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Less bump steer with '73+ suspension?

Posted By: AndyF

Less bump steer with '73+ suspension? - 12/17/17 10:28 PM

I have a '72 Duster which I'm working on. I don't like the early suspension design with the tapers point up since it is impossible to work on with headers. So I was going to switch to the '73 style steering linkage but I can't figure out what to do with the tie rods.

The '72 style has a shorter center link so the tie rod ends sit inboard of the LCA pivot. That would seem to be a bump steer issue? The '73 center link is longer so the tie rod ends are close to the LCA pivot. That seems like a much better design.

Anyone bump these two designs and compare them? I'm assuming when it came time to design the '73 suspension the engineers much have known the '72 style wasn't the best so that is why they changed it?

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Posted By: astjp2

Re: Less bump steer with '73+ suspension? - 12/18/17 02:34 AM

Andy, I think that was covered in the old Mopar circle track and the performance manuals...
Posted By: autoxcuda

Re: Less bump steer with '73+ suspension? - 12/18/17 05:17 AM

I bet you find the linkage points very close in both designs.

I don't think the critical points at the inner tie rods holes are a different width center to center.

I've bump steered the 73 style, but the tie rod ends where hotchkis spherical set up.
Posted By: AndyF

Re: Less bump steer with '73+ suspension? - 12/18/17 09:23 PM

The inner tie rod holes are different between '72 and '73 by a fair amount. At least 1/2 inch, maybe closer to 3/4 inch on each side. So it looks like the '72 style would have more bump steer than the '73 style since the inner tie rod is located inboard of the LCA pivot.
Posted By: AndyF

Re: Less bump steer with '73+ suspension? - 12/18/17 09:30 PM

Now the big question is can I use the '73 center link with the '72 lower ball joint? It all fits but there is a lot of angle on the tie rod since the inner taper points down and the outer taper points up.

I could remachine the lower ball joint so the taper points down but that is a lot of work.

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Posted By: autoxcuda

Re: Less bump steer with '73+ suspension? - 12/18/17 09:45 PM

Originally Posted By AndyF
Now the big question is can I use the '73 center link with the '72 lower ball joint? It all fits but there is a lot of angle on the tie rod since the inner taper points down and the outer taper points up.

I could remachine the lower ball joint so the taper points down but that is a lot of work.


Ok. Didn't think there was a width difference between the center links.

The 73-76 LBJ also has the tie rod taper pointing up. What's the geometry difference between the 67-72 disc and the 73-76 disc LBJ's (steering arms) ??
Posted By: autoxcuda

Re: Less bump steer with '73+ suspension? - 12/18/17 11:04 PM

Would be interesting to have an adjustable center link like thir ines we used in circle track racing.

Mopars center links are non wear...Less expensive to build/design.

Like this but pitman and idler to the inside of the tie rod pick up points



Then you adjust with these slugs

Posted By: DoctorDiff

Re: Less bump steer with '73+ suspension? - 12/19/17 07:36 AM

Tie rod end design changed for the 1970 model year.

In other words, '70-'76 tie rod ends interchange, but they are different than the'69 and older version.
Posted By: autoxcuda

Re: Less bump steer with '73+ suspension? - 12/19/17 10:33 AM

Originally Posted By DoctorDiff
Tie rod end design changed for the 1970 model year.

In other words, '70-'76 tie rod ends interchange, but they are different than the'69 and older version.


But the centerlink didn't change until 1973.
Posted By: CSK

Re: Less bump steer with '73+ suspension? - 12/20/17 04:25 PM

if you move the outer tie rod to the top it will have more bad bump steer
Posted By: TC@HP2

Re: Less bump steer with '73+ suspension? - 12/20/17 05:13 PM

Without actually mapping it out, I'd suspect they actually have similar bump characteristics.

Looking at the suspension from the front or rear, if you project a line from the upper arm inner bushings through the lower arm inner bushing, and the upper ball joint through the lower ball joint, you would find the tie rod ends are still inside this plane,which is the preferred place to minimizing the bump issue. Is it slightly better or worse, I don't know right off hand.
Posted By: AndyF

Re: Less bump steer with '73+ suspension? - 12/22/17 02:02 AM

I'm going to put the suspension back in the car and see what it looks like. I'll also swap in a set of '73+ knuckles to see what they look like. I don't really like the '72 center link since it is impossible to work on with the headers in the car. I'm not so sure I like the '73 tie rod setup with the one taper up and one taper down but I guess that is the way the engineers designed it.

If the '73 center link doesn't fit with my oil pan and headers then I might need to make my own or find someone who makes them. I've been told that the NHRA Super Stock guys fab their own centerlinks so I'll check there first.
Posted By: NV69B7RR

Re: Less bump steer with '73+ suspension? - 02/01/18 11:16 PM

Which of the two center links is best for optimal handling when using the 73+ Disc brakes and ball joints?
Posted By: autoxcuda

Re: Less bump steer with '73+ suspension? - 02/02/18 08:48 PM

Originally Posted By NV69B7RR
Which of the two center links is best for optimal handling when using the 73+ Disc brakes and ball joints?


Would have to bump both setups in same car. Must align it for each change.

Need to do it on many cars to confirm a trend.

There are other variables effecting the bump steer. Alignment, suspension parts used, alignment adjustments, straightness of frame, production tolerances may all, some, or one effect the bumpsteer results.

Posted By: AndyF

Re: Less bump steer with '73+ suspension? - 02/04/18 06:29 PM

My tape measure says the '73 center link reduces bump steer. The engineers moved the inner tie rod location out slightly on each side so it is more in line with the lca pivot. That should reduce bump steer. I don't know how much since I didn't actually measure the bump steer.
Posted By: ThermoQuad

Re: Less bump steer with '73+ suspension? - 02/13/18 12:35 PM

Ask Bill Reilly of RMS if you want some good data and information.
Bill was very helpful with technical support when i built the 2 cars to run Watkins Glenn
Posted By: 375inStroke

Re: Less bump steer with '73+ suspension? - 03/01/18 03:37 AM

Aren't the lower ball joints different, too? If the tie rods are the same length between the two styles because the tie rod mount location on the ball joint is relocated the same amount that it is on the drag link, bumpsteer would be the same, right?
Posted By: AndyF

Re: Less bump steer with '73+ suspension? - 03/01/18 07:19 PM

The tie rod length is not the same between the two designs. The '73 style inner pivot point is moved out so it lines up with the LCA pivot point. Looks to me like the design engineers figured out that they had a problem with the '72 design and they fixed in the '73 design.
Posted By: EagleDuster

Re: Less bump steer with '73+ suspension? - 03/01/18 09:34 PM

So this makes me curious about my car, I have a '70 Duster that I upgraded to '73-up spindles and brakes, if I'm still using the older-style steering centerlink will that cause bump steer issues?
Posted By: autoxcuda

Re: Less bump steer with '73+ suspension? - 03/01/18 11:14 PM

Originally Posted By EagleDuster
So this makes me curious about my car, I have a '70 Duster that I upgraded to '73-up spindles and brakes, if I'm still using the older-style steering centerlink will that cause bump steer issues?


No “issues”

It will be in an acceptable range of a stock setup.

Very few of these cars will have the same exact bumpsteer to the 0.001 inch.
Posted By: Rick_Ehrenberg

Re: Less bump steer with '73+ suspension? - 04/08/18 05:33 AM

If you take the time to blueprint it, usually by shimming the box and/or ovaling the idler arm holes (weld washers when done), it is amazing how low you can get the bump steer.

I was online with a British-Midlands F1 engineer during some 4-post tests on a B-body a while back. He was amazed to learn that this "near ideal' (his words) suspension geometry was on a stock '60s US sedan!

Incidentally, it is well known that the '73-up A-Body K's has incredibly lousy quality control, as stated, they were all over the map! I could tell you a story...

Rick
Posted By: CKessel

Re: Less bump steer with '73+ suspension? - 04/08/18 05:07 PM

Originally Posted By Rick_Ehrenberg
If you take the time to blueprint it, usually by shimming the box and/or ovaling the idler arm holes (weld washers when done), it is amazing how low you can get the bump steer.

I was online with a British-Midlands F1 engineer during some 4-post tests on a B-body a while back. He was amazed to learn that this "near ideal' (his words) suspension geometry was on a stock '60s US sedan!

Incidentally, it is well known that the '73-up A-Body K's has incredibly lousy quality control, as stated, they were all over the map! I could tell you a story...

Rick
Speaking of suspensions, 3-5 years ago you started an article on suspensions and mods. You talked about the various spindles and control arm availabilities, bump steer etc. You never finished the article after teasing about a sequel. Now that you are gainfully employed again, how about that sequel?
Posted By: NV69B7RR

Re: Less bump steer with '73+ suspension? - 11/17/18 06:07 AM

Can you use a 71 (340, reinforced) A body K frame with the 73+ center link, ball joints etc? Or would it be better to stick with the 73+ K frame?

I just got the 340 K frame and already have the 73+ spool style, so I can use either one. I plan to add the steering box reinforcements to whichever would work best. I have 11/16 tie rods and the 73 spindles & ball joints.
Posted By: autoxcuda

Re: Less bump steer with '73+ suspension? - 11/17/18 07:50 AM

Originally Posted By NV69B7RR
Can you use a 71 (340, reinforced) A body K frame with the 73+ center link, ball joints etc?


Yes. But you have to use the 67-72 sway bar that does not go through the K-member.


Quote:
Or would it be better to stick with the 73+ K frame?


To me the advantage of the 73+ is narrower sway bar to allow high backspace rims and the safety and durability of the spool motor mounts. They do make locking biscuit motor mounts so that negates much of that benefit. If you are only running 4.25” backspace rims with 245 wide or less tires, the backspace deal might not be an issue.

Disadvantage of 73+ is it fits much tighter around the Milodon Road Race oil pan.


I just got the 340 K frame and already have the 73+ spool style, so I can use either one. I plan to add the steering box reinforcements to whichever would work best. I have 11/16 tie rods and the 73 spindles & ball joints.
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