Warning: Cracking Frame on 818R at Track

[Mulry]

Good afternoon everybody. Wanted to pass along a warning about a non-contact frame crack on an 818R. Two weekends ago I was attending the NASA races at Motorsport Ranch in Cresson, Texas to help a buddy with his ST5 Miata. Turns out that Wade McBride had his 818R out there and was racing in ST2. We got to talking and as he was showing me around his car, he said that he had a problem with a frame crack during Friday practice but a shop at the track was able to repair it overnight by sleeving the tube with solid bar and then welding it in place. I didn't get a photo of it, but here's the location (on my chassis) where Wade described the problem:

IMG_6638.jpg

Wade said that it cracked right behind the weld at that angled joint (toward the rear of the car -- looks like the photo is getting rotated 90 CCW by the forum software) and that it was the flexing of the transmission that he thinks was responsible for causing it. Wade is not running a wing on his chassis, and didn't get hit, so there is little stress at that junction other than the transmission. We are working on a solution for our chassis to reinforce that location (the bolt in that photo is an attachment point for a wing), I'll add to this post when we have the design implemented. We think we are going to gusset the triangular area so that the weld isn't in tension and also weld some tube underneath to increase the strength of the bar overall and increase the angle of (something my mechanical engineer described but I can't remember now). But I wanted to get this warning out to other 818R builders. Cheers.

Pat

[Mitch Wright]

That's for the heads up Pat, I will give my car a good once over.

[Zach34]

Thanks for the info! I added some bracketry in that corner and that's going to be the main attachment point for my wing, too, along with some other bracing. Makes sense that the transmission stress probably caused it. I suspect the stress there is increased due to the elimination of the upper engine brace (dogbone). Has anybody figured out a solution to re-implement the dogbone from the WRX?

[Mitch Wright]

Checked out and gave my R a good inspection and no issues. Question is Wade running a 6 speed and was the X bracing removed/changed if a 6 speed is installed? This inspection is added to my list, I just wonder if there is more info that needs to added to the story.

[Mulry]

Mitch, Wade did tell me that he is running the 6-speed, but I don't know (and didn't look to see) if the x-bracing was removed.

That said, we came up with a small gusset that I cut on the CNC plasma cutter, then used a dimple die on. In addition, we welded some tube stock to the underside of the trans support bars. We think that these in tandem should be sufficient to prevent an issue back there, particularly since we are also mounting the wing just aft of that location. Here are a couple of photos of the parts and the installation.

Here is one of the support bars we welded below. This first weld was pretty porous thanks to not quite getting all the powdercoat 100% off the chassis prior to welding, but it should still be effective:

IMG_6665.jpg

We cut the gusset from 11 gauge mild steel plate (because that's what we had on hand), then bent the lower angle and dimple died the hole. I could share the .stl file if anybody else wants to use this gusset design. We templated in cardboard then drew it in Fusion 360. I think the bend worked out to about 135 degrees: Here's a photo of the gussets after using the die:

IMG_6659.jpg

Here's what they look like installed and primed:

IMG_6662.jpg IMG_6663.jpg

Hope that helps. Cheers guys.

[Mitch Wright]

Thanks for the update, was the break between the lower pick up point and the rear most diagonal or behind the diagonal?. If behind I would think adding an additional diagonal to add additional support for the trans mount pad something like this.Rear frame support.jpg If around the suspension pick up point that could be another thing to consider. Understand this is shooting from the hip without spending much time studying the area or seeing where the frame break was. If Wade to post additional photo's that would be helpful and to understand if additional items are mounted in the rear that are being supported by or near the trans pad such as oil coolers, dry sump tanks.

[Mulry]

IIRC, he didn't have anything mounted back there other than the transmission. Certainly not anything very heavy like an oil tank. I don't remember seeing any coolers back there either, but I may be misremembering. The crack was in this location (this is from my chassis, not his):

re chassis break.JPG