diameter .3" larger. . the weight saving was 7 lbs per rotor, and there was nothing I could feel or tell that I changed, and why would I..... that tire and wheel is 50lbs, the suspension unsprung is probably another 20, and the rotor is 15lbs now. saving 5 is not going to change the earth, but 5lbs rotating, x 2 for both sides, is like 15lbs as if was in the car... heck that's 1.5hp and ill take that any day! (and that rotor is only 13" diameter, much smaller than the rim, so even less effect) handlingwise, its not a big deal for the tracks we visit. very smooth and not very many hard transitional turns.

again, there is no basis for thinking that any weight saved on the wheel and tire can help fuel efficiency. again, its like 1.2 to 1.5 is sitting in the car next to you and that's not going to help much.