First ya gotta look at the chassis. It's a front heavy pig with the front & rear connected by a flexible, sheet-metal floor pan. If you want the suspension to work, ya gotta stabilize the chassis.
Subframe connectors are a must, and a lower chassis brace really helps the stock k-member. While you'reunder there, get the torque boxes welded up... the factory just tacks them together.
The rear suspension is a diverging/converging 4 link. It controls lateral motion by binding up.
Unfortunatly, it also binds while the suspension rolls. Along with the high roll centre (much higher than the front) & poor weight distribution, this is why the rear springs (200# stock) are so much less than the fronts (400# stock). Other problems include the rear brake jacking (from the upper control arms) & the snap oversteer (when lateral forces compress the rubber bushings & abruptly unload on the axle/wheels/tires).
Why does ford use this rear arrangement? It's cheap & easy, and it does provide lots of anti-squat for the front heavy beast (so you can accelerate quite well).
As you've probably noticed, the UCA's cause some problems, and if you wanna handle, they gotta go.
BUt this removes the lateral location, windup control & some antisquat.
Most people go for a panhard bar for lateral location (cheap & easy).
To control the pinion angle/windup, you can go for a 3 or 4 link, or a torque arm. To gain some of the antisquat back, the torque arm is probably the best choice (although weight & ground clearance may be an issue).
IRS - Independent Rear Suspension like on the 99+ Cobra's... far less unsprung weight, better ride, but it'll take your money too! (the stock setup is pricie, and needs some work). You get some camber in the corners, but the wheels won't be flat under acceleration & braking.
When the mustang went to the fox3 chassis in 1979, they cheaped out, and ditched the short-long-arm /
double-wishbone suspension and any hope of a dynamically beneficial suspension. Okay, maybe that's a little harsh. They did reduce weight & simplify the components.
The front is composed of a modified MacPherson strut. I believe it's "modified" cause it's not a
coil-over... it's got a heavier inboard spring between the LCA & chassis.
When you corner, the lateral forces try to roll the tire off the side of the rim. THis decreases the contact patch, and your ability to turn. This is why lower profile tires (bigger wheels) are better for the corners.
With the SLA suspension, it's compression actually adds negative camber, which negates the tire deflection & maintains the contact patch. (I'd really like an SLA!)
With the strut, the camber goes positive as the body rolls, making the contact patch even worse. This is why the racers will dial in large amounts of static -ve camber. They also run high spring rates, which decreases the amount of compression/geometry change.
If you add more caster, you can get a little bit of -ve camber from steering... but you can start to add some bump-steer too. This is why some people have found offset-rack bushings to be helpful.
Another problem is ackerman angle. When you're turning, the inside tire is making a tighter radius than the outside, but ford's geometry didn't take this into account (it was thought that the tire scrub provided better turn-in response). This can improved by moving the rack rearward (like with MM's unique rack bushings).
Back to roll centres. The rear is quite high, but the front is pretty low. To make things worse, the front's centre of gravity (COG) isn't nearly as low, so you get plenty front roll, which wrecks the geometry. This is why the front sway bar is so heavy.
A lower COG offers better handling/performance, but lowering the front cause the front roll centre to go MUCH lower than the COG (as the LCA's get more angled). This means you have to go to disproportionately heavier front springs, or change the LCA locations (get a new k-member).
K-members often improve geometry & reduce weight... partly with lighter LCA's & coil-overs. I'm not too keen on putting the spring loads on the strut towers, but many people do it.
The struts & shocks are crucial, and should not be skimped on.
SPRING RATE & WHEEL RATE?
Torque = Force * radial_distance
Force = spring_constant * compression
Compression also depends on radial_distance (just as circumference = 2pi * R)
So you've got torque = K*R*R = k*R^2, where k is just a constant (spring constant*2pi).
So the torque on the arm is dependent on the radius squared.
To determine the wheel rate, you need to look at the square of the motion ratio.
The rear lower control arm has the spring located around 70% of it's length. So the wheel rate is only actually 0.7^2 ~ 0.5 (ie half the spring rate).
The stock springs are about 200#, so the wheel rate is 100#.
Up front, the wheel rate is about 1/4 for a spring in the stock location (it's about 9/10 for a coil-over).
The stock springs are about 425#, and the wheel rate is around 100#.
The added advantage of the coil-over is that it decreases the load on the control arm bushings, so they articulate
freely, providing a smooth ride.
Pitching (for/aft sea-sickness) occurs when the front:back weight to wheel rate isn't balanced. If your shocks/struts are
overdamped, this may not be an issue. The stock mustang has a weight distribution of about 60:40, so if you're running a rear
rate of 100#, you should really run 150# up front. This corresponds to around 600# in the stock location.
The ranking/industry standard is frequently a scale from 1-8. They'll test from 11"/sec to 24"/sec!!!!
rating = lbs force (at some speed)
Carrera - Bil(20.5"/s?) - Edel(10"/sec) Rebound/Compression
1 - 100 - 80
2 - 135 - 120
3 - 175 - 160 - 165/80
4 - 220 - 210 - 200/150
5 - 265 - 260 - 250/220
6 - 320 - 320 - 320/260
7 - 390 - 380 - 400/275
8 - 475 - 500 - 470/325
9 - 600
Koni rates at 13"/s, and their rebound tends to adjust from 3 to 8/9.
1 Newton = 0.2248 lbs
Bilstein lists half steps, which is probably a good idea, as they're not adjustable.
sport H = 1300/540 N = 292/121 lbs = 5.5 / 2
cobra R = 1440/450 N = 323/101 lbs = 6 / 1.5
IRS H = 1600/745 N = 360/167 lbs = 6.5 / 3
IRS R = 2280/1145 N = 512/257 lbs = 8 / 5
(these are mounted inboard, so the wheel rate isn't quite as extreme).
The struts seem a little 'light', as other companies recommend an "8" for mustang applications. Perhaps they were just measured at lower piston velocities? Or maybe it's due to the digressive valving? (they'll handle the heavier springs, but 'feel' like a "6" or whatever)...
Obviously, as you increase spring rate, you'll wanna increase the rebound... but you might actually decrease the compression to maintain a suitable ride.
Of course, you can change the transient response by decreasing rebound (easy-up), or increasing rebound (tie-down) etc. The koni's actually have a lot of rebound adjustment.
Cheap shocks are often linear valved, yielding more resistance at higher piston velocities.
Digressive valving generally provides more resistance at low piston velocities (improved cornering & braking, capable of handling higher spring rates). Additionally, they usually
have a less resistance to higher piston velocities, so you get a smoother ride... definitely preferable on the street. So don't skimp on the shocks! Here's a nice graph illustrating their dynamic behaviour
(courtesy of Rick Kean from www.corner-carvers.com).
Griggs Racing has engineered the entire suspension, and does quite well.
Maximum Motorsports has some great products, and their new k-member should complete their package.
Kenny Brown - new k-members look REALLY good. He's 'fixed' the IRS as well... so you
could get a complete package.
Steeda - has a lot of bolt-in band-aid products, including an interesting 5link (4link +
panhard). They offer some decent stuff, but I feel it's over-priced, and under-engineered (rather than starting from a clean slate, it seems to be bandaged together to try and remain competitive in specific racing classes).
Global West has some good stuff (subframe connectors), but a lot of band-aid compromises too.
The mustang-specific stuff is often pricie... as it's engineered to a particular chassis, and it's a straight bolt-in. There's also many hotrod suppliers with coil-overs, SLA front suspensions & nice 4-links, which could also be adapted for a decent price (welding required).
Chris Alstons Chassis Works
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