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Rear end Question....IRS vs live axle

Old 01-22-04, 07:57 PM
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Rear end Question....IRS vs live axle

assuming you have two first gens with identical setups minus rear ends

both have bridge 12a making like ~230-50rwhp coil overs and properly setup suspension for the track conditions.

IRS say from a 2nd gen converted to work on a fb

or a

modified ford 8.8 with 4.30's or 3.90's


please give pros and cons, both setup with a similar clutch style LSD or spool if you like
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Old 01-22-04, 08:01 PM
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Wow, FC IRS swap is not an easy job!
That would basically ban you from almost every race sanction I know, except for all the "unlimited" racing classes.


-Ted
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Old 01-22-04, 10:04 PM
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yea I was mainly convincing my self that a 8.8 ford swap wont hurt my ability to corner assuming the suspension is set up correctly.

that and to see if someone with more knowledge than myself could explain pros and cons of each
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Old 01-23-04, 07:59 AM
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Well, with an IRS you can set it up to gain camber in a turn. That's impossible with a live axle because it will always try to sit flat.
Another down side to a solid axle is that one wheel affects the other. Hit a bump with one tire, it decreases the contact patch of the other tire.
The live axle is heavy, so if the tracks you normally race at have bumps (especially mid corner) then and IRS will be better (less unsprung weight).
Driveline torque is also transmitted to the tire with a solid axle.
But you really have to consider the rules of whatever class you are running in. Changing to an IRS may move the car to another class where it will be completely uncompetitive.
You can do just fine in autocross with a solid axle. Just make the attachement points of the panhard rod adjustable, so you can play with the roll center.
Good luck

*edit* I see you're from Waco. I think Peter Fehn and David Butler autocross in your area in a 1st Gen... they are NOT slow with their solid rear axle.

Last edited by Travis R; 01-23-04 at 08:01 AM.
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Old 01-24-04, 09:11 PM
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Originally posted by Travis R
Driveline torque is also transmitted to the tire with a solid axle.
That's an *up*side, not a downside. You can get way more anti-squat with a solid axle than with an IRS precisely because of the torque reaction. Means you can accelerate harder out of corners...
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Old 01-25-04, 06:31 PM
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you can also bend the solid rear for 1-2 degrees
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Old 01-25-04, 06:48 PM
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Originally posted by j9fd3s
you can also bend the solid rear for 1-2 degrees
Heh, heard that from Jim Susko... that still sounds weird if I think about it.

I just have to ask, since this thread's live, anyone ever swapped over the Ford 7.5? From what I've heard, it's quite a lot ligher than the 8.8...
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Old 01-25-04, 07:26 PM
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Originally posted by Travis R

*edit* I see you're from Waco. I think Peter Fehn and David Butler autocross in your area in a 1st Gen... they are NOT slow with their solid rear axle.
I run in the Texas Region with Peter and David. The FP car is certainly not slow, but then again it is an FB that began life with a solid axle and there is enough rubber on that car that the tires stick well out of the fenders. It's also a 3 link; not near stock.
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Old 01-26-04, 11:22 AM
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Originally posted by peejay
That's an *up*side, not a downside. You can get way more anti-squat with a solid axle than with an IRS precisely because of the torque reaction. Means you can accelerate harder out of corners...
Why do you want anti squat on a solid rear axle?
I was refering to the torque along the center line of the car... so it pushes down on the left wheel and lifts the right wheel. So whether or not you can accelerate harder depends on which direction you are turning.
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Old 01-26-04, 11:39 AM
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Originally posted by Travis R
Why do you want anti squat on a solid rear axle?
Same reason you want it on any car. Anti-squat adds load to the rear tires under acceleration without using up suspension travel. This puts the power down better.


Originally posted by Travis R

I was refering to the torque along the center line of the car... so it pushes down on the left wheel and lifts the right wheel. So whether or not you can accelerate harder depends on which direction you are turning.
You lost me here??? All the torque reaction is through the ring and pinion. The live axle itself does not see any torque that would cause what you describe.
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Old 01-26-04, 11:56 AM
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My understanding of anti-dive and anti squat was to prevent camber change, so the tire doesn't lose contact patch... camber change is obviously not a problem with a solid axle.
It's all about happy tires right? Anti squat effectively increase the rate of weight transfer, and to me, that says you're going to overload (shock) the tire more quickly. Like you said, it adds load.
If you're having a problem with suspension travel, that is a seperate issue to me... stiffer springs, cut the bump stops, run a smaller dia. tire or trim the fender.
I retract my statements about torque through the diff. See I'm a reasonable guy.
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Old 01-26-04, 01:14 PM
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Originally posted by Travis R
My understanding of anti-dive and anti squat was to prevent camber change, so the tire doesn't lose contact patch.
That is partially incorrect. Anti-dive and anti squat help keep the chassis from pitching under brake and accel. The real benefit is not the camber control, but the management of chassis pitch.


Originally posted by Travis R
Anti squat effectively increase the rate of weight transfer, and to me, that says you're going to overload (shock) the tire more quickly.
Incorrect. Tire grip is directly related to how much force (weight) is pushing that tire against the ground. Wings are one way of adding additional tire force. Anti-squat and anti-dive do the same thing by geometrically managing chassis loads through the suspension.

Look at the pic below.




Every force has an equal and opposite reaction. Under hard acceleration if the tires are turning forward, than the axle housing it turning rearward with an equal and opposite force (this reaction occurs through the snout of the axle housing where the pinion gear it trying to climb the ring gear). The links from the axle housing will be pushing upward where they connect to the chassis. This force directly opposes the natural tendency of the rear end to squat down under hard acceleration. The thing to remember is that it DOES NOT affect weight transfer. Weight transfer will still go to the rear under acceleration, but the anti-squat prevents the weight transfer from collapsing the rear suspension travel. From the pic you can see that as you move the point of intersection (instant center) with the chassis you can control how much anti-squat is in the chassis.

All of this is still true for multi-link and double wishbone suspensions, but they accomplish it in slightly different ways. Always remember that anti-squat and anti-dive only manage chassis movement, they do nothing to weight transfer. Weight transfer is a direct result of the car's CG being a point in space higher than the tires' contact patches. The higher the CG, the more weight transfer you have. A go cart has a solidly fixed suspension but it experiences the same weight transfer nonetheless. If you removed the springs and welded steel bars in their place on any car you would have no dive or squat, but you would still have the same amount of weight transfer.
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Old 01-26-04, 01:54 PM
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I see your arguement, but I said it increases the *rate* of weight transfer, not the amount. I already knew everything in your last two paragraphs.
If you allow the rear end to squat, then you can control the *rate* (and thus tire loading) with the shocks.
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Old 01-26-04, 02:11 PM
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Originally posted by Travis R
I see your arguement, but I said it increases the *rate* of weight transfer, not the amount.
Than it's still wrong. The *rate* of weight transfer is due to the amount of accel and the damper settings (assuming there is a suspension; weight transfer in a kart is purely due to the magnitude of the force). Suspension geometry (and therefore anti-squat or dive) does not affect the rate of weight transfer.

Originally posted by Travis R
If you allow the rear end to squat, then you can control the *rate* (and thus tire loading) with the shocks.
Yes. But if your suspension squats enough to use up the available travel and you hit the bumpstops your spring rates just went to somewhere around infinity. So the tire would gradually absorb the power and transmit it to the ground until the suspension bottomed out and instantly the tire breaks loose.

Originally posted by Travis R
I already knew everything in your last two paragraphs.
And yet you said "I was refering to the torque along the center line of the car... so it pushes down on the left wheel and lifts the right wheel. So whether or not you can accelerate harder depends on which direction you are turning". Maybe you were up too late last night
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Old 01-26-04, 03:00 PM
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Hey, I already said I retracted that "torque along the centerline" statement.
See my previous posts for my stance on bottoming out.
I disagree with your point about antisquat not affecting the rate of weight transfer. There is obviously a range here...
Zero antisquat does not affect the rate.
100% antisquat (rigid) has instant weight transfer.
So as you move through the range of more or less antisquat, it changes the rate of weight transfer.
This debate has obviously gone past the point of the original posters intent. The bottom line is you can make your car handle fine with either. Just make sure of the rules for the class you want to run in.
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Old 01-27-04, 07:11 AM
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Originally posted by Travis R
Zero antisquat does not affect the rate.
100% antisquat (rigid) has instant weight transfer.
You are still confused. This is absolutely not true.
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Old 01-27-04, 07:20 AM
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Originally posted by DamonB
The *rate* of weight transfer is due to the amount of accel and the damper settings (assuming there is a suspension; weight transfer in a kart is purely due to the magnitude of the force).
rigid = no dampener = instant weight transfer
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Old 01-27-04, 08:13 AM
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The Physics of Racing by Brian Beckman Explanation of vehicle and tire dynamics
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Old 01-31-04, 03:23 PM
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You are correct about the torquing along the vehicle's center line. (the driveshaft tries to spin the rear axle like a propeller) However, that's generally not as strong as the other torque reaction, the one that we can use to generate anti-squat.
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Old 01-31-04, 04:18 PM
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Originally posted by peejay
You are correct about the torquing along the vehicle's center line. (the driveshaft tries to spin the rear axle like a propeller)
There is a torque reaction that goes back through the driveshaft because it is in line with the torque applied from the motor. The ring and pinion however turns this torque 90 degrees inside the diff/axle housing. The axle housing does not see any real torque that tries to rotate it about the driveshaft. The torque it sees is due to tire loads and so the only point it will twist around is the axles, not the driveshaft.

The only forces that could possibly generate a torque that would spin the axle housing like a propeller would be from drag in the pinion bearings and inertia effects which are practically nil.
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Old 02-01-04, 03:30 AM
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Originally posted by DamonB
There is a torque reaction that goes back through the driveshaft because it is in line with the torque applied from the motor. The ring and pinion however turns this torque 90 degrees inside the diff/axle housing. The axle housing does not see any real torque that tries to rotate it about the driveshaft. The torque it sees is due to tire loads and so the only point it will twist around is the axles, not the driveshaft.

The only forces that could possibly generate a torque that would spin the axle housing like a propeller would be from drag in the pinion bearings and inertia effects which are practically nil.
Any time there is a reaction, there is an equal and opposite reaction.

Let's say you're pushing 200lb-ft of torque from your engine through a 2:1 gear. That is 400lb-ft of torque going through the driveshaft. That is 400lb-ft of torque trying to rotate the engine/transmission, and 400lb-ft of torque trying to rotate the rearend.

Of course, the engine/trans are somewhat solidly mounted. But they DO torque over until the motor mounts prevent further motion, or preferably until your torque bar/strap prevents any motion at all. The same happens to the differential, whether it is in a solid axle or an IRS - 400lb-ft of torque trying to rotate the diff like a propeller on the end of the driveshaft. In an IRS, the diff compresses the bushings holding it in, or just stresses everything if it's solidly mounted. In a solid axle, it tries to move everything.

Once that's taken up, the torque goes towards pulling down on the ring gear. If there was a 4:1 gear in the rear, then there'd be 1600lb-ft of torque pulling down on the ring gear, which also means there's 1600lb-ft of torque pushing up on the nose of the pumpkin. Again, that is taken up with bushing flex on an IRS, and in a solid axle it works against the suspension. If the suspension links are designed so that the pinion angle raises when the suspension droops, then a raising pinion angle tries to push the axle downwards. Voila, anti-squat!

After all that is balanced out, the torque goes to the axles, and awaaay you go!
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Old 02-01-04, 11:29 AM
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I agree with what you said peejay. I was trying to show how the forces present that could make the entire axle housing spin like a propeller at the end of the driveshaft are nearly nonexistant in the scheme of things.
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Old 02-01-04, 12:20 PM
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They're equally as existent as the forces trying to flip the engine over along its shaft axis. Not nearly nonexistent.

The torque in the rearend along the drive axle axis *is* much stronger, by a factor of whatever your rearend gearing happens to be, but it's not the only torque reaction happening.

- Pete (of course, it's partially negated by the engine trying to torque the left front tire off of the ground... )
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