But when it so happens that the power output of the engine is exhausted by the time the bike is supposed to hit its top speed on 5th gear itself but the engine seems to be running at quite a higher RPM, then in that case an additional gear cog is provided which is called an overdrive gear. The main use of the gear is to keep the engine speed down while not compromising on the top speed or performance of the bike. But that does not mean that the top gear of every bike is an overdrive gear. It depends on the machine at the end of the day.
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To get into some more technical specifics, let’s consider an example. Let’s say you have a bike with a 6 speed transmission. The rated top speed of the bike is at 150 kmph. You are on the highway cruising at 110 kmph on top gear at 6000 RPM when suddenly you come across an open wide road. Now you get a sudden pang to take your bike to the limits to get that adrenaline rush. You slot your gearbox to 4th and pull on the throttle and your bike does 130 kmph at 10000 RPM, now you slot into 5th and take the bike to its 150 kmph top speed at 11000 RPM. Now that is it, you have reached to your peak power and peak torque output and the bike has nowhere else to get power from. You have made it and suddenly you get to know that you have reached there with only 5 gears, then why is the 6th gear even required? But glance a look to your odometer, it is running at 11000 RPM. Most probably that is the part where the reading is marked in RED that means it might be harmful to the engine to let the bike rev at such high RPM. So what could be done now?
This is the time the 6th gear comes to the rescue. You switch to your 6th gear and you see the speed of the bike remains on 150 kmph but your RPM is now down from 11000 RPM to say 8500-9000 RPM. What did this do? Your bike is now running a lot more relaxed and the engine is at a lesser stress level. The vibrations from the bike reduce as well. What you just experienced was the bike running in overdrive.
This is because in your gearbox, the teeth on the gears are always in a certain ratio of:
1st gear - 1:4
2nd gear - 1:3
3rd gear - 1:2
4th gear - 1:1.5
5th gear - 1:1
6th gear - 0.75:1
(Do note that these are not the exact specs for any gearbox and are used only for representation purpose) You can notice that the ratio happens to keep on increasing as you move up the gears because below the gears the bike needs a lot of torque to get all that mass of the bike moving. But once the bike is moving, it can take it easy a bit so the increase in ratios; the same reason why the bike provides a higher fuel efficiency figure on the higher gears than on the lower ones.
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If you happen to notice the ratio of the 6th gear in the table above, you can check out that the input gear is lower than the output gear while in all other cases, the input is more than the output. This is only to reduce the engine speed without hampering the actual travelling speed of the bike and most importantly this ultimately results in higher fuel efficiency. So suppose if in 5th gear and running a speed of 150 kmph, your bike was returning figures of 20 Kmpl, then in the 6th at overdrive it would return something close to 30-35 Kmpl possibly depending on the bike at the end of the day.
So this is what overdrive is and how it works. Do hit us in the comments section below if you have any more queries about it and do let us know what bike you have and if it has an overdrive gear or not. Do remember that many refer the top gear of any bike as an overdrive gear, but it really is not the case. Many bikes even use the 6th gear to keep providing output to the bikes and hence are not running in overdrive. In overdrive the bike does not gain any power but keep running a steady pace using less engine output.
By: Pratik Patole