Sprocket & Chains
Sprockets and chains provide a cost effective way of reducing the speed of rotation of the motor to the speed required by the driven axle.
Chain types
The are many different types of chain and they have to be matched to the sprocket
Common types in this application are T8F , #25
Going too large in the chain size (distance between links) will limit the maximum number of teeth for the sprocket on the axle (it will be dragging on the ground!).
Going too small in the chain size will limit the size of the teeth and therefore the strength of them. This means they can wear out quickly.
Sizing your sprockets
The max speed of a Hacky is usually 25 Miles per hour. This is largely due to the trade off between torque / top speed / track length. With the limited power budget (30A) it limits the motor current and therefore the available torque. Corners come regularly in a Hacky race so a high top speed is often less important than good drive out of corners
So lets work the numbers out -
25 Mph = 1.60934*25 = 40.2 Kph
40.2 Kph = 40.2 * 1000 / (60 * 60) = 11.18 metres per second
Now we need to factor in the tyre diameter
Lets take a mid-sized tyre 310 mm
=310*3.142/1000 = 0.974 m
Now we have this we can calculate the rotations per second the wheel has to make
= 11.18 / 0.974 = 11.48 rotations per second
Motors have a speed range and if you are using a MY1020 2000W brushless motor then they have a speed range where their peak performance is around 4300RPM.
This will vary and its best to check the motor specs although these are usually not well documented from the typical Chinese suppliers.
4300 RPM = 71.66 rotations per second
The motor are usually supplied with an 11 tooth sprocket but different ones from 9 – 13 teeth are readily available (Note: if it’s a MY1020 motor the retaining nut is left hand thread)
So what is the size of the larger sprocket? Well you have done the hard work now
= (71.66 / 11.48) * 11 = 68.66
Larger T8F sprockets are readily available – search Ali for “T8F Chain Rear Sprocket 38 44 54 58 64 74 Tooth”
You can also search for plate sprockets.
Going for a slightly smaller sprocket on the motor e.g. 9 teeth would give a theoretical 56 tooth on the axle , therefore 54 or 58 would be a decent compromise
Chain adjustment
All chains stretch and you need to have adjustment. This is usually easiest accommodated in the motor mounts rather than the axle.
Keep enough tension so that the chain doesn’t come off but not so that its fully under-tension all the way round. Its worth checking this by rotating the wheel 90 degrees (4 times) and check the chain tension as things are never perfectly round.
Chain alignment
This is critical. If you are used to Shimano style sprockets on a bicycle then they work by using chain mis-alignment to move up and down the sprockets. If you don’t get the alignment of the sprockets correct then you run the risk of the chain coming off the sprockets. Check both the alignment to each other and the axle
Chain lubrication
If chains are not lubricated they can rust and the links start sticking. On the other hand Hackys tend to race in lightly mowed fields so get exposed to dust and dirt. Use a thin lubricant like WD40 and apply more frequenly