Understanding the IRS and Drag Racing

[arichmond1003]

Good morning,
My setup is a 19 a10 car that’s FBO and a converter. I’m running e85, have all the BMR Suspension goodies, drag springs along with viking crusaders in the rear. I’m also running a 28” tall Hoosier bracket radial as a tire
I’m not asking for advice on settings but more of an understanding on how the IRS is supposed yo work with drag racing. With an IRS car are we looking for squat, or anti squat and smashing the tire into the surface like with a live axel car?

Sponsored

 

[Lads1719]

The physics of the squat is counterproductive to launching. Like a sprinter. They’re in the blocks before the race starts, it wouldn’t make sense for them to start standing up, then get down.

How That translates into IRS is beyond me, I just remember the nerdy stuff from school

 

[arichmond1003]

The physics of the squat is counterproductive to launching. Like a sprinter. They’re in the blocks before the race starts, it wouldn’t make sense for them to start standing up, then get down.

How That translates into IRS is beyond me, I just remember the nerdy stuff from school

Appreciate it! I’ve just been doing some research and I know the “squat” is counter productive on 4 link and live axel cars, but I’ve read and listened to how Supra’s and s550s want squat. Or so I’m led to believe

 

[Norm Peterson]

Good morning,
My setup is a 19 a10 car that’s FBO and a converter. I’m running e85, have all the BMR Suspension goodies, drag springs along with viking crusaders in the rear. I’m also running a 28” tall Hoosier bracket radial as a tire
I’m not asking for advice on settings but more of an understanding on how the IRS is supposed yo work with drag racing. With an IRS car are we looking for squat, or anti squat and smashing the tire into the surface like with a live axel car?

As you are probably aware, anti-squat is a rear suspension side view geometry thing, roughly defined as "resistance to squat during rearward load transfer".

What the actual anti-squat effect is, is to allow near-instantaneous rearward load transfer directly to rear tire vertical loading and rapidly increase the amount of available forward bite.

But anti-squat (geometric load transfer, to a suspension engineer) is only one of at least three effects that are going on.

Another involves the suspension spring stiffnesses (perhaps more accurately, rear suspension wheel rates). Since the spring (and wheel) forces from this effect do not fully develop until the suspension has found its new, lower equilibrium ride height, this takes a bit longer, perhaps a third of a second or so. Plenty of time for incipient wheelspin to go all loose on a hard launch in the absence of sufficient anti-squat help.

In between are the effects of rear shocks. Shocks resist force in proportion to suspension velocity, which peaks somewhere between the other two effects.

It's really easy to build anti-squat into most stick-axle suspension arrangements by simply relocating a few linkage pivot points. This approach isn't as applicable to most IRS designs for a couple of reasons. One is that for IRS, the anti-squat construction line passes through the axle centerline (side view here), where the stick-axle line starts much lower at the contact patches. This means that the slope of the IRS construction line is much gentler (lower anti-squat %). In the S550's integral link IRS, anti-squat is primarily determined by the LCA, with (I think) a small contribution from the integral link's orientation as well. I'm not seeing big increases in A-S for the S550 until/unless somebody figures out how to relocate an LCA pivot without completely hosing other geometric effects.

FWIW, 100% A-S means no squat on launch and no delayed help form springs or shocks. More that 100% means 'separation' for as long as there's enough bite and axle torque to maintain it. After that, down she comes taking some forward bite along with it.

Incidentally, adding a huge rear antiroll bar generally isn't the answer for an IRS car. They work in a stick-axle car by carrying back more of the engine's torque reaction and using it to "re-plant" the RR tire for more even side to side grip (and a little more in total). Losing the front bar achieves a similar effect. In an IRS car, the engine torque reaction is completely resolved at the differential's chassis mounting and never gets past all those U-joints to the wheels.


Norm

 

[Norm Peterson]

Appreciate it! I’ve just been doing some research and I know the “squat” is counter productive on 4 link and live axel cars, but I’ve read and listened to how Supra’s and s550s want squat. Or so I’m led to believe

OE IRS geometries simply don't allow for stick-axle levels of anti-squat. Never have. Supposedly the S550 has more A-S then the S197 does in full OE configuration. Then again, having more than about 32% sure isn't saying much as far as dragstrip use is concerned.


Norm

 

[arichmond1003]

Norm
So then if you take an IRS car that’s an auto and just the suspension so that you have minimal squat, after the suspension is preloaded before launch, when you launch with rebound to push the tire into the strip would that be ideal?

 

[Norm Peterson]

Norm
So then if you take an IRS car that’s an auto and just the suspension so that you have minimal squat, after the suspension is preloaded before launch, when you launch with rebound to push the tire into the strip would that be ideal?

Seems to me that minimal squat (movement) with IRS is going to involve only spring rate and shock bump tuning. Which will probably involve shock rebound-side tuning as well to avoid unloading the tires if the suspension overshoots the right amount of bump travel.

I'm really a corner-carver guy, but I do have some idea of the physics involved for the straight line stuff.


Norm

 

[80FoxCoupe]

Seems to me that minimal squat (movement) with IRS is going to involve only spring rate and shock bump tuning. Which will probably involve shock rebound-side tuning as well to avoid unloading the tires if the suspension overshoots the right amount of bump travel.

I'm really a corner-carver guy, but I do have some idea of the physics involved for the straight line stuff.


Norm

Agreed. Once you hit a certain point in the rear travel, the alignment starts to change drastically with increased negative camber and toe in. You ultimately want to limit the bump travel to avoid this drastic alignment change. And to do so increased spring rate and compression and rebound dampening is necessary assuming a lower than stock ride height.

There is lots of ways to skin a cat. Many combinations are successful such as 15-in wheel with a 28-in tire at stock ride height, lower and stiffer rear suspension with a radial on a 17-in rim etc.

Also other factors to consider is power output, trans brake and surface prep.

 

[80FoxCoupe]

The theory for my setup is to run the rear stiffer, and put the power to the tire as soon as possible and hold it there. My car delivers power in a very linear fashion, so typically if I can get the car to move it's going to stay glued the whole way down.

 

[Norm Peterson]

Agreed. Once you hit a certain point in the rear travel, the alignment starts to change drastically with increased negative camber and toe in. You ultimately want to limit the bump travel to avoid this drastic alignment change. And to do so increased spring rate and compression and rebound dampening is necessary assuming a lower than stock ride height.

Maybe experiment with setting little or no static rear negative camber - or even a little bit of static positive camber. It'd probably take a whole new rear cradle to let you lengthen the upper lateral link enough to slow down the rate of camber change.

There is lots of ways to skin a cat. Many combinations are successful such as 15-in wheel with a 28-in tire at stock ride height, lower and stiffer rear suspension with a radial on a 17-in rim etc.

There may be a little help available from changing the front to rear rake, but I'm not sure how much (or even which way - more nose down rake would pull the anti-squat construction line down - less A-S unless the CG drops fast enough).

Also other factors to consider is power output, trans brake and surface prep.

Now you're getting outside my wheelhouse . . .


Norm

 

[80FoxCoupe]

Maybe experiment with setting little or no static rear negative camber - or even a little bit of static positive camber. It'd probably take a whole new rear cradle to let you lengthen the upper lateral link enough to slow down the rate of camber change.



There may be a little help available from changing the front to rear rake, but I'm not sure how much (or even which way - more nose down rake would pull the anti-squat construction line down - less A-S unless the CG drops fast enough).



Now you're getting outside my wheelhouse . . .


Norm

Norm, now you are getting into the secrets.....

 

[Norm Peterson]

Norm, now you are getting into the secrets.....

Guess that's the long-term fallout from being an engineer by training and profession. Mixed with a long-running serious curiosity and self-study regarding chassis, suspension, wheels, tires, and brake topics.

Quite a lot of the physics for forward acceleration are analogous to those for cornering, though you do have to take slightly different interpretations and aim for somewhat different targets. Sorry if I inadvertently exposed any secrets, but it's pretty much right there for a bit of thinking.


Norm

 

[arichmond1003]

I appreciate everyone input!

The theory for my setup is to run the rear stiffer, and put the power to the tire as soon as possible and hold it there. My car delivers power in a very linear fashion, so typically if I can get the car to move it's going to stay glued the whole way down.

My car also delivers power in a linear way
So what I’m gathering and assuming is the idea setup would be zero squat zero spin. If that’s not achievable the add squat until spin is gone?
and I’m talking just the compression side here.

 

[80FoxCoupe]

I appreciate everyone input!

My car also delivers power in a linear way
So what I’m gathering and assuming is the idea setup would be zero squat zero spin. If that’s not achievable the add squat until spin is gone?
and I’m talking just the compression side here.

I don't know that i would ever "add" squat. Several ways to do that if you wanted to; lower the spring rate, soften compression dampening, add weight to trunk and raise front ride height.

I personally would lower launch rpm, lower rear tire pressure and or raise front.

 

[80FoxCoupe]

Keep in mind, folks go fast in s550's with all kinds of setups. But i will say cars will killer 60ft's have one thing in common, killer surface..... Good tire adhesion can mask many suspension shortcomings.

Sponsored