Spent the last month fighting alignment and bumpsteer problems. I won! On Aug 4th, the total bump steer totaled almost 1″. Now it is less than .040″
Just about the time I was doing my rough alignment, the article in Kit Builder magazine by Dean Lampe regarding bump steer appeared. I should have adhered to the old premise, “ignorance is bliss”. There are many Hurricane kits being successfully driven and no complaints on the owners site. Many happy campers, huh.
I studied how bumpsteer is measured and the affects it has on a vehicle. I then measured my kit, as assembled, and found it to be drastically incorrect. At normal settings, I found the bump steer to be approximately 3/8″-1/2″ at 2″ of bump or jounce. And even worse, those movements were crossing neutral. As the car rolled, the inner wheel would toe out 3/8″ and the outer wheel would toe in 1/2″. This would have been one squirrelly ride.
I consulted with Dean, and a couple others. All felt it was something that should be addressed, especially if I ever wanted to take the car onto a track. The paradox is there are about 50 Hurricane kits presently on the road and no one had ever complained of it. Dean built one of the first cars and he acknowledged he never checked bump steer and felt the car handled OK. But with that same sentence, he added, “I don’t think most manufacturers even check it. I thought my new GT40 handled OK but after correcting for bumpsteer, I am amazed how much better it handles.”
As I studied the car and design and finally determined the steering rack was mounted too high, by about 1″. Moving it would be a major task since the left mounting boss sits on the lower frame extension. Moving the standard Mustang II rack down meant cutting and welding this part, something I didn’t want to do. I decided to modify the rack by cutting the boss off and fabricating a clamp mount for it.
In the process, I learned the rack was also mounted off center about 1/2″ Moving the rack till the boss on the right side touches the frame rail will remove about 3/8″ of the offset. Close enough.
As part of the initial analysis, I had purchased a Steeda bumpsteer kit. It would move the outer tie rod pivot up and down. But problems: I need to move the point up, right to the middle of the steering arm, and the Steeda kit is really designed for the Fox body and SN95 cars. The threads (9/16-18) are correct but the adjuster is about 1 1/2″ too long. I liked the idea of having some adjustment so I cut both the inner and outer ends to shorten it.
The mounting clamp on the drivers side is fabricated from 1 x 2 x 4 aluminum. Two pieces were clamped together and drilled for the mounting bolts. It was then bored on the centerline for the rack. The assembly is then bolted thru the frame extension. On the passengers side, I welded a 2 x 2 x 3/16″ gusset below the front crossmember, in line with it. I then drilled a new mounting holes. I used a die grinder to relieve the edges of the rack to miss some welds.
Once the mods were completed, the true test came – did I fix it or have I just made the problem worse. Measuring my bump steer at the finish (and adjusting it to optimum settings), I ended up with .034″ max at 2″ of bump and .012 max at 2″ of droop. I was one happy camper. Measuring the right side, I wasn’t as fortunate, I was getting .022 max at 2″ of bump but at droop, the toe was going to .048 at 1 1/2″ then reversing and going to .020″ at the 2″ point. The tie rod requires some tweaking. Part of the analysis revealed the inner tie rod pivots are about 1/2″ too far out. The Mustang II rack is 24.50″ between pivots. On my build, it would steer better if the pivots were 23.50″. For now, I’ll just live with what I have.
On the passenger side, I fabricated a nylon bushing to hard mount the rack. I chose to move the rack back about 1/4″ to imrove ackerman settings.
The modification to do this was not complicated but it was a little more than bolt on parts. I’ll be happy to share details with anyone who decides to tackle it.
My car also has an Independent Rear Suspension, and it can add to the steering issues. I was pretty fortunate, the roll steer is less than .030″ in four inches of movement. A trick when setting the rear, it is opposite of the front – set the toe first, then set camber by adjusting in the heim joints an equal amount on each.
These links proved very useful in my research of Bump Steer and how to correct it.
Morris Clements comments: http://www.gasholes.com/smf/index.php?topic=3232.0
this is a very lengthy thread, the bumpsteer section is around page 20 and 30. It is worth the time to read the whole thing. Morris provides the .021″ per inch tolerance basis.
More comments by Morris on the ClubCobra site: http://clubcobra.com/forums/showthread.php?t=60445&highlight=roll+steer&page=2
Longacre Racing: http://www.longacreracing.com/articles
This info provided by Longacre tells how to measure and what needs to change to correct what.
Modified Mustangs: http://www.mustangandfords.com/techarticles/mufp_0611_bumpsteer_explained/index.html
A good article, very helpful
str-427-cobra.blogspot :Click here
About half way down the blog, he discusses his findings. The use of a magnet laser light on the the rotor for rough evaluation really sped up the process of tuning.