Need a little Suspension advice...

[Rosey1968]

I bought '87 GN recently and I am trying sort out the suspension on the car. The car drives great except at higher speeds. I believe I am getting a fair amount of bump steer at speed. At 65+ when I hit bumps it shakes the wheel more than I would expect.

The car has brand new tires and the front suspension is tight. I actually went back to the place that did my tires and had them inspect it again. Everything is tight.

I think the car is riding a little lower than stock, see pics. From the ground to the bottom of the center of the front wheel well it is 26 5/8". The rear is riding lower at 26 3/8". The air bags in the rear spring are not currently working, I am hoping that is why the rear is a bit lower.

The upper control arms are not stock, they are a tubular. I am not sure who made them, see attached pictures. The spindles and lower arms are stock. Not sure about the front spring but I do know the back springs are Eibach (with bags/2008). Front spring were replaced in 2008 as with the shocks, they are KYB. Shocks seem good still.

All the rear control arms are stock. I will be upgrading soon to boxed lowers and tubular uppers this summer.

A little more about the car and my expectations… Engine has 10K on a rebuild and it dynoed at 420 HP and 560 toque when it was fresh. I am sure it has lost some… but it still has a lot. I will probably never take the car to the track. It is mainly to cruse and make trouble (fun) on the street.

My questions…
Are the upper front control arms my issue with bump steer?
Or could it be the front springs?

Basically I am looking for the best setup, what to replace first to keep the car about the same height and reduce the bump steer.

Thanks in advance for any advice.

 

[Chuck Leeper]

New tires. Fresh alignment? Alignment specs are??
Those uppers don't have a bump stop in them, and at full drop are stretching the brake hoses. Look like sat nite specials for a stock car.
I just put a set of UMI's , from GNS Performance. NICE pieces, w/ poly bump stops, 1/2" long ball joints. Hope to regain good alignment specs. 30 y/o frames sag!
Why the air bags?

 

[Rosey1968]

Chuck,

Thanks for the feedback.

It was aligned but I don't have the specs in front of me. As for the air bags, not sure of the purpose, they came with the car.

Is there any way to measure the upper arm to find out what drop they are so I can find a similar (drop/ride height) replacement?

I think I may just have to bite the bullet and buy new upper arms, spring and shocks to match... may be the smartest thing to do.

Thanks.

 

[Rosey1968]

So I think I may have stumbled upon the make of the upper arms:
http://www.gbodyparts.com/product_info.php?cPath=46_21_52&products_id=134

 

[Chuck Leeper]

Shoot Scot at GNS Performance a pm. He can help. He has several systems available.

 

[scot w.]

Rosey1968, Give us a call at the number in our signature and we can discuss your options. We have several suspension packages that "Work" and actually have bump steer correction numbers, not just some babble over the phone to sell ya parts.

 

[michael evans]

Bump steer is caused by tie rod location. You said you changed the front springs. If yu did, you also change the geometry of the front end.

Best way to see it is but it all together without the spring. With a dial indicator on the arm so you can raise and lower the arms watch the indicator as you move them.

Simple explanation:

 

[1KWIKSIX]

So I think I may have stumbled upon the make of the upper arms:
http://www.gbodyparts.com/product_info.php?cPath=46_21_52&products_id=134

Either those or the Speedway Motors tubular upper arms which are pretty much the same thing.
http://www.speedwaymotors.com/GM-Me...pper-Control-Arms-Steel-Cross-Shaft,5584.html

 

[marleyskater420]

OP- here is a writeup I did on Bumpsteer. Make sure to pay attention to not only the raised ball joints, but the physical location of the ball joint in relation to the stock location. If you read below, you will see how that is the most crucial aspect to fixing the bump steer.

1) Bump Steer- causes, effects, and how to correct it
A) Our cars use what is described as a SLA front suspension. This stands for Short Long Arm. This means that our lower control arms are longer than our upper control arms. Our suspension pivots along certain points called Pickup Points. A Pickup Point is the pivot axis of each bushing, center of the ball joint, and tie rod ends. All of these are PP's that the suspension travels along. This means that if you were to look at your suspension from the front, the suspension pivots on these PP's and the location of the PP's create certain arcs, or paths of travel, that the suspension will move along.
B) Bump steer is defined as "..an unwanted change in wheel direction or toe as the suspension travels."
C) As our suspension travels through its' arc the lower control arm pivot point and the tie rod pickup point swing in an arc as well (because they are Pickup Points of the suspension-they are the pivoting axis along which the suspension may travel). If the arcs of the two points-the LCA (lower control arm) PP and the tie rod PP are at different heights, they will have different arcs. This is illustrated in the following drawing from Jerry Bickel's book:


You can see how the tie rod PP and the LCA PP on the spindle have arcs that they swing along during travel. In the picture it shows the tie rod and LCA PP's are almost the same height, which allows them to have an almost parallel arc. The near-same height of the LCA and tie rod PP's allows for an almost zero-bump steer situation, because their arcs they swing along are almost parallel.
D) Bump Steer in our cars is caused because our tie rod and LCA PP's are not at the same height. To quote Bickel, "Any differences between the spindle and tie rod arcs will cause the tie rod to push or pull on the steering arm. This changes the direction of the wheel, creating bump steer." In a suspension with the LCA and tie rod PP's not equal heights, as the tie rod moves and pulls on the steering arm, it is causing either toe-in or toe-out (this is where the actual direction your wheel is pointing is changed) and thus the change in direction of the car. Here is another drawing from Bickel's book discussing this:



E) How do we fix this bump steer, now that we know what can cause it?
a) To fix this issue we need to find control arms that help to relocate the LCA PP to a more level position as the tie rod PP sits. A good example of this would the SPC LCAs with taller ball joints that are offered from SC&C. This design does not simply add a taller ball joint and calls it a day, but it physically locates the ball joint to a more favorable position for suspension travel-locating it to a more level height as the tie rod sits when you use the taller ball joints offered with the arms, improving the arc the PP swings along relative to the tie rod. This LCA also addresses caster and camber issues as well, but I won't touch on that right now.. Once we have a lower suspension that has the tie rod PP and the LCA PP to a more level height, we can significantly reduce and almost completely rid ourselves of any bump steer the car may have. Note: simply adding a taller ball joint to the LCA will not fix this issue. The actual physical position of the ball joint must be changed as well. This is what separates suspension pieces like the SPC lower control arms from the rest of today's offerings- they physically change the location of the ball joint AND use the taller ball joint to locate the LCA pick up point and tie rod pick up point to a more level location. BEWARE of other companies products offering taller ball joints- you must make sure that they are changing the ball joint location as well, which is the real key here.

 

[marleyskater420]

Chuck,

Thanks for the feedback.

It was aligned but I don't have the specs in front of me. As for the air bags, not sure of the purpose, they came with the car.

Is there any way to measure the upper arm to find out what drop they are so I can find a similar (drop/ride height) replacement?

I think I may just have to bite the bullet and buy new upper arms, spring and shocks to match... may be the smartest thing to do.

Thanks.

Read my post above and you will see that by choosing the correct upper control arm, which relocates the ball joint pickup points to a more level location with the tie rod, that you do not need to buy new shocks and springs to correct your issue.

I will put $100 that if you buy the SC&C upper control arms with tall ball joints and the tall ball joints for the stock lower control arms, that you will fix your bump steer issue more than if you were to buy another companies uppers AND lower control arms...

But hey, what do I know...I only provided information written by the guy who built the chassis for the world's fastest doorslammer drag car at 5.42 in the 1/4 mile..... I'm not trying to sell you anything, and I have no financial vestment in any part you choose, but I am trying to show you what the true cause of bumpsteer is, and save you hundreds of dollars...

 

[Rosey1968]

Thank you all that chimed in... your info and advice is appreciated.

I need to read the above a few time while I am on the road this week to fully digest.

Thanks again gentleman.

 

[scot w.]

Read my post above and you will see that by choosing the correct upper control arm, which relocates the ball joint pickup points to a more level location with the tie rod, that you do not need to buy new shocks and springs to correct your issue.

I will put $100 that if you buy the SC&C upper control arms with tall ball joints and the tall ball joints for the stock lower control arms, that you will fix your bump steer issue more than if you were to buy another companies uppers AND lower control arms...

But hey, what do I know...I only provided information written by the guy who built the chassis for the world's fastest doorslammer drag car at 5.42 in the 1/4 mile..... I'm not trying to sell you anything, and I have no financial vestment in any part you choose, but I am trying to show you what the true cause of bumpsteer is, and save you hundreds of dollars...

Marley, MANY of the large well known suspension manufactures out there today that build upper and lower A arms for the G-bodies already move the ball joint pick up points and some even have done the research to provide actual bumpsteer correction numbers to prove just how much there suspension "Corrects" the bumpsteer on a g-body car. Many of these companies are out there today proving how well their suspensions work on a weekly basis in front of thousands of spectators on autocross & road courses all over the country winning events all over the place.

 

[marleyskater420]

Marley, MANY of the large well known suspension manufactures out there today that build upper and lower A arms for the G-bodies already move the ball joint pick up points and some even have done the research to provide actual bumpsteer correction numbers to prove just how much there suspension "Corrects" the bumpsteer on a g-body car. Many of these companies are out there today proving how well their suspensions work on a weekly basis in front of thousands of spectators on autocross & road courses all over the country winning events all over the place.

That is awesome to hear!

It is always good for us consumers when we learn things like this, and have access to the same findings that the companies discover. I think it really helps the community as a whole!

 

[Turbo6inKY]

I bought '87 GN recently and I am trying sort out the suspension on the car. The car drives great except at higher speeds. I believe I am getting a fair amount of bump steer at speed. At 65+ when I hit bumps it shakes the wheel more than I would expect.

The car has brand new tires and the front suspension is tight. I actually went back to the place that did my tires and had them inspect it again. Everything is tight.

"Tight" is relative. The nuts and bolts could be tight, but the ball joints in the steering linkage could be shot. Have the idler arm and transverse link and tie rod ends really inspected, get a second opinion from another shop if you have to.

The shake could also be bad shocks. Have those checked out.

All the rear control arms are stock. I will be upgrading soon to boxed lowers and tubular uppers this summer.

Be careful. The rear suspension design expects those c-channel arms to deform slightly in order to function properly.