Silent reducer in Tarpan Honker - is it possible?

The sound of a working reducer is associated with both the 4011/4021 Tarpan Honkers and the later 4012/4022 versions produced at the Poznan FSR. Some call it a monstrous noise that is difficult to bear, others call it a whistling, howling or even singing noise that intensifies during acceleration and fast driving. The good news, however, is that there is a way to make the honker reducer of this generation much quieter and at the same time improve its reliability.

I was prompted to address the topic by two important factors. The first (key) was the spouse's comments about too much noise while driving. A the second, more accidental. I acquired an additional reducer. It was to be a 2nd generation device, that is, by design, quieter. Unfortunately, it turned out that components presented this reducer as a first-generation machine, that is, the one I had installed in my Tarpan Honker 4012.Already during the replacing the bearings in the reducer, which we had installed in the car during the rebuilding the vehicle, my attention was drawn to the types of bearings used in the reducer. I was puzzled by me by their, for my taste and knowledge, erroneous selection. At the time, however, I did not have time to analyze the issue in more detail, since the main idea was to bring the vehicle to a running condition as soon as possible. Now having an already operational vehicle I had more time to think about the causes of the noisy operation of the reducer that we experienced during the family drive. I therefore assumed that if I could fix the incorrect way of bearing, the performance of the of the reducer should noticeably improve. This is how I got involved in the "silent 1st generation reducer project in Tarpan Honker". For comparison purposes comparison just before replacing the reducer with the modified one, we made a recording of the of the drive with the reducer before the modification. Here it is:

As you can see, or rather hear - it was not quiet. The bad impression increased with speed, and driving above 50km/h became difficult unbearable in the long run. So I set about systematizing available information on the reducers used in Tarpan Honkers 4011, 4012, and later in Honkers 2324 and 2000.For obvious reasons, I dealt with only the reducers produced by Polmo Gniezno, and not the Borg Warner reducer, which was occasionally installed in some vehicles.

Reducers before 1 generation

The first reducers used in the PW1 prototype car were devices based in design on gearboxes and reducers produced by, for example, the Tczew factory and used in cars of Soviet production (e.g., UAZ, ZIŁ 157).

These were gears with spur gears. They used a simple bearing arrangement based on ball bearings and roller bearings.

Fig.1. T-550 gearbox

Fig.2 UAZ reducer

As the castings of all reducer bodies bear the FSR (Factory of Agricultural Vehicles in Poznań) markings, one can it can be assumed that the molds and prototype gears were produced in Poznań. In the later period, however, their already serial production was carried out in the POLMO plant in Gniezno.

The Tarpan Honker 4011 and 4021 user manuals list two types of reducers:

1) 4-wheel drive with inter-axle differential

2) Rear drive with option to attach front drive (very rare version later replaced by Borg Warner reducer)

In the illustrative drawings in the aforementioned manual from 1989, the gears in the gear reducers are marked as having straight teeth. However, in the gearbox, this component has oblique teeth. We have a similar situation also in spare parts catalogs - there, too, gears with straight teeth are marked straight on all shafts of the reducer.

Fig.3. 1989 instruction manual - diagram of the reducer

Fig.4. Diagram of the reducer - spare parts catalog.

1st generation reducers (1989 - 1996)

Most likely, while still testing the prototype it was decided to make changes to the reducer. The gears with straight teeth were changed to gears with oblique teeth (about 20 degrees).

Fig. 5. Bevel teeth in the 1st generation reducer

This treatment, on the one hand, strengthened the gears by increasing the mating surface, while on the other hand it introduced axial forces on the shafts of the reducer, not previously present, axial forces. At the time, this was not a major problem. Therefore, it can be assumed that the change in tooth type did not had an impact on the deterioration of driving comfort (mainly noise), since at the time cars were equipped with a 1500cc gasoline engine. Powered by gasoline engine, the car did not develop high speeds and therefore did not put excessive strain on the reducer. Therefore, with a high degree of probability it can be conclude that the nuisance whistles did not manifest themselves to the team responsible for implementing the implementing the modification. Or perhaps the engineering team's task was merely to strengthening the gears. This task was accomplished, unfortunately, at the expense of reduction in ride comfort. This became strongly noticeable after the use of more powerful drive units in later years.

In the early 1990s, it was determined that it was necessary to to apply a more powerful engine to the vehicle this time diesel. At that time users received new versions of the car designated as 4012 and 4022. The cars were equipped with reliable Iveco Sofim 2.5 and Iveco Sofim 2.5 diesel engines. were equipped with reliable diesel engines Iveco Sofim 2.5 and Iveco Sofim 2.5 TD. Compared to the gasoline unit, they provided higher power and more torque. Reducers therefore had to face completely new for them them operating ranges. These were very demanding for the intermediate shaft bearings.

On the input and output shaft there are bearings roller and ball bearings, which are able to transfer the axial forces that occur at the expense of excessive wear on the ball bearings. On the other hand, on the intermediate shaft intermediate shaft, 2 roller bearings are used. They carry only radial loads radial loads, so axial forces change their operation to some extent from rolling to sliding and lead to their much faster wear. The accompanying effect is the resulting annoying noise. It appears as information about a malfunctioning malfunctioning system.

This whistling (or howling if you prefer) reaches its moment of climax when the car is accelerated at speeds above 40-50 km/h. The loads that develop above these speeds cause the lubricating wedge on the roller bearings (the side surfaces of the rollers that work slidingly, not not rolling) begin to be ripped off by axial forces. The bearings rub the sides of the rollers against the thrust washers. Thus, there is dry or mixed friction instead of sliding, and thus vibrations occur. These cause increasingly loud whining. It intensifies as the speed increases and thus the greater load on the reducer. On the other hand, if we stop the acceleration process and stabilize the speed (the car will neither slow down nor accelerate) - the reducer will stop howling, because it is no longer affected by the high loads with which it has to deal in moments of gaining momentum.

It is also worth adding that the friction of the sides of the rollers negatively affects the service life of the intermediate shaft roller bearings and the oil of the transmission oil, which is degraded by contact with the overheating bearing.

2nd generation reducers

Probably at the end of the production of Tarpan Honkers 4012 at the Poznań factory, that is, around 1995, it was decided to eliminate the the imperfection described above. The cars began to be fitted with a modified structurally modified reducer, the so-called 2nd generation reducer.

The changes between the 1st and 2nd generation reducers were major, although invisible from the outside. Among other things, the gears were modified by increasing the inclination of the teeth to about 40 degrees. This made it possible to increase in the number of teeth, and this in turn translated into increased strength of these elements and, very importantly, also reduced the intensity of noise generated by the gears.

Fig.6. Modified gears - greater tooth inclination and greater number of teeth

The changes also resulted in higher axial forces on the shafts of the reducer. Thus, the bearing of the intermediate shaft was changed. Instead of one of the roller bearings, a double-row spherical roller bearing was used.

Unfortunately, it is difficult to say why, but no changes in the bearings of the input and output shafts. The bearings used there are designed for gears with straight teeth.

Another change was the shortening of the input shaft, which was most likely elongated in earlier versions and had a multi-spline for easy use with a power take-off attachment (e.g., an additional alternator, hydraulic pump, winch, etc.). In the 2nd generation reducers this option was abandoned by ending the shaft on a bearing. Also simplified was the sliding bearing of the input shaft gears, which unfortunately negatively affected the on its service life.

All the changes described were made without changing the shapes of the reducer housing castings and bearing covers. For today's users of Tarpan Honkers, this means that in theory the internal mechanisms are interchangeable in sets between reducers of all generations. However, one must take into account that the position of one of the circlips may be different on 1st versus 2nd and 3rd generation reducers.

3rd generation reducers

This brings us to the 21st century, when there is another, already the third change in reducers. This time, however, it was of a more cosmetic. As was already indicated in the first and second generation, the gears of the road and off-road gears and the input shaft were supported by plain bearings. During the modification work of the second generation, the lubrication mechanism for these bearings was simplified. This was probably most common reason for their rapid seizure in the absence of periodic transmission oil changes or operating the car with low oil levels. W modification work, it was therefore decided to replace the plain bearings for needle bearings.

Fig.7. Needle roller bearings in the 3rd generation reducer

It's hard to say what really guided it, because the the upgrade had no positive effect on the bearing of the input shaft gears input shaft. The reason for this was the limited access of oil of gear oil to the needle bearings. It was very difficult or even impossible also spontaneous removal of contamination caused by operation and corrosion. For this reason for this reason, most regenerations of reducers of this generation ended with a change of the bearing to plain bearings, so to speak, undoing the modification.

Changing the bearing arrangement to adapted to gears with helical teeth.

At this point, one might be tempted to conclude. Everything that has been said above unequivocally indicates that throughout the entire period of production of reducers, no one has seriously considered the cause of their noisy operation and their high failure rate. Perhaps there was no will to introduce modifications eliminating the identified infirmities. Even before 1989, after changing the type of gears gears, the entire bearing arrangement should have been converted to one that could transfer axial forces freely.

Fig.8 Modified 1st generation reducer

In most gear reducers where gears with angular gears, tapered roller bearings (angular contact roller bearings) are used, which, when properly tensioned, can proper tension are capable of carrying both significant axial loads, as well as radial loads. (Photo from

As part of the modification of the first-generation reducer, I made a a bearing change to the one that should be used in these reducers. All three shafts were fitted with tapered roller bearings and adjusted between the covers to get their proper preload. After installation of the reducer, I decided to conduct a road test. Its video and at the same time audio recording I present below. For the rest, see for yourself:

The difference in volume is noticeable. The use of appropriate bearings has greatly quieted the operation of the reducer, and the characteristic whine, whistling is heard in a very small range natural for gears. As it is, the car without inconvenience associated with noise, you can cover considerably greater distances. distances. Indeed, a road test showed that the first-generation reducer stopped howling at speeds above 40-50km/h and thus provides very similar travel comfort as the second-generation reducer, with which I had the opportunity to to compare driving comfort.

In conclusion, in my opinion, the simplest and at the same time most effective method of silencing the honker reducer is to modify the of the three shafts. It consists in adjusting their dimensions to those required by the tapered roller bearings require. From a theoretical point of view, the use of bearings tapered roller bearings should also extend the life and durability of the entire system. This however, will only be possible to confirm after some time. Undoubtedly, the change has a positive effect on increasing travel comfort. The difference really is big, and the joy of solving the problem is also gratifying. Anyone interested in details are invited to contact me via social media.

Tomasz Hryniów

P.S. Thanks to Wojtek Wojas for his help in obtaining materials and sharing his knowledge and photo materials on the about the different versions of the reducer, to Jarek Chojnacki for his help in editing my unstructured thoughts and processing the videos, and to Wife for her support with the project.

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