The design of trucks and special equipment often involves the use of non-standard solutions to increase load capacity and stability. One such solution is group of closely spaced dual axles — an engineering solution that allows you to distribute the load on the roadway and improve vehicle controllability. In this article we will look at what this design is, where it is used, and what nuances are important to consider during operation and repair.
Twin axles, located at a minimum distance from each other, allow you to increase the number of wheels without significantly lengthening the vehicle's base. This is especially true for dump trucks, road trains and other equipment that requires high load capacity while maintaining maneuverability. However, this design has its own characteristics: from the specifics of suspension settings to the requirements for tires and the braking system.
If you own a truck with a similar configuration or are planning an upgrade, this article will help you understand the intricacies of the device, the benefits and potential problems that you may encounter.
What is a group of closely spaced dual axles?
A group of closely spaced dual axles is two axles installed at a minimum distance from each other (usually 1 to 1.3 meters), which function as a single unit. This arrangement allows:
- 📦 Increase the carrying capacity without changing the dimensions of the vehicle.
- 🛣️ Reduce the load on the road surface by distributing the weight over more wheels.
- 🚛 Increase vehicle stability, especially when driving off-road or at high speeds.
- 🔧 Simplify the design compared to three-axis schemes, where additional balancing is required.
It is important to understand that contiguous axes are different from tandem (where the distance between the axles is greater, for example, 1.8–2.5 m). In a tandem arrangement, the axes can operate independently, while in a close arrangement they are rigidly connected and perceive the load as a single whole.
This design is often found in:
- 🚚 Dump trucks (for example, KAMAZ-6520, Scania R470).
- 🚜 Agricultural machinery (combine harvesters, tractors with trailers).
- 🏗️ Construction equipment (truck cranes, concrete mixers).
- 🚛 Road trains with increased load capacity.
Advantages and disadvantages of close tandem axles
Like any technical solution, a group of closely spaced axes has its pros and cons. Let's look at them in more detail so that you can evaluate the feasibility of such a design for your tasks.
| Benefits | Disadvantages |
|---|---|
| ✅ Increase in load capacity by 20–30% without lengthening the frame. | ❌ Complicated wheel balancing and increased tire wear. |
| ✅ Better distribution of load on the road (less damage to asphalt). | ❌ Higher maintenance cost due to increased number of parts. |
| ✅ Increased stability when cornering and braking. | ❌ Difficulties with passing technical inspection (axle load requirements). |
| ✅ Possibility of use in off-road conditions. | ❌ Increased fuel consumption due to additional weight and drag. |
A critical nuance: if the suspension is incorrectly configured, close axles can cause the car to “yaw” at high speeds (the “snake” effect). This is due to the fact that a small gap between the axles leads to resonant vibrations if the shock absorbers or springs are worn out.
Another important point - legal restrictions. Some countries (eg the EU) have strict maximum axle load regulations. Closed axles overcome these limitations, but require certification and regular inspections.
When purchasing equipment with close axles, check for documentation on the permissible load on each axle - this will help avoid fines during transportation.
Design and principle of operation
Structurally, the group of closely spaced dual axles includes:
- 🔧 Bridges (master or slave, depending on the model).
- 🛞 Suspension (spring, pneumatic or combined).
- 🔗 Balancing mechanism (distributes the load between the axles).
- 🦾 Brake system (disc or drum brakes on each axle).
- 🔄 Differentials and gearboxes (for driving axes).
The operating principle is based on rigid connection between axes, which perceive the load as a single whole. A balancing mechanism (often in the form of an oscillating beam) ensures even weight distribution, preventing overload on one of the axles. Used in drive axles center differential, which allows the wheels to rotate at different speeds when cornering.
Design Features:
- 🔄 Center lock (optional on some models, e.g. MAN TGS) - improves off-road performance.
- 🛠️ Air suspension (in modern trucks) - allows you to adjust ground clearance and rigidity.
- 🔧 Reinforced hubs and bearings - due to increased loads.
One of the key problems during operation is uneven tire wear. Due to the close proximity of the axles, the inner and outer wheels experience different loads, which requires regular tire rotation (every 10–15 thousand km).
Why does a car with close axles “yaw”?
This effect occurs due to resonant vibrations when the frequency of vibration of the front and rear axles in the group coincides. Causes: worn shock absorbers, incorrect tire pressure, wheel imbalance or damaged suspension. Elimination requires diagnostics on a bench and replacement of worn elements.
Typical faults and their symptoms
Operation of equipment with close axes requires increased attention to diagnostics. Let's look at the most common problems and their symptoms:
| Malfunction | Signs | Possible reasons |
|---|---|---|
| Uneven tire wear | "Bald" spots on the tread, squealing when turning | Improper balancing, low tire pressure, worn bearings |
| "Yaw" at speed | The car wobbles when driving in a straight line | Worn shock absorbers, wheel imbalance, damaged frame |
| Knocking in the suspension | Metallic impacts when driving over bumps | Worn bushings, cracks in springs, play in beams |
| Brake overheating | Burning smell, decreased braking efficiency | Seized calipers, worn pads, faulty pneumatic system |
Particular attention should be paid center differential (if it is in the design). Its malfunction appears as:
- 🔊 Crunching or grinding when turning.
- 🚗 “Wedging” of wheels on one of the axles.
- 💨 Increased fuel consumption due to increased resistance.
Warning: if you ignore knocks in the suspension, this can lead to destruction of bridge fastenings and emergency situation. For example, on KAMAZ-65115 There are known cases when, due to wear of the beam bushings, the bridge shifted, which led to the wheels jamming.
☑️ Diagnostics of the suspension of close axles
Maintenance and repair: key points
Regular maintenance of vehicles with close axes includes several critical procedures:
- Checking and adjusting tire pressure. The pressure should be the same on all wheels of the group (recommended values are indicated in
Operating manual). A difference of even 0.2 atm can cause uneven wear. - Suspension diagnostics. Every 20 thousand km check:
- 🔧 Condition of springs and air cylinders.
- 🔗 Play in hinges and bushings.
- 🛠️ Tightness of shock absorbers.
wheel bearings and constant velocity joints (CV joints).When repairing, it is important to consider:
- 🔧 Replacing wheel bearings must be carried out in pairs (on both axes of the group).
- 🛠️ Center differential repair requires specialized equipment.
- 🔗 Replacing springs must be carried out taking into account the load (reinforced springs for close axles differ from standard ones).
Attention ⚠️ When replacing suspension parts (such as shock absorbers), use only spare parts certified by the manufacturer. Installing non-original parts may result in loss of warranty and increased risk of accidents.
Regular rotation of tires (in a crisscross pattern) increases their service life by 20–30% and prevents uneven wear.
Legal aspects and requirements for axle loads
In Russia and the EAEU countries there are strict standards for axle loads for freight transport. For closely spaced axes, the following rules apply:
- 📜 Maximum axle load: 10 tons (for a single axle) or 16 tons (for a double group with a distance between axles up to 1.3 m).
- 📏 Distance between axles: if it exceeds 1.3 m, each axle is considered separately (maximum 10 t per axle).
- 🚛 Total weight of the road train: up to 40 tons (for 5-axle trains).
If these standards are exceeded, the driver may be fined up to 50,000 rubles (under Article 12.21.1 of the Code of Administrative Offenses of the Russian Federation). In addition, the vehicle may be sent to the impound lot until the violation is eliminated.
To avoid problems:
- 📋 Always carry with you
certificate of approval of the vehicle for operation(it indicates the permitted axial load). - ⚖️ Use weight control points to check the load before the flight.
- 📊 Lead axial load log (requirement for commercial vehicles).
Attention ⚠️ In some regions (for example, Moscow and St. Petersburg) there are additional restrictions for freight transport. For example, a ban on driving on the Moscow Ring Road during the daytime for vehicles with an axle load of more than 6 tons.
Modernization and tuning: when is it justified?
In some cases, equipment owners decide to upgrade the suspension by adding closer axles to increase load capacity. However, such tuning has its own nuances:
When modernization is justified:
- 🚜 For agricultural machinerywhere high throughput is required.
- 🏗️ For construction dump trucksworking off-road.
- 🚛 For road trainstransporting heavy loads over long distances.
When is it better to refuse:
- ❌ If the equipment is operated in a city with strict restrictions on axial loads.
- ❌ If the car frame is not designed for additional loads (reinforcement is required).
- ❌ If the budget does not allow you to regularly service an increased number of nodes.
Example of successful modernization: on Ural-4320 Often a third axle is installed close together to increase the load capacity to 15 tons. However, this requires:
- Frame reinforcement.
- Replacing the rear axle with a model with a higher load (for example, Ural-43206).
- Installing air suspension for even weight distribution.
Warning ⚠️ Independent installation of additional axles without approval from the traffic police is equivalent to illegal modification of a vehicle and entails a fine of up to 50,000 rubles + the need to return equipment to its original state.
FAQ: Frequently asked questions about close tandem axles
Is it possible to install closely spaced axles on passenger cars?
No, this design is intended exclusively for freight transport and special equipment. For passenger cars, tandem axles are not used due to:
- Lack of technical necessity (passenger cars do not transport such cargo).
- Legal restrictions (there are no rules in the traffic rules for such modifications of passenger vehicles).
- Deterioration in controllability (increases the risk of skidding).
How often do you need to change tires on closely spaced axles?
Tire life depends on operating conditions, but on average:
- With regular rearrangement (every 10–15 thousand km) - 80–100 thousand km.
- Without rearrangement - 50–60 thousand km (due to uneven wear).
It is recommended to use tires with a reinforced carcass (e.g. Michelin X Works or Continental HD3).
What kind of oil should I use in dual axle axles?
For drive axles with close axles, use gear oils of the class GL-5 with viscosity 80W-90 or 75W-140 (depending on climate). Popular brands:
- Castrol Syntrax Universal Plus.
- Mobil Delvac 1 Gear Oil.
- Liqui Moly Hypoid-Getriebeoil.
Replacement interval - every 60–80 thousand km or once every 2 years.
Is it possible to lock the center differential on asphalt?
No, locking the center differential on hard surfaces leads to:
- Increased wear of tires and transmission.
- Deterioration of controllability (risk of skidding).
- Differential overheating.
The lock may only be engaged on off-road or slippery surfaces (snow, mud).
How to check the balancing of closely spaced axles?
To check balancing:
- Raise the car on a lift and spin the wheels until
60–80 km/h(on the balancing stand). - Check for vibration or vibration.
- Use laser balancer for fine tuning.
Permissible imbalance - no more 5–10 grams on the wheel.