## Bushie Tyres

**TYRES**

“Queensland Code of Practice – Vehicle Modifications, Transport and Main Roads, Version 2.6 July 2015”, in “Section 3.4 Wheels and Tyres”, pages 43 and 44, includes the following quote (note the category for Bushie Ute is ‘NA’):

*“The overall tyre diameter can be increased to allow an increase of 7.5mm in vehicle height for passenger vehicles and no more than 25mm in vehicle height for four wheel drive vehicles (typically MC ADR category).*

*Tyres fitted to off-road passenger and light goods vehicles (MC, NA, NB ADR category) must not be more than 50% wider than the vehicle manufacturer’s widest optional tyre. Tyres fitted to passenger vehicles must not be more than 30% wider than the vehicle manufacturer’s widest optional tyre.*

*The rim width must match the recommendations for the tyre fitted.”*

The most vulnerable part of Bushie Ute, like any typical four wheel drive vehicle, is the tyre sidewall, and the second most vulnerable part is the tyre tread. Bushie Ute’s tyres need to be suitable for general use and long trips, often including difficult or low traction terrain in remote areas. Compromises are inevitable when selecting tyres and features considered include, in no particular order, durability, initial cost, wear/life, fuel economy, and traction on numerous terrain types.

Having used many different makes over the years, this time I have chosen Nitto “Trail Grappler M/T” tyres. Size range, prices and user reports are very good. Ideally, funds permitting, Bushie Ute would have a set of wheels with AT (all terrain) tyres for general, mostly on road use, and another set with MT (maximum traction, or mud terrain) tyres for low traction terrain, or terrain that demands a rugged tyre construction. Like some other modern 4×4 tyres, the Nitto Trail Grappler is a good compromise between AT and MT styles, and their strength is exceptional (the main downsides are increased weight and fuel consumption).

Bushie Ute has 16″ x 8″ steel wheels from Eastern Wheel Works in Victoria. Backspacing is standard Toyota Landcruiser, and the offset is -11 mm, which increases the track width by 22 mm (stock Landcruiser offset is 0 mm).

The following table shows the details for Nitto “Trail Grappler” tyres :

The following table lists the Load Index vs maximum tyre load in kg *when the tyre is inflated to the maximum rated* *inflation pressure*, and Speed Symbol vs maximum allowable speed in km/hr *when the tyre is inflated to the maximum rated pressure, and the load is less than the maximum rated load*:

The performance graphs shown above from Toyo illustrate the compromises when choosing between an A/T and M/T tyre. Likewise there are compromises when choosing between tyre sizes.

Increasing tyre diameter (in no particular order):

- reduces braking and tractive effort for the same torque at the wheels
- reduces acceleration due to the increase in polar moment of inertia
- reduces rolling resistance
- increases mobility on low strength soils
- increases off-road mobility by increasing ground clearance, approach, departure and break-over angles
- reduces stability by increasing the centre of gravity height

Increasing tyre width (in no particular order):

- reduces acceleration due to the increase in polar moment of inertia
- increases rolling resistance on hard surfaces
- reduces rolling resistance on low strength soils
- increases mobility on low strength soils

**TYRE LOAD and INFLATION**

A tyre placard attached to a vehicle provides the correct tyre inflation pressure for the stated load and tyre size. The the tyre manufacturer states the maximum load and inflation pressure on the sidewall of LT tyres. By providing that information, what both the vehicle and tyre manufacturers are primarily concerned with is the correct, or optimum vertical deflection of the loaded tyre.

The information on the tyre placard will not ensure the correct vertical deflection if a different size tyre is used. The deflection is calculated by dividing the vertical load on the tyre by vertical stiffness of the tyre. The vertical stiffness of the tyre is a function of tyre size (volume of air), inflation pressure, and stiffness of the tyre carcase. For the same load, a larger tyre requires a lower inflation pressure for its optimum deflection. The TRA (Tire and Rim Association) publish load vs inflation pressure data for LT tyres based on the formula: Load = Max Load * (Inflation Pressure/Max Pressure)^0.7

The inflation pressure obtained from TRA data, or formula, will result in vertical deflections greater than optimum when the load is considerably less than the maximum load marked on an LT tyre.

Stephen M. Padula, Michelin North America proposed an empirical formula, based on published research by Dr. Tim Rhyne (Michelin Americas Research and Development Corporation) for the vertical stiffness of tyres, to calculate inflation pressure. For more information see chapter Chapter 5 “Tire Load Capacity” in USA Dept. of Transportation – DOT HS 810 561 The Pneumatic Tire (9.64 MB pdf file)

The following load vs inflation pressure data from the TRA (Tire and Rim Association) “Light Truck Inflation Table” taken from a Toyo document “Guidelines for the Application of Load and Inflation Tables” and converted to SI units.

For an LT285/75R16 tyre with the following cold inflation pressures the maximum allowed single tyre loads are:

- 241 kPa (35 psi): 966 kg (2130 lb)
- 276 kPa (40 psi): 1061 kg (2340 lb)
- 310 kPa (45 psi): 1152 kg (2540 lb)
- 345 kPa (50 psi): 1250 kg (2755 lb) Note: this is the maximum for a Load Range ‘C’ (6 ply rating) tyre

For a LT315/75R16 tyre with the following cold inflation pressures the maximum allowed single tyre loads are:

- 241 kPa (35 psi): 1150 kg (2535 lb)
- 276 kPa (40 psi): 1232 kg (2715 lb)
- 310 kPa (45 psi): 1338 kg (2950 lb)
- 345 kPa (50 psi): 1449 kg (3195 lb) Note: this is the maximum for a Load Range ‘D’ (8 ply rating) tyre

The load rating at 240 kPa (35 psi) for LT285/75R16 and LT315/75R16 tyres exceed the load capacities of both the front and rear axles at the allowable GVM (gross vehicle mass).

The tyres of interest here are all load range “E” (old 10 ply rating) and the following chart, based upon TRA data, plots the maximum allowed single tyre load vs inflation pressure for each of the three sizes. The TRA formula for determining the inflation pressure of LT tyres, for a load considerably less than the maximum rating, leads to greater tyre deflection than optimum for highway speeds.

The three charts below graph the same inflation pressure vs single tyre load limit data, separately for different tyre sizes, and over the likely range of operating loads.

The maximum load allowed on the front axle is 1500 kg, or 750 kg per tyre. For the rear axle it is 1900 kg, or 950 kg per tyre. Therefore all of the chosen tyres, will adequately support the maximum axle load.

Gas permeates through polymer membranes, “Fick’s first law of diffusion”, causing tyre inflation pressure to reduce over time, generally 7 to 14 kPa (1 to 2 psi) per month. Lack of regular checking leads to an all too common issue of under inflation, adversely affecting tyre durability (on road), increasing rolling resistance and fuel consumption. The following graphs show how inflation pressure (a passenger car tyre in this example), affect rolling resistance and tyre deflection.

Tyre load and inflation pressure affect the understeer, or oversteer (handling) characteristics at highway speeds. Under-steer is required for safety reasons.

For the front axle:

- increasing the load will increase understeer
- increasing the tyre inflation pressure will reduce the understeer

For the rear axle:

- increasing the load will reduce understeer
- increasing the tyre inflation pressure will increase the understeer

Because of the distribution of the load carrying space, adding payload usually increases the load on the rear axle more than for the front. While tyre inflation pressures need to be increased to adequately carry a greater load, to correct the affect on understeer from a greater load increase on the rear tyres, it is necessary to increase further the inflation pressure in the rear tyres.

**TYRE DEFLECTION**

The following diagram shows an unloaded tyre on the left and on the right the deflection that occurs when a load is applied. In this diagram:

- ‘ro’ is original radius
- ‘rL’ is loaded radius
- ‘d’ is the deflection (d = ro – rL)
- ‘P’ is the load
- ‘K’ is the ‘spring constant’, a function of inflation pressure and tyre size (volume of air)

For a four wheel drive vehicle there are many times when it is necessary to reduce tyre inflation pressures, e.g.:

- to reduce the risk of tyre damage, reduce loads on vehicle components, and improve ride comfort when traveling over rough offroad tracks, and many “outback” roads in need of a grader
- to improve mobility on low strength soils and rough offroad tracks

In some cases the inflation pressure will need to be very low, e.g. large desert dunes with soft sand. Speed needs to be reduced accordingly when tyre pressures are significantly lower than appropriate for normal roads. Inflate tyres to an appropriate pressure when track conditions improve and before speed is increased.

Pressures can’t be reduced as much when a vehicle is heavily loaded with extra fuel, water, equipment and provisions for extended touring. Weight is a formidable enemy when venturing offroad, it reduces mobility, increases risk of mechanical or structural failures, wear and tear. It should be kept as low as reasonably possible, e.g. carry adequate reserves and spares, but don’t go overboard, cull heavy and bulky items and leave behind any camping gear that is rarely used.

To read more about tyres, open or download the following files:

USA Dept. of Transportation – DOT HS 810 561 The Pneumatic Tire (9.64 MB pdf file)

European Tyre & Rim Technical Organisation – Standards Manual 2003 (7.27 pdf file)

**TYRE SIZE AND GEARING VS ENGINE RPM AT 100 km/hr**

The engine in “Bushie Ute” has a power band from 2200 rpm (peak torque) to 3000 rpm (peak power). From the Toyo specifications the number of tyre revolutions required to travel 1 km is 370 for LT315R16 M/T tyres. For a road speed of 100 km/hr, the engine speed with available axle/diff ratios are:

- 4th gear; 2535 rpm (4.111:1), 2852 rpm (4.625:1), 3006 rpm (4.875:1)
- 5th gear; 1846 rpm (4.111:1), 2076 rpm (4.625:1), 2189 rpm (4.875:1)
- 6th gear; 1587 rpm (4.111:1), 1785 rpm (4.625:1), 1882 rpm (4.875:1)

These figures show that 5th gear with the 4.111:1 diff ratio should feel like 6th gear with the 4.875:1. It will be useful to see how the performance is in 5th gear, before making a final decision whether to change the ratio to 4.625:1 0r 4.875:1.

The accuracy of vehicle speedometers is covered by Australian Design Rule (ADR) 18.

For vehicles manufactured before July 2006:

- An accuracy of +/- 10 percent of the vehicle’s true speed was needed when a vehicle was traveling above 40km/h. This means if a vehicle was traveling at a true speed of 100km/h, the speedometer was allowed to indicate a reading between 90km/h and 110km/h.
- An odometer accuracy of +/- 4 percent was also a requirement.

From 1 July 2006 all newly introduced models of a vehicle available on the market must comply with ADR 18/03:

- The speedometer must not indicate a speed less than the vehicle’s true speed or a speed greater than the vehicle’s true speed by an amount more than 10 percent plus 4 km/h.
- The speedometer must always read ‘safe’, meaning the vehicle’s true speed must not be higher than the speed indicated by the speedometer. So if a vehicle traveling at a true speed of 100km/h, the speedometer must read between 100km/h and 114km/h. Another way of looking at this is if the speedometer indicates a speed of 100km/h, the vehicle’s true speed must be between 87.3 km/h and 100km/h.
- There is now no requirement to have an odometer, and so there is no accuracy requirement.

**TRACTION AND MOBILITY ON SOFT-SOIL
**

A considerable amount of research into the ability of vehicles to traverse soft-soil (low cohesive strength) terrain has been carried out over many years by the US Army, UK MOD (Ministry of Defense) and others interested in vehicle mobility in agriculture and forestry industries.

With regards to tyres, the research shows that increasing tyre diameter and width improves a vehicles ability to traverse soft-soil terrain. The normal pressure on the soil is a function of weight acting on the tyre and the area of the tyre footprint. The relationship between the normal pressure and the amount that the tyre sinks into the soil shows that the sinkage is inversely proportional to the smaller dimension of the tyre footprint. The tyre footprint area is a function of weight, tyre diameter and width, carcass stiffness, and inflation pressure. Increasing the diameter and width of the tyre, and reducing the tyre inflation pressure will increase the footprint area. As inflation pressure is reduced, the dimensions of the footprint area will increase primarily in the lengthwise direction, and to a lesser extent the width. Increasing the tyre width, unless it is exceptionally large and stiff, will increase the important smaller dimension of the footprint, further reducing the amount the tyre sinks into the soil.

Reducing the weight acting on the tyres, increasing the area of the tyre footprint, and increasing the smallest dimension of the footprint will allow the vehicle to traverse terrain that has a lower cohesive strength.

Of particular interest for “Bushie Ute” are the empirical relations from NRMM II (Nato Reference Mobility Model version II) that relate to the traction performance of wheeled vehicles on coarse-grained soils (sand), see ‘Table 9’ below.

These relations were used to compare the relative performance of LT267/75R16, LT285/75R16, and LT315/75R16 tyres. The same soil strength, CI (cone index) of 30 was used for all tyres. The formula in ‘Table 9’ are based on imperial units, however the results were converted to SI for the charts below.

Drawbar pull is the sustained towing force the powered wheel can produce on the given surface (D = Tractive Effort – Motion Resistances).

The results for powered wheels show what is well known, i.e. increasing tyre deflection by lowering inflation pressure, and reducing the load increases mobility. Not so well known is how much tyre tyre size affects mobility.

The results for un-powered wheels show how tyre deflection and size affect rolling resistance.

Download spreadsheet for ‘Mobility on Sand’ calculations

Read more on Vehicle Cone Index, Mobility Index, and Mean Maximum Pressure

Read more on Nato Reference Mobility Model (large pdf file)

Read more on tyre-soil interaction