Thread: Too much emphasis on suspension????

Originally Posted by Buggerluggs

JC

One point that has not come out in this thread is the effect of unsprung mass and the direct coupling of the two wheels. A live axle setup will lead to a sympathetic tramp action over corrugations. Every time a wheel hits a bump it takes off very slightly then bites again as it hits the dirt. This bite results in a significant twist reaction directly into the suspension. With both wheels directly linked they tend to do it together in sympathy. This is axle tramp. The heavier the unsprung weight the higher the reaction forces tend to be.

If suspended on a link system such as the LR coil setup tramp is minimised by the links. The four bushes mounted on the Disco arms for example have a small degree of flex and the arms much less. The result is that there is quite a resistance to the tramp action, but it is still there.

With leaf springs the tramp is resisted by the leaf which itself will add to the tramp action. Leaf springs with long travel are usually very long springs. The longer the spring the more it will 'wind up'. The leaf takes a sort of "S" shape on tramp. The axle will twist relative to the ground much more in this situation. Add to this that if you have the shockers mounted on the same side of the axle the spring "S" shape is undamped and will oscillate itself as a function of the leaf. The end result is that tramp action is far worse with leaf springs and either no shocks, or with shocks mounted on the same side (e.g. front only). The solution is to use an anti tramp bar and alternate mounted shocks (one in front and the other behind the axle).

The main advantage of independent suspension is that it reduces unsprung weight if done correctly and also eliminates the direct coupling of the two wheels and hence the sympathetic action resulting in minimised tramp.

Geometry is another aspect of suspension that impacts on how the trailer will handle, but is for another topic/book!

Unless you are either driving the wheels or braking axle tramp is either non-existent or irrelevant, as there is no torque to rotate the axle against the springs (if you have ever driven a car subject to severe axle tramp you will remember how it stopped instantly when you lifted your right foot!).

But your point about unsprung weight is the major advantage of independent suspension on trailers, not axle tramp.

And, as pointed out, the major problems with trailer suspensions are durability (which can be designed into any type of suspension) and ride - which is a lot easier to combine with this in an independent setup.

John

I agree, Durability is one huge factor, but having less moving parts, bushes, arms and anchor points for the aforementioned, means less complex gusseting and 'weight' in the chassis. There are only two mounting points on each side for a leaf pack, and 1 for the shockabsorber bracketry, as opposed to lateral location,( Panhard or watts link, ) upper and lower control arm of some sort, and shocks also ( For live axle).

I still maintain that a properly set up leaf sprung shockabsorbed trailer will be as reliable and as durable as anything, and available parts are important in remote areas. How easy is it to roadside repair a broken leaf spring? a lot easier than trying to reconstruct the set up with a broken panhard rod or trailing arm... As opposed to tying the pack together and/ or turnbuckling the axle in place incase of a U bolt failure. Also, phoning a wrecker for a secondhand hilux rear spring pack would be a whole lot easier than getting hold of the manufacturer for a spare part that has to be 'flown in' because of the lack of local bits.
I'm not poo pooing the major offroad trailer manufacturers here, as most of them won't have breakages anyway, but IF it happens, there is less of a headache to get the holiday/ trip back on the track again.


JC

Yep, totally agree.

I have just finished building my offroad trailer and on the recomendation from the MD at Alko I used leaf suspension with shocks. He believes that the only reason many of the "expensive" offroad trailers use independent coils is because thats what people expect...not that it is better.

The important thing is to use leaf springs that are matched to the load you are carrying. Many leaf spring trailers have spring rated to 2000kg + and only carry a 1000kg so they bounce around. Use long springs, have them matched to the load and use good shocks and it should be sweet.

I just towed mine from melbourne to brisbane then around Morton Island and it was great. The suspension worked well and towed fantastically. The real test will be on the corrugations later this year.

Another point in favour of leaf springs - coils, rubber torsion members or torsion bars carry the entire weight of each side at a single point, which means that the trailer frame has to transfer all the weight to these two points. Leaf springs carry the load divided between two widely separated points, so that (unless carrying a single fixed, concentrated load such as a motor or genset) the trailer frame is simpler and lighter -, which however, does not help the unsprung weight problem! If the independent suspension trailer frame is no more complex than the leaf spring trailer frame, it is either weak near the spring mount, or unnecessarily heavy away from the spring mount.

The same comments incidentally apply to cars, and taking the case of the coil spring 90/110 compared to the leaf spring Series - I suspect the coil spring chassis is significantly heavier than the leaf spring chassis, certainly the vehicle as a whole is, although there are other reasons for this as well - heavier engines and a lot more equipment.

John

Unless you are either driving the wheels or braking axle tramp is either non-existent or irrelevant, as there is no torque to rotate the axle against the springs (if you have ever driven a car subject to severe axle tramp you will remember how it stopped instantly when you lifted your right foot!).

You are right John. I was getting ahead of myself. It is the unsprung weight and the fact that the two wheels are attached that have the greatest effect.

Another point in favour of leaf springs - coils, rubber torsion members or torsion bars carry the entire weight of each side at a single point, which means that the trailer frame has to transfer all the weight to these two points. Leaf springs carry the load divided between two widely separated points, so that (unless carrying a single fixed, concentrated load such as a motor or genset) the trailer frame is simpler and lighter -, which however, does not help the unsprung weight problem! If the independent suspension trailer frame is no more complex than the leaf spring trailer frame, it is either weak near the spring mount, or unnecessarily heavy away from the spring mount.

When it comes to load sharing into the chassis, there are three load points per side on a leaf spring. Each end of the spring and the shocker mount. The loads are different in nature. The ends of the spring input loads in three planes. The ends of the shocker input loads in the verticle plane. On a coil sprung suspension the coil input is in the verticle plane and depending on what the geometry actually is, other points input in two planes or one plane e.g. the panhard rod inputs on one side in the transverse plane, a watts link is similar but on both sides and half the magnitude. At the end of the day it all comes down to how much of a compromise you make in designing the total system and how good your stress anaylysis is.

Back in the late 70's the Ford Falcon went from leaf to coil sprung rear end. Part of the design criteria was to ensure that it could tow as heavy a load and carry as much luggage in the boot. The end result was a lighter vehicle, but with a better use of the structure. It did all it was designed to do. The rest is history. At about the same time (earlier from memory) Holden introduced the Commode which had a propensity to bend at the rear spring mount when used at design limits and driven over good old Aussie outback roads. This required significant strengthening and alteration to the vehicle in its early years.

In summary, I like the simplicity of the leaf spring. It needs to be long and soft, hence closely matched to the vehicle weight characteristics. It needs to be damped with double acting shockers. The axle needs some carefull thought. The concept of a solid square bar is wrong in my opinion. The most significant aspect ot the axle is to resist bending. A solid anything is not the strongest method of achieving this. A tube is! A tube of say 75mm od and suitable wall thickness can be much stronger than a solid bar of 50mm, yet much lighter for the same duty. As unsprung weight is the critical factor then a tube would be the better alternative. At you reduce the unsprung mass you reduce the requirements of the shocker and therefor the loads transmitted into the chassis. This allows the chassis to be refined and be made lighter, which means the springs can be smaller etc....

My one concern with a leaf spring is the quality of manufacture. As it is both a load suspension and location device should it fail you are totally up the creek. Leaf springs are a relatively low technology to make, but to ensure high quality they need more than a blacksmith approach. If you get a product made for the high volume automotive market then the manufacturer will have quality levels of a high degree of confidence. If you get one made up by a smaller operator then you take your chances. There are a hell of a lot of things that will influence the durability of any spring, but more so the leaf spring. If a leaf spring fails at the eye there is little you can do (hence the military wrap). If a coil or air bag fails you can block it up and still get home.

WOW what a good read, I was going to fit the ALKO independent to a trailer that I have plans of building (gunna)

However after this read I may be reconsidering this. Don't know if i would put in Yota parts as they shook you apart in the ute and would prob do the same in to the trailer.

The Army 1/2 tonne trailers were over sprung, I remember Mortors loading one up when I was in and from memory it weighed in at over a tonne and the springs hadn't sagged at all. Use to bounce over the road when empty as well. Tough though.

Back in the late 70's the Ford Falcon went from leaf to coil sprung rear end. Part of the design criteria was to ensure that it could tow as heavy a load and carry as much luggage in the boot. The end result was a lighter vehicle, but with a better use of the structure. It did all it was designed to do. The rest is history. At about the same time (earlier from memory) Holden introduced the Commode which had a propensity to bend at the rear spring mount when used at design limits and driven over good old Aussie outback roads. This required significant strengthening and alteration to the vehicle in its early years.

Ford introduced the coil spring with the XE which was released in 1981/1982. It may have been on the plans for the XD (1979) but didn't make it, couldn't even fit a decent esky in the boot.I do remember the bending Commodores from towing one of the main things my boss looked for when trading them.

Ford introduced the coil spring with the XE which was released in 1981/1982. It may have been on the plans for the XD (1979) but didn't make it,

Splitting hairs, but it was designed in '79 and released in '80. I know because I did the original design. The last job I did for Henry.

Holden went to 4 coil suspension with the Torana in 1969, and the HQ in 1971.