Technical Frame flex (twist)

I’ve for a 31 Model A coupe on a boxed 1932 frame. Recently completed and now has 5200 miles on it. I see the radiator moving around a lot-then I noticed that the headlights are moving with it-then I noticed that the air cleaner (the whole engine) is moving with it. Then I looked at the front (fenderless) tires, and all that movement corresponds with bumps in the road. How much frame flex should I accept? Should I try to soften my front spring pack? Should I add a cage with forward braces into the engine compartment? What are y’all’s experiences?

 

What kind of X member is in the middle of the frame? If it is flimsy, that’s probably the problem.

 

The central crossmember is tubular and substantial. It attaches to both the top edge of the boxed frame and to the bottom edge of the boxed section. It is a Kiwi Connection brand frame. The flexing is in front of the firewall and I see it relative to the firewall. I wish I could add x-bracing forward of the firewall, but there’s that pesky engine in the way…

 

Before I'd do any spring work I'd want to know how it drives. Does it drive good? As in going thru pot holes, railroad tracks etc. Does it seem harsh and sharp suspension reactions going thru them? If so then softening the spring may help or even going to a softer shock. Letting the suspension do the work and not the frame.

If it drives really nice thru the above situations then I'd start looking at the frame and ways to maybe stiffen it. Like said above look at middle crossmember, I've even looked at ways to run supports from the middle of my front xmember back to the side rails towards and under the front of the motor in my 31' Tudor should I need to with the 440 in my car.

That's one thing about cars as they developed, designers started using the core-support, fenders etc as structure to help support the frame and control twist and flex.

We don't have that luxury with our old cars, especially going fenderless. I don't see anyway to get away from frame flex, it's something we have to allow for and expect. Controlling how much it does is another matter....

...

 

@Choppedcoupe
Is frame full boxed firewall forward ?
& double tubed on each side crossmember ?
I do not know for sure if fenders help
But I ran a Stock A frame I boxed with
Just a X 1x2 @ around middle front seat & little be-hide rear trans mount
DD & dragged with 427 , did not have /see flex
& 2 32 with 427's Same,but full box 32 frame Double tube crossmember
Fenders , No flex

 

So if your X member is anything like the one in this thread, it should be substantial enough: https://www.jalopyjournal.com/forum/threads/kiwi-konnection-california-roadster-co.943790/

As suggested above, maybe your spring or shocks are too stiff and not allowing the axle to cycle. They might be transferring the force through to the frame.

 

Up through the early '50s, roads were narrow, rough and still many unpaved; thus, frame flex was seen as a good thing and was designed in. The idea of a stiff frame on an A-bone is a 21st century idea/problem.

jack vines

 

Yes, same frame.

 

The frame twist is causing me to repeatedly break the bracket on the radiator where the firewall support rods connect.

So, if I’m gonna try softening front spring rate (to reduce frame flex), in what order should I remove leaves?

 

A photo of the frame would be a real help. What kind of steering does it have?

 

The radiator is most likely mounted on springs or rubber (cushioned) and is supposed to move somewhat in order not to fracture welds/braze of the tanks. I'd check that first...could be too soft of a mount.

 

That kiwi connection frame should have almost no flex.
Are there rod supports from the radiator tank to the firewall?
The radiator should be mounted to the cross member with springs that allow some flex, but no movement when you grip the tank and shake it.
I'd start looking for a weld crack in the center crossmember cage or the front cross member.
Was it this way from new, or has it gotten noticeably worse as you passed 5000 miles?
What's the air pressure in the front tires?

Put a small tie wrap in each front shock, down against the shock body. Let us know how far it gets pushed up after a rough ride. That would help us know how far the front suspension is moving.
Just a bunch of ideas from a Deuce long haul guy.

 

Been this way since new. Not getting worse, but I’m hating it worse-lol! Yes, I have a spring mount kit on radiator, with rubber underneath. Yes, I have support rods between radiator and firewall. I can’t put a zip tie on my shock rod because there is a cover over the rod (hooded shocks). But I see where you’re going with trying to measure the suspension travel. I wish there was a way to measure spring rate on a leaf the way ya can on a coil. I guess I could put in-hooded shocks on it in order to get an answer.

 

thousands of cars built like that, including the originals, although the front cross members of the originals had a bit more bracing in them than today’s after market ones, but were only riveted in.
But then the original chassis weren’t boxed either.

How is your radiator mounted, is it spring loaded like an original to allow for the flexing or solid mounted ?

The first and foremost concern (if it was my car), would be to check if any welds or other stress points are starting to crack in the chassis, - that will possibly tell you more about what is going on.

is there possibly something not correct within the suspension or steering geometry ?

Last suggestion - don’t drive it so hard ………..

 

I assume it’s a hi boy with no hood. Add fenders and a hood and you won’t see any movement.

 

I just climbed up on the front spreader bar and it barely moves under my 200 pounds of added weight. It’s scary as hell at 140mph!

 

Did you just get on it, or did you bounce yourself on it? Bouncing you should get a feel for the travel / stiffness, but not so or less so just standing on it.

Chris

 

Too much spring?

 

That’s what I’m entertaining. 149mph to be exact-at Bonneville on a super rough track. Headed to Arkansas Mile in 2 weeks to see how she is on concrete. Hoping to go over 150 if I can get a tailwind. Any idea which springs to remove first?

 

Not an option!

 

Sorry, I have no experience to share regarding creating a safe and smooth ride at double the highway speed limit.

 

I've lost count how many deuce chassis I built in the 40 plus years I was in the chassis business. In the center X bigger is not always better. Once I settled on a center X design I always used 1.25 120 wall DOM tube.with a short piece of 1.50 tying the bottom X tubes together which also mounted the ladder bars. I never had a complaint about frame flex.

 

if I can get it smoother at 65, it will be smoother at higher speeds too! You e given me a lot to think about and I appreciate it. I think my shackles may be binding, and it was you who opened my mind to the possibility that my front suspension is basically fixed (locked). Thank you so much.

 

Seems like your getting it figured out the reason I asked for what steering set up because if you were running cross steer with no pan hard bar you could see a lot of frame flex.

 

I wouldn’t say that I’ve got it figured out-lol. But I’m not ready to give up yet. The car has side steer with a F1 box. I have a dead perch instead of a panhard bar. The spring seemed a little softer if I loosened the big bolt that attaches the shackles to the axle and radius rods. But the sticky bind seemed to return when I tightened it back up. Perhaps that pin is free to rotate and I may have to find a way to restrain it from rotation as I tighten it? I’ll start removing spring leaves and see how it behaves.

 

Hmm how curious. I suppose it could happen. Might look at your shackle bushings to see if they are getting worn oblong?

 

I would expect standing on the front spreader bar at 140mph to be scary……..

I’m 85 kgs and just stood on mine, it didn’t move, but jiggling up and down it moved about an inch if that helps. (guessing)
It has about 40mm (1&1/2”) of travel normally.

this maybe one of those times where posting photos of suspension and chassis may help………
.

 

@Choppedcoupe
Is the spring spacer thick enough
Between spring & front crossmember ?
I am No engineer,
I feel there is alot of Flex / movement potential points in that Frame set up
Front to Rear including suspension.
Many Little adds up to alot
Especially for High speed , Like 115 or so Plus,
How much rake in Front to rear?
Play with Caster , more for High speed ,
Will be Heavy @ low speed .
How much camber in the front crossmember when in stalled ,?
If not enough for Rake & you need more caster there will get to point front spring will bind , Resistance and movement. & may require a wedge
To get the caster needed so
No twist /bind, resistance.

Suspension components
Long steer arms = Flex ,
Size of Tube , wall thickness vs Length
= Flex Tie rod , Drag bar /tube
cross steer etc.
Mounting points size & length
Same @ Rear .

My self I will not run my car up pass
115 , No cage , Full fenders No chop .
I Drag & Drive quite frequently
1/8 only for Drag , witch is 100-115 mph.
I tried one time for 1/4 , both Coupe & Sedan started moving around , around the 800 ft , Made me Uncomfortable,
So I Rolled out of pedal.. maybe air Under Fenders ?
I know whats its like to go over 200 in 1/4 ,
When I say Uncomfortable maybe
Also knowing No cages & in Pre War
Body/ frame Not Like a purpose Built
Frame / Cage design.

And Land speed 1 mile to 5 miles
@ 140 plus

 

How is your bodywork attached to the frame? Is it possible that you have some fairly soft rubber donuts, so it's the body jouncing around and not the frame and suspension so much?

 

I made 8 passes (from 123 to 149) over 2 days at Bonneville with the ECTA back in July. Drove the car 2000 miles each way. The car was actually more comfortable as I went faster, probably because it was skipping over half the bumps at that speed-lol. Imagine driving down a cobblestone street in old 1800s London-that’s what it was like. The car beat the crap out of me at 70mph on the salt and on the concrete sections of interstate-but at higher speeds it was less jarring. I just tried to be gentle (no abrupt inputs) with steering and let the car move as the wind and uneven surface moved it around. Eased into the throttle slowly (rear tires just wanted to spin) trying to have it on the floor in high gear by the half mile and held it there for the next mile to the end of the course at 1-1/2 mile. With weight transferred to the rear axle, the car wasn’t too bad, but when decelerating, the weight would transfer more onto the front suspension, and that nearly knocked all my fillings out. I’ve got coilovers on the rear axle, and zip tie “tell-tales” on the shock shafts indicate that I was using 3 inches of travel on the rear axle.