What the Hell is the Point of Interleaved Road Wheels?

[TokyoMorose]

17 hours ago, Toxn said:

Sadly no - I used the 1972 MMP equations and noodled around a bit with variables. From a gaming sums perspective, what they do is introduce a new term (tire deflection) which increases effective track contact area. Even a few centimetres makes a big difference here.

 

Realistically, of course, you're right - having soft, flexible bits on road wheels seems like a recipe for issues unless the vehicle is very light to begin with (at which point MMP more or less sorts itself out). 

 

Some of the French 40s and 50s prototypes had pneumatic roadwheels, and I wonder if that was influence from the German 'expertise' they agglomerated post war. They did after all, also build some interleaved suspensions at this time - as well as desperately tried to get the HL 230 family to work to spec.

[Toxn]

9 hours ago, TokyoMorose said:

 

Some of the French 40s and 50s prototypes had pneumatic roadwheels, and I wonder if that was influence from the German 'expertise' they agglomerated post war. They did after all, also build some interleaved suspensions at this time - as well as desperately tried to get the HL 230 family to work to spec.

I was wondering about that myself, actually.

 

I think part of it was that they had a bunch of hulls, plans, jigs and engineers on hand who'd worked on the things. So, you know, use what you have.

 

I think the other thing (as can be seen with the AMX 50 and Bat Chat) is that they were on a big drive to put a really powerful gun on a mobile chassis and still have it come out to less than 50mt (presumably while also being rail-transportable). So pneumatic road wheels and torsion bars must have appealed simply to keep dimensions down.

 

Pity that, like most German wunderwaffle tech, it was all a technological dead end.

[Toxn]

So here's a question: is it better, without using interleaving or whatever, to have a few big road wheels or lots of little ones in terms of MMP? Here I'm going to ignore things like travel and have no spacing between the roadwheels.

 

For a 5m track contact length, 40t vehicle with the other specs kept the same (track width 0.6m, track pitch 0.15m), you find that ground pressure rises quickly and then tails off as the size of the road wheels decreases. This rapidly leads into diminishing returns: 25 axles with 20cm roadwheels gets you an MMP of 95KPa, 50 and 10cm road wheels gets you 67, and 100 axles with 5cm road wheels gets you 48.

If you restrict things further to sane territory (12-4 axles), on the other hand, you get the following:

• 12 axles/0.42m roadwheels: 137 KPa
• 11 axles/0.45m roadwheels: 143 KPa
• 10 axles/0.5m roadwheels:  150 KPa
• 9 axles/0.56m roadwheels:  159 KPa
• 8 axles/0.63m roadwheels: 168 KPa
• 7 axles/0.71m roadwheels: 180 KPa
• 6 axles/0.83m roadwheels: 194 KPa
• 5 axles/1.00m roadwheels: 213 KPa
• 4 axles/1.25m roadwheels: 238 KPa

So in this part of the range the relationship is more or less linear. It's also clear that the easiest way to improve ground pressure, mutatis mutandis, is to pack as many wheels as possible onto a given length of track.

 

 

[Toxn]

Whoops, ended up quoting when I was trying to edit.

[N-L-M]

You can see from the MMP equation that the term for number of wheels is linear but the term for diameter is a square root.

Seeing as the tradeoff between the two is inverse linear, it evidently makes sense to maximize the linear one at the cost of the square root one, all else being equal. 

Now, where do we see this in practice? In heavy earth moving or construction equipment, primarily.

 

[Toxn]

44 minutes ago, N-L-M said:

You can see from the MMP equation that the term for number of wheels is linear but the term for diameter is a square root.

Seeing as the tradeoff between the two is inverse linear, it evidently makes sense to maximize the linear one at the cost of the square root one, all else being equal. 

Now, where do we see this in practice? In heavy earth moving or construction equipment, primarily.

 

Exactly. Earthmovers also don't have to worry about suspension travel and such, meaning that you can pack your extra wheels in aggressively.

[TokyoMorose]

15 hours ago, N-L-M said:

 

Seeing as the tradeoff between the two is inverse linear, it evidently makes sense to maximize the linear one at the cost of the square root one, all else being equal. 

 

By god, the British were right all along, Churchill coming in with the best MMP suspension of all time.

[Collimatrix]

20 hours ago, Toxn said:

Exactly. Earthmovers also don't have to worry about suspension travel and such, meaning that you can pack your extra wheels in aggressively.

Small wheels also have higher rolling resistance, and at high speeds tend to wear their rubber out faster.

[Toxn]

2 hours ago, Collimatrix said:

Small wheels also have higher rolling resistance, and at high speeds tend to wear their rubber out faster.

Indeed. Lots of little wheels != fast

[N-L-M]

8 hours ago, TokyoMorose said:

 

By god, the British were right all along, Churchill coming in with the best MMP suspension of all time.

The Churchill did indeed have very good MMP for its time, combined with a track link design with very aggressive grousers meant a good ability to cross muddy ground. That, combined with the low low gear ratios allowed it to slowly but surely cross obstacles many other tanks couldn't.

The British didn't get everything wrong.

[Toxn]

1 minute ago, N-L-M said:

The Churchill did indeed have very good MMP for its time, combined with a track link design with very aggressive grousers meant a good ability to cross muddy ground. That, combined with the low low gear ratios allowed it to slowly but surely cross obstacles many other tanks couldn't.

The British didn't get everything wrong.

If only any of their vehicles had had a turret ring worth talking about, something decent could have emerged before war's end.

[N-L-M]

It is indeed a recurring tragedy. Small turret rings and internal mantlets.

[Toxn]

31 minutes ago, N-L-M said:

It is indeed a recurring tragedy. Small turret rings and internal mantlets.

Cromwell/Comet being the most egregious example.

[LoooSeR]

2 hours ago, N-L-M said:

The Churchill did indeed have very good MMP for its time, combined with a track link design with very aggressive grousers meant a good ability to cross muddy ground. That, combined with the low low gear ratios allowed it to slowly but surely cross obstacles many other tanks couldn't.

The British didn't get everything wrong.

That sound more usefull for WW1, not exactly as crucial in WW2.

[Toxn]

1 hour ago, LoooSeR said:

That sound more usefull for WW1, not exactly as crucial in WW2.

Let's just say that the British hedged hard against the possibility of WW1 breaking out again. I mean, look at TOG's original requirements...

[Lord_James]

1 hour ago, Toxn said:

Let's just say that the British hedged hard against the possibility of WW1 breaking out again. 

The French did, too

[Toxn]

2 hours ago, Lord_James said:

The French did, too

Indeed. Not a good bet, as it turned out.

 

But you can't blame them for at least hedging. The French, at least, went into WW1 adamant that it would a war of offensive manoeuvre (based on their previous experience with the Prussians), and it was for a month or two. And then it wasn't and they lost a generation of young men.

 

If I'd been bled white and had chunks of my country rendered permanently uninhabitable because I didn't put money on the defensive being dominant in the last war...

[delete013]

5 hours ago, LoooSeR said:

That sound more usefull for WW1, not exactly as crucial in WW2.

 

It was indeed meant to climb trenches in a fashion of 1918. It seems that climbing was still quite useful for infantry support in ww2. Churchills made it much easier for the British to evercome the hills around Tunis. Germans felt safe on the peaks and the British simply drove up to them including to the 754th Grenadier Regiment HQ, bagging the officers. Navigating heavy terrain widens the maneuver space and the tactical solutions available. Lowlands are not everywhere.

But as the rest already mentioned, the small turret ring dictated the size of a weapon in the turret and Churchill's ended up being woefully undergunned, which is not very healthy for such a slow vehicle.

[delete013]

28 minutes ago, Toxn said:

Indeed. Not a good bet, as it turned out.

 

But you can't blame them for at least hedging. The French, at least, went into WW1 adamant that it would a war of offensive manoeuvre (based on their previous experience with the Prussians), and it was for a month or two. And then it wasn't and they lost a generation of young men.

 

If I'd been bled white and had chunks of my country rendered permanently uninhabitable because I didn't put money on the defensive being dominant in the last war...

 

It is still quite untypical of the French to behave so timidly. They won ww1 but apparently lost the battle of the will, because Napoleon was precisely the opposite.

[Sturgeon]

On 4/9/2021 at 3:19 AM, Toxn said:

I think part of it was that they had a bunch of hulls, plans, jigs and engineers on hand who'd worked on the things. So, you know, use what you have.

 

That's exactly what happened in all sectors. The French (for a time) absorbed the German small arms collective as well, and played around with adapting their late war designs. In the end, like the tanks, they didn't end up using any of the German ideas, partly because unlike the French tank sector, the French small arms developments were still quite technologically competitive.

[Sturgeon]

1 hour ago, delete013 said:

It was indeed meant to climb trenches in a fashion of 1918. It seems that climbing was still quite useful for infantry support in ww2. Churchills made it much easier for the British to evercome the hills around Tunis. Germans felt safe on the peaks and the British simply drove up to them including to the 754th Grenadier Regiment HQ, bagging the officers. Navigating heavy terrain widens the maneuver space and the tactical solutions available. Lowlands are not everywhere.

 

You have absolutely lost the privilege of speaking authoritatively on anything, even if you're right.

[Lord_James]

1 hour ago, Toxn said:

Indeed. Not a good bet, as it turned out.

 

But you can't blame them for at least hedging. The French, at least, went into WW1 adamant that it would a war of offensive manoeuvre (based on their previous experience with the Prussians), and it was for a month or two. And then it wasn't and they lost a generation of young men.

 

If I'd been bled white and had chunks of my country rendered permanently uninhabitable because I didn't put money on the defensive being dominant in the last war...

True, but from my understanding, the Renault FT had a better showing than the British Marks and Saint Chamond: being faster, cheaper, requiring fewer crew, and generally more reliable. 

[Beer]

 

2 hours ago, Toxn said:

Indeed. Not a good bet, as it turned out.

 

But you can't blame them for at least hedging. The French, at least, went into WW1 adamant that it would a war of offensive manoeuvre (based on their previous experience with the Prussians), and it was for a month or two. And then it wasn't and they lost a generation of young men.

 

If I'd been bled white and had chunks of my country rendered permanently uninhabitable because I didn't put money on the defensive being dominant in the last war...

 

I think that it's a bit of misconception. The French certainly saw that tanks and airforce were very important in the modern warfare, after all they had the largest tank force in the world by 1940 and their airforce was pretty well equipped. The issue was that for various reasons they mostly stayed basically with FT-17 and its more modern clones and on top of that had completely disfunctional command and communication chain. 

 

24 minutes ago, Lord_James said:

True, but from my understanding, the Renault FT had a better showing than the British Marks and Saint Chamond: being faster, cheaper, requiring fewer crew, and generally more reliable. 

 

FT was for sure the best tank of WW1 but by may 1940 there were still 1300 of them in French army service, of that roughly 530 in frontline units facing the German invasion...  

 

[Toxn]

57 minutes ago, Beer said:

 

 

I think that it's a bit of misconception. The French certainly saw that tanks and airforce were very important in the modern warfare, after all they had the largest tank force in the world by 1940 and their airforce was pretty well equipped. The issue was that for various reasons they mostly stayed basically with FT-17 and its more modern clones and on top of that had completely disfunctional command and communication chain. 

 

Ya, but part of the reason for that is because the Char B1 was sucking up all of the resources that should have gone into making more modern medium designs.

 

As for their doctrine and C&C, it was perfect for fighting WW1 all over again - built around a process of slow, deliberate, centrally-coordinated movements focused around artillery fire massing and coordination. It just wasn't up to the task of orchestrating manoeuvre warfare (and the very real dysfunction didn't help).

[Beer]

14 minutes ago, Toxn said:

Ya, but part of the reason for that is because the Char B1 was sucking up all of the resources that should have gone into making more modern medium designs.

 

Tend to disagree here. Up to the very last moment the military kept buying literally thousands of 2-men tanks both for the infantry support and cavalry eqipped with the same vintage gun as the FT-17 (R-35, H-35, H-38, FCM-36). That means that by may 1940 the military had close to 3000 2-men tanks with useless guns which were by large majority produced during late 30'. That's a looot of resources vasted.