So i am not sure how far the frame should flex. Seeing it is suspended at each end by the springs and if you jump up and down upon the center it seems to flex more than i was expecting? This makes me want to reinforce it somehow but do not want to if is it unnecessary. How much flex is normal?
Less than this:
Unless it's been seriously eroded by rust, the stock frame should be fine.
Kep, the frame will twist alarmingly, but it is built to withstand this and work with it. The suspension adds to this effect. If you mean up and down flexing in the chassis rails, I would suspect there should not be much at all.
Hope this helps.
Allan from down under.
Up and down movement. The body had rub mark from where the tops of the doors contacted the body. So how thick were the first 1909-10 frames? i hear they had to rivet plates the length of them to stiffen them up.
Most bodies develop the sagging door syndrome you have experienced. It is largely due to sagging in the frame rails over a long period of time. This is why it is essential to straighten a frame before restoration.
An interim fix can be achieved by inserting washers between the body mounts and the body to jack the body up. Other body makers solved the problem by removing the top corner of the door panel so it couldn't drag on the pillar. My Holden bodied touter is like this.
Hope this helps.
Allan rom down under.
Frames are often permanently bent downwards by the rear engine mounts. When rebuilding you'll have to straighten it. The sill plates were put on the first 2,500 frames, after that the frame material was made thicker and no substantial design change had to be made until the end of model T production when bodies got heavier so deformation of frames became more of an issue - the frames were made out of even thicker material by mid 1926 production:
"The engineering department had been running some tests. It seems that the bodies on the Improved Fords were much heavier than those on the earlier models and they found that the frames with the standard thickness of steel tended to sag in the middle. The first plan was to put in a reinforcing plate in the side members, similar to that used on the very early 1909 models. By February 28 there was a note to the Midland Steel Products Parish and Bingham division to stop shipping frames until the rails could be made of "Type L" steel with a thickness of .180 to .200 inches. The Ford engineers suggested using the same gauge stock as was used on the ton-truck frames."
You might notice a twisting when you go over bumps in a dirt road. The seat back feels like it is moving independently from the floor. This is normal.
Attempts to re-inforce the frame could cause more damage than it would prevent. If you stiffen one part, you would transfer the torsion to another part so that instead of being distributed along the length of the frame it would be concentrated in a smaller area which would cause metal fatigue.
Best thing to do would be at the time of restoration and the body is removed from the frame, you measure diagonally and from front to back to see if the frame is level and straight in each direction. Then bend to get as straight as you can. When you re-install the body, start with the radiator and firewall. Get the hood to fit by shiming or sanding down body blocks as necessary. After you get the hood fitted, work on the front doors, then the rear doors. Note, sagging doors can be either or both worn or bent hinges, or sagging door posts.
Twisting along the length from rt. to lft. must have been substantial. Minimal from frnt to rear. Just guessing because of the suspension. As stated it shouldn't have a dip or bend along the length of the rails frnt to rear.
This would be considered a bit extreme length wise although many here would be willing to give it a shot.
There's really no answer to your question, at least as asked. However, as others have stated, a stock frame in good condition will be all you need. It seems to have worked well for all the other T's, (except those early ones).
As to your doors showing rub marks, the body does at least as much flexing as the frame and the doors also have their own degree of freedom. It's all the "nature of the beast".
Remember, what doesn't bend, must break.
Stan and Ollie were sure hard on these poor cars for their comedies.
There is a way to measure how much is normal, The weight to bow the center downward a millimeter with quantity of weight over the center would be a measurement.
Unless your frame is seriously weakened by rust the weight needed to bow it down a certain amount would be about the same as all frames made from early 1909 until the spring of 1926. (The C- sections of the beams must also hold their shape to keep the strength.)
Are the rivets tight in the front and rear crossmembers? Loose rivets there would reduce torsional stiffness.