Watts link overview paragraph:
Look closely at the axle and spring suspension set up on your leaf spring car and you'll see that it is designed similar to a back yard swing. With the leaf springs anchored at the front and back only, the axle housing is allowed to swing side to side.
Imagine what happens to the suspension geometry when you go hard into a corner or get into a threshold braking situation approaching one G with a 2800 to 3600 lb. car. The shocks, springs and tires are giving you an oversteer/understeer condition as you go through the corner. Even the best spring and shock upgrades will not be working at their maximum
The F2S Watts Link lowers your Mustang's roll center and helps keep your car's handling response symmetrical in both left and right cornering. Because the F2S Watts Link keeps the axle centered under the car, the rear of the car now "follows" the front suspension rather than fighting it.
The watts link will keep the axle centered under the car, lower your roll center and allow you to take better advantage of further suspension upgrades. Often tuners will find that a watts link will allow you to upgrade with springs and shocks that control without overloading the suspension. Don't make shocks and springs do work they werenÕt designed for.
We chose to mount the F2S Watts Link "propeller" to the chassis so the car's roll center and center of gravity do not change in relation to each other. This relationship is known as "roll couple" and has a great effect on your Mustang's handling under cornering, braking and acceleration.
Rear Leaf Spring dynamics at the subsequent effects on car handling
My comments are meant to be from the practical and simple perspective. There are volumes written to explain suspension geometry: bump steer, ackerman, scrub radius, tow-in/tow-out, camber, caster, roll center and roll couple. Most of us just want a straight forward explanation as to why we get out of control under braking, cornering and acceleration. The problems the average reader experiences are minimized compared to a Pro-Stock or GT-1/TransAm/ALMS car, so we'll assume most of us drive the more pedestrian versions. But understanding what's happening with rear axle is the best way to fix the problem.
1. What does a rear suspension do on a rear-wheel drive car? What happens under acceleration, braking, cornering, and combinations thereof?
1) In a rear drive suspension car, the live axle effect on handling can vary from 100% to as little as 10% or less and here's why. Under hard acceleration a properly set up drag car will apply maximum weight transfer to the rear axle and in many cases lift the front wheels. In this extreme case, the rear suspension is controlling acceleration, steering and braking. A lot of work to do in a 2800 to 3800 lb Mustang. Depending on the sophistication of the suspension (stock leafs or coils, add-on traction bars, pinion snubbers, torque links, high density bushings or a complex 3-link or 4-link) the effect can be a good launch or up in smoke. As the drive shaft rotates and applies torque and horse power to the differential, multiple forces are in effect as the car begins to move forward. Immediately the tires fight for traction and with a good differential, in theory, the effect should be equal side to side. Not so. The rotating driveshaft, ring and pinion and axles apply more force (torque) to one tire than the other. This rotation also causes weight transfer from one side to the other. Plus, under acceleration, the front of the differential is being forced up cause spring wrap, and not evenly from side to side. Want more? Most likely the axle is also moving side to side. So in addition to acceleration, the rear suspension is now attempting to steer the car. All this and you're barely through the 60 ft mark. Now repeat this scenario at each hard shift and the 1/4 mile can be a real handful. The more horse power the bigger the handful.
1320 ft later you're hard on the brakes. All of the above now acts in reverse with two important additions. Most of the carÕs weight has now transferred to the front suspension and the rear brakes are trying to lock up the tires and add more drama to the unloaded springs and shocks. Add in, reverse torque factors which are now trying to drive the pinion into the pavement and you need a handful of steering input just to keep the car in a straight line.
Now lets throw in a corner. Remember the tires can only work at 100% of their capacity and up to now the tires have only had to accelerate and brake. All of the above forces continue to apply as a car approaches a corner. When you set up the car to turn in, clip an apex and follow the proper line through the turn out, new gravitational forces come into play. Your transition from straight line acceleration, to straight line braking, then cornering (with or with out trail braking) to acceleration off the corner are putting heavy side loads on your suspension and tires. If you suddenly need 50% or more of your tires traction to corner it leaves less than 50%, probably more like 20% or 30% for braking or acceleration. The body is trying to slide to the outside of the corner while the axle and tires fight to stay on line. Most likely you've just introduced a condition commonly known has "dog tracking" to your road racer. We've all been behind a car going down the street with the rear tires misaligned with the fronts. That's "dog tracking". And with out the proper suspension aids your car will "dog track" every time you go through a corner. Yet another way the rear axle tries to steer the car.Accelerate out of the corner and all of the forces described above regarding acceleration take over again.
The effect of the rear axle in "steering" the car cannot be overlooked if you are going to build a great handling road, track or race car. When you realize that the rear axle transmits100% of the power to the pavement, it is extremely important that the forces I described are minimized. Fortunately there are plenty of suspension add-ons available. Heavy duty springs, adjustable shocks, sway bars, traction bars, torque links, panhard bars and watts links when selected properly will change a loose and unpredictable car into a car that accelerates straight, corners predictably and stops with confidence.
2. How have Mustang rear suspensions changed over the years? The early cars are very crude, how did that evolve through the Fox, SN95 and the current generation cars?
2) The Fox bodies went to coil springs for packaging reasons and to make the ride more compliant. Quickly Ford had to add the Quad-Shock set up to control wheel hop when the HO Mustangs started making over 200 hp again. I believe the suspension evolution in the SN95 models where just that, evolution. Since the later Mustangs are very big in the drag racing scene, all the improvements where made to reduce wheel hop and improve axle control under acceleration. Built in control arms control the axle wide up under both acceleration and braking and the aftermarket is flooded with upgrades. IÕve driven stock SN95s and they handle and stop much better than a stock early Mustang but remember that the Mustang platform was designed in the 1950Õs when the Falcon was concepted. A great deal of what makes the later Mustangs stock suspension setup so much better is the built-in chassis stiffness and the front suspension design. The whole geometry of where the the shocks and springs are mounted on the control arms, steering box location and built in camber and caster angles deserve itÕs own article. Everyone knows the late models work better but the reasons are not always clear with out diagrams and suspension theories to explain why it works.
3. What are the drawbacks and benefits of a live axle? Why don't Mustangs have IRS?
3) The question of why Ford doesnÕt fit all V8 Mustangs with a IRS has been around since rumors of an IRS for the 1965 chassis. In my opinion the answer is quite simple: Cost. The Corvette has had an IRS since the 60s and many high end sports cars have had them forever so there must be an advantage. There is. First off, most of the inherent problems with a live axle car are eliminated with an IRS. Because the differential is mounted to the chassis, side to side movement of the axle and many of the related problems are eliminated. The differential transmits the power to the wheels through half shafts to the axle spindles. Because the suspension now has to control only the wheel hub, axle bearing components, wheel and the tire the unsprung weight is greatly reduced. Unsprung weight consists of all the drive line components the springs and shocks have to control. The goal in road racing is to reduce unsprung weight. The less unsprung weight the more control the springs and shocks have in keeping the tires firmly glued to the pavement. Light weight wheels are important in road racing for this very reason.
With an IRS, the axles swing on an arch with the differential being the pivot point. As the car goes through a corner and the body rolls, the tires are able to maintain a flatter contact with the pavement. In a live axle chassis, the tires remain parallel to each other. What you want ideally is for each tire to find it's own best contact angle. The IRS has a big advantage in this area. One more advantage the an IRS has over a live axle is a softer and more compliant ride, although this issue is more important in a luxury ride than a corner carver.
But IRS suspensions are not without their own unique problems. They are by their very design more complicated and therefore more expensive. Most IRS suspension will have continuing Camber, caster and toe-in changes as the suspension moves through its travel range. The axle shaft in a live axle set up is replaced by a half shaft with two u-joints; maintenance and breakage potential. Because the wheel is pivoting on the end of a half shaft additional parts are need to keep the wheels aligned properly. The rear suspension can be out of alignment similar to the front and this alignment can be a source of handling nightmares when it goes out.You could have a situation where the rear is steering left and the front is steering right. Most of us can dial in a live axle rear suspension much much easier than an IRS and this alone is a vote for the live axle. Keep in mind that the incredibly fast GT1 and Trans Am cars run live axle suspensions.
In a drag racing environment the live axle is king. From Top Fuel to pro Stock to bracket racing live axles are used because they are stronger (less parts to break) and they are tunable for weight transfer. Since many of the Mustang owners are into drag racing the IRS would offer no advantage. This is not to say that the IRS won't work on the drag strip but it certainly is at a disadvantage.
Because of manufacturing costs, over 50% of the Mustangs sold are 6 cylinder automatics, the IRS is just not justifiable on a production line car. The Cobra Mustangs sold with IRS have certainly shown well for themselves but the owners paid a dollar premium and on any track day you will see live axle cars running as fast or faster than the IRS Mustangs. With properly selected suspension components: springs, shocks, bushings, panhard bar or watts link, the live axle car will corner with complete confidence and control.
4. What influence do tires and wheels have on rear suspension tuning? What about tire pressures?
4) Tires and wheels are the number one area for unsprung weight reduction. The lighter the wheel and tire combination the more control the springs and shocks can excerpt in keeping the tire planted firmly on the track. Wider wheels will help spread the tires contact patch and must be matched to the tire manufacturers recommendation. Too narrow a wheel width can cause the tire to roll over in a corner. Two wide a wheel can prevent the tires full contact across the surface. Both situations on the rear axle can contribute to oversteer.
Wider wheels are often used on the rear because the rear tires are a bigger size than the front. The wider and taller tires provided a better bite on acceleration and more control on a cornering situation. With most Mustangs having more weight on the front axle than the rear axle it would seem that the bigger tire should go up front. But because the front suspension design and adjustments allow the tires to do a better job in cornering forces than the rears the smaller tires can work well in controlling the braking and cornering forces.
The rear tires have a much different job to do than the fronts and as such their height, tread width and cross section widths need to be greater. Braking, cornering and acceleration all put unique demands on the rear suspension. Under hard braking weight transfer takes weight off the rear axle. Because rear braking is extremely important in car control the wider rear tires put more rubber on the pavement and help counter act weight transfer braking effects, and the car stops with more control. Going through the corner, momentum affects the rear like a pendulum and wants to swing the 45% to 50% of a front drive cars rear weight around. The more rubber on the back axle, the less pendulum effect. Acceleration off the corner requires good bite and here again the wider, bigger tire is necessary to lower lap times.
Tires are the what keep you on the track. After all the suspension tuning has been applied and the driving schools paid for it still comes down to four tires transmitting all the information from the pavement to the driver. When you watch a NASCAR race and hear the the crew chief is taking one pound of air out of a corner it's not because the portable air tank is running low. Tire tuning is probably the most overlooked area of suspension tuning in a trackday or road race car. Probably because it requires back to back testing and detailed notes. Its easier to put 30 lbs in the front and 26 in the back and just go out and drive hard and have fun. In an autocross event, tire pressure effects can be easier to monitor and can make a big difference. Taking a pound or three out of a rear tire can help make the rear suspension softer, induce some oversteer and help transition the car through a tight apex. Every car and every driving style will react differently but do not over look how variations in the tire pressure will affect your cars handling.
Two measurement are very important when tuning tires for handling. Accurately recording the pressure before you go out on the track and when you come in for a pit stop or finish your session can tell you a lot about your chassis set up. Ideally you would like all four tires to gain the same amount of pressure. If the fronts are gaining considerably more pressure the the rears, you are probably experiencing a great deal of push or understeer. Conversely if the rears are heating up much more than the fronts you are probably fighting a loose or oversteering car.
Contrary to what you make think, a tire pressure gauge is not the only tool for monitoring tire performance. A tire pyrometer can give you a tremendous amount of information. Taking the temperature across the surface will tell you if the pressure is high or low. A higher temperature in the center of the tread than the outsides indicates too much pressure and vice versa. Higher temperature on the outside of a rear tire indicates an oversteer problem. Higher temperature on the outside of a front tire indicates an understeer problem. Here is where a stable rear suspension can help. A watts link will keep the axle centered under the car, lower the cars roll center and prevent excessive weight transfer in a corner. Never underestimate what tire tuning and monitoring can do to improve handling.
5. What influence do shock and spring rates have on tuning handling?
6. How does the front suspension interact with the rear? For example, how does a softly sprung front affect the rear?
7. What are the differences in tuning of the rear suspension for a road racer versus a drag racer?
5, 6 & 7) Shocks and spring can be the most complicated suspension pieces on a race suspension. Check out a professional race car hauler and youÕll find a big selection of springs and shocks. Every professional race team now has a shock dyno to properly tune for compression and rebound. If you can afford it, purchase single or double adjustable shocks and ask around to see what your competition is having good results with.
A giant myth is that stiffer is always better. Both springs and shocks need to CONTROL the car not "shock" the handling. Most owners of street cars will decide to improve their stock suspensions with stiffer springs and shocks. Race cars have firm suspensions so the firmer the better, right? Not always. Remember that handling is about control. The idea is to have the suspension controlling car movement while the tires stay in contact with the pavement. I like to look to other types of racing for inspiration and ideas. Off-roaders race over the most demanding terrain in racing. Even though the road race track is smooth in comparison, it still has bumps, elevation changes, stones, debris and expansion cracks we have to deal with. Current off-road technology features soft long travel suspensions with multiple shocks to control without "shocking" the vehicle. They've learned it's all about controlling the race car and letting the tires do their job.
Start your suspension modifications by addressing the primary problem. In most live axle, rear drive suspensions, the axles lateral movement is not properly controlled. If you have a factory panhard bar, replace the bushings with firmer aftermarket pieces. Better yet, replace the panhard bar with a watts link. If you do not have a panhard bar, start with a watts link. The watts link will keep the axle centered under the car, lower your roll center and allow you to take better advantage of further suspension upgrades. Often tuners will find that a watts link will allow you to upgrade with springs and shocks that control without overloading the suspension. Don't make shocks and springs do work they weren't designed for.
Shock tuning and spring rates can dial in understeer, oversteer or neutral handling. Softening the rear rates can often induce more oversteer for better turn in but too much and youÕll also have oversteer coming out of the corner. Every car will react differently because of weight distribution, tire compound and driving style. But remember that tuning does not always mean stiffening.
Shock valving can have a dramatic effect on car attitude and handling. Soft compression shocks on the front of a car and soft rebound on the rear will allow more and quicker weight transfer under braking. When you take weight off the rear axle it can lead to over steer and increased rotation in a corner. Good if you need more help in getting the car turned, bad if you all ready have oversteer. The same shock set up can help by keeping the front of the car tight to the track. This can help with aerodynamics and top speed on a long fast track. Spring and shock rates will also affect chassis roll which can help or hinder cornering. Soft compression shocks will allow quicker weight transfer from side to side and a firmer compression will help slow weight transfer. Ultimately the springs will limit the amount of body lean but the shocks will dampen the rate at which it occurs and help tune the chassis.
Drag racers have different but equally challenging objectives. Study a current day pro-stock suspension set up and you'll see it's designed to do an awful lot in just 1320 ft.. Drag racers deal with acceleration, straight line control and to a much less extent stopping. The rear suspension in a drag car has to control huge amounts of torque and horsepower applied instantly to the axle and tires. Traction bar devices will control axle rotation and properly valved shocks will allow the car to apply as much weight as possible to the tires. The tires in a drag car will also suffer from the old "wheel hop" dilemma. The shocks need to keep the tires planted on the stirp, allow the chassis to settle and the help keep the car straight at well over 100 mph. Front springs and shocks will allow the weight to transfer and then let the cars front end quickly settle down on the track for maximum aerodynamics and top speed. Shock valving is as important here as it is on a NASCAR track or ALMS course only in a different way. It is obvious the drag race suspension technology has come a long way since the old "gasser stance". Stiff rear leaf springs and I-beam front ends limit top speed and braking but only look great if your at a vintage drag race.
Rear suspension in a road race or track day car has different objectives than a drag car but some theories still apply. The acceleration loads are not as great or at least they shouldnÕt be. If you launch your road racer so hard that you suffer a great deal of wheel hop you probably need a driving school. Car control in the road racer is necessary under acceleration, braking and cornering and often two out of the three at the same time. I've discussed how the live rear axle forces affect the road racer and drag racer earlier. The most important point to keep in mind is to make changes to your suspension one at a time, keep notes on the results and always go for less than more.