My thoughts on the Camaro rear cradle.

[willhe64]

I'm a former structural engineer. I've built 3 complete tube chassis drag cars in my life, with 4 link rears. I've read the "door slammers handbook", the quintessential volume on building drag cars. That's it for my qualifications.

In order to improve on a structural part, you have to examine the forces that are applied to it.
Tension and compression, forces trying to stretch or squeeze something.
Deflection, forces trying to bend something.
Torsion, forces trying to twist something.

I've got the rear cradle out of my 45th vert, put a tvs2300 in it and need to beef it up. I've looked it up down and sideways and here are my observations. Correct me if I'm wrong.

Cradle bushings. Definite unwanted movement, there just for vibration control. Stiffening these up will improve handling (stop the rear tires from steering the vehicle) and your 60 foot times. I fail too see how this mod will help with wheel hop though. It may lessen the effect once it starts but it does nothing to control your wheel geometry. Also, this is the only place where rear cradle vibration will be transmitted through to the body ( a bit from the sway bars though). Stiffening the other components may add some vibration noise, but you won't feel it.

Diff bushings. When you hammer the throttle, the nose of your differential wants to twist upwards while this is moving your tires aren't turning. Stiffening these up will improve your holeshot and 60 foot times. They will not improve handling. In fact the softer these are the less shock your driveshaft, axles and gears will receive.

Lower control arms. These experience all forces. Tire rotation wants to twist them. Forward Motion, bumps and cornering wants to deflect and push/pull them. The trailing arm is there to triangulate it and help. We need these parts as stiff as possible.

Trailing arms. Here's where I'm going to get in trouble. The only force I can see on these is tension and compression. Bushing movement is very unwanted but aside from miniscule gains if they are lighter (they move faster), I can't see how aftermarket trailing arms help over stock. Factory arms do not stretch or compress, they are in fact over designed so they won't bend if you hit the odd curb at low speeds.

Toe rods, same as trailing arms. Any aftermarket toe rod with with bushings will offer nothing over stiffening up the bushings in the factory piece. The adjustable ones with solid mounts are great though. Zero movement, but way overpriced. I can build a set with 2 eyelets and a threaded tube for $25.

Upper control arms, same as the lower. But these are triangulated only by their width on the inboard side, those bushings definitely need to be as stiff as possible.

I looked at the hotchkis chassis max brace. It bolts the bottom of the cradle to the body. Stops lateral cradle movement, but still this will be flexing a lot as the cradle goes up and down. transmits a lot of vibration too I bet as it adds a solid metal connection to what once was a rubber isolated assembly. A proper solution would be a triangulated piece with bushings or eyelets.

Well, those are my thoughts. Having said that. I've ordered Cradle inserts (compromise between performance and comfort) and urethane replacements for every bushing in the cradle except differential. We'll see how that goes.

[Scott@Bjorn3D]

On the Hotchkis brace where it goes to the cradle there are some rubber pieces that the bolts go thru to cut down on the vibration. I helped with my wheel hop allot, but there still is some there if it is cold outside.

I also read that some after market axles are made to cut out wheel hop. Have not investigated into them yet though.

[LSxJunkie]

Wheel hop on IRS cars is a function of harmonics. I've never understood why people buy that bushings will solve it. The attempted GM cure of differently weighted axles to destroy the harmonics makes much more sense than stiffening random bushings hoping that something will take.

[willhe64]

The continuation of wheel hop is a function of harmonics. The start of wheel hop is caused by changes to the tires geometry under load. Fix the tires geometry and stop wheel hop.

[Norm Peterson]

Bushing compliance allows energy to be stored and ultimately released, and this can be viewed as a "force times distance" thing. So as you force the bushing compliance "distance" under load → zero, the energy stored also tends toward zero. No stored energy (or at some point only a small enough amount of it) and the hop generally goes away.

This is also the mechanism by which engine mount stiffness plays a part in wheel hop. Only in this case as those bushings load and unload it directly affects the driveshaft torque.


On edit - what the anti-squat curve looks like as the rear of the chassis squats also matters. If you're losing anti-squat as squat occurs, the tires momentarily lose a little grip, which makes it a little easier for the tires to start spinning, which in turn makes it easier for the bushings to unload. Don't forget that you'd be operating on the back side of the tires' "mu-slip curve" at that point.


There's a little structural engineering and analysis on my resume . . .


Norm

[willhe64]

Just finished dismantling the rear cradle and I am now totally convinced that aftermarket toe rods and trailing arms are a giant waste of money, unless you replace them with solid, non bushed parts. There is no way these stock parts deflect or bend, or stretch or collapse (the primary force) under loading.

[caverman]

I'm not going to say your wrong or get into an engineering debate because frankly I have no clue from an engineering standpoint.

What I can comment on is my own real world experience. I took my car to the drag strip a couple times with the stock TAs using stock wheels/tires. I couldn't get a good launch and all and my times were dog slow because I would get wheel hop and would have to let out some to let the tires hook up again. I did a little research and changed out my TAs to aftermarket ones. Went back to the track and bam! 90% of the wheel hop was gone. I still got a little going from 2nd - 3rd and I was probably still getting some off the line but it was reduced enough I couldn't feel it any more.

So, not trying to convince you that you have to have them. That's your decision. All I know is they worked for me and I'm convinced they were worth money and effort in my situation.

[JLE58]

Yeah my trailing arms cut out a lot of wheel hop that's for sure.

[67motorcat]

Originally Posted by willhe64

Just finished dismantling the rear cradle and I am now totally convinced that aftermarket toe rods and trailing arms are a giant waste of money, unless you replace them with solid, non bushed parts. There is no way these stock parts deflect or bend, or stretch or collapse (the primary force) under loading.

I agree with you here for sure.The "Metal" parts are not what is flexing contributing to wheel hop,it's the soft stock rubber bushings.

I purchased BMR trailing arms a the very first mod for my Car to control wheel hop and they worked.Even with the stock outer bushing left in place,Also replacing those at this time,I probably could have just replaced the bushings on the stock arms to get the same effect, but the BMR one's look cool

Now that I have way more power than stock it was time to deal with the ass end pitching left...so dealing with the subframe cradle bushing now as well.

As a compromise I'm going to try the inserts and agree that the cradle movement has got to be the biggest problem we have getting these cars to hook correctly.

[Norm Peterson]

Originally Posted by willhe64

Just finished dismantling the rear cradle and I am now totally convinced that aftermarket toe rods and trailing arms are a giant waste of money, unless you replace them with solid, non bushed parts. There is no way these stock parts deflect or bend, or stretch or collapse (the primary force) under loading.

The arms themselves don't stretch or shrink by enough to ever matter (maybe on the order of ±0.001"), and the loading that they see doesn't introduce enough bending to matter (yes there is a little bending going on unless you've got zero-friction rod ends at both ends of the arm).

It's the bushing compliance - their "squishiness" if you will - that gives rise to things like hop and deviations from the theoretical geometry. Think of the "give" in OE bushings as effectively lengthening and/or shortening the arms by up to maybe 0.10" per bushing, 0.20" total (or about a couple hundred times what the arms' own deflection amounts to).

When you try to improve the "operational" geometry and reduce hop by reducing bushing compliance in only one direction is when things get complicated.


Norm

[Synner]

Drag racers were bending toe rods so they're not all that awesome for serious abuse. The OEM ZL1 unit would be a cheap upgrade. But I do agree that bushings are the primary benefit of aftermarket ones.

Full cradle bushings were a massive difference, didn't notice the toe/trailing arms at all when I did mine. Although I didn't have track days immediately before/after for comparison purposes of those. The cradle bushing difference was apparent on the street as well as track. What was most important was removing any chance of wheel hop since I have an M6 which did happen. Had horrific hop when stock the first time I tried heating up the tires. I thought my car pooped out an IRS.

[429]

Complaining about wheel-hop if you have or had street tires is stupid.

Put a real tire on the car, and it will go away, 99% of the time. I've cut 1.4s without cradle or diff bushings with zero wheel hop. I'm sure I'll get around to them eventually as an unnecessary supporting modification/improvement, but the best two things you can do are add a super stiff drag bar and a set of good LCAs. (other then tires)

[67motorcat]

Originally Posted by 429

Complaining about wheel-hop if you have or had street tires is stupid.

I don't think Stupid is the rite term here my friend

95% of us Camaro owners have "Street" tires and need street tires because we drive on the street 95% of the time.

Curing wheel hop and sideways motion is the goal here with higher than stock HP with "Street" tires.

Sure, a dedicated Camaro set up to perform optimally on the 1/4 mile track is a different animal.

[willhe64]

Originally Posted by 67motorcat

I agree with you here for sure.The "Metal" parts are not what is flexing contributing to wheel hop,it's the soft stock rubber bushings.

I purchased BMR trailing arms a the very first mod for my Car to control wheel hop and they worked.Even with the stock outer bushing left in place,Also replacing those at this time,I probably could have just replaced the bushings on the stock arms to get the same effect, but the BMR one's look cool

Now that I have way more power than stock it was time to deal with the ass end pitching left...so dealing with the subframe cradle bushing now as well.

As a compromise I'm going to try the inserts and agree that the cradle movement has got to be the biggest problem we have getting these cars to hook correctly.

This is what I'm saying. I bought the energy suspension bushing kit for $150. Has every bushing for the control arms, trailing arms and toe rods. Going to do inserts as well.

My convertible has a reinforcing bar that bolts right through the front cradle bushings too. This will anchor the cradle quite well.