Scrub Angle issues.. crazy front wheel vibration

[9secondart]

I recently added a rack and pinion system to my Mopar. The bump steer and Ackerman are very close. The bump steer is .054 on the drivers side and .066 on the passenger side. The Ackerman is about 1-2 degree short of the angle differences at a full turn. Close enough for a drag car..... The issue I have is at the big end when I apply the brake I get a vibration on the front end and the front wheels oscilate. It goes away after a few seconds. I have come to the conclusion it is "Scrub Angle" or "Intersection Angle" . The million dollar question is how do I improve it and what is the ideal intersection point at the itre to ground contact point? I already changed the from rim offset to get closer to center but other than heating up the spindle and bending it to increase the angle i am stuck.

Any help or suggestions are greatly appriciated. If you think it is another problem....please offer it up.


Ray
9.42 @ 142mph
1.26 60 foot
Small Block Chrysler all motor

[John_Heard]

Ray it sounds like you may be experiencing a toe-in to toe-out change when under braking conditions. You might have to back and look to see how much toe in you have at ride height and give it a bit more, perhaps when the springs compress some it is toeing out.

[9secondart]

I run a negative 1/32 toe in and the bump steer is good so it is not a bump steer problem which would change the toe during its travel. I agree that running more toe in would help. I will try that as well, maybe go -1/8?

Thanks for your input.

[John_Heard]

1/8 sounds fine, and if it gets rid of the problem that's a good indicator what was going on. Not many people check to see what bump is doing under compression. You could pull shocks and springs and recheck it again above and below ride height to see.

One way that really shows how and where it moves is to mount a laser on your spindle. Shoot it at a right angle across the garage and map out the movement of the spindle on some paper as it moves from full compression to full extension. The distance will really amplify the curve and make it easy to see improvements. You might be able to make some changes that puts the most neutral part of the curve closer to where the car runs at.

[DOTracer]

Buddy of mine struggled with this all season. His wound up being the caster all screwed up (different from one side to the other), plus bump steer under braking.

[demented]

That does sound like a caster angle problem. Caster is the angle that is responsible for high speed stabillity. If you don't have enough caster set into the front end, what will happen is that going down the track under power the caster angle goes positive because of angles the car is in(nose up).Once you go into coast at the end of the run, if the caster was borderline during the run, then the caster will go negative under braking, its called a death wobble, some motorcycles experience it escpecially the ones that arn't designed for high speed runs. Caster for a drag car should be around 2-4 degrees positive. My preference for caster is a little more than that because it has a tendency to drive very straight with minimal effort but tends need more muscle to turn. As for toe on the front, it depends where your rack is set up at. If it is in front of of the axle centerline then it should be toed in very slightly less than a 1/16th. If the rack is behind the axle ceterline then it should have a touch more toe in. Tierods work better and stay truer under tension than compression. But the goal is to have the tires as as parallel to each other as posibble when going down the track. Anytime I have seen someone that adjusts toe to compensate for a problem usually is has something else out of whack. Race cars fit on alignment racks at your local tire store and can have some great results from getting an alignment on a state of the art alighmnet machine. 15 years ago when I worked at goodyear, It was not uncommon for me to do an alignment on a drag car at least once a week. Guys would trailer them in and we would spend some time doing a great alignment on it. The would bring it to me as a last ditch effort to try and correct a problem they would encounter going down the track. Almost always it would be a caster related problem. I even do alignments on my mud drag car and other mud trucks and it does make a difference on them as well.

Hope this helps

[John_Heard]

Good idea to check to see if it's got enough caster as well.

Speaking of caster angles, have any of you considered what large amounts of caster do to the chassis when the car gets loose and you're steering into a slide? I was thinking about that the other day and was wondering what effect it has. One of these days I'm going to get the car on the scales and see if it moves weight around or not. For example, less caster would have less effect on weight movement if you're having to work to correct the car. I've got 7* on my car, just wondering how much is really necessary.

[demented]

I would imaginge that the more caster you have the more weight it would move. In theory, you are actually picking up on a corner when you are steering. Also more caster the harder it is to steer (escpecially when using manual steering) so you might not be as quick to steer out of a problem because of theincrease in effort to turn the wheel. I noticed this on my mud car when I have lots of caster in it. There is a point of having too much caster. You only need enough to keep the car straight under speed. Anything more just makes it harder on parts and the driver.

[9secondart]

I run 3 1/2 to 4 degree of caster and 0 camber. Generally a 1/32 toe in. I changed front rims and increased the offset. The shake improved dramaticly. This would change the intersection point closer to the centerline of the tire. When I project the centeline of the upper ball joint and lower ball joint to the ground the tire contact point is on the inside by 2". I believe this is a contributing factor but just not sure. It is a forward steer rack and the geometry is pretty good. Not great but good. At ride height the rack arms are at 18 degrees to be parallel with the lower control arms. They also kick rearward to get to the steering arm so there is a coumpound angle. If I could move the rack back I would but on a Chrysler it is impossible without major reconstruction.

That all the facts hope that helps with where I am at. During the run, straight as an arrow. Only during braking does it shake.

Ray

[demented]

How are the rotors on the front? Have you had them checked to make sure they are not warped? If they are that could be what starts off the wobble.

[9secondart]

I have had them remachined and checked all the possible connection points, bearings and so on.

I found this online. This is nuts but the video definaetly shows what is going on. My situation is no where like this but it better explains what is experienced.

http://videos.streetfire.net/video/145946.htm

[Moparious Maximus]

Not sure if its relavent since we run a straight axle, we found a good steering dampener will help out alot.

We had problems in the past with front end shake during braking and usually below 40 mph. It would also do it when the front wheels landed after a launch, one time my arms hurt for days is shook so bad.

[Dave Morgan]

Dear 9,
As far as your scrub radius, you would like to have the SAI (steering axis inclination) line to aim for the tire contact patch in the middle third of the area. It sounds like you understand the effect as the cure is to do exactly what you did, get different offset wheels.
I agree with the approach to look at caster angles, you might have too much.
John's approach with bumpsteer is valid also, but if your car is acting like the one in the video, the rattling did not occur during ride height change, which would invalidate bumpsteer as the problem.
There is one more caster-related issue, it's called Percentage of Anti-Dive and is most affected by the front-to-rear angle of the lower control arm (if this is a strut car.)
Dave Morgan

[9secondart]

Dave, thanks for weighing in. I have been studying the dive as a possible culprit along with the (SAI). My thoughts are to machine .250 off of the lower ball joint thus increasing the angle and mimicking the increase in the offset wheel. The offsets available are liminted and a combination of both may be required. The second thought is to adjust the compression side of my shck to slow the weight transfer from rear to front. I could add a urethane shock limiter that has a derometer such that is lows the transfer with out acting like a stop. this would be tough coming down from a wheel stand. The intention of this is to slow the camber change while braking and reduce the transfer. We will also be adding in an adjustable proportioing valve to do the same thing.

Do you think I am on the right track? Figuratively speaking (LOL)

Ray