In heavy machinery transport, the physical diameter of the running gear tires directly dictates the working deck height of a lowbed (lowboy) semi-trailer. Choosing between traditional tube-type 8.25R20 steel radial tires and modern low-profile tubeless 235/75R17.5 tires determines whether tall 40 to 80-ton excavators, drilling rigs, and industrial boilers clear 4.2-meter highway overpasses or require hazardous, expensive long-distance detour routes.
1. Technical Specification Comparison: 8.25R20 vs. 235/75R17.5 Lowbed Tires
Rolling diameters, loading deck height impacts, and axle load capacities across lowbed tire platforms:
| Engineering Parameter | 8.25R20 Tube-Type Steel Radial (Traditional Standard) | 235/75R17.5 Low-Profile Tubeless Radial (Ultra-Low) |
|---|---|---|
| Overall Tire Outer Diameter | approx. 974 mm (38.3 Inches outer diameter) | approx. 797 mm (31.4 Inches compact diameter) |
| Working Deck Height Over Axles | 1,050 mm – 1,150 mm deck height | 800 mm – 900 mm ultra-low deck height (Saves 250mm vertical space) |
| Wheel Rim Specification | 6.5-20 Multi-piece split lock ring steel rim | 6.75 x 17.5 One-piece drop-center tubeless wheel rim |
| Load Index (Single / Dual Mount) | 136 / 134 (Dual: 2,120 kg per tire / 8,480 kg per dual-wheel axle) | 143 / 141 (Dual: 2,575 kg per tire / 10,300 kg per dual-wheel axle) |
| Cold Inflation Pressure | 8.3 bar (approx. 120 psi) | 8.6 – 9.0 bar (approx. 125 – 130 psi high pressure) |
| Operating Heat Dissipation | Moderate (Internal friction between tube and casing) | Requires careful cooling (High rotational speed on highway runs) |
| Tare Weight per Wheel Assembly | approx. 68 – 75 kg (Tire + tube + flap + 20" steel rim) | approx. 48 – 54 kg (Saves approx. 20 kg per wheel position) |
| Optimal Machinery Haulage | Standard bulldozers, wheel loaders, mining quarry haul | Tall crawler excavators (CAT 330/349), high-cab drilling rigs, transformers |
2. The 250mm Height Advantage: Clearing Highway Overpasses
Vertical bridge clearance represents the most critical constraint in abnormal heavy transport:
- The Overhead Bridge Conflict: A Caterpillar CAT 336 or Komatsu PC350 excavator has a transport shipping height of 3,180 mm. Mounted onto an 8.25R20 lowbed trailer (deck height 1,150 mm), the total traveling height reaches 4,330 mm (4.33 meters)—exceeding the legal 4.2-meter height limit across African and European highway bridges.
- The 235/75R17.5 Solution: Riding on compact 17.5-inch tires, the lowbed deck height drops to 850 mm. The exact same excavator now travels at an overall height of 4,030 mm (4.03 meters), easily clearing standard 4.2m overhead overpasses, bridge beams, and toll plaza gantries without police escort detours.
3. High Revolutions & Heat Dissipation Challenges on Small 17.5" Tires
Small-diameter tires rotate significantly faster at equivalent highway traveling speeds:
- Higher Rotational Velocity: At 80 km/h, a 235/75R17.5 tire completes approximately 535 revolutions per kilometer, compared to only 440 revolutions for an 8.25R20 tire (a 21% increase in rotational speed).
- Managing Thermal Buildup: More revolutions per minute generate intense internal flex heat. Fleets running 17.5-inch tires across hot tropical regions (e.g., Sudan, northern Nigeria, Atacama desert) must enforce strict 70-80 km/h cruising limits and maintain exact 9.0 bar cold inflation pressures to prevent tire tread delamination.
4. Multiple Axle Lines: Distributing 80-Ton Heavy Machinery Loads
Because individual small tires carry high point pressures:
- Chengda engineers 17.5-inch lowbed trailers across 3-axle, 4-axle, and 5-axle configurations (with optional front dollies).
- Distributing an 80-ton industrial boiler across 16 or 20 tire contact patches maintains ground bearing pressure well within legal highway weighbridge limits while preserving ultra-low deck clearance.
Frequently Asked Questions
Q: Can 235/75R17.5 tires be used on rough unpaved mining gravel tracks?
A: Yes, but drivers must travel at reduced speeds. Small-diameter tires have lower approach roll-over angles against large sharp boulders. For extreme unpaved quarry roads with minimal overhead bridge restrictions, 8.25R20 or 11.00R20 tires remain preferred.
Q: What is the load capacity of a 4-axle lowbed equipped with 235/75R17.5 tires?
A: With 16 tires (dual-tire setup across 4 axles), each tire rated at 2,575 kg in dual formation, the axle group safely supports 41,200 kg, allowing legal 60 to 70-ton excavator transport when combined with tractor fifth-wheel capacity.
Q: Where can heavy equipment haulers order lowbed trailers with 17.5" or 20" tires?
A: Browse our complete heavy-haul catalog in the Semi-Trailers Category, view our Heavy Lowboy Trailers, or consult our International FAQ Center.
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