Question: “I have this go kart and I want to make the rear tires bigger. If I change the diameter of rear sprocket so that it increases correspondingly with the rear tire, will the performance of the go kart suffer? In other words will the go kart go slower than before and will the go kart have really poor off the line acceleration?”
Good question, I have often thought about that myself. I have often said that the rule of thumb for a go kart to have optimum performance is to have the rear sprocket be nearly 85% of the rear wheel diameter. The question is, does that apply just for 12 inch diameter wheels or does it apply to 16 inch rear wheels too?
The answer to that question may surprise you. It surprised me. When I ran through the calculations a funny pattern emerged:
– A 12 inch diameter tire with a 12 inch diameter rear sprocket yielded a speed of 22 miles per hour at 7.3 feet per second squared of acceleration.
– A 16 inch diameter tire with a 16 inch diameter rear sprocket yielded a speed of 22 miles per hour at 7.3 feet per second squared of acceleration.
I thought that was erroneous so I ran a whole slew of tires sizes with corresponding sprocket sizes and still came up with the same numbers.
So the answer is no. No, your performance will not suffer if you increase your sprocket size correspondingly with the wheel size.
What will happen if you do not increase your sprocket size is that the clutch will become over loaded and smoke. The go kart will require pushing to start and will have horrible low end performance.
But if you increase the diameter of the sprocket so that it is 85% of the rear tires diameter, the performance should stay nearly constant.
That is an unexpected result, but one you can depend on if you are designing a go kart from scratch. The rule of thumb stands: “The rear sprocket should be nearly 85% of the rear tire size for optimum acceleration and speed performance.”
The belt drive system for example is quite dependent on this relationship. The reason is that the drive pulley for a belt drive can be quite large, 1.75 inches in diameter, and requires that the rear pulley be as large as possible.
In the case of the wood go kart that I designed, in order to keep the ratio a simple one to one system (versus adding a jack-shaft) the rear pulley had to be the same size as the rear tire. I was able to get through this problem by making a wood pulley which was durable enough to handle road conditions, cheap enough to make myself and best of all gave the performance that was needed for good acceleration and overall speed performance.