Suspension mods and the real world
01-20-2007, 09:11 AM
#1
Suspension mods and the real world There has been an interesting trend in the suspension forums lately. It's the "I want it all" syndrome. We see posts looking for the best ride, best ride with improved handling, posts where someone’s 18" or 19" wheels don't ride as smooth as stock, posts where people want to lower cars and not hurt their drag times, posts where people want to use lowering springs with drag shocks, etc.
The chassis is one place that the laws of physics can not be bent. Everything involves a trade off and you have to decide if that trade off is right for you and your use. I'm going to try to explain some of these trade offs below.
Lowering springs: The springs job is to keep the car off of the ground. The spring has to be stiff enough to prevent bottoming out. Springs do provide some additional roll stiffness, but that's primarily the job of a sway bar. So, in order to keep a car from bottoming out when you lower it, you need to increase the spring rate. Meaning, you use stiffer springs. If you eliminate 50% of the suspension travel by lowering the car (from ride height to the bump stops), you are asking the spring to do the job in 1/2 the distance, and you'll need something approaching twice the spring rate (give or take a bit). This increased spring rate is very important to your choice of shocks to go with those springs. Lowering the car also puts you closer to the bump stops. Hitting the bump stops is normal; hitting them hard and often will cause a rough ride. This is often one of the major causes of the rough ride on a lowered car. Getting rid of the SS/WS6 bump stops and changing them out for the V6/Z28 bump stops can help. Removing them is not usually the right answer since the next thing you hit will be solid and can really make the ride rough (if you don't break something instead).
Springs and drag racing. A soft spring will be a long spring (as long as the weight of the car is the same). If I have a 100 in lb spring and that corner of the car weighs 750 lbs, it will compress that spring 7.5 inches. If I have a 1,000 in lb spring, it will compress that spring 3/4 of an inch (I'm avoiding the tilt of the spring/shock assembly and the impact it has on the actual travel to avoid making this overly complicated). In order to help raise the nose when you launch, the spring needs to be a lower rate/longer spring. The 100 in lb spring in our example will have 7.5" where it can help push the nose up. As the nose comes up, the spring will be extending and helping the nose rise. The 1,000 in lb spring can help for 3/4" before it's just more dead weight on the front of the car. This is why drag springs are typically around 275 in lbs and not 600+.
For performance handling, this is the opposite of what you want. You want the car to be planted and stable. You don't want the nose to rise during acceleration, at least not very much. You don't want the tail to rise during braking (soft springs in the rear can push the rear up under braking, just like drag springs in the front), etc. The applications of these springs are completely different for drag racing and road racing. Though many guys report running acceptable numbers, performance/handling based lowering springs will always slow your drag times down slightly over a setup using softer springs.
Shocks: You need shocks that can damp your spring rates and the unsprung weight on each corner. Those big 19" wheels and tires are unsprung weight and the shock has to control that too. The stiffer the spring, the more control you will ask your shocks to provide. The shock is a damper and its job it to reduce chassis motion and oscillations. You don't want to be bouncing down the road like some of the old beaters we've all seen, the ones that hit a bump and bounce down the road for another 1/2 mile. That's one of the jobs of the shock absorber. With drag shocks, you can have a car that acts like that and is constantly crashing into the bump stops due to the greatly reduced damping of the drag shocks. This can lead to an unpleasant ride. However, the opposite can be true. If we get way too much compression and/or rebound, we can make the ride harsh by effectively limiting or eliminating the suspension motion. This is one reason that quality adjustable shocks are nice, you can get what you need/want and nothing more (or less). Do not confuse shocks that are "stiff" with shocks that are properly damped. There can be a big difference between the two and often will be. Just because a shock can be turned up until it is very stiff does not mean that it is actually controlling the suspension properly. Remember that reduction in suspension motion from too much compression or rebound that was mentioned a few lines ago?
Shocks and drag racing. In a typical drag application, we want the nose to rise quickly to put weight on the back tires. This is helped by using soft/low spring rate springs (as discussed above). This is also helped by using shocks with very little rebound damping. Rebound is the valving you use when you are extending a shock, such as letting the springs extend to push the nose up. Too much rebound damping will limit how quickly those springs can move. If they can't overpower the shocks and extend the wheels quickly, the benefit of those softer/longer drag springs will be lost. So, drag shocks typically offer very little rebound resistance. This can also cause them to allow the car to bounce up in the air over bumps. Drag shocks can act similar to worn out shocks in many cases. Some of the better quality ones have velocity specific valving to attempt to sense the difference between a pothole and a drag launch, others don't.
For performance handling, this is the opposite of what you want. You want rebound damping primarily to control the roll rate of the car in a turn and it also will impact how quickly the nose and tail rise under acceleration and braking. We want a "lot" of rebound damping to prevent the car from feeling like it's just flopping around when we turn into a corner. We also need enough compression damping to control the weight of the wheels, tires and suspension parts (unsprung weight). Springs keep the car from bottoming out, compression damping controls the weight of the suspension parts and wheels when they are traveling upwards, such as after hitting a bump with one wheel that tries to bounce that wheel and tire up off of the ground. Though many guys report running acceptable numbers, performance/handling based shocks will always slow your drag times down slightly over a setup using softer shocks.
Sway bars: The job of the sway bar is to limit body roll. The rebound resistance of the shock will control how fast a car can roll in a turn, the sway bars control how far a car can roll in a corner (they get some help from the springs with this, but it is primarily limited by the sway bars). They also control how the weight is distributed when a car is turning a corner. A stiffer bar will transfer more weight on that end of the chassis to the tire contact patches. If you transfer too much, you can cause that end of the vehicle to lose traction before the opposite end.
Sway bars and drag racing. For drag use, you want to remove as much weight as you can from the nose, you also want to free up the wheels and tires to extend as easily as possible (to take advantage of the low rate springs and the drag shocks reduced rebound damping to lift the nose). So, most guys take the sway bar off of the front of the car for serious drag use. This eliminates several pounds of weight, and the extra resistance of having to overcome the grip of the bushings that hold the sway bar to the frame. On the rear of the car, you will often find a 25mm "drag bar" or even larger than that. The rear bar helps keep the axle square and level under the chassis during the launch. It reduces the tendency of the right rear of the chassis to be pulled down over the wheel and can help the car to launch straight. An airbag can be employed in the right rear spring to help level the car during launches as well.
For handling, we want the sway bars to keep the car level in turns and we adjust their sizes to balance the chassis so that the car is neutral (one end doesn't slide before the other). When the front of the chassis rolls in a turn, the outside front tire loses camber (it rolls onto the outside edge of the tread). When a tire is not sitting flat, or close to it, on the ground, you are losing grip. By installing a 35mm front bar, we are limiting the amount of body roll in the front of the car and helping to keep that tire closer to flat on the ground. This helps keep the tire working effectively in corners and can lead to a great increase in front grip. However, the added weight and "stiction" of the poly bushings used on such a bar will limit the speed that the wheels can extend (like you'd want for drag use). For handling, this is not much of an issue and the positives far outweigh the negatives. In the rear, we want enough bar to balance the front. This is very often a 19mm solid, a 21mm solid or a 22mm hollow bar on a 4th gen, 3rd gens will often be similar. The 25mm rear bars are just too big for most handling applications and can cause the back of the car to slide long before the front, this will limit total grip. If you always have to drive below the limits of one end of the car, the package isn't balanced.
Note: Sway bars actually have little impact on ride quality in most situations.
The chassis is one place that the laws of physics can not be bent. Everything involves a trade off and you have to decide if that trade off is right for you and your use. I'm going to try to explain some of these trade offs below.
Lowering springs: The springs job is to keep the car off of the ground. The spring has to be stiff enough to prevent bottoming out. Springs do provide some additional roll stiffness, but that's primarily the job of a sway bar. So, in order to keep a car from bottoming out when you lower it, you need to increase the spring rate. Meaning, you use stiffer springs. If you eliminate 50% of the suspension travel by lowering the car (from ride height to the bump stops), you are asking the spring to do the job in 1/2 the distance, and you'll need something approaching twice the spring rate (give or take a bit). This increased spring rate is very important to your choice of shocks to go with those springs. Lowering the car also puts you closer to the bump stops. Hitting the bump stops is normal; hitting them hard and often will cause a rough ride. This is often one of the major causes of the rough ride on a lowered car. Getting rid of the SS/WS6 bump stops and changing them out for the V6/Z28 bump stops can help. Removing them is not usually the right answer since the next thing you hit will be solid and can really make the ride rough (if you don't break something instead).
Springs and drag racing. A soft spring will be a long spring (as long as the weight of the car is the same). If I have a 100 in lb spring and that corner of the car weighs 750 lbs, it will compress that spring 7.5 inches. If I have a 1,000 in lb spring, it will compress that spring 3/4 of an inch (I'm avoiding the tilt of the spring/shock assembly and the impact it has on the actual travel to avoid making this overly complicated). In order to help raise the nose when you launch, the spring needs to be a lower rate/longer spring. The 100 in lb spring in our example will have 7.5" where it can help push the nose up. As the nose comes up, the spring will be extending and helping the nose rise. The 1,000 in lb spring can help for 3/4" before it's just more dead weight on the front of the car. This is why drag springs are typically around 275 in lbs and not 600+.
For performance handling, this is the opposite of what you want. You want the car to be planted and stable. You don't want the nose to rise during acceleration, at least not very much. You don't want the tail to rise during braking (soft springs in the rear can push the rear up under braking, just like drag springs in the front), etc. The applications of these springs are completely different for drag racing and road racing. Though many guys report running acceptable numbers, performance/handling based lowering springs will always slow your drag times down slightly over a setup using softer springs.
Shocks: You need shocks that can damp your spring rates and the unsprung weight on each corner. Those big 19" wheels and tires are unsprung weight and the shock has to control that too. The stiffer the spring, the more control you will ask your shocks to provide. The shock is a damper and its job it to reduce chassis motion and oscillations. You don't want to be bouncing down the road like some of the old beaters we've all seen, the ones that hit a bump and bounce down the road for another 1/2 mile. That's one of the jobs of the shock absorber. With drag shocks, you can have a car that acts like that and is constantly crashing into the bump stops due to the greatly reduced damping of the drag shocks. This can lead to an unpleasant ride. However, the opposite can be true. If we get way too much compression and/or rebound, we can make the ride harsh by effectively limiting or eliminating the suspension motion. This is one reason that quality adjustable shocks are nice, you can get what you need/want and nothing more (or less). Do not confuse shocks that are "stiff" with shocks that are properly damped. There can be a big difference between the two and often will be. Just because a shock can be turned up until it is very stiff does not mean that it is actually controlling the suspension properly. Remember that reduction in suspension motion from too much compression or rebound that was mentioned a few lines ago?
Shocks and drag racing. In a typical drag application, we want the nose to rise quickly to put weight on the back tires. This is helped by using soft/low spring rate springs (as discussed above). This is also helped by using shocks with very little rebound damping. Rebound is the valving you use when you are extending a shock, such as letting the springs extend to push the nose up. Too much rebound damping will limit how quickly those springs can move. If they can't overpower the shocks and extend the wheels quickly, the benefit of those softer/longer drag springs will be lost. So, drag shocks typically offer very little rebound resistance. This can also cause them to allow the car to bounce up in the air over bumps. Drag shocks can act similar to worn out shocks in many cases. Some of the better quality ones have velocity specific valving to attempt to sense the difference between a pothole and a drag launch, others don't.
For performance handling, this is the opposite of what you want. You want rebound damping primarily to control the roll rate of the car in a turn and it also will impact how quickly the nose and tail rise under acceleration and braking. We want a "lot" of rebound damping to prevent the car from feeling like it's just flopping around when we turn into a corner. We also need enough compression damping to control the weight of the wheels, tires and suspension parts (unsprung weight). Springs keep the car from bottoming out, compression damping controls the weight of the suspension parts and wheels when they are traveling upwards, such as after hitting a bump with one wheel that tries to bounce that wheel and tire up off of the ground. Though many guys report running acceptable numbers, performance/handling based shocks will always slow your drag times down slightly over a setup using softer shocks.
Sway bars: The job of the sway bar is to limit body roll. The rebound resistance of the shock will control how fast a car can roll in a turn, the sway bars control how far a car can roll in a corner (they get some help from the springs with this, but it is primarily limited by the sway bars). They also control how the weight is distributed when a car is turning a corner. A stiffer bar will transfer more weight on that end of the chassis to the tire contact patches. If you transfer too much, you can cause that end of the vehicle to lose traction before the opposite end.
Sway bars and drag racing. For drag use, you want to remove as much weight as you can from the nose, you also want to free up the wheels and tires to extend as easily as possible (to take advantage of the low rate springs and the drag shocks reduced rebound damping to lift the nose). So, most guys take the sway bar off of the front of the car for serious drag use. This eliminates several pounds of weight, and the extra resistance of having to overcome the grip of the bushings that hold the sway bar to the frame. On the rear of the car, you will often find a 25mm "drag bar" or even larger than that. The rear bar helps keep the axle square and level under the chassis during the launch. It reduces the tendency of the right rear of the chassis to be pulled down over the wheel and can help the car to launch straight. An airbag can be employed in the right rear spring to help level the car during launches as well.
For handling, we want the sway bars to keep the car level in turns and we adjust their sizes to balance the chassis so that the car is neutral (one end doesn't slide before the other). When the front of the chassis rolls in a turn, the outside front tire loses camber (it rolls onto the outside edge of the tread). When a tire is not sitting flat, or close to it, on the ground, you are losing grip. By installing a 35mm front bar, we are limiting the amount of body roll in the front of the car and helping to keep that tire closer to flat on the ground. This helps keep the tire working effectively in corners and can lead to a great increase in front grip. However, the added weight and "stiction" of the poly bushings used on such a bar will limit the speed that the wheels can extend (like you'd want for drag use). For handling, this is not much of an issue and the positives far outweigh the negatives. In the rear, we want enough bar to balance the front. This is very often a 19mm solid, a 21mm solid or a 22mm hollow bar on a 4th gen, 3rd gens will often be similar. The 25mm rear bars are just too big for most handling applications and can cause the back of the car to slide long before the front, this will limit total grip. If you always have to drive below the limits of one end of the car, the package isn't balanced.
Note: Sway bars actually have little impact on ride quality in most situations.
Last edited by DONAIMIAN; 01-21-2007 at 09:45 PM.
01-20-2007, 09:12 AM
#2
Wheels and tires: Large wheels and tires are heavy and low profile tires are stiff. Adding weight to the car in the form of 18" or 19" wheels and tires will increase what we are asking of the shocks compression valving. In an effort to keep those heavy wheels and tires from being launched into the bump stops every time we hit a pebble in the road, we need to increase the compression damping of the shocks. If the shocks can't control the weight of the wheels and tires, we can wind up with a harsh ride since the bump stops are being used to limit the upward travel of the suspension over bumps. Also, 20 and 30 series tires have very little sidewall and tend to be very stiff. Where a 40 or 50 series tire might flex over small bumps, the lower profile tires are more likely to transmit that energy into the chassis. This causes noise, harshness and a rough ride. It's the nature of the tire, some tires might be a little softer than others, but they will all be pretty similar in most cases. If you want it to ride like a luxury car, stick to 17" or smaller wheels.
The suspension is a package. You will get the best results when you match things up. If I'm serious enough about drag racing to buy drag shocks, I really should put drag springs on there too and at least an air bag in the right rear. The car may not handle well on the street, but it will go very well in a straight line as it was intended to do with this package.
By mixing and matching parts, you can wind up with a car that does nothing well. If I put road race springs on drag shocks, I lose the benefit of the drag springs to lift the nose, and I don't have shocks to properly damp the car for handling use. So, it doesn't go as quickly as it could down the strip, and it won't get around a road race course either. And, I'll probably hate the ride.
The suspension is a package. You will get the best results when you match things up. If I'm serious enough about drag racing to buy drag shocks, I really should put drag springs on there too and at least an air bag in the right rear. The car may not handle well on the street, but it will go very well in a straight line as it was intended to do with this package.
By mixing and matching parts, you can wind up with a car that does nothing well. If I put road race springs on drag shocks, I lose the benefit of the drag springs to lift the nose, and I don't have shocks to properly damp the car for handling use. So, it doesn't go as quickly as it could down the strip, and it won't get around a road race course either. And, I'll probably hate the ride.
