I have a 67 Camaro that is equipped with Speedway Motors G-Comp X front suspension and Speedway Motors torque arm rear suspension. It has Penskes all the way around, 315/30-18 tires front and rear and I'm pretty serious about autocross and competing at SCCA Solo Nats in CAM-T class. I have a 6 speed trans and engine makes about 575 hp to the wheels. I am considering going full IRS and would be designing and building the whole thing myself. If I pull the trigger on this, I would run inboard brakes and would scratch build upper and lower control arms and the uprights using C7 Vette hubs. What should I consider for ideal geometry? Does the geometry of the front affect what I should do in the rear? My suspension is all entered into Performance Trends Suspension program, although I have made some changes and should probably double check all those measurements again.

Great question Mark,

Half of the answer needs to pull information from your current straight axle setup.  The other half will be IRS specific.

When you're laying out your basic geometry in a software program, two things you'll want to design in are your target anti-squat & rear roll center. Since you are already racing your car with the straight axle, let's pull some details from what you're currently doing. 

Questions for you:
A. What is your current Anti-Squat
B. Do you feel you need more rear grip on corner entry (under braking) or exit (under throttle)?
C. What is your current rear roll center?
D. Do you feel your mid-corner handling is most often neutral, tight or free handling?
E. Would you like to tighten up, or free up, the mid-corner grip with your IRS?

Lastly (for today only), what are your goals with transitioning to IRS? What do you hope to achieve?

Great question Mark,

Half of the answer needs to pull information from your current straight axle setup.  The other half will be IRS specific.

When you're laying out your basic geometry in a software program, two things you'll want to design in are your target anti-squat & rear roll center. Since you are already racing your car with the straight axle, let's pull some details from what you're currently doing. 

Questions for you:
A. What is your current Anti-Squat
B. Do you feel you need more rear grip on corner entry (under braking) or exit (under throttle)?
C. What is your current rear roll center?
D. Do you feel your mid-corner handling is most often neutral, tight or free handling?
E. Would you like to tighten up, or free up, the mid-corner grip with your IRS?

Lastly (for today only), what are your goals with transitioning to IRS? What do you hope to achieve?

A. Anti-Squat is 93.8%
B. Car is good on corner entry and exit but over the winter I attended Radford Racing school and they really emphasized trail braking and I imagine my change in driving style is going to lead to corner entry oversteer
C. Roll Center height is 12.75"
D. Neutral
E. To be honest, I don't know

the goal is to make the car more stable on bumpy surfaces and feel more planted and confidence inspiring overall.

Heidts makes a complete kit for my car but it's outrageously expensive. Take a look at it here https://www.hotrod.com/how-to/heidts-pro-g-irs-bolts-in-and-adds-modern-performance-to-your-classic-chevy

Great question Mark,

Half of the answer needs to pull information from your current straight axle setup.  The other half will be IRS specific.

When you're laying out your basic geometry in a software program, two things you'll want to design in are your target anti-squat & rear roll center. Since you are already racing your car with the straight axle, let's pull some details from what you're currently doing. 

Questions for you:
A. What is your current Anti-Squat
B. Do you feel you need more rear grip on corner entry (under braking) or exit (under throttle)?
C. What is your current rear roll center?
D. Do you feel your mid-corner handling is most often neutral, tight or free handling?
E. Would you like to tighten up, or free up, the mid-corner grip with your IRS?

Lastly (for today only), what are your goals with transitioning to IRS? What do you hope to achieve?

A. Anti-Squat is 93.8%
B. Car is good on corner entry and exit but over the winter I attended Radford Racing school and they really emphasized trail braking and I imagine my change in driving style is going to lead to corner entry oversteer
C. Roll Center height is 12.75"
D. Neutral
E. To be honest, I don't know

the goal is to make the car more stable on bumpy surfaces and feel more planted and confidence inspiring overall.

Heidts makes a complete kit for my car but it's outrageously expensive. Take a look at it here https://www.hotrod.com/how-to/heidts-pro-g-irs-bolts-in-and-adds-modern-performance-to-your-classic-chevy

I'm going to answer these in a different order.

B. Trail braking will make you significantly faster.  Significantly!  I'm sure you also learned in the class, that you can moderate your trail braking. That will be key in driving cars with different setups and especially different amounts of anti-squat.  As outlined in my rear suspension thread on here, the higher the anti-squat, the more rear grip we have on exit & less on entry. Conversely, the lower the anti-squat, the less rear grip we have on exit & more on entry.

A. 93.8%. Your Anti-Squat is 93.8%! Holy cow. That's the kind of anti-squat numbers (at or near 100%) we run with decoupled 3-links. In any other rear suspension I would have said you are loose-loose-loose on entry. But you saying you're not trail braking clarifies everything. You are spot on correct, when you migrate to trail braking, you will need more rear grip on corner entry. 

Here is my general guidelines. For all but decoupled suspensions, the happy window for Anti-Squat is in the 30% to 70% range.  30% has more grip on corner entry & allows the driver go deep & brake more. (More can be harder and/or longer) The issue will be getting out of the corner. Light, low powered cars can make out OK. But heavier, high powered cars will struggle for grip & acceleration. The driver needs to be egg under the throttle pedal gentle & patient as Job. Throttle control skills are critical with low anti-squat.

70% Anti-Squat has less grip on corner entry & requires the driver brake earlier & softer if trail braking. Brake modulation skills are critical with high anti-squat. Getting the car to accelerate out of the corner is easier with higher anti-squat (like 70%). The Driver can roll on the throttle much quicker without losing grip. Therefore corner exit acceleration is better.

Unless decoupled, anti-squat is always a compromise.  Think of your range of targets as 30%, 40%, 50%, 60% & 70% (in general).  Obviously 50% is a compromise in the middle. It tends to provide the best overall compromise, especially is the car is heavy (2000# & up) and high powered (400HP +).

Right now in the planning stages, you get to decide on the balance of rear grip you want. Think about the percentages & how you'll drive it. If you think getting off the corner quicker is a priority, lean to the higher anti-squat percentages. Just be prepared to brake earlier & softer. If you think getting into the corner deeper with more more speed is a priority, lean to the lower anti-squat percentages. Just be prepared to baby the car out of the corners. Where do you think will work best for your driving goals & the courses you compete on?

C.  12.75" is pretty high. Before I respond with suggestions, can I ask what spring rates you're currently running in the rear? Do you have a rear sway bar?  If yes, what is the rate range on it?

D.  Sweet!  Let's plan to keep that. Anti-squat is not an factor in the middle. But roll center, track width, CG, spring rate & sway bar rate will be the big factors affecting mid-corner handling.

E. The goal is to make the car more stable on bumpy surfaces and feel more planted and confidence inspiring overall is a very good goal. The IRS can help with the bump surfaces for sure. Of course there are things you can do to increase grip over the bumpy surfaces with your straight axle setup as well.  Let me know if you want to explore those.

I'm holding off on bringing up topics that are are IRS specific until we get these items ironed out. Answer the questions in A & C.

I am running 800 lb/in springs in the front and 250 lb/in springs in the rear. Sway bar is 525 lb/in front and 494 lb/in rear.

I am running 800 lb/in springs in the front and 250 lb/in springs in the rear. Sway bar is 525 lb/in front and 494 lb/in rear.

That's too much rear sway bar rate for the rear spring rate.  While the 800# front springs should be keeping the rear tires planted firmly, you still may be experiencing the inside rear tire coming unloaded.

Mark have you run the car with the rear sway bar disconnected?  And if yes, how was the handling specifically in each section of the corners ... entry, mid-corner & exit?

I am running 800 lb/in springs in the front and 250 lb/in springs in the rear. Sway bar is 525 lb/in front and 494 lb/in rear.

That's too much rear sway bar rate for the rear spring rate.  While the 800# front springs should be keeping the rear tires planted firmly, you still may be experiencing the inside rear tire coming unloaded.

Mark have you run the car with the rear sway bar disconnected?  And if yes, how was the handling specifically in each section of the corners ... entry, mid-corner & exit?

I had 850 lb front springs and 225 lb rear sway bar but the car could be pushy so I went stiffer on rear sway bar and the balance was good but the front sway bar was at full soft and I had no place to go with it. I softened front spring to 800 lb and it has nice balance now. Tire temps were even front and rear and across the tread so I left it. I am switching tire brands (From Vitour to Bridgestones) and I think my driving style has changed a lot over the winter as a result of racing school and many hours of simulator time. I may find the balance to be not good. Also, I have moved weight balance of car about 1 or 1.5% more rearward since I last drove the car.

I am running 800 lb/in springs in the front and 250 lb/in springs in the rear. Sway bar is 525 lb/in front and 494 lb/in rear.

That's too much rear sway bar rate for the rear spring rate.  While the 800# front springs should be keeping the rear tires planted firmly, you still may be experiencing the inside rear tire coming unloaded.

Mark have you run the car with the rear sway bar disconnected?  And if yes, how was the handling specifically in each section of the corners ... entry, mid-corner & exit?

I had 850 lb front springs and 225 lb rear sway bar but the car could be pushy so I went stiffer on rear sway bar and the balance was good but the front sway bar was at full soft and I had no place to go with it. I softened front spring to 800 lb and it has nice balance now. Tire temps were even front and rear and across the tread so I left it.

That makes sense.  I would have done a different tuning change, with increasing spring rate, instead of sway bar.

Here's why: In the rear, when the car is in diagonal roll ... combination of pitch/dive & roll when braking & turning on corner entry ... if the rear sway bar "wheel rate" is close to or exceeds the spring rate, the inside rear tire will lift off the track surface.

I suspect your car is either lifting the inside rear tire currently, or close to doing it, and the high 12.75" rear roll center is the only thing preventing it.

If you stick with the straight axle for awhile (before going to IRS) ... and work on introducing trail braking to your driving ... I would suggest:
  A. Reducing the Anti-Squat to around 50%
  B. Increasing the rear spring rate to 325#
  C. Reinstall the 225# rear sway bar

Then:
1. If the car is loose everywhere, lower the rear panhard bar until the handling is neutral.
2. If the car is tight or pushy on entry, soften the front spring rate.
3. If the car is loose on entry only, stiffen the front spring rate.

Here are the topics that are are IRS specific:

1. Rear Roll Center
2. Rear Anti-Squat
3. Rear Camber Changes
4. Rear Bump Steer
5. Rear Roll Steer

1. In most cases, the Rear Roll Center is not tunable. When I designed the Extreme IRS for Speedtech, I designed it where the lower control arms had two different front mounting positions. One was a lower rear roll center & lower anti-squat ... for optimum autocross performance. The second mounting position provided a higher rear roll center & higher anti-squat ... for optimum road course performance. For some reason they choose to not include both mounts. If you design your own Mark, I'd suggest you do the same as I did with two mounts. Or, you may just want to prioritize autocross. 

Once you have your straight axle combination dialed in, I think you will have a lower rear roll center than you currently do.  Some people may think of that rear roll center as your ideal target for your new IRS package. They'd be wrong. Successful IRS combinations always have lower rear roll centers. (I'm sure there is some exception out there. But I've never seen it.) The reason is IRS typically does not create as much grip as a straight axle for various mechanical reasons.  By typical, I mean OEM systems being ran in autocross and/or on road courses.

The reasons the rear roll center "typically" needs to be lower, for more rear grip, is many other factors in the "typical" IRS geometry are less than optimum. I typically see them with too much camber change (good or OK on one side & bad the other) & too much rear roll steer. These reduce the grip, so the lower rear roll center is needed.  Frankly, that's not enough on faster tracks like road courses and high speed open road runs.  So you BIG WANGS on most IRS cars. This is due to the lower grip of the IRS geometry not being optimum.

If you design the geometry to optimize the camber change on both sides ... and have neutral roll steer ... you'll want the rear roll center a little higher. Now, I'll throw you a curve ball & clarify what the goals & strategies should be.

Curve ball: The goal of IRS should not be to simply create more grip. The only place IRS can create more grip that a straight axle can, is over rough surfaces. This is an advantage, because all track surfaces are "rough" to some degree. Even tracks we consider "smooth" have undulations we don't see unless we put a long straight edge on a section of track.  See image below. The race tires & suspension of a really well refined straight axle suspension does a great job of keeping the tires loaded, driving over all the track surface undulations.  A really well refined IRS will do a better job.



The bigger advantage of an Independent Rear Suspension is we can configure it to help us turn better in the tight corners. This is where Roll Steer comes into play.  I'll go into more detail later, but the short story is, we can design in positive rear steer (tires pointing toward the outside of the corner) to help the front end. If you know what you're doing, you can use this roll steer to make the car drive around the tight stuff measurably better than a straight axle. But in doing do, you reduce the overall rear grip. This is why the big ol' rear wing is needed. The IRS car that is great in the tight stuff, will be scary loose in the high speed corners, unless we add rear aero downforce.

Take this into account as you work out your IRS game plan. I'll be back soon to discuss items 2-5.



2. Rear Anti-Squat
3. Rear Camber Changes
4. Rear Bump Steer
5. Rear Roll Steer