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Ground Pressure


WAKEN

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Hi all,

 

I'm new to this forum. I'm looking for the ground pressure for the following WW2 British armoured cars:

 

Humber Mk IV

Humber Scout

Daimler Mk II

Daimler Scout

 

Does any body have this info?

 

If not I believe I can calculate ground pressure using the following 2 formulas:

 

1. Calculate average ground contact area by multiplying length of tyre by width of tyre

 

2. Calculate average ground pressure by dividing load by average ground contact area

 

I know nothing about tyres and a quick google search leaves me baffled! If anybody knows the length and width of the tyres these vehicles used please educate me!

 

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Ground pressure depends on tyre pressure. Let down the tyres a bit, more tread contacts the ground, ground pressure is reduced. (off roaders do it all the time, and many Military and Emergency vehicles can alter tyre pressure on the move)

 

The best you can do is work out theoretical values, but how accurate these are going to be is questionable since different versions of manuals for military vehicles often contain totally different tyre pressures. A vehicle with Run flat tyres (RF) will have far thicker sidewalls, so for the same tyre pressure there will be less deflection hence RF's always have a higher ground pressure than a normal construction tyre of the same size.

 

Secondly your formula only works if weight is evenly distributed front and rear, which is unlikely.

 

Ground pressure for front axle will not be the same as ground pressure for rear axle.

 

You need to divide each axle loading by the tyre contact area for the wheels on that axle, and work on the highest figure as being the ground pressure.

 

Lastly if your tyre is a bartread for example, at low ground loadings on hard ground the contact area is totally on the bars of the tread. At higher loadings or on on soft ground, the bars forming the treads sink totally into the ground, and weight is then taken also on the valleys between the bars.

 

How long is a piece of string?

Edited by antarmike
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This is all old ground (pressure) , one of the best basic references is from page 138 of OFF ROAD HANDBOOK with back country travel tips. by Bob Waar 1975. ISBN 0-912656-15-8 . Obviously an American publication but they are more into tyres (or should I say tires).

 

Has all the theoretical , how to arrive at your "net-to-gross contact ratios" etc. Basically you ink the tire and lay a sheet of paper over a sheet of alloy or steel or ply or whatever and drive over it, no need for string at all..

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But length of tyre depends on tyre pressure and this could have varied widely and length depends upon weight applied so "length" will not be the same for a particular tyre type fitted to one vehicle as the same tyre fitted to another vehicle, which I why I say how long is a piece of string. There is no standard length a given tyre size is going to be.

 

No-one will have accurate information as to the length of a particular tyre type adopts at a certain loading, you will have to find a vehicle and measure it for that vehicle.

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I have a fair number of official manuals covering vehicle movement from the ASC through to RLC, nowhere can I find any reference to the term of ground pressure.

 

However the term might be of use with a tracked vehicle in consideration of soft ground, at least here the surface area of the tracks & weight would be known.

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When I tried to google the tyre types these armoured cars used I found this info:

 

Humber Mk IV - 10.50 x 20 RF

Humber Scout Car - 9.00-16 or 9.25-16

Daimler Mk II - 9.00-16 or 9.25-16

Daimler Scout Car - 7.00-18

 

I don't know whether that helps anyone answer my query or not?

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When I tried to google the tyre types these armoured cars used I found this info:

 

Humber Mk IV - 10.50 x 20 RF

Humber Scout Car - 9.00-16 or 9.25-16

Daimler Mk II - 9.00-16 or 9.25-16

Daimler Scout Car - 7.00-18

 

I don't know whether that helps anyone answer my query or not?

 

Well it means you know fairly accurately the width's of the tyres associated with the vehicles. This being the first figure in each case, measured in inches.

 

The Humber MkIV shows it has run flats, so these will have incredibly strong rigid sidewalls, so the length of contact will be shorter than comparable 10.5" wide tyres fitted to 20" diameter rims.

 

The problem you will have is determining the length of thread in contact with the ground, and this, in my opinion really is "how long is a piece of string?"

 

Also 9.25" wide tyres are nearly 3% wider than 9.0" tyres so you will end up with two different ground pressures for some of these vehicles depending on which tyres where on the vehicle at the time. (although there may well be a different length for either width which further muddies the waters....)

 

And seeing that ground pressure varies between front and rear axles you need laden axle weights for these vehicles.

 

I note also your query about Half track. To be technically accurate you cannot work out an exact ground pressure for any tracked vehicle since each road wheel will carry a different weight, the vehicle being nose or tail heavy, and seeing as the track flexes, some road wheels will be further in the ground, and the tracks around that area will have a higher local ground pressure than other areas of the track where the road wheels are carrying less weight. Maybe not overly important, but another piece of string to try and measure. Tank or halftrack ground pressure is an average figure, and even when accelerating the torque tends to lift and lighten the front of the tracks and put a higher ground pressure under the rear of the track. (true also of armoured cars where ground pressure per wheels varies constantly under acceleration and braking.) Good luck in your mission.

Edited by antarmike
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Excuse my complete ignorance on tyres but what is the second figure from the values that I found?

 

Thanks,

 

Neil

This is the internal diameter of the tyre, which is the same as the diameter of the rim onto which it fits. (this diameter does not include the upstanding flange alongside the bead of the tyre, it is the diameter of the well into which the tyre fits.)

 

It does not have any relationship to outside diameter of the tyre. This gives you further problems.

 

A British 14.00x 20 Bar grip tyre is 14" wide , fits on a 20" diameter rim and is roughly 48" outside diameter. The U.S. made version of a 14.00 x 20 is also 14" wide, fits on a 20" rim but is nearer 50" outside diameter.

 

A British road pattern 14,00 x 20 is also nearer 50" outside diameter, so there will be big variations in the "length" of a tyre, according to the tread pattern, and in which country it was made.

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Humber Mk IV - 10.50 x 20 RF this is 10 1/2" wide tyre on 20" rim

Humber Scout Car - 9.00-16 or 9.25-16 this is 9 or 9 1/4" wide tyre on 16" diameter rim

Daimler Mk II - 9.00-16 or 9.25-16 as above

Daimler Scout Car - 7.00-18 This is 7" wide tyre on an 18" diameter rim.

 

Forgot to say there was no standard specification for Daimler Scout car and different roofs were fitted , early ones had four wheel steering and two steering wheels, later changed to two wheel steering, single steering wheel, so you haven't even got a standard weight for the vehicle. At the time of the D day landings some Dingos had wading screen to allow them to wade to a depth of 10 feet.

Edited by antarmike
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A bit of background info on my request: I'm working on a project to calculate the combat power of units that fought Normandy. In the model that I am using one of the factors that goes in to the equation to derive the combat power of an AFV is its mobility. One of the components that go in to the mobility equation is ground pressure. From books that I have and the internet I have found the ground pressure for all the AFVs that I'm interested in bar the 4 in this post and 2 German AFVs.

 

I appreciate these may be 'how long is a piece of string' type questions but a 'best guess' based on something tangible is what I'm hoping to achieve.

 

Thanks,

 

Neil

Edited by WAKEN
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"I have a fair number of official manuals covering vehicle movement from the ASC through to RLC, nowhere can I find any reference to the term of ground pressure."

Clive,

 

Do any of your manuals cover these 4 vehicles and do they provide any information on tyre pressure?

 

Thanks,

Neil

Edited by WAKEN
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Don't know about the theoretical side of all this but I can vouch for the fact that the Dingo is not very clever on the (sandy) Normandy beaches, especially on a rising tide when stationary and just left ticking over for ten minutes or so. - Could only pull it out with a Stuart.

 

The almost solid Dunlop RFE tyres offer no "spread" and of course as there is no air to let out the footprint remains that of a sharp blade. In contrast though if you can keep moving the fluid flywheel transmission and smooth "take up" means that they don't bog that easily.

Edited by David B.
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Clive, Do any of your manuals cover these 4 vehicles and do they provide any information on tyre pressure?

 

Thanks,

Neil

 

Neil I'm sorry I don't have any manuals on wartime vehicles, my vehicle interests are more recent. The manuals I was referring to cover more general logistical & administrative subjects I'm afraid.

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The only WW2 vehicle I've ever seen that makes an issue out of ground pressure is , yep, the Dodge. That is only in the media hype, it says that ' The Dodge has less ground pressure than a camel!'. Dosen't say wether it is an unladen camel or not though. There are jsut so many variables in the question. Suspension type, load, tyre type, pressure, and most importantly the nature of the ground itself. In WW1 when tanks first operated oficers would walk in front tetsing the ground with walking sticks.

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The only WW2 vehicle I've ever seen that makes an issue out of ground pressure is , yep, the Dodge. That is only in the media hype, it says that ' The Dodge has less ground pressure than a camel!'. Dosen't say wether it is an unladen camel or not though. There are jsut so many variables in the question. Suspension type, load, tyre type, pressure, and most importantly the nature of the ground itself. In WW1 when tanks first operated oficers would walk in front tetsing the ground with walking sticks.

Yes but a walking camel has a higher ground pressure than one which has decided to lie down and stubbornly refuses to get up.... That ground pressure would be hard to beat!

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When I tried to google the tyre types these armoured cars used I found this info:

 

Humber Mk IV - 10.50 x 20 RF

Humber Scout Car - 9.00-16 or 9.25-16

Daimler Mk II - 9.00-16 or 9.25-16

Daimler Scout Car - 7.00-18

 

I don't know whether that helps anyone answer my query or not?

 

Some mistakes in this list, The Daimler armoured car MkII was fitted with 10.50-20 tyres, the Humber Scout was 9.25-16 originally ( it is only in postwar years that 9.00 were used as 9.25 was obsolete). As for the Daimler Scout Car, these tyres did not have air in them, so they were same footprint all the time.

 

In your calculations, a tyre standing on a hard surface will have a relative small point of contact, but one standing on a soft surface will spread load further as tyre settles in, imposing load on those lower parts of the tread as well.

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Forgot to say there was no standard specification for Daimler Scout car and different roofs were fitted , early ones had four wheel steering and two steering wheels, later changed to two wheel steering, single steering wheel, so you haven't even got a standard weight for the vehicle.

 

Bit of confusion here. The early Daimler Scout Car with 4 wheel steering did not have a second steering wheel.

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Bit of confusion here. The early Daimler Scout Car with 4 wheel steering did not have a second steering wheel.

 

My mistake then, I though they had swivelling drivers seat, two steering wheels and in either direction one axle trailed, and self centred on the castor angle. in effect only one axle steered depending on direction of travel. I accept I may have this wrong/

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  • 2 weeks later...

Just read this thread, by length of tyre..........i take it that folks mean the actual area of tyre in contact with the ground? Not the actual length of tyre as in curcumfrence.Please redirect to appropriate stupid questions thread if applicable!Regards Tim

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