A Arms in the Rear!

[geoff]

why doesnt anyone ever put a arms in the back? All i see is 4 links 3 links or trailing arms. The only rear A arm truck ive seen was that one paris dakar KIA which was pretty clean, although i cant remember who built it.

It just seems to me to be a REALLY simple and effective way to do things... any reasons why not??

[mfs]

cant get as much travel with cv joints u can only run so much angle 22 to 25" is the most and thats with all the high doller stuff & relly wide too , plus transaxles are costly and dont hold up as well inless its lower hp.

[Scrapiron]

The Elite 10 car is A-arm front and rear. I have been past by them in some big stuff and was realy impressed at how the car worked. Look for pics on trackside under the name Aymar. They are getting ready to built a new one and judging by the new prerunner they have it will be very sweet. They claim to have no cv plunge. I have no idea what this guy is talking about with angles of cv's last I checked we all ran the same 930's or 935's. They also run a Mendeola 5 speed just like the rest of us.

[matt_helton]

ever seen ivans trophy truck?

check out Castle Fab. he is building an IRS ranger.

[geoff]

i was thinking about using a supra motor and a turbo400 trans with a real short driveshaft connecting the trans to a supra third member, that way i wouldnt worry about a transaxle and it would move the weight forward a bit. For axles, i could definately go with a 930, but thats the least of my worries. We use the supra rears in drag cars and they never break, one car is running 7.89 on street tires and the original OEM rear (lsd) and oem axles.

that zero CV plunge is a really interesting idea tho... i dont see how you could get the susp geometry to do that, but maybe im missing something

i am looking for the pics but i dont know where trackside is?

[mexracer10]

Look at alot of the sand cars that have rear a arms, some are using u joints insted of cv's. The u-joint axles dont plunge, so all you have to do is make your a-arms work the same as your axle, since the a-arms work on a radius perpendicular to the car so will the axles.

[ChuckH]

http://www.tracksidephoto.com

[mexracer10]

Also corvets have the irs set up with a gm 12 bolt center section with short "driveshaft" like axles to the wheels. Corvets have ran mean hp numbers to the wheels and dont break the rear ends. I had always toyed with the idea of biulding a 4 seater with the corvet rear end

[mfs]

cant get as much travel with cv joints u can only run so much angle 22 to 25" is the most and thats with all the high doller stuff & relly wide too , plus transaxles are costly and dont hold up as well inless its lower hp.

edit: 22-25" travel.

rear 3- link with rear end 28" to 36" travel

[geoff]

yeah a corvette rear would definately be a viable option.

i dont see much wrong with a 25" travel rear end, particularly if the suspension is setup really well with properly valved shocks. The advantages of independant suspension are very appealing

[geoff]

http://www.tracksidephoto.com


looking at the site still not sure where you see aymar?

[1450-ranger]

http://www.tracksidephoto.com/races/2005MDRWildWash250/images/Aymar_0529.jpg
http://www.tracksidephoto.com/races/2005MDRWildWash250/images/Aymar_0532.jpg
http://www.tracksidephoto.com/races/2005MDRWildWash250/images/Aymar_0533.jpg
http://www.tracksidephoto.com/races/2005MDRWildWash250/images/Aymar_0536.jpg

[1450-ranger]

http://www.tracksidephoto.com/races/2005MDRWildWash250/images/Aymar_0539.jpg

[geoff]

that thing looks sick. It looks like it has a lot of angle on the lowers?

[mfs]

how about this diff its a toyota 7.5 w 930 adapters and 4.88 gears. its for sale.

[mfs]

i dont see much wrong with a 25" travel rear end, particularly if the suspension is setup really well with properly valved shocks. The advantages of independant suspension are very appealing


25" with big buget and it would have to be really wide. i would say why u dont see it that much is cost.

[grantdcol]

I don't think it has to be all that costly if you keep your standard drivetrain and use an OEM IRS differential, especially compared to a comperable 3/4 link.

Perhaps a better option than a semi-rare corvette transaxle is an 8.8" centersection from a ford thunderbird. The new Explorers and Expeditions with IRS open up two other options for a rearend - I believe the Expendition uses an IRS version of the 9.75" truck axle.

Also, a trailing arm setup would be the way to go instead of true a-arms. More rearward travel, and possibly reduced axle angles if engineered right.

-Colin

[Stephen]

Dondel's TT is IRS also.

[mexracer10]

The Mike Tieman truck also irs and Danny lettner also irs and the old Fortin tt also irs which ibarra has i think, and toyota tt also irs there must be something there that works

[geoff]

they all look like big RC10s

[ntsqd]

Hey! Those work!

[geoff]

hey thom!

any idea of what the scrub radius should look like in the back? It sems to me that a near-zero scrub radius would be ideal?

[FABRICATOR]

There are two discussions going on here. Trailing arms have many advantages over A-arms in the rear, including strength and superior wheel movement. You are more likely to see variations on trailing arms than a shift to A-arms.

It’s just a matter of time before IRS overtakes solid axle setups, even on trucks. IRS travel is more effective than solid axle, so you don’t need as much.

[FullsizeFun]

hey thom!

any idea of what the scrub radius should look like in the back? It sems to me that a near-zero scrub radius would be ideal?

I dont think Scrub would be an issue in the rear, unless you plan on having rear steering.

[zjohnson]

The ideal way to get the wheel travel, with the least amount of moving parts, is a trailing arm setup. Going super wide is the only way to get large travel numbers though, because the internal cv angle is what kills you. Running a non plunging CV allows for greater angulation. That in combination with a plunging axle, like the one from McKenzies will easily get you 25"+ of rear wheel travel w/out going to a super wide track width. If you don't mind going wide, 30"+ is easily possible with a wide trans.
In my opinion A arms aren't what you are looking for in Off road, to many parts to break, and the strength doesn't seem to be as good. Plus, with a raised torsion housing, you can get a good arc on your travel.

[grantdcol]

I think a torsion setup would be a step in the wrong direction for a truck. Coilovers would be the way to go.

But yes, trailing arms can get you a great travel arc with a highly progressive motion ratio to the shocks, and most importantly, rearward travel.

-Colin

[zjohnson]

"torsion housing"- the axis in which a trailing arm rotates around . . . Not using a torsion bar, just refer to this part of the chassis as the torsion housing because that is what is was called . . .

[grantdcol]

You pointed out that CV joint angles are limited Plus they get weaker as the angle increases. In a IRS, wouldn't it be possible to use a double cardan joint (constant velocity) or a Cornay joint? Both are capable of up to 90 degree angles. The MacPherson TT used Cornay joints in the front for an AWD system:

http://www.drivetechnologies.com/page6.html

-Colin

[FABRICATOR]

CV’s do loose strength with more angle but so does every other joint. You will get more travel using a narrow trans than a wide one. That’s why Corvette and other existing rears won’t work with long travel. A double Cardon will go more angle than a simple U-joint, but it won’t go anywhere near 90 degrees or even as much as a common non-plunging (Rzeppa) joint. The Cornay seems to be a bit of an illusion.

[Scrapiron]

Here is the Aymars web site, and they have a good pic of there frame on therewww.eliteoffroad.com (http://www.eliteoffroad.com)

[mosebilt]

LOOK! rear a-arms w/26" wheel travel,25 degree cv angle,2" ground clearance full compression w/35" tires,5 degree camber change through full stroke of suspension,outside tire stays vertical in corners and sway bar gives it the x-tra bite.works bitchen! pricey though.

[grantdcol]

Nice setup. That's a rear-engine buggy with a transaxle though, not a truck.

-Colin

[mosebilt]

we are going to be putting that set up in a "truck"-using a redline LS-1 w/auto mounted mid-engine to a Winters quick change rear end.keeping all the weight in the center of the vehicle (low polar moment) makes the car steer/corner well-but sacrificing straight line stability-the opposite applies to spreading the eight out in the car.the Winters is only 1.5" wider(.750 per side) than a Mendeola-can still use long axles and get wheel travel.the car runs 32" long axles w/Kartek mid board-micro-stubs.with the quick change rear end the gearing can be changed in 20 minutes and the availability of different gears makes "tuning" the gear ratio easier-almost limitless.rear end costs about $3000.00

[ELITE10CAR]

I feel there are some misconceptions here about a arms. Many people seem to believe that a arms are not as strong as trailing arms, or that they do not work as good. We have never had an a-arm crack, break or fail in the three years we have raced our car, yet we have seen numerous trailing arms literally come off (two cars in our class at laughlin). Some believe that they do not work as good, but at laughlin we passed 17 cars on day two, in 44 miles, that can attest that our a arms do work. We finished fourth dispite the time lost passing those vehicles(just over a minute down). Many of our fellow racers have taken notice that a arms might not be such a bad idea after all. The facts are that trailing arms do work very well, but there is always something better. I feel the rear a arm design is like all other items in history, if it defies the social norm, it will take a while to become accepted. Others have used this design with success, yet it still has not been accpeted.

[mosebilt]

:) :) :) :) :) :) :) :) :)

[zjohnson]

Just one question for you A-arm rear suspension guys: Is there a succesful A-arm class one car? From my point of view, a trailing arm is more simple, and if built right, as strong as a well built set of A-arms But to me, I think that an A-Arm rear is just too many moving parts, and also to many parts to replace when you prep. I also dig the fact that a trailing arm does just that, moves back for most of its up travel if designed correct. I am just not sold on rear A-Arms, the scrub and the complexity is what bothers me. I am sure that enough time and money can result in a competitive car, but I like A-Arms for the front. Also, if you consider the angle of the torsion housing on trailing arm cars, your CV angle is determined by your tranny 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.
Question for Mosebuilt: What is the scrub in the rear of your car, size CV, and power running through the trans? Just nice to know what people are trying. Also, not to pick, but 5 degree of camber change seems like a lot . . . how wide is the car?

[ELITE10CAR]

A rear a-arm setup is in fact more complex than a trailing arm. There are more moving parts but that doesn't mean that there are more parts to break. If built right (as we have done) they are just as reliable as anything else out there. Just as stated before, I have seen and heard of numerous trailing arms break...probably because there are far more of them out there to break. If a car is well prepped and checked for cracks, breaks, etc, neither of them will break. The fact is that we are the only Class 10 or 1 running competitively with an a-arm rear....again...we have never broken or even bent our rear a-arms. There are just as many moving parts on our rear suspension as our front and everyone elses front. And for the record, you can get more down travel with an a-arm rear than with a trailing arms rear. This is do to the fact the axle can be perfectly perpendicular to your transmission and that is the only direction of travel. The front to rear angle on the axle never changes as it does with a trailing arm, therefore you will get more down travel. The CV never has to deal with a compound angle. You can also run plungeless CV's on a rear a-arm setup without $10,000 plunging axles. These are just my experiences as I have personally raced thousands of miles in our A-arm rear class 10 car. I have tons of in car videos (showing us pass many class 10 winners/champions) and as soon as I can get someone to make a few clips I will post them.

[mosebilt]

5 degree's camber change through the entire stroke(-3 full droop,0 mid stroke,-4 full compression) isnt a lot when you consider irs is usually 3-4 degrees.we are running 934 cv's,as far as scrub is concerned i designed the arms to have aprox. 1.5" per side-to keep a more consistent plunge on the axle,so consistent that we might use non-plunging cv's next time.you dont notice the scrub in the back since your not steering and it might actually give you more bite.a-arms in the back need to be mounted level with the ground otherwise you loose a lot of traction,the raised torsion housing on irs cars works well for that suspension(i do it in all my cars) but the a-arms dont seem to like it-you get a lot of wheel-spin.we are putting anywhere from 400-850 to the ground through this stuff.car is 92" wide. your not picking.

[geoff]

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?

In my opinion there seems to only be benefits to be had once people open their minds to IRS especially in desert racing of all off-road motorsports. I feel it takes the same amount of money to make a really good 4 link rear as a good a arm rear.

thanks for your opinions tho guys its a really interesting thread

[elcaprerunner]

Any overall contending independant rear race car I've ever seen has a rearend kick. On some cars it's not so bad, on others it's really bad. I think that is a big reason you don't see too many IRS A-arm Trophy trucks. The two best I've seen were Ivan's and Skilton's (Kia). Letner's is fast but has a nice kick in the big holes, Dondel's had a really bad kick when it was first built. I think rearend kick is a big turn-off, having the front end go up in the air is much better than having the rear go up in the air.

[geoff]

Also, if you consider the angle of the torsion housing on trailing arm cars, your CV angle is determined by your tranny 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]

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]

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]

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]

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]

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]

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

I wonder what it actually does?

Josh

[matt_helton]

dangit Josh, its angle, not angel. lol

nice question btw.....lol

[zjohnson]

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]

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]

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.

[elqdeasu]

That's a good point zjohnson. I never thought about it that way. Thanks.

[FABRICATOR]

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]

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.

[FABRICATOR]

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]

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]

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]

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]

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]

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

[tim_krueger]

ok lets try this again

josh i'd like to see a drag race between the green weenie and kung fu

for some reason kung fu feels like it hooks up better than most, even under boost. i liked being pinned in the seat rather than just shooting a roost (not thet the weenie dosnt pull)

[matt_helton]

oh hell..........

[Josh_K]

Tim...
How do I say this so it doesn’t insult anyone...?

Dan’s truck is really nice and well built but I don’t think that would be a far comparison. After all my truck weighs about 6k and his weighs about 4,000-4,500lbs (guessing).

I do know that Dan told me the most speed he could hit the ror jump with was 65 and I had 67 mph once or twice. So there pretty close in acceleration.

[ntsqd]

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!

Thanks for the lesson in what I already knew. The reason I brought it up is b/c it runs counter to everything I've seen and done. If it isn't discussed, then it can't be dismissed or considered for what they're really changing. For instance, how do they change pinion angle on a dirt oval car? My guess is that they alter the suspension linkage in some way. Odds are high that whatever they do changes the IC's location.

As to Anti-Dive/Squat and locking out suspension movement, I agree. But, I think a small amount of one or the other can be an advantage. You don't want the Anti-Dive to lock out the suspension when you suddenly discover that that Whoop got deeper since that last lap, but it would be handy if you had just enough AD that you don't smash the front end down into the oil-pan grabbing Lunkhead just b4 the ramp into that Whoop.