Front end problems
06-13-2011, 12:04 PM
#1
Front end problems ok as some of you know it's been a long haul to get this car back on the road. I have another issue I have been fighting since 2006. First I have rebuilt the front end trying to fix this, I have done bushings, ball joints, inner and outer tie rods, intermediate steering shaft between the rack and column base, I even put another rack in it. Here is what it does. I get it aligned and it feels great, tracts straight, brakes straight, wont grab every rut and groove in the road. That last anywhere from a day to a couple of hours. Then it's right back to tracking all over the place and under braking it is almost scary. I wont let the wife drive it just because it's that erratic in trying to change lanes. Tires are wearing on the outside edges and I have alignments done by 4 different places, last 2 times at the local dealer and once they did what they call a carriage (K member) adjustment. Same thing each time. I am about to take it back to the dealer again but I do not understand a lot about alignment other then making the wheels and car track correct. I understand camber and toe but not so much about how the caster affects the car. What am I missing here? This thing should feel like it did when it was brand new. It aligns every time and the guys say there was no problem adjusting it. It's I have taken a look at some of the sheets and it seems it needs it everytime like something is changeing in the front end. The rack is tight to the K member, the K member is tight to the unibody, the adjustments are tight at the lower control arms. Any Ideas?
06-13-2011, 02:18 PM
#2
Here's what I remember about caster:
Steering axis inclination (SAI) is how much the relative position of the lower control point (lower ball joint) and upper point (upper strut mount) varies from 0*. if these points were directly above/below one another that would be 0* inclination. Since the lower control point of the steering axis is further forward than the upper, you have positive inclination toward the rear of the car. This resembles the way bicycle forks and handle bars are tilted rearward. This helps the front wheel return to center under forward movement. A shopping cart's front wheels are a good example of no or zero SAI but negative caster. They center and point rearward under forward movement, but the point wherethey attach is parallel to the floor. Your strut mounts lean back a tiny bit (as well as inward).
A car has a pair of casters- the front wheels. Since their steering axis is tilted rearward, they return to center together, by way of the center link/tie rods/sway bar. Caster is measured in positive degrees (think lower point in front of the upper point is positive, opposite is negative caster like shopping cart wheels).
Caster is not a wearing angle; however, it will pull to the side with the least amount of positive caster. The side with the least rearward tilt of the steering axis will be the side it pulls to. Cross-caster is the difference between the 2 sides. There is usually a tiny amount of difference in the alignment spec to allow for road crown.
If your outside edges are wearing, I suspect you have a camber problem, and the wandering weak feeling you get is related to insufficient positive caster. If they moved the K member as far forward as it will go and it still feels wrong, you might need to talk to someone who knows the use of camber/caster eccentric sleeves to put around the lower ball joints. This allows adjustment beyond the factory limitations to accomodate the need for additional negative camber and additional postive caster.
I used to see this all the time on full size Ford vans back in the mid- late 90s. The only way we could cure them was to sleeve them with a camber/caster sleeve.
Hope this all makes sense.
Steering axis inclination (SAI) is how much the relative position of the lower control point (lower ball joint) and upper point (upper strut mount) varies from 0*. if these points were directly above/below one another that would be 0* inclination. Since the lower control point of the steering axis is further forward than the upper, you have positive inclination toward the rear of the car. This resembles the way bicycle forks and handle bars are tilted rearward. This helps the front wheel return to center under forward movement. A shopping cart's front wheels are a good example of no or zero SAI but negative caster. They center and point rearward under forward movement, but the point wherethey attach is parallel to the floor. Your strut mounts lean back a tiny bit (as well as inward).
A car has a pair of casters- the front wheels. Since their steering axis is tilted rearward, they return to center together, by way of the center link/tie rods/sway bar. Caster is measured in positive degrees (think lower point in front of the upper point is positive, opposite is negative caster like shopping cart wheels).
Caster is not a wearing angle; however, it will pull to the side with the least amount of positive caster. The side with the least rearward tilt of the steering axis will be the side it pulls to. Cross-caster is the difference between the 2 sides. There is usually a tiny amount of difference in the alignment spec to allow for road crown.
If your outside edges are wearing, I suspect you have a camber problem, and the wandering weak feeling you get is related to insufficient positive caster. If they moved the K member as far forward as it will go and it still feels wrong, you might need to talk to someone who knows the use of camber/caster eccentric sleeves to put around the lower ball joints. This allows adjustment beyond the factory limitations to accomodate the need for additional negative camber and additional postive caster.
I used to see this all the time on full size Ford vans back in the mid- late 90s. The only way we could cure them was to sleeve them with a camber/caster sleeve.
Hope this all makes sense.
06-17-2011, 11:37 AM
#3
I am going to take it back and see where it is at and then take it back every couple of days so I can create a log of what it is doing with alignment printouts. Thanks for the info it makes sense and there is a question in my mind if the sway bar may have something to do but I'm not sure. This was my step dad's car and he did hit a small tree with it on the right front tire. They had to replace the k member, tie rod had snap so that as well and they may have had to pull it on a frame machine but I have no documentation and I had no idea he was going to leave it to me so I did not get to ask him. Now one of the first things I did was LS1 brake upgrade and I went ahead and did new ball joint and all that listed in the original post. The strut was bent at the very top when I replaced it. it had a very noticeable bend in it and I wondering if the sway bar is bent as well because it was an absolute bitch to get back in and is the only thing besides the control arms I have not replaced. It aligns every time like I said and talking with the guys in the frame shop they say if it aligns then there is no problem with it because they can return the front end to factory settings. I am just wondering why it seems to fall out of alignment. I don’t have enough alignment print outs close enough together to prove this but something is going on because as soon as I change the toe out a bit the car seems to react fine. It's almost like something is flexing.
06-20-2011, 12:45 PM
#5
During an alignment check a good tech will perform an "included angle" sweep. This measures the variances in SAI change from side to side. It helps pinpoint bent or misplaced front suspension components. The sweep is done manually by moving the wheels from lock to lock with the alignment heads on the wheels with the steering wheel unlocked. This is done after the toe is set so the steering wheel stays at center relative to the steering rack like it's supposed to.
If the tech does not perfrom an IA sweep as part of the procedure, it's entirely possible to go through an alignment and set all the specs with the wheels centered and have everything come up 'looking' normal and within spec, having essentially moved the adjustments to make up for the bent or misplaced components, statically. However, when things move, especially when under the force of dynamic flex, you have behavior that defies what you've been shown on the alignment printout sheet before driving it. Dynamic flex describes how things move when the weight of the car moving at speed forces the suspension to flex a bit. This is one of the reasons toe is usually set a tiny bit negative (leading edges of the tires closer than the trailing edges). The force of movement and weight moves the leading edges apart a fraction of a degree while underway. Camber and caster change a tiny bit under dynamic flex as well.
A practice I used to hate seeing was slotting the holes in the upper strut mount. Sure, it lets you move the upper strut mount back further to make extra positive caster, but if the problem that made it necessary was the lower mount point being too far back, you've essentially created setback, where the wheel assembly is a hair further rearward than the opposite wheel. This is common practice in oval track race setups but doesn't have any business on a street driven car.
You might get with someone who does unibody correction. This will most likely be a collision specialist who knows how to correct unibody damage, usually using laser measurement equipment. Even then, to get it perfect it may still be necessary to use a camber/caster sleeve on that side where the damage was.
If the tech does not perfrom an IA sweep as part of the procedure, it's entirely possible to go through an alignment and set all the specs with the wheels centered and have everything come up 'looking' normal and within spec, having essentially moved the adjustments to make up for the bent or misplaced components, statically. However, when things move, especially when under the force of dynamic flex, you have behavior that defies what you've been shown on the alignment printout sheet before driving it. Dynamic flex describes how things move when the weight of the car moving at speed forces the suspension to flex a bit. This is one of the reasons toe is usually set a tiny bit negative (leading edges of the tires closer than the trailing edges). The force of movement and weight moves the leading edges apart a fraction of a degree while underway. Camber and caster change a tiny bit under dynamic flex as well.
A practice I used to hate seeing was slotting the holes in the upper strut mount. Sure, it lets you move the upper strut mount back further to make extra positive caster, but if the problem that made it necessary was the lower mount point being too far back, you've essentially created setback, where the wheel assembly is a hair further rearward than the opposite wheel. This is common practice in oval track race setups but doesn't have any business on a street driven car.
You might get with someone who does unibody correction. This will most likely be a collision specialist who knows how to correct unibody damage, usually using laser measurement equipment. Even then, to get it perfect it may still be necessary to use a camber/caster sleeve on that side where the damage was.
