Defining Ground Pressure

[demosthenes]

What exactly am I seeing when I look at a value for ground pressure? It is known that the pressure is not uniform across the entire track, and that it is higher beneath each road wheel. So am I being provided values for average pressure, maximum pressure or something else? Also, do different nations have different definitions and interpretations of what they mean by ground pressure?

[rmgill]

Softer ground is going to increase your foot print because the parts of the track NOT under a road wheel are going to "sag" upwards only so much and then they're going to come into play for pressure/support. Unless you're on pavement I suspect that you get some support from such points.

 

Softer ground also gives more support for tires as you sink in. This is why sand tires are big balloon tires. (understood even in WWII as evidenced by the wider sand tires on the LRDG Chevy 30cwts).

[Lieste]

For initial sinkage, or for pressure on hard ground the MMP is a good guide. For softer ground a tracked vehicle will tend towards the NGP ~ but with some unevenness of pressure still present.

 

Published figures are almost exclusively non-penetrating NGP ~ which is as much use as a chocolate fireguard IMO, particularly for determining resistance to motion and probable speed made good.

 

There are a few public domain VCI figures which have been published in some older US Gov 'engineering' papers ~ these are referring to XM1 and XM2 as 'future' vehicles, and have a small sample of additional vehicles... there are some later papers that present 'lumped' models with 'High Mobility, Tracked' based on values for M1A1/T80 etc...

 

From what I gather from the various 'snippets' and with reference the Wong - The Theory of Ground Vehicles is that relative MMP is a good predictor of when a vehicle begins to suffer performance wise, while relative NGP might be a better measure of relative limiting mobility condition (bogged vehicle).

[Guest Jason L]

Practically speaking variability in the ground itself probably results in far greater ground pressure distribution than the distribution between spanning track and the road wheels. Real life isn't usually a very well quantified sandbox. Is NGP for tracked vehicles also calculated at 15% sinkage ?

[TTK Ciar]

Ogorkiewicz has a chapter in ToT about it:

 

http://ciar.org/ttk/mbt/soil-mechanics/

[Guest Jason L]

Ogorkiewicz has a chapter in ToT about it:

 

http://ciar.org/ttk/...soil-mechanics/

 

Woah, that's a huge difference between MMP and NGP for even modest road wheel spacing! I had no idea it was that high. Figured it was like a factor of 2 max.

Edited August 31, 2012 by Jason L

[Misfire]

What exactly am I seeing when I look at a value for ground pressure? It is known that the pressure is not uniform across the entire track, and that it is higher beneath each road wheel. So am I being provided values for average pressure, maximum pressure or something else? Also, do different nations have different definitions and interpretations of what they mean by ground pressure?

 

http://www.sciencedirect.com/science/article/pii/002248988190015X

 

http://www.sciencedirect.com/science/article/pii/002248988190015X

 

http://books.google.com/books?id=Blp2D1DteTYC&pg=PA354&lpg=PA354&dq=ground+pressure+tracked+vehicles&source=bl&ots=Xrpt_kT5ib&sig=yYEpQU8Qw9a3nriK5J0KRatNhgg&hl=en#v=onepage&q=ground%20pressure%20tracked%20vehicles&f=false

 

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

 

http://warships1discussionboards.yuku.com/topic/5643#.UEGDe6DAHos

[Leo Niehorster]

The pages from Ogorkiewicz were very enlightening, although I didn't try to follow the supporting math itself.

 

On a similar vein, what about dual wheels vs single wheels per axle, particularly the rear axle(s)?

Seems that the theory would favor dual wheels, (less ground pressure per wheel), but hasn't practice shown that single wheels operate more effectively, say, for example, over soft ground, like sand or mud?

 

Leo

[CaptLuke]

 

On a similar vein, what about dual wheels vs single wheels per axle, particularly the rear axle(s)?

Seems that the theory would favor dual wheels, (less ground pressure per wheel), but hasn't practice shown that single wheels operate more effectively, say, for example, over soft ground, like sand or mud?

 

Leo

 

Duals are going to decrease your GP, all other things being equal, but there are a variety of issues with them:

• The extra width means either a wider vehicle or less internal volume, both of which have drawbacks
  
• With short, narrow wheels, this may be manageable but these are the worst wheels for cross country work in the first place
  
• The taller and wider the wheels get (both good for cross country work), the worse the space issue becomes
  
• You can't use CTIS since you have to allow enough space between the tires so that they clear each other on an emergency low pressure setting
  

All this sets up a catch 22: if you care enough about cross country performance, you don't use tires small enough to dual them up and there's no way in hell you would give up CTIS (assuming it was practical in the time frame you are looking at), but if you don't care about cross country performance why hassle with the extra cost and maintenance? This is an oversimplification of course, the US used duals successfully on cargo trucks for years and the Soviet BA-10 armored car used them too, but I think by and large it holds.

 

It's also important to remember that going to an additional axle to get two more tires on the ground, instead of using a dual wheel, gets you other cross country benefits that have nothing to do with ground pressure. Trench crossing goes up dramatically when you go from two axles to three and even more if you go to four. There is also less chance of the vehicle getting stuck by bottoming out going over a step or mound type obstacle.

 

I think maybe it's accurate to say that more axles, more evenly spaced have proven superior for cross country mobility, as have larger tires, and the combination of these factors and the emergence of CTIS has driven duals from the military scene but I don't know of any study, or even empirical comments, that dual wheels wouldn't give superior mud/sand/snow etc. performance than single tires of equivalent size.

Edited September 2, 2012 by CaptLuke

[Leo Niehorster]

Thanks CaptLuke.

[Lieste]

Actually they have a generally increased resistance to motion ~ while a twin axle single wheel vehicle will drive with the rear wheel in the ruts of the front, this isn't possible for a dual wheel setup.

 

Rowland indicates that he considers "Dual wheels ineffective" in his paper 'A Review of vehicle design for soft ground operation' (circa 1972-74?)

I have something more 'concrete' with a value for the influence of duals somewhere, but as I recall they did increase the "effective MMP" in the same manner as not driving some axles (concentrating driving torque and soil stress on fewer axles).

[CaptLuke]

Actually they have a generally increased resistance to motion ~ while a twin axle single wheel vehicle will drive with the rear wheel in the ruts of the front, this isn't possible for a dual wheel setup.

 

True enough on soft soils, though a separate issue from MMP/NGP/etc. I would imagine the harder the surface the smaller the effect, otherwise long haul semi trucks wouldn't use them, since efficiency is king for that kind of work.

 

 

Rowland indicates that he considers "Dual wheels ineffective" in his paper 'A Review of vehicle design for soft ground operation' (circa 1972-74?)

I have something more 'concrete' with a value for the influence of duals somewhere, but as I recall they did increase the "effective MMP" in the same manner as not driving some axles (concentrating driving torque and soil stress on fewer axles).

 

I'm not sure what "effective MMP" means so I can't really comment. As I said, I know of some reasons duals are less effective than one might expect from just a raw tire footprint measure, so I'm certainly willing to believe there are others. However in Rowland's own wheeled MMP factor he penalized larger number of axels, IIRC for the same reason: they were travelling in ruts already established by the preceding wheels. This is shown in his "K factor", which increases the MMP, and actually goes up as the number of axles increases (or at least that's how I interpreted it).

 

I think a thought experiment is in order. Take an American 6x6 2 1/2 ton truck, which has dual wheels on both the rear axles. Run it over soft ground with all the wheels on. Then take the outer wheels off of the rear axles and, because this is just a thought experiment, you can even line the remaining back wheels up so that they line up with the front wheels. Then run it over the soft ground again. Which version of the vehicle is going to have the better soft ground performance?

 

Finally, this is the summary on dual wheels from Cranfield University Wheels and Tracks Study (10 - 25 Ton Armoured Fighting Vehicles), Report No. RMCS/ESD/PCB/266/00, March 2000, Engineering Systems Department, Royal Military College of Science (which references a lot of Rowland's work):

 

 

 

Civilian load carriers often utilise dual wheels on the rear axles, but this arrangement is not

normally found on wheeled AFVs since it leads to additional hull intrusion. For a given ground

pressure, compared to large diameter single wheels, dual wheels increase resistance to motion on

soft soils because they leave much wider ruts in the soil. There is some experimental evidence to

show that trafficability is more sensitive to wheel diameter than tyre width (see Annex 1C, Section

1C.2.4). Therefore the layout usually adopted for AFVs involves the use of large diameter single

wheels with the same track on all axles Large diameter wheels also enhance obstacle crossing

capability.

[rmgill]

 

I think a thought experiment is in order. Take an American 6x6 2 1/2 ton truck, which has dual wheels on both the rear axles. Run it over soft ground with all the wheels on. Then take the outer wheels off of the rear axles and, because this is just a thought experiment, you can even line the remaining back wheels up so that they line up with the front wheels. Then run it over the soft ground again. Which version of the vehicle is going to have the better soft ground performance?

 

 

The one with the more balanced drive line, the singled truck. The problem for the dualed out truck is that the front axle, which has a very large proportion of it's weight up front over the singles is still at a high ground pressure and you're spending traction not just pushing the front wheels through the muck, but the rear wheels are doing so as well since they're not in the same rut of the mud. In addition they're floating higher on the muck.

 

With super singles, you're pushing the fronts and the rears are hopefully further down in the muck and getting some traction. They're also not making a wider rut in the process since they're lined up with the track of the front wheels so you're not doubling the force needed to move forwards against the muck.

 

The guys on steel soldiers have looked at this and several of them are engineers in their former careers. The general consensus has been that singled deuces and 5 tons have slightly better cross country performance. Singled out to larger tires is better in general though.