Big Wheels and Tyres

 

A lot of people like the look of Jeeps with their suspension jacked up and big shiny wheels fitted with big fat tyres. I must admit I think it adds a look of purpose to the already rugged appearance. I must also admit that mine has all the aforementioned attributes and a few more, but lets make the point clear, I’m not keen on monster trucks and mine certainly isn’t one, and I think subtlety is always better then blatant showiness. This leads me on to the reason for the article. I have been asked by lots of people what is best way to go about modifying a Jeep to give it that taller wider style that looks so appealing. Hopefully I will give you something to think about and try, but this will not be an instruction manual.

To go about these types of modification, takes two things, work and money! The more you use of one has a reducing effect on the other, i.e. if you spend more money, then you do less work. Conversely, the more work you do, the less money you will spend. Unfortunately it will cost money at some point no matter what you are capable of, but if you have lots of money, it is possible to do away with all the work.

Tyres

Tyres have three important sizes to consider the first is the height, second the width and thirdly the rim size the tyre will fit. Now this can be confusing because tyres come in metric and imperial sizes. I use 33 x 12.50 R15 AT’s. This means that my tyres are 33 inches high from the ground to the top, 12.5 inches wide and fit on a 15 inch wheel rim. Standard fitment to Mahindra Jeeps are metric 235 / 75 R15. This means that the tyre is 235mm wide, the distance from the wheel rim to the tread of the tyre is 75% of the tyre width and fits on a 15 inch rim. Notice that metric tyre sizes still use imperial rim sizes !! This means that the standard tyre fitted to Mahindra’s has an overall tyre height of about 28.5 inches by 9 inches wide. So my tyres are 4.5 inches taller than standard giving a lift of 2 ¼ inches. So you can lift your Jeep just by changing the tyres.  

Other options in tyre sizes are available and what size you can fit is dictated by what will fit under the wheel arches. Even a modest difference in tyre size has an effect, my 31 x 10.50 MT's fouled the rear wheel arches when offroading, so I had to fit wheel spacers to keep them away from the bodywork.  

Wheels

Wheels come in two types of metal, steel or alloy. There are other materials available, but these are only used by the likes of Michael Schumacher and command an income of his type to purchase them.

Wheels have four sizes to deal with and they are:

Width, The width needed is dependant on how wide your tyres are and is described in inches. 

Diameter, The diameter is the size that correlates directly with the tyre size and is usually described in inches.

Offset. Offset is described in millimetres just to confuse. The offset describes the distance from the centre of the wheel to the mating face that buts up to the hub.

PCD, This stands for Pin Centre Diameter. Basically it’s the size of the circle that has the five wheel stud holes drilled in. A Mahindra Jeep’s PCD is 5.5 inches, which is the same as the CJ variant of Chrysler Jeep and the same as Suzuki SJ’s and Vitara’s, not to mention Diahatsu Fourtrak’s. 

My steel rims are 15 J10 00 this means they are 15 inches tall, 10 inches wide with no offset, they are fitted with 33 x 12.50 R15  mud terrain tyres. My alloys are 15 J10 –60 this means they are the same 15 inches tall, 10 inches wide but have a minus 30mm offset meaning they sit further out from the car. These are fitted with 33 x 12.50 R15 All Terrain tyres. The 'J' in the size description refers to the bead profile and correlates to the tyre bead.

  offset

Fitting larger tyres is the only way of increasing the ground clearance of your axles because they are directly connected to the ground and your axles. Put simply, fitting 31 inch tyres gives you a 1¼ inch improvement in ground clearance and like wise fitting a 33 inch tyre will lift you another inch. Yes only one inch, because the other inch is at the top of the tyre.

Now you know what wheel sizes are and what tyre sizes are. So you now know that a 31 x 10.50 R15 tyre is the same overall height as a 31 x 10.50 R16. You also know that a 265 / 75 R15 is smaller overall than a 265 / 75 R16. What's that you don’t ?    go to the start and read it again and pay attention this time!

Suspension lifts and body lifts.

Most suspension and body lifts are for gaining not ground clearance but tyre clearance. I.e. you fitted the larger tyres to gain ground clearance and they hit the body of your jeep. My Jeep has a 2 inch body lift, 4 inch spring lift and a 2 inch shackle lift. This means that my axles are now well over 6 inches further away from my bodywork than they were when the Jeep was standard. Apart from tyre clearance this also improves ramp over, approach and departure angles. The illustration should explain what these are. As the Jeep goes up so do the angles.

  angles

Body Lifts.

Fitting a body lift is quite simply putting spacers between the body tub and the chassis. These are fitted at the body tub bolt points. The height to fit is simply a matter of choice, but there are more problems encountered the higher you go. I fitted two inch spacers to mine and I had to modify the body tub aperture where the steering column goes through it. The transfer box levers fouled the body tub as well, so that aperture had to be widened too. Fitting a smaller spacer would have eased if not negated these modifications, likewise bigger spacers would have caused more severe problems.

Spring Lifts.

Spring lifts are simply (but expensively) done by fitting new parabolic springs. These have to have longer shock absorbers fitted because the standard shocks won’t be long enough. I have fitted this type of lift to my Jeep, and they give me an increase of four inches in height from standard. No modifications are needed for this type of lift if you stay below two inches of lift. Beyond that you may need to tilt your rear axle and your gearbox to straighten the line of the rear prop shaft due to the excessive vibration it may cause at normal road speeds. If you have to make this modification, you will need to fit free wheeling hubs to the front wheels. This is due to much increased angle of the front propshaft. Normal road speeds will cause enormous vibration and wear on the Universal joints. Free wheeling hubs allow them to stand still during normal road driving and then you can re-engage them for offroad driving, where speeds are low and vibration not an issue.

Shackle lifts.

These are quite simply longer shackle plates and bolt straight on. Increasing the length up to two inches is considered to be quite safe, beyond that and steering geometry becomes compromised and dangerous side effects may start to appear. Mine are two inches oversize. If you want to make this modification, buy the ready made versions for sale. I do not recommend making your own, as the quality of the welding and steel used is critical in this type of modification. DON'T RISK IT, IMAGINE WHAT WOULD HAPPEN IF IT BROKE GOING AROUND A CORNER !!!!!!

Spring over axle Lifts.

This is achieved by unbolting the axles from the springs, putting the axle underneath the spring and bolting it back on again. Plates have to be welded to the axles for the springs to mate against and problems can occur with the differential boxes on some axles. This is a problem on our Jeeps. This will get you a lift equal to the springs depth and the axles tube depth, about 4.5 inches. This is the least favoured lift technique due to side effects caused by the springs trying to 'wrap' and causing dangerous 'bump steer' I have only experienced one of our jeeps with this type of conversion and it was ….. adrenalin inducing on the road. I definitely wouldn't recommend this type of conversion on our vehicles.