CK5 - The Full-Size GM Off Road Community

This weekend I am planning on helping my father build a new crossmember/belly skid for his K5. We are running a doubler, so we also need to make a new t-case crossmember but the one problem we have run into is how to make it so that we dont have to solid mount the drivetrain and still run a stock style easy to find mount.

So this had me thinking, what have you guys done? and what would you change about it to make the crossmember more functional? Also what works good about it? and what are possible design considerations I should look at while making the mount?

For the cross members that I have built, I solid mounted the tranny, t-case, or both, and used YJ polyurethane spring bushings mounted to tabs on the side of the frame to give you your bushing. Basically I welded mounting plates to tube to mount the drivetrain, then used the YJ spring bushings on each side of the cross member, in the tube, then welded tabs on the inside of the frame to bolt the YJ spring bushings too. Those bushings work well because they fit perfectly in 1.5" .120" wall tube. If you think that isn't stout enough, some Chevrolet bushings mount inside 1.75" .120 wall tube. Kind of hard to explain, but I hope you get the idea.

One thing to consider would be cutting the center section out of a couple of stock crossmembers and the using the stock urethane t-case mounts. You can fab a little higher clearance end for the crossmembers where they come up to the frame but still use the center built for the stock type bushings.

I don't particularly care for the mounting system using the bushings at the frame rails, I like to have the mount stiffness matched up between the motor and t-case mounts and running the bushings really wide at the frame rail makes that mount effectively very stiff. My Doubler is mounted on a pair of bushing assemblies on roughly a 14" centerline under/beside the T-cases, then the crossmember is bolted solid to the frame rails. A side effect of this is you can build a really strong belly pan and you won't have to worry about flexing the drivetrain if you sit/bounce the truck on it.

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I don't particularly care for the mounting system using the bushings at the frame rails, I like to have the mount stiffness matched up between the motor and t-case mounts and running the bushings really wide at the frame rail makes that mount effectively very stiff.

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I totally agree, but I've grown tired of arguing it...

Well after I ripped my first three stock cross member/bushing assemblies into pieces and tore up an AA adapter, I decided it was time to do something. Mounting the bushings at the frame puts far less leverage on the mounts. It would however transfer more stress to the engine mounts. If your engine mounts are tired, they will become the weak link. I have had zero problems with this new set up, and argueably beat on my junk just as hard or harder than anyone. In my new ride, I integrated these type of mounts on the engine and transmission/t-case adapter. We will see how it holds up.

Hey Jason, I've got my skid plate off right now so I can make modifications to it for the larger capacity transmission pan. If you want to help me with the mods to the skid plate I can bring it to your place and you can look at it yourself to get some ideas. Otherwise, I can send you some pics if that would help. Let me know.

PICS!!!!!!!!!!!!!!!!!!! /forums/images/graemlins/deal.gif

I've always thought of this when designing a replacement crossmember at the tranny. Use tube, bend it in relation to the crossmember. Then for the mount, solid plate there, what would a good thickness there be? Then solid plate on the bottom, probably not complete coverage, but at least under the mount.

As for the mounts themselves, I've read that a good rule to follow is to use the same material throughout the whole drivetrain. If urethane is the choice, then tranny and motor mounts need to be urethane, and the same if solid mounts are chosen. Correct?

That's a good start if they are in the stock location.

But you also have to consider leverage (or lack of) when building custom mounts. Cross members with mounts located outboard of the stock case location are effectively stiffening the trans/case mount considerably. All other things being equal, mounts located near the center will allow the trans and case to torque as needed to stay aligned with the engine with little or no stress on the case. However, the same mounts located out by the rails will make the case mount almost rigid with respect to engine torque induced rotation. If the engine mounts (or frame) allow ANY movement of the engine relative to the frame at the cross member mounting location (not the same as at the engine mount location), then the trans and case are going to have to physically restrain that torque movement as soon as the “give” in the cross member has been exceeded. (Yes, this is over simplified but accurate enough for this discussion).

To say it a different way, with mounts in the center, you will have more angular freedom in the trans/case for a given displacement in the isolating mount. The same displacement in a mount located by the frame rails will result in dramatically less angular freedom for the same amount of displacement in the mount. Just a WAG for discussion, to provide equivalent angular freedom at the trans/case, the inboard mounts could be made of hard poly and match outboard mounts made of thick, soft rubber...

Adding to Russ (baddog),
If the x-member is hard mounted to the tranny/t-case and the bushings are at the frame there is indeed a lesser amount of give resulting in higher stress finding a way out. Basically the math plays out like this:
If the tranny needs to move 3° laterally (frame flexin') a stock mounting position will need to pivot 3° at the bushing. If the bushings are at the frame the resulting pivot could be in the range of 10 times the lateral angle (your new x-member would act like a lever arm)

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That's a good start if they are in the stock location.

But you also have to consider leverage (or lack of) when building custom mounts. Cross members with mounts located outboard of the stock case location are effectively stiffening the trans/case mount considerably. All other things being equal, mounts located near the center will allow the trans and case to torque as needed to stay aligned with the engine with little or no stress on the case. However, the same mounts located out by the rails will make the case mount almost rigid with respect to engine torque induced rotation. If the engine mounts (or frame) allow ANY movement of the engine relative to the frame at the cross member mounting location (not the same as at the engine mount location), then the trans and case are going to have to physically restrain that torque movement as soon as the “give” in the cross member has been exceeded. (Yes, this is over simplified but accurate enough for this discussion).

To say it a different way, with mounts in the center, you will have more angular freedom in the trans/case for a given displacement in the isolating mount. The same displacement in a mount located by the frame rails will result in dramatically less angular freedom for the same amount of displacement in the mount. Just a WAG for discussion, to provide equivalent angular freedom at the trans/case, the inboard mounts could be made of hard poly and match outboard mounts made of thick, soft rubber...

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Thanks Russ,

You probably just saved me from busting a bunch of parts!.....I hadn't even considered that mounts on a long "lever arm" are not as compliant as stock ones in a stock location.

I had mocked up a doubler crossmember with two traditional tranny adapter mounts (one on each side)....tucked up on top of the lower "C-channel" of the frame. It was a nice out-of-the-way spot, but in retrospect NOT a smart decision from an engineering perspective.


God, I love this forum! /forums/images/graemlins/thumb.gif

What about this...
http://www.coloradok5.com/doubler/4x4ironside.jpg

I know you guys have seen it, but it hasn't come up in this discussion. I am planning on doing something like it for mine.

there is a way to get all of the benifits of a stock mount and a super skid. You can use your current x-member and cut the middle 10 inches or so (the peice with the bushing mounts), weld that to your skid exactly where your support lands and hard mount (bolts) your skid to the frame rails. This will put the x-member right under your support (stock, doubler, atlas, doesn't matter now) and make your frame more rigid and if mounted well can be used to connect links for coils or tractions bars. The biggest benefit is you can use auto store, over-the-counter, stock bushings. I know it doesn't have the drool factor of "super tube man and bushing boy" but does have it's benefits.

Hope I am helpin' and not steppin' on toes.

p.s. As some already know I do have crass sarcasim that can be difficult to convey when written.

Blazen,

Even though it's a cool piece (4x4 Iron) it would be considered a "bolt on" solution, so it really wouldn't leave us much to discuss in here..... /forums/images/graemlins/thumb.gif /forums/images/graemlins/deal.gif




AZ,

Bushing-Boy!.....I like that. So who is who? Is Watson the super tube man?

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What about this...
http://www.coloradok5.com/doubler/4x4ironside.jpg

I know you guys have seen it, but it hasn't come up in this discussion. I am planning on doing something like it for mine.

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The only thing I don't like about the 4x4iron piece is that there isn't a crossmember support at the 203 adapter. Also, I think this crossmember only has bushing's under the adapter and not under the mounting plate; this would greatly increase the chances of breaking an adapter.

I am working on a new crossmember setup this weekend and will snap some pics of it for you.

Az-K5,

That is a bitchin idea, I really love the idea of being able to use a stock mount. The way I see it is that the General designed them that way for a reason so you might as well try to retain the stock mounts. With using this design I can push the doubler up a little higher since we need to cut the floor out as the doubler hits the floor with the stock crossmember in.

Mr. Watson suggested to my father when he purchased the doubler that he should use AR-360 steel as a skid. My dad found some AR-400 steel at the local steel supply, so we will be using that.

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Blazen,

Even though it's a cool piece (4x4 Iron) it would be considered a "bolt on" solution, so it really wouldn't leave us much to discuss in here..... /forums/images/graemlins/thumb.gif /forums/images/graemlins/deal.gif

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I agree(as the forum rules go) but it could be a good piece to focus your design on. I am not going to buy one, but I like the way it looks and with some modifications to it's design, it will work well for what I want. And I like how it looks. /forums/images/graemlins/thumb.gif

Zcarczar,
That is an idea that has been rollin around in my head a while. It is on my to do list, right after x-over. There are some other benefits of the stock mounts that I remember from an earlier diesign, the stock x-member has ovaled holes. GM was expecting our frames would move. W/O the ovals you would crack a tranny or adapter, maybe even the t-case. I am a little anal about my mounting procedure. I use poly on the springs, the cage-frame tie ins and body-mounts, but rubber on the egine and t-case only, this allows less vibrations and a place to relieve stress points in the system. The rubber wares out faster but a mount is only $11.00 or so. I should have some in process and installed pics up by Jan. I am out of school for a month now and I might as well put some of my egineering classes to good use. you mentioned AR-360 as a proper steel, what exactly is the difference, or what should I be asking for? I have only worked with mild steel, no alloys. I imagine it is a rockwell rating but I have another year of school before I get into metals.

Thanks for the interest.

oh, in case anybody was wondering I am studying for a mechanical BSE and possibly aerospace. /forums/images/graemlins/thinking.gif /forums/images/graemlins/pimp1.gif

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oh, in case anybody was wondering I am studying for a mechanical BSE and possibly aerospace. /forums/images/graemlins/thinking.gif /forums/images/graemlins/pimp1.gif

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I knew we should have called the place "Rocket Science"....

/forums/images/graemlins/cool.gif

The AR-400 is the Abrasion Resistant steel. I geuss its used in mining applications for long life and Mr. Watson said that it is very hard so that the rocks dont dig into it like typical mild steel. I searched on the internet about it and it also appears that it has very good impact resistance too. The only downside is that its really expensive and I geuss its harder to weld than mild steel..

The 400 represents the Bruhenl ratings of the metal, which is the equivalent of rockwell rating in steel I assume. Typical mild steel is around 200. Maybe Mr. Watson will chime in and give us the true reason for using it.

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One thing to consider would be cutting the center section out of a couple of stock crossmembers and the using the stock urethane t-case mounts. You can fab a little higher clearance end for the crossmembers where they come up to the frame but still use the center built for the stock type bushings.

I don't particularly care for the mounting system using the bushings at the frame rails, I like to have the mount stiffness matched up between the motor and t-case mounts and running the bushings really wide at the frame rail makes that mount effectively very stiff. My Doubler is mounted on a pair of bushing assemblies on roughly a 14" centerline under/beside the T-cases, then the crossmember is bolted solid to the frame rails. A side effect of this is you can build a really strong belly pan and you won't have to worry about flexing the drivetrain if you sit/bounce the truck on it.



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I have a picture of your skid plate in my pictures from TTC - I was with Brian when we were B-S'n with you at SEMA and meant to ask you about how you mounted it. Now I know, thanks.

I have some pics of the crossmember somewhere I think. I lost most of my build up pics from before TTC.
I used the AR360 for the hardness and impact resistance, which is what it's for. It's basically an alloy very similar to 4340 and is sold in a VERY hard form. This stuff is really tough to bend, in fact, we have to slit the bend line with a torch, bend it and then weld the corner up after the fact. But rocks just don't dig into it.

The thinnest material I was able to find was 3/16". I wanted 1/8" but I kind of like the thicker material since it is really tough. I've bounced on it pretty good and it's stayed solid. Part of that has to do with the crossmember structure too, I used 1.5 x 2 x 1/8th wall rect. tube and there's quite a bit of it.

On the 203 foot mount subject: ask Beck sometime how he feels about mounting the 203 foot but not having that mounting area tied solid to the frame. He busted a tranny adapter and the back of the TH350 when he set the buggy down on a rock on the crossmember near the front where the 203 mounts up. Now it has a very solid brace running up into the main cage structure and you can jack the vehicle up with a floor jack at that point and it doesn't flex.

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The only thing I don't like about the 4x4iron piece is that there isn't a crossmember support at the 203 adapter. Also, I think this crossmember only has bushing's under the adapter and not under the mounting plate; this would greatly increase the chances of breaking an adapter

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It has bushings under both the tranny adapter and the doubler adapter. There is not a crossmember at the 203 due to driveshaft clearance problems.

Good post. Its been on the to do list, but got moved up last weekend after teetering on my somewhat reenforced stock setup. To answer your question this is what I'm shooting for when I fab my new one up.

1. Strong enough under the tcase to take the weight of the truck dropping onto it. This is a must!

2. Use the stock transfer case mount.

3. Keep it tight to maximize ground clearance.

4. Skid plate the entire bottom, all the way up to the frame mounting points, no exposed structure to snag on rocks. The front and back angled to ramp over rocks.

5. Mount to the frame over a large area to distribute load on the frame better.

6. Still allow servicing the tcase.

A minor consideration is making it somewhat cleanable, so its not just a mud and stone bucket. As part of the project I want to reenforce the frame's lower flanges all the way back the rear spring mounts, since they are getting bent up from the rocks. Might reenforce the flanges from the front spring shackles to the rear spring mounts first and then mount the tcase crossmember/skid plate. /forums/images/graemlins/thinking.gif

Did I miss anything important?

I like your ideas. I have a thougt on the clean and service end of it. You can make a web type structure (out of 1" square or whatever you prefer) for the main part going to the center frame section, the foward most frame section, and right at the rear spring mount (the web will look like a british flag). Tie the web onto the stock bushing part, Weld on a nut or thread some steel at all six web ends (3 on both sides) and some (2-4) at the center of the whole thing. Make a skid to match the outline and drill to fit the threaded holes. Use button head allen screws or counter sink the bolts, to mount up the skid and now it is removeable and slidable. Not having a full skid on a DD (in my case at least) whill keep the un-carpeted floorboard heat down to a minimum. The ablity to remove it will allow full access for maintenance an a quick clean up with the garden hose. Making the skid separate will keep the difficult welding of AR steel to a minimum also. I am by no means telling anybody this is the only way, or the best way. I just really like this new forum and want to try and open up the possibilites of Driveway Driven Fabrication, (copywright pending on the merger of D.D.F. and C.E.E. /forums/images/graemlins/deal.gif /forums/images/graemlins/rotfl.gif, not really but some day maybe)

here is my booty fab x member . it uses stock mounts .it isnt finished yet tho either . /forums/images/graemlins/smirk.gif

PIC (http://us.f2.yahoofs.com/users/3f5a9304_7f86/bc/Unfiled+Photos/__sr_/P1010009.jpg?phBP41_AxT1RTnaf)

Can't get your link to work /forums/images/graemlins/confused.gif