What are the Effects of Tire Growth and Offroad Driving?

Dear off-road and nature enthusiast friends, we are together with another sharing of our Off-road Basic Technical Information article series. The information I will provide; These are concentrated, technical-based, lived experiences, and some of the details that will find a name in the off-road literature for the first time. With the hope that it will benefit the readers.

Our topic today is “ What are the Effects of Tire Growing and Offroad Driving? ” :

We can think of the first step of vehicle modification as changing tires. If we have not bought a ready-made vehicle, if we have bought a standard vehicle, the first thing we will do is to change the tire.
First of all, let’s take a brief look at the tires we use in the field. AT (All Terrain), MT (Mud Terrain), XT (Extreme Tarrain) Since this article is not about tires, it’s only about tire enlargement, I won’t go into extraneous details.

I want to enlarge my tires, how much should I enlarge and why should the tire be enlarged in the field , let’s try to find the answers together.

Large tires have greater obstacle clearance than smaller tires. So as the tire gets bigger, your ability to pass over the obstacle will increase. If we examine the attached picture, it can be seen that the heights that the tire can exceed at the same angle position are different in different diameter tires and, as the tire diameter increases, the height corresponding to the same angle also increases. If we examine the values in the table, we can see what level this increase is. To give a numerical example, at a fixed angle of 75º, which will create the same difficulty for the tire, a 27″ tire can overcome a 25.4 cm obstacle, while a 35″ tire can overcome a 32.9 cm obstacle. When we look at it in centimeters, this difference, which seems small, is 30%. When we go to 37”, the difference will be 38%. That is, an obstacle that is insurmountable for the small tire, such as a flat wall, will be an obstacle that can be climbed and rolled over for the large tire.

The net result here is that the bigger tire is always more capable in terms of off-road ability.

Tire diameter and tire width should not be confused with each other. The two are very different concepts in terms of benefit/harm, depending on the background. But for commercially available tires, in general, the larger the tire diameter, the larger the width.

Especially in solid axle vehicles, there is no “reasonable” way to move the differential watermelon off the ground, other than to increase the tire diameter. Portal axis-like applications are utopian solutions for the vehicles we are talking about.

Assuming that the differential watermelon of a normal vehicle is ~ 16 cm from the ground, for a vehicle with an increased tire size of 3” (eg from 30” to 33”);

3 inches x 2.54 cm/inch = 7.62 cm

7.62/2=3.81cm
3.81/16= 23.81% is a considerable increase.
Sometimes you need 1 cm to move forward without hitting the bottom of the vehicle. If you have that 1cm you will pass without hitting, otherwise you will hit pass or you will fail.

It is an indisputable fact that for successful off-road driving, it is necessary to make our tires as large as possible.

If we examine these elements under the headings:

The tire must physically fit inside the hood and fender. Or this should be achieved with minor shaving. You can make bigfoot with very deep modifications, there is no limit here. Our assessments are for modifications to be made within reasonable limits.
If the upgrade kit is applied, I recommend doing this without going too far.

Does being able to physically place the tire inside the hood or cut it a little bit will allow us to enlarge the size as much as we want? For example, can we put 42” tires on the jimny?

There is no obstacle to wearing, the important thing is can it be used healthily? Especially in Istanbul groups, it is possible to come across Suzukis fitted with sergeant pattern large tires. It is useful to look at these vehicles and know that these tires are not mounted by simply shaving the hood and fender. Many of them are vehicles with profound modifications to the drivetrain.

On the one hand, while growing the tire, on the other hand, it is necessary to ensure that the power of the vehicle flowing over the engine-transmission-road transmission-shafts-differential-axles is transferred to the tires without forcing the vehicle to break.

If we change the final drive ratio as we enlarge the tire relative to the original, we will reasonably reduce the risks for the system up to the differential. This means: If we increase the tires of a vehicle with final bevel ratio 4.10 and original tire 32” to 35”, the new ratio will be 4.10*35/32 =4.48. That is, the new final final ratio must be greater than 4.48 and the closest ratio. It could be 4.56 for example. After this change is made, the additional forceful effect created by enlarging the tire will be eliminated in terms of engine components-gearbox-road transmission-shafts. However, this will not be the case for the inside of the differential and the axles. If the inner casing of the differential, idler gears and shafts, axle gears and axles, U-joint or CV-joint connections at the front do not have the strength to meet the increased load, if you do not drive the car very naively, it will say “checkerboard” somewhere and break.

In accordance with the impulse-reaction principle, a force equal to the force exerted by the tires on the ground to propel the vehicle is applied by the ground to the outer wall of the tire. In other words, as the tire diameter increases, the moment arm of this force will also increase, so the vehicle will need more power with the growing tire and will be forced more.

The upper limit of the force transmitted to the axles will cause a torsional moment equal to the friction force * the radius of the wheel. Since the radius is a factor in the equation, the larger the diameter, the greater the opposing torsional moment. If we’ve made the tire too large for the vehicle’s infrastructure to handle, this opposing torsional moment can cut the axle. If the axle is strong enough, the differential can fall apart, and if it is strong enough, it will break whatever it finds weak, such as the shaft cross, the gearbox, the gearbox, the engine. If it breaks none, the vehicle’s infrastructure is already strong enough to handle that size.

This is not the only parameter that forces the tool. To a lesser extent, there is also the mass moment of inertia of the tire, which is a parameter of the tire’s mass, which opposes the power from the engine during starting. It is the counter-mass resistance that occurs when we try to move the tire at rest. Since the mass is obtained by dividing the weight by the gravitational acceleration and the gravitational acceleration is constant, we can also evaluate the change in the moment of inertia directly as a parameter of the weight.
In other words, the mass of the tire is directly proportional to the moment of inertia, its weight and the square of its diameter. (I=mr^2/2)

This resistance will increase as the tire gets heavier and its diameter increases.

This means that ; After the tire has grown, do not press the gas too hard, you will cut the axle. This situation is very common in ZJ and WJ group vehicles with DANA 30 Front differential.

That’s why, with the drivetrain unmodified, the persistence of continuing to drive that “majestic” defender with tiny tyres.

Your vehicle, which you have modified by installing large off-road tires, is now; It has become a vehicle that consumes more fuel, the road grip on asphalt is weakened, the speed limits have been reduced significantly for safe use, and it has become a vehicle that strays more than its track even in bends on asphalt. If you want to be affected by these factors at the lowest level, it is useful to know that you have to make your tire less big.

Conclusion and recommendations:
As all landlords know, as a vehicle approaches the terrain, it moves away from the asphalt. If I have asphalt performance and safety, and my off-road performance is very high, I feel like I’m full and I have a cake. There is no such option.

Generally, we can say that the bigger the tire, the better off-road performance.

If you’re going off-road, upgrade as much as the vehicle’s infrastructure allows and fit the largest tire.

Grow your tires, but don’t make the decision about how big you want to make just on the basis of your ability to physically fit the tire to the vehicle. Enlarge your tires by modifying these transmission parts, if necessary, within the tolerances of the vehicle’s existing drivetrain systems.

The key word is “the vehicle’s infrastructure allows”.

Large tires also affect the vehicle image very positively. However, I do not think that our friends who are actively on the field grow tires just for the vehicle image.

When you increase the tire, press the gas a little more moderately, avoid sudden loads. Knowing that throttle reaction devices increase the level of spikes, avoid using devices that reduce throttle reaction times, especially on vehicles with oversized tires to the vehicle mechanical limits.

Do not ignore the general recognition that non-rubbing tire sizes of vehicles in moderately elevated states are also the manufacturer’s maximum sizes that can be used in drivetrains without additional action.

Changing the tire size and/or final drive will cause deviations between the actual speed of the vehicle and the gauge speed. While corrections can be made with software dexterity in new generation vehicles, this problem can be eliminated in old generation vehicles by changing the speed wire gear corresponding to the tire size and final drive ratio. The arrangement made with software dexterity not only corrects the speed, but also regulates the gear shift revolutions according to the new situation, if the vehicle is an automatic transmission.

Large tires used for offroad purposes are generally wider as well. If the vehicle has the original tire 245mm wide and mounted on a 7″ wide rim, you will likely need new 9″ offset wheels if you have increased the tire to 35″x12.5″. In the data sheets of the tires, there is information about the min-max rim widths that they can be fitted. Do not rely too much on the information of the sellers, get the information about the recommended rim width of the tire you choose directly from the manufacturer’s site.

Even if you don’t have a problem with flat road driving with enlarged tires, there is a possibility that you may have problems with articulation and/or hood rubbing at full turn of the steering wheel or rubbing directly against the chassis or bodywork. This situation may also differ according to tire brands and types. Sometimes even the lift kit application may be insufficient. Try it out if possible.

Best Regards,

16/03/2020