Tank suspensions; fight!

[z42]

There has been a few threads were the topic of different tank suspension mechanisms has been breached. So let's have a kerfuffle were we can argue the merits and demerits of various types of suspensions used on tanks.

I'll start with a short list:

• Torsion bars: Used extensively by German tanks (except Pz IV). And many contemporary tanks as well (e.g. German Leopard 1/2, US Abrams, most post-WWII Soviet/Russian tanks). Generally fairly compact, although they do take up space in the bottom of the hull, leading to a higher tank. Another suggested disadvantage is that if the vehicle hits a mine, the highly loaded torsion bars can spall and send fragments flying inside the tank. This can of course be countered by putting the torsion bars inside some kind of armor tube, but that might make maintenance difficult?
• Christie: Each road wheel individually suspended on a bell crank, which is connected to a coil spring suspension, usually inside the hull. Used by British cruiser tanks, and the famous Soviet T-34, among others. Should allow for high speed mobility over harsh terrain. However it does use quite a lot of volume in the hull, and particularly if it's inside the armor box it "wastes" a considerable weight of armor around the suspension components.
• Horstmann: A kind of bogie suspension, where you have two wheels on individual bellcranks connected via a spring. While this results in poorer ride quality than independent suspension like Christie, the bogie helps spread the load (if one wheel is pressed up, the other wheel on the bogie is pressed down). And crucially, the suspension is contained between the tracks, so it doesn't use volume inside the armored hull like torsion bars or Christie. Used on many British tanks.
• Vertical/horizontal volute spring suspension. Bogie style suspension similar to Horstman (particular the horizontal variant), except using volute springs rather than coil springs. Used on some American tanks, in particular the M4.
• Leaf springs and bogies: The German Pz IV and derivaties used a bogie style suspension a bit similar to the Horstmann or volute spring suspensions, except using leaf springs.
• (Hydropneumatic): Using gas instead of coils or torsion bar. I'm not aware of any WWII era tank that used it, although the basic technology itself was in widespread usage by then in aircraft landing gear in the form of "oleo struts". Used on some modern tanks, like the British Challenger, French Leclerc, and South Korean K2. The latest iteration of this technology called "in-arm suspension", evidently manages to fit the entire suspension system inside the bell crank for each road wheel, leading to a very compact system. And with modern electronically controlled stuff, can also be used to help elevate or depress the main gun by lowering/raising the front/rear of the tank separately.

 

[Admiral Beez]

Was it necessary for the bogie types to have two wheels connected? Having the suspension system outside of the tank offers benefits for repair and replacement, but could you have just one wheel connected to an external spring, without torsion bars?

 

[z42]

Was it necessary for the bogie types to have two wheels connected? Having the suspension system outside of the tank offers benefits for repair and replacement, but could you have just one wheel connected to an external spring?

You mean something like the Israeli Merkava tank suspension, which has individually suspended wheels with coil springs? If each wheel is individually suspended, perhaps it would then be classified as a Christie style suspension? Not sure where the border lies exactly.

 

[Admiral Beez]

You mean something like the Israeli Merkava tank suspension, which as individually suspended wheels with coil springs? If each wheel is individually suspended, perhaps it would then be classified as a Christie style suspension? Not sure where the border lies exactly.

Yes, like the Merkava. I don't think any tanks in WW2 had this layout? Why not, I wonder.

The suspension components are still external of the hull, so not Christie. Interestingly, Chrysler looked at putting Christie type suspension into the M4.

Chrysler's Improved Suspension M4A4 - Tank Encyclopedia

In 1942, the Chrysler company developed a new 'big wheel' suspension system for use of the M4A4 model of Sherman. None were built.

tanks-encyclopedia.com

 

[z42]

Yes, like the Merkava. I don't think any tanks in WW2 had this layout? Why not, I wonder.

I'm not sure really. It does seems like a decent design, in that it manages to combine several nice features:

1. All the suspension mounted within the track oval, so it uses space that otherwise would be wasted. Similar to the WWII era bogie suspensions.
2. Tried and tested coils spring technology, which was widely used long before WWII.
3. Each wheel has individual suspension, so ought to give good mobility.

The suspension components are still external of the hull, so not Christie. Interestingly, Chrysler looked at putting Christie type suspension into the M4.

Chrysler's Improved Suspension M4A4 - Tank Encyclopedia

In 1942, the Chrysler company developed a new 'big wheel' suspension system for use of the M4A4 model of Sherman. None were built.

tanks-encyclopedia.com

Crucially, here the springs are so long they're not able to fit them inside the track oval, and thus it increased the width of the tank.

 

[WARSPITER]

The Merkava suspension is basically a derivative of the original Horstmann suspension which does
allow external access and has been well sorted for decades.

 

[ThomasP]

Useful for a basic definition of the Horstmann suspension system:

"Horstmann suspension - Wikipedia"

Technically speaking, it appears that the Horstmann type was used on some of the Vickers Light Tank series - as well as the early Vickers Cruisers Mk I and Mk II and the Valentine in the Slow Motion form.

Yes/No?

 

[WARSPITER]

More or less and other suspensions used the same method with different spring and shock absorber setup
handling a pair of wheels. The Sherman system was basically the same thing but with volute springs rather than
coils.

The Christie type was good up to certain weights but was hard to maintain as the longer spring / shock absorber
gave more travel but was separated from the wheels, usually inside the hull which made maintenance difficult.

Tortion bar suspension had its ups and downs as well. Access to the bars being difficult when they broke and weight
distribution being another problem, especially when tanks were built with heavy armour and large guns which placed
a lot of strain on the front bars.

These systems were all significant improvements over the earliest tanks which didn't have any suspension. Apparently
they didn't need any because they were basically very slow. From film I've seen of WWI tanks going over uneven ground
I get the feeling the crews may have disagreed.

 

[z42]

The Merkava suspension is basically a derivative of the original Horstmann suspension which does
allow external access and has been well sorted for decades.

If you want to nitpick, I think one could argue that the Merkava suspension is not a Horstmann style suspension, as there's no bogie and all the wheels are independently sprung. If you look at the picture that A Admiral Beez posted above, it might seem like a bogie, but it's actually that two wheels share the fixed hull mounting stuff, and both wheels move completely independent of each other.

 

[WARSPITER]

In a way yes and in another not really as the moving bar is compensated for by the double coil springs.
Specification descriptions still describe it as Horstmann but maybe so to differentiate from other vehicle
types which designate independent suspension to be wheels on the same axle to have independent
suspension.

Marketing ?

 

[Elan Vital]

The US Engineering Design Handbook The Automotive Assembly has a section on suspensions which is a good starting point although some nuances or actually-built suspensions are not mentionned and the section mostly focuses on the US experience: https://apps.dtic.mil/sti/pdfs/AD0830268.pdf

I would note that "Christie" suspension gathers many very different layouts so the advantages and disadvantages typically claimed for this suspension are not the same.

You have the traditional style seen on British tanks with internal springs mounted between two armor plates with cross tubes running accross the floor and serving as the pivot.
The T-34 had them internally, but behind a single thick plate to provide the best ballistic efficiency compared to double-skin construction.
The US GMC T49/T67, the Christie Sherman concept, the French AMX-40 cavalry tank project, the Panzer IIIA and the Leichttraktor with independent coil spring suspension all had them externally, which can mitigate some of the loss of space induced by coil springs. I will note that bogies do not necessarily save width compared to independent coil springs.

The British A32 (extra heavily armored Cromwell), A35 (same for Comet) and A36 (same for A30) used a Christie suspension with "straddle mounted pivot shaft bearings". Unfortunately no archives explaining this particular system have come to light yet, but we know that it improved the weight limit (A35 weighed 36 tons). There was a 17 pounder Cruiser tank concept using an external Christie suspension of such a type which weighed around 40 tons if I recall. The Merkava shows that independent coil springs remain suitable even for very heavy tanks, and it is unlikely that 40 tons even was the absolute limit of the technology in WW2, just that nobody tried to go beyond. I don't have PM Knight's book on the matter to read again.

Both AMX-40 and apparently that last Cruiser (and possibly all the straddle-mounted types) dispensed with the cross tubes which take some room on the floor. The latter had a similar space claim to torsion bars, such that all Christie tanks featuring cross tubes could instead have been designed for torsion bars but with a net reduction in suspension space claim since the coil springs are deleted and the TBs use already-occupied-space.
Other independent suspensions with metallic springs existed. The A25 Harry Hopkins and its derivatives, the Vickers Vanguard and Valiant all used independent coil springs with wishbone mounts. The Panzer II used independent leaf springs.

Bogies were originally selected mostly out of familiarity as they already existed on trains. They can equalize the load and have a useful walking-beam effect at less than 10 mph (16 kph) which makes them (per the Americans) superior to independent suspensions below this speed. However, independent suspensions are straight up superior at higher speeds because they can allow greater wheel travel, and unlike bogies the compression of a first wheel does not halve the wheel travel of the next wheel in the bogie. Independent suspensions will also be lighter, simpler, easier to maintain and less susceptible to damage (might apply only to internally-mounted types) than bogies. Moreover, damage on a bogie has a much bigger effect than losing a wheel.

Compared to leaf, volute and coil springs, torsion bars are lighter and simpler, and they take space in height only which is far less problematic than width for the other springs (and height and width for springs with cross tubes). They are a lot more durable, are internal so less exposed to fire, rocks and other elements. Most torsion bar tanks incorporate easy removal methods. Maintenance requirements are nil for the spring itself, but TBs typically have fewer bearings than bogies or external springs so they are less maintenance intensive. I will note that repair/suspension replacement is not something that the crew can do, and will require specialized personnel and equipment regardless of the suspension type, and probably factory level repairs, so external springs are not particularly better here.

Hydropneumatic suspension saves weight compared to TBs, have more dynamic properties and can more easily handle weight increases, on top of adding extra functionalities. The main issue was ensuring they would remain sealed for a decent reliability life, which was not really feasible until the 60's. In modern times it's really just a choice between torsion bars and hydrop.

There is practically no reason to not use torsion bars the moment a suitable suspension is tested (mid 30s). Save for tanks with woefully insufficient power to weight ratios to run well beyond 10 mph, all WW2 tanks employing bogies (and worse, small wheels) in the interwar, WW2 and postwar periods are actively sacrificing high speed potential and even suspension durability (especially for small road wheels which wear faster and will take more damage from rocks). The Soviets tried to fix existing designs with independent suspension twice (T-26 with T-46 and T-28 with T-29 and torsion bar derivatives). Those efforts failed, but not due to the suspension itself.

The problem is that before the war, tank design did not always take a holistic approach where the impact of design decisions on other aspects of the tank are taken into account; and where all possible configurations of a given solution are not always researched. This resulted in multiple instances where independent suspensions (coil springs or otherwise) did not spread either because the associated tank in general was rejected, or because people were looking only at the "traditional Christie layout" which introduced a lot of extra disadvantages, or because a given suspension was rigidly reserved to a single tank class; e.g how Christie was reserved to light and fast tanks only when it would have been viable at much heavier weights if only the idea had been thought of or attempted.

 

[WARSPITER]

Good points on what was and wasn't feasible during the Second World War.

Production facilities come into play here as well. Changing systems that are reasonably durable probably
wasn't getting much traction when the war was already on due to the need to produce equipment.

Agree with the Hydro pneumatic suspensions. Japan has put a lot of work into these and the MBT's they
have show it off nicely. Again though, the 1930's / 40's weren't the time when these were feasible.

 

[EwenS]

Yes, like the Merkava. I don't think any tanks in WW2 had this layout? Why not, I wonder.

View attachment 832013

The suspension components are still external of the hull, so not Christie.

The Merkeva is a unique Israeli design born of their experience in the tank battles of the 1967 & 1973 Arab- Israeli wars and the circumstances in which they expected to fight the next.

The paramount feature of the design was the personal protection of the crew (Israel lost too many trained tank crews in its wars and they were not easily replacable). Firepower came second and mobility third. And it had to be large enough for its crew of 4 to remain in it in comfort for long hours of operational duty before they even got to combat. Every part of the tank (fuel tanks, ammunition, & tools included) had to play its part in crew protection. And experience showed that it needed more ammunition than other tanks. So you get a number of features.

1. Engine up front (to protect the driver). It was also designed for easy access and replacement if necessary (under 60 mins in combat conditions was the target for replacment so that the tank wouldn't be lost so easily to minor faults / damage)
2. Driver in the main crew compartment rather than separated so offering access to the rear exit.
3. Spacious crew compartment at the back end with a door offering a safer escape route than climbing out the hatches. This spacious crew compartment could also be used to carry additional ammo into action (battles in the Golan and Sinai showed Israeli armour tended to run out of ammo faster than they could be resupplied) or a small number of infantry (useful in Israel's cross border raids) or to evacuate casulaties.
4. The suspension was designed to cope with the rough terrain in the mountains of to the north of Israel as well as the hills to the east. And note from the photo just how much of the lower side armour is protected by all that suspension componentry (every part designed with a view to crew protection)
5. Hydraulics were minimised and placed in the turret bustle away from the crew, because war experience showed that when existing tanks were hit, hydraulic fluid got sprayed around inside the tank and burnt easily. All the ammunition was carried below the level of the turret ring.
6. It was a vehicle built for cautious fighting not speed, again something indicated by where it was expected to fight in the steep valleys of Lebanon.
7. It was heavy (62 tons) and relatively slow (46 kph v 72kph for an M1 or a Leopard 2) despite its powerful engine, designed to help it climb well in mountainous terrain.

The whole concept proved its worth in 3 months of fighting in the Lebanon in 1982. 7 Merkeva I were lost (6 to AT mines and the last to a barrage of RPG and Sagger AT missile hits). Only a handful of Merkeva crewmen became casualties. Far fewer than in Centurion & Patton units fitted with Blazer reactive armour.

 

[z42]

In a way yes and in another not really as the moving bar is compensated for by the double coil springs.

Hmm, are you saying that the thing where the top ends of the coil springs attach to can pivot, and thus the two wheels are connected? I hadn't thought of that, and now that I look at the picture I suppose it's possible?

In any case, this was the suspension for the Merkava Mk I and Mk II, for Mk III and IV they switched to a slightly different kind of suspension, which at least certainly is independent.

Picture of Merkava IV suspension assembly:

 

[WARSPITER]

Yes they did which is good because it means everybody is right for a change.

 

[Elan Vital]

My previous link, and this one summarizing US automotive trials go further on the American experience: https://worldoftanks.com/dcont/fb/document/tests_v6.pdf



A major weakness of VVSS is that it featured more exposed sliding surfaces, such that damping characteristics would change depending on environmental conditions and would prevent controlled damping. HVSS featured far less exposed sliding surfaces and was more conducive to the use of shock absorbers, allowing controlled damping.

Local experience showed that torsion bars were superior in many respects to volute spring bogies. They adjusted themselves more readily to irregular terrain and:

Other trials showed larger or lesser gains and more details depending on the state of development of HVSS. But it goes to show that no two bogie suspensions are the same. 1945 and 1946 trials demonstrated the potential for even greater reliability/durability and somewhat better ride for HVSS, but Shermans used in the postwar period didn't benefit from this.

The torsion bar setup on the M4A2E4 did not prevail in large part because of a flawed installation and the fact that this suspension was easier to set up in a new tank like the T20 series rather than an existing tank designed with bogies in mind. The ground clearance was only 8", half of the minimum normally necessary.

1945-46 developments further improved torsion bar durability for M26, but this was also not applied on this tank. Had the war continued or had M26 continued production in peacetime, it could have received substantial improvements.

 

[z42]

Thanks. It's interesting though that torsion bars were once if not widespread at least not entirely unused on cars. E.g. Porsche 911 used to have it, see diagrams on

https://www.airbrigade.com/post/porsche-torsion-bar-suspension

But for some reason they have disappeared from use. Maybe if you combine a coil spring with a shock absorber ("coil-over" shocks) you can claw back much of the "wasted" volume for coil springs vs torsion bars?

Formula 1 does seem to use torsion bar suspension.

Maybe it's all more a question of the geometry and how you can efficiently package the suspension components into whatever vehicle one is discussing?

 

[WARSPITER]

The 69 Dodge Charger had torsion bars on the front which allowed the ride height to be changed by adjusting the torsion.
The ride was good but the mounting system had to be hefty and there was always the possibility of something giving way
or the bar failing.

As cars have been required to be lighter to help fuel efficiency the Macpherson strut system and other coil spring types
have become more popular.

One explanation I have seen is that the bar or coil is the same thing anyway - the coil spring is just a coiled torsion bar or
the torsion bar is just a straightened coil spring.

Coil springs on road cars do have the advantage of being variable rate by having different spacings between the coils so you
get softer springing over smaller bumps and progressively harder tension over larger ones or when cornering harder.

On a formula one car, commercial truck, or military / heavy vehicle the torsion bars work very well as the conditions are different.
The race car is used on a good surface and has wide tyres with little to no body roll when cornering. Larger vehicles roll less due
to being heavy in the first place.

In other words - what he said....

Maybe it's all more a question of the geometry and how you can efficiently package the suspension components into whatever vehicle one is discussing?

 

[z42]

The ride was good but the mounting system had to be hefty and there was always the possibility of something giving way
or the bar failing.

As cars have been required to be lighter to help fuel efficiency the Macpherson strut system and other coil spring types
have become more popular.

Also in cars with coil springs you need reinforced "turrets" to hold the suspension and take the forces generated. No free lunch etc. Perhaps the closest thing to a free lunch with coil springs is that the springs and shock absorbers can use the same mounting.

One explanation I have seen is that the bar or coil is the same thing anyway - the coil spring is just a coiled torsion bar or
the torsion bar is just a straightened coil spring.

Yes. If you think about it, when you compress or extend a coil spring, the spring energy is stored as torsion in the coil.