Limping Jagdtiger

The German Jagdtiger tank destroyer was built in two variants: one with a Porsche suspension and one with a Henschel one. The former was deemed poor and removed from production. This fact is stated in every publication dedicated to the Jagdtiger, but is not discussed in detail. In this article we won't stop at brief descriptions, questionable overviews, and conclusions, but instead will discuss how German late war tank suspensions were developed, cover the Porsche suspension, then present the history of the Jagdtiger's suspension in the appropriate context. Our tale will be based on an analysis of the design rather than compilations of opinions of other authors.

Development of running gear

One of the biggest issues the Germans came across in their work was the insufficient lifespan of rubber tires. One attempt to improve their lifespan was the use of additional interleaved road wheels. This worked, but as new tanks became heavier and heavier even this approach failed. The outermost row of the 57 ton Tiger H1's road wheels had to be made removable so it could fit into the railway gauge for transport, but even then the lifespan of the rubber tires left much to be desired. Meanwhile, the future Tiger II was going to weigh over 65 tons.

Drawings of the Tiger II's road wheels with internal shock absorption.

The Germans turned to road wheels with internal shock absorption. The steel rims of these wheels had a rubber lining between them and the wheel disk. Normal rubber rims offer better shock absorption, but only a small part of the rubber is active at any given time. Due to these higher loads it overheats and wears out faster. Road wheels with internal shock absorption have rubber lining where the entire mass works at the same time, and thus they can withstand much greater loads and wear out slower.

A unification of the Panther II and Tiger II road wheels was performed during their development. 800 mm diameter road wheels with internal shock absorption were designed. The dual road wheels were arranged in two rows, so at least the outer row could be removed without issue. Compared to the Panther and Tiger H1, the number of road wheels was reduced, and now they no longer stuck out past the railroad gauge. However, there was an issue. The existing tracks were developed for wheels with rubber tires, and did not last long with the new wheels, particularly due to the guide horns breaking. Designers developed tracks with two types of alternating links (they are often called composite, two-part, or doubled in various sources). One track link was cast with two guide horns and prominent grousers. The connecting link, as the name implies, merely connected two of the other links together. It had no guide horn or grouser. It consisted of several forged parts with space for the drive sprocket in between them.

Trials of the new wheels and tracks on the first Panther prototype, summer 1943.

The Panther II tracks made of Gg 24/660/300 links were also used as transport tracks for the Tiger II. The connecting links were assembled from two parts: a larger one and three smaller ones. This allowed the Germans to do the following. The Panther II tracks could be used as transport tracks for the Tiger II, plus the 660 and 800 mm tracks used the same connecting track links. The left and right tracks now used interchangeable links, unlike the early Tiger H1, which had different track links for the left and right tracks. Finally, the connecting links weighed a lot less than the main ones, and so the tracks became lighter. The first MAN Panther prototype was converted to use the steel rimmed wheels and tracks with alternating track links.

Assembling transport and combat tracks on the Tiger II and Jagdtiger.

In addition to the Panther II and Tiger II, the Schwerer Wehrmachtschlepper heavy halftrack and Maus and E-100 superheavy tanks used tracks like these. Another favourite pastime of German designers was to transition to single wheels from double ones. All tanks of the E-series except the E-100 were designed with single steel rimmed road wheels. The final Pz.Kpfw.Maus also had running gear with single road wheels. The Porsche Jagdtiger suspension was designed according to the same principle.

Porsche suspension with parallel torsion bars

A distinguishing feature of Porsche's wartime designs was a bogey suspension with parallel torsion bars. Some publications characterize it as compact, easy to repair, and effective. One is left to wonder what the definition of effectiveness is in this case, as no one used it after the war. Other articles criticize it as complex and unreliable, as the short torsion bars were prone to breaking. We won't repeat such evaluations here, but instead take apart the design of the suspension and explain how it worked.

Cutaway drawing of the suspension bogey.

A Porsche suspension consists of a suspension arm, casing, torsion bar, rubber dampening pad, the torsion bar, and the cam gear that twists the torsion bar. The suspension arm is affixed to the hull with a rotating joint. One end of the arm is fixed rigidly to the axle of the first road wheel. A rubber pad is affixed to the other end. A tilting casing is installed on the axle of the first wheel. The axle of the second wheel is attached to the axle of the second wheel. The casing contains the torsion bar and the cam gear that twists it. The torsion bar is attached with a spline coupling to the casing on the side of the second wheel. On the side of the first wheel, it is held in a pipe with a ball bearing. A lug is attached to the pipe that presses against a lug rigidly attached to the axle of the first wheel.

Suspension drawings (via Harold Biondo).

The suspension works thusly. When the tank hits an obstacle, the road wheel will move and move the casing with it. The lug on the pipe will press against the lug on the axle of the first wheel, which will make it turn relative to the torsion bar and twist it. On the side of the second wheel, the casing presses against a rubber pad that limits the suspension travel. After the pad is compressed, all impacts will be rigidly transferred to the hull of the tank. The casing has a divot with a rubber pad that prevents it tilting in the opposite direction.

Blueprint of the first type of bogey with a skid (via Harold Biondo).

Note the following features of this suspension. There is only one swing arm per two wheels. The wheel axles are always the same distance away from one another. The torsion bar twists as the angle between the casing and swing arm changes (knee action). The Porsche suspension was very compact and easy to install. It does not take up space inside the hull and is protected by the road wheels from the outside. However, it has serious drawbacks. The short torsion bar can't twist very much, and so the suspension travel is short. The suspension is very rigid and not appropriate for high speed vehicles. The road wheels can only travel a large distance if their opposite road wheel travels the same distance in the opposite direction. The sprung weight of the tank rests on 6-8 compact cams, which requires the use of expensive hardened steel. The relative complexity of the bogey is hardly a drawback, since it's compensated by the smaller amount of assemblies and ease of installation. There were also no anomalies with the torsion bars, as the rubber pads saved them from excess weight.

A promising alternative

Having covered all of the above, we can now understand the history of the Jagdtiger. This SPG was to be assembled at Nibelungenwerke. This factory assembled the Tiger (P), Ferdinand, and Pz.Kpfw.IV: fighting vehicles with external suspensions and traditional non-overlapping road wheels. The Jagdtiger required different technologies. The Henschel suspension had its torsion bar ball bearings resting on the armour plates, which meant that a large amount of openings had to be precisely cut and finished. This was a complex and expensive process, not to mention that it was impossible to do ahead of time due to deformations caused by welding. 

A British drawing of the first suspension variant with skids that supported the upper run of the track. This suspension was tested on chassis #305001 (via Harold Biondo).

Ferdinand Porsche convinced Hitler to allow the installation of a simplified Tiger (P) suspension with parallel torsion bars on the Jagdtiger. It reduced the weight of the vehicle by 2680 kg (1200 kg according to other sources), reduced the time required to finish the hull and suspension elements by 450 hours, and reduced the cost of the tools required by 404,000 Reichmarks, or almost half. Repairs were also simplified, since a bogey could be replaced in front line conditions relatively easily. These claimed advantages should be taken with a grain of salt, since we don't know how well they were implemented in practice. For example, the Jagdtiger's hull was minimally changed, and therefore the freed up space was not used rationally. It was not possible to reduce the height of the hull or increase the clearance like on the Tiger (P), since it would require changing the transmission's air channel, engine frame, etc.

Chassis #305001 on trials. A gun was later installed for a full comparison with the Henschel chassis. The skid of the first bogey can be seen.

The proposal was accepted. Porsche designed a new running gear with four bogeys and eight wheels per side. The torsion bars were 1075 mm long (to compare, the torsion bars on Henschel's suspension were 1960 mm long). The 700 mm diameter road wheels (Henschel's wheels were 800 mm in diameter) were installed in two rows, overlapping each other. A row of guide horns ran between them. The width of the steel rim was greater than on Henschel's wheels: 88 vs 75 mm. The full suspension travel was 165 mm (73 mm static, 92 mm dynamic), which corresponded to the torsion bar twisting by about 13 degrees. This running gear was similar to the E-75 project and followed the latest trends in German tank development: external suspension, single steel rimmed road wheels instead of dual rubber rimmed ones, and tracks with one guide horn on every other link.

A Henschel type idler photographed on chassis #305001

Nibelungenwerke built the first two Jagdtigers in February of 1944: #305001 with the Porsche suspension and 305002 with the Henschel suspension. Both SPGs were unique. Jagdtiger #305001 had skids on top of its bogeys which were later dropped. The idler was borrowed from the Henschel suspension, which required keeping both rows of guide horns. The Henschel running gear had two guide horns per track to match the two rows of wheels. Since Porsche's track guide horns ran between two rows of road wheels, there was no need for an external row. Henschel's idler ran on the outer row of guide horns, since the last wheel was in the outer row. The Jagdtiger #305001 had to have tracks with two layers of guide horns, with the inner row partially trimmed. In the future, the Porsche suspension used a different idler which ran on the inner row of guide horns, and the outer could be almost fully removed.

A Porsche type idler used starting with chassis #305003.

Jagdtiger #305002 was also unique. It was built with a Henschel suspension, but its side armour was initially prepared for the Porsche suspension. The Porsche suspension was already in production, and SPG #305002 was only built to compare the designs.

A lineup of problems

Comparative trials of Jagdtigers #305001 and 305002 began on May 5th, 1944. The Jagdtiger with a Porsche suspension shook during motion. The new tracks made of Gg 24/800/300 links were suspected. Jagdtiger #3005003 was built with Ferdinand tracks made of Kgs 62/640/130 links and a new idler for single guide horn tracks. Trials once again showed significant shaking, which died down as the SPG accelerated to 14-15 kph on a level road. As a result, a decision was made to produce tank destroyers with the Henschel suspension. Some also say that the breaking of a bogey had an effect on the conclusions, but the author had not seen any direct confirmation. This breakdown happened on Jagdtiger #305004, while suspension problems were discovered on SPGs #305001 and 305003. Furthermore, the part that broke was the rib, rather than any element specific to the Porsche suspension such as the torsion bar cam gear.

Jagdtiger #305003 with Ferdinand tracks and Porsche idlers. Switching out the tracks didn't help.

As confirmed by end users, the switch to the Henschel suspension was correct. The 653rd Tank Destroyer Battalion that received 7 Jagdtigers with the Porsche suspension observed the following issues:

1. The shaking from the alternating guide horn tracks was enough to upset the calibration of the gun. The suspension was too rigid.
2. The singular road wheels loaded the tracks unevenly, as a result of which track links bent and track pins broke when driving cross-country.
3. The presence of different and incompatible types of running gear on the same vehicle made repairs more complicated.

Erwin Aders, the head of the Henschel design bureau, prepared a note on the development of the Tiger II, which was conducted under his supervision, in February of 1945. Aders wrote that the running gear with alternating composite track links and two layers of double road wheels first caused concern. The road wheels did not load the tracks evenly, and the tracks were even wider than on the Tiger H1. The composite tracks were also not very rigid, and so issues with breaking track links and track pins were expected from the start. Finally, there was significant wear on every other drive sprocket tooth when they were used. All of the above applied to the 68 ton Tiger II with nine dual road wheels per side. One can infer the severity of these problems on the 74 ton Jagdtiger with eight single road wheels per side.

Gg 26/800/300 tracks close up, courtesy of Panzer Fakten group (https://www.facebook.com/groups/PanzerFacts)

After assembling 11 Jagdtigers with the Porsche suspension, the Nibelungenwerke factory switched to the Henschel suspension in September of 1944. It already changed by that point. New Gg 26/800/300 cast tracks were designed. The connecting links consisted of one part instead of four. This made them much tougher, but there was no opening for drive sprocket teeth. As a result, tanks with these tracks had nine teeth instead of 18. Delivery of Gg 26/800/300 track links began in July of 1944. Old drive sprockets simply had every other tooth cut off. As for the Jagdtigers, new tracks and drive sprockets were introduced in September along with the new suspension.

The second Tiger II prototype with Kgs 73/800/152 track links. The can be distinguished from the Gg 24 and Gg 26 by the presence of guide horns on each track link as opposed to every other one.

The introduction of new tracks solved one problem and caused another. Load from such a heavy vehicle was too much for only 9 drive sprocket teeth. The idea of alternating track links lead to a dead end and a new design was needed. The Tiger II was equipped with new Kgs 73/800/152 track links starting in March of 1945, which allowed the return of 18 drive sprocket teeth. These were never introduced on Jagdtigers. Erwin Aders noted continuing issues with the steel rimmed wheels, as the bearings easily shifted and jammed. The amount that the wheels could shift compared to the guide horns was also less than on the Tiger H1.

Conclusions

Evaluation of the Porsche suspension usually lands in one of two camps. One says that the Porsche suspension was a reasonable alternative, but mass production was cancelled thanks to a prolonged conflict between Ferdinand Porsche and the Ordnance Directorate, exacerbated by the failing bogey. The other camp claims that the Porsche suspension was already poor when torsion bars started to fail on the Tiger (P). The fact that the Germans used the same suspension on the Jagdtiger merely meant that they learned nothing from comparative trials of the Tiger tanks. Note that in both cases the problems with tracks and drive sprockets are not mentioned.

Jagdtiger #305004 running gear, Bovington tank museum.

The aforementioned facts cause us to reject both of these common theories. On one hand, the Porsche suspension was indeed a bad match for the Jagdtiger and moving to Henschel suspensions was the right move. On the other hand, it was not just one technical solution such as the bogey suspension with a parallel torsion bar that was wrong, but all the solutions put together. The narrow single road wheels put uneven pressure on the wide tracks, which was exacerbated by their position in two rows. The tracks with composite links had poor toughness and bent easily.

Individually, these solutions could have worked. The Pz.Kpfw.Maus had a running gear with single road wheels and composite tracks, but the wheels were spread out among four rows to evenly distribute the load. The Ferdinand had a parallel torsion bar suspension, but it was coupled with regular tracks and doubled steel wheels with wide rims (84 mm vs 75 on Henschel running gear) and therefore worked well. In case of the Jagdtiger, the issues made one another worse.

Suspension, running gear, and tracks of the Maus tank. The composite alternating tracks can be seen.

It's impossible to blame Ferdinand Porsche alone for the failure of the Porsche suspension on the Jagdtiger. It was the 6th Department of the Ordnance Directorate led by Heinrich Kniepkamp that promoted the idea of single road wheels split into two rows and composite tracks. A similar suspension with eight single road wheels in two rows and four bogeys was planned for the E-75. The load on each road wheel for the SPG planned on this chassis was close to that of the Jagdtiger (4160 kg vs 4100). Porsche tested this concept and demonstrated that even though the E-50 and E-75 were not even ready on paper, they already had to be redone. Erwin Aders from Henschel also spoke of the Ordnance Directorate's influence.

Finishing hulls at Nibelungenwerke. The factory received Henschel's complicated suspension design without much enthusiasm.

The use of the Henschel suspension on the Jagdtiger replaced a terrible design with a bad one. Oberstleutnant Johannis noted in his report on January 16th, 1945, that Nibelungenwerke considers the design to be foreign and repulsive. Despite all the aforementioned changes, the Germans never managed to get the Tiger II and Jagdtiger running gear right, even though the late Jagdtigers essentially had a completely different running gear than the old ones. 

Original article by Dmitry Zaitsev.