RC Gear Ratios Explained! Pinion and Spur for max performance
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In another video, that you can watch here, we talked about all the ways to make your Rc car faster. But let’s just quickly talk about pinions and spurs again – that’s right, this is gear ratios explained.
The pinion gear on your RC car is the gear that is attached to your motor and usually smaller, and the spur gear is the usually larger gear that the pinion drives. Spur gears are sometimes integrated with a slipper clutch or with a differential but the concept of ratios stays the same.
There are other gears in your car that also dictate how fast your car goes, like the diff gears and even wheel size, but for simplicity, we’ll just talk about these two as they are the easiest to change on most cars.
Warning – there is maths in this, but I’ll try not to give anyone a headache, including myself – let's do this.
This pinion gear has 20 teeth, and this spur has 100 teeth. 100 divided by 20 is 5, so this is a pinion to spur ratio of 5 to 1. That means that the pinion (and therefore your motor) has to complete 5 revolutions to make the spur complete just one.
Unless you have a quick motor, this isn’t very fast gearing. It’s like riding a bike; if you start off in 1st, your legs will be pedaling really fast and it’s easy to move, but you aren’t going to win that Cadel Evans thing.
If we swap this 20 tooth pinion out for a 40 tooth one, but keep the same 100t spur, the gear ratio would now be 2.5:1. That means the pinion (and motor) only has to turn 2 and a half times to make the spur spin once.
But now if the pinion turns 5 times, like with the old ratio, the spur will double its original speed by spinning twice.
In some cases, you may run out of adjustment and will not be able to fit a bigger pinion. You can also change the spur. Let's keep the 40 tooth pinion but go down to a smaller spur of 80 teeth – this is a gear ratio of 2:1.
The closer you get to a 1:1 gear ratio, the faster your car will go.
But it takes a lot of torque to have these fast gear ratios, so go too far for the motor, and you’ll overheat it and start burning components. ESC, Battery Pack, Motor etc. Think of trying to start pedaling that bike in 15th gear – unless you’re a pro, it’s way harder than the lower gears.
Most manufacturers have their cars in a very safe place from factory – the Traxxas Slash 2wd with the brushed motor has a pinion/spur ratio of 5.625:1, and with the rest of its gearbox, has an overall drive ratio of 15.3:1. So every 15 turns of the motor, the wheels spin once. Bring that ratio down and the top speed will be higher, at the expense of some acceleration, and the motor and electronics having to work harder.
Here’s the main takeaways, for most cases:
Bigger Pinion or smaller spur – go faster, less acceleration and torque, higher temps and shorter runtimes.
Smaller pinion or bigger spur – go slower, more acceleration and torque, lower temps and longer runtimes.
Now, we have spoken about the most common method of changing your vehicles gear ratio by using different size pinion and spur gears, but it’s also important to understand that other components can affect your vehicle’s final drive ratio, like the Differential pinion and ring gear.
Some manufacturers produce different ratio diff gear sets, using the same concept as the discussed pinion and spur. These are typically found in crawlers or drift cars to create overdrive setups, where the rear wheels spin faster than the front.
Another, more common component that is changed without much thought given to the effects it may have on the final gearing ratio, wheels! Take these 1.9 Canyon trails, they have an outer diameter of 116mm, compare the roll out against these 2.2 Canyons with a 135mm diameter.
If you go too fast on the gearing for your motor and esc, they’ll just get hot and might not even make it to their peak rpm. There’s a “sweet spot” for every chassis and electronics combo that will give you the best balance of reliability, speed and runtimes.
Going hardcore stupid gearing also voids the warranty from most manufacturers – in fact keep in mind that upping the gearing of the pinion will, in most cases, void the warranty on an esc in an RTR vehicle. If you aren’t sure, don’t be afraid to ask your local hobby shop. I’m not saying don’t do it, a properly fast car is awesome! But a 5th scale basher probably isn’t meant to constantly do 120kmh, so don’t be surprised if you push the limit, and the limit pushes back.
That’s it! Go mess around with your gear ratios and see if you can get your car perfectly dialed to win your backyard grand prix! Bye!
