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Redesign a Pontiac Fiero suspension for better geometry

Redesign a Pontiac Fiero suspension for better geometry

Redesign a Pontiac Fiero suspension for better geometry

I want to learn about suspension design and what goes into it. I figured the best way to do it is with an actual practical application, rather than just complete theory.

I always wondered if with modern CAD/CAM software it would be possible to design a more modern suspension for the Pontiac Fiero.
A ) Double wishbone front + Double wishbone rear
B ) Double wishbone front + fancy multi-link rear
C ) another option I hadn't considered.

It would be great if it were to (almost) bolt in to the vehicle. Maybe use factory rear cradle attachment points and replace the rear cradle. The front end may be replaced as well, while utilizing the attachment points.

The goal is to apply what engineers have learned about suspension design in the last 30 years since the Fiero was built.

Let's put considerations such as: feasibility of actually making it a real life product, costs/reward, starting over with a tube-frame chassis, etc. on the back burner to let creativity flow.

Reasons I thought it would be a great idea to do this:
1) It is a great way to learn/share ideas about modern suspension theory
2) Learn suspension design applied to a specific project (with applied constraints), rather than just complete theory, or a scratch build vehicle.
2) hopefully get a good debate going (please without name calling, etc. let's keep this civil)
3) See how open source/collaborative thinking would work out in the Fiero world
4) provide an avenue to share technical information/content for people with similar interests.

Here's my original thread. (as you can see it was derailed by quite a bit sadly. :( )


I'm sure you're not familiar with the Pontiac Fiero, so feel free to ask me questions, and I'll try to answer as much as I can.


RE: Redesign a Pontiac Fiero suspension for better geometry

Although I don't know the intimate details, I do know that the front already uses an upper and lower A-arm arrangement. In the early ones it was similar to the Chevette's front suspension. The rear suspension uses essentially X-car (Chevrolet Citation) front suspension transferred to the rear with the tie-rods attached to a fixed location rather than to a steering rack. I also know that there was some sort of major change in the last year or two of production which addressed some of the handling flaws of the earlier models. What the nature of that change was ... I don't know, but if you're dealing with an early one, it's probably worth finding out.

One of the issues with MacPherson in general, is that in bump travel (or in compression, on the side of a car on the outside of a corner) the camber curve tends to go in the wrong direction. Upper and lower A-arms with the upper arms shorter than the lower, and with the upper arms inclined with the outer end higher than the inner, generally have a camber curve that goes in the right direction. When used as a rear suspension, camber curve in the wrong direction is generally not good for grip (i.e. it will tend to oversteer). On a mid engine car, you need the rear suspension to be as good as it can possibly be, as far as the geometry is concerned, to minimize the tendency for the car to swap ends.

Another issue is that they generally don't have strong anti-dive (or anti-lift, if you prefer, when used as a rear suspension). The rear multilink designs nowadays generally have the spindle attached to what is more-or-less a trailing arm, with links guiding the spindle holder in both the camber direction and the toe-in/out direction. This results in very strong resistance to the rear end of the car lifting up under braking, and many cars so equipped don't lift the rear at all under braking. This is a good thing, because usually the camber goes in the wrong direction if the rear of the car lifts up. Having it stay near nominal ride height under braking keeps this from happening so hopefully it can be less squirrelly under braking.

To minimize the tendency for the back end to come around, you want the rear geometry to move slightly towards toe-in under both bump and rebound travel and also you want the bushing deflection when the brakes are applied to move slightly towards toe-in. All of this is easy to arrange with a multilink design. The tricky bit is figuring how much is the right amount, but that's what adjustable pivot locations are for.

Take a look at the rear suspensions of: Ford Focus, VW Golf Mk5 or Mk6 (the ones with the multilink rear - not the twist-axle generations, which are all the rest of them), BMW 3-series, Chevrolet Equinox (the current one), or the last couple generations of Porsche 911.

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