• Off-Roading 101: Tire diameter

    What is too tall or too small


    Offroading20150609In my article last week, we discussed tire width and its effects on a four-wheel-drive (4WD) vehicle. We touched on how wide tires make off-roaders look stout and macho, and we also talked about the pros and cons of the different tire widths.  In this column, we will look at the other aspect of tire size, which is height.

    The diameter of the tire also relates to the ride height and no matter how much lift you install on your vehicle, in the end it is still tire size that will dictate ground clearance.  Always remember the height of the tire is what provides separation between the vehicle’s underside and the ground.

    Here are some things you need to know before getting a new set of tires. First, not all tire diameters are the same even if they are advertised as so. Some manufacturers sell tires that are advertised as a certain size. Once they are fitted on the wheels, however, the tire height ends up to be much lower than anticipated. There are many reasons why this happens.  It could be the wheels where the tires are mounted on are too wide, forcing it to stand lower even when fully inflated. The best thing to do is undertake a trial fitting (on similar-sized wheels) to determine how tall the tire will stand.  In off-roading remember every inch counts. For those fitting their off-roaders with much taller tires than the stock ones, do expect your vehicle to have slower acceleration because of the additional distance a tire needs to complete on a full revolution.

    With taller tires, expect brake performance to slightly suffer. This because of the increased leverage added by the taller tire on the brakes. For a 4WD with no computer-controlled braking systems, the above-mentioned handicap can be offset by changing your vehicle’s final drive gear ratio, which can be done by replacing the ring and pinion gear. Remember though that both front and rear axle ratios have to be replaced.

    Effects on brakes, components
    On computer-controlled braking systems, expect your instrument panel to light up like a Christmas tree because all the sensors will not read the proper values and will get the wrong feedback, hence a lower performance level of the system. Do not fret when this happens.  Your vehicle will still stay running. Only the braking system will under perform. In some brands, you can actually buy a program to correct the values to match the dimensions of the taller tires.

    When measuring how much additional ground clearance you have added by installing taller tires, always cut this in half since the tire is round and you only get half of what you added. So let’s say your standard tires are 30 inches in diameter and you installed 33-inch tires – in reality, you only added 1.5 inches in ground clearance from the lowest point of your vehicle that is usually the differential housing of the axles.

    Taller tires mean a higher center of gravity. So when you are negotiating a slope sideways, the vehicle will have a higher tendency to roll on its side. Having taller tires will also make the tires rub the wheel well. This can range from slight rubbing when turning, or a full braking effect. This happens under more extreme conditions when the suspension gets fully compressed while the wheels are fully turned in any direction. Having taller tires will also dictate how much suspension lift you need to put in your vehicle to accommodate the desired tire size.

    The additional weight of taller tires will affect the longevity of your wheel bearings. Vehicle axles have stress limits and a taller and much heavier tire can stress your 4WD’s axle to the breaking point, especially during difficult trail conditions.  Understand the limitations of your axle before installing very large and tall tires. A 15-percent increase in tire height should be the threshold when you should begin to reevaluate your driveline.

    Vehicle manufacturers design their vehicles to address the general needs of the public so there is no such thing as a vehicle that fits all needs. It is for that reason you have to understand that once you start doing upgrades on your vehicle, some may be friendly to upgrades while others are not. This can be best gauged by the availability of aftermarket upgrades available to a particular brand. However, this is not necessarily the case either. Some aftermarket upgrades may not be popular locally, not because it is bad but most probably because of cost. Aftermarket importers usually shy away from these expensive parts because of lack of market interest.


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