Disclaimer... People like to write fancy disclaimers these days, as if they have a great body of work that could be taken seriously. Imagine a young guy standing in court, explaining how he modified his car according to Jonnys webpage, which then caused him to crash and die. The judge would fall off his chair laughing. So yeah, sue me. I take full responsibility! for the speling on these pages.
Cars are often made with soft comfortable suspension and designed to understeer a bit around corners with a bit too much body roll. Combine that with saging springs, dead shock absorbers, old rubber bushes and tall skinny tyres and you have a car not pleasant or safe to drive. Many car enthusiests want to change their car into a sports car, but lowering the car alone (often by cutting the springs) can make the handling worse. Particualy in the front, there are many suspension and steering angles that need to be known about when changing anything as they are all inter-related and effect each other.
In lowering a car with different (or cut) springs, you will often also be changeing the roll centre, camber, camber change with travel etc... In changing the camber or caster, you could also be changing the bump steer and Achermann angle (in a very minor way, I'm just being legalistic/silly).
K = (W^4 x 12000000) / (8ND^3)
K = spring stiffness in lb/"
W = wire diameter in inches
N = number of active coils (not including the end coils) plus 1/2 coil
D = coil diameter in inches
With the formula, you can see how much stiffer your springs will become if you chop a coil off for lowering. A smaller wire diameter than stock should not be used as this will put too much stress in the spring, which could lead to a break. With this formular, you can shop at the wreckers for springs rather than pay for colourfull ones. (A good starting point for AE86, 6kg/mm front, 4kg/mm rear)
Cutting only a small amount off your coils to both lower the car and stiffen the springs can be a good thing, if its only 1" lower. It has been said that you can lower your car without changing the spring rate, by compressing the springs and putting them in a oven that way for a few minuits, and experiment with length, but its not something I'm about to do. People also use a oxy torch on the spring while its in the car, which can never be recomended. If the car is less than 100mm off the ground, you may start to have problems.
When changing to stiffer springs, stiffer shocks should also be used to match. Incase you didn't know, 'gas' shocks also have oil in them and operate as normal oil shocks, but the gas keeps the oil under preasure so there is less chance of air bubbles getting into the oil. You can tell gas shocks by the way they expand out when not on the car, and it is hard to push them in. With some struts, the shock can be replaced with new gas shock cartridge which makes things very simple and good. But the old oil shock can be made good simply by replacing the oil and resealing them, provided the problem was leaking shocks or lack of oil. To stiffen the shock a thicker oil could be used, proveded the valve is ok. Standard shock oil is ATF, or Dextron. You can buy motorcyle shock oil in different weights, the thickest is 30 weight which I have used.
Often called anti-sway bars or stabilizer bars. They act as a torsion spring to keep a pair of wheels acting together and to reduce body roll. On a RWD car, the front bar is thicker than the rear. This maybe to let the rear axle move more to keep the power to the ground, as race cars often get the inside front wheel off the ground, .. and for other reasons. Adjustable bars are often used for tuning. Spring rates are also used to control roll and understeer/oversteer charicteristics. Old cars such as 70's corollas, only have a Anti-roll bar on the front and none on the rear. Many suspension companies ofer a upgrade of a larger front bar, and a rear bar to match. For a car like a old Corolla to be used in a sporting way, and 1" (25.4mm) front bar and a 3/4" (19mm) bar at the rear would be the stiffest I would go. You don't have to spend much money with aftermarket new ones though, there is often a sway bar to be found at the wreckers if you know what to look for.
Bushes are the rubber things that are fitted between some suspension components to absorb shock and to allow movement. When these bushes age and wear, they can leave room for exsesive movement and ruin the handleing of a car. These days old rubber bushes are replaced with polyurathane which is much harder, sliperyier and longer lasting. Replacing all the rubber bushes with polyurathane can make a tight rattle free car thats more predictable than new (you would hope). Using stiffer bushes can make the car more predictable as movement in old bushes can cause a change in steering and suspension angles when under load.
Standard tyres are often selected to give average grip a soft ride, and good mileage. Any enthusiest will go for wider tyres and alloy wheels. Alloy wheels are lighter and reduce the unsprung weight, but are mostly bought for looks. If you want good handleing but have little money, save the wheels to very last. But, you can still get wider tyres with a lower profile. A tall tyre will flex sideways causeing the car body to roll and reduce tyre grip, so a tyre of a lower wall profile is often needed. To maintain the same tyre outside diameter, it may be needed to go to larger wheel rims when getting wider lower profile tyres. I recomend getting new tyres on cheap steel rims from the wreckers if needed. The minimum requirement for a little performance 4cyl car would be 185/60/14 tyres. One step up would be 195/50/15 tyres which would have close to the same diameter and be better for smooth roads. The next step being 205/40/16 etc. but wider than this is not often needed on a 4cyl car and the lower profile you go to the harsher the ride but better the corner holding on smooth surfaces. Wider tyres can also take too long to heat up for max grip.
The static roll centre is the imaginary point at one end of the car that the car will roll about when pushed or subject to G forces. The front and rear static roll centres can be easily found, and a line passing through the front and rear roll centres is called the roll axis.
For optimum handling, the front roll centre should be a little lower than the rear roll centre. The higher the roll centre, the more the body will tend to roll when turning a corner. The effect of the roll centre height can be simmilar to the effect of anti-roll bars. Some race cars have a very low static roll centre, which could mean that they would stay very flat. But they then tend to roll sideways more, which is then controlled by stiff anti-roll bars. Changing the anti-roll bar rates can than be used to tune the suspension.
Rack and pinion steering is a simpler and more direct system that is now prefered over older linkage style systems. A linkage system can have many joints, all with a small amount of movement which can make for a vage feeling. Rack and pinion is far more direct and responsive and able to give instand feedback. Power steering is not often needed for a little sports car.
Camber is the angle the wheels tilt to the ground. A bit of negative camber (tilting in at the top) is desirable to help the outside wheel grip in a corner. Excesive camber (more than -1.5) causes uneven tyre wear and reduced stright line stopping ability. Camber can be changed (on a car with struts) by changing the control arm length or moving the top mounting position. Older cars have been designed to have some positive camber (tilting out at the top). Positive camber is used in anticipation of the passangers weight to lower the car and change the camber position back to zero. In many systems, camber will change with the wheel travel up and down. It is often desirable for a wheel to have more negative camber as that corner of the car is loaded. This will be the case when the control arms slope up towards the centre of the car.
The greater the caster angle, the greater the dynamic camber change. Caster is great, because it changes the camber of the wheels when you turn to best put your tyres flat on the ground. When you turn to the left, the outside right wheel will get more negative camber, and the inside left wheel will get more positive camber. Caster also keeps the wheels stable and in a straight line at speed. Too much caster makes the steering a bit heavy as the front of the car is actualy lifted up when you turn. For this reason larger cars that have power steering can run lots of caster, and smaller cars are less likley to need as much. If the caster angles differ from one side to the other, the car will tend to pull to one side.
Can't be changed, unless you change the struts.
Imagine a line from the top ball joint or strut pivot, through the lower ball joint, and down to a point on the road. The distance from this point to the middle of the tyre is the scrub radius. A large scrub radius should be avoided and will make steering heavy. Putting spacers behind the front wheel, or using wheels of a different offset will change the scrub radius. Some scrub radius is designed into some cars, for stability or something.
The toe-in should be the last thing to adjust, and should always be chaecked if any other adjustment have been made. It is usually best to have the toe-in at close to zero, or slightly toed in for RWD, or slight toe-out for FWD. Toe-out will usualy make the car unstable and sensitive with steering movement, with toe-in giving the car more stability.
When the front wheels move up and down with the road, the steering should not be effected much. If the toe-in of the front wheels changes as the wheel moves up and down, the car will have bump steer and be dangerouse to drive. Other than being effected over bumps, the car can become ultra sensitive to steering movements and vear across the road with minimal steering imput. Zero bump steer is desirable, particualy with offroad racing with long suspension travel. To achive zero bump steer, the steering tie-rods must be the same length as the lower control arm, and the tie-rod and control arms must be parallel. You can usualy only change this if you a building a new system or doing large modifications. Many cars come with some bump steer (arms of different lengths). It is more important for the arms to be parallel. In some cars, the arms are different lengths in such a way that the car has bump steer in the oposite way. This makes the car more stable and less responsive to steering input, and can make the car tend to understeer.
Ackermann Toe-out on Turns
Ackermann "toe-out on turns" is the principle used to change the front wheel toe-in when the wheels are at an angle. From the diagram, you can see that the front wheels need to be at different angles to roll freely around a corner without scrubing. This is achived by having the steering knuckles offset so the piviot points line up as in the diagram. Many cars do not come with geometry strictly to this principle, but with less steering knuckle offset, so as the reduce the turning circle when the wheels are turned to full lock. For a race car, it is not important to have the propper Ackermann angles as the outside front tyre does most of the griping, the inside wheel may be off the ground at times, and both wheels do a fair bit of slipping anyway. Some people building a purpose built race car will design it with NO Ackermann effect at all (parallel steering) for better feel. But if you modify a sedan for parallel steering you will often end up with less steering lock which is a bad thing.
18:1? 14:1? The first time I played on gravel I thought "I realy need a quicker ratio" because I was all over the place spinning that big steering wheel. But with experience you get to know what to do with the wheel and have time to antisipate steering needed. A quicker ratio may be good for rally or any racing, but leaves less room for error. eg, with a quick ratio I may get very sideways around a gravel corner, exit nicely but as I almost straighten out a lazy error can send me fishtailing all over again. I know, I suck and no one will get in the car with me. Different cars come with different ratios, with sporty variants haveing quicker ratios, both with old linkage steering and rack and pinion. "Quick racks" can be obtained for popular conversions. The steering ratio can also be modified by changing the steering knuckle length, although this would often make a street car illegal.
I have not ever measured corner weights or done anything to correct this. But, you use scales under each wheel. If there is more weight on the front than the back, you can only shift the engine, relocate the battery, things like that. If a diagonal pair of wheels (front left, rear right) carry more or less weight combined that the other diagonal pair, then this is not very good. You can correct it by changing the left or right ride heights with your coilovers. You do all this with the sway bars disconected, maybe with the driver seated.
Good settings for small RWD car?
25mm anti-roll bar front, 19mm for rear.
Front springs slightly stiffer than rear, and all slightly stiffer than stock.
better shocks to suit.
front roll centre lower than rear (should be checked if the car is lowered.)
-0.5 to -2.5 degrees camber, 2.5 to 5 degrees caster
I think... drum brakes were used alot for front and rear up into the 70's. Front disk brakes used were often 2 piston fixed calipers, with one piston on either side pushing on the pads. In the 80's single piston sliding caliper disk brakes became much more common. These are used because they save weight using only one piston, but are just as effective as they have a floating caliper still aplying preasure on both pads. Vented disks are a good thing for reducing fade and are much more common now, especialy in heavyer cars and anything ment to be sporty. Race cars use huge vented disks with 4 or 6 spot calipers. These are of the old fixed caliper design, only with multiple pistons on either side to give more even force over the large pads they use.
There are also 2 spot calipers which have a floating caliper and 2 pistons on the one side. Many little old cars that have had engine transplants need bigger brakes as well, and many people go the whole way and get big vented disks on the front with 4 spot calipers, and disk brakes of some sort on the back. It is easy to change the whole strut assembly to something off a larger car to get the bigger brakes. 10" vented disks can be found on many Jap vans and light trucks as well as large passange cars (like Commodore, Falcon, Magna in Australia). The single piston calipers these come with will usualy be good enough for the street, but many get after market 4 spot calipers or Mazda RX7 4 spot calipers. 4 spot calipers can also be found on Volvos, BMW, 4WD Hilux are the same as Landcruiser etc. Good semi metalic pads can increase the effective heat range and reduce fade. This is a effective upgrade even with standard brakes.
It is a good idea to get the master cylinder off the same car you steal brakes from, or at least note what size it is. Many master cylinders have the size written on them in inches, eg. 3/4" or 13/16". The front pistons probably want to be about 2 times the diameter of the master cylinder? With the rear drum cylinders about the same size as the master cylinder? not sure.
When disk brakes are used with drum brakes on the rear, a rear proportioning valve is often used in the rear brake line. This is because the drum brakes have a dendency to lock up before the front, so a pressure limiting valve is put in the rear line. In a race car, this valve is often replaced with a adjustable type mounted in the cabin. Some cars and many light trucks and vans have a adjustable proportioning valve which is adjusted by the vehicles ride height. One of these valves could be modifyed to use in building a race car instead of buying a expensive adjustable unit. Some old non adjusting drum brakes have a check valve in the line to keep some preasure in the line. When these brakes are replaced with disk brakes the valve should be taken out, and the proportioning valve may have to be changed also.