The Stovebolt Site Forums The Parking Lot Big Bolts extending frame and support plate

I'm at the point on my chassis that I can cut and extend as I planned. Please give as much feed back as possible. The cut would be directly behind the cab and would be 4ft.The frame will be completely boxed including the extension. The old support plates will be repaced with new. I'm concerned with the welds becoming the weak point in the chassis. I have added a small drwawing to aid my question. The support plate will also be welded to the frame with 1 inch spacters to allow water to drip away rather than rust as many have seen when they were bolted/rivited to the frame from the factory. Even with the boxing and the support plate will gussets be needed? Will the frame being welded have stress issues at the welds. Im hopeing that the rigidness will be my aid: boxed and supported plate. Pleaese respond.




http://i226.photobucket.com/albums/dd15/46cabover/boxframeandsupportplate.jpg

Hy CAJUN CRAW FISH, for those of us not familliar with your project, what kind/size of frame are we talking about?

Sorry 3B, Ihave a 1946 1.5ton with lift bed
http://i226.photobucket.com/albums/dd15/46cabover/12-15-08_1103.jpg

I like the idea of spacing the factory gusset plates away from the frame some. Might not be as strong as original, but would solve the rust issues.
Might I suggest space it only 1/4" away, use large 1/4" thick "washers" at all the original rivet locations, just use flanged frame bolts and nuts this time, or rivets again if you like. I think 1" is excessive, 1/4" should be all you need.

A proper welding job on the frame and the new gusset plates should be more that sufficient, not sure boxing the frame is necessary or even a good idea??

Grigg

Thanks grigg, and as always your opinion is very much appreciated.

I have always heard that the frame should be cut on a 45 degree angle, not straight up and down. This allows the new joint to have a longer weld, which makes it stronger.

Mike B

I've seen some frames with a step pattern at the joint.

down-over-down-over-down

Or a notch.

down-over right-down-over left-down

Hy CAJUN CRAW FISH and guys, I agree with Grigg on the spacing of your reinforcement, and I agree with Mike B and Builder on the angle or notch cut, personally I prefer the angle, one direction on one side the opposite direction on the other. I would not box the frame, it may make it too rigid, and transfer stress to an area outside the boxing. Most dump kits come with some kind of a subframe to bear the stresses of the lifting action, so boxing the frame is not required. There is a section on frame repairs in most maintenance manuals, have a look at that, there are some useful tips on welding frames and reinforcements, hope that helps.

Thanks for all the input guys. I will look into the notching a little more it sounds interesting and I like the interlocking idea of the notch weld.

I don't know why boxing the frame isn't a good idea. Making the frame more rigid makes the frame stronger. That is why frames are boxed in and crossmembers are added.

Rigid is not always stronger. The original frame is designed to flex some, and that's OK.

If you box a portion of the frame it will still flex before and after the boxed part, possibly more so. The transition from boxed to open may cause problems or grow cracks.

Give it some thought before haphazardly changing the design and operating characteristics of a frame. Personally I think it deserves a lot of thought and some structural/mechanical engineering background too.

Semi trucks don't have boxed frames and they carry lots of weight for hundreds of thousands of miles.
There must be more to a simple C-channel frame than meets the eye...

Grigg

If you don't think a truck frame flexes, watch a semi take off from a red light. I have been working on semi frames for 45 years. When I stretch a frame, I cut the web at an angle and weld it. Then I bolt a glove over it with flange bolts and possibly inside also, depending on the application. The more you weld a frame, the easier it will break a weld. If you box it in one spot, it will put more stress on other places. If you look at semi frames, they were bolted with 5/8 flange bolts until the mid 90's and then Huck bolted with 5/8 Huck bolts. Since a Huck puller to pull 5/8 grade 8 Hucks costs around $50,000, I still use flange bolts.

I've had a lot of experience with this sort of work too. A lifetime's worth to be exact. Having done, engineered, supervised and instructed others on hundreds of frame modifications I would NEVER cut a frame " directly behind the cab". Most all frame failures occur within 18" of the back of the cab so it is best to avoid cutting that area and place the cut as far behind the cab as practical.
Things to consider when doing any frame modification;
A. what are you wanting to do with the "new" frame? Simply increasing/shortening the wheelbase or adding equipment?
B. what new equipment will be added and what are it's capacity and limitations?
C. what is the GVWR of the unit as manufactured?
D. what is the new higher GVWR desired?
E. how will the truck be used?
F. special considerations involved such as off road use, cranes, winches, etc.
G. frame materials & application

With that information one may make a solid and serious evaluation of the requirements and methods. The engineering work involves meeting the RBM of the frame as designed or increasing it and knowing the section modulus of the material.

Here is where I agree with previous comments: Always install reinforcements with a 45 degree angle cut on the end. I also agree that frames can be made too rigid and need to flex. Having a mechanical and engineering background is alway helpful and especially so here since so many variables are present. Good math skills are a must as you need to calculate the RBM.

Here is where I disagree with previous comments: bolting to the flange. ALWAYS bolt to the web of the material, NEVER bolt or weld on the flange of a reinforcement area. It is quite doubtful that you will be using original frame material which may have a much higher tensile strength than the replacement material so caution must be used not to weaken it with welding or drilling. Frame flanges should never be welded anyway. Always keep the two materials tight against each other. There is no real rust concern if properly cleaned and painted. Most new truck frames never see much paint anyway. Do NOT weld the two materials together, only bolted.

The reinforcement area should never be less than 18" either side of the cut if at all possible. Reinforcement can be with angle or a full channel on the inside or the outside depending upon the design criteria. For an older truck as you describe you may be able to use an angle on the inside with the flange on top and the then bolt it to the web section with a minimum three bolt pattern at the ends. Center bolts are not needed unless the length exceeds 24". 1/4" mild steel plate (36000 lb tensile strength) can be used for the like of an old farm truck, etc. Weld should be done uphand, DC and with a rod material of suitable strength. We normally used 70 series or stronger. An old chassis may only need 50 series and MIG can be used. Leaving a gap between the materials allows for the flexing in a direction you do not want, i.e., outward. Al welding needs to be done by an experienced hand with good penetration and appearance. Grind the material to a Vee and use fillet welds with appropriate rod size so it is done in one pass on these light materials.

The reason I go into such detail is obvious to most but never to all. This is the backbone of the truck. If it fails it will have dire consequences that can involve the operators life or worse, others lives. I do not intend to scare anyone off from doing this work but like so many things it is necessary that you know what you are doing.
Ruining a frame is probably going to create an irreplaceable situation. That and the fact that none of us wants to see an old truck screwed up, only rescued and driven.

I will be adding a lifting arm with winch to lift approximately 800lbs and possibly a small generator. This will be placed behind the cab. The chassis stretch plan is 4ft. Rear axle weight capacity is 10,500lbs " specs from BRADS 41-46 CHEVY". front axle is 3,500lbs.
Im adding a 5.9L cummins gen2, approx 1300lbs. New springs being made for the weight. P30 rear axle, dually. Truck will be used to pull a boat and two car trailer often. The lift bed will be used to dumb dirt on occasion.
Winch will be installed and a generator.
New support plates will be installed and boxing the frame is the plan as the chassis was not built to handle the front engine weight or torque and the addition of a tow trailer. Please let all the experience/knowledge you have flow on this topic as I want to build once if possible and I like using what was supplied by the factory rather than scraping a chassis and putting the body on another late model chassis. If I can improve on it then I would rather sweat and get dirty.

Is your chassis long enough to move the rear axle back 4ft and then add length to the frame behind the rear axle? I see this done all the time on medium duty box trucks. It seems to me that there are less strains on the frame behind the rear axle than there are between the front and rear axles.

No, not enough space to move the axle back but thanks for the input. There is a pic of the truck in the link at the bottom of my post

Do you have a spare frame, or the back half of one?
You could cut two apart and put them back together as one longer one with only one weld joint. Make the front section of the frame as long as possible.

If you need a new section of frame made these folks can do it, and fully understand how and why, might even give you some suggestions or pointers.
I got a quote from them to form some 4" channel and some crossmembers that I'll make into a sub-frame for my goose neck hitch.
http://www.pgadams.com:80/

Grigg

Thanks Grigg I will call asap and email them at the end of this post. I do actualy have another frame of the same year and type of truck to use.

53GMCPhil's comment about sliding the axle to the rear is a good one and it was always my first choice on a straight frame truck. Old chassis all had a taper over the axle that precluded doing this. Some of the 70's IHC & F*&d chassis had straight frames and it could be done easily on them but not on medium duty GM's. Actually it can be done on tapered frames too but it involves a lot of extra work that most customers simply would not pay for. Also I only did it on non dumping applications.
Any steel service company can form plate to the channel dimension you need so no reason to leave your locale. Just be sure the radius matches the frame so they must have the proper forming dies.

Your winch requirement is so small that it will not need additional reinforcement, Crawfish. I would NOT box the frame but rather do it as I first described by an angle inside the frame. You will need to add one additional crossmember to support the midship stub hanger bearing. Build it as close as you can to the original and it will not only work but look correct as well. Good Luck, I know you can do this.

hey cajun
on my 56 it came w/ a dump and there was a flat sheet of metal about 1/4 inch thick bolted to the outside of the frame w/ cutouts where the rear leafs ride.this plate gose from the rear of the frame all the way up to about 1 ft under the cab.

My frame has the same and I plan to replace it with new to fit the length of the frame after extension.