Truckarms...again How can we fix them?

[dontlifttoshift]

I'm new. I have searched. I have read every thread that pertained to truck arms and even skimmed over some of the related drama. Here is what I learned:

Inherent bind, the whole system is in a bind. Depends on what side you stand on I guess it could be good (no arb) or bad....duh, bind is bad.

Heavy unsprung weight - self explanatory, it's like having an extra 9" pumpkin along for the ride

Packaging - opinions vary, on some vehicles it isn't a big deal, like the '48 chevy pickup I had a few years back.

I have work arounds for the three things listed above and consider those problems solved. So what I really want to know, from the guys that know, is what is wrong with the geometry? My priorities are comfort (I drive) and autocrossing and it seems to me that geometrically this suspension is well equipped for those two things.

FWIW right now rear roll center is 15" off of the ground and antisquat numbers are a theoretical 90%, sorry best guesstimate. The car weighs about 2,600 hundred punds with me in it if that makes a difference. That is about as technical as I can get right now, I don't have any real measurements until I scale the car and blow it apart for the winter.

Oh, thanks for not blasting me too bad:fingersx:

Donny

[MonzaRacer]

So if you have bind in a truck arm set up install johnny joints in the front pivot points. then reduced bind, the bind is based over several feet in a truck arm set up and is negligible in my opinion. as for weight then you either need to redesign the arms to weigh less either in materials or actual design. or install a different suspension.

[ArtosDracon]

I like truck-arm suspensions. I've considered it for my blazer. Just use a spherical end on the front, and modify the design\materials slightly and there is nothing wrong with it.

[monteboy84]

Turn it into a Torque Arm style suspension and it should work fine :idea:

-matt

[Mingus]

I know Hotrods To Hell have a "correction" kit for stock 60's GM truckarms. I don't know if it's good, bad or otherwise I just remember seeing it advertised somewhere.

[Payton King]

He is running a truck arm and his car is quick around the big track and autocross.

[dontlifttoshift]

He is running a truck arm and his car is quick around the big track and autocross.


Who is?



I have got the bind problem solved. After reading my post I guess I should have been clearer. What is the best geoemetry for a truckarm style setup.

Please no "just turn it into a three link, 4link, torque arm, IRS." that doesn't hlep me do what I am trying to do. Us street rod types have been using this setup for 25 years or so, i just don't think there was any science behind their application....it was all based on simplicity and packaging......I am looking to optimize it.

Thanks
Donny

[John Wright]

Who is?



Donny,
Here ya go:
https://www.pro-touring.com/forum/showthread.php?t=38420

[dontlifttoshift]

DSE swivel link in the front, I wittled some delrin bushings for the front but I am going back to rubber this winter for NVH purposes. In the back is a left hand/right hand threaded adjuster for wheelbase and pinion angle adjustments and a 1/2" clevis bolts it to the rear axle with shoulder bolts.

I hope this helps.

[dontlifttoshift]

Donny,
Here ya go:
https://www.pro-touring.com/forum/showthread.php?t=38420


Thanks I didn't see that one, I will have to wait for lunch to read that one though.

[Bryce]

Those look like longer ladder bars. I guess they would work the same. and lighter than a normal truck arm. I would say for geometry it would be similar to other systems. Road race application would be 50-60% antisquat. IC is fixed at the front pivot points. Maybe run a watts link and a swaybar for tunning. i know typically you would adjust the height of the panhard bar, to adjust roll center and stiffen or loosen the rear end. Also what is your CG height? That may determine what kind of lateral locator you use. You might need something really low.

[monteboy84]

Any reason for not running spherical rod ends/bushings at all 6 points? I think if you went that route it would alleviate any bind issues associated with other applications, where the truckarms are bolted rigid to the housing, as yours will be.

-matt

[DynoDon]

Who is?



I have got the bind problem solved. After reading my post I guess I should have been clearer. What is the best geoemetry for a truckarm style setup.

Please no "just turn it into a three link, 4link, torque arm, IRS." that doesn't hlep me do what I am trying to do. Us street rod types have been using this setup for 25 years or so, i just don't think there was any science behind their application....it was all based on simplicity and packaging......I am looking to optimize it.

Thanks
Donny
So are you running Truck Arms or Triangulated Ladder bars?
I have noticed when setting up Street Rods with triangulated ladder bars, that if I use rod ends instead of bushings in the front location, that the cars get a bit of a rocking motion side to side, especially without a sway bar, which many street rod guys do not want for aesthetics.

[dontlifttoshift]

Those look like longer ladder bars. I guess they would work the same. and lighter than a normal truck arm. I would say for geometry it would be similar to other systems. Road race application would be 50-60% antisquat. IC is fixed at the front pivot points. Maybe run a watts link and a swaybar for tunning. i know typically you would adjust the height of the panhard bar, to adjust roll center and stiffen or loosen the rear end. Also what is your CG height? That may determine what kind of lateral locator you use. You might need something really low.


Why such a low value for anti-squat? Isn't traction king?

Is there a significant advantage with the watts vs. a panhard in reagrds to truck arm geometry or is the difference the same for all.

Sway bar would tune better than changing roll centers, correct? Or does it take a combination of both.

CG is estimated at 20" to 22". The car is near neutral front to rear, actually a tad heavy on the rear.....I don't have the actual numbers in front of me.

Thanks, this is what I was looking for and is what is lacking from most forums I hang out at.

[dontlifttoshift]

[
QUOTE=DynoDon;726296]So are you running Truck Arms or Triangulated Ladder bars?

Forgive me, I don't see a difference....geometrically speaking. I am running ladder bars.



I have noticed when setting up Street Rods with triangulated ladder bars, that if I use rod ends instead of bushings in the front location, that the cars get a bit of a rocking motion side to side, especially without a sway bar, which many street rod guys do not want for aesthetics.

Isn't that what we want, bind free articulation?

[dontlifttoshift]

Any reason for not running spherical rod ends/bushings at all 6 points? I think if you went that route it would alleviate any bind issues associated with other applications, where the truckarms are bolted rigid to the housing, as yours will be.

-matt


Never really thought about it. From my limited experience, the swivels at the front cure the bind issues.

[DynoDon]

[

Forgive me, I don't see a difference....geometrically speaking. I am running ladder bars.
Not sure if there is really any difference in the geometry, but they are two different things physically and I just wanted to make sure what you were working with


Isn't that what we want, bind free articulation?

According to my NASCAR sources, the truck arm design is never bind free, by design. The arms move in two different arcs and that causes a twisting motion in the axle that the truck arm handles by twisting itself. It is why the arms are made of an I Beam section, not square or round tubing*. By design the truck arm suspension acts like a big sway bar. They use truck arms of different section area to change rear roll resistance and use different front bushings or sometimes mono balls to also change the roll stiffness.
Ladder bars offer almost no resistance to twist and therefore require sway bars and stiff front bushings to help control rear body boll.
* There are some companies making "Street" oriented truck arms from square, rectangular, and or round tubing and the roll resistance of these is very high and they are susceptible to cracking over time.
At least this is how it has been explained to me by a NASCAR suspension expert.

[dontlifttoshift]

Yeah, we are on the same page. We've done a bunch of cars with the the Pete and jakes type ladder bars and for a cruiser they work great.

Instead of twisting I beams we have a smooth radial motion with the swivel link at the front of the ladder bar.....about as bind free as you can get with this set up.

[Bryce]

Why such a low value for anti-squat? Isn't traction king?


Yes, but you are not starting from a standstill at all times. You dont need that much antisquat at 30mph as you need at 0 mph. The more A/S the more brake hop.



Is there a significant advantage with the watts vs. a panhard in reagrds to truck arm geometry or is the difference the same for all.


I think Truck Arms will react the same as other suspensions with a panhard and watts. Watts is less bind and less, almost zero lateral movement.



Sway bar would tune better than changing roll centers, correct? Or does it take a combination of both.


The torque arm that cause body roll is the lenght between RC and CG. The sway bar counteracts that torque arm. The lower the RC the more body roll you could experience due to the longer torque arm. You would increase your sway bar stiffness to counteract the increase in body roll.



CG is estimated at 20" to 22". The car is near neutral front to rear, actually a tad heavy on the rear.....I don't have the actual numbers in front of me.

Thanks, this is what I was looking for and is what is lacking from most forums I hang out at.

[silver69camaro]

Any reason for not running spherical rod ends/bushings at all 6 points? I think if you went that route it would alleviate any bind issues associated with other applications, where the truckarms are bolted rigid to the housing, as yours will be.

-matt

I'd avoid sphericals if possible due to NVH issues. They can be awful on the street.

[John Wright]

NVH issues.
My un-educated guess is that this stands for Noise, Vibration, or Harmonics?..or maybe Harshness?

[silver69camaro]

My un-educated guess is that this stands for Noise, Vibration, or Harmonics?..or maybe Harshness?

Harshness, yes.

[dontlifttoshift]

Matt, care to weigh in here. If remember right, you are not a fan of truck arms. Other than previously stated issues with weight, bind, and packaging, what would you do to optimize?

Bryce, as you get further away from brake hop with less anti squat, do you get closer to wheel hop under acceleration?

[silver69camaro]

That's a tough one.

Well, I'd have to come up with a design that minimized the weight but still stiff enough to avoid goofy resonant vibrations, which can be a serious problem with those if they are too light weight. The design would have to be flexable torsionally, but not exhibit any columnar bending (think I-beam shapes here).

On to bushings, I think I'd seriously look at rubber. This is one of those applications that require movement at either the suspension members themselves, or the bushings. I know rubber bushings aren't sexy, but they have their place I think this may be one of them. They would reduce bind, reduce NVH, and give a good ride. Stiffer bushings can be used in your lateral link to reduce deflection there and make it feel less like a typical rubber-bushed suspension. Large diameter bushings are the OEM norm these days - for good reason - and I'd look into something like that.

If you can package it, I'd keep the anti around 60% and use lightweight drivetrain components (driveshaft, wheels, tires, aluminum pumpkin, etc) to offset the extra weight.

Truckarms are like any other suspension - spring it right and it will work great on a track...but it's dealing with the heavy weight that is tough on the street. Too heavy of an arm and it will need a high spring rate, too light and it they can resonate.

[Mean 69]

The arms move in two different arcs and that causes a twisting motion in the axle that the truck arm handles by twisting itself. It is why the arms are made of an I Beam section, not square or round tubing

Exactly, this is specifically why the truck arms are I beam sections. Other mechanical configurations may "work," but this is the best application for constructing the arms themselves. I recall an aftermarket company (not Hot Rods to Hell) that made some sexy looking tubular pieces a few years back, clearly didn't understand the mechanicals, or didn't care for aestethic reasons.

For a good source for rubber bushings, look to Howe racing. They make a piece that uses a rubber bushing, in a steel housing with a stud on it. Excellent pieces.

Also, with the very light weight of your car, and the relative "high" unspring weight of the truck arms typically found (i.e. circle track pieces), you might find yourself constructing your own arms if you wish to go the i beam approach. Stock cars weigh WELL over 3k lbs, but then again, Ron's car uses race pieces and apparently works pretty well (haven't seen it run personally).

Have fun!
Mark

[Derek69SS]

On to bushings, I think I'd seriously look at rubber. This is one of those applications that require movement at either the suspension members themselves, or the bushings. I know rubber bushings aren't sexy, but they have their place I think this may be one of them. They would reduce bind, reduce NVH, and give a good ride. Stiffer bushings can be used in your lateral link to reduce deflection there and make it feel less like a typical rubber-bushed suspension. Large diameter bushings are the OEM norm these days - for good reason - and I'd look into something like that.


I agree... all this talk of spherical joints, but they would actually INCREASE BIND in this application. Rubber is the only way to reduce bind in a truck-arm setup.

[dontlifttoshift]

Well, I'd have to come up with a design that minimized the weight but still stiff enough to avoid goofy resonant vibrations, which can be a serious problem with those if they are too light weight. The design would have to be flexable torsionally, but not exhibit any columnar bending (think I-beam shapes here).

On to bushings, I think I'd seriously look at rubber. This is one of those applications that require movement at either the suspension members themselves, or the bushings. I know rubber bushings aren't sexy, but they have their place I think this may be one of them. They would reduce bind, reduce NVH, and give a good ride. Stiffer bushings can be used in your lateral link to reduce deflection there and make it feel less like a typical rubber-bushed suspension. Large diameter bushings are the OEM norm these days - for good reason - and I'd look into something like that.


See post 9 in this thread, I think I have addressed those concerns. Bushed in Delrin now but I am going back to a rubber bushings.



If you can package it, I'd keep the anti around 60% and use lightweight drivetrain components (driveshaft, wheels, tires, aluminum pumpkin, etc) to offset the extra weight.



60%, a little higher than what Bryce recommended, maybe i'll try both.


Truckarms are like any other suspension - spring it right and it will work great on a track...but it's dealing with the heavy weight that is tough on the street. Too heavy of an arm and it will need a high spring rate, too light and it they can resonate.

This I can attest to, now add a 295/45-20 on a 20 x 10 into the mix. Do we all dance on the line between aesthetics, comfort, and quickness or am I retarded.

[Twentyover]

Since the main load carrying members of the truck arms arethe upper and lower flanges of the I-Beam section, can drilling the web be an effective method for decreasing weight?

What are the disadvantages I'm not seeing?

[MonzaRacer]

read this JOHNNY JOINTS

[ArtosDracon]

I have to agree with matt across the board, with the exception that sphericals on all 6 points of your ladder bars will be too much NVH for a street car. A lot of that aspect is preference, though, and since that front joint appears to thread on, you may try both rubber and a spherical, you could even interchange them, rubber for street use and sphericals for the track.

Geometrically speaking, the only major difference between a ladder bar and a truck arm is that a truck arm is fixed in both length and pinion angle, where as ladder bars can be adjusted, often for both of those aspects.

Truck arms do intentionally use the twist of the I-beam as part of the suspension characteristics, however ladder bars do not, and generally can not, which is why the ladder bars will require more give in the front joint than the truck-arms.

I typically like ~70% for street and 60% for track, so you may want to build some adjustability into your front mount, if at all possible. Just don't forget to check and reset your pinon angle when adjusting AS.

[ArtosDracon]

I agree... all this talk of spherical joints, but they would actually INCREASE BIND in this application. Rubber is the only way to reduce bind in a truck-arm setup.

I'm pretty sure he was refering to poly or delrin bushings, not sphericals. With the ball in socket motion of a spherical joint, in both truck-arm and ladder bar suspensions, the bind is better, if not nearly eliminated.

[ArtosDracon]

Since the main load carrying members of the truck arms arethe upper and lower flanges of the I-Beam section, can drilling the web be an effective method for decreasing weight?

What are the disadvantages I'm not seeing?

The upper and lower lateral sections of the I-beam carry the main torsional load of the suspension, but the vertical section carries the torque, in both acceleration and braking, so I would say no.

[ArtosDracon]

This I can attest to, now add a 295/45-20 on a 20 x 10 into the mix. Do we all dance on the line between aesthetics, comfort, and quickness or am I retarded.

That tire on that large of a wheel is definitely going to make finding the balance of NVH that is right for you on the application much, much harder. Hopefully that is at least a lightweight alloy wheel?

[silver69camaro]

See post 9 in this thread, I think I have addressed those concerns. Bushed in Delrin now but I am going back to a rubber bushings.



Last night I was trying to think about how spherical "ends" (Johnny Joints, sphericals, Swivel-links, etc) would reduce bind in a truckarm. I think it would reduce it somewhat, but not completely. When the housing rotates in body roll, the arms will travel in an eliptical arc and will need to change length for complete freedom. Rubber would allow the length change due to the soft durometer.

The more I think about it, I'm just not sure. I'd love to see the result when you get it done and how it works.

[dontlifttoshift]

I am glad I got you thinking.

I feel it works pretty well.I've got about 10,000 miles on these ladder bars so far and have won two Goodguys autocrosses with it. I ran within 1.4 seconds of Ryan Mathews and Chris Jacobs at said Goodguys events with reasonable consistency. I have also run some SCCA stuff (solo). On a course of that size my lack of acceleration and inability to turn the car quick enough is a hinderance and I usually give 4 to 5 seconds to Chris on a 60 second course.

In the never ending effort to get better/faster I will definitely change to rubber bushings and build in some adjustment for AS and RC

[DynoDon]

Last night I was trying to think about how spherical "ends" (Johnny Joints, sphericals, Swivel-links, etc) would reduce bind in a truckarm. I think it would reduce it somewhat, but not completely. When the housing rotates in body roll, the arms will travel in an elliptical arc and will need to change length for complete freedom. Rubber would allow the length change due to the soft durometer.

The more I think about it, I'm just not sure. I'd love to see the result when you get it done and how it works.

I was thinking the same thing and agree with the statement about rubber. It depends on how much articulation he is getting as to whether or not it shows much difference in the real world. A stiff car with little body roll wouldn't show much, a heavy body roll would definitely cause bind that would manifest itself in more twisting of the I beam or in his case ladder bar.

[DynoDon]

I however ladder bars do not, and generally can not, which is why the ladder bars will require more give in the front joint than the truck-arms.

I think the ladder bars will twist in actual use - they just aren't designed to and given enough twisting motion over a long enough period of time, will probably develop cracks and fail. Just depends on how much roll there is.

[silver69camaro]

I think the ladder bars will twist in actual use - they just aren't designed to and given enough twisting motion over a long enough period of time, will probably develop cracks and fail. Just depends on how much roll there is.

Pretty much. In our experience the chassis bracket will tear first.

[exwestracer]

Never really thought about it. From my limited experience, the swivels at the front cure the bind issues.

Might as well just solid mount the axle ends. Sphericals or bushings aren't doing anything at the rear anyway...provided the lateral locator is rigid enough. If you really want to cure the bind, mount the front pivots as close together as possible, and split the bars so you have a pivot at each end of the upper link, like the Jeep long arm kit:
https://static1.pt-content.com/images/pt/2010/10/8135d1258674226bds65longarmvsskyjacker6l-1.jpg

What exactly are you asking? If your CG is 20-22" with a 100-110 WB, you're never going to get the front pivots high enough with any sort of bar length, hence the low anti-squat figure (I'm assuming you're not going to put the mounts above the floor?). Short bars are going to give you a violent reaction and a lot of pinion angle change with travel.

A watts link would be a good choice, to keep the R/C and front pivot intersection on chassis centerline.

[dontlifttoshift]

Wheelbase is 106". Assuming a cg height of 21" that puts the anti squat line at 8" from the ground at the point at which the ladder bars attach to the frame, unless I am doing it wrong. They are mounted very close together at the front and measure 44" in length.

I graduated Wyo tech (Laramie) in '99. That's when Gordon was quoting Herb Adams book for "advanced chassis theory" after class.

Thanks for the input

Donny

[MonzaRacer]

http://www.amazon.com/How-Make-Your-Car-Handle/dp/0912656468/ref=sr_1_1?s=gateway&ie=UTF8&qid=1285920082&sr=8-1
This book taught me tons about handling and actually unconfused me about doing alignments, which I do now as well as help set up race chassis sometimes.

[MonzaRacer]

http://www.amazon.com/How-Make-Your-Car-Handle/dp/0912656468/ref=sr_1_1?s=gateway&ie=UTF8&qid=1285920082&sr=8-1
This book taught me tons about handling and actually unconfused me about doing alignments, which I do now as well as help set up race chassis sometimes.

[BillyShope]

If you really want to cure the bind, mount the front pivots as close together as possible....
And, as you do, it might dawn on you that what you have is the old Ford torque tube suspension, but adapted to an open driveshaft.

(I think this might be why the NASCAR boys agreed to its mandatory use. They already knew how to work with the Ford torque tube suspension.)

So, if you REALLY really want to cure the bind, I suppose you could bring the arms together and adapt the Ford ball and socket from the torque tube. Can't recall there being any serious problems with them, though they might not stand up under dragracing loads.

As has been pointed out, you'll need more swaybar to make up for the loss in roll stiffness.
http://www.racetec.cc/shope

[DynoDon]

If you put the pivot points together as Ford did to create a single pivot point you completely eliminate bind. As soon as you split the bones you start to create bind no matter how close they are. The further apart the more potential bind. The problem with a single point is it usually ends up too low under the driveshaft.

[exwestracer]

And, as you do, it might dawn on you that what you have is the old Ford torque tube suspension, but adapted to an open driveshaft.

(I think this might be why the NASCAR boys agreed to its mandatory use. They already knew how to work with the Ford torque tube suspension.)

So, if you REALLY really want to cure the bind, I suppose you could bring the arms together and adapt the Ford ball and socket from the torque tube. Can't recall there being any serious problems with them, though they might not stand up under dragracing loads.

As has been pointed out, you'll need more swaybar to make up for the loss in roll stiffness.
http://www.racetec.cc/shope

Yeah. They are still using this design (Monoball) in offroading. My only concern with it is the sturdiness of the lateral locator and mounts. If that lets go, the whole works is totally free to rotate right out from under you....

[exwestracer]

Wheelbase is 106". Assuming a cg height of 21" that puts the anti squat line at 8" from the ground at the point at which the ladder bars attach to the frame, unless I am doing it wrong. They are mounted very close together at the front and measure 44" in length.

I graduated Wyo tech (Laramie) in '99. That's when Gordon was quoting Herb Adams book for "advanced chassis theory" after class.

Thanks for the input

Donny

Your neutral line height sounds about right (without knowing the rest of the info), so to get above that, you'd need a pretty tall ride height or pivots above the frame. Bar length is nice and long, so it sounds like you've done about all you can with it already...

[shortrack]

I agree... all this talk of spherical joints, but they would actually INCREASE BIND in this application. Rubber is the only way to reduce bind in a truck-arm setup.

a little late but FWIW my car (which is straight off the track) has rubber bushings in the truck arms....

[AMC Racer]

Does anyone have a weight comparison of a complete truck arm suspension with panhard vs. a typical 4-link or 3-link watts arrangement?

[shortrack]

I know my truck arms weigh 23lbs each....never weighed it but the panhard bar prob 15 or so then shocks and springs.

[Tig Man]

When I worked at Laughlins chassis shop, we used to drill the arms with a 1 3/4" hole saw front to rear on short track stuff before they outlawed it! Also changed the front rubber bushing to an even bigger rubber busing because the monoball arms shocked the tires to hard.

[monteboy84]

All our circle track stuff is 3-link, but I would think there'd be some tricks to making truckarms work. One of the keys to getting out of the corner in oval track racing is to tighten the car with rear steer on exit of the corner, which we accomplish using arm angles and live action from our liftbar. Either using a rubber bushing in the RS truckarm and a solid in the LS, or just using a softer bushing in the right than the left would accomplish this.

But I digress. The holes you drilled in them would lessen the roll stiffness by decreasing the torsional strength of the arms, which seems to be the key to success with these. If you can decrease roll stiffness to where it is less than what you need, you should be able to go back up in roll stiffness using different size rear swaybars to do your tuning.

-matt