Suspensions: technical question

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Leopard 2
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Suspensions: technical question

Post by Leopard 2 » 07 Oct 2004 10:06

Hi forum

I've read recently that they exist two types of suspensions/chassis that I thought it was only one: Panzers with overlaped road weels... and Panzers with interleaved road weels...
What is the diference between interleaved and overlapped?
(I hope I've used the corect words)...
I've seen this problem when studying late prototypes of Panzer II, until the Luchs... (VK901, 903, 1601, 1303).

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Post by Christian Ankerstjerne » 07 Oct 2004 12:05

I think that the possible difference could be that the interleaved roadwheels consists of more than two rows of wheels, but I don't see why there should be any actual difference...

Christian

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Post by Leopard 2 » 07 Oct 2004 12:43

I'm confused... because English is not my regular language. That's the reason why I got problems with those two words...
If someone could explain me that (in french would be a dream!)...
I'm so sorry, I don't remember where I've found that there was a difference... because I've that on some paper I've printed from the net some time ago... here it is (about the Luchs):
" A new type of suspension was used with 5 large overlapping suspension wheels with no return rollers. Torsion bar springing was used, and this kind of suspension led ultimately to the Panther and Tiger suspension where overlapping became interleaving, a necessary step to reduce ground pressure."
In that way, you may be right...?

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Re: Suspensions: technical question

Post by cbo » 07 Oct 2004 13:10

Leopard 2 wrote:Hi forum

I've read recently that they exist two types of suspensions/chassis that I thought it was only one: Panzers with overlaped road weels... and Panzers with interleaved road weels...
What is the diference between interleaved and overlapped?
(I hope I've used the corect words)...
I've seen this problem when studying late prototypes of Panzer II, until the Luchs... (VK901, 903, 1601, 1303).


This is an example of overlapping roadwheels. It is the Tiger II.

Image

This is an example of interleaved roadwheels. It is the Tiger I.

Image

I'm not sure what prompted the change from interleaved to overlapping wheels from Tiger I to Tiger II, but overlapping wheels came with a number of problems including uneven loading of the suspension arms and uneven pressure on the tracks and trackpins, causing the latter to bend and increased resistance in the track. The bent pins were difficult to remove and the track more likely to break. Some of these problems were also due to or aggrevated by the use of steel-rimmed roadwheels in the Tiger II.
The problems may not have been as severe in a ligher vehicle, though.

I'm not convinced, that there would be much difference between overlapping and interleaved roadwheels in terms of ground pressure as long as the number of roadwheels were the same?

Claus B

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Post by Leopard 2 » 07 Oct 2004 13:19

Woaw! Thank you for the pics and for your explanation, even I'm not sure to have understand it at 100%! :wink:

So, it appears that interleaved wheels were used at the beginning, and replaced on the Tiger II by overlapping wheels... Meanwhile, the piece of texte I've copied above means that there was an overlapping system before this interleaved system... Does it mean that the interleaved wheels would have been a progresse, but, because of those problemes you've explained, they gave up and returned with the overlaped wheels on the Tiger II?

Anyway, thank you very much, I understand it now!

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Post by Christian Ankerstjerne » 07 Oct 2004 13:21

As long as the tracks are the same, I don't see how the number of road wheels could change the ground pressure. The weight would be more evenly distributed on the tracks, but the ground pressure should be the same...

Christian

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Post by Leopard 2 » 07 Oct 2004 13:28

I think he's right about this idea of the number of road wheels and their ground pressure...
Ground pressure of a tank is not the same of the one of a car for example. The ground pressure of the car is the one that are doing the four wheels on the road... But the one of a tank is much more the pressure that the amount of road wheels are exercing on the tracks, not on the ground... I think you have ton consider the tracks themselves as the "ground", because the wheels are suporting the weight of the tank, not really the tracks...
Am I wrong??? (is there any physician in the forum?)
:P

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Post by Lkefct » 07 Oct 2004 15:54

IN terms of the ioverlapping roadwheels, the load is not going to be distributed evenly across the track. The outer wheels would put the load mostly on the outer portion of the track, the inner wheels on the inner portion.

When the roadwheels are interwoven, the load is more evenly distributed acorss the track on every suspention arm. IN addition the other moments of the load that are distributed acorss the track, llike those in the pins that hold the tacks together, should be better distributed as well.

I suspect there is not much real world difference in the ground pressure. On very soft terrain, it might make a small but important contribution.

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Post by cbo » 07 Oct 2004 16:20

Christian Ankerstjerne wrote:As long as the tracks are the same, I don't see how the number of road wheels could change the ground pressure. The weight would be more evenly distributed on the tracks, but the ground pressure should be the same...


It would depend whether you measure ground pressure in NGP (Nominal Ground Pressure) or MMP (Mean Maximum Pressure). NGP is a usefull figure to describe the tanks performance on a hard surface, as it treats the tanks tracks as if they were solid, but it does a poor job of describing the tanks performance on softer surfaces because here, the flexibility of the tracks come into play and hence the number of roadwheels also becomes important, as the pressure under the roadwheels is much higher than between them. The more roadwheels, the lesser the pressure under each wheel and hence better off-road performance. Here, MMP does a much better job at describing the relative performance of different systems.
Two tanks, who have similar NGP values, could very well have radically different MMP values, reflecting how track design and the number of roadwheels influences off-road performance. The classic example quoted by Ogorkiewics is that few ever complained of the off-road capabilities of the British Matilda tank which served admirably in all sorts of terrain. Its NGP was 1.12, very close to the British Covenanter tank which was notorious for bogging down with an NGP of 1.02. The difference between the two is betrayed by the MMP values: Matilda 252, Covenanter 390. In this case, the superior performance of the Matilda is mainly due to all its little roadwheels, which help distribute its weight evenly along the track, while the Covenanters large Christie-type roadwheels would limit the pressure to a few points along the track.
Another example would be the Panther with an NGP of about 0.88 while the Sherman 75mm with VVSS suspension was about 1.05 depending on model. A rather modest difference. Yet, the Panther was renowned for driving across ground where Shermans would helplessly sink. The MMP values tells us why. The Panthers was at 154 while the Shermans was at 252 - a massive difference. Of course, the number of roadwheels was not the only parameter - the T34 only had five, but its trackdesign assured that its 0.71 NGP translated into a very low MMP value of 174.

So when it comes to real off road capabilities, the roadwheels matter quite a lot, but it is not reflected in the Nominal Ground Pressure, only in the Mean Maximum Pressure and other, more complex, measurements.

Claus B

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Post by Leopard 2 » 07 Oct 2004 17:38

8O 8O 8O

Very impressive... Thanks a lot!

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Post by cbo » 08 Oct 2004 10:06

Leopard 2 wrote:I'm so sorry, I don't remember where I've found that there was a difference... because I've that on some paper I've printed from the net some time ago... here it is (about the Luchs):
" A new type of suspension was used with 5 large overlapping suspension wheels with no return rollers. Torsion bar springing was used, and this kind of suspension led ultimately to the Panther and Tiger suspension where overlapping became interleaving, a necessary step to reduce ground pressure."


The whole idea about using large, overlapping or interleaved roadwheels came from the German half-tracks of the 1930ies which had interleaved roadwheels. I think the idea came about due to a desire of combining the use of large roadwheels for speed (like in the designs by Christie in the US - large roadwheels are not as hard on the bearings as smaller ones) and many roadwheels to reduce the load on each axle. It is possible, that the superior off-road performance of the system was, at least initially, an added and not very well understood bonus.

In any case, interleaving and overlapping both fulfilled the basic criteria for speed and low axle loading, so I assume that the Germans used whatever they saw as the most usefull system in each tank design. The problems with overlapping wheels in the Tiger II was probably due to a combination of weight and steel-rimmed roadwheels. But the Luchs (Pz II Ausf. L) had rubber-rimmed wheels and was very light, so proabably, overlapping roadwheels did not cause any of the troubles they were later to do in the Tiger II.

The reason for adopting the overlapping wheels in the Tiger II may have been the problems of mud-packing in the Tiger and Panther but Overlapping wheels also made the exchange of damaged roadwheels much easier. As you can see on the drawings I posted, if one of the inner wheels on the Tiger II was damaged, you only needed to remove the two adjecent outer wheel combinations to replace the damaged wheel. If you damaged the inner center roadwheels on the Tiger I, you had to remove 9 wheels or two-wheel combinations to get to the damaged roadwheel. Furthermore, in order to change to transport tracks on the Tiger I, you had to remove the outer wheels (those crossed out in the drawing) while you could just change the tracks on the Tiger II.

Claus B

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Post by LegalAssassin » 08 Oct 2004 21:24

I would guess it has something to do with the fact that it's more complex to build the interleaved type. Let's face it, with the first you can just slap on two wheels on the same "pin", on the other you need a lot of extra pieces.

I would guess that the reason is simply easier construction rather than making tank's performance better. Then again, it's a "semi-qualified" guess.

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Post by The Argus » 12 Oct 2004 13:36

Sorry for edit.

Ground pressure is usually calculated by dividing the weight of a vehicle by the area of it's 'contact patch.' As a crude generalisation it has it's place, but the more accepted method is measured in units of MMP.

MMP is Maximum Mean Ground Pressure, a tank track isn't ridged; it's a chain that flexes as each road wheel passes over each link.

If you put a sensor under every link of a track then ran a tank over the top of it, each link would register a spike for every wheel. If you doubled the number of sensors and put one under every track join too, you'd get another set of spikes matching the wheels but larger than those from under the links.

The point of highest pressure is not under the links (the minimum spikes), it's between them (the maximum spikes).

The wheels always exert the same pressure, but the track is best able to spread the load in the middle of a link and least effective over the joints.

MMP is the mean of these Max spikes and represents the worst pressure the ground under a track might experience.

To reduce MMP, you need more wheels to spread the weight over more points on the track. Becuase there is only so much weight so the more Min spikes it's spread over the smaller those spikes will be. And you need fewer joints between links to reduce the number of maximum spikes.

Look under a bulldozer, lots of little wheels and long track links. It's the same as a Churchill or any of the WWI tanks.

The problem is little wheels and long links are SLOW. The faster they move the more vibration they set up.

This isn't 100% accurate but think it like this: A wheel is round, wrap a track around it and it becomes a polygon. The longer the links or the smaller the wheel, the fewer sides the polygon has; and square wheels don't make for a smooth ride. And obviously there is more pressure on the corners than on the flats.

A fast tank needs big wheels with short pitched track to put as many sides on that polygon as possiable; and big wheels have a lower rolling resistance anyway.

Now big wheels alone can make a long pitch track run quicker than small wheels, after all they still add more sides to the polygon even if not as many as a shorter pitch would do on the same diameter. This is how the T-34 managed to combined big wheels, narrow tracks and good ground pressure.

The Cromwell in contrast used the same Christie derived suspension but used a much shorter pitched track. It was alot more comfortable to ride around in at any speed, but it got bogged alot more often than a T-34.

The problem with big wheels is you can't get as many into the same length of hull as you can small ones, it's a simple matter of diameter. But that only aplies if they are on the same plane. If you bring one wheel forward, its neighbours back and then shuffle the axels closer together so the wheels overlap; well you can keep your big wheels and get more of them into the same length of track.

Making tracks wider actually has nowhere near as much effect as making the pitch longer, dispite the increase in area. What is important is the realtionship between two adjacent links of track and the wheel passing over them . In the case of the Tiger/Panthers, the track was going to be wide anyway thanks to the overlaping wheels, and between this width and the extra wheel stations per side, they didn't need longer pitched track to reach a respectable MMP.

Keeping the short pitch made the more comfortable to ride and easier to move quickly.

shane

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Re: Suspensions: technical question

Post by tigre » 23 Apr 2020 23:29

Hello to all :D; bumping up.......................

Interleaved road weels!

Source: Armor Modeling. Sd Kfz 251 Family. K. Townsend.

Cheers. Raúl M 8-).
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