Kart COG Calculations

The background story:

We had one driver that i had been working closely with in rotax that had not finished outside the top 5 at a state title for over a year. And we had another driver that couldn't get the kart to go around the corner if their life depended on it.

We had access to some very good corner weight scales and the karts with their respective drivers seated were within 0.5% on front to back weights.

The question:

Did we have a dud frame? Was it just that one driver was just useless?

The answer to both of these questions was no. We had the Center of gravity height wrong. The kart that was working well had a taller driver that was built more solid in the shoulder area

I made a spreadsheet that allowed us to calculate cog height it showed that the kart that was not working had a much lower cog height. We raised the seat in the kart that wasn't working by 25mm and all of a sudden it was drivable.

cog height spreadsheet in xls format

The method of measuring the data is quite simple. I have decided not to elaborate on the how and why as information is available online regarding cog calculations and i am quite bad at explaining maths problems.

All weights are taken with driver seated preferably with race gear on (helmet). It is not a one person job.

We place the kart on scales and weigh it to get the total weight.

We place the kart with only the rear on the scales to get the rear weight  (kart MUST be level)

Measure and input your tyre diameter (yes it effects the result)

Measure your karts wheelbase

Now you must raise the front of the kart a set amount with blocks under the tyres contact patches. you need to get to about 25cm or higher. The numbers wont be accurate if you don't get a good height change. This will increase the weight on the rear axle. so enter that into the spreadsheet

Once you have input the above data the spreadsheet will do the rest. It will tell you the rear weight bias and also the center of gravity height in cm from the contact patches of the tyres.

These numbers mean nothing by themselves unless you have a few karts / drivers to try it on. eg your mates kart always handles better than yours so you might scale them both up and realize he has a lower or higher center of gravity or different rear percentage. This would help you chose to head in a certain direction with your seat position.

On a final thought i would just like to say that numbers are just numbers. I have seen karts that don't scale up that great (everyone said it had too much rear weight) almost win the nationals and karts that are that magic 58% rear that everyone talks about that just wouldn't work until the seat was moved. Regardless of how much information you have on seat potion from the manufacturer or forum posts is is still a bit of a trial and error. But knowing where you are can help decide what to try