Tread Be­yond Roads

In this month's se­ries on off-road mod­i­fi­ca­tions, we take a look at gear ra­tios and tyre sizes

Evo India - - COLUMNS - JASKI­RAT SINGH NAGRA Jaski­rat Singh Nagra is the CEO of Sar­bloh Mo­tors, re­spon­si­ble for bring­ing the Scor­pion to In­dia @js­n4x4

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! ⌧

Newspapers in English

Newspapers from India

© PressReader. All rights reserved.