Why would you need more moving force (torque) when putting larger wheels on a 4WD ?

Picture an imaginary lever arm between the center of your wheel and the ground.
The engineers of your car have carefully calculated the torque of your engine, the gear ratios of your transmission, the gear ratios of your transfer case, and the gear ratio of your differentials in relation to vehicle weight and tire size (they care only about the radius - the "lever arm" in our example here). The engineers did all this to give you pleasing acceleration and good off-road performance with a low crawl ratio.

If you would later put large tires on your vehicle and the radius/lever arm increases (in the image below twice as long as before) your moving force decreases proportional to the increase of the radius/lever arm (in this example creating only half the moving force power). This results in sluggish acceleration and a fast crawl speed plus less torque to climb obstacles.

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In order to regain decent moving force you will need to modify your drive system. To put in an engine with more torque will only partially solve the problem. The delicate balance between engine rpm, fuel consumption, smooth shifting and good off-road performance is still way off.
The only way to regain what you lost is by recreating the same (or better) gear ratio/crawl ratio that the engineers had in mind. That is done mainly by installing lower (numerically higher) differential gears.

As a rule of thumb you can expect to lose about 3.5% of torque/gear ratio for every inch of tire size increase.

For example, if you replace your stock tires of 28" with 35" tires your loss is 7 x 3.5% = 24.5%. If your stock gear ratio in your diffs is 4.9:1 you will have to get diff gears that are numerically about 25% higher. 4.9 x 25% = 1.225 - so, your new gears should be 6.125:1

The gear ratio examples I have used here were for a Mercedes G - and Mercedes offers diff gears that are pretty close to that desired number - 6.17:1

If you don't have a calculator - or don't know hoe to use one, here is a web site that helps you find the right gears.

I have also described the procedure of fitting larger 35" tires to my Mercedes G.


Effect of larger wheels on your stopping force

 


Ten stupid things men do to mess up their 4WD




 
More basic off-road driving tips coming soon

Topics so far:

tire safety

mudflaps

when to engage 4WD

where to mount chains



4WD glossary




who invented 4WD ?

most unique 4x4


future of 4WD?


easy trips

tough trips


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