Broken Corvette Rear Lower Control Arms

[crash]

In preparation for this year's 25 Hours of Thunderhill event we have been exercising the FFR PDG GTM #5 race car in some short endurance racing events. (less than 5 hours generally)

Since we started racing the GTM we have had a couple of issues with rear lower control arms and shock mounts. Some may recall that we broke the rear shock mounts almost immediately after starting to race the GTM in endurance events. This led FFR to redesign the rear shock mounts, which seems to have solved the issue for most street focused GTMs.

For us, as a race team, we take stressing the GTM to extremes. This last race we ran on a track that had curbs and has a rather rough track surface. During a 4 hour race we ended up hitting a curb at about the 2 hour mark and breaking the left rear lower control arm. We had previously broken a right side rear lower arm, and I thought it was an anomaly so simply replaced and continued. Now with a second failure it was time to look at this closer.

Because of the design of the shock mounts that we run, the shock mount held the fractured arm together and allowed us to finish that race even with a completely broken lower control arm. Unfortunately the car became VERY difficult to drive and we ended up in second place out of 4 in our class. We had been leading previous to this.

I will say that our rear lower control arm shock mount design was an improved version of the second generation FFR part and it did it's job fairly well. (These were the QRP designed and built shock mounts) Evidence being that we were able to complete the race with a broken control arm. That said, we did have the issue of the car being almost undriveable, so I went about redesigning the lower shock mount again.

What I have come up with attaches both sides of the lower control arm with steel parts. It doesn't remove the "weak link" that is the weak point on the lower control arm where fracture occurs. I contemplated strengthening this location, but ended up staying away from this area of the suspension arm. I felt this fracture point is a necessary element to avoid transferring too much load into the frame mounts of the suspension and causing potentially larger issues with significant repairs to the frame. So the concept is that under extreme loads, such as a curb impact, the lower control arm can still absorb that impact, and even fracture if necessary, yet will remain "together" and also transfer shock loading more evenly to both side of the lower control arm so that droop from shock mount bending is reduced after LCA failure. Thus, hopefully, keeping the car much more driveable, even with broken suspension components.

A picture of the old and new mounts is attached and you can really see that the increased strength is focused on transferring much of the shock load back to the original Vette shock mounting area of the lower control arm.

I am not selling these parts as they are "one off" type of things, but thought I would share so that others can see one of the things we are still doing in development of the GTM to be stronger and more robust.

Of course anyone is welcome to recreate what we have done here if they choose to.IMG_20230306_174832104 resize.jpgIMG_20230516_181332741_HDR resize.jpg

I will try to remember to get a shot of the fully assembled lower control arm tonight so it is more clear where everything is when installed.

[J R Jones]

With no GTM experience I can offer only generic failure comment. I do not understand how the blue steel parts relate to the control arms.
The shock loads go from the shock mount to the ball joint and the wheel/tire, no break there. The break is between the wheel/tire and the OEM spring mount, assuming you are using the OEM spring mount to support the car. The control arm fracture is in tension under acceleration. It is in compression while braking and curb impact. The fracture was bending under suspension load.
If the failure is fatigue you may have dark and light sections across the fracture. If that is the case, replace the control arms at 1/2 your current service life.
jim

[crash]

With no GTM experience I can offer only generic failure comment. I do not understand how the blue steel parts relate to the control arms.
The shock loads go from the shock mount to the ball joint and the wheel/tire, no break there. The break is between the wheel/tire and the OEM spring mount, assuming you are using the OEM spring mount to support the car. The control arm fracture is in tension under acceleration. It is in compression while braking and curb impact. The fracture was bending under suspension load.
If the failure is fatigue you may have dark and light sections across the fracture. If that is the case, replace the control arms at 1/2 your current service life.
jim

Yes you are not understanding the specifics of how the GTM is set up from FFR. Because of the reversed upper control arms and the location of the axle shaft the shock and coil over spring have moved from the traditional location to the opposite side of the arm. This would mean the shock now mounts where the original spring mount would be on a Vette. Not only this, but it actually has to be moved so far to that side of the arm that the shock bolt is now in single shear. This creates a torsional force on the weaker side of the suspension arm. The original QRP redesign addressed this rotational force by significantly increasing the link between the old shock mount and the new one. What it did not address is arm collapse upon destructive impact forces. This new design does this. It is bigger, it is heavier, but it is also much more substantial and robust.

Again, things will be much clearer, especially for someone that does not know the GTM setup, once I post an as assembled picture.

[beeman]

Thanks for sharing, Mike.
Looks like the suspension switched from a wishbone type suspension to a trailing-arm-type suspension (albeit very crude and unstable) when the wishbone broke?

[J R Jones]

Yes my assumption was the Corvette configuration.
Do you mean "reversed upper control arms " or reversed lower control arms? I assume, not flipped upside down, but FFR exchanges right and left? Chevrolet designed with Finite Element Analysis and specific inputs; many are now opposite of design parameters. Under those circumstances my thoughts are fabricated steel lower control arms.
jim

[J R Jones]

Putting a finer point on this application. Chevrolet designed this control arm to be as strong as necessary, and as light as possible. It is therefore heavier on the (corvette) front leg which takes the highest tension loads AND the shock loads.
By reversing the installation the lighter leg gets the highest tension load AND the spring load AND the shock load. Of course you likely have more weight bias aft and racing duty loads.
If you are mounting the shock/spring off the leg center, yes, you are also twisting the light duty leg. I would not drive it on the street like that. You need steel lower control arms.

[lance corsi]

Putting a finer point on this application. Chevrolet designed this control arm to be as strong as necessary, and as light as possible. It is therefore heavier on the (corvette) front leg which takes the highest tension loads AND the shock loads.
By reversing the installation the lighter leg gets the highest tension load AND the spring load AND the shock load. Of course you likely have more weight bias aft and racing duty loads.
If you are mounting the shock/spring off the leg center, yes, you are also twisting the light duty leg. I would not drive it on the street like that. You need steel lower control arms.

Jr, the upper corvette control arms are designed for anti-dive, wherein the caster increases upon compression. By reversing or swapping sides , the suspension becomes more straightforward, eliminating the caster increase. My C-4 suspension was the same way except the shock mount is centered on the lower a-arm, which allows even distribution of the loads. That’s the front suspension, however I’m not sure if this applies to the C-5 and newer rear elements. My C-4 rear suspension is a trailing arm, different from what I’m seeing here. In any case, sharing the load forces with both sides of the lower A-arm would be a good as could be expected with the factory a-arms. Beyond that, a fabricated chromoly steel arm would the obvious next step.

[J R Jones]

Lance, I have C4 Corvette F & R suspensions on my LS/6spd Studebaker Avanti. I fabricated the frame. Caster change in IRS can be unstable (especially the 818). I use the transverse composite leaf springs. They work well. My rear shocks mount to the hub carriers which also works well. I strive for durability in all things suspension.

[crash]

Here are some shots before it went into the car, and also once bolted up.

You can also see the 4130 fabricated uppers that I have made so that camber adjustments are much more easily made at the track. I have moved the upper shock mount for better shock angle, and had the shock been in this location when I originally built those upper arms I certainly would have made them differently and much more adjustable, but such is the process of system evolution.

As for the fabricated LCA...I could certainly go that route and redesign the entire rear suspension system, but I have four arms as spares for this setup sitting on the shelf and at this point, keeping the car running races while not recreating the wheel, so to speak, is a higher priority.

I mean let's realize what I am doing here...making the system more survivable after a huge impact partially takes out a suspension LCA. I think the system is working pretty well and a complete redesign is not really called for here at this time.

[J R Jones]

Crash, It is your budget and your responsibility. Not how I would do it, consequences go beyond a DNF.
Realize that your suspension travel stops appear to be the shock/spring. Curbs are not the only load spikes. When the suspension bottoms or unloads, your offset fabricated mount takes all the load.
jim

[lance corsi]

It also may help to keep the lower shock mount as close to the wheel hub as possible, lessening the impact on the mid line of the lca. Don’t know if it’s possible with your current setup. Btw, are the suspension components forged? I would think so, rather than just cast.

[doug_porsche]

After you get done beta testing them, will they be commercially available?

I only do 1 to 2 track days a year, and usually avoid the sleeping policemen (that is NOT where I a losing my time) so I dont need them for the racetrack, but....

I live in Denver, where the potholes are as deep as the mountains are high.

The main street near me is about to be repaved, so they just got done roto milling it. The roto milling alone improved the condition of the street!

[crash]

I do believe that the C5 and C6 LCAs are forged aluminum. All Vette suspension arms are forged as far as I know.

I am not going to produce these and I know you and I (Doug) have had phone discussions previously. You are one of the reasons I posted this thread. I know if you were asking that there are others out there, and I know my answer of, "just contact Ted" was probably not what you wanted, and now that I have redesigned what Ted had done I thought it important to put it out there that we have found that even more strength is needed if you are going to track these cars.

So what I have done there I believe really does two things.

1) Because it links both sides of the LCA together with substantial steel components it makes the entire lower control arm setup stronger and better able to deal with regular stresses that the arms see in normal usage on the GTM

2) Again because both sides of the arms are connected more solidly should there be a failure at the weakest point on the LCA during an impact or other extraordinary load the arm is much less likely to come apart into two separate pieces, and therefore the shock should stay connected to the part of the arm that is connected to the upright in a stronger fashion, and therefore cause less of a disruption in the suspension system operation.

The new part should do this without causing the LCA to become significantly stronger and less able to absorb an impact and break. In other words, I want the arm to still break as it has during an extraordinary event, yet not disrupt the handling of the car so much as it has previously. This should allow us to finish races, and because we are still not transferring the critical load to the frame, and therefore not likely bending LCA mounts, it will leave the system to be repaired as easily as replacing an LCA. No modifications have been done to the LCAs in order to use this new mount. The only thing required is the removal of the stock Corvette rubber shock mount bushing.

I hope this helps others as that is why I posted this thread.

[beeman]

Mike, do you think the LCA failure was catastrophic failure of an intact aluminum part, or rather failure through an area of fatigue micro-cracking? Might make sense to just swap the LCAs out yearly or whatever timeline, they are relatively cheap.

[lance corsi]

Crash, I’d have to agree with you on the forgings. After thinking about your situation, I would recommend a loop which straddles your driveshaft, and incorporate a lower shock mount on top of the loop. This would evenly distribute load forces to both A-arm sides equally. With your current setup, a side load looks unavoidable.

[crash]

Mike, do you think the LCA failure was catastrophic failure of an intact aluminum part, or rather failure through an area of fatigue micro-cracking? Might make sense to just swap the LCAs out yearly or whatever timeline, they are relatively cheap.

Both instances of failure were due to impacts on hard objects. This one was so significant that the driver radioed "ouch, I hit a curb and that hurt effing bad!" It seemed to drive okay at first so we didn't bring him in to evaluate, and, like I said, the car finished the race although developed a mean push. Then after the race we were looking at how it was sitting and noticed the droop of the suspension which led us to the broken LCA. Here is the order of development and consequently the order of strength of the mounts:

1) Original FFR mount. Okay for street use, but if you change shocks and do any track days, etc. then the FFR redesigned mount should be considered.

2) FFR redesigned mount. Obviously they saw what we did on the race track and decided to improve the design and strength for later kits.

3) The QRP Ted designed mount. Ted decided that an even stronger mount would be good on the race only FFR PDG GTM. His design has worked for many years and won a lot of races. In over 12 years of use this mount has never failed. It held the suspension together and the car finished both races where the lower control arm failed.

4) The My Race Shop designed mount. This is just the strongest version in this mount evolution. Is it the perfect solution to the fact that if we hit something hard with the GTM rear suspension we might break the rear lower control arm? No. But there always has to be a weakest link and the fact that the relatively cheap and plentiful Corvette C5/C6 arms are this weakest link is not a bad thing. This mount just tries to contain the pieces of the LCA when it does break under unusually severe circumstances so that we might finish races before looking at the car in the shop and making a "the customer stated" video. It is actually really incredible that we can run for 2 hours after breaking a LCA and finish races. We are still putting well better than 1 G on these suspension parts with them being broken. In my opinion, it really shows how strong the rest of the car is to NOT have other areas bent or damaged after a suspension breaking impact.

[crash]

Crash, I’d have to agree with you on the forgings. After thinking about your situation, I would recommend a loop which straddles your driveshaft, and incorporate a lower shock mount on top of the loop. This would evenly distribute load forces to both A-arm sides equally. With your current setup, a side load looks unavoidable.

Yes, the issues with that is that the upper control arm will not allow for that because of space considerations, and the shock would end up being too high to give the required suspension travel.

I did consider a push rod with a cam link and frame mounted shocks, but, again, significant change and redo, where the issue here really does not come up but once in a blue moon when a driver does something he/she is not supposed to. I think of this modification as more of a second chance safety device for this race car, as opposed to something that is absolutely needed.

[lance corsi]

Yes, the issues with that is that the upper control arm will not allow for that because of space considerations, and the shock would end up being too high to give the required suspension travel.

I did consider a push rod with a cam link and frame mounted shocks, but, again, significant change and redo, where the issue here really does not come up but once in a blue moon when a driver does something he/she is not supposed to. I think of this modification as more of a second chance safety device for this race car, as opposed to something that is absolutely needed.

I would agree that my suggestion would necessitate a redesign. Good luck with it!