What the Hell is the Point of Interleaved Road Wheels?

[Collimatrix]

I'm sure that all the SH regulars will know this backwards and forwards, so this is more for the benefit of newer people, or people who stumble in via google, or people who want a quick link they can throw out as an answer to anyone who asks the question.

 

So, what's with the goofy-ass road wheel design on German WWII AFVs?

 

A puzzled and terrified worker struggles to comprehend and assemble the suspension of a tiger I

 

You may have run into a variety of explanations for this running gear design; that it provided a smoother ride, that the design saved rubber, or possibly some other rubbish.  Like the myth that frontal drive sprockets provide more traction (seriously, how in the hell is that supposed to make any sense?), these wrong explanations of the merits of interleaved road wheels seem to rise from some quote taken out of context.

 

The interleaved road wheel running gear may have saved some rubber relative to an alternative design that was particularly wasteful of it.  But interleaved road wheels are not particularly economic in this respect because, and I realize this is a complicated concept to explain so I'll try my best, they have more wheels.  Interleaved road wheels do allow for large wheel diameters, and a larger diameter wheel will spread wear out over a larger circumference.  So interleaved road wheels might allow for the rubber on the wheels to last longer, although their construction would require more in the first place.

 

Interleaved road wheels would not improve ride quality either.  The ride quality of a tank is not a function of the size or number of wheels it possesses, but of how they are sprung.  So, it is possible that in certain competitive trials an interleaved road wheel design outperformed a design that lacked this feature.  I could readily believe, for instance, that the tiger (H) had a better ride quality on rough terrain than the tiger (P), or that the SDKFZ. 251 had a smoother ride than the M3.  However, this would be because the tiger (H) and SDKFZ. 251 have independently sprung road wheels on torsion bars while the tiger (P) and M3 do not.

 

 

Torsion bar layout of the tiger II

 

Volute spring suspension of the M3 half track

 

So, what do interleaved road wheels do?

 

They have two principal effects; one is a small benefit, and the other is an enormous detriment.

 

The small benefit of interleaved road wheels is that they spread the weight of the vehicle out more evenly on the track links:

 

 

The weight of a tank is not completely evenly spread out on the contact area of its tracks.  This is because tracks are not rigid.  If they were, they would be mainly ornamental and tanks' engines would just be for show.  More of the weight of a tank is concentrated under the parts of the track that the road wheels are sitting directly on top of.  Additionally, once a tank starts to sink into the soil a bit, larger road wheels work better than smaller ones because the larger ones have more contact area.  But you can only fit so many large diameter road wheels in the space of a tank's hull.

 

Dynamic!

 

 

So, the only way to have lots of road wheels and have big road wheels at the same time is to interleave them.  Simple as that.

 

If you would like an exhaustive look at the development of the semi-empirical MMP equation, read this.

 

The major, crippling downside to interleaved road wheels is that it makes changing the road wheels extremely time consuming.  

 

A pair of workers perform maintenance on a panther tank, and contemplate the futility of all human achievement

 

Lucas Friedli reprints in his book on big cat maintenance a report from a training unit complaining that replacing the inner road wheels of a tiger tank took ten hours.  That is completely outrageous, and was a contributor to the poor operational availability of the big cats.

 

For this reason, interleaved road wheels have rarely been used after World War Two; only on a few French prototypes and a Swedish APC:

 

PBV 302 variant with interleaved road wheels

 

Some bizarre French tank

[EnsignExpendable]

If you're on a per-deliverable kickback scheme, getting kickbacks for the number of wheels is a pretty decent move.

[Walter_Sobchak]

I hate to be a pedant, but there is a difference between interleaved wheels such as the suspension on the Tiger I and overlapping wheels such as the Tiger II.  

 

Here is the interleaved roadwheels of the Tiger I.  Notice how the wheels don't just overlap, they actually surround each other on both sides.

 

 

 

This is different from the image in the OP which is for a Tiger II.  The Tiger II wheels overlap, but they are not interleaved.

[Collimatrix]

Sure.  Panther and Tiger I have three layers (originally 4 for the tiger), while the tigger II has only two layers.

 

From a weight distribution standpoint it makes no difference.  It does mean that in the aristocratic tigger the tracks are being twisted, since each road wheel station is hitting the tracks off center.  This means that the tracks don't last as long.

 

It's probably a bit easier to maintain, but if one of your inner wheels goes you're still looking at pulling two others to get at it.  So it's a pretty similar trade-off in the end.

[Meplat]

"What the Hell is the Point of Interleaved Road Wheels?"

 

More wheels+ more bearings=More money.

 

Look at Nazi Germany as less of a government, and more of a huge ponzi scheme.

[Collimatrix]

If you're on a per-deliverable kickback scheme, getting kickbacks for the number of wheels is a pretty decent move.

 

Better still if the road wheels in question wear out quickly and need frequent replacement.  As the ones on panther did.

[EnsignExpendable]

Especially if the wheels wear unevenly so you can get more maintenance contracts!

[Walter_Sobchak]

To quote from the interview of Robert Forczyk that I recently posted on  my site (gotta work in that self promotion where I can!):

 

Also, it’s easy for people who never had to change a road wheel or a torsion bar to admire the big gun on the Tiger, but ignore the idiocy of the interleaved road wheel design. Yes, it provided a more stable gun platform, but increased the weight and made maintenance that much more difficult.

[Legiondude]

Noticed in one of Yuri Pasholok's articles that the guy behind the E-25 just wouldn't let the interleaved suspension concept go. He tried to get basically the E 25 patented in the US

 

[Collimatrix]

Great find legiondude!

 

I'm actually pretty partial to the E-series suspension concept, as it appears to solve or at least ameliorate the big problem with interleaved/overlapping road wheels.

 

 

There are horizontal tubes filled with belleville washers pressing against a piston.  The piston is attached to the swing arm via that rack and pinion toothed mechanism.  This would probably work fine for the E-10 and E-25, but surely I am not the only one who thinks that those teeth would strip right the fuck off if E-50 or E-75 hit a bump.  Especially with late-war German metallurgy.

 

Anyway, to pull a bad road wheel you pull the bogie of two road wheels off.  Even if they're in fully overlapping configuration, you only need to tilt the bogie off diagonally to get it off the hull without disturbing the other road wheels.  This does mean that you have to take the road wheels off as pairs, and with their associated springs, but this is basically the same procedure for changing a road wheel on a tank with Horstmann suspension:

 

 

And you'll see paeons to the centurion raving about how much easier it was to swap out busted centurion suspension units than it was to change out road wheels and torsion bars on the M48.

[Xoon]

Great find legiondude!

 

I'm actually pretty partial to the E-series suspension concept, as it appears to solve or at least ameliorate the big problem with interleaved/overlapping road wheels.

 

 

There are horizontal tubes filled with belleville washers pressing against a piston.  The piston is attached to the swing arm via that rack and pinion toothed mechanism.  This would probably work fine for the E-10 and E-25, but surely I am not the only one who thinks that those teeth would strip right the fuck off if E-50 or E-75 hit a bump.  Especially with late-war German metallurgy.

 

Anyway, to pull a bad road wheel you pull the bogie of two road wheels off.  Even if they're in fully overlapping configuration, you only need to tilt the bogie off diagonally to get it off the hull without disturbing the other road wheels.  This does mean that you have to take the road wheels off as pairs, and with their associated springs, but this is basically the same procedure for changing a road wheel on a tank with Horstmann suspension:

 

 

And you'll see paeons to the centurion raving about how much easier it was to swap out busted centurion suspension units than it was to change out road wheels and torsion bars on the M48.

This might be a bit off topic but:

Does anyone know why the Centurion was regarded as such a great machine?

To me it only seems like a fatter slightly upgraded Panther tank.

 

To get on topic again:

How does the Horstman suspension compare to torsion bar, Hydropeumatic  and other suspensions? (weight and off road capability)

[Collimatrix]

Centurion had the good luck to be crewed by generally competent crews and sent against enemies who were generally incompetent.  The IDF's prowess at tanking hardly needs reiteration at this point, and the Indian Army's opponents were, shall we say, not well known for their tactical acumen when it comes to armored combat.

 

Also, the sho't kal is a seriously hot-rodded machine that is a hell of a lot better than a 1945 vintage centurion (which I agree is roughly comparable to a panther, except that the centurion actually works).  In addition to the bigger gun and all the extra armor, it has an entirely new (American-made!) engine and transmission.  They also received new fire control, fire extinguishers, turret motors and gun elevation equipment, although I don't know if they had all that stuff in 1973.  They had the American engine and transmission for sure though.  Most nations that operated the centurion after the 1970s upgraded theirs to Israeli spec (e.g. Sweden).

 

Horstmann suspension was OK by WWII standards, but outdated thereafter.  The biggest problem with it is that the road wheels are not independently suspended.  As you can see in the picture above, the pairs of road wheels share a common spring.

 

This has a few small advantages, such as being lighter as well as providing more resistance to vehicle pitching oscillation.  If you look at this commonly referenced German test of tank resistance to pitching, you can see that the vehicles without independent suspension tend to do better.

 

This is undercut by one enormous disadvantage; non-independent suspensions have horrible ride quality at high speeds.  What happens is that the pairs of wheels sharing a spring will both be compressed by obstacles at the same time, effectively doubling the spring constant and making it much more likely that the road wheels bottom out.

 

Granted, centurion and chieftain had such ass power to weight ratios that this was probably not a huge concern.  But still; it was not the way forward.

 

Horstmann does have the advantage of not taking up room inside the hull at all, and being (supposedly) easy to repair.  The prevalence of torsion bars in post-war tank design suggests that these are secondary considerations.  Also, there are ways of getting torsion bars to be less obnoxious, like on the AMX-30 where the third swing arm is reversed so that the turret basket sits between the torsion bars:

 

Picture courtesy of Walter

 

Other than that, it's hard to make generalizations.  A description like "torsion bar suspension" really only tell you what shape the spring suspending the roadwheel swing arms is.  That doesn't really tell you much about the important parameters of suspension performance like spring constants, travel length, unsprung mass, damping coefficients or harmonic period.  It just tells you that the swing arm is attached to a cylindrical piece of steel that it twists.

[Xoon]

Centurion had the good luck to be crewed by generally competent crews and sent against enemies who were generally incompetent.  The IDF's prowess at tanking hardly needs reiteration at this point, and the Indian Army's opponents were, shall we say, not well known for their tactical acumen when it comes to armored combat.

 

Also, the sho't kal is a seriously hot-rodded machine that is a hell of a lot better than a 1945 vintage centurion (which I agree is roughly comparable to a panther, except that the centurion actually works).  In addition to the bigger gun and all the extra armor, it has an entirely new (American-made!) engine and transmission.  They also received new fire control, fire extinguishers, turret motors and gun elevation equipment, although I don't know if they had all that stuff in 1973.  They had the American engine and transmission for sure though.  Most nations that operated the centurion after the 1970s upgraded theirs to Israeli spec (e.g. Sweden).

 

Horstmann suspension was OK by WWII standards, but outdated thereafter.  The biggest problem with it is that the road wheels are not independently suspended.  As you can see in the picture above, the pairs of road wheels share a common spring.

 

This has a few small advantages, such as being lighter as well as providing more resistance to vehicle pitching oscillation.  If you look at this commonly referenced German test of tank resistance to pitching, you can see that the vehicles without independent suspension tend to do better.

 

This is undercut by one enormous disadvantage; non-independent suspensions have horrible ride quality at high speeds.  What happens is that the pairs of wheels sharing a spring will both be compressed by obstacles at the same time, effectively doubling the spring constant and making it much more likely that the road wheels bottom out.

 

Granted, centurion and chieftain had such ass power to weight ratios that this was probably not a huge concern.  But still; it was not the way forward.

 

Horstmann does have the advantage of not taking up room inside the hull at all, and being (supposedly) easy to repair.  The prevalence of torsion bars in post-war tank design suggests that these are secondary considerations.  Also, there are ways of getting torsion bars to be less obnoxious, like on the AMX-30 where the third swing arm is reversed so that the turret basket sits between the torsion bars:

 

Picture courtesy of Walter

 

Other than that, it's hard to make generalizations.  A description like "torsion bar suspension" really only tell you what shape the spring suspending the roadwheel swing arms is.  That doesn't really tell you much about the important parameters of suspension performance like spring constants, travel length, unsprung mass, damping coefficients or harmonic period.  It just tells you that the swing arm is attached to a cylindrical piece of steel that it twists.

Did the British ever design a great, noteworthy tank?

 

From my point of view, the British wasn't the best tank designers.

 

Thanks for the info by the way!

[Walter_Sobchak]

This might be a bit off topic but:

Does anyone know why the Centurion was regarded as such a great machine?

To me it only seems like a fatter slightly upgraded Panther tank.

 

To get on topic again:

How does the Horstman suspension compare to torsion bar, Hydropeumatic  and other suspensions? (weight and off road capability)

In anything, the Panther is the "fatter" tank.  Really, the two vehicles have little in common.  The Panther is a very conventional German design of the period, other than the use of slopped armor.  Due to the double torsion bar suspension, the front mounted transmission and the bow gunner, the hull of the Panther is quite a bit taller than Centurion and the Panther also has sponsons, whereas the Centurion hull rests entirely between the tracks.  Look at the front glacis plate of the Panther compared to the glacis of the Centurion, the Panther plate is at least twice as big.  Since this is generally the thickest piece of armor on a tank, you can see how keeping it smaller is a good weight saver.  Despite the sponsons on the Panther, it has a surprisingly small turret ring and turret, whereas the Centurion turret was big enough to allow it to be up-gunned twice over its long career.  

[Meplat]

Great find legiondude!

 

I'm actually pretty partial to the E-series suspension concept, as it appears to solve or at least ameliorate the big problem with interleaved/overlapping road wheels.

 

 

There are horizontal tubes filled with belleville washers pressing against a piston.  The piston is attached to the swing arm via that rack and pinion toothed mechanism.  This would probably work fine for the E-10 and E-25, but surely I am not the only one who thinks that those teeth would strip right the fuck off if E-50 or E-75 hit a bump.  Especially with late-war German metallurgy.

 

 

 

 

 

Speaking of metallurgy.. I'd worry more about the Bellevilles failing with a obnoxious "crunch" before the teeth sheared.

[Xoon]

In anything, the Panther is the "fatter" tank.  Really, the two vehicles have little in common.  The Panther is a very conventional German design of the period, other than the use of slopped armor.  Due to the double torsion bar suspension, the front mounted transmission and the bow gunner, the hull of the Panther is quite a bit taller than Centurion and the Panther also has sponsons, whereas the Centurion hull rests entirely between the tracks.  Look at the front glacis plate of the Panther compared to the glacis of the Centurion, the Panther plate is at least twice as big.  Since this is generally the thickest piece of armor on a tank, you can see how keeping it smaller is a good weight saver.  Despite the sponsons on the Panther, it has a surprisingly small turret ring and turret, whereas the Centurion turret was big enough to allow it to be up-gunned twice over its long career.  

When I was saying "fatter" I was referring to the weight of the centurion, at 51 ton. That is 6 ton more than the Panther, for around 20mm thicker UFP and 50mm thicker turret and 5mm thicker lower sides. 

 

Being in general smaller than the Panther, I find the design inefficient, armor wise, compared to the Panther.  Also it is 10kp/h slower than the Panther, I am unsure of why that is. 

 

In general I feel like the Centurion is like a upgraded Panther, a little better armor, same gun performance (later up gunned to the long 88s performance), slower, heavier, smaller and more room for upgrades.

[Priory_of_Sion]

Walter's Forczyk quote touched on this, but about how what is the overall weight increase by having interleaved roadwheels? 

[Collimatrix]

When I was saying "fatter" I was referring to the weight of the centurion, at 51 ton. That is 6 ton more than the Panther, for around 20mm thicker UFP and 50mm thicker turret and 5mm thicker lower sides. 

 

Being in general smaller than the Panther, I find the design inefficient, armor wise, compared to the Panther.  Also it is 10kp/h slower than the Panther, I am unsure of why that is. 

 

In general I feel like the Centurion is like a upgraded Panther, a little better armor, same gun performance (later up gunned to the long 88s performance), slower, heavier, smaller and more room for upgrades.

 

The panther's nominal top speed is misleading.  The transmission has extra gears that allow it to have an extremely good top speed for a vehicle of its class.  However, at those high gear ratios the panther would have so little torque that it can really only hit those speeds on paved surfaces.  Its off-road performance is much more comparable to its peers.

 

Walter's Forczyk quote touched on this, but about how what is the overall weight increase by having interleaved roadwheels? 

 

Interleaved road wheels would add a significant amount of weight, considering that you're almost doubling the number of swing arms, road wheels and torsion bars.  Suspension accounts for about 10% of the weight of a tank, so total weight gain might be on the order of 5%, as a rough guess.

[Priory_of_Sion]

 

Interleaved road wheels would add a significant amount of weight, considering that you're almost doubling the number of swing arms, road wheels and torsion bars.  Suspension accounts for about 10% of the weight of a tank, so total weight gain might be on the order of 5%, as a rough guess.

That's a non-trivial amount. Imagine if Panther weighed ~42 tons instead of ~45, it could have made it a whopping 175 km without failure instead of 150 km!

[Walter_Sobchak]

When I was saying "fatter" I was referring to the weight of the centurion, at 51 ton. That is 6 ton more than the Panther, for around 20mm thicker UFP and 50mm thicker turret and 5mm thicker lower sides. 

 

Being in general smaller than the Panther, I find the design inefficient, armor wise, compared to the Panther.  Also it is 10kp/h slower than the Panther, I am unsure of why that is. 

 

In general I feel like the Centurion is like a upgraded Panther, a little better armor, same gun performance (later up gunned to the long 88s performance), slower, heavier, smaller and more room for upgrades.

 

I would suspect that the extra weight of the Centurion vs the Panther is due to the Centurion having a heavier and better armored turret and side skirts over the suspension.  Side protection on the Centurion is most likely much better than Panther.  

[Collimatrix]

The cent's turret is considerably bigger.  Relative to their gigantic hulls the big cats have teeny tiny turrets.  The panther actually has a smaller turret ring than the sherman.  Panther's side protection is better on its sponsons (50mm inclined 30 degrees for Ausf. G vs 51mm at twelve degrees for cent mk X), but worse underneath the sponsons (40mm flat).

 

The cent mk 1 and the panther are roughly comparable in overall mass, armament and protection.  But the cent had a lot more growth potential.  I can't imagine that the panther would have been as amenable to up-gunning as the cent was given its tiny turret ring.  As for up-armoring, forget it.  The design was already much heavier than the final drives could handle.  Furthermore, improving the armor would cost the panther more weight because its glacis plate is gigantic.

 

Upgrading the engine would have been a trick too.  The space within the engine decks on the big cats is not very large.  The Maybach motors they used delivered extremely good horsepower relative to their size, so naturally the designers didn't leave any space for anything with less specific power (and, you know, maybe a little less break-y).  This is why all the proposals to re-engine the king tiger with a diesel and all the E-series napkinwaffe would have required a complete redesign of the engine deck.

[Xoon]

I would suspect that the extra weight of the Centurion vs the Panther is due to the Centurion having a heavier and better armored turret and side skirts over the suspension.  Side protection on the Centurion is most likely much better than Panther.  

 

Actually, The Panther has side skirts, 5mm side skirts. And when it comes to hull armor, the Centurion and Panther (Ausf. F) is pretty much identical, the Centurion having very slightly better overall side armor, while the upper sides of the Panther being a 5mm thicker and sloped, making them a little better. So, pretty much identical. 

 

On a second look, they have identical front armor, side armor, and the panther has a little thicker rear armor (45mm compared to 38mm). 

 

But yes, the Centurion obviously has a much more heavily armored turret, having about 30mm thicker sides, cheeks and rear, and between 150-250mm thick turret face. 

 

Mvh

Xoon.

[Walter_Sobchak]

The cent's turret is considerably bigger.  Relative to their gigantic hulls the big cats have teeny tiny turrets.  The panther actually has a smaller turret ring than the sherman.  Panther's side protection is better on its sponsons (50mm inclined 30 degrees for Ausf. G vs 51mm at twelve degrees for cent mk X), but worse underneath the sponsons (40mm flat).

 

The cent mk 1 and the panther are roughly comparable in overall mass, armament and protection.  But the cent had a lot more growth potential.  I can't imagine that the panther would have been as amenable to up-gunning as the cent was given its tiny turret ring.  As for up-armoring, forget it.  The design was already much heavier than the final drives could handle.  Furthermore, improving the armor would cost the panther more weight because its glacis plate is gigantic.

 

Upgrading the engine would have been a trick too.  The space within the engine decks on the big cats is not very large.  The Maybach motors they used delivered extremely good horsepower relative to their size, so naturally the designers didn't leave any space for anything with less specific power (and, you know, maybe a little less break-y).  This is why all the proposals to re-engine the king tiger with a diesel and all the E-series napkinwaffe would have required a complete redesign of the engine deck.

Yep, Colli hit it on the head with this.  I always thought it a bit odd that the German's went through the trouble of creating the most compact, high power engine of the period (HL-230) and then put it in completely over-sized vehicles.  If they had put that engine in something around 30 tons or so, they could have had a vehicle as fast as the Comet.  

 

Fun Fact: When they put the AVDS/1790 engine and CD850 transmission into the Centurion, they had the mount the powerpack at about a 3 degree angle sloping down toward the rear of the vehicle in order to position the final drives in the correct location.    

[Toxn]

The cent mk 1 and the panther are roughly comparable in overall mass, armament and protection.  But the cent had a lot more growth potential.  I can't imagine that the panther would have been as amenable to up-gunning as the cent was given its tiny turret ring.  As for up-armoring, forget it.  The design was already much heavier than the final drives could handle.  Furthermore, improving the armor would cost the panther more weight because its glacis plate is gigantic.

 

This. There is a significant difference between two equally-sized tanks when one of them is at the end of it's development life already while the other still has plenty of upgrade space. Think pzIV versus sherman.

 

The cent carried on so long because it has lots of spare load capacity on the suspension, a generous turret ring and a big engine bay. The panther didn't carry on much past a few years with the french because changing anything would have required a redesign of the entire vehicle.

[Collimatrix]

If you're on a per-deliverable kickback scheme, getting kickbacks for the number of wheels is a pretty decent move.

 

I'm actually not 100% clear on why the Germans were so obsessed with this suspension design.  It wasn't just the big cats that they stuck with these silly things, it was every single Sdkfz 251.  That was probably even worse for them, since those were supposed to be mainstay medium transport.

 

If the Germans ditched the stupid suspension and were able to make a few hundred extra tiger IIs in 1944, I can't see how that would make much difference.  If they had ditched the stupid suspension and made a few thousand more Sdfkz 251s in 1939, that might have material impact.

 

Forczyk mentioned in his T-34 vs panther book that there was some loon in the German armor development bureaucracy who was completely obsessed with torsion bars and interleaved road wheels, and would basically reject any design that didn't have them.  This is apparently why the DB panther design gained them later in development.  But it's not clear to me why this guy had so much say in tank design, and why everyone around him didn't point and laugh.