GEAR RATIOS
By Don Holmboe
Gearing a kart is an issue that many new karters don’t try to deal with. Most people will ask around the local track to see what everyone else is running and run that gear. However, what if you are testing at a new track and don’t have any locals around?Moreover, how do you know that the gear they give you is really the optimum one for your situation? This month I am going to review some rules of thumb based on my experience, which will give some insight on the task of establishing the optimum gear for a kart.
The first thing to remember, obviously, is the goal the whole exercise is to obtain the minimum lap time. It is not necessarily trying to get to maximum RPM at the end of the longest straight. Depending on the track, the gear that gives maximum RPM at the end of the longest straight may not give the minimum lap time. I’ll get back to that point later.
First, some background. Let’s assume a dyno pull shows that peak torque for the engine we’re gearing is about 10,300 RPM. Although it does not officially have a redline, we will use 15,000 RPM as our redline figure.
Also, when you are running a gear test you need to have the carburetor tuned for race conditions and be sure to keep in tune with race conditions throughout the test. The test will still work with the carburetor set a little rich as long as it is consistent during the test, but the closer to optimum setting the better the test will be. Pick a point at the end of the longest straight to look at your RPM gauge (preferably on the data download to assure consistency).
Remember, if you are looking at peak RPM for each lap there will be some variation as the peak RPM is dependent on the RPM at the beginning of the straight: the more RPM at the beginning of the straight the more at the end of the straight. This is why many beginning karters find themselves dropping down to smaller gears as their driving skills improve. While a beginning driver might find that a certain gear gives a good maximum RPM number on the straight, an experienced driver might come on to the straight with more RPM and find that the same gear gives them too much RPM at the end.
So pick a gear, based on another similar track or a local recommendation and run some laps. Let’s say that the maximum RPM at the end of the longest straight is 14,300. Next, drop two teeth and try the test again. The reason I say to always drop teeth is because the biggest error I see is running too big of gear. Contrary to what you might think, running a bigger gear does not always result in increased RPM. I have found through experimentation that if the initial gear selection is way too big (defined as approximately five teeth more than optimum) most engines will not pull more RPM. In fact, going to a smaller gear can cause the RPM to increase.
The concept of a larger gear not allowing for more RPM at first blush seems incorrect. Remember that we are talking about a starting gear that has been incorrectly selected and is about 5 teeth bigger than optimum. The explanation involves the horsepower curves of the motors. All kart motors achieve their maximum horsepower at less RPM than they are capable of running. The higher you try to rev the motor the less the horsepower. Think of the horsepower it takes to maintain 60 MPH. If it takes 15 horsepower to run at 60 MPH and you are geared to be at an RPM (higher) that gives you 14 horsepower at 60 MPH you obviously won’t achieve 60 MPH thus not reaching as much RPM. You can prove this to yourself by adding 3-5 teeth to your normal gear and seeing the affect.
When in doubt as to add or take teeth off: always drop two teeth. As a rule of thumb, I look for a decrease of less than 200 RPM per tooth when reducing teeth or an increase of less than a 200 RPM per tooth when adding teeth when you have made the right choice.
There is a trade-off involved with gearing. The lower the gear, the closer you are running to the peak torque number. However, the bigger the gear the better the acceleration off the slow corners. The layout of the track will define the optimum maximum RPM. For example, at a relatively fast track here in the northwest, our best lap times correspond to a maximum RPM of 14,700. We have found if there are no tight turns to slow the engine down than it is best to operate a little closer to the peak torque number. At another, tighter track, our best times correspond to a maximum RPM of 15,300. At the tighter track we allow our maximum RPM to get farther away from the peak torque number in order to pick up some acceleration off the slow corners.
To recap, pick a gear and drop two teeth. Keep dropping two teeth until less than a 400 RPM drop is recorded. Play with that final gear plus one tooth to figure out which gives the optimum lap time. If the first two tooth drop results in less than a 400 RPM decrease, be sure to add two teeth to the original gear to be sure that original gear was not too small. If that increase results in less than a 400 RPM increase than the original gear was close. Try it plus and minus one tooth to find the optimum lap time.