Commander Tyre Size Guide (document updated 04/02/2010) Tyre sizes that can be fitted to the Jeep Commander are limited due to rubbing on the upper control arm ball joint bolt, the subframe or the rear part of the front guard inner plastic liner. Several factors complicate what will fit such as: overall wheel diameter, tyre width, rim offset, rim width & the amount of lift. Depending on the size tyre, different offset rims may be required to clear the ball joint bolt and subframe. Also the tyre sidewall height [profile(%) x width], may require the rear lower portion of the front guard inner plastic liner and the pinch weld minor modification (trimming/bending). Note that before adding wider tyres, these tyres have short footprints (short tread length ground contact patches) which may adversely affect ride comfort as they ride over bumps more abruptly and hit bumps missed by narrower (longer footprint) tyres and may also reduce braking and acceleration performance. Offset & Backspacing Offset specifies where the hub mounting surface sits with respect to the centreline of the rim. Changing the offset moves the rim in or out, changes the amount of mechanical leverage on axle and suspension components, wheel track and of course backspacing. Deviating too far from the vehicle's design specs (more negative) can place excessive stress on suspension and axle components and can result in premature failure. Standard practice is to have the offset stamped on the inner surface of the rim i.e. “ET50.8” (ET is an abbreviation from the German word “Einpresstiefe” literally translating as “insertion depth”) Rim offset can be one of three settings: • • • Zero offset:The hub mounting surface is even with the centreline of the rim. Positive offset: The hub mounting surface is toward the front of the wheel. Positive offset wheels are generally found on front-wheel drive cars Negative offset: The hub mounting surface is toward the back or brake side of the wheel. "Deep dish" wheels are typically negative offset. Negative offset is often found on standard rear-wheel-drive vehicles and on so-called reversed rims. Backspacing is the measurement from the outer edge of the rim's inner (inboard) flange to the hub mounting pad and is necessary to ascertain where the rim's inner flange will sit with respect to suspension components and so, must be measured using the actual overall width of the rim, not the stated width. Since the hub mounting pad is always at a fixed distance from suspension components. Because the rims' hub mounting surface is with respect to its centreline (offset), backspacing varies with changes in both offset and overall rim width. In general terms, most rims are about 1” (25.4mm) wider than stated ie: a 7.5” rim will actually measure 8.5” and with zero offset it would have a 4.25” backspacing. If the same width rim had a + 50.8 mm offset then its backspacing would be 6.5”. So increasing positive offset = increased backspacing meaning the rim's inner flange is closer to the suspension components. Alternatively, if the rim width is changed it will immediately affect these clearances e.g: a 8" wide rim with 4.5" of backspacing has the same 0mm offset as a 9" wide rim with 5.00" backspacing. The difficulty here is that in specifying rims, it is clear that backspacing alone does not present the whole story simply because suspension clearances will be different from vehicle to vehicle and with tyre width. While backspacing is an essential measurement needed to calculate clearances, offset (ET) relative to the centreline of the rim, is the accepted standard when specifying a rim and from which backspacing can then be calculated as well as additional axle loading. Backspacing (in) = Rim Width/2 (in) + Flange Width (approx 0.5 in) + offset (mm) x 0.0393 Note: The center bore of the rim (CB) centres the wheel on the hub. Since most rims are mass produced with a large centre bore to accommodate a wide range of vehicles, hub centric rings (hard plastic rings) in various sizes are available to centre alloy wheel when mounted. If the centre bore does not fit the hub diameter and wheel is not centred then eccentric vibrations and load stresses as the wheel rotates will eventually damage the wheel's bolt holes and studs. Maximum Tyre Size Guide The following is a guide only and is indicative of the maximum tyre sizes that can be fitted. 50mm to 75mm lift with OE 17x7.5” +43.8mm offset rims: (6.0” backspacing) 245/70R17 255/65R17 265/65R17 (minor rubbing - depends on tyre type & brand) 50mm to 75mm lift with 17x8.5” +25mm offset rims:(5.75” backspacing) 245/75R17 255/70R17 265/65R17 265/70R17 (trimming required) 100mm lift with +25mm, +10mm offset or less (5.5” backspacing preferred maximum): 255/75R17 265/70R 17 275/65R 17 275/70R 17 (rubbing - depends on rim offset) 285/70R 17 (trimming required) Commander Tyre Size Comparison Chart (Complied using data from www.1010tyres.com Tyre Size Calculator) Tyre Size 245/65R17 245/70R17 245/75R17 255/65R17 255/70R17 255/75R17 265/65R17 265/70R17 Overall Diameter 750.5mm 774.7mm 799.06mm 763.27mm 788.67mm 814.07mm 776.22mm 802.64mm Sidewall Height 159.00mm 171.45mm 183.64mm 165.60mm 178.30mm 191.00mm 172.21mm 185.42mm Speedo @ 100km/h 100km/h 96.8km/h 93.8km/h 98.2km/h 96.1km/h 92.1km/h 96.6km/h 93.4km/h Jeep Grand Cherokee & Commander Wheel Specifications The Grand Cherokee was the first to receive the 5x127mm PCD common to many GM and Ford vehicles and now the Commander and JK Wrangler. Note that the Grand and Commander have different offsets and caution should be exercised if intending to fit Grand wheels to the Commander which potentially could foul suspension components. WH Grand Cherokee: ET + 50.8mm: backspacing 6.25": CB 71.5mm XH Commander: ET +43.8mm: backspacing 6.0": CB 71.5mm AEV 17x8.5”: +25mm offset vs OE 17x7.5” +43.8mm offset (image from www.rimsntires.com tyre Size Calculator Caution: Check state laws on 4x4 wheel track modification before changing rim offsets. The Australian NOCP only allows a 25mm wheel track variation for except for dependent suspensions (beam axles) which are allowed 50mm maximum. No reduction is allowed. A simple formula for calculating wider tyre rim offsets: New Rim Offset = Eo + (Ew/2) – (Pw/2) Eo = Existing Rim Offset Ew = Existing Tyre Section Width Pw = Proposed Wide Tyre Section Width Correct Tyre Inflation Pressure Regardless of its size, every tyre's load capability, durability, traction and handling depends on using the right inflation pressure for various operating conditions. Since too little or too much inflation pressure sacrifices some of the tyre's performance, maintaining the "right" inflation pressure is extremely important. Vehicle manufacturers state their recommended tyre pressures on the vehicle's placard. These tyre pressures (Cold Inflation Pressures) are OK for most situations with the OE tyre sizes but if the vehicle is lifted and equipped with bigger tyres, the vehicle's weight distribution will be different so these recommended pressures won't necessarily apply. Every tyre has a maximum inflation pressure rating stamped on the sidewall. This is the maximum permissible pressure a tyre can be safely inflated to at its maximum load capacity (Load Index). It is not the pressure it should be inflate to for normal operation. 1) The 4 psi Rule Boyle's Law states that for a given volume of air, as temperature increases, pressure will increase, and if temperature decreases, so will pressure. Therefore it follows that temperature differences will affect the tyre pressure. For every 7oC change in temperature there is a pressure change of about 1psi (7kpa). Tyre pressures will increase as the vehicle is driven due temperature rise caused by friction. The “4 psi Rule” uses this temperature effect with speed over time to determine the correct operating pressures for a particular tyre and vehicle load: 1. 2. 3. 4. Inflate tyres to vehicle placard cold inflation pressures Drive for 1 hour at normal highway speed (say 100 km/h) Stop and check hot pressures If tyre pressure has: - increased by 4psi then the cold inflation pressure is just right - increased by more than 4 psi then the initial pressure is too low - not increased by more than 4 psi then initial pressure is too high Note: • • • • Tyre pressures should always be based on cold inflation pressures measured in the shade after the vehicle has been idle for at least 3 hrs or driven less than 3 km from cold. The Jeep Commander Owner's Manual states the 2 psi – 6 psi is the acceptable pressure rise with 4 psi being the optimum. Boyle's law is one of three gas laws which describe the behavior of gases under varying temperatures, pressures and volumes. A change of 1oC equals a Fahrenheit change of 9/5 = 1.8oF 2) Chalk Method. 1. 2. 3. 4. Make a mark all the way across each tyre. Drive the Jeep for a short distance. A straight line is best. Note the wear pattern in the chalk: - chalk left at the edge the tyre is over inflated. - chalk left in the center means it's under-inflated. - a clean pattern indicates the pressure is just right. Make a note of the pressure for future reference. 3) Tyre Load Index Method** When loading up a vehicle, tyre pressures will need to be adjusted to suit the vehicle's weight & load distribution between axles. The tyre's cold inflation pressures can be closely approximated using the tyre's load index (maximum load) and maximum pressure. Both can be found on the tyre's sidewall: 1. 2. 3. Take the max weight that the tyre can support and multiply that by 4 to get the maximum loadcarrying capacity all four wheels can support. Next take the loaded weight of your Jeep plus load safety margin (say 20%) and divide that into the maximum 4 wheel load-carrying capacity. Now divide this number into the maximum pressure stated on the sidewall and the result is the ideal cold inflation pressure. Note: Gross Vehicle Mass (GVM) - the maximum laden mass of a vehicle when loaded according to manufacturer’s specifications including the kerb mass of the vehicle plus full fuel, passengers, accessories, cargo, tow hitch and ball weight. Kerb Mass - the mass of a vehicle, ready for service, unoccupied and unladen with all fluid reservoirs filled to nominal capacity except for fuel, which should be only 10 litres, plus all standard and optional equipment fitted on delivery. Tare Mass - the mass or weight of an empty vehicle, caravan or trailer i.e. the kerb mass or unladen mass Payload - the maximum capacity of the vehicle for the transport of all non-standard items including the vehicle's cargo, occupants, accessories (bull bars, spot lights, roof racks, tow bars, tray, canopy etc) and tow ball mass. Maximum Available Payload - the maximum weight of the cargo which can be placed upon a vehicle calculated by subtracting from the vehicles GVM, the vehicles kerb mass, the total weight of the occupants and the weight of any fitted accessories. Tow Ball Mass - The weight imposed on the rear of the tow vehicle’s tow ball from a caravan/trailer. Mass - the property of a body that causes it to have weight in a gravitational field. In a general context it can be considered as weight Weight - the vertical force exerted by a mass as a result of gravity In the following examples the vehicle's normal operational weight is assumed to be its kerb mass plus half payload. Manufacture's Kerb Mass = 2242 kg Manufacturer's GVM = 2948 kg Vehicle's Max Payload = (GVM – Kerb Mass) = (2948 - 2242) = 706 kg Assumed Laden Mass = Kerb Mass + 706 = 2242 + 353 = 2595 kg Plus 20% Safety Margin = 2595 X 1.2 = 3114 kg Example 1 (3.0 litre CRD Commander Ltd with Cooper 265/65R17 ATR tyres - Load Index 112): Tyre Load Index Rating = 1120 kg at 44 psi Max Tyre 4 wheel Total = 1120 x 4 = 4480 kg Tyre Loading Factor = 4480/3114 = 1.44 Ideal Inflation Pressure = 44/1.44 = 30 psi (2.0 bar) Example 2 (3.0 litre CRD Commander with Cooper 255/65R17 ATR tyres - Load Index 112): Tyre Load Index Rating = 1060kg at 44 psi Max Tyre 4 wheel Total = 1060 x 4 = 4240 kg Tyre Loading Factor = 4240/3114 = 1.36 Ideal Inflation Pressure = 44/1.36 = 32 psi (2.2 bar) Example 3 (3.0 litre CRD Commander with OE Goodyear 245/65R17 Wrangler - Load Index 105): Tyre Load Index Rating = 925kg at 44 psi Max Tyre 4 wheel Total = 925 x 4 = 3700 kg Tyre Loading Factor = 3700/3114 = 1.18 Ideal Inflation Pressure = 44/1.18 = 37 psi (2.5 bar) **This method will give a good starting point to refine operating pressures using the “4 psi Rule “ Commander Technical Specifications Tyre Load Index Tyre Terminology Only the tyre's internal air pressure and the vehicle's weight infulence a tyre's footprint (contact patch area). When loading up a vehicle the tyre infaltion pressures will need adjusting to cater for the additional load. The following methods can be used to determine laden pressures: 1) Footprint Method For loaded vehicles, inflate all tyres to their cold inflation pressure then measure the tyre tread lengths before packing. After loading, adjust each tyre pressure, to bring its footprint to the same length as it was unloaded. The tyre tread length can be measured by placing a straight edge at the front and rear of the tyre where it meets the road surface. Simply measure between these two reference points. 2) Load Ratio Method Most vehicles, when loaded, will not have its load equally distributed over each axle, the front and rear tyre pressures can be adjusted using the axle load distribution ratio. If front to rear load distribution of the vehicle is 45:55 then: Front Pressure = (45/50) x Cold Inflation Pressure Rear Pressure = (55/50) x Cold Inflationn Pressure Assume: 50:50 Cold Inflation Pressure = 35 psi (2.4 bar) Therefore: Front Pressure = 45 ÷ 50 x 35 = 31 psi (2.2 bar) Rear Pressure = 55 ÷ 50 x 35 = 38 psi (2.6 bar) 3) Tyre Radius Method This method is similar to the “Footprint Method” but uses the Static Laden Radius often quoted by tyre manufacturers for large tyres and measured from the road surface to the centre of the axle. When inflated to this measurement, tyres will be at their correct laden pressure. If not quoted or unknown then, before loading up, measure the from the ground to the axle centre with the tyres at their recommended cold inflation pressure and then after loading, adjust tyre pressures to bring them back to that measurement. Caution: Cold Inflation Pressure should only be measured in the shade (preferably 25°C) and after the vehicle has been idle for at least 3 hrs or driven less than 3 km from cold. Since air temperature has a significant effect on tyre pressures, the recommended rule is to set tyre pressures in the morning before rising temperatures or radiant heat can affect pressures. Tyre pressure can change approximately 7 kPa (1 psi) for every 7°C change in tyre temperature – it will go down with lower temperatures, and up with higher. If tyres are inflated to placard pressures when they are hot or in the heat of the day, cold pressures will probably be dangerously low the following morning. Alternatively if a tyre is inflated on a cold morning and the day high is very hot, tyre pressures after driving at length, may exceed the tyre’s maximum rating. This can also happen when desert driving in very hot sand. Tyre pressures should be checked in the shade when the vehicle hasn't been driven for at least 3 hours or travelled less than 3 km at low speed. If air must be added to hot tyres, the accepted rule is to add add 4 to 5 psi (0.3 bar) to the placard pressure and recheck when cold. Disclaimer: This document was compiled for education purposes only. Some information contained within is from either the Internet, manufacturer's brochures or other printed material. Information presented here is in either its original text form or edited to reflect UK English spelling, metric conversion, or re-arranged to make it's presentation easier to read. This document has been produced purely to assist 4x4 owners and for no other reason. No claim is made to the originality of text, diagrams & photographs other than the collecting, editing & arrangement of such. The omission of a bibliography is due to the difficulty in assigning true authorship acknowledgement. Where text is used verbatim and not directly attributed to it's author then it is considered implicit and gratefully extended to that author. Accuracy of information has not been verified and is USED AT OWN RISK. No liability is accepted in any form for any damage caused by the use of this information or its accuracy.