A Arms in the Rear!

geoff

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zjohnson said:
Also, if you consider the angle of the torsion housing on trailing arm cars, your CV angle is determined by your transmission output flange location, and not the typical negative camber needed on a an A-arm car. (allowing for a greater magnitude of down travel, which is the most important travel number for a rear independent car.

thats a really good point. if its gonna have a arms and a camber curve its gonna need some $$ axles. lesser setups would need to use trailing arms to get travel..
 

geoff

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what do you mean a "bad kick", like the tail end came flying up and the nose falls? I would not expect the front to fall at all on a mid engine IRS vehicle because all the weight is in the rear
 

elcaprerunner

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Not so much that the front falls, but the back end of the car will jump violently. Even the most correct cars like a Jimco will do it, one reason you never see them go real fast over a steep mogul or jump, they are at risk of going over. Combine the violent rear kick with the speed the car is moving and the upward momentum of a rear engine and it is going over. But just about any truck of any class can hit that same steep mogul or jump just as fast as he can hit anything else and it will pop the front in the air (well designed buggies can do this too). there is not much risk of cartwheeling with the nose in the air at a high rate of speed as much as you are the back.
 

ChuckH

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Throttle control can lessen rear end hop, some MX riders and people with lots of seat time have much less of a problem, seat time helps getting a rear engine car into and around a corner fast also.

Basically if you let off the gas your going to unload the rear, and if you do it at the wrong time while going fast your going to be looking at the ground.
 

mosebilt

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weight transfer is the main factor for why the buggies/rear a-arm cars "kick".the "trucks have better weight transfer with the engine in the front ,driver/drivers in the middle,fuel cell and spares in the back.the weight is pretty evenly distributed(still more in the front),so when you hit that hole/jump you dont need to pre-load the suspension as much as with a buggy/a-arm car-truck.with the motor in the front,when you hit the hole/jump the upward movement of the suspension moves the vehicle up,popping the front up and usually keeping the vehicle fairly level,with rear engine there isnt much weight on the front suspension,so you hit the same jump/hole it forces the front end up more,then the rear hits where all the weight is,compresses the rear suspension more than the front-the resulting rebound kicks the back up.if your good/lucky you can pre-load the suspension immediately before the jump/hole and go over it fairly flat.expirience and properly valved shocks have most to do with that.rear a-arm cars have a whole different weight transfer problem-rear more than mid.with mid-engine the weight is more centered in the car so the weight transfer(which is usually straight up or back a little) doesnt affect the car with a kick that much(usually),rear engine -where all the weight is hanging off the back of the car is harder to control(most of the weight is behind or close to the rear wheels).expirience and shock valving come into major play here.
 

FABRICATOR

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geoff said:
Fabricator -- what is the superior wheel movement that can be had with trailing arms? I would think that a camber cuve would be a good thing?
Not sure about camber change in the rear with A-arms, does not seem very relavant. A-arms work great, but looking from the side (which is the way the bumps go by) they go straight up and down. Similar to a 3 or 4 link system, trailing arms swing away from the bumps through about half their travel. In a long travel situation, it is easier and more efficient for a chassis to accommodate the loads from a rear swing arm system than A-arm. It is also easier to keep things rock resistant and fairly light. So far, rear A-arm setups have been limited to smaller, lighter cars which don't need really long travel anyway.
 

Josh_K

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Can some one here explain the benefits of pinion angle change with a linked reared?

I wonder what it actually does?

Josh
 

zjohnson

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why would you want 1000lb solid axle messin' up your race car? j/k
No Idea about pinion angle change, I believe its needed to help the U-joints live, not sure, don't really know . . .
 

elqdeasu

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Good question geoff,
I had also wondered about IRS on trucks. Does anyone know if IRS would be legal on a class 7 open or class 8 open truck? I know it would be tougher to design and build due to the required stock frame. But, regardless of a good or bad design, I was wondering if it is even legal in these classes. I have the Score rulebook and all it says for both these classes is:

"Suspension must retain the original stock concept (A-Arm, MacPherson Strut, I-Beam, etc.). Suspension support systems are open (leaf, coil, torsion bars, etc.)"

A 3-link or 4-link rear suspension is not the original concept but almost everyone has them. So I believe that trailing arms or A-arms on the rear would also be legal. The support system would be switched from leaf to coil just like in a 3-link or 4-link design. What do you guys think? Legal or not?
 

zjohnson

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I think by stock concept they mean you must retain the solid axle of a truck. I think that 4-link vs leaf spring is the same Idea, both are kind of outdated (if you ask me, and I don't want to open another barrell). I think that if you choose the 7open or 8open class, you need to retain that rear solid axle, until they come out with an independent rear truck.
 

FABRICATOR

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I’d like to hear about the pinion angle thing too as some here mention it regularly. IMHO, at least some of it originates from leaf springs, wheel hop, and pavement issues. Ideally, the pinion shaft should stay more or less parallel to the transmission output shaft. This keeps the U-joints and drive line running as smoothly as possible. I only know of two things you can do with pinion angle when working with long travel. The first is to make sure the limits of the U-joints, etc, can cover the entire range of axle movement. The second is to take whatever is left over from the first and adjust it to provide the most longevity.
 

Superfab

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Pinion angle has a direct effect on u-joint life. You need approx 2 deg. otherwise the joint goes into harmonics and destroys it self. An excessive amount will cause it to bind and break. If you look at passenger cars that have an offset pinion they usually have the driveshaft in a straight line to the pinion at ride height. The offset from trans output to pinion center provides them with the angle on the u-joint. On a truck at some point the pinion may pass through a point where there is no angle. As long as it is only momentary it won't cause any problems.
 
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FABRICATOR

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Right on! But the part that is a mystery is how (or if) pinion angle affects the operation of a 3 or 4 link rear suspension. Some claim it does.
 

ntsqd

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I've come across info in Circle Track racing that pinion angle can change the fwd 'bite' of the car. The condition of the track will cause them to change the angle. Never have seen any guide or documented investigation into this though. Maybe one of the circle track chassis books talks about it?
 

Superfab

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On a 3 or 4 link it's not pinion angle but the location of instant center that determines the forward bite. It's referred to anti-squat. The lower instant center is the less anti-squat you will have and also less forward bite. More anti-squat = higher instant center = more bite. The more bite it has the faster you can apply throttle. NHRA Pro-stock drivers have extremely adjustable 4-link setups so they can get the car to hook up on all the different tracks they run. The traction is never the same from track to track. There are several good books on the subject but the whole thing boils down to if you want it correct you gotta do the math. That's why guys like Trevor Harris make the big bucks!
 

Josh_K

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Superfab said:
On a 3 or 4 link it's not pinion angle but the location of instant center that determines the forward bite. It's referred to anti-squat. The lower instant center is the less anti-squat you will have and also less forward bite. More anti-squat = higher instant center = more bite. The more bite it has the faster you can apply throttle. NHRA Pro-stock drivers have extremely adjustable 4-link setups so they can get the car to hook up on all the different tracks they run. The traction is never the same from track to track. There are several good books on the subject but the whole thing boils down to if you want it correct you gotta do the math. That's why guys like Trevor Harris make the big bucks!


DUDE...

Take your your prostock car and hit some 30" whoops at about 90 mph. Then please explain to me what launching from a tree has to do with a off road 3/4 link rear.

Josh
 

zjohnson

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I know I'm cracking open the can of worms, but why inhibit the suspension travel of an off road car. That is how antisquat and antidive work, they prevent the suspension from traveling under specific scenarios . . . I am more concerned about the car going straight, and then I'll just adjust my shocks (thats why you got them) to cover the squat and dive of the car.
As far as applying roundy round theory, its a nice idea, just get nascar, or anyone else, to race on a 3D course and we'd understand it all . . . I understand the best thing that can be built into a 3/4 link is traction, but a lot of that has to do with mass transfer an CM.
 

Josh_K

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First off, I am not going to tell you much, but what I will tell you is that on a full-blown v8 powered race truck/truggy traction plays almost no role in the formula. Just look at the tires a t.t. uses. They are built to slip and lose traction. All this drag race & roundly round traction BS is a major misleading concern. In off road if you want to go fast do like a dirt bike does as it climbs a hill. Spin the tire”s”.

As far as the pinion thing keep thinking about it. It’s kind of like rear cantilever arms. There is more to it than meets the eye.

Josh
 
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