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IRS and anti squat measuring

Suspension Tuning & Optimization

Relevant Module: Suspension Geometry > Anti Effects

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Discussion and questions related to the course Suspension Tuning & Optimization

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I have 2 different questions relating to measuring anti squat. The first is how to measure it. The video shows the anti squat as a percentage, with the line drawn from the rear tire contact patch, to the roll center. I have seen several places say to measure it as either squat or anti squat, with a neutral line drawn from the contact patch to the center of gravity height above the front center of wheel location, and where the roll center lands (above or below the line). I'm wondering which one is right, or more accurate, or if they are the same answer measured two different ways.

The second question is regarding measuring anti squat on an IRS car. Does the neutral line get drawn from the tire contact patch, the center of the wheel, or the center of the axle at the differential?

Nothing from HP Academy?


Our apologies for the delay in response. I've been working Pikes Peak Hill Climb and Andre is on vacation. Andre will return next week and I've left him a link to this thread so he will be in touch soon.

Thank you for your patience,

In the meantime, Sam has created this graphic to hopefully clear things up a bit.

Attached Files

Hi Charles, I'm back on deck now. The method demonstrated in the course will provide you with the correct anti squat percentage. Did the graphic posted above by Mike/Sam help clarify your questions? Do you still need further help?

Thank you for the responses and visual. It sounds like the ways I described it are incorrect then? I'm asking because of how I described it is in the book "Chassis Engineering" by Herb Adams. I know that book is referenced at some schools for chassis design and engineering.

Regarding my second question, it seems you are also saying that it doesn't matter if the rear suspension is IRS or not. Anti squat is still figured from the line drawn from the rear tire contact patch and not center of the diff or axle.

Attached Files

Hi Charles, the method you described is not incorrect.

As Tim mentions in the course at 1:20, there are several ways to define/quantify anti-effects and we've chosen the method that most simply and comprehensively describes their contribution to suspension tuning and optimisation to suit this course.

'Chassis Engineering' - Herb Adams and 'Race Car Vehicle Dynamics' - Milliken & Milliken, both describe anti-effects from the perspective of chassis and suspension design, taking brake and drive torque into account which are very important to understand when designing a system (e.g. IRS/Live Axle, inboard/outboard brakes) but difficult to understand, change and tune in practical terms since they're part of the chassis configuration.

The method we use is covered in Carroll Smith's - 'Tune To Win' and focuses on anti-effects as a tuneable function of suspension geometry, in reaction to longitudinal load transfer which is the most appropriate method to understand for this course. I've attached an excerpt from the book below.

In simple terms, longitudinal force is applied through the COG at a distance above the ground creating what Carroll Smith calls the 'Squat/Dive Moment'. If our side view swing arm is in-line with the COG, longitudinal load is directed along the swing arm to the tire contact patch and we have effectively 100% anti-dive (no elastic contribution to load transfer reaction/locked suspension). As that virtual swing arm intersects lower along the dive moment more of that longitudinal load is transferred into the elastic components of our suspension.

Hopefully that helps provide some context for why we use the method we do to define anti-effects as a suspension tuning tool, rather than a chassis design tool for this course?

Attached Files
  • Anti-Effects-_-Tune-To-Win-Carroll-Smith.jpg
  • Attachments may only be downloaded by paid Gold members. Read more about becoming a Gold member here.

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