| SPRING BASICS | ||
| When we think of a car's suspension, the first thing we think of is springs. This is probably the best time to reiterate the division of vehicle weight we're dealing with: | ||
| 1- UNSPRUNG WEIGHT: The portion of the total weight of the vehicle not supported by the suspension springs. It is comprised of the wheels, tires, hubs, hub carriers, and brakes, plus approximately 50% of the weight of the suspension links, drive shafts, springs, and shocks. | ||
| 2- SPRUNG WEIGHT: Is that portion of the total vehicle weight which is supported by the suspension springs. this includes the chassis, engine, driver, fuel, gearbox, etc. - in other words - most of it. | ||
| Carroll Smith - 'Tune to Win' - pg.29 | ||
| Basically, the biggest pain in the ass with tuning a car's handling is the dynamic we have between tire load and body roll. When we set up our cars, we need to decide how much roll resistance we're going to assign to our springs, and how much we're going to assign to our ARBs. | ||
| The dilemma isn't as complicated to understand as it is to fix. It goes like this: | ||
| Lateral load transfer is BAD. Body roll is BAD. |
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| Lateral load transfer is bad because as we see in the sections on TIRES as well as DYNAMICS, a pair of tires will ALWAYS have the most grip when the weight on them is evenly distributed. The more you load the outside tire while cornering, the more grip you get on that tire, but the less overall grip your WHEEL PAIR has. | ||
| So logic would dictate, why not set up our car to transfer the least amount of lateral load possible? Why not run our car with little or no ARBs? This way we'd keep our front and rear wheel pairs more evenly laden, and maximize our cornering grip. | ||
| Why? | ||
| Because ARBs are called 'Anti Roll Bars' for a reason. Without them, we'd have excessive body roll, which is BAD, right? | ||
| There are a lot of misconceptions about the negative consequences of body roll. One school of thought is the more the body rolls, the more weight (or load) gets over the laden wheels, which adds cornering grip. True, in theory, but not really. There's some significant negative trade-offs that go along with it that we don't want: | ||
| 1- Body roll increases dynamic camber angles which gets the wheels up on their outside edges, shrinks the tire's contact patch, and lessens our grip. |
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| 2- The car is sloppier and less responsive in lateral transitions. | ||
| So the next thing we look at to solve our dilemma is our spring rates. Spring rates also affect body roll, so why not run a really stiff spring rate to reduce body roll to acceptable levels, and throw the ARBs in the trash? We reduce roll with our springs, instead, and without ARBs, we'll keep our wheel pairs more evenly laden when we turn the car. | ||
| Not so simple. The problem is best summed up by the experts, and I quote: | ||
| "We have seen that chassis roll is restricted by a combination of the compression of the suspension springs due to load transfer and the resistance of the anti-roll bar. We need an anti-roll bar because, if the suspension springs are stiff enough to limit roll to our desired maximum, the (spring) rate in ride inevitably would be too high for tire compliance." | ||
| Carroll Smith - 'Tune To Win' - pg.66 | ||
| Our cars are complicated machines, and there's a lot going on. If we look at our cars' springs and what they do, they perform more than one task: | ||
| 1-Reduce body roll, but to a lesser extent and with greater inefficiency than the anti-roll bars. |
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| 2- We can adjust spring rates to balance oversteer/understeer characteristics of our cars. | ||
| 3- We can adjust our spring rates (along with our shocks) to increase or decrease vehicle responsiveness and driver feel. | ||
| But, of all the things we tune our springs to do, simply put. | ||
| OUR SPRINGS' PRIMARY FUNCTION IS TO MAXIMZE TIRE COMPLIANCE WITH THE ROAD, THROUGH ADEQUATE SUSPENSION TRAVEL. | ||
| Setting spring rates can get confusing because of all the above. They perform many functions. But we need to keep in mind that the first three functions are secondary to the need to set our rates for maximum tire compliance. | ||
| A spring rate that is too soft feels mushy, sloppy, and unresponsive. And let's not forget our dynamics. Springs don't just control responsiveness in the lateral sense like our anti-roll bars do, they also regulate vehicle pitch characteristics under the brakes (dive) and under acceleration (squat). Springs that are too soft will: | ||
| 1- Cause the nose of the car to scrape the road under heavy braking. |
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| 2- Increase dynamic, negative camber, which will reduce tire compliance under both brakes and acceleration. | ||
| 3- Cause the car to bottom out in bump travel and unload the tires on rough roads. | ||
| 4- Cause the suspension to droop excessively in rebound, reducing tire compliance over undulating road surfaces. | ||
| Stiffening the suspension springs will fix the above conditions, as a stiffer spring rate helps reduce body roll, and gives the driver increased feel and responsiveness to steering inputs. The car will take its set faster when going from a given steering angle back to neutral steer. The car will transfer weight faster in the corners, as well as from front to rear under power, and from back to front under the brakes. | ||
| However, too stiff a spring rate leads to the following: | ||
| 1- A car that is twitchy and more unforgiving for a given steering input. | ||
| 2- Harsh choppy ride that doesn't have adequate suspension travel to maintain tire compliance. | ||
| 3- Car feels unstable and slides a lot. | ||
| 4- Difficult to put the power down on corner exit. | ||
| 5- Difficult to get car to turn in on corner entry. | ||