Drag Racing on IRS Sucks.

[motorhead]

Not that this subject is news or anything. I've been thinking about this issue a lot lately (and I already touched on it in another thread: https://www.hellcatforum.org/thread...sier-28x10-5-17dbr-comparison.5060/post-62175), especially since I put the "drag pack" on my car and got an opportunity to take a close look at the suspension. The recurring theme is either: my car hooks up well all the time or my car hooks up some of the time and then traction goes away. It seems that 99% of the time people look at their tires as the direct cause of the loss of traction, and don't bother to look at the root cause. So to quote myself:

Has anyone here tuned the instant centre and anti-squat in an IRS car? 4 links, ladder bars, hell even leaf springs, sure. All day (the Drag Pack cars come with solid axles for a reason). But, the factory HC IRS? Nope. I bet most folks wouldn't even begin to know where to start, much less how to alter it. So, we ultimately attempt to mask the inherent traction built issues into the factory HC suspension with the biggest tires possible, inducing weight transfer, and locking out the suspension (as camber gain is not ideal). This covers it pretty good: https://www.svtperformance.com/threads/drag-racing-irs-in-here-need-tips.1070658/#post-15155552

Beyond changing tires, the a few factors that are well within the average person's control are altering spring stiffness, controlling movement at the differential, and setting the suspension angles (camber and toe). The latter is achievable with either aftermarket arms and/or a camber correction kit from MOPAR, and an alignment machine. The general rule of thumb is that you want 0* of camber at the hit (when the car squats or rises) to ensure the best contact patch, the Demon is a bit higher in the back for just this reason. And 0 toe for the most efficient use of the tires/power/etc and better control. Controlling the pinion angle, this either done by minimizing movement the pinion side (a snubber) or the ring side of the housing – a differential brace. Typically most folks use a diff brace, but the cradle lockouts help in reducing deflection too. Now spring stiffness (and associated shock characteristics) are an interesting one. Yes, you can swap in "Demon" springs and potentially swap the BCM over to have "drag mode" but that is hardly practical for most people who are 95% street. So what to do? Well that's where it gets interesting. As mentioned above you can either lock the suspension travel out and hit the tires or soften it up and let the car squat in an attempt to transfer the weight. As we have seen so often, especially during burnouts these cars bob up and down in a most nauseating fashion - it looks stupid and it isn't efficient having the suspension harmonically load and unload.

To visualize the porpoising (example only, not a criticism):

Schools of thought: https://www.yellowbullet.com/threads/irs-setup.615945/post-12822806 & https://www.yellowbullet.com/threads/irs-setup.615945/post-12825152

However, “an IRS with significant anti-squat generally has bad roll (bump) steer characteristics” (https://grassrootsmotorsports.com/articles/suspension-mythbusting-solid-axle-vs-irs/), making this an impractical solution especially considering it would require re-engineering the car and ruining it for the street.

So if you take it back to basics, you should be asking yourself why this is occurring. This thread explains it beautifully with some baseline engineering considerations: https://www.eng-tips.com/viewthread.cfm?qid=286119 What is really good is Dodge applied the different length/diameter half-shafts to the LX/LA platforms to address the frequency issue, and if you are fortunate enough to have a Demon or a Widebody your car came with further upgraded half-shafts. To expand on the concept more these threads offer some possible mitigation strategies: https://www.ls1gto.com/threads/soft-or-stiff-rear-springs.784073/post-19633161 & https://www.ls1gto.com/threads/gforce-coil-over-set-up.424437/post-8043948

So what?

I believe there are a few simple non-permanent things we can do to quickly stiffen up the rear suspension.

From a home brew mindset we can pretty easily install coil spring boosters (https://www.amazon.ca/dp/B000AMBOFS?tag=duc12-20&linkCode=osi&th=1&psc=1) to raise the rearend of the car (which most HCs have about 1-1.5* of negative camber off the assembly line) to better square up the tires/suspension and increase the spring rate by isolating a coil or two). There are calculators available to determine the estimated effects. Another option is to install air bags/drag bags/a football into the springs and tune the rate with pressurized air. From what I’ve the factory springs are very narrow internally and may not facilitate this option. The outlier here are the shock settings.

It is commonly seen and anecdotally accepted as though “Street” is the preferred suspension setting as it allegedly promotes the most weight transfer. While it is highly likely it does soften the front up to encourage weight transfer, it will likely do the exact same in the rear where it may be more desirable to slow the compression rate. It is the old 90/10 front and 50/50 rear shock concept. The inability to tune compression and rebound independently front to rear is problematic.

Here is an interesting engineered Band-Aid that could assist in augmenting the shocks and dampen the unwanted movements: http://www.hopnot.net/InstallationI...tallationInstructions/tabid/2244/Default.aspx & http://www.hopnot.net/LinkClick.aspx?fileticket=D6JmvRWQhO4=&tabid=201

To sum up, there is lot more trial and error waiting to be undertaken to overcome the inherent limitations in the stock suspension. Of course you can start swapping in loads of parts in hopes of influencing this issue and never addressing the root cause – and that’s the owner’s prerogative. For me, I just want to find a simple reversible solution for that 5% of the time I am at the drag strip – so that I can enjoy the car 100% of the time.

Of course for the hardcore among us (but, not so hardcore as to cut up their car and fix the issue with a 4-link or the like) there is always the ability to invent a $12,000 "solution": https://shop.watsonracing.com/S550-MUSTANG-9-INCH-LIGHTWEIGHT-IRS-2015-2019-p/wr-15-rearsubfrm.htm

 

[Boris]

I think you just need to decide what you want. These newer cars are designed to be a "jack of all trades" with ride comfort, cornering ability, and straight line performance. The IRS is great for all of that but if you really want to drag race then you should be looking at 9", 8.8, 12 bolt etc with proper shocks and cal-tracs.

I look forward to your updates on this project.

 

[Boris]

Now that i think on it, wasn't the old Carlyle orange z06 on IRS when it ran low 7s/high 6s?

 

[motorhead]

Now that i think on it, wasn't the old Carlyle orange z06 on IRS when it ran low 7s/high 6s?

Stock suspension? Probably used an advanced traction control system to manage slippage inducing hop.

 

[Blkout]

IRS has never been ideal for drag racing, but it has come a long ways since the earliest days. That being said, IRS is designed for road racing and ride quality.

 

[Jack_Toepfer]

The IRS - untouched - has been deep into the 1.3s, right? With a tire swap you have enough traction to burn up the clutch after a few passes. Once you upgrade the clutch the transmission will fail around it.

How deep are you trying to go in the first 60' with 4500lb cars?
Just curious where the target is, so that we know how far off the mark we are starting from.

 

[motorhead]

The IRS - untouched - has been deep into the 1.3s, right? With a tire swap you have enough traction to burn up the clutch after a few passes. Once you upgrade the clutch the transmission will fail around it.

How deep are you trying to go in the first 60' with 4500lb cars?
Just curious where the target is, so that we know how far off the mark we are starting from.

Mechanical limitations (and the sheer silliness of having a 4500lb dedicated drag car) notwithstanding, I am looking at this from the perspective of promoting consistency throughout the run for most folks. This is the why, the what, and the how.

 

[WhipCat]

That’s a pretty good write up you did.

a couple questions to identify how deep you wanna go would be:

how hard do you want to launch it?

if you can pull a 1.45 60’ on average, I would consider that a very competitive launch for a street car with occasional track usage. If your searching for 1.2x - 1.3x than you are talking allot of stress on the rear end components and you have to really put some solid parts and upgrade allot of components to handle those launches, especially if it will be done frequently.

are you going to mod the rest of the car? Or just the rear end?

You can run a fast 1/4 mile time in a bunch of different ways, launching out of the hole very quickly is one of the ways, but there are other ways to get down the track with the same or better time.

advanced engine and trans tuning can get a car to launch well, but also allow the engine to get into its powerband as soon as possible once it leaves the line, as well as providing clean shifts that give the car the best power without over revving or damaging the engine.

power adders like adding more boost on stock blower, aftermarket superchargers, turbos or nitrous, or a combination of them can get the car accelerating very quickly in the last 1000’. This can make up for allot difficulty launching the car and dealing with all the inconsistencies that drag racing with an IRS poses.

I speak all of this from experience.

if your car is primarily a street car that you enjoy driving around town and taking friends and family for a cruise, with the occasional 1/4 mile visit 4-8 times a year for fun. Then setting up a crazy modded rear ended setup at the expense of ride quality with added noise and vibrations may not be the best option. But maybe adding a power added like above to give the car better acceleration with a good engine and trans tune to get the car moving and into power as soon as possible, will compensate for allot of inconsistent launching problems.

Attached is a time slip for a street setup with nothing more for suspension then a diff brace, control arms and some wheels and tires.

getting the IRS to hook up reliably is a pain in the ass, especially if you wanna keep the car street able and enjoyable.

I’m curious if you can find other areas to improve the launching of the IRS cars. I especially likes where your talking about weight transfer and springs on the car.

 

[motorhead]

That’s a pretty good write up you did.

a couple questions to identify how deep you wanna go would be:

how hard do you want to launch it?

if you can pull a 1.45 60’ on average, I would consider that a very competitive launch for a street car with occasional track usage. If your searching for 1.2x - 1.3x than you are talking allot of stress on the rear end components and you have to really put some solid parts and upgrade allot of components to handle those launches, especially if it will be done frequently.

are you going to mod the rest of the car? Or just the rear end?

You can run a fast 1/4 mile time in a bunch of different ways, launching out of the whole very quickly is one of the ways, but there are other ways to get down the track with the same or better time.

advanced engine and trans tuning can get a car to launch well, but also allow the engine to get into its powerband as soon as possible once it leaves the line, as well as providing clean shifts that give the car the best power without over revving or damaging the engine.

power adders like adding more boost on stock blower, aftermarket superchargers, turbos or nitrous, or a combination of them can get the car accelerating very quickly in the last 1000’. This can make up for allot difficulty launching the car and dealing with all the inconsistencies that drag racing with an IRS poses.

I speak all of this from experience.

if your car is primarily a street car that you enjoy driving around town and taking friends and family for a cruise, with the occasional 1/4 mile visit 4-8 times a year for fun. Then setting up a crazy modded rear ended setup at the expense of ride quality with added noise and vibrations may not be the best option. But maybe adding a power added like above to give the car better acceleration with a good engine and trans tune to get the car moving and into power as soon as possible, will compensate for allot of inconsistent launching problems.

Attached is a time slip for a street setup with nothing more for suspension then a diff brace, control arms and some wheels and tires.

getting the IRS to hook up reliably is a pain in the ass, especially if you wanna keep the car street able and enjoyable.

I’m curious if you can find other areas to improve the launching of the IRS cars. I especially likes where your talking about weight transfer and springs on the car.

Thanks, but I cannot express enough that I am just scratching the surface at the moment and I have no specific goals in mind other than testing some basic theories out. A huge consideration is that, at least for the time being (until a Demon owner makes their BCM available to a AlfaOBD user), we are all pretty well stuck with the factory shock programming (street, sport, and track) limiting us. I am also going to shy away from throwing big money at this - spending a few bucks on some spring inserts and air bags or building a few parts (like a pinion snubber or a diff brace) is well within reason.

I am also up against some issues being a clutch car (where the biggest variable will likely be the driver) - but I do have some local guys with the automagics who could really benefit from increased consistency. So R&D for mad launches may come later on. And I absolutely agree that power management is a paramount consideration especially if you add more to compensate for a softer launch; different ways to skin a cat. I am mostly interested in efficiency and consistency, plus ease of assembly/disassembly for a 95% street car.

I just wanted to get this idea out of my head.

 

[WhipCat]

Thanks, but I cannot express enough that I am just scratching the surface at the moment and I have no specific goals in mind other than testing some basic theories out. A huge consideration is that, at least for the time being (until a Demon owner makes their BCM available to a AlfaOBD user), we are all pretty well stuck with the factory shock programming (street, sport, and track) limiting us. I am also going to shy away from throwing big money at this - spending a few bucks on some spring inserts and air bags or building a few parts (like a pinion snubber or a diff brace) is well within reason.

I am also up against some issues being a clutch car (where the biggest variable will like be the driver) - but I do have some local guys with the automagics who could really benefit from increased consistency. So R&D for mad launches may come later on. And I absolutely agree that power management is a paramount consideration especially if you add more to compensate for a softer launch; different ways to skin a cat. I am mostly interested in efficiency and consistency, plus ease of assembly/disassembly for a 95% street car.

I just wanted to get this idea out of my head.

i think what I have found in doing hundreds of drag passes on this car, is that as I took power out of my car I was able to improve my launch. By the time I was getting really good at launching and learning to make this thing hook up, my stock trans finally gave out and started slipping. I have since swapped out to a brand new trans. One of my theories which I believe is correct, is that these cars are ridiculously hard to launch because of the instant torque factor with the blowers we have. They bring power on so hard that getting the tires to hold is darn near impossible when you go full send at the track. I think a proper drag tune, which is basically a tune that pulls power on the hit pretty significantly, and then brings the power in after the 1-2 shift, would be the ticket.

I agree that getting the demon drag settings on the hellcat would be awesome, I’ve seen that talked about on the other forum before. That would help, as the SRT engineers obviously have done their homework to get the demon to launch well.

I think allot of what you are seeking, is not so much a mechanical fix, but more of a tuning and parameter fix on the cars.
Yes you need to strengthen the rear end and get it setup as best as you can for weight transfer and holding the power and torque, but I think it comes to tuning. Especially for higher modified cars.

 

[Magnified]

I went through this kind of thinking 15 years back with the GTO. After building it up the IRS just could not take the hit (even on the street, really). The only cost effective solution (cause I'm poor) for that set up, which, most importantly, was not electronically adjustable, was a set of drag bags to physically prevent the spring from allowing more than a desired compression. With all the electronics and other interconnected systems involved in the suspensions on the HC, and other factors, I would hesitate to go with so simple a solution. Never know what other unintended consequences will be engendered.

Of course, for me, honestly, I see the easy fix for this to be with the cars electronics and software. Properly programmed and tweaked (just like the launch control) a HC should be able to launch every day on the street like a GT500 (I'm not saying it's a better car, but its launch software is far superior) and at the strip like a long time pro. I was actually kind of surprised that no one at Dodge / SRT had put together an informal tutorial on how to get the best out of the systems / program settings in X situation. Do they not care? Are they afraid of liability? For a fraction of the cost of physical changes to the car improved software (including suspension tuning) is for me the low hanging fruit. Or perhaps they're waiting for the next HC model to introduce this as a mind blowing "improvement."

Amazing write up by the way. Did you work on that off line, or just spew stream of consciousness onto the forum? Either way, a stand up job man.

 

[motorhead]

I went through this kind of thinking 15 years back with the GTO. After building it up the IRS just could not take the hit (even on the street, really). The only cost effective solution (cause I'm poor) for that set up, which, most importantly, was not electronically adjustable, was a set of drag bags to physically prevent the spring from allowing more than a desired compression. With all the electronics and other interconnected systems involved in the suspensions on the HC, and other factors, I would hesitate to go with so simple a solution. Never know what other unintended consequences will be engendered.

Of course, for me, honestly, I see the easy fix for this to be with the cars electronics and software. Properly programmed and tweaked (just like the launch control) a HC should be able to launch every day on the street like a GT500 (I'm not saying it's a better car, but its launch software is far superior) and at the strip like a long time pro. I was actually kind of surprised that no one at Dodge / SRT had put together an informal tutorial on how to get the best out of the systems / program settings in X situation. Do they not care? Are they afraid of liability? For a fraction of the cost of physical changes to the car improved software (including suspension tuning) is for me the low hanging fruit. Or perhaps they're waiting for the next HC model to introduce this as a mind blowing "improvement."

Amazing write up by the way. Did you work on that off line, or just spew stream of consciousness onto the forum? Either way, a stand up job man.

That's the thing this issue has been at our feet since about 2004 when the Holden platform debuted on our shores (that reminds me I should look into what the Aussies have figured out), then the 03/04 Cobras, Caddys, Camaros, and such that all share similar issues. The problem is that these cars are so new, and lots of inexperienced and poorly informed people bought them, that I doubt they ever thought to look into it. I agree that that are definite benefits in using technology to our advantage (and it exists in the BCM, or an interface/bypass in the physical shock control mechanism (what happens if you simply unplug it?)), but there are still mechanical considerations that can be influenced. I don't suspect that upping the rear spring rate would have massive adverse effects on the handling characteristics of the car in a straight line - at least not catastrophic ones.

And thanks, this is all just stuff that floats around in my headspace - it just took time to do the supporting "research".

 

[motorhead]

I did a little poking around the differential yesterday, took some measurements and came up with a few ideas to tighten things up. But, that's for later.

In the meantime I did some digging into the Adaptive Damping Suspension (ADS) in these cars. I started looking for a standalone system that would use the Bilstein shocks and allow the independent control of the rebound and compression functions. Unfortunately there isn't anything publicly available on Bilsteins' website (even doing a little backdoor tactics).

So here's what they gave us - note the leads to the tops and bottoms of the remote cylinder, those are the (electromagnetic(?)) adjustment for the valving. For reference Bilstein typically uses a monotube design for their shocks, which typically limits the amount you can lower a car and maintain functionality.

The ADS shock assemblies are similar to conventional shock absorbers with the addition of a shock solenoid assembly and the associated wiring mounted to the shock absorber body. Please note that because the shock absorber is a monotube design, it is important to never support the shock by its tubular housing.

While some ADS components look similar to standard components, it is important to remember that these parts should not be interchanged.

So what is ADS and how does it work? Well per FCA ADS offers an assortment of modes.

Outstanding ride, handling and capability

New for 2013 is an improved Adaptive Damping Suspension (ADS) system that is expanded to three modes and tuned specifically for the Dodge Challenger SRT8 to provide an everyday performance ride or a much firmer race track-capable damping for more challenging driving situations.

In “Auto” (we now know this as Street) mode, a wide range of on-road and driver inputs –such as vehicle speed, steering angle, steering speed, brake torque, throttle position and longitudinal/lateral/vertical accelerations –automatically tune the suspension for specific conditions.

In “Sport” mode, the damping system rebound and compression is locked to a higher damping rate. Drivers can easily choose between settings with a press of a button on the centre stack to quickly change the shock damping characteristics from “Auto” for everyday commuting to “Sport” for more spirited driving situations.

In “Track” mode, the higher damping rates combine with a performance shifting and gear holding feature that allows the driver full shift control when using the steering wheel mounted paddle controls or Auto/Stick™.

The ADS system on the Dodge Challenger SRT8 392 features the fastest turn-in and most nimble handling characteristics of any SRT product. In the “Sport” or “Track” settings, the system provides the highest damping rates, allowing the driver to push the handling envelope to the extreme.

SRT-tuned Three-mode Adaptive Damping Suspension (ADS)
Street Mode = sporty, compliant ride
Sport Mode = firm, improved handling
Track Mode = firmest, maximum handling (SRT 392 and SRT Hellcat

Great for a standard road car, and quite likely the reason why folks default to "Street" mode for drag racing in a normal Hellcat or Redeye. But, what about the Demon?

The Bilstein Adaptive Damping suspension adjusts the front and rear compression and rebound to optimize weight transfer onto the rear tires, softening front rebound while firming rear compression.

To prevent wheelhop, the launch control targets the wheel speed sensors to detect the driveline oscillations before they ruin the first 60 feet (maintaining full throttle, but cutting spark or fuel as necessary)

Firming up the rear compression, eh? Sounds familiar. Looking a little deeper we find that it boils down to the 90/10 and 50/50 in the Drag mode suspension setting.

SRT Demon-tuned Three-mode ADS Suspension
Auto (Street) mode = compliant ride (Compression/rebound -front & rear: soft/soft)
Sport mode = sporty ride and improved handling (front & rear: soft/firm)
Drag mode = maximum weight transfer (front: firm/soft & rear: firm/firm)

Another interesting note is that Demon uses four unique shocks (which are available through Rockauto at $600 each - LOL):

68316053AA - Rear Left
68316054AA - Rear Right
68316052AA - Front Left
68316051AA - Front Right

Meaning that it may not be a simple software change. May not. Or it may be but only within the design limitations of the standard HC/RE ADS shocks. But, again (as above) simply swapping part A for part B or changing some flags in the BCM may not be sufficient or effective, or at least as effective as an all Demon suspension swap.

For your additional reading pleasure I've added some files on the ADS shocks that I found from a 2015 thread on the other site. From what I have seen these cars don't react poorly to having the shocks disconnected, trouble codes - sure, but they can be swapped out. So, if that's the case than there is a potential to disconnect from the control module and manually manipulate the settings (assuming they are indeed 12v electromotors).

 

[DGatzby]

The linear rear spring rate of 778 lbs/in on the HC is the most stiff of the Mopar family.

Progressive rates available that are better ONLY on full compression are:
MP 514-828
Eibach Pro 314-927

Up front where the weight of the car is, the HC is soft at 313.
KW shines above the rest here with their progressive rate of 413-536.

 

[motorhead]

The linear rear spring rate of 778 lbs/in on the HC is the most stiff of the Mopar family.

Progressive rates available that are better ONLY on full compression are:
MP 514-828
Eibach Pro 314-927

Up front where the weight of the car is, the HC is soft at 313.
KW shines above the rest here with their progressive rate of 413-536.

Indeed - chart attached for reference.

But, its about how the energy is managed. And until we can start adjusting our shocks independent of the stock parameters we are stuck testing other methods. A 50/50 bias in the rear makes the spring behave in a very neutral fashion which is great for a Demon - in the instance of using a HC/RE (as delivered) in Street mode we have to consider that the potential compression is going to be significantly softer than a Demon in Drag mode. Which means that locking out the rear and preventing squat is a reasonable objective, while leaving the front loose. It is a mechanical lock-out procedure for a suspension with very little room for adjustment to begin with - a simple case of fucking with the cock you got, not the cock you want.

For those interested the distance between the stock HC/RE rear coils is 3/4" at full droop (no tire) - so 1" booster blocks should squeak in there with a bit of lube (LOL).

 

[DGatzby]

To supplement your thread, here are the aftermarket spring rates.

 

[16GoManGoHC2]

This some of the reason these tires looked like this after this days “practice” lol? Not worn evenly across?

 

[DGatzby]

This some of the reason these tires looked like this after this days “practice” lol? Not worn evenly across?


View attachment 14260 View attachment 14261 View attachment 14262

Is that worn in the inside or outside?

 

[motorhead]

This some of the reason these tires looked like this after this days “practice” lol? Not worn evenly across?


View attachment 14260 View attachment 14261 View attachment 14262

I would imagine so. Possibly your toe setting a bit too. But mostly camber gain when it squats, exacerbated if the car is lowered. Why pay for the whole tire if you are only going use 3/4 of it?

 

[DGatzby]

I would imagine so. Possibly your toe setting a bit too. But mostly camber gain when it squats, exacerbated if the car is lowered. Why pay for the whole tire if you are only going use 3/4 of it?

The gentlemen who is going to set my car up and I had an in depth talk today. They are working on it this week. Does anyone know if taking the camber to one degree more than spec negative without any other adjustments will change the toe? Regardless we are going to see what happens if all four need -1 more, since I have the full set of AAD adjustable arms front and back.

If that is the inside of that tire, what does the other side look like? What did the track look like?