Technical Frame flex (twist)

I’m interested to see if the frame is flexing and why. As I slowly progress with my speedster build with custom made C channels (1/8” thk) the frame flexes quite a bit, but the engine and trans (C59A flathead/T5) really stiffens up the front half of the chassis even with the Ford engine and trans rubber mounts. I would imagine the same is true for most any engine/trans/chassis combination.

 

It’s flexing (twisting) for sure. Im exploring the possibility of front suspension bind.

 

Thats impressive & amazing more stable @ those speeds , & bumpy .
Again I do not know if the fenders play apart in what I experienced in both body types.
Some here say vehicles have to be low center of gravity & heavy ,
Just wonder @ what speed that start's @.
Your wheel base around 106 ?
On one of Four car I built
@ around 55-65 mph Front would start to shake front end Real bad like you
describe seemed No bump needed for it to happen , this particular one was Vega
Box cross steer ,
Tried all typical adjustment I'm looking at parts , I put 10 deg of caster ,
It narrow to like right @ 63 mph.
Played with air pressure ,
3-1/2 inch wide wheels 50/50 ..
Then I seem to Notice play in Box ,
Would adjust , few 100 miles , agin @ 63 ,
After 3 time , I replaced box ,
No issue so far ,only 1,000 miles on box.
But I still will not drag over 1/8 mile .
If I ever get around to it & add a
8 to 10 point cage , front engine plate then I might go for 1/4 , should be low 9 mid 8 , with another engine .
As now I am fine with low 6's

 

When I first got to driving my car, I had a front end shake at 51 mph. Very re-producable. Went away below and above that speed, but it felt like hell right at 51. I had one front rim (steel wheel) that was out of round. Changed that-no improvement. Took it to a shop that shaves tires to true them. They took a lot off of it. It was a cheap Asian radial that was T speed rated. They also balanced it on the car (I now need to clock the wheel to the hub when I run those wheels), but it is smooth as silk now. Funny thing is, I have 2 sets of aluminum wheels, one with radials and one with Coker Firestone bias plies, and I didn’t do a thing to them and they are smooth as silk. Low 6s is no BS. I’ve just got a 4-pt roll bar to the chassis, and I’m trying to decide if I want a full cage. I’ve got one on my real race car and I’m losing that lovin’ feeling about climbing over door bars. But, I wanna do Drag Week some time, so I’m kinda torn about my safety situation. Bonneville was a bucket list trip, so I just sent it and blocked out the fear that I had when going down the course on a practice run at 70. For my next runs, I just drove as technically as I could and was as surprised as anybody when I got the timing slip. Then I went out and did it again.

 

This is a frame I did. I tested it and it was super stiff. Also cheap and easy. You don't need threaded inserts for the body bolts. A sheet metal shop bent the channels for very little money. It is easy to check the stffness. Just put it up on three supports and put a twist load on it and measure.

 

Sure enough fellas, my frame is cracking where the front crossmember attaches to the boxing plate. I disconnected the shocks and there’s a bit more movement when I bounce up and down on the spreader bar. I’m going to completely remove the shocks and go for a careful drive and report back. Looks like I’ve got some all-nighters ahead!

 

Stress concentration?

 

Gussets on the back corners of the front crossmember would be a good addition for your high stress application. More like the original Henry crossmember that had a full length horizontal lip on the back of the crossmember.

Andy’s frame is a great example of tying the center of the X member together. As one corner tries to rise, it will try to shove the opposite corner down. If the center of the X is tied on top and bottom it will keep the two sides from operating separately. It will be better in keeping the whole frame in a singular flat plane. A tube or two underneath don’t have the same strength.

 

Hard to tell but is the frame not boxed after the crossmember? If not, for sure it’s a stress concentration area.
Is this also a crack?

 

Yes, it looks like another crack where you indicated. It kinda looks like whoever put this crossmember in the frame thought that welding the back half of the crossmember to the boxing plates and not to bother welding it to the c-channel AND the boxing plates. I took the car for a drive with the shocks removed and there was still a lot of radiator shake over bumps. I’ve torn down the front spring and am working thru removing one or two leafs. I’ve been very proud of how this car has had plenty of clearance between the frame and the suspension. I may have to use up some of that clearance in softening the front suspension. Spring bind doesn’t seem to have been an issue. But, there is a lot of “stiction” before free movement in the plastic shackle bushings. I also likely had them tightened too tight. (I’m gonna get this suspension nice and slick!). But they don’t seem worn. What do you guys grease those with?

 

I'm wondering if the dead perch is a contributing factor in the cracking. Great for smooth asphalt and really great as first designed for circle track use but you basically have no spring movement on that end because there no place to go as there is on the end with the shackle. Meaning that if you set up a ruler/yardstick/tape measure between the frame and the tire on the side that the dead perch is on and stack weight of the front as the suspension goes down it is pushing the frame away from the tire and on the rough salt or rough roads it is doing that every time you hit the least bit of a bump.

 

Are the front shocks still uninstalled?

How hard are they to compress and expand with your two hands? If this is less than difficult, throw them out. You need a pair of Bilstein shocks.

Are they the SoCal shocks with the covers?

 

I don’t understand the question…?

 

I have been running Competition Engineering 3-way adjustable drag shocks. I’ve run them on the street 60/40 setting. I haven’t really been happy with them because they are so difficult to adjust, and as you’ve implied, I do t like having the hoods over the shaft. So, today I ordered some shocks from Speedway, and I hope they are just barely stiff enough to keep the tire in contact with the road-lol. I drove the car a bit tonight (even without shocks) and it is SO much better with all the front end components greased and lubricsted and no longer binding. I had assembled everything dry so that it would stay neat and clean. But, I’d rather have it greasy and smooth in the end.

 

Frames aren’t supposed to flex?

 

You guys have been super helpful. No, a frame isn’t supposed to flex so much that is breaks components and cracks itself.

After chatting with yall and doing a front suspension tear down and reassembly, it turns out that the front suspension was bound up and basically not functioning, so every imperfection in the road (remember that I drove across the rough interstate of the Midwest (Missouri, Nebraska, Colorado, Utah highways, especially punishing was I-80) at 80 to 110 mph, covering 4,000 miles over a week. I beat the tar out of this car, with the front suspension pretty much completely bound. It had to have a consequence.

Places where the binding was occurring: over -tightened shackle bolts and over-tightened shock bolts, both without any lubricant. Also, plastic Posies Superglide plastic pucks stuck to the paint on the spring leaves, not allowing them to slide or “glide.” I took everything apart and greased it and was more thoughtful about how tight I tightened parts that really need to be allowed to move. I also think that my F1 style shock mounts were contributing to a lot of stress on the frame rails and that may have contributed to some of the cracking. I reassembled the lubricated and not over-tightened (so that’s why the shackles have nylock nuts?) suspension, I took it for another test drive without any shocks and the car drove much nicer. There’s still some radiator movement, but the amplitude of the movement is much smaller, and my doors and door glass weren’t rattling and my shifter wasn’t beating me to death, and the shock mounts weren’t groaning and squeaking on every bump. I bought this car from someone else, and I really just tried to complete the build, rather than re-inventing the wheel on all the prior builders work. Some stuff was really well done (like the chop), but it seems that the front suspension needed revisiting. I think I can repair it just fine. I’ve got some (hopefully better) shock mounts on order, which I’ll gusset and weld on rather than having bolts. I’ve got some shocks (which I hope are less aggressively valved, though I may budget for some double adjustable shocks in the future, so that I can adjust them for as soft as possible). I’m also going to repair the cracks in the boxing plates and do some gusset- ting of the front crossmember to share the stresses between the c-channel of the frame and the boxing plates. I may do a little boxing in front of the cross member as well. 6 days til the AR Mile, where I hope to lay down some 150mph passes. It’s 5 hours each way and I’ll go thru a couple of industrial towns in Tennessee (avoiding Memphis’s terrible roads) and spending about an hour on I-55, which gets all the heavy truck traffic between Memphis and St Louis). I’ll be rolling 80 the whole way. I’ve got a fabricator coming by tonight to help me put my welding plan together, then I’ll tear it all back down and get to work.

 

Thanks for keeping us updated!

 

Running 150 you must have some kind of roll bar or cage. I would expect from fire wall back to be stiff as a brick. What are body mounts? & if you expect to continue running high speed; tie front frame into cage

 

If you want a smoother ride over bumps, replace the dead perch with a normal perch/shackle. All the movement/compliance from that side of the suspension is being transmitted sideways through the spring to the frame. That will make your radiator dance...

Good luck getting it all sorted out!

 

I thought the shackle nuts tighten the shackles against the shoulders of the shackle bolts...so there shouldn't be any overtightening. I've never left them "loose" enough to allow for movement.

 

in the original post (number 1) and some subsequent posts, it was easy to assume that he was dealing with a street driven car that possibly never went over the speed limit.
When reading that he drives almost 150 miles an hour, it opens up a completely different scenario.

the advice given in the @AmishMike quote above is probably the best yet for what the car is intended for………..maybe worth consideration.

 

Ford made Model "A" frames made to flex-twist it's part of the suspension.

 

Op Coupe Looks as would of been built in 62,
If Op post pic's you will be surprised