Liquid vs. Iron Ballast

These father/son arguments seem to be abundant. The only consolation that I get is that even though I am alway wrong according to dad, in several months he will prove my point to himself and preach it as the gospel according to his experience.

Any and all opinions are welcome and the more the better. Buickanddeere, I hope you respond so your "buddy" Ron can show us his intelligence.

We have 2 of those lawn rollers for getting bumps out of the yard in the spring time. One is made of concrete, the other is the type you fill with water. They are real similar in weight. To me, the concrete roller seems to pull a lot easier behind the lawn tractor than the water-filled roller. It also seems to take less toll on the tractor, as it is an even mass being pulled. Especially when going up hills, the water roller will push-pull-push-pull all the way up, which can't be good on the tractor.

Just my 2 cents.

JP

I printed your question out & took it to my earlier engineering meeting & it kind of became the topic of the morning.

Here�s the consensus of an entire group of automotive engineers [either your lucky or unlucky day]..

What was �estimated� [no lab work or testing involved] was:

The liquid would take less energy to initially put in motion due to fluid slippage inside the tire,, the cast weight would retain slightly more kinetic energy due to a tighter coupling of the mass to the axle/wheel so could transfer slightly more stored energy,, the fluid filled would take slightly more energy to keep in motion due to the fluid slippage inside the tire & fluid turbulence,, the fluid filled could [we didn�t all agree on this point] also take more energy to keep in motion due to tire deformation due to fluid displacement while rotating.. This was all figured at a wheel RPM below the point that the fluid would start acting as a solid due to centrifugal forces.. We all agreed that once the fluid coupled more solidly to the tire at high RPM�s there would be a change in properties.

My personal input/experience on the steel vs. fluid is:

-The fluid places the majority of the mass slightly lower so the general stability & roll over stability is better..

-If you have even tried to manually roll a tire/wheel [or push a tractor with fluid in it] you would find the fluid filled much harder to move than cast weights.

-The fluid ride is worse due to less air volume in the tire.

-The fluid can have a big effect on tire traction depending on the tire air pressure involved as the tire foot print spreads out differently on the bottom..

-The obvious inherent concerns of corrosion & tire puncture repair..

Keep in mind the above is just an educated theory & would need lab or field testing to become a proven fact..

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I am in complete agreement with your conclusions. Part of the disagreement is based on the fact that the characteristics of the liquid ballast will change as ground speed changes. There is going to be a huge difference between 3 mph and say 18 mph with a fluid filled tire. I haven't tried to calculate anything but I would imagine that the hp requirements to move the fluid would increase exponentially as speed increases. Also the iron wheel weights would tend to maintain a flywheel effect as their position is constant and on a shorter radius to the axle.

As I responded to G-Man below, our argument does not take into consideration the other virtues of the different types of ballast material, only the hp required to move them and maintain movement.

We play around with a little bit of antique tractor pulling. By no means are we on a competitive level. While I agree with dad that the fluid ballast will put the weight directly on the tire footprint and take away some of the tire flexibility/tread deflection issues, I simply stated that a fluid filled tire requires more hp to move. Dad's belief is that as long as the tire is not completely filled the water is never actually moved by the tire and requires no hp to move. He also feels that the iron weights must actually be moved as more mass on the tractor and thus requires more hp.

Besides tractor pulling, the tractors are used for pulling wagons occasionally. This is where I believe the liquid ballast to be less convenient. At road speeds, the liquid will require more hp and cause more tire wear than the easily removable wheel weights.

Anyway, thanks to you and your engineering group for the input. In reality, each has it's own merits and benefits for different applications and I'm not looking to use any of this to "prove dad wrong" in any way. I just wanted to hear from my JD peers.

As for the center of gravity not being lower.. I have to disagree with you there... If the fluid is higher in the rear [or even the front] the other side would be equally lower unless you know something about fluid dynamics I don�t.. Even if that wasn�t the case & the fluid is actually at 11 & 3 o'clock there would still be more fluid below the wheel center than above it [do to the air space at the top].. That would lower the center of gravity to below the wheel center,, on a cast weight there is always the same mass above & below center [or real close anyways] so the center of gravity on a cast weight would always be at wheel center..

Good discussion going here�.

Clooney

First the axle shafts are machined to accept strain type gauging, then slip rings are installed on the shaft to allow readings while the axle is in motion, then the axles are mounted in a rigid fixture & known fixed torque loads are put on the end of the axle [or the position the wheel would mount, then the output is put through an amplifier, then the reading instruments are calibrated to those known fixed loads.. Then the axles are installed & the readout is adjusted for the installation & static load off-set.. Then the tractor would be run without any weights for a baseline [probably under various working loads & conditions to match customer usage]. Then the cast weight would be added & runs made under the same conditions as no weight, then the cast removed & the test repeated with the fluid added.. Then all removed & another baseline run to look for any off-sets due to bent parts or instrument problems..

In some cases the above could be run using wheel torque transducers instead of instrumented axle shafts [if transducers were on hand that would fit the application].

If a quick & dirty test were needed to just prove the difference in power required to move either weight set-up the tractor would just be towed with another tractor at a few [exact] different speeds with a load transducer in a towbar & the drawbar pull weight compared..

We do similar tests every day on rear axles, transmissions, towing type trucks, wheel rims, drive shafts etc. Not something a person would go through to just see what happens but most are needed for towing recommendations, durability concerns, part capacity, overall safety of a part or system, or government requirements..

The fluid vs cast weight power requirements could also be estimated using data from previous testing, known fluid dynamic & fluid friction charts & formulas, rotational mass energy storage formulas & acceleration formulas etc. But usually testing is still needed to back the estimation up & provide hard data..

Clooney

Quote: "That's all well and good, but that would be of little to no value in this case"..... Answ, I disagree with that statement as it would be a very good way to measure the torque [converted to horsepower if you like] required to turn both fluid filled & cast weight equipped wheels. That statement was in response to your original statements of: " but will it be measurable?� & �And if you were able to measure the loss, how would you determine if it were due to the actual fluid in the tires, or perhaps due to the weight of the fluid lowering slippage too much?�

Quote: �Most tractor power train components, particularly final drives and axles, are so extremely overbuilt that they are fully expected to survive in even the most adverse conditions��. Answ, that is a true statement but has nothing to do with measuring the torque to turn the different weight set-ups.. In the testing mentioned above the axles were strain gauged to use as calibrated torque measuring devices not to measure destructive loads or abusive loads [that�s another type of test].. Take the axles out of the testing & use wheel torque measuring adapters if you like..

Quote: �Different soil types take different amounts of horsepower to move across� �.. Answ, again true but the tests to compare the torque to turn should really be measured under those different types of soil conditions to see if the difference in torque to turn varies by soil conditions or work loads.. That was mentioned in the first test set-up post.

Quote: �The operator in the cab isn't going to notice even a 1% difference in horsepower requirements between types of ballast� �. Answ, probably not but the fuel usage over the life of the tractor, drive train wear, engine durability might [not will but �might�].. We won�t know if the horsepower difference is 1% or maybe more or even maybe less until the tests are run.

Quote: �If it was of any consequence, you can be sure one of the tractor or tire manufacturers would have done the testing and published the results� �.. Answ, how do we know they haven�t.. Obviously the power loss isn�t great or there would be all kinds of published data.. The originating post of the thread wasn�t how much power loss but if there was a difference in power to turn either weight set-up..

Quote: �Tractor makers always recommend cast-iron ballast, but know that guys will use fluid��..Answ, there must be a reason for that!

I understand where you are coming from on this issue but we aren�t comparing real world power losses that could be under 1%.. We were [are] discussing what weight package [assuming the same total weight] would take less [or more] horse power to turn & what set-up would be more efficient..

Clooney

I guess it's much like the old "Tootsie Pop" commercial. The world may never know!

Been a long time since my college physics but thought I would interject some ideas for your responses.

I agree with Buick concerning the surface tension and friction of the water against the tube. The water is going to move and movement costs energy. Due to the wave motion of the fluid, I would think it would take more energy to stop the fluid over cast weights. I keep thinking about the guys who haul water around here, of course we are talking about much slower speeds.

The majority of the fluid will always be at the bottom of wheel at a greater radius than cast weights. I would think this would hold the CG lower than cast weights.

The radius of the fluid mass is at a greater distance from the axle than cast weights. I don't think the fluid filled tire would have much of a flywheel affect. But at that slow of RPM does the cast weights have much of an effect?

Cast or fluid, the added weight is not being carried by the axle bearings etc. since the weight is attached directly to the wheel.

As the tractor moves over different terrain and the fluid sloshes around this energy is going to be transmitted back to the drive train. I wonder what the long term affects are... With the substantial gears and bearings, not much I suspect.

Again, I am not anything like an expert or pretending to be one but this is a good thread and wanted to inject my thoughts.

Jason

It wasn't my intention to sound as if I was against what you were saying. I simply meant that our argument is about the difference in hp lost to turning the specific types of ballast(liquid vs. iron). We are not figuring the other virtues of the different materials such as cost, corrosion, ride, component life or convenience.

I guess this is much the same as the long vs. short drawbar comparison for tractor pulling.