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CG location effect for steady state cornering

  1. Jun 8, 2015 #1
    I am having trouble understanding this.

    The question was how to limit breakaway oversteer

    I know that putting the CG to the rear will increase the rear traction and induce understeer as there is less grip on the front tyres.

    However, I have also found that to reduce oversteer you want to limit rear load transfer as this increases the cornering force (good) but can overload it causing breakaway? (which I assume is non-progressive oversteer and is snappy).

    So generally, you would want the CG to the rear but not if the car is prone to breakaway oversteer? in which case you want it to the front and just limit the load transfer to rear? Or do you want to leave CG at rear and just limit further weight transfer there? Confusing as they seem contradictory
     
  2. jcsd
  3. Jun 8, 2015 #2

    cjl

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    I don't think I agree that rear CG promotes understeer - if anything, I'd tend to think the opposite, since front heavy cars tend to understeer more in my experience. Yes, front grip is reduced with a rear CG, but the amount of front grip required for a given lateral acceleration is reduced as well.
     
  4. Jun 9, 2015 #3

    jack action

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    I'm with cjl, the end with more weight should breakaway first. Here's a typical graph for the available lateral force vs the vertical load on a tire:

    515660d1256951847-losi-xxx-s-tips-tricks-open-mod-etc-graph-cornering-traction-vs-load001.jpg

    Here are the same data, but now it's the coefficient of friction of the tire (i.e. the cornering force divided by the vertical load) vs the vertical load:

    515661d1256951859-losi-xxx-s-tips-tricks-open-mod-etc-graph-cornering-gs-vs-vertical-load001.jpg

    We can see that a tire with less weight on it has a higher coefficient of friction and thus can support greater lateral accelerations.

    For example, with the previous tire on all corners of a 3000 lb car, with 750 lb on each corner the car would be able to sustain 1.2 g of lateral acceleration. With 2 front tires with 500 lb each and 2 rear tires with 1000 lb each, the front end would be able to hold until 1.4 g, but the rear end would loose it at 1.0 g.

    Worst, if we assume there is a total weight transfer (all the weight on the outside wheels), the front outside wheel has 1000 lb (1.0 g) and the rear outside wheel, with 2000 lb on it, will hold until only 0.75 g. But if you tuned your suspension to transfer weight from the rear end to the front, you could have 1500 lb on each outside tire, giving you a 0.8 g limit back and front.
     
  5. Jun 12, 2015 #4

    Ranger Mike

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    Mekyy, Nice question and welcome to the forum. I suggest you read Race Car Suspension class in the Automotive sub forum on this post. As I read your question , many subjects need to be addressed before we get to the nub of the matter. Here is a summary of the questions you wish to address.

    How to limit over steer and specifically a snap loose condition.

    Where is best location of CG for maximum cornering ability.



    The following is from Race car suspension class post #811 on page 41



    When you are in a race car turning in at corner entry, you feel like you're being pushed toward the outside of the turn. Most of us refer to this as the centrifugal force. WRONG. There isn't any force pushing you outward. Centrifugal force is what physicists call a pseudo or a fictitious force, because it doesn't really exist. More specifically, in Newtonian mechanics, the term centrifugal force is used to refer to one of two distinct concepts: an inertial force (also called a "fictitious" force) observed in a non-inertial reference frame, and also the equal and opposite reaction to a centripetal force.
    So is the centrifugal force isn't real, why do you feel like there's something pushing you out the right-side window when you make a high speed left turn? The answers lie in Newton's laws of motion. An object going straight will keep going straight unless a force makes it change speed, direction or both. When a driver is bombing down the straightaway and starts to turn, the centripetal force makes the car turn and, because he's buckled tightly into the car, he turns also. The force he feels is because his body is trying to keep going straight. The seat and shoulder straps, lap belt and sub straps tied to the car are all exerting a force on him toward the inside of the turn while he's trying to go straight. The net result is that the driver perceives a force to be acting outward, but it is actually acting inward.
    Got it?
    The force that makes a car turn is called the centripetal force. Centripetal literally means "toward the center". Imagine you had a rubber ball with a string attached to it. Whirl the ball over your head in a horizontal circle. What makes the ball go in a circle instead of flying away from you is the force the string exerts on the ball, which pulls the ball in a circle.

    A race car doesn't have a string attached to make it go in a circle but it does have TIRES. The tires contact the pavement exerting force toward the center of the turn. Engineers talk about lateral force. The lateral force is perpendicular to the direction the car is going at any moment.

    The size of the centripetal force is given by multiplying the mass of the car by the speed of the car squared, and then dividing by the radius of the turn.

    F= MV2 / R where centripetal force equals the mass of the car, v is the speed of the car and r is the turn radius.

    Without going into a lot of math , the faster you go, the more force you need to be able to turn. Tighter turns require more force. Just like Aerodynamics, the force isn't linearly dependent on the speed. If you double your speed, the force needed to turn goes up by a factor of four. If you triple your speed, the force increases by a factor of nine.

    pls re-read post # 691 on page 34 on weight jacking.
    When a race car goes into a turn three things can happen and two are bad.
    1. Tires don’t have enough down force and will slip.
    2. Tires have too much down force and will overheat the right front tire and eventually will slip.
    3. The car completes phase one turn entry and enters mid turn phase two.

    The key to this event is to keep maximum tire contact during the dive and roll. This is why the right front tire goes negative camber and the left front tire goes positive camber in the turn. We want both front tires to carry the same amount of load when turning. This is why we bias the car with left side weight. We purposely offset the weight up to 60% static when we place the car on the weight scales. We do this knowing that this need to be done to counter weight transfer during cornering.
    WRONG! No “weight” is transferred. The tires react like weight was transferred but what we are really dealing with is FORCE as described above.
    Back to the race car racing down the back straight at 90 MPH. When we go in to turn entry phase one we change both speed and direction via the tires. The car wants to continue going straight. The suspension and tires are the only tools we have to deal with this force. During the turning event the body will roll to the right side in the typical left turn. It rolls through the front and rear Roll Centers (RC). Some of the momentum Force is scrubbed off by the coil springs ,ARB (sway bar) and dampers (shock absorbers) compressing and converting the force to heat. Once the body has taken a set the tires are left to deal with the rest of the force. If we look at the post # 691 on page 34, we see the force vectors of straight sideways lateral force shearing the tires and the right front tire contact patch countering the body roll force. If we have the front roll center located too far to the right side we start to lift the left front tire in a jacking effect. If the front roll center is located too far to the left there is not enough leverage angle to counter body roll and the force shears the tire contact patch. If we have the front roll center located properly, we have the maximum down force possible to stick the right front tire and provide maximum tire adhesion to counter the force and we beat the other race cars out there.



    So now we know never to say weight transfer regarding a race cars handling. Back to your question - How to limit over steer and specifically a snap loose condition. Answer is to spread the cornering forces over the four tires as evenly as possible to maximize each tires cornering capability. To do this we have to understand what one tire does when cornering. To save time please post if you are familiar with Coefficient of Friction , tire slip angle, slip ratio, cornering force?
     
  6. Jun 14, 2015 #5

    Ranger Mike

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    The typical Go Kart. Solid front end with no moving suspension parts and one piece rear solid axel, West Bend 2 cycle engine and twin tilotson carbs.
    The Law of Momentum – Product of Mass and velocity. Mass traveling in one direction will continue in that direction at the same speed until acted upon by other forces.
    The law of conservation of energy - total energy of an isolated system remains constant. Energy can be neither created nor be destroyed, but it transforms from one form to another.
    In the case of the Go Kart we have a means of accelerating a Mass to a given velocity. It will remain traveling in that direction until we turn the steering wheel or the wheels bump into something or we have side ways wind or the like. It will continue at the same velocity until gravity and aerodynamics scrub off speed or we apply the brakes ( hit the whoa pedal in pit talk). When we apply the brakes we are converting forward momentum to heat as a reaction to tires contacting the pavement and brake system components causing controlled friction to slow the velocity.

    If we race around the traffic cones we placed at the Wal-Mart parking lot in a left hand turn we feel force trying to lift the inside (left) tires and plant the outside tires. If we can run fast enough and the tires are sticky enough to keep contact, eventually we would lift both left side tires. In pit talk this called 100 percent weight transfer of UNSPRUNG Weight. A good analogy and easy to understand but not necessarily true.

    Momentum – Product of Mass and velocity. Mass traveling in one direction will continue in that direction at the same speed until acted upon by other forces.

    In this case we have a go kart traveling straight until we enter a turn. When we turn left ,the vehicle wants to continue to travel straight but the tires are in contact with the track and push back. Now we have lateral force ( sideways force).
    This force depends on the radius and speed your running, and your track width and center of gravity.
    The tire sidewall acts like a spring to a small degree to soften the load being dumped on the tire. Lateral Force is transferred to the pavement until such time as the tire contact patch can not longer maintain adhesion. When this limit is exceeded we have the tire sliding on the pavement. Again the force of momentum transfers to heat in the tire sidewalls and tire contact patches. If lateral force reaches the point of inability to over come centripetal force of the tires ,we have lifting of the Mass as a reaction to jacking forces of the sticky tires. Think Pole vaulter. At this point the driver usually opts to apply the brakes. In any event we have dealt with the conversion of momentum to another state.

    Things are not that simple in real life.
    In this case it’s the 200 pound fat kid from next door, driving the cart . Put down the Big Mac , Lumpy! For the purposes of this discussion when you think of Lumpy, think Center of Gravity (COG). Don’t forget the things that impact on your ability to corner are the radius of the track, your speed, your track width and center of gravity.

    Now lets modify the go kart frame to have a center pivot for the front axle ( the tube connecting both front spindles). If we modify the rear of the go kart frame to have a center pivot for the rear tube and mount the drive axle so it will work properly and if both pivot points are the same height, we have the essential of our modern race car chassis. One problem. The chassis will flop to one side or the other due to gravity. By placing coil springs on each side of the pivot points front and rear we can “suspend” this arrangement so the chassis is centered.

    If we repeat the cone course test we note that we can attain a bit more speed before both left side tires lift. The springs dampen the fat kid momentum to a slight degree by increasing the time it takes for the transferred weight to finally cause tire shear. The springs convert some of the lateral force to heat. The suspension effects the percent of load the front takes versus the rear.


    Lets really mess things up and put a heavier spring on the right side of the front of our go kart and try the cone killer course.
    We find that we now have an under steer condition as the right front tire wants to wash out or snow plow when we get going fast. This is because the right front spring is resisting the Momentum MORE THAN the right rear which has had more time ( as miniscule as it is ) to deal with the force transfer due to the springs coil compression. The right front ( compared to the right rear) is acting like a solid link and only the tires side walls are dampening the impact on the weight loading the tire contact patch.

    We could go on with more examples like moving the rear pivot point higher ( ROLL CENTER ).
    This is a whole new subject though.
     
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