Drag Racing IRS In-Here, Need tips

[03mustang92]

Setup is basically stock IRS, brace, bumpsteer kit MT ET Drag Stiffwall. The question is, on 12psi in the tires and dumping the Mcleod RXT @ 4k, the car spins, then bogs down real hard then hooks. Would you just lower pressure down to 10psi? Or is the trick to actually slip the clutch to get it moving?

 

[bubblehead93]

what are you running for rear shocks and springs?

what sort of burnout are you doing? tire pressure goes up with the burnout... so you may be starting with cold 12 psi and ending up somewhere north of that... you need to figure out how much heat the tire needs for grip, the burnout that will give you that heat, how much pressure that burnout adds to your tire due to the heat, what the optimum tire pressure at launch is, and then start at a pressure which will give you your optimum tire pressure after the burnout, i.e. start at 13# to get 15# after the burnout...

are you hopping the rear end at all once it hooks?

the right tire with the right pressure and a good shock in the rear to help in weight transfer... I launch at 3.8K to 4.0K... I used to have a similar problem until I switched to a Hoosier 28" drag slick and the right tire pressure (12# cold)... just completed changed my setup to QA1 DA coil-overs front and rear after grenading a questionable set of stock rear half-shafts (good deal, too good I guess)... have DSS half-shafts now, just need to get the car back to the track... I was running H&R race springs and MM MM2 dampers all the way around, the dampers really limited my weight transfer to the rear... I now run a auto-x coil-over setup with the MM struts and shocks and have fitted the QA1 front/rear coil-over setup for drag racing... recent typhoon out here took away this weekend to take the car to the track with the new suspension...

 

[03mustang92]

I am doing the burnout mickey thompson recommends to do with stick cars which is a to do "enough to haze the tires, then stage immediately.". I am running tubes in my slicks if that makes a difference. Also Running H&r race springs, and factory 80k miles shocks. I have air bags in my springs from LML also but not aired up bc im not sure where to start with them.

i do not believe it is hopping but i could be wrong, ill post a video of the launch

 

[03mustang92]

https://youtu.be/RHVZ8PR74x4

 

[bubblehead93]

the effect of the tube is to stiffen the sidewall further, I don't have any experience with the Micky Thompson ET Drags, but I run my Hoosiers tubeless... I would defer to others on tube or not with the slick you are using...

as far as your burnout, I think you may want to haze the tires a bit more, JMHO, not sure from the video if you are getting enough heat into the surface of the tire... I think you may need a little more haze to the tires, hence the spin then hook and bog... also right after launch your nose is coming down unloading the rear, probably also contributing to you spinning the tires... the stock shocks are not helping you, especially at 80K, you may want to consider some Viking B225 DA shocks, you can adjust the rebound so that the rear comes up more slowly, keeping the weight transfer aft...

 

[Tractionless1]

Dumping the clutch is an invitation to breaking axles, the posts above are great advice. 1.51 60' here launching Eaton at 3k and less than 10 passes on teh car.

 

[351stang]

Try Airbags and other "old racer" tips

This is a old drag racer trick and is cheap. I don't know why it hasn't caught on with the Cobra crowd.

http://www.summitracing.com/parts/a...04/make/ford/model/mustang/submodel/svt-cobra

They're cheap and allow you to adjust to changing track conditions or different tires. Use about 2 PSI more air in the right rear. Try 13 psi LR/15 psi RR to start and play around with it. I'd call Summit to verify that these are 2003-2004 compatible, because I can't remember the PN I bought years ago. Run the schrader valves to the trunk and label them. Be careful not to over inflate with high pressure air. You don't even have to drop the springs to install them.

Your burn out doesn't look sufficient. If you've got 3.73 gears or numerically higher do the burn out in 2nd gear. Put your side mirrors so you can see the rear tires, drive around the water box, back into the water, pull out of the water 2 feet, then do at least a 10 second burn out or until you start to see smoke wisp off the tires. DO NOT LET OFF THE GAS. Let off the brake and let the car go forward a few feet or until the tires bark. Line up with the heavy rubber on the track and tree up as soon as possible.

Try not to make a pass after some noob or import racer that drives through the water box with treaded tires.

The other thing you can try is to vary the tire pressure slightly from right to left (I'm talking like 0.5 to 1 psig), but you need a good pressure gauge with .5 psig increments. I personally wouldn't run less than 12.5 psi.

http://www.summitracing.com/parts/mor-89555/overview/

Essentially you are trying to compensate for the rotation of the engine compressing the right rear side of the car and unloading the left tire. You also want to minimize the rear of the car "squatting" and potentially altering the contact patch.


Are you running tubes or tubeless? I recommend tubeless.

[EDIT: Just saw that you have air bags, but not inflated. This is easy money for you, follow the instructions above.]

[EDIT2: Don't listen to the crap on the Mickey Thompson page. Do a burn out as above.]

 

[Wicked46]

I can get you a pair of double adjustable Viking shocks for your IRS!

 

[351stang]

I think you'll see a big improvement with air in the bags and a proper burn out.

 

[Riddick]

What kind d of 60s are you currently cutting, gear ratio, And what size rear tire? On a 26in tire and the stock 3.55 I have found that slipping the Clutch from about 4500 works well. I tried dumping it at 5k and the car bogged. I have heard dumping from 5500 works well so next time out I'll try that. With your stiff sidewall I would recommend slipping the clutch instead of dumping it. I run a bias ply Hoosier so it has some give to it, which absorbs a lot of the initial hit.

 

[broke7]

I run a similar combo and have found the following and my car 60ft's in the mid 1.4's

Small burnout, enough to see smoke in side mirrors and fill the wheelwells, roll out feathering gas and press clutch it once they bite, no John Force tactics.

I have had great results at 11-11.5 psi, and even 10.5 on a shitty track (post run pressure).

I also run airbags with stock springs. 7psi in passenger bag, 5psi drivers. I have a set of single adjustable no name shocks I run on full stiff.

I launch my car at 2800-3000, will try more this coming weekend if the track is there. Damn near a dump/slip. You actually want the slick to spin just a tad.


Here are a couple of recent runs.

15psi in tires, too much slips and actually spun a tad in 1st gear. 3200 launch. 1.51 60'

http://youtu.be/oT2mgBYaHY4


11psi. 2900rpm launch, tons better. 1.44 60'

http://youtu.be/hF2KokdXJPE


Gear ratio and tire height play a big role also

I prefer 3.55's and 26" stiffwalls on a car that traps 135 and below.

135+ I like 28's and 3.73's....(I have those) I dont run tubes either.

 

[351stang]

Stock half shafts??? How many passes on em? :rockon:

 

[72SBC]

Ditch the tubes. I run 12 psi cold on a 28x9 m/t soft sidewall, you might need to run slightly more in the stiff sidewall. If you are getting too much wobble go up a psi till it is manageable. You can dump it but you'll need to go a little higher rpm, 4.5k-5k. Your clutch and half shafts might hate you though lol. My buddy dumps the clutch on his but it's off a 2 step at 4.5k on a 28 inch tall tire. Every car is different and just need to find what works for you, the stiffer the tire the harder you need to hit it to plant it. Also run rim screws, lots of traction problems I've fixed just with screws. It helps plant the tire and helps with you spinning on the hit.

 

[broke7]

Stock half shafts??? How many passes on em? :rockon:

Probably close to 50 this go around.


No 2 step for me either. There is a good mix between too much rpm and not enough

 

[Wicked46]

A slick is easier on the IRS than a drag radial. A slick won't wheel hop and prematurely break a halfshaft. I always recommend Slicks to those with an IRS.

 

[Stage 4.6]

I posted a similar thread on another part of the forums but didn't get much response? wicked46 wondering if you can give me some guidance? I still have the IRS and run a Drag star setup 17x4 and 17x9.5 I have the MT/ET streets 26 x 11.5 x 17 what tire pressure you recommend for this tire? I am getting wheel hop when hitting second gear? Tire pressure at the time was 16psi I only have 2 passes on my Cobra this year best trap was 120-121 mph launching at 2xxx rpm 60ft 1.99 I know horrible considering the tire I am running.
- H&R springs
- Stock Bilstein shocks
- FTBR kit w/ FRPP diff cover
- DSS halfshaft
- 4:10 FRPP gears

Any advice from anyone would be appreciated.

 

[03mustang92]

Holy hell, this thread blew up from the last time i looked at it in july! Thank you for all the information given out! Headed to a prepped track this weekend to see what she can do.

wicked46, i may have to take you up on those vikings this winter! Going to drop the irs and rebuild the diff along with a set of ftbr bushings too!

 

[ClubVenom1]

A lot of interesting comments here. I noticed some say "I think"... don't think, KNOW. Do some research. This is info taken from proven setups from real drag racers. This might shock some of you, but like I said... do some research.

Drag Racing an IRS 101​

Solid-Axle VS IRS

Under most circumstances, rigidly mounting the differential assembly to the chassis in an IRS-equipped car is a good thing since it reduces unsprung weight. This arrangement allows running softer springs rates and shock valving, which improves ride quality without sacrificing handling. However, planting thousands of horsepower from a standstill falls outside the realm of most circumstances, and the huge drawback of an IRS is that it limits the amount of anti-squat built into the suspension in a drag-racing application.

As a car accelerates forward, weight transfer causes the rear suspension to squat. Since the rearend housing and differential assembly are not mounted directly to the chassis in a solid-axle car, the rearend can separate (move downward) from the body to resist squatting. That’s what suspension designers and drag racers refer to as anti-squat. The icing on the cake is that the torque of the engine in a solid-axle car is transmitted through the suspension, which drives the rearend assembly and tires into the ground to enhance traction. According to legendary suspension engineer Herb Adams, manipulating the instant center allows a solid-axle car to achieve over 100 percent anti-squat, whereas an IRS is typically limited to about 25 percent anti-squat.

Suspension Recipe

Now lets get to the meat and potatoes. Setting up an independent rear suspension like a solid axle suspension is a recipe for disaster. An IRS requires a completely different suspension setup than a solid-axle car, and managing weight transfer is way more important. With a solid-axle car, you can adjust the instant center to make the body separate from the rearend, and smash the tires into the track. Solid-axle cars don’t squat at all, they just separate, lift the rearend, and take off. You can’t do that with an IRS. Instead, you have to MAKE THE CAR SQUAT, and transfer the weight rearward.

The Squat Factor

A solid-axle car and an IRS car work completely opposite of each other. A solid-axle car works on the premise of separation to plant the rearend into the ground, but the only way to get an IRS car to launch is to transfer the weight to the back and keep it there through the entire pass. Since you can’t adjust the instant center on an IRS car, the most effective adjustment you can make is the shock valving. The initial hit at launch doesn’t drive the tires into the ground nearly as hard as in a solid-axle car, so the goal is to valve the shocks so the car squats very quickly. From there, you need to control the extension of the shocks to prevent the weight from transferring back up to the front.

Double Adjustable Shocks

Setup the rear shocks compression valving soft enough so that the car can squat evenly at a very rapid rate out of the hole but not so much that its going to drag the gas tank. Once the car squats evenly, adjust the rear shocks to tighten up the rebound. Normally, the rear shocks will try to extend back up at a faster rate than what you want thus unloading the rear tires and result in immediate tire spin. Double adjustables shocks in the rear are a must have for effective and repeatable 60 foots in an IRS equiped vehicle.

The front shocks also rely on correct shock valving, but for the exact opposite purpose as the rears. The goal with the front shocks is to control the rate of extension. You need to keep the front shocks extended over a very long period of time, and don’t use travel limiters.
A good double adjustable rear shock can help get your 60-foot times down to 1.5 seconds or less depending on car weight, tire type, and HP/TQ numbers.” Although more exotic shocks are available for more extreme applications, off-the-shelf double-adjustable shocks have proven effective in many 8-second combos.

Weight Distribution

If weight transfer is so important in an IRS car, why not just put more weight in the rear of the car in the first place? That’s exactly what George Farkouh decided to do with the ARH/Farks Supercars Camaro, and the results... Well, I will let him tell you. Notice what he says in a magazine article I pulled online......

“Since you can’t drive the tires into the ground with an IRS car, the percentage of static weight over the rear tires is very important. That’s why we put the intercooler core, water tank, and ballast in the back of our car to keep as much weight over the rear tires as possible,” says George. “When you get an IRS car setup right, it squats and stays that way the entire pass. Our best pass to date is 7.67 at 179 mph on 275mm drag radials, which makes it the fastest IRS-equipped, fifth-gen Camaro in the world. This combo started out pulling 1.32-second 60-foot times in its first season, but now we have that down to 1.20 seconds.”

Yeah I know its a Chevy quoted here but the exact same principle applies to ANY IRS equipped car. And note that he did this on Drag Radials folks. Thats impressive. So stick your battery, intercooler tank, nitrous bottles, water meth tank, and girl friend in the trunk of your car. Well maybe not your girl friend... but possibly your EX instead.... LOL.

DEFLECTION PREVENTION

Compared to a solid axle, an independent rear suspension relies on a multitude of control arms, links, and rods to locate the rear wheels. These links utilize bushings to prevent road noise and vibrations from being transmitted into the chassis. The downside is that as these bushings deflect under load, wheel control is compromised.

With the stock suspension, the entire spindle and differential moves back and forth during a hard launch. This causes wheelhop and tire spin. To prevent this from happening, install a quality bushing kit, replace the stock rear toe links with stronger pieces, and upgrading the stock anti-roll/sway bar to a much larger/stiffer bar than stock like 1.5 inches. Now all the alignment specs don’t change at all during launch, and your 60-foot times will be very consistent.”

Full Tilt Boogie Racing sells a complete IRS bushing kit with delrin and aluminum bushings, but any other quality bushing kit will work to your advantage in this area. Even Polyurethane is better than the stock rubber ones.

Rear Camber

To assist with cornering grip, the negative camber in a modern IRS system increases as the suspension is compressed. While this increases the contact patch of the tires while cornering, it actually decreases the contract patch during hard straight line acceleration. Dialing in some positive camber seems like a logical solution, but some tips are hard to come by.

I don’t know how many secrets I want to reveal to the world as far as alignment settings are concerned. What I will say is that how a car looks at a rest is completely different than how a car looks while going down the track. All that matters is how the car looks as it’s going down the track. With that note..... Hint, Hint... put some positive caster in the car, but not that much. It’s less than one degree.

STIFFNESS

Let’s say you want to try your hand at going fast with an IRS, but have no intentions of running 6s. Custom race shocks aren’t always necessary for less extreme applications. In fact, experimenting with modestly priced springs and shocks are a good starting point for managing weight transfer.

Compared to a solid-axle car, an IRS car benefits from softer front springs. This gives them more stored energy, which helps with weight transfer. The front shock valving should be similar to a solid-axle car, but the rear shocks in an IRS must have much stiffer rebound valving. Some IRS drag racers I read about online went through a phase with their cars, it would hookup on the initial hit, then go up in smoke ten feet into a run after the rear tires unloaded. By stiffening up the rebound valving, most were able to keep their cars squatted and hooked up the entire pass. Now their cars just squat and go.

Anti-Roll/Sway Bar.... Good To Have

Big anti-roll/sway bars in an IRS-equipped car perform the same function as they do in a solid-axle car. In addition to loading the left-and right-side tires evenly at launch, they help force the tires downward into the pavement. This is one of the few ways an IRS can generate at least a small degree of anti-squat.

I thought this was very interesting info so I looked at numerous videos of IRS and SRA cobras launching and noted the HUGE amount of squat that the successful suspension tuned IRS guys got. NONE of them lifted and planted the tire like the solid-axle cobras did.

I'm sure most of you have had good success with the tried combos that got you the 60' times, and in no way am I saying that those combos don't work. Just posting the research I scoured from reading.

I dont take any credit for this information. Its from multiple articles I have read on the subject both online and in magazines and books.

Hope this helps everyone and happy track times.... Now Go Kill a Few Chevys at the track.

 

[TP 112]

I learn something lurking around here everyday. Great info, thanks!

 

[ClubVenom1]

No problem... Maybe a Moderator should Sticky that info.

Or I might just repost for others later.