Technical Straight Axle Caster on a drag car, lets talk about the mechanics

So, I have been pondering what I should do to the Fly when it undergoes an overhaul. When I originally welded in a 35-40 Ford front crossmember, It was set up with about 4 degrees of caster.

I am using a 36 front axle with an early 30's front spring. I moved the spring on the top of the axle and used longer perch pins.

I am thinking I need to add more caster, somewhere in the neighborhood of 6-8 degrees. The question that is plaguing me is this: Do I need to remove the front crossmember and add more caster to it, allowing the spring to be in a more relaxed state? Or do I build a machined wedge to compensate for the angle of the spring?

Make sense? Think of the angle of the spring relative to the crossmember.

 

Could you use adjustable perches?
Speedway 910-33042

 

Fine tuning caster using wedges is normal for 4x4 front axles. The only problems seems to be if the wedge is made of too soft of a material (aluminum). Phil

 

Also, keep in mind this is a transverse leaf.

 

If you mount the spring ends with a single bolt that can pivot you can change it when ever you want and it never binds.

 

What I found in mocking up the x-member and spring in the '31 was if you wedge the spring to increase caster, make sure there's no interference between the spring and front of the x-member.

 

I agree with Mr T body.
A three degree wedge isn't much but if you are going to have problems that is where it will be at.
Pete

 

Sorry I don't have a better pic, but my avatar pic kinda shows the setup.

 

Is there some handling difficulty you're having that warrants a caster change ??

 

Yes, I get a death wobble on deceleration. Once the chassis unloads it shakes violently. I have a snubber on there, but I would like to just correct the issue. Trying a few different things.

 

I understand what it is your trying to get done. Also we know fixed ends and moving parts need to swing on a smooth arc. That said changing the front crossmember angle could be part of the fix. I know that isn't so easy to do. A wedge isn't a real good choice in a closed crossmember like yours, would work on a single spring perch. What I do take notice of on your photos in #8 is lots of Spring arc and shocks that let you have a lot of travel. I'm going to guess that with the wheels Up and shackles horizontal that the shock travel is bottomed out. At static load the shackles are moderatly downhill. On landing after launch they keep traveling down and maybe even further on decell. This said means your Caster is constantly changing more so with the Hair Pins at there angle. All this movement puts the Shackles under a lot of load. What I have seen in the past and may be part of an easy fix for you is to loose the stock shackles and make your own out of male and female Heim Joints for all 4 plates. Now you can adjust the length of the front plate and the back plate to add Caster without twisting the spring and the Axle can travel front to back a bit as the Hair Pins push and pull. Noting needed at the crossmember itself.
The Wizzard

 

Shock travel really isn't an issue. We built a set of friction type shocks (lever arms with a spring and leather) that basically have a lot of range of motion. Tube shocks would probably help dampen some of the oscillation. I am planning a redesign on the front spring. It wasn't designed with this type of launch in mind. That part came as a surprise. Thanks for the input. Good stuff.

 

Roothawg, while I have never dealt with death wobble on a race car, I have fought it since the beginning of my career on numerous vehicles. It is a phenomen that manifests it’s self on straight axles with a straight tie rod.
This has been discussed at length on other posts with a lot of different opinions. Sometimes even with everything “ right”, it will still show up upon occasion.
I have cured death wobble many times by changing castor. Even though it may show perfect by specs, a peticular vehicle, may need to increase or decrease the angle a bit to eliminate the problem. Of course this is assuming that all parts are tight and the tires are balanced and round.
The angle changes somewhat due to acceleration (frame going up in relation to axle) and de acceleration ( frame going closer to axle). Since yours is only doing the death wobble on de acceleration and not on acceleration, you should just have to adjust your angle slightly.


Just my experiences.



Bones

 

I have been told by the old timers 12 degrees on the front axel, it will go straight and still can turn it at the end of the track. so that's where I set mine. hasn't been down a track yet. just sayin.

 

I remember when they used to push start the top fuel cars. When they turned around on the track there was so much caster on them that the front wheels seemed to fall over, not turn.

 

Looking at the position of your steering arm rod, it looks like the car will steer to the right when you hit the throttle and to the left when you let off. What I am describing is bump steer not death wobble. Bump steer MIGHT initiate death wobble or it may have nothing to do with what you have going on.

Try 7 or 8 degrees and see what it does. A small wedge should add the 3 degrees you need and not drag the front of the spring on the crossmember.

In my experience 90% of death wobble comes from some unknown slop in the drag link/steering rod/steering box, the other 10% is caster.

Good luck, -Abone.

 

Not only will you need to shim the spring, but you will need to lower the hairpin mount, unless you have clevices with enough adjustment on them. It will be good to see the Fly again, its been a long time.

 

Had a friend running a 36 that ran 10.20`s that was doing the same thing. Had him bolt on a steering stabilizer and cured it immediately.

Everything on his was new, bearings, rotors, tie rods etc and running 8.5° caster. So I knew the front was in good condition so that's why I had him try the stabilizer. Costs him 45.00 & an hour of his time.

 

Why do you say that Jeff? See something obvious?

I actually made the original radius rods non-adjustable, I plan on adding some adjustment on the top tube.

I had one on it, but I wanted to try and leave it off the next rendition. It did cure the effects, but it probably just masked the original problem.

 

In cases like this I honestly believe that it's a combination of things, natural wear, natural play in front end parts, steering gear and steering column components so that it's hard to find one cause. But I believe the biggest issue is weight vs. Load.

These cars don't weigh a lot on the nose so you don't have that mass forcing the tires into the pavement causing that needed friction to force the tires back against the natural play. Once the nose comes down, the tires oscillate and not enough weight/friction to stop or to momentarily break the pattern to stop it and that's why I think the stabilizer works and works well doing it.

 

I kinda wonder if this rod running uphill has any effect on it? It’s adjustable, but it will reduce the turning speed.
Here are a few more pics. It’s in a sad state.
Here’s the story on the steering box. I built the Fly in 6 weeks. I told Ryan if he would host a race, I would build a car for it, so it was a whirlwind.

The steering box was a swap meet find from my buddy Mowtoomuch. We fabbed everything and when we finally got around to installing the steering wheel a week before the first HAMB drags, we found out it was a boat box. Yep, steered backwards. So we designed a bellcrank to reverse the motion. I probably need to spend some time and see if I can rebuild the box to remove any play. the main leaf is too long also. I need to get another one made. Of course, it used to have a lot more arch in it.

I misspoke earlier, it has almost 7 degrees of caster in it.

 

Kind of hard to tell in the picture, but it looks like the steering rod is pretty close to horizontal to the planet and close to parallel to the hairpins. If that is the case, when the front axle goes up and down through its arc using the rear hairpin mount as a fulcrum, it will push the steering rod forwards and backwards. That motion actually turns the pittman arm and steering wheel causing the car to want to head towards the guardrail.

BUT, your front end might be wrong in the picture and correct in real life. And, even if it is giving you bump steer, that may not have anything to do with your death wobble.

Do you have a profile shot ?

-Abone.

 

You hole shotted me on the pictures, I forgot you had that crazy steering set up. Here two cheap things to try:

1) Easiest and free. Lock down the steering wheel so that it cannot move at all, not even a little bit, then jack the front end up with jackstands on the axle beam itself and wiggle the shit out of the front tires. Wiggle the drivers and passengers separately to the left and right. If there is any movement at all, it can induce death wobble. You have 2 rod ends on your tie rod end, 4 rod ends on your steer linkage and a big trapeze bracket. The chances of some unwanted movement are pretty good.

2) Free, but not as easy. Remove the leaf spring from the car completely, cycle the front axle from top to bottom and check for any clockwise of counterclockwise movement in the steering wheel.

Again, IF you have bumpsteer, it may or may not have anything todo with your death wobble.

-Abone.

 

Gotcha. That’s a good winter day project.

 

Are those Rubber bushings in the Axle ends of the hair pins as well as a rubber sleeve mounting the Steering gear in the Rod end at the gear box?
The Wizzard

 

Just throwing this in being your hunting. Is the bottom Bat Wing eye machined for a Flat nut or counter sunk for a Lug Nut style nut? It just looks suspect to me along with the flat washers for shims between the Axle and top Bat Wing eye.
The Wizzard

 

The rod ends are poly. At the gearbox that is splined, no rubber there. The bat wings have tapered pins in them. I used the long perch bolts that normally are used if you want to add a shock mount below.

I probably should machine a spacer to replace the washers under the batwing. It’s a 36 axle, which is narrower at the pin perches,compared to the 37-41 axle. That might actually be a good place to start. I built the Fly in 2002, so I forgot those washers were even in there!

 

What looks like rubber at the steering box is in this photo between the Rod cap and the boss the sector shaft comes through.

 

Oh, that is just an insulator we made to keep from transmitting the vibration through the steering box.

They make spacers, so no need to tool up to fab my own.
https://www.speedwaymotors.com/2-Inch-Axle-Boss-to-2-1-4-Inch-Bracket-Spacers,31781.html

 

I'd run wheeley bars in the back to keep the front end from going up no more than a inch to reduce the sudden shock to the front suspension when it comes down.