Technical Straight axle truck steering gets stiffer when brakes are applied

It's generally a very twitchy handling truck at the moment. I don't think I've specifically noticed it on bumps, but it's a handful.

 

Before you get carried away with trying things have you got access to a pair of skinny street tread tires on stock 8 lug wheels that you can bolt on and try?
The reason being is that I think you are pissing in the wind and expecting to not get wet when the issue is you are hard on the brake pedal with big high traction tires that are not going to slip on pavement or gravel. You might try it on smooth wet cement and see what happens. You might also check to see if the belt is slipping when you go to turn, they don't always squeal when they slip.

 

This makes me suspect that you have near zero caster, or negative caster.

When you put on the brakes, weight shifts to the front, causing the nose to dive a bit. This will change the caster angle. If it is already close to zero, this can make it negative. If it is already negative, this can make it difficult to steer.

When you are not under braking, this can make the steering quicker, less predictable, and less likely to self center when exiting a corner.

Check you caster angle with the vehicle on the ground. If it is not positive 3-5 degrees, or so, shim the leaf spring pads to get it there. Wedge shaped shims are cheap and easy to get. They are most commonly used to correct pinion angle, but can be used for this, too.

 

I get what you're saying, and I really have no baseline for this truck. When I got it it was a factory p/s truck which was a ram assist system that was so worn out the steering wheel had like 20° of play. I rebuilt the frontend and did the disc conversion before I put 10 miles on it. Drove it manual for a bit, then did the CPP conversion. I may be expecting too much, but there's no issue in trying to clearance it enough to ditch the 1/4" spacers. I do not have any other tires to try out it, although it needs a new set any time now...

I have a cheap bubble-gauge for caster. The truck had 1° before the p/s conversion (assuming I was using the gauge correctly). I added 3° shims when I did the p/s conversion (thick end towards the back) so it should have 4° now. I feel like it needs more, but I might also be fighting something else.

 

Thick end towards the back would increase caster. I would double-check that they are in in the correct orientation, and what your current measured caster is.

 

Ya, that would positive bias the caster, axle needs to tilt back for negative

 

maybe this will help.

 

Hmnn, I always thought the opposite, tilting axle back being negative degrees

Back to my hole...

 

Since the truck is spring-over-axle putting the thick end of the shim at the rear gave me positive caster (per the posted diagram) which I thought is what I wanted? Like a shopping cart, the wheel centerline trails behind the steering centerline (the way all motorcycles are). I haven't put the gauge on it in a couple years since I installed the shims but I'll double check what I've got.

 

Nope. Not as I understand it.

 

You are correct. I had it backwards.

Damned dyslexia.

 

I wrote it backwards. The beam axle needs to tip backwards at the top.

 

Lol, I get it. I second guessed myself a couple times putting those shims in and even got into an argument with my buddy who was helping me. It helps me to think about it like a motorcycle neck.

Sent from my moto g(6) using Tapatalk

 

What bike is that in your profile picture?

 

That is an early 60's BMW /2 modified with a singleport 1200 VW motor. It didn't do anything better than stock (start, ride, stop, etc) but I wish I still had that bike

 

Very interesting thread. Negative caster is like a shopping cart. Positive caster is like a motorcycle. Very positive caster is like a chopper. When we were kids we used to extend the forks on our bicycles to make "choppers" . If you have done this and ride the "chopper" you will understand what positive caster does- very stable at speed( easy to ride no handed) but steers harder when going slow. Also we have all seen the "death wobble" a shopping card gets when pushed fast across the parking lot- but very easy to steer in the tight store. This scrub radius is very interesting to me. I added a disc conversion to my 56 F100 and everything came out fine. I guess I never knew how lucky I was...

 

That bike is pretty cool, if OT around here, but I like it...

 

Now that you're aware of it, you'll see it all the time, it's a very common condition on hot rods where builders are unaware of it. You'll even see it on some very high end builds.

 

For the "twitchy" condition, I had something similar on mine and after re-taking measurements I found my drag link was angled just enough to give me bump steer. I made an "adjustment" to the spindle steering arm to correct that but also made my own adjustable drag link that would allow me to get the pitman arm in a vertical position when the truck was pointed straight.

 

I'll take a look at that also because the steering wheel is also not straight after the CPP install. Neither of the available positions put the pitman arm in quite the same spot as the stock box, and no one makes an off-the-shelf adjustable draglink for these. I could also maybe tweak the angle of the steering arm forward or back a bit to correct that.

 

I'm thinking I may have added closer to 3/4" of positive scrub to the truck between the wheels and the brakes. I found a pic of a stock drum and it sits below the lip on the spindle that locates it. The rotors sit at least 1/4" above that lip (will measure more exactly later) so the brakes probably added 3/8"+ to the scrub and the wheel spacers add another 1/4" for a total of maybe as much as 3/4" of positive scrub per side. I'm thinking a set of stock '94+ Dodge steel wheels (which coincidentally is what the calipers and rotors are from) might be the ticket. They're +6mm more offset than the Ford wheels I'm running and should clear the caliper without spacers (since they're the wheels that would have gone with these brakes) which should net me 1/2" less scrub per side. Just have to figure out if they clear the draglink enough and how to make them look good.

 

I have a 1965 R69S, all original, with 15,000 miles on it. I knew every owner since new.

 

If nothing else works make sure when applying the brakes you're not lifting your leg so high that your knee is resting under the steering wheel preventing it from turning.

 

I think this one is called contact patch

 

I concur the twitchy steering may be caused by the lesser caster - had that / been there and don't want to go back. Also need to realize when the scrub radius is increased by wider wheels on a straight axle you are actually trying to lift the whole front end when you turn, mostly by the wheel/tire in the direction you're turning. You also noted 10# residual valves. These are needed to keep fluid up to the caliper when the master cylinder is lower than the caliper or wheel cylinder. 2# for disc brakes and 10# for drum brakes is what I have seen. If a 10# is being used where you only need the 2# that could cause some issues as well. If your master cylinder is on the firewall you shouldn't need these as gravity will be your friend. You may need a proportioning valve to back off the rear brakes some tho. Just a few thoughts.

 

I had an issue after the disc conversion with spongy brakes. I added a 10# residual valve to the rear line to make sure the rear brakes weren't retracting too far. I've made many changes to the brakes since then including trying several different m/c's (currently running an early 70's Ford F250 1 1/4" bore), changing from a GM-style PV2 prop valve to a Wilwood adjustable, etc. The m/c has no residual pressure valves internally and I don't have any added to the front brakes, so it should be ok. I'm not confident I have the rear prop valve set 100% correctly but I've got it adjusted a little light on the rear brakes right now if anything, definitely not too heavy on the rears.

 

That's not what residual valves are for.

They are to keep the seals preloaded. Modern seals render these unnecessary.

That which keeps the shoes from retracting too much is the adjuster, via the star wheel.

In the majority of rear drums, slack is taken up via a mechanism that turns the star wheel when you apply the parking brake.

Of course, all of that needs to be in-place, functional, and in-use for this to happen.

Prior to that, you need to manually set the shoe drag, via manually by manipulating the star wheels.

If you aren't using the parking brake, you need to continually adjust your rear brakes.

Believe it or not, spongy brakes are a symptom of improperly set rear shoe drag.

 

The adjuster also tightens when you are backing up and apply the brakes. If it only relied on the use of a park brake, would not get much adjustment. In my younger days, I would adjust up the brakes by repeatedly backing and hit the brakes.

 

Speaking of "death wobble" I remember when I was young and stupid my buddy and I talked an even dumber kid into getting in a shopping cart that was left outside one night and we pulled him across the parking lot with a '67 Chevelle. It was fun and games up to about 10 or 15 mph when the cart "cartwheeled". Lucky we didn't kill him...

 

Post a photo of said mechanism, and describe how what you suggest happens.