Technical Straight axle truck steering gets stiffer when brakes are applied

Having a weird issue with my sweptline dodge, but being a straight axle truck I figure this is an issue that might have come up with other guys' installs. I have never been happy with the way the truck steers. I did a complete frontend rebuild right after I bought it because it was undrivable before (kingpins, tie rod ends, drag link, spring shackles and bushings, etc) and a scarebird disc conversion was done at the same time, so I'm not sure if this issue is a) normal, b) something that predates my ownership, or c) something I screwed up. The truck has a CPP power steering box, but I suspect this issue would be present with the original manual box as well.

What I realized this weekend while driving it is that when the brakes are applied the truck does not want to static-steer. Even revving it up so I've got extra pump flow the box struggles to turn the wheels while the brakes are applied, even on a somewhat loose surface like gravel. Take your foot off the brake and the truck steers easily, even if it's still completely static (not rolling). This makes sense to me somewhat because with any caster the wheels will want to rotate a little bit as they are steered, but I was surprised it was so pronounced and I was surprised it was an issue even on gravel/grass. You can hit the brakes off an on while steering and feel the steering just get tighter and hear the p/s pump relieve as the pressure spikes. Only while stopped as far as I can tell, but it's hard to gauge while rolling since it steers so easily anyway. There's no connection between the brakes and the steering as far as I can tell since everything is mounted to the spindles. Anyone ever experienced anything like this before?

 

What size are the front tires? How much offset do the wheels have?

 

What Squirrel said. If you have offset wheels, that move center of tread away from kingpin. They will have to drag around an arc instead of pivot on the tire. There is a specific term for it but, it escapes at the moment

 

What do the wheels do when you turn the steering? ie do they more or less pivot on the spot or are they dragged around in a sweeping arc - I suspect the latter either as a wheel offset matter, where Squirrel is coming from, or a track width issue that the Scarebird brake conversion might have introduced, or a bit of both maybe.

It must be pretty bad though. I have a car with the unmentionable front end that arcs the wheel somewhat but its no match for the power steering.

Chris

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The wheels are 80's F250 stock steel rims. I don't recall the tire size offhand, maybe 245-75's but the rims are 6x16's, nothing crazy, and pretty little offset (THIS link claims 13mm offset but it seems like they have a little more than that). I had to run 1/4" spacers to clear the calipers to run them, but I don't think the rotors moved the WMC much compared to the drum setup. Regardless it's nothing out of line.

Pump is a stock Saginaw, not sure the specific p/n. I had issues static steering at all when I first put the setup together so I bumped the pump pressure up to 1400 psi (was like 1150 out of the box) using a p/s pump pressure tester setup and the truck steers fine now until I hit the brakes. I don't know what pressure the box is capable of standing (CPP 400-series box), but 1400 psi should be more than enough to run it.

 

Scrub radius is the term that was alluding both you and me!! Chuck into the mix some king pin inclination, or does that just complicated matters??

Chris

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It doesn't arc them noticably. I set the fronted up using the homemade turn-plate method (a pair of greased vinyl floor tiles under each wheel) when I measured the caster and it's not like it slid the tiles apart or anything. Has about 4° of caster assuming my cheapo bubble-gauge is correct. Had less than a degree when it was a manual and I added 3° shims.

 

How are tires wearing? Any feathered edges?

 

The wheels and tires I bought used, and I haven't driven the truck more than a couple hundred miles on them, but nothing odd as far as I can tell.

What is considered "a lot" of offset? If the wheels were +1/2" and I ran a 1/4" spacer for caliper clearance is that getting into "a lot" of offset, or are you talking about several inches?

 

Here are two pics of what @squirrel is talking about.

 

Okay, it sounds like you may also have a hydroboost brake system. If it is plumbed as the last thing in the hydraulic system, you will lose much of your steering assist when the brakes are applied. You can still steer, just with less hydraulic assist. If you have to really jump on the brakes, it is better to slow the vehicle down than to give you power steering. If you lock the front wheels it won't steer anyway. This effect is more pronounced when the vehicle is setting still. It is also the correct way to plumb the system.

 

Should have noted it in the original post, but it's got vacuum-boost brakes. Only thing the pump runs is the steering box.

 

Is the front axle above, or below the leaf springs, and what front axle is it?

 

It's called "scrub radius".

 

Stock axle ('61-'71 Dodge) spring-over-axle. Stock spindles and arms, just machined to accept hub-centric rotors instead of drums. Stock springs and ride-height too. I would say looking back at the pics of when I did the machine work that the discs didn't affect the WMS location at all. Stock wheels should have been 6" wide on ~3 1/2" backspacing and the ones I've got now are 4", but with the 1/4" spacers I should be within 1/4"-1/2" of the original scrub radius.

 

It's pretty common for disc brake swaps to cause an increase in scrub radius, and that matches up to your description of the complaint.

 

Bummer. I can't put the wheels any farther in without them rubbing the draglink and caliper. I guess my only option is to find something more like a 16x5 with the same backspacing? How sensitive is scrub radius? Do I need to get back to within 1/8" of stock, 1/2", ??

 

From what I am reading it turns great at all times either rolling down the road or static unless you have the brakes locked up at a full static sitting position.

Try this = put a piece or two of tape on the tire straight up and down with the tires straight ahead. Get a helper to turn the steering wheel from full lock to full lock without pushing the brake and measure how much the tire moves/rolls when it turns. Then have them return it to center and push the brake and try it and see what goes on. I'm thinking that the tires rotate far more than you thought they did plus the extra caster over stock adds to the leverage factor a bit as you are having to lift the truck to override the caster.

I'm thinking that it is as simple as the wheel/tire brake combo you have locks the tire up and those tires have enough grip on the ground that they don't slide like most tires do in similar situations. Nothing wrong you just have to have a light touch on the brake when you are turning the wheels when static.

 

There should be no interconnection between the brake system and the steering system if one is vac boosted and the other is pump boosted. But there could be an influence.
You are standing still (static) and you clamp the brakes, then attempt to steer. The brakes are locked and so are the tire patches. So, the contact to the pavement could resist rotating in place a little bit. Perhaps enough to be noticed but not enough to inhibit steering. This will vary with lighter weight and smaller patches but it's simple physics.
Your symptoms suggest problems in the steering box.

 

For grins, what happens with the axle in the air on stands and you apply the brakes and turn the wheel?

 

The reason I ask, I wonder if a brake line might be kinking and holding one caliper closed.

 

Have you considered the effect of increased scrub radius? In such a case the wheels actually have to roll around a pivot point, not just pivot in place; the greater the scrub radius the more rolling around a pivot point is required. With brakes applied the wheels can't roll, hence the original complaint. With brakes off the wheels are free to roll and there is no steering problem. It all points to scrub radius.

 

Your illustrations help me grasp it better. I couldn't visualize that much spread in my mind.

 

CPP says their box requires a minimum of 800-1200 PSI for it to operate properly but they do not say what the max (not to exceed) pressure is. If you're using a wider tire (8” or bigger) they recommend a pump that is "high volume" because of the added drag on the steering unit. Sure sounds like the brakes are causing just enough additional drag that the system can't overcome it. Have you checked the set screw adjustment on the box to see that it's not overly tight? Have you run the test that Budget36 suggested?

 

Haven't tried it up on stands yet. Couldn't get out to the shop last night, but I'll have some time tonight for sure. I haven't messed with the box lash adjustment (and I'd prefer not to honestly) but I'll say that with the truck off the ground I can turn the whole steering by grabbing one of the tires. Seems nice and smooth, no binding. The steering issue seems the same whether I'm in the middle of the box or towards one of the extremes so I doubt it's a box lash issue but I suppose anything is possible. I've not been impressed with CPP's quality control. This was one of the boxes that they machined the hose ports too deep on and I had bad leaks until I got some adapter seats from them.

 

My money is on a wheel/spindle combo of offsets and angles that forces the tire to roll when turned. Should be easy to see that happen. Mark the floor and a spot on the tire with the tire pointing straight ahead. without moving anything turn the the steering full lock in either direction. If the tire mark is not still straight down the tire rolled when turned. With the brakes applied that would really stress the steering.

I had an early Dakota with wide tires on Centerline fakes with too much offset that did something similar.

 

Totally off the wall but could it be that something (kingpin?) is binding when the brakes are on and the forces on the axle are pushing the kingpin forward on the top and backward on the bottom? I would be curious if the problem occurs only with the wheels straight ahead or if it also does it near full lock.

 

Ok, everyone who said "scrub radius" gets a cookie. I tried jacking the wheels off the ground first and the issue went away. Then I used a level and tape to mark the tire with a vertical stripe when the wheels were straight and then cranked them hard right and it rotated probably 2" at the tire edge (~10°). With the new 11" rotors and 4 piston calipers it's no surprise it doesn't want to steer. The driver's side tire rotates CCW when I steer left, which I think means a significant amount of positive scrub radius? this is also probably contributing to the truck's twitchy steering at higher speeds. Anyone know if it should have a little positive or negative scrub radius (ideally) and how much is too much? I guess first I can try to clearance the calipers enough to get rid of the 1/4" spacers up front and see how much that helps. I had some old American Racing wheels on the truck before these that cleared the calipers even though they were also 16's. I gotta figure out where they're hitting and see if I can do anything to get rid of those spacers. Never would have thought with this mild wheel combo I would have ended up with them rotating this much during steering.

 

Wouldn't taller tires also reduce positive scrub radius?

 

Ideal scrub radius would be zero. The other thing you might not have noticed since you say you haven't driven it much- when you hit a bump or pothole with one wheel it will try to turn the wheels to that side.