Join Date: Jan 2007
Location: South Carolina
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I've seen this over at atvconnection.com a couple of times... I thought it was time we posted it here as well... A special thanks to Buckaroo (from atvconnection.com) for putting this together...
"A lot of people have written to me, and in person have asked me about the handling characteristics of the AC's. How to lessen the tippy feeling of the AC's and how to make it a fun 'buzz-around' machine. I will tell you there is no magic too it, some basic principles of geometry, physics and logic can have you running around having a great deal of fun and really enjoying your quad without having to put up with tippy, ill handling.
First of all lets start with tires. All tires 'roll-under' some roll more than others. 2 ply will roll more than 4 ply and 4 ply more than 6 ply etc. Why? because 6 ply have stiffer side walls than either 2 or 4 ply. The stiffer the sidewall the less roll-under. What does roll-under do - it causes oversteer (makes you feel like it is doing a nose dive over the outside wheel while turning) or (the sensation that the front is tucking under). If taken to extreme the machine will roll onto its side. In a nut shell 'roll-unders' can cause 'roll-overs' How do you stiffen sidewalls? Simply, add more air or add tubes or both. With that said.
On to profiles (sidewall height) - a low profile tire will always have less 'roll-under' than a tall profile tire. A 25 inch tire has a lower profile than a 26 inch tire and a 26 has a lower profile than a 27 inch etc. So in a nut shell again a 25 inch tire will always give you superior handling characteristics over a 26 inch and taller tires. A side note here, a 25 inch tire will also accelerate faster because it maintains a lower gear ratio. And also if your engine because of the gear ratio change may not be able to pull the full RPM range, you may find that the 25 inch tire will also run a faster top speed. It will certainly get to the limiter quicker and it will also put more power to the ground. (Same principle that you are putting more power to the ground in low-range than in high-range).
Now let's talk about springs and suspension. General rule - soft suspension grips and stiff suspension doesn't. (The physics behind this is that gravity is a constant downward pressure, when you soften a suspension you are making it heavier because gravity can act on it more, if you make your suspension stiffer then your are resisting gravity and making the load lighter - heavy is traction/grip) If you look at the setups on the AC you will see different physics and geometry on the front as apposed to the rear. You get most all the stability from the rear suspension. The rear suspension is attached to the lower A-arms and the springs/shocks are at a different angle as apposed to the front. The front suspension is by design made to move up and down and to compensate to an ever changing camber, it is not designed to stabilize the machine (to offer more stability the shock would have to have more angle, which would limit it's ability to compensate for the changing camber and also limit its ability to move up and down and to absorb weight transfer without compromising the camber). So by stiffening the front you are counter productive in making the machine stable and in fact making it more unstable, and if taken to extreme it would make your machine mushy in the rear like you had a flat tire back there and the rear would float and feel squirrely. Because the shocks on the rear are attached to the lower A-arms and have a sharper angle to the frame, by design it is there to stiffen the frame and to stop the machine from tipping. (this is why sway-bars are attached to the rear and not the front).
So what actually happens when you turn? Well to a degree you need to slow down, this causes some weight to be transferred from the rear to the front. and when you actually initiate a turn some of the weight from the inside is transferred to the outside. This puts a heavy load on the outside front wheel. Remember soft grips and stiff doesn't, if your springs are too stiff (which you may think would assist in handling the weight transfer and make the machine more stable - it doesn't) the machine will plow or want to go straight. Remember the rear does the stability not the front. If the front is soft it will absorb the weight transfer and grip, a stiffer rear will keep it from feeling tippy. (we actually do have some extra stiffness in the front by having appropriate air in the front tire to stop 'roll-under' - tires to some degree are like mini springs - but without the control of rebound or compression that a shock offers - all we can do is to remove some those properties by making the tire stiff). This way the shock can do its job in a controlled fashion, with minimal affect from the tires rebounding and compressing uncontrollably. The front and rear do just the opposite as well as the left and right. When we load the front we are unloading the rear and visa versa. When we load the left we are unloading the right and visa versa. What is loading and unloading - simply, transferring weight.
Ok now we go into a turn.
If the rear is too stiff or the front too soft we overload the front causing the rear to be excessively unloaded (light - loose) - results are the rear will prematurely slide and the front will oversteer and we may actually have to counter-steer (steer opposite of the turn) to keep the machine from spinning out or having a tuck and roll if the tires are too soft and 'roll-under'. This is called 'loose going in'.
If the front is too stiff and the rear too soft we have the opposite in that the front will understeer or plow because it is not being loaded enough (too light - too stiff) and if the tires are too soft and 'roll-under' we will get the feeling that we want to do a handstand on the handle bars and the inside rear tire may in fact want to leave the ground because the right rear is loaded to heavily and wants to go straight and the inside rear wants to get to the outside (or jump over the outside rear - because the outside rear is planted too firmly - it is soft and has grip and has not given up its weight). This is called 'tight going in'.
A good turn is to have the front soft, load the weight, grip and head into the turn. The rear needs to be stiff enough to resist gravity and give up its weight to the front but not so stiff as to go loose but only to follow the front to the apex (the point at which the turn basically is complete and we have negotiated the entrance) of the turn, at this point we are ready to hit the gas to unload the front and to load the rear again. On occasion this may cause the rear to break loose or loose traction and slid a bit. Once that is complete and the transfer of weight is almost done the rear will load to the point of hooking up (it has no enough weight to get traction), the front has unloaded and now rebounding and becoming light, we may even be able to loft the inside front tire off the ground. We are basically about out of the turn completely and headed straight again.
On to track width. A wider track will always offer better handling. Here is the simplistic physics and geometry behind that principle. A wide track offers stability from left and right weight transfers by decreasing the angle of tippiness. Instead of the weight wanting to go to the outside of the tire it is directed to the inside of the tire causing the suspension to squat rather than roll. Thus in effect simulating a lower center of gravity and not transferring as much weight. Thus confining the transfer of weight within the track width and lessening the transfer to the outside. You can get a wider track with offset wheels or the addition of wheel spacers.
The basics in a nut shell.
1)stop the roll-under from the front tires - low profile and stiff sidewalls.
2)soften the front suspension - to counter the stiffer tires and to more easily absorb the loading of weight without throwing it to the outside (tippy - tuck) or resisting to accept the transfer (plowing - too stiff)
3)stiffen the rear suspension - to enhance the unloading of weight and resist gravity and become light enough (give up some traction) to follow the front (without premature loose going in) and to be light enough with less traction to be able to slide if necessary (at the apex) in order to complete the turn and to get heavy again only when the gas is applied and to hook up."