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 SuspensionLast  on September 01 2008How to make your Fiesta handle Its all about preventing the shell from rolling too much, and trying to keep as big a contact patch at the front at all times (so you can keep the throttle pedal buried and get round corners even faster) I'll start at the rear of the car. Rear suspension The tubular rear axle on the Mk1 Fiesta is a traditional design, shared amongst many manufacturers. Using a Panhard rod to keep the axle square to the ground, the system works quite well. Linked with an uprated rear anti-roll bar however, considerable improvements can be found, in keeping the car straight and level when cornering hard. I wanted to give the rear wheels some negative camber, and had thought about bending the axle, or cutting it in two, and using a steel sleeve, welding the axle back together, with a preset amount of negative camber. What I settled on was a system that joined the two halves of the axle back together, but allowing adjustment of the camber and toe-in, in situ using two wedge shaped aluminium rings. The trick is machining the rings so when they're bolted back together, and sandwiched between the two halves of the axle, the resultant angle gives -1.5° negative camber on each wheel.  This picture shows the end result. I also had the axle and all parts powder coated whilst it was away. And all fastners are 12.9 grade steel for added strength and safety.
 My Fiesta also has an adjustable and rose jointed Panhard Rod (above), supplied by Trans Auto Sport (TAS). And it has a pair of adjustable and rose jointed rear trailing arms (left), again supplied by TAS. All the rose joints add up to a very noisy ride, but the suspension is completely adjustable and the settings cant drift once adjusted.
 Since fitting the modified rear axle and anti-roll bar, I've adjusted the anti-roll bar drop links so that they are vertical when the car is sitting on all four wheels, which reduces the risk of the links binding and causing the rear end to lock up when cornering. I've also shortened the bar. As you can see, with five holes, the bar is rather close to the axle. By shortening the bar, I've reduced the number of adjustments available, but I still have from 5 times to 8 times the stiffness of the original XR2 anti-roll bar.I'm using SPAX gas adjustable rear shock absorbers. Together with 450 pound shortened rear springs, and the anti-roll bar set on 5 times the stiffness of the standard anti-roll bar, the handling has been transformed. As you can see from the photograph below, the Fiesta now has a tendancy to lift a rear wheel on hard cornering. This is, believe it or not, to some extent desirable (read on...) Making a FWD car go round corners Since a FWD car is nose heavy, it must be setup to work the front tyres as evenly as possible. That means it must corner with the inside rear tyre very lightly loaded or airborne. We trade away lateral grip at the rear to gain more at the front, where we need it. We also gain drive traction on the inside front wheel. This is important in a FWD car, because we cannot use limited slip differentials that lock too firmly or abrubptly, unless the driver has great tolerance for steering fight. It is important to note that once the inside rear wheel is airborne, the rear suspension has contributed all the anti-roll moment it can, and any further roll resistance has to come from the front. Up to the point of rear wheel lift, rear load transfer builds faster than the front load transfer. Beyond that point, rear load transfer is 100%, and front load transfer builds rapidly. So does roll angle. So does understeer. As a general rule, to get a car that has good consistency as grip varies, we want the inside rear wheel to lift just a little in steady state cornering, when grip is good. If it lifts more than that, we are likely to have a relatively loose car when grip is poor and a much tighter car when grip is good.  Nov 2004: I am toying with the idea of fitting a softer front anti-roll bar to complement the rear. Now compare this picture with the one shown below, taken the year before, and you can easily see the difference in the body roll and the behaviour of the rear axle. Look at the gap between the rear tyre and the top of the wheel arch. The addition of the stiffer springs and rear anti roll bar have made the biggest differences. The -ve camber rear axle helps too. Finally, another picture showing the behaviour of the stiffer rear suspension, this time at Shelsley Walsh hillclimb, July 2004. The rear tyre is off the ground as the car approaches the top S at full speed.
The front suspension I started with a clean sheet of paper when designing the front suspension. I wanted to reduce weight, unsprung weight too, and give the car more caster and camber. So, using tubular steel, the front end was redesigned. The standard bottom ball joint was replaced with a 20mm spherical bearing and a EN24 steel pin, which bolts in to the bottom of the front hubs, and which was also machined to lower the roll center of the car.  I'm using AVO 2.25" adjustable front shock absorbers, with 300 lb springs. The front suspension is adjusted to give 2° -ve camber, 3° of caster angle, and 0° toe-in. This picture shows the first incarnation of the front tie bars and track control arms. We opted for M10 rod ends for the tie bars, and M12's for the TCA's. However, during 2004, I bent the M12's, which were replaced with M14 rod ends, and I also bent the M10's (bottom right of the picture), which have also now been replaced with M14's. Just goes to show that there are some significant forces at work on the front suspension.The accelerometer in the Race Technology datalogger regularly records >1G lateral acceleration under hard cornering, which is a force in excess of 700KG at work against the rod ends. This would explain perhaps why the rod ends on the TCA's bent, but the ones on the ends of the tie bars?  This picture shows the Track Control Arm and Tie Bar. Note the spherical bearing and the extra long EN24 pin which helps lower the roll centre of the car by lowering the ends of the track control arms to make them more parallel with the ground.
 To mount the rose jointed tie bars, we had to fabricate a new front cross member. It bolts to the underside of the chassis, and is held in place with six bolts. A photo of the front suspension fitted to the car (car is off the ground and the wheel is removed)
During 2004 (and 2005) I recorded new personal bests at all tracks I competed on, which proves that the changes to the suspension have made a significant increase in the performance of the car. I aim to improve further still during 2005, and so far I have again set new PB's at the tracks I've visited. All work on the front and rear suspension has developed in conjunction with Rutland Performance Vehicles |
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