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 see BIG WINGS 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

I had 50% anti-squat but changed it because my 60 foot times were terrible at Pro-Solo (drag race start) and the car would aggressively squat down in the back when I got on the gas. I have not had an inside wheelspin problem but it has happened briefly. I have a Torsen T2R diff

I had 50% anti-squat but changed it because my 60 foot times were terrible at Pro-Solo (drag race start) and the car would aggressively squat down in the back when I got on the gas. I have not had an inside wheelspin problem but it has happened briefly. I have a Torsen T2R diff

That makes sense for sure. The higher anti-squat definitely helps drag starts.  It's usually not a wheel spin problem when the sway bar picks up the inside tire. It just unloads it, freeing the car up. I think it will start being an issue, only once you're trail braking.

Back to the 5 items ...

2. Rear Anti-Squat

One would think to run the same anti-squat as a stick axle if the car was balanced. But most do not. That is because knowledgeable Racers are managing a "total grip package" with IRS. We are freeing up the car (reducing rear grip) with rear roll steer, to make the car turn better. We can get that lost grip back, by simply lowering the rear roll center. (This works on straight axle as well). I think you'll find most successful IRS Autocross cars in the 4"-7" rear roll center range.  Just remember if you ran this with no rear roll steer & perfect camber, it would be too tight. 

So in your "total grip package" with IRS & a low rear roll cenetr, you can run less anti-squat as well.  Very common to see around 40% on winning cars. The lower anti-squat increases rear tire grip on corner entry, helping with trail braking.  The lower rear roll center provides the needed grip to literally drive around the corner & accelerate quicker on the corner exit.

3. Rear Camber Changes:
For most IRS autocross cars, we run "around" -1° Static Camber. The final amount depends on what the final geometry does to the camber with body roll.  Man!  I have seen a zillion different final IRS geometry setups. IRS setups where the negative camber is lost on the outside tire & gained on the inside tire is most common. It is correct if disengaging the inside rear tire to assist in turning is your goal.  Which it usually is.  It is backwards of what you want, if you're trying to increase total rear grip when cornering.  It is the nature of most IRS designs.

There are rare IRS setups, where the outside tire gains negative camber & the inside tire loses negative camber. This creates the most TOTAL rear grip, but is not always possible to create with set length control arms and/or mounting points. Some odd setups gain negative camber on both sides, which also reduces grip on the inside rear tire & increases it on the outside tire. Now of course, how much body roll someone's car has, affects the camber change amount. 

I have about 10-12 IRS designs specifically for road race cars with different body width rules & ride height rules. The images below show a narrow (relative for a modern race car) track width (59.5" track / 73.5" Outside Sidewall).  The first image shows the IRS rear suspension (from the rear view) at static ride height.

The second image shows the car in body roll. It is only 0.7° of rear body roll, which is super low, due to the low ride height & aggressive race setup. Typical IRS autocross cars would roll much more. In the second image, I have the rear roll steer set to "Neutral", meaning zero roll steer.  (Notice NO toe changes from static.)

In the third image below, I have the same setup, but with significant positive rear roll steer. (Notice the toe changes from static.) This would be a rocket around tight corners, but require a wing to not kill yourself on the 150mph sweeper.

I will not be making a decision on this until September but if I do pull the trigger, I'll start putting together a design and share it here to get some feedback

To finish up the 5 items of consideration in designing (or configuring) your new IRS ...

4. Rear Bump Steer
5. Rear Roll Steer

4. Rear Bump Steer is similar to front bump steer, referring to when the wheel & suspension go up or down, due to bumps in the track surface. In an ideal world, we would want zero wheel angle change from bump steer & whatever our target rear roll steer is. But that's not possible. If you have one, you have the other. 

The reason for both of these is tire contact patch angle. Ideally we would want tire contact patch angle change from bump steer & whatever tire contact patch angle change our target rear roll steer creates. Again, that's not possible. If you have one, you have the other. 

Now it is possible to have zero wheel angle change from bump steer AND wheel angle change from roll steer. I achieve these two all the time in working out where the rear toe link mounts on the chassis & upright steering arm. But if you want rear roll steer, you will have rear bump steer.

Because we're mechanically changing the tire contact patch angle during both, we want to run the minimum necessary to achieve our rear steer goal. We don't want the tire contact patch angle changing back & forth a lot as the car drives over bumps. If we did have a lot, we would experience more grip loss over bumps than we already do.

5. Moving on to Rear Roll Steer.  This is a strategy decision if there every was one. I don't casually play with Rear Roll Steer.  It is a strategy decision based on the handling needs of the race car.  Two examples:

Positive Rear Roll Steer:  If we need the car to turn better than we can achieve working on the front end, then we can make the decision to add in positive Rear Roll Steer. That is when both tires change from running straight (or toed in) to pointing to some small degree towards the outside of the corner. This helps the car turn sharper, like the rear steer on a forklift.

This also makes the car have less rear grip. So in our strategic decision making, we have to account for this. On higher speed corners (think 100+ MPH sweepers) the solution is adding rear grip through the downforce of a properly designed rear wing or spoiler. This is why you see tall spoilers or big wings on so many IRS Corvettes. The aero is less effective in the tighter, slower corner where we needed better turning. So the combination of positive Rear Roll Steer & rear aero downforce help both of those areas. 

The one area still needing attention is forward grip. You need to make sure you dial in the rear anti-squat to help the car have enough forward driving grip as you accelerate out of the corner. So like so many things in car racing, it the the COMBINATION of key items that dictate the overall performance of the race car.  More true in IRS than a straight axle.

Negative Rear Roll Steer (also called Counter Rear Roll Steer) is the when both tires change from running straight (or toed in) to pointing to some small degree towards the INSIDE of the corner. This INCREASES rear tire grip, BUT makes the car HARDER to turn sharp.

I have only used Negative Rear Roll Steer in two unique situations. Both are considered to be tuning "Band-Aids" to some degree. The first situation is when you need more forward driving grip off the corner ... and you have run out of all other tuning options to achieve it. Adding some Negative Rear Roll Steer will help you achieve that. (This can also be achieved in straight axle suspensions with the lower links.) Unfortunately, I found myself doing this on both oval track & road race cars, when we ran out of adjustment. We would always correct the car back at the shop, so as to not need this band-aid next time.

The second is when I worked out a unique setup for Tom Whalen's C5 Corvette. He wanted to win the Silver State &
Big Bend Unlimited Class with only 700HP LS & NO AERO.  He had crashed & totaled a C5 at 180mph trying this & almost killed himself. I designed a setup with counter rear steer, so that as Tom drove the corners at 160-180mph, the chassis rolled 1.5° & the C5 rear counter rear steered 3/16" toward the inside of the corner, creating plenty rear grip. Tom Whalen won the Silver State Unlimited Class once in 2016 & the Big Bend Unlimited Class twice in 2016 & 2018, with this setup.

Hope this helps clarify your areas of focus when you build your IRS.