truckarm sketch-need input

[skeevay]

I have been trying to decide on a rear suspension for my 65 Impala. The truckarm setup is one that I am researching. This is all new to me so I am looking for a little expertise from anyone willing to give their 2 cents. Check out the link below (hopefully it works) and let me know what you think of the layout. What should I do different....tips...recommendations...etc. Thanks!

Oh yea, the front suspension is from a 96 Camaro....stock geometry...etc.



http://img358.imageshack.us/img358/1...ullsize4np.jpg

[JMarsa]

Nice drawing, do it yourself? I've got a HTH truck arm kit but none of it is installed yet. BTW, your putting a strut set-up on your Imp? Why ditch the A-arm set up?

--JMarsa

[skeevay]

Thanks, I did the layout in Corel Draw....trying to figure out what I need to change adjust...etc. before I start fabbing.
The Camaro set-up isnt really a strut type suspension. It is an unequal length A-arm set up with coilovers. Much better geometry than the 40 yr. old Impala version. Plus now I have rack & pinion steering, disc brakes, and a pluthera of aftermarket upgrade parts available. Not to mention it was cheap (labor intensive though! :rolleyes5 ) and different....chances of seeing one like it are slim.

[Norm Peterson]

Pictures ARE worth 1k words.

I'd give some thought toward lowering that rear RC a bit. That 4th gen F-body front RC is somewhere in the 1.7-ish range and is matched with about an 11" rear RCH, this at a road course lapping daily driver (read: level) stance. Your wheelbase is somewhat longer, meaning that a slightly higher rear RCH would work. That said, the serious F-body and Mustang guys are lowering the rear roll centers below 10", and at there's at least one G-body effort in the 12" range for a 108" WB (down from 17-ish). I think a combination of dropping the height of the truckarm chassis pickups and lowering the PHB can get you perhaps into the 13" range without giving up too much ground clearance, requiring excessive PHB attachment structure, or getting too close to zero axle rollsteer. You'd then want more rear spring (arguably the preferable way here, to keep the rollsteer from changing as much with added rear seat and trunk loading) or a bit more rear sta-bar. Perhaps a little of both.

Norm

[JMarsa]

Norm,

You blow my mind! How do you know all this stuff? For all the truck arm discussion that's gone on here between Skip's drawing and your response I think that's the most substantial data posted here.

I better get drawing because I think double checking my HTH kit might be in my best interest.

--JMarsa

[skeevay]

Norm, your obvious knowledge of suspension design is much appreciated....thank you for the help.
I was wondering what your thoughts are about the anti-squat for this particular layout. Is it a less important element compared to axle roll steer? In a revised drawing I have the trailing arm mounts in the same location and lowered the PH bar to a position 2 3/4" above the virtual intersection of the arms.The rear RC height changes to about 13" and anti-squat increases slightly. What are your thoughts on this?
Also, What is you opinion on changing the angles of the arms slightly in plan view by widening the spacing in the back (so that my springs will hit the arms square and utilize the factory frame spring pockets)? Would this create too much added bind? I was leaning towards rubber bushings for less harshness.
Finally, you mentioned "more rear sta-bar". I was under the impression that one was not required on a truck-arm set up. Evidently this is not always true?

Here's the link to the revised sketch with the above changes and the rear of the arms angled out slightly http://img83.imageshack.us/img83/319...llsize24sx.jpg

[CoryM]

I own a 70 C10 with the original "truck arm" suspension, and I also own another 70 C10 which is almost exactly the same except for it has rear leafs. The leaf truck has much less bodyroll from the rear. I ASSume the leafs bind and reduce the bodyroll where the coil sprung truck has less bind (and more articulation). Long story short, the coil sprung trucks need a lot larger rear swaybar to keep them level.

If you are interested here are a couple pics of the factory truck arms from when I parked it rather badly
http://www3.telus.net/public/corym/crash/crash1.JPG
http://www3.telus.net/public/corym/crash/crash5.JPG

You will notice that the factory trailing arms are not straight, maybe you can do the same to get your spring perches where you want them.
Cheers.

[JodysTransmissions]

Cory,

Yes, it seems you have terrible body roll with the coil spring set-up! I hope you weren't in her at the time you parked?

Regards, Jody

[CoryM]

Cory,

Yes, it seems you have terrible body roll with the coil spring set-up!

Now thats funny! Yes I was in the truck, wasnt hurt other than whiplash/bruises. Landed roof first at about 45mph and stopped in about 2ft. Hit hard enough that my eyeballs were sore for a few days which was a new feeling to me.
http://www3.telus.net/public/corym/crash/

Skeevay, I meant to ask, how long are you planning on making those trailing arms? Shorter arms will give you better antisquat but also more brakehop unless you use a pullbar or similar. Just wondering if you had accounted for that, and if so how you ended up figuring out what length was best.
Cheers.

[skeevay]

Cory, after seeing your pictures I'm thinking I might need an anti-roll bar and a roll bar
As far as length goes for the trailing arms.....I am using measurements for the stock Chevrolet arms. If I go this route I would rather buy some then make them. Although, depending on what I can conclude from all my research and your guys' help, I will definitely fabricate some if it proves to be worth while.
On the subject of shortening the arms for better anti-squat, and the possibility of brake hop, I think that would be detrimental to the design of this suspension system in general because the long arms are designed to flex torsionally (affecting/limiting body roll). The shorter the arms and shorter the "triangle" seems to me would just create worse bind conditions and wouldnt allow the arms to twist. Depending on how much shorter we're talking of course. I think the only way to figure the "best" length would be a suspension program .......or maybe a group of NASA engineers. Ha Ha :rolleyes5....or Norm

[Norm Peterson]

I was wondering what your thoughts are about the anti-squat for this particular layout. Is it a less important element compared to axle roll steer? In a revised drawing I have the trailing arm mounts in the same location and lowered the PH bar to a position 2 3/4" above the virtual intersection of the arms.The rear RC height changes to about 13" and anti-squat increases slightly. What are your thoughts on this?

I like the axle rollsteer figure for this better than the original (about 3.6% vs 6.6% based on scaling dimensions off the screen, where numbers beyond ~7% have been identified as detrimental to slalom-type maneuvers). But I'm a little confused about the A-S increasing from only a PHB height change. That's fully defined by the truck arms, at least kinematically, and with the assumption that the WL has zero stiffness against changes in pinion angle.

For a street or road course/auto-x driven vehicle, I'd prefer the lower roll center with less vehicle roll understeer. As Cory has already mentioned, A-S has its evil twin, anti-lift and its companion, brake hop. Sticky rear tires will exaggerate any problem here. You can rebalance your brake system in various ways to eliminate this, but you throw away some of your potential overall braking capability when you do so.

In the far-out extreme, A-Mod auto-x, there's even some current thought toward intentionally using less A-S so that you can gain forward bite faster (using the falling of the rear RCH to shift the lateral load transfer forward, leaving more grip available for traction at the rear). But that's a situation where 0.002 seconds in 40 matters, you only get 3 tries per day to get everything right, and may involve a slightly different driving technique.

Also, What is you opinion on changing the angles of the arms slightly in plan view by widening the spacing in the back (so that my springs will hit the arms square and utilize the factory frame spring pockets)? Would this create too much added bind? I was leaning towards rubber bushings for less harshness.

Some bushing compliance makes this work better, and relieves the arms of having to provide all of the torsional flexibility between the chassis and the axle. But more compliant bushings allow slightly more roll and also suggest an increase in total rear roll stiffness. Perhaps the answer lies in the use of bushings that are not of equal stiffness in all directions (stiff longitudinally, soft against roll, which can be done). Widening the spacing in the back while leaving the chassis side pivots in the same place will move the virtual intersection point rearward and increase anti-squat slightly. Rollsteer will increase a little as well.

Finally, you mentioned "more rear sta-bar". I was under the impression that one was not required on a truck-arm set up. Evidently this is not always true?

I was speaking generally, in that if you lower the rear RCH you'll want more roll stiffness back there from either the springs or a bar. With a high enough RC, coil springs alone might be enough (but your rollsteer would be horrible). If you have an overriding requirement for forward traction while still cornering fairly hard, you might not want one either, assuming that you can balance the chassis otherwise. On a purely theoretical note, no rear bar represents something of a desireable goal. But it's apt to be the lesser evil.

The leaf spring situation is slightly different, as leaves also offer some sta-bar effect by virtue of their bending and torsional stiffnesses. This effect is increased by the use of harder/solid front spring eye bushings, BTW. Call this a case of the sta-bar hiding in plain sight. I think this was discussed a while back.

Norm

[skeevay]

Thanks again Norm for your expertise!

I'm a little confused about the A-S increasing from only a PHB height change.

The revised drawing also reflects the widening of the rear of the arms per some of my inquiries on that subject. Sorry, I was unclear when I stated the arm mounts were left in the same position. I was referring to the front mounts in side view. Because of the greater angle between the two arms the convering point moved back a little, hence a little more anti-squat. (Which you touched on later on in your post)

A-S has its evil twin, anti-lift and its companion, brake hop.

I am hoping this will be less of a concern, as the arms reach at least 47" from the axle centerline in side view in all the layouts I have done so far. Which is still pretty close to the stock GM configuration.

It might help also to state the intended use for the car. I plan on putting a lot of miles on it and the chance of it being on a drag strip would probably outweigh it ever seeing a road course (but if opportunity arises..... ). I would also like the "sports car" feel and definitely some "spirited" driving is in the future. I also have a large family and intend on loading up and road tripping occassionally, which I would definitely like to account for before building the rear suspension.

The unequal compliant bushings that you speak of.....is there a good source that you know of for something like this?

Norm, The rear sway bar issue appears to be something I need to concentrate on a little more. It sounds like your opinion would be more spring then bar if possible. Which is less expensive, less work, and might also have an added benefit when adding passenger weight...etc. Although, ride quality within reason is still of importance. I still haven't figued spring rates etc. yet, so I guess I need to dive a little deeper into this area.

[Norm Peterson]

If it's more likely to see strip time rather than auto-x or open-tracking, then skewing your choices to favor anti-squat may better suit you. But don't go overboard (read: not over 100%) and try to keep the rollsteer rate under 4 or 5 percent. It looks like your anti-squat is ~80% at this point, and somewhat above OE (50% - 65% . . ).

I know that more than one option exists for connecting truckarms to the chassis, but not the specific details. As far as unequal compliance goes, you'd be looking for what the OE mfrs refer to as "voided bushings". Or for a way to modify regular ones in the right places after the fact (albeit with some penalty in bushing lifespan a likely consequence).

Norm

[funcars]

Stock car products sells a variety of components for truck arms including rubber and monoball bushings along with the arms themselves and other pieces. The quality of the components they fabricate is good.

[Kenny]

Newbie here, I would also look at adjustable panhard bar mounts, we use some pretty trick stuff on late model stocks. At least you would not be locked in, and have some built in adjustment. Every driver I talk to prefers the rubber bushings, but they also tell me that three links are faster. One thing I do know is you have a bit of bind with these things.

[skeevay]

I'm still not sold completely on the truck arm set-up. I have looked into a 3-link (upper arm length problems, cant sacrifice the back seat) and the satchell link (not much info available) but all the truck arm pluses may outweigh the other options. After doing some more floor pan and frame measurements maybe I will do a layout of these other set-ups.

At any rate, Do you think the last drawing would be a good starting point with 3.5% axle roll steer and 80% A.S.?

Kenny, good suggestion. I do plan to make the PHB adjustable and think it will be necessary to dial things in just right. And if I go the truck arm route I think I will start with the stock rubber bushings and experiment from there if needed.

Funcars, I have searched quite a bit for truckarm parts. Stockcar Products will be who I go with if I order new. There does not seem to be many places that even offer these parts.

[Kenny]

Yes, those numbers are a vast improvement over stock. we use a basic driveshaft hoop/ x-member with the arms only as far apart as required for a practical sized safety hoop. We do not use big spring setups, our cars are running Penske coilovers with the shock mounts as far outboard as is practical and as vertical as possible, even though adjustable upper shock mounts (so you could vary shock angle) could be handy for tuning, especially on a dual purpose car. We always place the bottom shock mount 7" below axle centerline. the reason for the extreme out board positioning is simple leverage geometry, this allows the least amount of spring rate to have maximum control over roll motion and retain a reasonably smooth ride. With this setup you can also run a lighter bar with the same results minimizing "precieved" stiffness. Suspension blocks don't have much of an impact good or bad, it will allow you to do less floor fabrication. The arm angle is dictated by the housing pads, so you could go ahead and fab up a x-member and mock this up outside the car, bolt it up and scribe all the real estate that needs clearing. As you can see by the AS numbers, forward and side bite are not gonna be this systems strongest point. The upside to this system is ease of installation(it's idiot proof),good strength and cost effectiveness. I will never understand how some of these companies can knock their customers over the head so hard with their prices, and then to couple all this up with carrera or qa-1, which is the joke of the racing industry...... That should start a fight! Peace

[skeevay]

the shock mounts as far outboard as is practical and as vertical as possible

Makes good sense. Do you mount the shocks to the rear of the trailing arms, or do you usually have room to the outside and fab a mount to the rear end housing?

the reason for the extreme out board positioning is simple leverage geometry

This logic for the shocks/springs seems could be applied to the rear end housing mounts for the truck arms as well. Since the car is so wide and has the room I was thinking of widening the spacing of the rear arms slightly (modifying housing pads). Plus, as stated in a previous post this would line the arms up to the factory spring pockets in the frame. Although I am not sure if I will use shocks + springs as opposed to coilovers with this set up. Any one have an opinon on which would be better?

I know what you mean by some of the kit pricing. The parts are pretty basic, but I guess the convenience of making one phone call and receiving one package with everything you need inside is worth the extra cash to some. It would sure be nice....but I just dont have enough extra cash!

carrera or qa-1, which is the joke of the racing industry...... That should start a fight!

Hey! I'm one of those jokers who ordered some QA1's for the front. Since then I havent heard much good about them.....should have done more research....I'm stuck with them now.

[Kenny]

Yep, you're correct, but since the arms don't do anything except control housing rotation it's not critical.... But it's a moot point since what you'll probably end up with is shock mounts that are bolted to the bottom of the arm with the u-bolts. Did I forget to mention the shock brackets are available pre-made? Sorry about that,oh yeah,the shock hack was nasty too..... If you are trying to keep your floorpan intact, I would also explore the torque arm deal,it can have more forward and side bite and it has a well proven road race and dirt track record. I don't know if there are any of those camaro Dana 44's to be scored .....If you were planning to do an aftermarket rear its kinda a cool option to explore. Again,sorry about the shock crack....I'm not very good,but I'm slow!

[skeevay]

I am open to any sensible options at this point. I have a new Ford 9" with bare axle tubes on jack stands under the car right now. I haven't convinced myself yet that the truck arms are the way to go. The torque arm suspension seems to have some good points, but I have been led to believe that it is hard to set up and is more complex then it seems.....maybe a topic for a new post?!

A little more on topic....questions concerning the "centerdrive" benefits? Just as a front wheel drive car feels different then a rear wheel drive car. Since the car is pushed and driven more from the center of the chassis then from the rear, with all other factors aside, does this relate into any plus or minus points for the truck arm system? There are the claims that the car feels lighter (which would certainly be appealing in my case), has an easier to drive "feel"....etc... Does anyone have opinions or experience on this subject?