Accident Where Integrity of CMC (cab mount chop) Questioned?

"I would avoid a CMC if at all possible. It’s cheap and simple to have done so people seem to not think it’s a big deal, that mount is designed the way it is for a reason and I would not want to get in an accident with a CMC."

The above quote came from a recent thread and suggests that the CMC (aka: body mount chop) commonly performed during a mild lift/tire install might fail prematurely or unnecessarily in an impact accident. I disagree but, but think it's an interesting question. I'm curious if anyone has been in an accident either involving another vehicle or crashed into something solid where the CMC was a focus in assessment of damage. Particularly, either questioned by insurance regarding a claim, deformed unexpectedly, or was broken from the frame? Tying to see if there is any real liability in terms of insurance claims or significant risk to safety. Thanks!

 

What exactly are the credentials of the individual for whom this quote has been taken?

 

This question has been posed before many times on this board so let me see if I can summarize this correctly and efficiently:

Many folks who have done the CMC mod who have gotten into a wreck shortly thereafter or in any period of time after having it performed have never had that "structural repair" (because that is what it is classified as) fail or had it questioned when done correctly. Structural repairs are done on vehicles with body on frame style vehicles all the time and when done right it is stronger and much better than the areas surrounding it. Most insurance companies would have to have it specifically pointed out to them that the area was modified or "repaired" and even then the area would have to be highly questionable in integrity in order for them to deny you a claim.

In short, don't be afraid to do it from a safety standpoint. Make sure that whoever does it for you is a competent metal tech and if you ever get into a wreck, don't make it a point to go out of your way and highlight to an adjuster that you modified it and you will have no issues or delays in getting a claim approved.

 

The quote was me, I'm not a mechanical engineer but I did study it for a few years before changing majors and am a software engineer. So I guess you could call me semi-credentialed?

Anyway, the point I'm trying to make actually does hit common ground between any engineering discipline. Any complex engineering project, whether it's a piece of software, a bridge or a truck is a system built up of many interrelated parts. One thing I've learned actually working professionally as an engineer is that a lot of seemingly arbitrary or annoying design choices have a good reason behind them(not saying that they all do. There's a lot of poor engineering work out there). Changing one part even for the better(in this instance if we assume a well done CMC actually makes that particular mount stronger) can have unforseen consequences with others. When I say I would worry about it in an accident I don't mean like a mild or moderate one, I mean like a potentially deadly one. That cab mount might be designed to deform in a certain way in a collision to interact with the greater crash structure, making it weaker is obviously bad but making it stronger could also cause a problem in some probably very specific scenarios.

Now, I'm not saying that it's a huge risk. Hell my truck is lifted and I'm about to put sliders on it, I know these things also could have effects like that in particular circumstances. But that's the only way to achieve what I'm trying to achieve and I can live with the risk. If you're trying to fit 35s then yeah, chop away(well, have a good fabricator chop away) because that's the only way to achieve the performance you're after.

However if the mission is to fit 33s without rubbing under a 2-3" lift, this can be done the proper wheel offset and adjustable UCAs set up by someone who knows how to do an off road alignment. I think it's ridiculous to jump straight to chopping cab mounts when for just a little more money you can do it properly, not risk the cab mount and wind up with a truck that drives way better.

 

Writing as a retired engineer, software programmer and performance analyst. Most “things” (systems) are over engineered and take multiple failures for the system to collapse. Perhaps people are over thinking the issue.

 

Overthinking things is my job!

I don't mean "never do a CMC", I just think that people are way too fast to recommend it to fit 33s.

 

Over 15 years body shop experience. My favorite/most difficult customers were engineers. They always had a better way to do something.

I believe the cab mount concern would only come into play if a collision hits right on the tire. Hard enough for it to be pushed rearward into the cab. With a normal cab mount there is plenty of steel there to protect from cab intrusion. Without it, more likely for wheel/tire to enter the cab in an extreme case. One would have to be very unlucky for it to be an issue.

 

Truthfully if the entire suspension system on either side where the CMC was done is coming through the cab then you're most likely winding up in the hospital due to the severity of the collision as a whole. CMC or not, that tire was going to come through even in the OE form, lol. Forget the CMC. The truck is totaled without question and your health as a whole is now the paramount and only most important factor in these scenarios.

Why do I feel like literally every comment here, including my own has been echoed in other threads before? I'm going to go wash my hands of this now and go away because these threads are similar to the "what oil is best and should I run" kinds on here.

 

But what oil is best? And can I run it 10k miles between changes?


Seriously though, as a fellow enginear I enjoy this conversation. Engeneer? Enginere?

 

Engi-nerd!

 

For sure, the cab mount location is at a point where it will only accept stock, or slightly larger tires. In my opinion, the cab mount chop is far less of a concern compared to adding larger size tires. Adding larger size tires will greatly increase the chances of having an accident, thus, resulting in more injuries.

 

^^^^^^^^^^ THIS ^^^^^^^^^^



Also.... VERY OFTEN things are engineered to, simply, be less expensive to produce. That cab mount may not even be "designed" any other way than it is the fastest, most efficient, way to do it on the assembly line.

 

Who the hell is going to know you did a cmc? Unless you had a hack do it and the welds look like snot. This forum cracks me up.

 

I find it pretty hilarious that we are on a forum that the vast majority of us have a lift, and oversized tires... some have body lifts. There are those who have relocated complete assemblies of their suspensions (Solid axle swap, Linked rear ends) done Long Travel, added disc rear breaks. ALL of these have SIGNIFICANTLY CHANGED the "design" of the vehicle.

On top of that, we have those who have pushed the pay load up to, and past, capacity with racks, AGM batteries, skid plates, sliders, roof top tents, skottles, and maybe even blow up sex dolls.

And here we have pencil necks discussing the actuarial dilemma of cutting a 5 inch, useless, corner of their wheel well and welding back a plate that is most likely stronger and better than the original piece that came off.

AND THEN... we have those who want to be so OCD about it, they raise the concern that it being stronger, may be bad for the way it was engineered. News flash... they don't make crumple zones that allow the tire to be pushed back in to the passenger compartment foot wells and brake the passenger's legs... I think we're good there.

 

How do you know if someone's an engineer? Don't worry they'll tell you But seriously I think engineers are smart and appreciate their attention to detail. I'm OCD like that lol.

 

Ok it all started because I said that people are way too hasty to recommend CMCs to fit wide 33s with a lift, and @Dawelda thoughtfully made a new thread instead of hijacking that one.

If 285/75r16s on stock TRDOR rims with a 2 or 3" lift are rubbing on the CMC it's because the caster isn't set right. All I was saying is people are a too quick about recommending the CMC when they don't even understand what the real problem is.

Nothing wrong with elaborate builds and cutting or relocating stuff to achieve a goal and build something awesome. That's worthwhile. I don't think taking on possible risks from cutting structural pieces is worthwhile when you could stop the rubbing and make the truck drive way better by fixing the actual problem which is the fact that putting on a lift and bigger tires effectively decreases your caster angle which pulls the tire towards the back of the wheel well. Correcting the caster angle fixes the problem.

 

Sex dolls. That's what the group should collectively take away from this here. I think some of you in here may need to take that away here more than others.

 

Your pseudoexplanation makes some incorrect assumptions/guesses about the role of the cab mount in a collision. The mount is NOT a crumple zone designed to deform in a crash, it is meant to remain rigid to keep the wheel from pushing into the driver and passenger foot wells.

Shaving an inch off the mount and boxing back in with plate isn't going to drastically alter this ability of the mount. If anything it probably comes out stronger alot of the time as you mentioned because people tend to go way overkill and box it with much thicker plate than the frame was originally made of. I'd wager most people on TW arguing that CMCs are a safety liability have never actually seen one in person. Even a fairly aggressive one leaves most of the mount in place.

I did mine after about a decade on TW and reading threads like this for years. I remember looking at it when I was done and thinking "really that's what all this drama was about?"

Where you can start to debate the safety concerns are with cab mount relocation because at that point you've completely removed the stock cab mount and moved it further back where it's effectiveness at blocking a wheel is more debatable. But that's a different discussion and at that point people are pretty heavily modified and trying to stuff 35s or 37s with a good idea of what they're in for.

 

In a world that exists in shades of gray it's amazing how often engineers think they have the definitive answer.

Signed,
A scientist

 

I didn't say it but I will tell you a CMC, in a specific incident, could be detrimental to you and your passenger's safety.
The specific condition?
https://www.youtube.com/embed/eouRDQ417mU

Small offset crash. Notice how, as the vehicle continues forward, the wheel gets deflected downward and away from the cabin floor. The front cab mount does this. Making it shorter or heating the ultra high strength steel could cause it to weaken and allow the wheel to come into the cab through the floor.

My credentials? I understand physics.