Trailing Arms design & target Anti-Squat

[tsm1mt]

I found some information in the 3 vs 4-link thread that's going on right now, but I still have a few questions..

Mounting the shocks (and/or coil-overs) to the trailing arm seems very common, and as noted in the other thread, mounting them above the trailing arm, in the top of the arm, or at the bottom of the arm is a preference thing, largely.

..but why are "all" of the trailing arm pictures I see pictures of a "bent" trailing arm?

Rock-crawlers commonly bend their lower trailing arms "upside down" to get a flat belly, then curve down at the axle end for ground clearance.

The desert trucks typically have the opposite, with the trailing arm going down at an angle, then roughly leveling out to the rear axle.

I'm curious about the Whys.

I think the "drop, then level" arm puts the weight lower, and allows a lower mounting (or at least, lower overall height) of the coil-overs and shocks, which should make for a lower COG.

Is that the primary motivation?

I'm guessing that a "straight tube" style link proved either too weak to support the coil-overs at the mid-point, or once you've upgraded the material it was too heavy, so a "plated" link became the standard to get the strength, and the "drop" link turns out to be the lightest, lowest, and strongest design.

That said, I've read articles that indicated the TTs are often setup so that, at full bump, the belly of the truck (and/or some part of the trailing arms) actually touch the ground.

Again, I'm guessing this is aiming for lowest COG and lowest weight with the most wheel travel.

Related to trailing arm design is the link setup, and Squat vs Anti-Squat. Leaving the 3 vs 4 links out of the picture..

I notice a lot of the trucks tend to lift the front end under throttle.

Is this by design, or just a result of "that much power"?

I'm wondering if they actually engineer some *SQUAT* in, so that under throttle it causes the rear to purposefully dive. This would force weight transfer to the rear, making the nose effectively lighter, and pulling the front "up"... just the kind of thing you might want just before a jump.

(With my current leaf suspension, I induce this by lifting at the base of the jump, then flat-footing it and using the engine to lift the nose, launching under throttle.)

Or are they setup for roughly 0 (or more) AS, and it's just the massive amounts of power that causes it to still lift the front?

[CRAIGHALL]

As far as the issue of a link crowning up or down I feel a design that which more material is under the shock is simply easier.

Say the shock are center line with the pivot,with the material under C/L in stress the material is in tension (much stronger).Obviously you lose some ground clearance to get that but you shock ends can be normal not the extendeed type.(unless yo go below C/L).

With the arm built flat on the bottom (shock C/L) All your strength would come from the material on top which would be in compression and for all equal purposes would need to be beefier.You would also need extended ends on you shocks or a really wide arm to clear the coils.---Or you could build them equal height from C/L and use ane extended end as well.

[ntsqd]

Craig has it nailed, but I'll put it in different words in case that helps.

Picture a simple beam supported at each end and with a single point load in the exact middle. The greatest Shear in the beam is at the load and the greatest Bending Moment is at the load. So you design the beam to have enough cross section at the middle to handle those loads. When you look at the Shear and Bending Moment at the point halfway btwn the load and one of the supports, neither the Bending or the Shear is as large. And the Shear and Bending Moments at the supports are near zero. So the size of the beam can be progressively smaller at those locations. If you are building a bridge it's more economical to make the beam the same over it's full length even though you only need that cross section at the middle of the span. For a lower link that adds unneeded weight.

As noted, Steel loves tension and and only tolerates compression. In a perfect world all steel would be loaded in tension only. A compresive load that would collapse a steel tube wouldn't even phase the tube if it were tensile.

[tsm1mt]

Bending or the Shear is as large. And the Shear and Bending Moments at the supports are near zero. So the size of the beam can be progressively smaller at those locations. If you are building a bridge it's more economical to make the beam the same over it's full length even though you only need that cross section at the middle of the span. For a lower link that adds unneeded weight.


Gotcha. Same idea behind running the axle truss UNDER the axle, since it's in tension when you try to "U" the axle tubes, and you can build a lighter/stronger under-axle truss vs. an over-axle that's always in compression and by necessity has to be heavier.

The trailing arms have to be stronger/beefed up over a simple tube link because of the coil-overs, which are mounted to the trailing arm to maximize wheel travel for a given length of shock.

... so how about the squat/anti-squat?

Are you purposefully inducing squat with link setup to help carry the nose, or is it actualy set up with some anti-squat and they pick the front because there's still that much power getting to the ground?

I suspect the latter, and that if it were actually setup to induce squat, it would wind up bottoming the rear suspension every time you laid on the throttle - but it sure seems that the "right" amount of squat/AS would be a neat thing to accomplish to help make sure you can lift the nose on demand.

Thanks,

-Tom

[billymanfroy]

Are you purposefully inducing squat with link setup to help carry the nose, or is it actualy set up with some anti-squat and they pick the front because there's still that much power getting to the ground?

I don't pretend to know all the theory here, but I can tell you that trucks that are required to keep the stock frame (like class 7 or 8) really limit where the links can be mounted. This sometimes dictates many of the characteristics like squat, dive, etc. In order to get a decent amount of travel, minimize plunge and pinion angle change, yet still have ground clearance under the forward lower link mounts requires some sacrifices. I know it did on ours. I've looked at a few reg. cab toyotas, and using the same geometry we have on our ranger would put the upper links right about where your sternum strap would be!

We chose to concentrate on getting clean travel with less than 1" of plunge and less than 2 degrees of pinion change. The squat lines aren't perfect, but they aren't so bad as to make the truck drive poorly. Our biggest concern was the lack of lateral support from the top links since they are so long and anchored inside the frame rails. (there's not a whole lot of angle to them) We compensated a little by angling the lower links out towards the wheels.

[tsm1mt]

I don't pretend to know all the theory here, but I can tell you that trucks that are required to keep the stock frame (like class 7 or 8) really limit where the links can be mounted. This sometimes dictates many of the characteristics like squat, dive, etc.

Packaging is always a problem. :D

I don't suppose it's a big deal on a Trophy Truck, though.. and it seems every video of a TT on a section of pavement or a smooth dirt track seems to have the rear squatted and the nose stuck in the air... but again, it's likely just a result of throwing $$$ at the powerplant than by design. :D

[ntsqd]

I think packaging is a problem no matter what you're building. Clean sheet designs just start out with a couple fewer constraints.

I believe if you run too much AS on a loose surface you risk blowing the tires loose too easily. A pipey type high HP engine would make this even harder to drive. Take some AS out and now you're not loading the tires as hard, which should make for an easier to drive vehicle. Then you get squat on hardball.

It comes down to driver preference.

[billymanfroy]

We "only" have 510 hp and ~480 tq, but it does NOT hook up whatsoever on any surface until about 60 mph or 3/4 of the way through 2nd. If I modulate the throttle it's better, but it would be nice just to pin it and hang on. I am sure more attention to squat/dive would have helped this some.

We have made some adjustments and keep going larger on the tires. It's now up to 35x12.5 MTRs with a 6.00 gear. We are hoping the looser converter will help it transfer a little more on launch. But we are worried about blowing the tires off. Guess we'll see in Parker.

[FABRICATOR]

Acceleration, weight transfer, and traction are all one. With a compromised setup, you must shore up one to get the other two.

[ntsqd]

Keith Bontrager is quoted as once having said: "Cheap, Light, Strong; Pick any two."

[DPpatrol]

The squat anti-squat debate has been disscussed thourghly on this board. I think that is part of the reason people are reluctant to start talking about it again. Trying searching some old posts. I think you'll find an answer to your question.

jason

[Scott_F]

Another benefit of a curved arm, bent down, is that the lower shock mount can be below the centerline of the two pivots of the arm. This eliminates the "tilting" or "falling over" forces that the shock would apply to an above CL mount.

..but why are "all" of the trailing arm pictures I see pictures of a "bent" trailing arm?

The desert trucks typically have the opposite, with the trailing arm going down at an angle, then roughly leveling out to the rear axle.

I'm curious about the Whys.

I think the "drop, then level" arm puts the weight lower, and allows a lower mounting (or at least, lower overall height) of the coil-overs and shocks, which should make for a lower COG.

Is that the primary motivation?

I'm guessing that a "straight tube" style link proved either too weak to support the coil-overs at the mid-point, or once you've upgraded the material it was too heavy, so a "plated" link became the standard to get the strength, and the "drop" link turns out to be the lightest, lowest, and strongest design.

[Just4Fun]

You wouldnt have to worry about the arm flopping over if you run a bushing in the front, which will keep the arm from tilting/rocking over. Also I am not a bushing fan at all, just depends on what you want to use on the pivots.

[tsm1mt]

Another benefit of a curved arm, bent down, is that the lower shock mount can be below the centerline of the two pivots of the arm. This eliminates the "tilting" or "falling over" forces that the shock would apply to an above CL mount.

I found the "Lower links 101" thread Thread (http://race-dezert.com/forum/showthread.php?t=4050&highlight=lower+links) earlier today and saw the "flop/wiggle-arm" concept and the need to mount the shocks at, or below, the center line of the imaginary straight-line link to keep the trailing arm from wanting to roll all of the time.

Makes sense, though I'd never thought of it from that angle before.

[curt]

You wouldnt have to worry about the arm flopping over if you run a bushing in the front, which will keep the arm from tilting/rocking over. Also I am not a bushing fan at all, just depends on what you want to use on the pivots.

Camburg has a deep uniball cup that helps this also, it is about twice as deep and that allows you to run the uni as the pivot and shim the outsides with a stepped delrin bushing to give it "twist resist" stability, total rotation on our is maybe 5 degrees either way with the durability and strength of a uniball...Curt

[V8Ranger]

Re: "As noted, Steel loves tension and and only tolerates compression. In a perfect world all steel would be loaded in tension only. A compresive load that would collapse a steel tube wouldn't even phase the tube if it were tensile."


I hear this every once in a while but I have a tough time "digesting" it. Obviously, steel in compression is susceptible to buckling but so is any other material. Assuming buckling isn't a factor, I don't see where steel in compression would be any weaker than tension. All data I've seen notes that ductile materials have a compressive strength equal to its tensile strength.

[Just4Fun]

Camburg has a deep uniball cup that helps this also, it is about twice as deep and that allows you to run the uni as the pivot and shim the outsides with a stepped delrin bushing to give it "twist resist" stability, total rotation on our is maybe 5 degrees either way with the durability and strength of a uniball...Curt

Yes I know, HM makes them. Also, that is just 1 more thing to wear out. If designed right, you shouldn't have to rely on a piece of delron to keep your arms straight.

[ntsqd]

Re: "As noted, Steel loves tension and and only tolerates compression. In a perfect world all steel would be loaded in tension only. A compresive load that would collapse a steel tube wouldn't even phase the tube if it were tensile."


I hear this every once in a while but I have a tough time "digesting" it. Obviously, steel in compression is susceptible to buckling but so is any other material. Assuming buckling isn't a factor, I don't see where steel in compression would be any weaker than tension. All data I've seen notes that ductile materials have a compressive strength equal to its tensile strength.


Buckling is the failure mode for steel in compression. Steel in compression suffers buckling failures at a lower load than the same geometry would in tension. There is a thing called the "Slenderness Ratio" (length/diameter) that comes into play with compression, but has no meaning in tension. Concrete is incrediably poor in tension, but works exceptionally well in compression. Perhaps you've heard the term "Pre-stressed Concrete" ? What they're doing is putting a tensile loading on the rebar while the concrete sets up. Once the concrete has cured the steel is putting it in compression and the concrete is putting the steel in tension. Makes for a far stronger assembly than either on it's own or even mixed but not pre-stressed.

[Scott_F]

Thom, that is a very good explanation about pre-stressed concrete. I am not in construction, so I didn't know that. Thanks for the info.

[V8Ranger]

Re: "Perhaps you've heard the term "Pre-stressed Concrete" ? What they're doing is putting a tensile loading on the rebar while the concrete sets up. Once the concrete has cured the steel is putting it in compression and the concrete is putting the steel in tension. Makes for a far stronger assembly than either on it's own or even mixed but not pre-stressed."

I've heard of it but wasn't quite sure how they went about manufacturing it. Pretty ingenious. Kinda similar to tempered glass where they induce a compressive stress on the surface since the surface flaws in glass make it prone to failure in tension.

Re: "Buckling is the failure mode for steel in compression. Steel in compression suffers buckling failures at a lower load than the same geometry would in tension. There is a thing called the "Slenderness Ratio" (length/diameter) that comes into play with compression, but has no meaning in tension."

I guess what I'm asking is why does everyone single out steel as being bad in compression, especially if you are negating buckling (for example a short, stout structure).
Taking buckling into account, Euler's equation for buckling (which takes into account the slenderness ratio) is dependent on the length of the structure, the end condition of the structure (how its supported), the second moment of inertia of the structure and the modulus of elasticity of the material. Since steel is a fairly stiff material, or at least more so that many materials including aluminum, brass, titanium etc, I don't see why it gets the bad wrap about not being good in compression. Feel free to enlighten me.

[DPpatrol]

All materials are worse in compression due to buckling effects, there is no reason to single out steel. Now composites like carbon fiber, there's a material that does not like compression.

AS4/3501-6 Graphite/Epoxy Composite's failure stress when loaded in the direction of the fibers is about 310 ksi when loaded in tension and 225 ksi when loaded in compression.

jason

[ntsqd]

Re: "Buckling is the failure mode for steel in compression. Steel in compression suffers buckling failures at a lower load than the same geometry would in tension. There is a thing called the "Slenderness Ratio" (length/diameter) that comes into play with compression, but has no meaning in tension."

I guess what I'm asking is why does everyone single out steel as being bad in compression, especially if you are negating buckling (for example a short, stout structure).
Taking buckling into account, Euler's equation for buckling (which takes into account the slenderness ratio) is dependent on the length of the structure, the end condition of the structure (how its supported), the second moment of inertia of the structure and the modulus of elasticity of the material. Since steel is a fairly stiff material, or at least more so that many materials including aluminum, brass, titanium etc, I don't see why it gets the bad wrap about not being good in compression. Feel free to enlighten me.

Define where you cross over into "short, stout structure". Might have a feel for where it happens, but I do not Know where it happens.

Review Brittle Material behavior vs. Ductile Material behavior. I'm not singling out steel, it's been done for me since it is the material of choice to fab a racecar from. My point all along has been that steel is not as good in compressive loadings as other materials are. The basic design philosophy of designing a steel structure is to avoid compressive loadings where ever possible. That holds true for race cars too.