Truck doesn't want to steer while brakes are applied

[JLeather]

I noticed something weird the other day about my '67. It's got a CPP box and a little extra caster (~4° if I'm measuring it correctly). I've not had an entirely positive experience with the CPP steering, but I've gotten it to where I'm ok. The truck drives well, and I definitely prefer having the power assist. I've got the pump shimmed up to ~1400 psi and nothing has leaked for months (knock on wood). What I realized this weekend while driving it is that when the brakes are applied the truck does not want to 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 (you can hear the pump relieve almost immediately when steering while the brakes are applied). 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. Anyone else notice this either with the CPP box or the stock manual steering? Is there anything else at all that could cause the steering to me so much more difficult with just the brakes on/off? I wonder if pulling a little caster back out would make the truck steer easier? I hesitate to do this as it's still more nervous than I like at highway speeds and I was actually thinking of bumping it up a few more degrees.

[Wildergarten]

I noticed something weird the other day about my '67. It's got a CPP box and a little extra caster (~4° if I'm measuring it correctly). I've not had an entirely positive experience with the CPP steering, but I've gotten it to where I'm ok. The truck drives well, and I definitely prefer having the power assist. I've got the pump shimmed up to ~1400 psi and nothing has leaked for months (knock on wood). What I realized this weekend while driving it is that when the brakes are applied the truck does not want to 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 (you can hear the pump relieve almost immediately when steering while the brakes are applied). 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. Anyone else notice this either with the CPP box or the stock manual steering? Is there anything else at all that could cause the steering to me so much more difficult with just the brakes on/off? I wonder if pulling a little caster back out would make the truck steer easier? I hesitate to do this as it's still more nervous than I like at highway speeds and I was actually thinking of bumping it up a few more degrees.

Does the truck have a hydro-boost for the brakes?

[JLeather]

Does the truck have a hydro-boost for the brakes?

No, the brakes are vacuum assist. The power steering is a closed loop between just the pump and the box. Only thing connecting the steering and the brakes (as far as I can figure) is the tires...

[my5thmopar]

I don't have that issue with my CPP. I do have a Sid's and wide tires. Stock big block PS pump. I don't know what the shims are, they came with the kit. I really don't have any ideas What do you mean about being nervous? I can run 70+ without issue. Craig

[Series1Utiline]

Sounds like your pump may be on the edge of not having enough pressure. What pump are you using and how do you know you have 1400 psi?

I adapted a GM Type II pump for my CPP set-up and used a 6" aftermarket pulley. I had too much pressure initially and had to change out the spring in the pump to lower the pressure. All is fine now.

[JLeather]

I don't have that issue with my CPP. I do have a Sid's and wide tires. Stock big block PS pump. I don't know what the shims are, they came with the kit. I really don't have any ideas What do you mean about being nervous? I can run 70+ without issue. Craig

At higher speeds the steering is very light/sensitive. I can take my hands off at 70 mph and it does track straight, but it's just very sensitive to input at that speed. This is something I suspect more caster would improve. There's little to no play in the system between the faster CPP box and the all new frontend (kingpins, tie rod ends, drag link, spring bushings, etc).

I assume along with your dropped axle you got a shorter pitman arm for the CPP box? Mine is a stock-length 7 1/2". A shorter arm would improve the boxes mechanical advantage if this is really just a case of being on the edge of not strong enough.

Sounds like your pump may be on the edge of not having enough pressure. What pump are you using and how do you know you have 1400 psi?

I adapted a GM Type II pump for my CPP set-up and used a 6" aftermarket pulley. I had too much pressure initially and had to change out the spring in the pump to lower the pressure. All is fine now.

The pump is a standard Saginaw that I bought from the vendor that I cannot type into the forum. I had issues with any static-steering at first with the system and went through a second pump, but it didn't improve anything. In the course of diagnosing the install I bought a MAC-brand p/s pressure tester (the kind you plumb into the system and can temporarily dead-head the pump) and found my box-stock pump was making about 1150 psi. I incrementally removed shims in the pressure spool until it was up to about 1400 psi and my static steering improved, although at the time I never noticed a relationship between applying the brakes and the static steer effort as I was usually steering from outside the truck in the shop. I have an aftermarket March pulley that is about 5 1/4", similar to a typical stock pulley, which was the only one I could find with the offset necessary to match the existing 2-row engine pulley.

The frontend rebuild and scarebird disc conversion all happened at the same time. I only put a few miles on the truck before doing that work as it was essentially undrivable when I bought it (worn out stock p/s setup caused like 15° of steering wheel play and the engine ran poorly on top of that). I will say that I have never been happy with how hard this truck is to steer. I originally attributed it to the worn out frontend and my not having owned an old manual steering truck for years, but now I'm not so sure. I daily-drove a '66 Chevy pickup for years and never thought twice about the stock manual steering (I know that's a totally different animal with ball joints and all, but it's not like I lack the arm strength for a manual truck...).

[JLeather]

Related question, does anyone know what the stock front axle width (WMC-to-WMC) would be for a stock '67 2800 lb axle with stock brakes? I'm wondering if the scarebird setup changes the wheel offset any significant amount. Also anyone know what the stock rim width and backspacing is for a '67 D200?

[Hobcobble]

Sweptlines equipped with Power Assisted Steering from the factory had shims
installed on the axle. Did your truck originally come with P.A. Steering or was
it manual? If it was manual, did you add shims to the axle?

John

[JLeather]

This is (was) a factory p/s truck. When I got it the factory power assist was still in place, but was badly worn out and could not be repaired. Strangely, though, the truck did not have any axle shims in place and when I initially measured the caster after reassembling it as a manual-steering truck it had about 1° which seems correct for manual steering. I assume someone over the years had possibly replaced the springs or u-bolts and left out the original shims, or maybe the power assist was a dealer add-on and they never included the shims? Either way I have since installed 3° shims for a total of 4° of caster, however it didn't make much of a tangible difference to me when I did so. I will also confess the caster measurements have been made using a cheap bubble-level gauge from Amazon and some homemade turn-plates, but I've used it on several cars and it seems to work ok.

[Wildergarten]

Sweptlines equipped with Power Assisted Steering from the factory had shims installed on the axle.

You probably already know this but just to be clear: those shims were intended to increase steering effort. The reasons are: (1) for self-centering to work and (2) to increase the "feel" when steering such that the driver receives tactile feedback from the wheel. Therefore more counterforce was needed with power steering because the steering wheel has more mechanical advantage. Positive caster provides that counterforce. The shims make more positive caster.

[JLeather]

Sweptlines equipped with Power Assisted Steering from the factory had shims installed on the axle.

You probably already know this but just to be clear: those shims were intended to increase steering effort. The reasons are: (1) for self-centering to work and (2) to increase the "feel" when steering such that the driver receives tactile feedback from the wheel. Therefore more counterforce was needed with power steering because the steering wheel has more mechanical advantage. Positive caster provides that counterforce. The shims make more positive caster.

Right, that's why I would like to go up to 7° on the truck to make it feel more stable at highway speeds, but I need to figure out this other issue first and make sure it's not related to the caster I already added.

[PwrWgnDrvr]

That's easy. Take the shims out and go for a drive around the block.

[JLeather]

I tried jacking the wheels off the ground first and the issue went away, so definitely tire/wheel related. 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 with the tires rotating that much in a static turn. The driver's side tire rotates CCW when I steer left, which I think means it has positive scrub radius? This is also probably contributing to the truck's twitchy steering at higher speeds (postitive scrub is like running toe-out). Anyone know if it should originally 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.

Could someone tell me the width and backspacing of the original wheels? I never had original wheels on this truck. Or could someone with stock wheels, brakes, and tires tell me the measurement center-to-center of the tire contact patch? I can figure out from that how far out my scrub radius is. If I'm right about the scrub radius being too positive then taller tires would also help?

[soopernaut]

The original wheels and tires on these trucks were much narrower than the typical modern stuff being used now. You would have to specify the year and model to get a wheel specific size. I don't know if there are any written specs on the original back spacing, so it may require physically measuring wheels.

Scrub radius is related to camber, which cannot be adjusted on a stock I beam axle unless you bend the axle. The other thing it is related to is the center of the tire, which can be adjusted by wheel width, back spacing, the use of spacers, etc. I think you need to experiment by putting wheels/tires on that put the tire center closer to the axle and see if the problem lessons.

[JLeather]

These are 16x6 steel wheels with 3.50" backspacing and +1/2" offset. I thought they were very similar to stock wheels, but I couldn't find any specs. I'm going to try measuring an extra stock axle/brakes I have to see if the Scarebird conversion changed the WMS location much if at all. Only other thing that is pushing the scrub radius out is the spacers. I opted for 1/4" spacers over shaving down the calipers originally, but I'm going to recheck and see where they rub and how badly and see if I can't get these wheels fitted without spacers.

Tires are 235-85R16's. I can probably go a little skinnier, but again they aren't that far off stock (except of course being radial...).

[Wildergarten]

These are 16x6 steel wheels with 3.50" backspacing and +1/2" offset. I thought they were very similar to stock wheels, but I couldn't find any specs. I'm going to try measuring an extra stock axle/brakes I have to see if the Scarebird conversion changed the WMS location much if at all. Only other thing that is pushing the scrub radius out is the spacers. I opted for 1/4" spacers over shaving down the calipers originally, but I'm going to recheck and see where they rub and how badly and see if I can't get these wheels fitted without spacers.

Tires are 235-85R16's. I can probably go a little skinnier, but again they aren't that far off stock (except of course being radial...).

I run wheels and tires almost exactly that size on a W200 with manual steering and there are no problems.

[JLeather]

I run wheels and tires almost exactly that size on a W200 with manual steering and there are no problems.

Are you also still running stock brakes? I'm going to do some measuring. I'm wondering if the scarebird install pushes the WMS out farther than I had originally thought.

[Wildergarten]

Are you also still running stock brakes? I'm going to do some measuring. I'm wondering if the scarebird install pushes the WMS out farther than I had originally thought.

Bone stock.

[JLeather]

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.

[my5thmopar]

These are 16x6 steel wheels with 3.50" backspacing and +1/2" offset. I thought they were very similar to stock wheels, but I couldn't find any specs. I'm going to try measuring an extra stock axle/brakes I have to see if the Scarebird conversion changed the WMS location much if at all. Only other thing that is pushing the scrub radius out is the spacers. I opted for 1/4" spacers over shaving down the calipers originally, but I'm going to recheck and see where they rub and how badly and see if I can't get these wheels fitted without spacers.

Tires are 235-85R16's. I can probably go a little skinnier, but again they aren't that far off stock (except of course being radial...).

I had 16x6 with 4" BS and same tires on my power assist D200 and they would hit the drag link. I think stock was 3.5-3.75. I don't think the wheels are the issue. One thing I see is your remark about using a spacer, explain....... I'm guessing that you're using the crown Vic rotors and didn't machine the hub. Did you drill out the bottom hole in the bracket? I have seen the instructions so, maybe they have fixed some issues. There is a known issue of the pads hitting the brackets. (I had to fix mine on the D100) Is the spacer behind the rotor? If so, then you spaced the caliber out too or I think there would be binding. Craig