Trailing arms and camber angles

[Root Moose]

I've been waffling back and forth between buying something like a Starcraft 10RT or customizing one of my old tent trailers. I keep coming back to buying being spending a lot of money for a bunch of compromises that's I'd end up fixing anyway.

So tonight I've been thinking about fixing up my trailer.

I was thinking about changing the suspension to an independent trailing arm design. What I haven't seen is a lot of discussion about camber angles.

My understanding is that a trailer likes a bit of camber to track true. Let's say 3/4 of a degree.

If you set your camber on a trailing arm suspension such that it sits at the desired amount at loaded weight, doesn't the cycling of the suspension play havoc with the camber gain/loss curve through it's travel?

Or does it really not matter for short time duration perturbations? Am I over-thinking this?

TIA

 

[R_Lefebvre]

If it's a true trailing arm, the camber doesn't change as the suspension moves due to geometry. It would only move due to the loads twisting things (such as the arms). The same thing happens to solid axles even. I once put about 4000lbs in a trailer with a 1500lb axle (oops!) and had about 2 degrees camber.

I hadn't heard that trailers run better with camber? I figured 0 was best for lowest rolling resistance.

And, there's no practical way you're going to get a trailing arm suspension under a tent trailer without building a whole new frame under it. The existing frame just is not strong enough to take the point loading of a trailing arms setup.

 

[Root Moose]

If it's a true trailing arm, the camber doesn't change as the suspension moves due to geometry.

I'm thining the AT style swing arm suspension - used the wrong label. If the arms are say at... 20 degrees relative to the horizon (just for discussion) and the camber is set at that angle then as the hub travels down or up the camber has to change as the hub's relative vertical has changed.

And, there's no practical way you're going to get a trailing arm suspension under a tent trailer without building a whole new frame under it. The existing frame just is not strong enough to take the point loading of a trailing arms setup.

For certain. The frame on normal tent trailers is way to mickey mouse for that. I may tub the box while I'm at it. It's all a function of where things end up being relative to the Jeep. I'll build the "spine" of the trailer first and then get it loaded up to go from there.

 

[elmo_4_vt]

So it sounds to me like your thinking of CASTER, not chamber. Yes, I would assume that you'd want to keep some minimal caster in you setup, but it would definitely be minimal. I think you have the "straight tracking" effect of caster just by using a trailing-arm suspension, and if you have too high a degree of caster, your wheel would suspension movement would introduce a larger amount of horizontal wheel movement (front to back) within the wheel well.

In the end, I think I would just let the physical properties of the arm and frame set the caster (length of the arm, mounting to your frame, suspension travel) and leave it be. You should end up with anything more than 10-15 degrees.


-

 

[Root Moose]

So it sounds to me like your thinking of CASTER, not chamber. Yes, I would assume that you'd want to keep some minimal caster in you setup, but it would definitely be minimal. I think you have the "straight tracking" effect of caster just by using a trailing-arm suspension, and if you have too high a degree of caster, your wheel would suspension movement would introduce a larger amount of horizontal wheel movement (front to back) within the wheel well.

Actually, camber is what I'm concerned with (wheels pointed in at the top when viewed from front). That said...

In the end, I think I would just let the physical properties of the arm and frame set the caster (length of the arm, mounting to your frame, suspension travel) and leave it be.

...you may be right. My buddy is currently making fun of me on Yahoo IM for over thinking this.

 

[elmo_4_vt]

... If the arms are say at... 20 degrees relative to the horizon (just for discussion) and the camber is set at that angle then as the hub travels down or up the camber has to change as the hub's relative vertical has changed.

This is what made me think you were talking about Caster, sorry... I don't think that with the normal trailing arm setup that there is any camber change during suspension travel, at least not in an ideal engineering situation. It's not like a 2 or 3 point strut setup, the spring doesn't provide any support to the suspension. There is only one attachment point to the frame for articulation of the arm, and as long as that is placed square to the frame, it should move in an arc perpendicular to the ground.

Although there is a trailer build in here that used trailing arms that actually had issues with his camber due to the metal flexing. When he loaded his trailer the camber would change and was actually causing tire wear issues. I would shoot for zero camber during the setup, and any minimal flex that happens in the arms would give you a tiny bit.

As far as over thinking it, that's what's great about a place like this, lots of technically minded people to bounce ideas off of. Especially when we sitting in a waiting room with nothing to do but wait and think and surf...

-

 

[ntsqd]

The only way to get non-distortion induced camber change purely from cycling a trailing arm suspension is for the pivot axis to not be perpendicular to the frame's centerline. The so-called "IRS" used on the later VW Bettles was designed this way. There is a shallow angle between the pivot axis of the arm assembly and any line/feature that is perpendicular to the chassis centerline.

If the arm has some built-in camber, then the effective camber will change as the arm cycles because the plane of the maximum camber angle will rotate with the arm and about it's pivot axis, but the measured camber angle at any point in the travel is always in a vertical plane perpendicular to the ground.

The heavier duty application live trailer axles have some camber built into them. It is expected that loading such an axle will cause it to flex enough that the camber will be zero when going down the road.

With a trailing arm there may be some advantage to having a slight static toe-in setting. This is to compensate for distortion of the arm and the pivot bushings.

 

[R_Lefebvre]

I'm thining the AT style swing arm suspension - used the wrong label. If the arms are say at... 20 degrees relative to the horizon (just for discussion) and the camber is set at that angle then as the hub travels down or up the camber has to change as the hub's relative vertical has changed.

I see what you're saying. I think in most cases, the pivot line of the arms is dead square, no angle anywhere. So no camber change by geometry.

So it sounds to me like your thinking of CASTER, not chamber. Yes, I would assume that you'd want to keep some minimal caster in you setup, but it would definitely be minimal. I think you have the "straight tracking" effect of caster just by using a trailing-arm suspension,

No, there is no caster on a trailing arm. Caster is the rearward slant of the steering axis, and this suspension has no steering axis so...

and if you have too high a degree of caster, your wheel would suspension movement would introduce a larger amount of horizontal wheel movement (front to back) within the wheel well.

Actually, I think what you're thinking of here is Kinematic Recession. That is, the wheel moves back as it moves up. This makes the ride smoother. You'd achieve it in a trailing arm suspension by having the arms slope down to the back when the trailer is setting level. I think most trailing arms are set up this way by happy-chance.

I don't think that with the normal trailing arm setup that there is any camber change during suspension travel, at least not in an ideal engineering situation.

You're right, there is no camber change in a pure trailing arm suspension. But you can get it in a semi-trailing arm suspension. That is when the hinge line is on a 20-30° (typical) angle when viewed from above.

There is a shallow angle between the pivot axis of the arm assembly and any line/feature that is perpendicular to the chassis centerline.

Angle viewed from the top, or angle viewed from the back. I know about the angle viewed from the top (semi trailing arm I mentioned above... never thought about an angle viewed from the back.)

The heavier duty application live trailer axles have some camber built into them. It is expected that loading such an axle will cause it to flex enough that the camber will be zero when going down the road.

They build in positive camber, so that it bends to zero with load? Interesting.

With a trailing arm there may be some advantage to having a slight static toe-in setting. This is to compensate for distortion of the arm and the pivot bushings.

Good point. I think having some running toe-in would also help with stability, in exactly the same way it helps at the back of FWD cars.

 

[ntsqd]

There is a shallow angle between the pivot axis of the arm assembly and any line/feature that is perpendicular to the chassis centerline.

Angle viewed from the top, or angle viewed from the back. I know about the angle viewed from the top (semi trailing arm I mentioned above... never thought about an angle viewed from the back.)

I was thinking top view, but the same would hold true for a front/rear view.

[HIJACK]Personally so long as the pivot axis is between perpendicular to the chassis CL and less than 45* to it, it is a trailing arm system. Calling it a "semi-trailing arm" is confusing. To me anyway. Not the first time that those who get to name such things have, IMO, screwed up. [/HIJACK]

The heavier duty application live trailer axles have some camber built into them. It is expected that loading such an axle will cause it to flex enough that the camber will be zero when going down the road.

They build in positive camber, so that it bends to zero with load? Interesting.

Exactly. It is sometimes visible in pictures of the axles.

With a trailing arm there may be some advantage to having a slight static toe-in setting. This is to compensate for distortion of the arm and the pivot bushings.

Good point. I think having some running toe-in would also help with stability, in exactly the same way it helps at the back of FWD cars.

For stability purposes I had wondered about proposing enough static toe-in that drag from motion did not reduce it completely to zero. Can't get too carried away with this, but a little might be a good idea.
Toe-out would be BAD.

 

[R_Lefebvre]

For stability purposes I had wondered about proposing enough static toe-in that drag from motion did not reduce it completely to zero. Can't get too carried away with this, but a little might be a good idea.
Toe-out would be BAD.

That's exactly what I'm talking about. Rolling toe-out on the back of a FWD car makes it drive like a forklift.