Why the hang up's on I.R.S ?

[dot]

At the time I put in my Corvette rear I was seriously considering a Ford 9â€. I asked a friend who was building a circle track car and he advised me, that the difference between the two in racing setups was the way the cars handle the corner. In circle track the preferred way is to drift through it. In GTP or sports car racing although drifting is present, it is not the best way around varying radius corners. Drag racing has proven the requirement of a solid rear axle. There are plenty of books on the subject. It just depends on the application you are going to apply the car to.

In the early nineties Hot Rod put a ford 9†in a Z. They were happy with it.

Cheers…:::Glenn

[Guest Zachb55]

they did huh? ill have to look for that, i bet my step dad has the mag, just in a box out in the garage some place... The reason im beginning to look into all this stuff is because the next step on my car will be to improve the brakes, suspension, and driveline to prepare for an eventual engine swap... my goal for the car is going to be something that handles well but drags better than anything else, so im not too worried about the cornering aspects on this car. Any tips on where to get started with studying my different suspesion options? books/websites? thanks!

 

-Zach

[johnc]

NASCAR and most of the Circle Track race cars run a solid rear axle because the rules require it. They do not run a solid rear axle because of better handling. A properly setup IRS compared to a properly setup line axle will:

 

1. Allow better braking into corner because of better rear compliance. More braking load can be shifted to the rear. Ever hear of a NASCAR driver complaining about rear axle hop under braking when running Watkins Glenn or Sears Point? When that happens they tend to spin and go off track.

 

2. Allow better transition into and out of corners due to less rear unsprung mass. The chassis (shocks mostly) doesn't have to be tuned to control that additional weight. It can be tuned to provide better transitory response.

 

3. Allow better power application in corner exit because of better rear compliance and a separation of torque effects from the suspension. Circle track cars have to have the rear suspension tuned to control pinion windup during accleleration even to the point of letting the pinion move to soften the rear and get more grip. That pinion movement takes power away from the rear tires and affects acceleration off a corner.

 

Look at the top level series where rear suspension design is not limited to a specific type. IRL cars on ovals all run IRS. CART on ovals all run IRS. IRL, CART, F1, ALMS, etc. all run IRS on road courses. NASCAR engineers and designers have to run a live axle because of rule requirements, not because its the best choice.

[stony]

wow talking about coming back from the dead. this thread was 3 years old till the 16th when it was revived.

 

I was on an IRS trip for a while. spent gobs of money on custom axles modifying my struts to accept supposedly stronger 300z stub shafts. (26 spline) and a group of other things i felt would make it bulletproof. i failed. Now the car is in the shop getting a 8.8 with 35 spline axles, spool, 4.56 gears, billet caps, aluminum diff cover and wilwood disc brakes ladder bars, and fully adjustable coilover shocks. the rear end will measure 54 inches from the 2 wheel mounting surfaces. this will allow me to fit my 275 50 15s under the small overfender i have without having to doing an mini tubs. the car will look just as it does now except the rearend will not break when i launch

 

Im going to install a 2 step so i can launch consistently at 4000rpm

[Guest Aaron]

Stony, I will be interested to hear how well those ladder bars work on the street. I have heard that they are great for drag racing, but do not drive well because they do not allow the axle to twist and conform to irregular surfaces.

[johnc]

wow talking about coming back from the dead. this thread was 3 years old till the 16th when it was revived.

 

Wow! I didn't notice. I guess this gets our "HybridZ Back From the Dead" award.

[Dead Roman]

For an IRS unit the tire travels through an arc which changes the camber (more positive). Since this helps against side forces on the tire and counter acts body roll it is a good thing and improves handling. As Mikelly noted' date=' an IRS car squats on take-off but when it does it adds camber and the tires are no longer flat on the road which reduces traction.

 

.[/quote']

 

 

 

An IRS rear unit dosent travel in an arc as you have stated. on an IRS untit the camber can be adjusted but as the suspension compresses and decompresses it will stay where you have adjusted it to, camber will not change if it did, what would be the point of adjusting camber eh?. What you are referring to is an independant suspension like that used on the front end of a ford truck, a- arm suspension does travel in an arc and is similar to full IRS but its really not the same thing.

[JMortensen]

Uhhh, camber does change on a Z. A double wishbone IRS may have camber changes cancelled out, but any Z out there (not sure about the Z32 or the 350) is going to have a camber curve, where the camber will get more negative until the control arm goes horizontal, then less negative as it continues through its sweep.

 

ZX's have camber AND toe change like a Porsche or a 510.

 

Setting camber is not a final thing. You set where it starts out, then based on tire temps or tire rollover or whatever you adjust to get the best perfomance. It constantly changes as you drive.

 

Jon

[Guest ON3GO]

well guys i hate to just chime in but what would yiu guys think the perfect rear end for a street driven Z, that street races, and also does auto-x'ing and such on weekends.

my front suspension is already pretty much for a road racing car, and the car is very stiff.

reason i ask is i need a rear end, and with 400+hp in my new motor im afraid ill be blowing this thing up FAST (the rear end that is).

any ideas guys.

also take into note that if i could do this now i would, but if i have to wait to do it really right and such ill wait.

 

mike

[johnc]

An IRS rear unit dosent travel in an arc as you have stated. on an IRS untit the camber can be adjusted but as the suspension compresses and decompresses it will stay where you have adjusted it to, camber will not change if it did, what would be the point of adjusting camber eh?.

 

Camber does change as the front and rear suspensions of the 240Z moves through its range of travel. Both the front and rear suspensions are designed to gain negative camber as the suspension compresses. This is to offset the positive camber gain caused by chassis roll. An independent suspension that doesn't gain camber in bump is difficult to make handle well. The Ford Tempo and Mercury Topaz are fine examples of a poorly designed independent front suspension.

[blueovalz]

I'm going to add an insignificant tidbit here as well. If I understand the suspension correctly, the camber increase continues beyond a horizontal control arm configuration. The rate of increase decreases until the control arm is perpendicular to the strut tube and only then does in start to decrease, but no where I know of does any Z have this much compliance in the suspension.

[Dead Roman]

An IRS rear unit dosent travel in an arc as you have stated. on an IRS untit the camber can be adjusted but as the suspension compresses and decompresses it will stay where you have adjusted it to, camber will not change if it did, what would be the point of adjusting camber eh?.

 

Camber does change as the front and rear suspensions of the 240Z moves through its range of travel. Both the front and rear suspensions are designed to gain negative camber as the suspension compresses. This is to offset the positive camber gain caused by chassis roll. An independent suspension that doesn't gain camber in bump is difficult to make handle well. The Ford Tempo and Mercury Topaz are fine examples of a poorly designed independent front suspension.

 

Danke, you learn something new everyday

 

why is it that indy cars have a suspension that does not travel in an arc?? is it because they can afford to spend the time figuring out how to make it handle well? is the fact that you have teh whole footprint of the tire on the ground at all times worth the extra work?

[JMortensen]

I thought camber increased (relatively speaking) after the arms go horizontal, because as the suspension continues to move the angle effectively makes the control arm shorter and shorter, drawing the wheel further into the wheel well, shortening the distance from the bottom outside of the tire to the centerline of the car and reducing the camber.

 

Never seen a Z go past horizontal before? Take a look at a really low Z with sectioned struts (I think yours might even qualify). The fronts go past horizontal before the rears, but the rears can go past as well. Here's Tom Holt's website showing his rear end at full droop:

 

http://sth2.com/Z-car/fulldroop2.jpg

 

I can only imagine that this setup goes past horizontal pretty often, despite the stiff springs and heavy shocks. Probably past horizontal when you put it on the ground...

 

Also, my understanding is that once the arms go past horizontal then side loads on the tire will compress the suspension, but they don't if the angle of the control arms is down.

 

That's been my take on it for years. I'd be interested to know if I'm wrong, but in my own little mental geometry example it still seems that I am right.

 

Jon

[JMortensen]

Just to clarify, I'm saying camber increase, not negative camber increase. I see that we are using opposite terms there.

 

Jon

[blueovalz]

My understanding of the control arms used with struts is that as the control arm swings upward from a normal unloaded droop position, the angle (from vertical) of the strut, increases, which provides more negative camber as the arm swings upward. If you were to imagine this arc continuing upward to an extreme angle, you would be able to see a point in which the strut's centerline is on a tangent with this arc. At this point the angular (negative camber) increase has reached its farthest point and will now retract as the arm is swung further upward. Now this should not be confused with the lateral displacement of the axle. Yes, the axle's furthest point outward on the car is at the point in which the arm is horizontal, BUT, this is NOT the point at which maximum negative camber is reached. Once the arm travels upward beyond horizontal, the axle begins moving inboard (relative to the car) once again, but the negative camber increase continues (albeit at a very small rate at this point) until the arm is perpendicular to the centerline of the strut. This is because the strut is shortening (being compressed) during this time. The suspension is basically a triangle with one side's length (the strut itself) being shortened, but it's other two side being rigid in length. When the lower side of this triangle (control arm) is at a right angle to the strut side of this triangle (originally the hypotenuse in my simple triangle), the angle at the top (strut tower) is at it's maximum (equating to maximum negative camber).

 

My front arms are indeed beyond the horizontal position as mentioned above

[silicone boy]

All good comments. IRS is certainly better, more compliant in real world situations, as well as road racing over not so great surfaces. On the other hand, if you increase your spring rates to super high rates that some people use on some road racing setups, then IRS starts to resemble a solid axle, so the differences become less important (that's why its OK for Nascar cars to use them and maintain decent handling, yet maintain ultimate durability). I wouldn't trade, though. IRS is just neat.

[johnc]

why is it that indy cars have a suspension that does not travel in an arc??

 

Which "Indy" car? Dallara, Lola, March, G-force? Some have push rod suspesnions, some have pull rods, some are built with a rising rate, some have equal length control arms, some have unequal length control arms. Are we talking about flat oval, banked oval, road, or street course setup? All have the suspesnsion arms travel in an arc (they have to) but they might be adjusting suspension arm length to achieve certain things depending on the track.

 

With a banked oval track camber gain is less important because you get much less body roll, so the suspensions are adjusted to keept he tire vertical in bump. On a road course the suspensions are adjusted to gain negative camber in bump. And, remember, their suspension have, maybe, 4 inches of total travel so its hard to actually see camber change and the suspension moves.

 

And one more thing to confuse the issue - aerodynamics. When a vehicle uses aerodynamics as its major handling component then suspesnion design and setup favors that more then what we traditionally focus on when all we have to work with is mechanical grip. Ride height, pitch, and body roll are much more important when running big aero then tire camber.

[Zsane]

Sorry guy's, but you can't compare the best setup Z suspension with any modern Formula One or IRL setup. Yes those IRS systems are the best in the world, the letters IRS is where the comparison ends. Accuracy and consistency is where it's at in a suspension, even with your geometry correct on all accounts, without a torsionally ridged chassis the geometry goes right out the window with any flex, and bushings, do you have any in your setup? Well there goes your geometry as soon as you put a real load on it. I know, you car out handles anything on the road! it's all relative, see what it does behind a car built for handling. Just like a kid screaming about how fast his buddies 5.0 or Camaro is, then he goes to the drag races and after the Top Fuel cars run, the car looks like his granny is pushing it down the track with her walker. He then remembers that is was fast as hell pulling away from that housewives Volvo the other day, Hummm!

The IRS advantage: smoother ride over rough pavement, if set up properly, reduction in unsprung weight allows the tires to follow the surface better!

Disadvantages: a more complex system that bottom line cost more to setup properly, most production designs compromise optimum geometry and deflection characteristics for cost, the major disadvantage is the ability to put power to the ground, minimal anti squat! High powered cars without rear weight bias (like a Hybrid Z) will have PROBLEMS getting optimum performance and handling. Note the word, problems, I didn't say , it can't be done. The bottom line is both can work great or bad, it just depends on the setup, and on another note, MacPherson strut setups were originally created for cost and simplicity reasons not performance. Except where required by race sanctioning bodies in production-based classes, you don't find them on many race cars do you?

With that said, I still love my Z!