Custom cradle?

Real tech discussion on design, fabrication, testing, development of custom or adapted parts for Pontiac Fieros. Not questions about the power a CAI will give.

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ericjon262
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Custom cradle?

Post by ericjon262 »

looking at a thread on old europe, I saw where a member built his own cradle, modeled after a 88 cradle. To me, this doesn't seem like a terrible undertaking, but it seemed like a pretty good bit of work for a piece that doesn't offer any benefit over a stock cradle. if you were to engineer a replacement cradle, what changes would you employ to improve handling? doesn't have to be based on a stock cradle either.

Thanks-

Eric
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Re: Custom cradle?

Post by Aaron »

Aluminum, and built around my engine specifically. It'd be cool to use it for some form of fluid storage as well, maybe additional fuel capacity. But I doubt it'd add enough to be worth it.
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Re: Custom cradle?

Post by ericjon262 »

I wonder how much weight would be shed by going aluminum, while maintaining comparable strength/rigidity.
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Re: Custom cradle?

Post by Series8217 »

Doesn't the stock cradle only weigh 40 lbs or so? I remember carrying one out of the junkyard myself when I was a scrawny little kid.
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Re: Custom cradle?

Post by ericjon262 »

Series8217 wrote:Doesn't the stock cradle only weigh 40 lbs or so? I remember carrying one out of the junkyard myself when I was a scrawny little kid.

Me, I'm not too worried about the weight aspect, because as you said, it's not too heavy to begin with, and the weight is very low in the chassis. if it could drop >30% of the weight though, I'd consider aluminum. I'm more concerned with improving the handling of the car then the weight.
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Re: Custom cradle?

Post by fieroguru »

The stock 88 cradle is 50 lbs. The aluminum cradle from an LS4 car is 40 lbs for reference.

On the 88 rear cradle there isn't much more that can be done but raise the lateral link pivots to help with the rear camber gain.

The weak parts on the 88 cradle are the front and rear crossmembers, so I normally swap them out with some 2x3 tube and relocate the rear one to maximize the room for the exhaust while keeping the truck stock.

I do have an 88 cradle fixture from when I built my tubular one... lots of work. I use it for swap mockup and welding in the 2x3 crossmembers more than anything any more.
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Re: Custom cradle?

Post by ericjon262 »

fieroguru wrote:The stock 88 cradle is 50 lbs. The aluminum cradle from an LS4 car is 40 lbs for reference.

On the 88 rear cradle there isn't much more that can be done but raise the lateral link pivots to help with the rear camber gain.

The weak parts on the 88 cradle are the front and rear crossmembers, so I normally swap them out with some 2x3 tube and relocate the rear one to maximize the room for the exhaust while keeping the truck stock.

I do have an 88 cradle fixture from when I built my tubular one... lots of work. I use it for swap mockup and welding in the 2x3 crossmembers more than anything any more.
how about the early model cradles? any real improvement to be made by adjusting control arm/tierod mounts? I know the '88 is considered the ultimate, but I'd bet a second look could come up with something better.
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Re: Custom cradle?

Post by fieroguru »

ericjon262 wrote: how about the early model cradles? any real improvement to be made by adjusting control arm/tierod mounts? I know the '88 is considered the ultimate, but I'd bet a second look could come up with something better.
On the 84-87, you can change the angle of the lower a-arm mounts to lessen the pro-squat characteristic and play around with the placement and length of the tierod to reduce bump steer, and possibly raise the inner pivots to improve the camber curve and reduce roll slightly... but it won't make a huge difference.

The best part of the 88 rear suspension is that it isolated the lateral loads from the braking/acceleration ones. So you can run rod end lateral links to eliminate toe changes caused by bushing defection w/o impacting ride harshness. The strut moved further inboard also gave it ever so slightly more camber gain, but is still a huge net loss in camber with roll... You can play with the lateral link and upper strut mounts to make it "less bad", but it is still nothing close to what I would call ultimate.
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Re: Custom cradle?

Post by The Dark Side of Will »

ericjon262 wrote:looking at a thread on old europe, I saw where a member built his own cradle, modeled after a 88 cradle. To me, this doesn't seem like a terrible undertaking, but it seemed like a pretty good bit of work for a piece that doesn't offer any benefit over a stock cradle. if you were to engineer a replacement cradle, what changes would you employ to improve handling? doesn't have to be based on a stock cradle either.

Thanks-

Eric
As Series8217 is demonstrating with his car, lowering the outer pivots or raising the inner pivots on an '88 cradle and making appropriate suspension tuning changes (springs, dampers, bars) along with the right tire stagger results in a car that handles very well.
As Guru points out and as Series has also demonstrated, the '88 suspension so modified still doesn't have a very good camber curve and needs to be stiffer than might otherwise be ideal.

A while back I came up with an SLA design based on the '88 rear links, with the strut removed and the body lowered 1 inch. It gave perfect camber performance and kept the roll center in an area the size of the palm of your hand through 5 degrees of body roll. The static roll center was about 5" above the ground, IIRC, so (if used with a front suspension with the roll center at ground level) it also had a roll axis inclination that compliments the Fiero's weight distribution. The knuckle end pieces consisted of plates that would bolt to the camber bolt holes on the knuckle. These plates would carry the ball joint between the rotor and camber bolts while providing the lower pickup for a coil-over shock inboard of the original strut location. This would also allow ~1" wider rear wheels than the strut suspension allowed. The UCA would need to have mounts cut and welded into the lower frame rail... although it *might* be doable with bolt-on mounts that only require holes to be drilled.

I've been thinking I should dust that design off and build UCA's like Series' fully adjustable units ( http://realfierotech.com/phpBB/viewtopi ... =3&t=17936 ) in order to finally make the SLA rear for '88 cars a reality.

At that point the only weakness is the 5x100 hub bearings and small pattern outer CV joints.
ericjon262 wrote: how about the early model cradles? any real improvement to be made by adjusting control arm/tierod mounts? I know the '88 is considered the ultimate, but I'd bet a second look could come up with something better.
The early cradle is a different story...
By the time you move enough pivots and change enough components to fix the pro-squat, fix the bump steer, fix the roll center and fix the camber curve, it really is more practical to ditch the stock cradle and come up with a clean sheet design.

What I'm intending to do with The Mule is to build two crossmembers, one forward and one rear. The rear would bolt directly to the current cradle attachment points, but would have to be triangulated forward and aft along the frame rails. It would have a pair of powertrain mounts projecting foward, similar to the control arm or cradle bushings that Guru used to mount his LS4 powertrain. The crossmember powertrain mounts would be set up with corresponding brackets on the engine such that the engine could be dropped straight down without removing the crossmember.
This crossmember would interfere with any exhaust that works with the stock cradle, so installing it would mean building a new exhaust system. It also may not work with the common FWD exhaust setups like the LS4 has which merge both banks into a single outlet off the rear manifold.

The forward crossmember would actually fit in between the inboard mounting ears for the existing forward cradle mounts. I'm not sure if it would bolt to anything else to keep it from rotating, or if I could set the mounts up to pull/push straight through it so that it wouldn't need any rotational bracing.

The suspension would be a 5 link per side arrangement with upper and lower lateral links, a toe link and dual trailing arms.
For reasons related to symmetry, I would *like* to have the toe link in plane with the lower lateral link. If the height of the lower link is half the height of the upper link, then the loads on the lower link are twice the loads on the upper link. With the toe link and lower link in the same plane, each one sees half the "lower loads", meaning that all three links see the same loads.

However, packaging may dictate that the toe link is in plane with the upper link instead. The toe link should be in-plane with one of the lateral links rather than being in the middle between the uppers and lowers. It's typically easy to package this way and it completely eliminates bump steer.

Another thing I would *like* to do is use really long lateral links, similar to BMW's lateral links in the E36 and E46 rear suspensions. BMW placed the outer pivots directly above and below the axle centerline, but offset the inner pivots forward to clear the bulk of the diff. A Fiero could do something similar, but offset the inner pivots to the rear instead. I haven't done the modeling yet to know if that will package in the space available without cutting the frame rail.

EDIT: One thing I think will almost certainly be doable is to put the bottom shock mount at the bottom of the knuckle and the top shock mount on a weldment to the bottom or side of the frame rail. This should allow even more backspacing in the wheel, maybe even as much as 11.5 or 12" width. It might even be interesting to pull the hub flange inboard a bit to take advantage of common offsets in 18x11 and 18x12 wheels on the market (I think they're available for Mustangs), but then the left axle gets pretty short, especially if used with the F40 trans.[/edit]

The trailing arms could anchor to a component that would bolt into the front cradle mounts the way the Fiero cradle did. Because it's only anchored on that one bolt, this component may have to have a hefty mounting tab that reaches forward to be bolted to the floor pan--much like a subframe connector on a Mustang of F-body--or maybe upward to the firewall (or maybe both) in order to secure it against rotation. The mounting bolt would be installed from outboard, go through the trailing arm mount and thread into the forward crossmember.

The knuckle would be water jet cut from 1/2" (or thicker) aluminum plate, would have bolt-on mounts for the links and would accept a Corvette hub bearing (like the SKF units for the ZR1), use drum-in-hat parking brakes via ZR1 hats and two piece rotors. Rotors could be dimensional equivalents to C5 or C6 fronts or maybe even ZR1 rears if feeling spendy (and bought appropriate matching front units...)
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Re: Custom cradle?

Post by ericjon262 »

do you have any of that drawn up? sounds pretty damn cool!
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Re: Custom cradle?

Post by Series8217 »

The Dark Side of Will wrote: I've been thinking I should dust that design off and build UCA's like Series' fully adjustable units ( http://realfierotech.com/phpBB/viewtopi ... =3&t=17936 ) in order to finally make the SLA rear for '88 cars a reality.

At that point the only weakness is the 5x100 hub bearings and small pattern outer CV joints.
How about revising the design so that it utilizes the C5/C6 Corvette knuckles? Then we can use C5/C6 outer axle splines, hub bearings, and brakes -- including the parking brake setup.
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Re: Custom cradle?

Post by The Dark Side of Will »

Series8217 wrote:
The Dark Side of Will wrote: I've been thinking I should dust that design off and build UCA's like Series' fully adjustable units ( http://realfierotech.com/phpBB/viewtopi ... =3&t=17936 ) in order to finally make the SLA rear for '88 cars a reality.

At that point the only weakness is the 5x100 hub bearings and small pattern outer CV joints.
How about revising the design so that it utilizes the C5/C6 Corvette knuckles? Then we can use C5/C6 outer axle splines, hub bearings, and brakes -- including the parking brake setup.
That would mean reworking the cradle, which is what I was trying to avoid by simply adding an upper arm to the stock geometry.
Designing a replacement knuckle for the '88 SLA setup wouldn't be any harder than designing one for the early car replacement I described above.

For the '88, there are a couple of aluminum rear knuckles from FWD GM cars that look like possible replacements, but those options are more likely to use tiny FWD rear brakes, which are certainly not ideal on a Fiero. Of course they can be adapted, but so can anything else (meaning that the effort has to be balanced between the work necessary to adapt something that's not ideal and the work necessary to do a clean sheet design that *IS* ideal).
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Re: Custom cradle?

Post by The Dark Side of Will »

ericjon262 wrote:do you have any of that drawn up? sounds pretty damn cool!
Its all works in my head... does that count? :wink:

I'd been expecting to have to spend about $1000 on Rhino or similar to get the CAD capabilities I'd need... (and I've had more pressing things to spend money on...)

However, if Google Sketchup can produce the models you showed in your other thread, I may have to give that another look.

eMachine CAD is another possibility.
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Re: Custom cradle?

Post by ericjon262 »

The Dark Side of Will wrote:
ericjon262 wrote:do you have any of that drawn up? sounds pretty damn cool!
Its all works in my head... does that count? :wink:

I'd been expecting to have to spend about $1000 on Rhino or similar to get the CAD capabilities I'd need... (and I've had more pressing things to spend money on...)

However, if Google Sketchup can produce the models you showed in your other thread, I may have to give that another look.

eMachine CAD is another possibility.

Sketchup doesn't do to bad, if I could take them time to really learn the in's and out's of it, I'd bet it could be a decent setup, but I have no other CAD experience.
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Re: Custom cradle?

Post by The Dark Side of Will »

I used 3D modeling with boolean primitives in AutoCAD R14 (giant PITA, FYI)... I could make it sing then, but parametric interfaces have come SOOOOO far since the good ole days of 2.5D CAD.
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Re: Custom cradle?

Post by fieroguru »

The Dark Side of Will wrote: For the '88, there are a couple of aluminum rear knuckles from FWD GM cars that look like possible replacements, but those options are more likely to use tiny FWD rear brakes, which are certainly not ideal on a Fiero.
FWIW, almost all the tri-link rear uprights that come on a FWD car have the lateral link connection on the upright moved up much, much closer to the wheel bearing so using them with the stock cradle is no-go. Same with the ones used on Toyota's.
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Re: Custom cradle?

Post by The Dark Side of Will »

fieroguru wrote:
The Dark Side of Will wrote: For the '88, there are a couple of aluminum rear knuckles from FWD GM cars that look like possible replacements, but those options are more likely to use tiny FWD rear brakes, which are certainly not ideal on a Fiero.
FWIW, almost all the tri-link rear uprights that come on a FWD car have the lateral link connection on the upright moved up much, much closer to the wheel bearing so using them with the stock cradle is no-go. Same with the ones used on Toyota's.
And tend to mount the strut lower relative to the wheel centerline also...
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Re: Custom cradle?

Post by ericjon262 »

I know this idea will probably get shot to pieces, but is anything fundamentally wrong with leaf springs? I would imagine a transverse leaf would help with packaging quite a bit.
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Re: Custom cradle?

Post by The Dark Side of Will »

Wrong? Nothing. Corvettes use them very well. Tadge Juecter, the Corvette chief engineer said that the transverse composite leaf was the lowest weight highest performance spring setup the Corvette team evaluated. It also keeps its weight lower in the body than any other spring option.

However, rate adjustment is difficult, so the Corvette race cars still use coil springs so that they can change the spring rates easily.

It's a matter of how appropriate it is to the application. A cradle that could use a Vette leaf spring would need to have adjustable mounting in order to dial it in for the Fiero, which would add weight and complexity. When you consider that the leaf spring would basically be the lowest component in the chassis, and consider the loads it takes and what kind of structure would have to support it, you can see that using one would probably add a good bit of structural weight to any replacement cradle.
Being lighter than the stock cradle is actually pretty difficult (partly because the stock cradle doesn't have to take any spring loads)... and one of the reasons I'd like two crossmembers instead of a replacement cradle... keeping the "bounding box" of the load bearing structure as small as possible is the first step toward keeping the load bearing structure as light as possible.
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Re: Custom cradle?

Post by ericjon262 »

I see why it might be harder to implement, as far as adjust-ability, is concerned, couldn't one be pre-loaded with a U bolt to adjust ride height/quality?
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