Tread Beyond Roads
In this month's series on off-road modifications, we take a look at gear ratios and tyre sizes
OOFF-ROADING IN INDIA IS GOING THROUGH a wonderful phase. Last three months have witnessed some very well organised events across the country. Being privy to some of these, I can say with full conviction that they will soon place India at the forefront of this sport internationally.
In the first two editions of Tread Beyond Roads, the focus was on basic and necessary upgrades. But if you are serious Offroader, it is imperative that you start looking up at gearing and re-gearing of your vehicle.
If you have reached here, it is obvious that you have installed larger tyres. This increase in tyre size demands looking at the a) transmission ratio – number of turns made by the engine for one revolution of the transmission output, b) transfer case ratio – number of turns of the transmission output required to make the output of the transfer case to turn once, c) differential ratio – number of turns of the transfer case output required to make the tyre turn once, and d)final drive ratio (also called Crawl Ratio) – number of turns made by the engine to get the wheel to turn once.
FDR = TR x TC x DR
Always remember, tyre is the final gear in the scheme of things. So, if you take a larger tyre it would travel farther for one full turn than a smaller tyre. The vehicle would thus travel farther with each revolution. Now if we consider this along with the Final Drive Ratio, this implies that with larger tyres the vehicle will travel farther with the same engine RPM. Alternatively, at the same driving speed, the engine will have lower RPM with larger tyres.
The above sounds good, but now the flip side! Torque is the amount of force that is applied by a rotating member at exactly 1m from the axis. With the increase in the distance from axis (i.e. using larger tyre) the applied force decreases. And furthermore,force on an object is equal to its mass multiplied by its acceleration. Therefore increasing thetyre size, will result in a decrease in acceleration. So, for the same final drive ratio, a smaller tyre will turn faster but transmit more torque to the ground, and a larger tyre will turn slower and transmit less torque. And as we all know by now, offroading is all about torque. So how do we compensate for loss in torque?
Some use, crawler gears in the transfer case, others adjust the differential ratio, and the more technically gifted actually rebuild the transmissions. Basically all these methods try and compensate for the change in Final Drive Ratio, which effectively drops with the increase in try size.The goal of all of this gearing is to help the vehicle transmit more force to the ground.
The easiest way to compensate for the use of larger tyres is changing the differential ratio. The way to do it is:
[New Tyre Size] / [Old Tyre Size] X [Original Differential Ratio] = [New Differential Ratio]
For example, if you are running 29-inch tyres and the differential ratio is 4.27, and you want to upgrade to 33-inch tyres, then your differential ratio should change to 33 /29 x 4.27 = 4.86.
Since exact 4.86 ratio crown pinion set might not be available, one could choose a gear that approximates the requirement. Similarly, there are custom crawl gears available for certain vehicles that can be installed in the Transfer Case thus increasing the overall gear reduction.
Besides delivering more force to the ground, proper gearingallows the vehicle to traverse obstacles more slowly and with greater control. Since engines tend to produce peak power higher in their rev range (Power = Torque x RPM), in order to provide your vehicle with optimum torque to traverse steep gradients at low vehicle speeds, you need your engine to be able to spin adequately fast while your wheels spin slowly. This ensures that your engine delivers optimum power while your vehicle is at a low, speeds.
The benefits of proper gearing, or in case of off-roading, having a higher crawl ratio is not only limited to multiplication of engine’s torque as is passes from the flywheel through the various gears, but it also enables the engine to operate in the optimum power band while keeping the vehicle speed slow. This optimum power can then go through many gear reductions to deliver much desired torque at the wheels.
If you have successfully traversed the obstacle of gearing / regearing, you are about ready to jump into the daunting terrain of suspension upgrades. That, I reserving for the next edition of my column. Till then, crawl with ease! ⌧