B/F Spindles and Bumpsteer

[Lynn D. Brown]

On my '87 GN, I'm running into a bumpsteer problem with the B spindles, or maybe they are F Body, which I think they R the same except for bolt circle. My bump steer is pretty bad w this combo, so I tried the taller lower ball joint from SCNC (Mark Savitske). Well now because of the wider track of the B/F spindles, one of my front tires is hitting the outside of the wheel well. I reduced the tire size from 245-50 to 225-60, but it still hits on bumps, and it's even tearing up the new tires. I also have Hotchkis Upper A's.

So, I was looking at the 2 tall spindle kits from SCNC that are actually off of a C5 Corvette. These tall spindles (same height as B/F) have the narrower track like the G body short spindles to solve my hitting problem and supposedly the geometry will also solve the bumpsteer problem. Pretty damed expensive, though. But they require 13" Corvette rotors, however, so that means 17" wheels and tires. So now the total bill really gets out of hand.

I already have 12" 1LE brakes, and would like to reuse those and avoid the new wheel / tire expense, but apparently they don't fit the Corvette spindles. Don't know if any of U guys have any other alternatives to get rid of this bumpsteer ?

I did just get another tidbit from Hotchkis which I didn't know about. D Oliver said that U have to replace the Tie Rod ends with some other kind when U do the B/F spindle conversion. I didn't know that when I did the conversion 10 yrs ago. I have to ask him what TR ends I need. Anyone know about this ?

 

[UNGN]

I don't know what wheels you are running, but we switched to GTA Rears in front to get the tire off of the wheel opening with the wider brake rotors.

It won't solve the bumpsteer problem, but may give you more clearance with your current set up.

 

[Toady]

Lynn D. --

I'm no expert, but here's what I've gathered from reading here and on montecarloss.com.

As you say, the two Corvette-based packages, G-5 and AFX, require huge wheels to clear the 13" rotors.

If you're running 15" wheels, SC&C's Stage 1 or Stage 2 are your choices. I think those tall LBJs are for use with the stock G-body spindle - and the matching tall UBJs - not the F/B spindle. Because I want to keep the stock T wheels on my car, when it's time I'll be going Stage 2 with "ZQ8" Blazer spindles/brakes. HTH.

 

[FBSideoiler427]

Bump Steer woes

On my '87 GN, I'm running into a bumpsteer problem with the B spindles...?

Disclaimer: I'm no expert and I have an electrical engineering degree and my suspension R&D work has been on my 67 Mustang therefore my understanding of Bumpsteer is -

Bump Steer (also called toe steer because it is caused by a change in toe-out) is caused by a dissimilarity in geometry between the tie rods and the upper and lower A-arms.

The geometry of the upper and lower A-arms dictate what arc curve the wheel will scribe in its up and down path. For this purpose, we are interested in that wheel path arc scribed at the attachment point of the tie rod end to the spindle, (the outer tie rod path and the wheel center path will he exactly parallel because the two parts are rigidly attached.)

To avoid interference with the wheel movement, the tie rod must move in the same motion as the wheel path. In other words, the distance between the inner and outer tie rod ends must be the radius of the circular path the wheel makes. If it is not the center of the wheel arc, or it the tie rod is not at the center of the arc, the wheel will toe in or out when moving up or down.

In my feeble brain, there are only a few steering geometry design problems that exist to cause bump steer, and they are: tie rod too long, tie rod too short, outer tie rod end too high, outer tie rod end too low, or some combination of these problems. (To clarify some terminology before progressing any further, the outer tie rod end referred to is the end which attaches to the spindle whether it is en the left front or right front wheel, and inner tie rod end is the end which attaches to the steering cross shaft.)

If should be noted that caster and toe-out are quite closely related. When a caster change is made in the suspension, it automatically moves the tie rod up or down. Basically a one degree change in caster will result in a tow steer change of 3/32 of an inch.

Determining and correcting bump steer is done by moving the front wheels though their range of vertical travel, measuring the toe out change and plotting it on a graph. Then the deviations are corrected by changing the length and height of the tie rod ends. The shape or pattern of the line shown on the graph indicates what the cause of the problem is.

Setting the bump steer pattern can be done following these steps: (1) Adjust the car height and front end alignment to required specs. (2) Drive the front wheels on alignment plates, (3) Remove the coil springs and support the car's front end with a hydraulic jack. (4) Take a toe out reading at the car's lowest position (full jounce) and record it on the graph. (5) Raise car height one inch and take another reading, graph it, and continue in this manner in one inch increments until the wheels nearly lift off the ground. This should result in about eight readings with the fourth or fifth reading being the initial toe setting.

In making adjustments for toe steer, first adjust the left and right outer tie rod ends so they are both an equal height from the ground. Then additional corrections can be made. The tie rod end height change is accomplished by shimming the lie rod end.

When changing the toe out, always shorten or lengthen both tie rods an equal amount. If both tie rods are not the exact same length, the steering will be off-center and more play will he introduced into the steering wheel.

If the outer tie rod end is too high or the inner tie rod end is too low, shim the outer tie rod end to lower it. If the wheel is toeing-in on bump and out in rebound, the cause is the outer tie rod end is too low or inner is too high.

A convex curve like this indicates tie rod end heights are compatible but the tie rod is too short causing the wheel to toe-in in both bump and rebound The curve at the right is the same basic problem as the one at left except the tie rod is too long here causing toe-out.

HTH

 

[dwkohout]

I already have 12" 1LE brakes, and would like to reuse those and avoid the new wheel / tire expense, but apparently they don't fit the Corvette spindles. Don't know if any of U guys have any other alternatives to get rid of this bumpsteer ?

If your up for some fabrication or have a good welding shop just lower the inner tierod down about an inch on the draglink. Thats what I did 4yrs ago and no hint of bumpsteer ever since.

Heres anouther modded draglink and it's adjustable. I wouldn't be crazy about using heim joints on the street though.
http://jeffd.50megs.com/bump_steer_mods_page.htm

 

[TURBOPOWERED68]

If your up for some fabrication or have a good welding shop just lower the inner tierod down about an inch on the draglink. Thats what I did 4yrs ago and no hint of bumpsteer ever since.

Heres anouther modded draglink and it's adjustable. I wouldn't be crazy about using heim joints on the street though.
http://jeffd.50megs.com/bump_steer_mods_page.htm

i would buy 2 from you if you would make them for me. i could supply the center links or you supply them.

 

[marred4life]

i would buy 2 from you if you would make them for me. i could supply the center links or you supply them.

same here, but me in line if you have some

 

[Lynn D. Brown]

Bump Steer

Yes, I would be interested also, Jeff.

 

[dwkohout]

WOW! I not here to sell anything just share what I know. I would have no problems making these but from a legal standpoint I'm not sure if I would want to make them to sell. Also I don't think this mod will pass vehicle inspections which some states have.

 

[TURBOPOWERED68]

write the disclaimer how its not intended for use and its just a show peace to be placed on the wall.
and ill sign it.

 

[dwkohout]

I'll think about it.

 

[RAMAIR1]

dwkohout...I just have to say that I am really digging that Olds. Very tastefully done.

I have the Street Comp Stage 2 set up currently from Savitske. It was money well spent. When I get around to it (read: when I have some $$$ the wife doesnt know about) I will finish it off with the remainder of the G5 setup. I already have the 17" wheels. Marcus won't steer you wrong (no pun intended)

 

[dwkohout]

Thanks RAMAIR1! I have checked out Marcus's site. Good stuff! If only I had the $$$.

 

[Ormand]

Sideoiler, that's a good explanation of bump steer. There's another explanation in the back of the "Chevy Peformance" book that you buy at the chebby dealers. Or at least there was, in one edition.
One of the suspension/brake vendors sells tie rod ends to help resolve the bump steer with the tall spindles. They are called "Baertrackers", so that gives you an idea of which vendor!

 

[dwkohout]

One of the suspension/brake vendors sells tie rod ends to help resolve the bump steer with the tall spindles. They are called "Baertrackers", so that gives you an idea of which vendor!

Ormand if only it was that easy.

 

[MG8T7GN]

FWIW, bumpsteer is the amount of toe change during suspension jounce and rebound- not if the tires hit the body or chassis during suspension movement. I, too, ran into the same problem using the tall knuckles and Hotchkis upper control arms when I installed Baer's front brakes. I had them convert a set of stock knuckles to use the 1LE calipers and went back to stock upper control arms. I also needed a different caliper mount bracket for the stock knuckles.

Mark

 

[gmachinz]

On our B-body spindle swaps, I don't have any bumpsteer issues that are that extreme. Most of the time we use a heavier load rated front spring with a coil cut along with Bilstein shocks and when we upgrade to 17's up front, there just isn't any problems. I like to get the IC down a few inches (less body roll and more control in the turns) and this accomplished that. I end up fabricating new aluminum inner fenders for every car though-can't have the wide tires rubbing up front-sometimes we even trim off a 1/2" or so from the wheel lip opening but that's usually on a complete brake/suspension overhaul. -Jabin