Wednesday, March 2, 2011

Setup Questions from Readers

In response to a recent reader request, this post will kick off a more interactive way to blog.

The reader asked, "Would you mind answering some setup questions on the blog?"

You bet. Here we go. Send your questions in the comments to this post.

Let's do it this way:
1-Keep the questions short, punchy, and focused. It's difficult, if not impossible, to reply to something as broad as "How do I get rid of mid-corner understeer?" in a short answer.
2-If there are a lot of questions, I may have to pick and choose which ones to answer. If your question doesn't show up in the blog comments, it's either been delayed or set aside.
3-In this initial thread, why don't we limit questions to setup? Maybe we'll start another thread or two later for other topics, like "life as a race engineer", race strategy, organization and planning, etc. In other words, other topics in the scope of the blog.
4-I'm going to reserve the option to wrap up the thread, for later resurrection, if needed. My upcoming schedule is pretty busy and I'd rather avoid leaving a bunch of folks hanging.


  1. Does changing the angle of your lower trailing arms in a "3-link" rear suspension (ie. Stock Car) really change the roll center height?

    You need the angle of the lower trailing arms to determine your roll axis needed to define an intersection height at the rear axle plane. However, I have never heard anyone say that you need to recalculate your roll center when you change your lower control arm angles........

  2. IF 3-link means two lower longitudinal trailing arms and one upper...

    The lower trailing arms only affect the roll center if they converge on the center of the car, as viewed from above. If they are parallel to the centerline of the car, they don't change the roll center. The anti-squat/brake anti-lift and roll steer changes can and will be significant, though.

    Roll center will be determined by your Panhard rod, Watts link, Jacobs ladder, whatever you have. Don't forget that without an SLA suspension, lateral jacking is decoupled from roll center height. Some of what the world attributes to roll center is actually jacking.

  3. Thanks for the opportunity.

    For a hypothetical FWD car (whose susp. pro/anti is not known) is it possible to see both the front and rear ride heights go down during braking in telemetry?

  4. Yup.
    It's just not common for the front to have enough anti-dive to prevent the front from moving lower in braking. At the rear, some FWD cars, typically the less expensive ones, will have a beam/trailing arm suspension which will have A LOT of brake anti-lift. Maybe even enough to force the ride height down in braking? Well, you should take some simple measurements and run some calculations, but it's far from impossible. Remember the sprint cars, with a lot of anti-squat, that LIFT the rear ride on acceleration? Some principle, different direction.

  5. Buddy, if I do simple load transfer calculations for a trail-braking scenario, it looks like the outside front tire should have gained the most load, and the inside rear tire should have lost the most load. But if I think about trail-braking in terms of how to corner-balance a car, that would imply that the inside rear actually gained some load along with its diagonal corner. What's really happening here?

  6. Hi Buddy:

    How do you differentiate corner entry instability due to the driver overdriving the car (braking late for example), and the car just being at its limit? How do you know mechanically there is just nothing else to do to to get the car to go faster in that corner-except asking the driver to take it easier?


  7. Regarding the trail-braking load transfer, your initial calcs are probably correct. Load has indeed transferred to the front and the outside of the corner. In the absence of the effects of roll stiffness and roll centers, you would still have the same crossweight, just rearranged in where it falls. Outside front heavier, inside rear lighter, the other two somewhat closer to static. Try this coarse example:
    Static 1000 1000
    1000 1000
    Brake 1200 1200 transer 200 to front
    800 800
    Turn 1000 1400 transfer 200 to right
    600 1000
    Now, in the real world, the crossweight WILL change due to lateral load transfer distribution in the suspension.

  8. Oops, my formatting disappeared and I'm too lazy to retype the whole reply. Hope the weights are clear.

  9. JS, that's really a nice question. They would look similar in data. The driver would report entry O/S. And, there will be a late-corner push from not getting rotated early, having wound out steering to correct the O/S. One thing is for sure, an observation visit to the corner in question would help. It's also likely that it would respond more effectively to chassis tuning if it's caused by the car. Real entry instability comes from the obvious sources - too much front aero, too much rear brake, diff too open for the chassis setup, too low front roll center, etc., etc. A hard look at the design and setup of the car would be worthwhile. I would suspect that a G-G diagram would show too much combined Gs if it's the driver and not enough if it's the car. To be completely honest, it's been years since I worked with a driver who would blindly overdrive the entry. Many pros ask for more entry stability, to allow them to hustle the entry.

  10. Something is really slowing down your site while loading.

  11. Sorry, I can't find anything wrong. I just use Blogspot, and there's nothing I can do about performance in the options available to me.