How To Easily Measure Stagger
I always enjoy your articles. I seem to find something to think about from them. It is good mental exercise for an old mind. It beats doing crossword puzzles.
Please show your new readers why you measure the larger tire first. The pictures in this month's issue ("Stagger," Nov. '12) has them doing fraction calculations in order to get stagger. From 35 years in teaching math and more than 50 years in race cars, fractions are not the strong suit of many people. Newby's will bless you.
A person that might be a good story is Diandra Leslie-Pelecky. She wrote "the Physics of NASCAR" and has been doing a blog called "Building Speed" which I'm sure you have seen on Jayski.
I will look her up. She is also very involved in the discussion of concussions. I did a story on tires years ago and was over at my friends shop near my home. He said, "Do you want to see an easy way to measure stagger?" And he proceeded to show me what you are talking about.
You first measure your larger tire at the front or back of the car. Then remember that dimension only and wrap the tape around the smaller tire on the other side of the car. Where the larger tire dimension falls from the end of the tape on the smaller tire (the number will overlap past the zero end of tape on the smaller tire), is the stagger amount.
There is no subtracting needed, only the need to remember one number, the larger tire circumference. I have used that method many times and told that story numerous times to many racers. Thanks for reminding me.
Looking For a Clear Answer
I've been reading all your info for a while now. I am trying to wade thru all this roll/moment center info and I still don't see any clear answer. It seems a car with a higher center of gravity should also have a higher RC.
I built a car this winter for what we call the Thundercar class up here in Canada. It has stock suspension pick up points, tube chassis, fiberglass body, and a four-link rear. I opted for a '69 Chevelle front clip and rear pick up points, 6-inch ride height, and 108-inch wheelbase.
We run 3,200 pounds, 55 percent left, 50 percent rear, 8-inch Towel City retreads on Hoosier casings. I bought the big 13/8-inch Hotchkiss sway bar at 540 pounds and have a 600 LF, 650 RF, 200 LR,and 175 RR springs. I've been testing it half the season and no matter what, the front doesn't seem to ever totally hook up. I can make it worse, but never really get rid of the understeer. So, I am wondering about RC location.
The front runners here are saying their roll centers are left of center (metric chassis), and they are fast, but also look like their cars are skating quite often.
The way my outside tires get hot, I think I have too much roll, so wouldn't I need to raise the RC or move it to the right? It is currently about 3.5 inches off the ground, just right of center and only migrates down. I have removed the bumpsteer, and don't have a good method to measure Ackermann.
I was also wondered if the shocks were too stiff, I can't feel the chassis dive or roll while driving. Shock travel is almost an inch on front and almost even with no bar preload, it is loose. Help!
The front roll center should be left of centerline. When the car dives and rolls, the moment center always moves, usually to the right. So, your MC is most likely way to the right when the car is in the turns and that makes the front suspension very stiff.
To measure Ackermann, and I have shown this numerous times in the magazine, string out about ten feet in front of the car from the sides of both front tires. Mark a point on tape n the garage floor. Then turn the wheels and re-string the tires. Measure between the marks on both sides. If the measurement is the same, you have zero Ackermann.
Your car could stand to be up to a half inch difference at 10 feet out with the left turning more than the right, but no more than that.
Also, your crossweight should be around 51.7 to 52.0 with the 50 percent rear weight. If you can, compare the left side tire temperatures and adjust the rear spring split until the left front is the same temperature at the left rear tire. If you can move your MC, you might not need to do anything more to get that LF tire temperature up, it will heat up after the change.