Total Lateral Load Transfer Distribution

[hquackenboss]

@sleepeer68

I read your thread, and I want to suggest a sequence of changes. To start, I have some observations about your car based on the photos, and your depiction it understeers everywhere.

You don’t need to calculate your roll angle. You have enough information to actually measure it.

I pasted a copy of the picture that you said is a 1.2G 50MPH corner and measured the roll angle. I did this as follows:
1. I pasted the picture into PowerPoint. If you don’t have Microsoft Office, you can use Google Slides, construct a triangle, and calculate the angle with high school trigonometry. With PowerPoint, you can do it without any calculations, by:
1 . draw a horizontal line. You can make the line width as wide as you want and pick a bright color.
2. I rotated the photo and moved the horizontal line until the contact patches of the inner and outer front tires were on the horizontal line. With a little trial and error, typing in decimal fraction angles, and move the horizontal line until the it is aligned with the contact patches of the inside and outside front tire, you get a picture like I have uploaded. It is a little hard to discern exactly where the outside contact patch is, so I might be a little off, and I recommend you reproduce my analysis on your own computer.

1. Based on my analysis, while I don’t know the actually cornering force, the measured roll angle (you can see the measurement from PowerPoint to the right of the picture) is (180 degrees - 177.6 degrees) = 2.4 degrees. My conclusion is you are in the right ballpark.
2. It may be too much of a stretch to judge conclusively that your car understands just from the picture, but since the roll angle is high enough to indicate the lateral force is pretty high, your front steering angle is pretty high, so yes, based on the photo, it looks like it understeers.

Recommendations:

1. I would not raise the rear roll center height under any circumstances. Accepting the trade-off of an increase in front roll center height to get a camber gain improvement is one thing, but it is the wrong thing to do to balance the steady-state handling.

2. I wouldn’t change the spring rates, or geometry at least, yet. Optimizing spring rates in these kinds of cars is harder than it looks, and if you haven’t done anything crazy, there are more important things. Same answer for making further geometry changes. Your geometry isn't way out of whack from what good practice is, I think you are still at the point where you can make dramatic improvements without messing with geometry.

3. Your alignment specs with this geometry (-1.5 degrees negative camber, ~6 degrees of caster) are not way off. I would leave them until you at least do the following:

4. I would increase the rear anti-roll bar (ARB) stiffness a bunch, to the point where it clearly oversteers. I would do this with no other changes to start. If I understood your posts, you said slightly increased the spring rate. Whether or not you did, I would not back that off. I would, to repeat what I just wrote, increase the rear ARB stiffness.



5. If you cannot do (1) right away, try reducing the front tire pressures. Especially for autocrossing, you can get to the low 20 PSI range. This will reduce the ride rate (the tire is also a spring, and reducing the pressure lowers the spring rate of the tire). But you are going to need to to (1).

6. Regarding locking your inside front tire, you should realize that if the car understeers, it is going to be more prone to inside front lock up because of the lateral and longitudinal load transfer. If you still have problems after increasing the rear ARB stiffness, then there are a few things you can do. First, is to reduce pitch under braking (this might not be enough, but it is easy to do if you have adjustable shocks compared to alternatives.
a. Increase front damper (shock) compression stiffness. If you happen to have 4-way adjustable shocks, adjust the low-speed compression stiffer.
b. Increase rear damper rebound (extension) stiffness. Again, if you have 4-way adjustable shocks, adjust the low-speed rebound stiffer.

7. The front end of the car looks a little high. I can see your headers are kind of low, and I doubt you want to scrape going over speed bumps, but I would lower it. This does three things: First, since with this geometry, the rate of negative camber increase under compression increases the more it is compressed, lowering it will further increase the negative camber on the outside tire for a given roll angle. If you have modeled your geometry, you can confirm this. if you don't think this is correct, please post a curve. Even better, if you do happen to take the front springs out, cycle one of the front assembly with the springs out (so you can move it from full droop to full compression, and plot it. See number 8 (below) regarding ball joints.

If you lower the car, which will increase the rate of camber change as the outside front suspension compresses, this will reduce the positive camber on the outside front tire. The other two things this does, is lower the CG of the car, and lowers the roll center.

8. Given you have a way of measuring and adjusting bump steer (I mean measuring it on the car, not just from the model), then I would use a taller upper ball joint, which will increase the negative camber gain. In fact, you can also use a taller lower ball joint, which will have the effect of lowering the car, and further increasing the rate of negative camber gain under compression. As a general rule, if the toe steer (bump steer) goes very slightly positive toe out under compression, then you get a sort of automatic counter-steer, which can make it a little easier to stay at max lateral without so many small steering corrections.

9.. I am guessing you have aftermarket lower control arms, which have some form of non-rubber bushing. If in the off chance you are using stock lower control arms and bushings, your cornering forces are high enough to compress them to the point where your actual camber on your outside front wheel is more positive than your calculation. The stock arms are probably fine, but you need to replace the bushings, and my strong preference is not urethane. I am focusing on the lower control arm bushings because they see higher load than the upper control arm bushings do, but both are important. Also check for any slop in the ball joints. A number of aftermarket units have enough slop to cause problems. Also, if you do have solid bushings, , it is crucially important if you have solid bushings that they move freely. Even urethane ones can be lubricated with a high-pressure lube, and some have grease fittings.

10. Beyond the above, there are a set of relatively easy things you can do with spring rate tuning using jounce bumpers, and some other things, but rather than provide more opinions, the first step is to get to a near-neutral balanced car by increasing rear roll stiffness, and then the other things listed.

I have an observation in general about comments on the thread, and what I see from people that look at Milliken Moment Diagrams, and models with load transfer equations, spring rate calculations and so on. Bill (the now deceased father) and Doug Milliken's book, Race Car Vehicle Dynamics, is a fabulous book, but it has so much information, that it is pretty difficult to appreciate the big picture, and to figure out which changes have the biggest impact, and what to do first. Also, it doesn’t emphasize some crucially important to keep three things in mind, which can screw up the handling: deflection, slop, and stiction. Ball joints and steering linkages have slop, welds in aftermarket parts can crack, solid bushings can bind, and rubber ones can deflect.

Finally, I am a stranger on this message board, so you might ask, who is this yahoo with all these opinions? Here is an article I wrote that describes how I got my start: https://www.pontiacv8.com/blog/2019/...ry-quackenboss I also built a C4 street prepared Corvette that was national championship competitive, and these days I advise a collegiate Formula SAE team that in the past few years finished first among 80 North American colleges, and finished 6th of 120 global colleges.