Hey guys, I have a question on how to correctly measure to chop a top. I want to chop the roof on my truck about 4"-6" and after doing a lot of reading, I can't figure out the proper way to measure for the cut. The plan is to cut the A and B pillar and section the roof in the middle and stretch it both forward and sideways so that the pillars maintain their original dimensions. The B pillar seems pretty simple, mark two lines parallel to each other and cut. The A pillar on the other hand leans back on an angle and from what I've seen, you just mark the same amount parallel to the post and cut and that just doesn't seem right to me. Because the A pillar is on an angle, isn't the measurement that needs to get cut different than if it were parallel to the ground? I'm a machinist/fabricator by trade and according to my calculations, the distance that needs to get cut is actually the hypotenuse of the triangle? The 4"-6" measurement is actually on the adjacent side according to my diagram below. I may be over thinking this, but I just want to get it right. Any help would be greatly appreciated.
Interesting question.......looking forward to the replies that actually try to answer it. Depends on the profile of the roof in question. A Model A has a much more verticle "A" pillar than a '50 Merc. Ray
Welcome to the HAMB! The only body I've chopped was a '29 Ford Tudor, I had a sheetmetal shop sheer a long piece of sheetmetal the width of the cut. This gave me a piece to clamp on the body and scripe to parallel cut lines. Spray the area with layout die. Post photos of the project. Bob
Yes, you are correct. If I do, I have a feeling you guys won't help me (which is a shame because I've read a lot of good information on this site - and the reason why I posted here in the first place). Regardless of the year, make: the question is still relevant. Not my truck, but what I am trying to accomplish.
Your thought process is correct. I have never chopped a top, but I know the amount cut out differs. If you want to do the calculations, measure the angle of the A-pillar (from verticle) and use that in a trigonometry calculation. Use your triangle! The overall elevation change of the chop would be the "A" side, your A-pillar angle would be angle "X". The calculation would be: cos X = A / H or H = A / cos X PM me if you need anything else.....
I used a square. Mark your A post for the top cut. Hold the square with the one side vertical and line it up to the mark on the A post. Drop the square down the amount of the chop and make your bottom mark on the A post. Now you have your two marks, cut out what is in between. I would guess that you can figure it out with the calculations described above but I am to lazy and it makes my head hurt. Neal
There is no real need to get that complicated. Figure out how much you want to take out of it, mark it with tape and cut along the lines. Hopefully, you are not chopping a 67-72 Ford truck like in the picture as that looks horrible, but if you are, remember that those cabs (and lots of other trucks too) are taller in the front than in the rear. If so, you might need to take an extra 1/2 or maybe 3/4 out of the front. Also, hit the search button for lots of good top chopping tech. Read everything and make your own decisions. Remember, it's only metal.. Good luck and holler if you need anything. -Abone.
You might be able to tell in this pic... I wanted 4in out of the roof so I measured 4 inches out of the A, B and C pillar, marked it with tape and cut at the edge of the tape. Masking tape works bitchn for this. I cut 4 out of the posts, which is called a 4 inch chop. Yes, the actual drop is less than 4 inches, because of the pillar angle, but in the end it doesn't matter. Remember to cut the roof off first then cut off the 4 inch stubs. Trying to cut 4 inches off a roof flopping around on the floor sucks.. Good luck, -Abone. PS. Here she is done.
I usually separate & remove the outer skin on the a pillars. Then drop it the amount you want in the rear and adjust/cut the inner a. Then cut the outer skin and install after. I won't quarter a top unless I have another one for skin and roof rails. Much better to keep the chop mild and lean the post. Usually looks better too. IMHO anything that modern looks bad with more than 2" or so.
Why overthink it? on the angle of 99% of the windshields, in only 4-6 inches using all the math would it work out to more than a small fraction of an inch difference? (1/8-1/4 maybe?) Poorly planned you could be off more than that buy cutting and grinding (very poor but ive seen it) And I agree with another as stated above, quartering the roof would be my last option, that opens up a giant can of worms to screw up with and leaves you with a huge area of bodywork to be fixing, in a roof which flexes.
I've always understood that people measured the amount of the chop by the difference in window height. It doesn't matter as much in your case, but if I were to tell someone I chopped my shoebox 6" in the front (the hypotenuse of the triangle, or the length of the "A" pillar that I actually cut out) and 7.5" in the back they'd end up with a much different looking chop than what I've got. I (and I think most) consider the chop a 3.5" chop in the front and 5.5" in the rear, because that is the vertical drop of the roofline. It doesn't matter as much when talking about chopping a T, an A, or your '34 Ford. But, when talking about a '35 or newer car with a more angled "A" pillar, the theory makes all the difference.
Thanks for everyone's input. Yes, I am chopping a 67 F100 but not as much as shown in the pic. I'm looking for about 4" total. I've already installed a Crown Vic front end and plan on flipping the rear axle over the springs and doing a C notch. I know it's easier to pie cut and roll the A pillars back, but that's not what I want to do. It'll cause too many problems with the curvature of the front windshield and from the research I have done, sectioning the roof is the better way to do it. I have access to a spare roof if I need pieces to weld back in. If leaning the A pillars is a better way, please explain why. I'm not totally against it. I'm also removing the drip rails for a cleaner look along with shaving the door handles and filling in any trim/holes in the hood and fenders. I also have another front bumper and plan on removing and filling in the area for the front license plate to create a smooth line across the front of the truck. I've been a machinist, fabricator, CNC programmer for nearly 35 years and calculating the amount to cut out of the A pillar is not rocket science. It will also ensure that the side windows stay parallel to the roof line. I know it's probably not that much of a difference in length considering the small amount of the angle, but IMO it's the right way to do it. The truck just looks out of proportion IMO and bringing the top down a couple of inches will help. I've always wanted to try a chop, and I don't have a lot of $$ invested in this particular project so why not? Right now the bottom of the front bumper sits about 12" off of the floor with the PI springs. I'm going to wait to install the 5.0 engine and the rest of the front end to see how it sits. I may end up with a set of civilian springs or 1.5" drop Eaton PI springs depending on where it ends up. Some pics as of a couple days ago. I've added some sleeves to the front frame rails around the bolts and will be boxing the rails this weekend.
Here is a photo of one of the many 53-56 F-100's I've done. I'm not going to get into all that Math stuff you were going on about but will give you something to think about on the Quartering of the top itself. As you can see the A post has a good bit of lean much like your cab. On this truck and many others I've done I get a second roof. By doing so I can make only 1 seam from side to side and Not tip the top of the doors inboard. Also when it comes to the windshield you only need the bottom edge cut. No cuts on the sides or top. Sounds complicated but not really. To keep from pinching the top side to side. DO NOT CUT THE A post in the middle. Figguer out how much verticle drop you want and cut that out just below the drip rail. Now on the donner top cut that off the bottom end of A post. Cut the glass flange off the Cab post and the outside of the donner post and do a verticle weld inside and outside. By using 2 tops you can bring the front piece to the back piece and make 1 weld across it. A much easier to control weld seam. On the back side I most often take the complete window out in 1 piece about 3" away from the glass hole. I don't like the Mail slot look for the back glass. Then cut each side loose and a fill piece form top of window hole to the center cut line. Generally 1/2" to 1" depending on how much drop you do. The door top angle NEVER changes. Sounds complicated but before you start jumping all over me just go look at the cab and think about it. I've done many tops in this manner and it makes a better look over all to most. I hate a Pinched looking top and any time you can make less cuts on the windshield the better the chances of not braking the glass. The Wizzard
Leaning the posts is not necessarily better. It can be simpler IF it's a mild chop. What I see so often is tops that are quartered and pieces poorly made to fill the gaps in the inner structure. I spent my career in heavy collision. I WILL NOT section a pillar or roof side rail without a proper structural joint. Most of the time that involves sleeveing, requiring the extra parts from the doner roof. Also many guys add strips down the center, and their welding and metal finishing skills are not up to it. If you have the skills, and another top, Pist n broke explained well it in his post above.
I have to disagree with parts of this. leaning the posts only works with very mild chops on curved windshields. after that it throws the pinchweld out of alignment.
Agreed, I was referring to mild chops. Even then some alteration of the opening can be required. You just have to weigh the amount of work and desired look. Quartering a top correctly is labor intensive also, but necessary for what the OP wants to do.
3X on Pist-N-Broke's suggestion, but to the original question, and as it would apply universally to calculating material removal on other than vertical members (front or back) you can always count on the "sum of the squares" where with any triangle with one 90 degree corner, the sum of the square's of the sides equal the square of the hypotenuse. So, the two short sides squared and added together are the same as the long side squared. Level the car (or truck), grab a framing or other available square and a bubble type level. If you know, for instance, you want the front edge of the roof to drop 4" then hold the vertical edge of the square against the A pillar at 4". Level the horizontal leg of the square and see where it intersects the same line on the A pillar (down to the nearest fraction you care about). Let's just say it falls exactly at 3" for this example. Get out your slide rule (just kidding) and do the math (3 x 3)+(4 x 4)=the square of how much to cut which is (9)+(16)=25 and the square root of 25 is 5". It works no matter what numbers you use, as long as that vertical and horizontal initial measurement is accurate. If you're going to add rake to the chop then this needs to adjusted depending on where you take your measurements. In that situation a projector beam light source and some string lines will help with the first side, then you can transfer marks from actual measurements to match the other side.
It's called Pythagorean theorem and it only applies to right triangles. Regardless, I wasn't posting to ask how to calculate the dimensions. I've been a machinist/fabricator/tool and die maker for nearly 35 years and I'm quite proficient at trigonometry. The topic and question was the correct way to chop a top with angled pillars. I've seen lots of information on how other people did it with just marking the posts the same amount and that didn't seem correct to me. Granted the distance on a 10-20 angle doesn't seem like much and probably wouldn't matter on a project such as this, but considering the parts I have been machining all of these years that deviation would make a huge difference on high tolerance parts. The theorem below can be used to calculate any side of a triangle if you have any other 2 sides of a right triangle (where "a" and "b" intersect @ 90 degrees). "c" is always the hypotenuse. Sorry to get off track. I did not intend for this to be a trigonometry lesson.
BTW: to get really high tech I thought about using a laser transit to shoot a beam around the pillars on the truck (level it first of course). Mark the B pillars, measure up/down the amount of the chop, adjust the beam to this mark, shoot another one around the truck and mark the A pillars. No calculations required.
OK, So to answer this part of your Question "Because the A pillar is on an angle, isn't the measurement that needs to get cut different than if it were parallel to the ground?" Yes, you are correct. And, I've found it easier to cut 90 degrees to the actual post than level to the chassis. It does take a little fine tuning some times to get exactly what you want and a good fit. I think even with a laser you'll find some final adjustment is necessary. I understand why your thinking about all that Math stuff being your background as a machinist and all. I never got quite that far but it hasn't stopped me from turning out some pretty nice stuff. Something else I learned along the way is not to get all caught up in the inch number you decide to cut out. Go for a look not a number. I only use a tape measure to make sure of square and level side to side. I have yet to have a customer say I didn't cut exactly what they asked for and they seldom are. It's a look they are after even if they think it's a 4" Chop. The Wizzard
OK OK Go to Harbor Freight , your going to buy the heaviest sledge hammer they have to get the windshield to fit ! CUT THE WINDSHIELD FIRST !
Sorry, All the trigonometry in the world can't change the shape of the windshield ! Take a styrofoam cup, turn it upside down , cut a 1/4" high crossection out of the side. now see if you can put the top onto the bottom after you take the 1/4" out. This is what you are doing . Take a look at the truck i have in my album.
I agree with doing the math first. If you measure and cut it acording to the map, you will have far less to do than if you do it by eye. the guys that chop cars everyday can 'see' the problems as they go. The first timers not so much. I asked about chopping my car in another thread. Choprods straightened out my thinking. So much so that I will probably have him chop my top. http://www.jalopyjournal.com/forum/showthread.php?t=756248 Maybe he will see yur post and comment. He will probably convince you that Quartering the top is not the best way.
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