Drag link adjustment on 427

[Dominik]

Good morning!

Who can explain please who to determine the correct position of the forward point for the drag link? Buy welding the chassis one has to really be careful how to position this mount.

I wonder how much leeway there is to compensate after welding it. That will apply to factory tolerances from cars built in the 60s too, I guess.

We do know that toe will change by cornering in bump and droop, which is desired (in this case). But how much change is wanted?

[Dominik]

Did nobody ever adjust this?

There is about 1 5/8" room to play within the mount at the chassis. The initial toe-in at ride height can be re-adjusted, but the toe curve should be affected based on the position of the drag link, shouldn't it?

[mickmate]

Hey Dom, are you set up with toe gauges? The radius rod style is easy to adjust like a turnbuckle. I don't know what effect moving the mount location will have. I don't think you should move it from where it goes, Ford tried that back in the day on the computer with Negstadt and Arning. I think at least one of those guys is still around maybe you and I should conference call with him. I'll bet Bruce has some idea of what the rear adjustments will do for us.

[Dominik]

Thanks Nick!

Yes, I can plot the toe curve of the radius rod, but I don't know what's a good curve.
The designers back then must have had specs to adhere to based on handling behavior.

If it's not that critical, it would give us some slack to position this chassis bracket.
It's a pain to stay with in the drawing's tolerances (okay, this is now scratch builders stuff...)

[CHANMADD]

Well, I decided that if I mounted the links rearward I was able to double the length of the rod, which in turn would minimize the steering effect. Also I believe that the rods are stronger on tension than compression and can therefore be a much lighter piece too.With heim joints it also makes the toe infinately adjustable. Obviously the most important benefit is the elimination of axle tramp.My installation was quick ,between race days and could have been neater but hey 20 years later they are still just fine. Pic in gallery..

[Cobra #3170]

This is what I did on CSX3170 to get acceptable bump steer:

The original cars were a ***** to set rear toe on because changing the drag link changed toe but it also effected the caster angle of the rear upright. You had to adjust the rear inner pivot with those two large nuts on either side of the spherical bearing to correct the bump curve every time you changed the drag link. That would then effect toe so it was an iterative
process that could take hours to get right because you had to check both toe and bump steer after an adjustment. Moving the location of the drag link could help bump steer on a car with stock ride height.
This is what I did with my 18" race tire set up.

I set rear ride height at 3.75" from the rear of the frame tubes to ground and front height at 2.87" from the front frame tube to ground. I fabricated two new drag links using left and right NMB rod ends so that all you had to do was loosen the locking nuts and rotate the drag link tube to change its length. I bought two new lower rear A arms from Kirkham and then revised the inboard ends to get rid of that crappy spherical joint that moves around
because the fit is so bad. I set it up with a 3/4" NMB rod end so that I could also adjust the length of the lower arm. Initially I used a bunch of washers on either side of the rod end to locate it fore and aft in the frame double shear bracket. I then screwed around for days trying different caster angles by shifting washers back and forth on either side of the 3/4" rod end. I would then set the proper toe angle from .00 to - .090 and check the bump steer or toe change for the full range of travel about 1.8" down to 2.0" up.
I made up some charts and then picked my ideal caster angle from the data. I then fabricated 4130 spacers at the correct dimension to replace the ideal washer arrangement.
I found that I could get really good bump steer with this approach at that ride height.
If the car has a street ride height it might be beneficial to lower the the forward drag link pick up. I am not really sure, the best thing would be to fabricate a movable pick up point
and try it. If you don't have a good suspension analysis program and VERY accurate measurements of all the suspension points in 3 axis the program gives you garbage. I tried it both ways and found that my ability to measure locations in space on the car were not good enough. You really need a computerized measuring system to do it right. A bonus with my set up is I can adjust track width slightly by resetting the upper and lower arm lengths for minor tire clearance issues. I use less toe in the rear on an autox course and more in the rear on road courses to avoid the dreaded oversteer monster. .

[Dominik]

Excellent reply - thanks! Nobody really paid attention to this “minor” detail.

Morris changed the whole thing.
He took the toe chance (bump steer) out (which was wanted in the 60s), but 15” tires are so much more fogiving and the 17” need such “gimmicks” much less.

I check once my next car is on wheels. I need to research toe curve a bit... But by the progress I am making, I have plenty of time for research ;-)

[Cobra #3170]

Originally Posted by Dominik

Excellent reply - thanks! Nobody really paid attention to this “minor” detail.

Morris changed the whole thing.
He took the toe chance (bump steer) out (which was wanted in the 60s), but 15” tires are so much more fogiving and the 17” need such “gimmicks” much less.

I check once my next car is on wheels. I need to research toe curve a bit... But by the progress I am making, I have plenty of time for research ;-)

Original 427's had a rubber mount on the front of the drag link and a rod end at the rear. Under acceleration they would toe in because the rubber would deform making them more stable but causing lots of understeer under throttle in a corner. Geometry and torsional stiffness are very important no matter what tire you are using. Bump steer can be used to advantage but only in specific cases because it makes the car unpredictable under other conditions. It is much better to get it right and then tune with springs, bars, roll axis, shocks and aero ( if you can). One reason that modern Mustangs, Camaros, Corvettes and Vipers are much better than older Muscle cars including Cobras (on the same tires) is that manufacturers are paying attention to suspension geometry now.

[Dominik]

I didn't know about the front rubber mount. That changes a few things in my thinking.
But I got most of the needed info from your initial response. Very good!

Thank you, and thanks Nick for mailing that initial reply to me. I didn't get a notification.

[mickmate]

I just got the right guy on the thread. Thanks Bruce, VERY valuable input on the subject.