Simple trig for frame cuts, etc.

Okay...this ain't math cl***, it's a traditional hot rod forum, right? So, we'll keep it real simple.

If you've ever sat and scratched your head trying to figure out a step notch or frame Z cut, hopefully this will help. We'll use the example of cutting the frame at the firewall and raising it to clear the front axle. Keep in mind what we're really doing is creating a triangle between the old frame rail location and the new raised rail.

All we need to know to get the exact cut angle is distance (along the frame) between the first cut and the second, and the height we want to raise the rails. Let's say we want the first cut right at the firewall, and the second cut 30" from the firewall. The amount we're going to raise the front of the rails will be 5". What we have is a triangle that is 30" along one side (the bottom of the frame) and 5" along another side (the rise). The third side of the triangle is the path the new frame rail will follow when we're done. We'll figure out that length a little later.

Yes, we can draw all this out on the garage floor and measure everything directly; nothing wrong with that, but once you've done this a few times, you'll realize how much faster it is, and it works right every time.

The above photo shows a triangle with sides labeled "adj" (adjacent), "opp" (opposite), and "hyp" (hypotenuse, or longest side). The adjacent side would be the distance along the frame, and the opposite side would be the amount of rise in the new frame. We are looking for the angle marked in the drawing, which is the same angle we will cut out of the frame.

Time to dip our toe into trigonometry. The function of a triangle that we are going to use is called tangent or "tan". Tan is the opposite side divided by the adjacent side (tan = opp/adj). In our case, tan = 5"/30".

Time to fire up the Windows calculator. It's buried under accessories in the "all programs" menu. If you go to "view" and click "scientific", you'll get everything lit up that we're gonna need. IF you're got a real scientific calculator, more power to ya! First divide 5 by 30. Our answer is .166666666blahblahblah. Now, that is the ANSWER for the tangent. What we want is the ANGLE that number is the tangent for. So click on "Inv" on the calculator (to reverse the operation) and then the "tan" ****on. We get 9.46 or about 9 1/2 degrees of cut.

Now, back to the garage and get out the protractor. We want to cut half the angle on each side so everything fits back together right. Remember, a straight cut is 90deg to the frame rail, so we are going to set the protractor at 1/2 the angle off 90, or just over 85 deg. Use the protractor to mark a "V" on the frame centered at both cut locations, and break out the sawzall. Don't forget that the V at the front will be upside down. If you've measured and cut accurately, the cuts should close up and raise the front of the frame 5".

For any major cuts like a step notch, you're going to need to know the length of the replacement piece to make the transition. Back to the math... Square each of the short sides of the triangle and add those #s together. In our case 5X5=25 and 30X30=900, added together gives us 925. Now we take the square root of 925 (calculator again) which comes out to 30.41". More cut angle means more change in length of the piece needed to make the transition. In our case, the original rail section would probably work OK, but much more angle is going to require an extension or a whole new piece.

I could go on and on about how many uses this stuff has, but that's the biggie for this crowd. Try it a few times and I think you'll agree it's a pretty easy way to figure out these types of cuts.

 

cool....had to read it twice and will again when i need to....knowledge is power, thanks

 

Its the litte things that make life easier.............Thanks

 

Finally, a piece of trig that is actually usefull! Thanks.

 

That's kinda like saying, "Finally, a piece of my right arm that is actually useful." LOL

 

That infamous indian soh-cah-toa taught me everything I ever remembered about trig.

Sine = Opposite / Hypotenuse

Cosine = Adjacent / Hypotenuse

Tangent = Opposite / Adjacent

 

Thanks for taking the time to post that, but it doesn't take the place of a template that allows you to check the LOOK of a frame cut or Z section.

 

I lke the chalk on the floor better!!!!!!!

 

Math, you use it everyday...

 

I certainly agree

 

It's pretty awsome when you use the trig to figure cuts and everything fits when it's mocked up. Saves a lot of time, and can be done outside of the garage. Good tip!

 

is there a way to put this in a attachment so i can download it and put it in my files PLease?? This is very good information

 

Can always cut and paste in a word or notepad file.