THIS IS FROM A SAVED THREAD ON "CROSS CONFUSION". Thanks to Mike and Todd who's great information on Bob's is sadly gone. I have the entire Cross Confusion thread if anyone wants it, send me a pm or email
acblend@hotmail.com
Msquared
03-26-11,, 02:55 PM
Cross, I call it the great confuser. It is quite simple.
Cross has two primary functions. They are: dynamic weight distribution to the rightside tires and to control how the kart turns. Another function is that a higher cross setup will place more load on the right front which prevents the right rear from overloading on exit. A confusing thing associated with cross is which way loosens or tightens the kart? The answer is it depends on where you are at on cross relative to leftside weight. Anytime cross is moved closer to equaling leftside it tightens the kart. So if you run high cross, lowering cross will tighten the kart. If you run low cross then raising cross will tighten the kart.
Here are some examples: Cross 65% and leftside 58%. Lowering cross to 60% will tighten the kart because it is now closer to equaling the leftside weight. Raising cross will free up the kart because you are moving away from leftside weight. So, if you now had 50% cross and 55% leftside, increasing cross, moving it closer the equaling leftside will tighten the kart. Lowering cross will free the kart.
On a high grip track, low cross tends to give you a kart that does not get tight and handles well, but does not give you much speed off the turn. Why? Because you are controlling the left rear but not maximizing the right front, the handling is somewhat sluggish. You are mainly running off the right rear and keeping the left rear from causing the kart to push.
Medium cross (the dead zone) has inconsistent handling and is slow. Why? Not enough load is on the right front to relieve the load on the right rear. The right front does not load enough and the left rear reloads too soon resulting in a tight condition. This is due to what is referred to as a fight for dominance of the rear tires. Generally high cross is for tracks that have a lot of grip. Low cross is for tracks with less grip. For running on coke syrup indoor tracks you actually want to go negative on cross running 48% or lower. Trends have shown cross in the 52 to 58% range to work.
High cross gives you a quick reacting kart that plants the right front hard enough to relieve the right rear and control the left rear. The higher the grip that the race track has (the more G’s pulled) the more you have to pre-load the right front corner of the chassis (spring) with the high cross to get the kart to store and re-disperse the energy that it is receiving from the racetrack. Without the cross, the right rear generally wastes the stored energy (binds the kart) of the spring (chassis). That is what is meant by “freeing the right rear up.”
With a car there is a balance that has to be considered among the four corners of the chassis because each corner is independently suspended which is to say, it does not care about what connects one corner to the next. With a kart, each corner does have a suspension (the tires) but each corner, very much so, cares how it is connected to the next corner (the chassis itself). Because of this, you can’t think in terms of “balance” as it pertains to your setup percentages.
Here is an example of how cross works. The kart seemed to have a bit of a push (not bad). Track conditions were quite good with lots of grip. At the end of the night I set the kart on the scales to discover that the cross was almost identical the leftside percent (about 55%). This was not where I thought our set up was. I thought I had 53% left and 57% cross. I put 2 turns into the weight jacker (1 turn equals 1% of cross); I went back out in hot laps and tried it....it now pushed like a truck going in! I then took out the 2 turns and then took out a further 2 and tried it. It now turned in great but was a little loose coming off. At this point I figure another 1/2 to 1 turn in would be pretty close. If I continued to put cross in after the first 2 turns, that in theory would have eventually loosened up the kart?
What happened in the above is what is supposed to happen. Lower cross creates less load on the left rear. When the left rear is the predominate tire, after considering the weight transfer to the right side in the corner, you are going to push. The tire with the most load, has the most drive. That is simple physics. Knowing where your kart is at is great along with what changes do what. Don’t worry about the numbers; just know where you are at. That is what they are for.....to let you know where you are at. The feel at the track, tire temps and air pressure build-up can tell you if you are close to where you need to be. Many times, I will make changes without scales. I do not care what the numbers are. When it is all said and done, I will put the kart on the scales and see if my adjustments put me numerically where I figured it would be....in my mind. 9 times out of 10, I am within .2%. I scale to find what tires and adjustments do what. At the track, I make the adjustments the track asks for.....knowing in the back of my head where I am at. I might spend 120+ hours on the scales before the kart hits the track. After that, it might hit the scales just for curiosity’s sake....just to make sure I am not overlooking anything.
The track will dictate your cross. The determining factors are “grip”, “steering input”, and “geometry” of the track. Sometimes higher cross can work on lower grip tracks because of the corner geometry and steering input “making the kart mechanically transfer weight faster”. At the same time, sometimes a hard biting track, may allow low cross because of opposite conditions. The one key factor is to have a kart that is user friendly enough that you can “feel” which direction to go in.
Some general statements about cross would be:
1. High cross for high grip tracks.
2. Lower cross for low grip tracks.
3. The closer cross is to being equal to leftside, the tighter the kart is going to be.
4. DO NOT USE CROSS TO FORCE A KART TO HANDLE. Cross is for fine tuning ONLY.
Take the above at face value. To some high cross is 60% and to others it is 65%+. Use it as a guide to help you understand cross.
I generally adjust cross using the LF. If the kart has a rear weight jacker, I'll use it to fine tune with. Moving the LR up or down too much is going to effect the CG / roll center location which is a whole different ballgame.
As many have discovered, karts are very sensitive racing machines, so no matter what you do, it all matters.
The best thing for you to do is to put a good baseline setup in your kart and test what cross does to your kart.
I hope this helps.
Mike McCarty
Chassis Manual
Liquid Speed Tire Prep by Josh Philpott
www.kartcalc.com
www.220kartshop.com