#5
Beitrag
von felix » 2012-07-26 1:49:41
Hello allroads,
1124: Medium chassi, decent engine - good choice first of all!
The leaf spring issue is a bit tricky with old fire fighting vehicles. This type of trucks was build to carry an almost constant load so the springs are not, or less progressive than the configurations of normal commercial cargo trucks whose load differs all the time between empty and fully loaded.
But you are lucky, at least your rear suspension is made of two independent springs, one for the empty truck and one that needs some more load to get connected to bearings at the frame so both springs are working together. It cound be worse. At your front suspension the second spring is made of rubber, but this one is more to avoid a potential catastrophe than to improve comfort. But it is two springs creating a progressive system at the front tough.
I assume that your goal is more suspension comfort. I can't solve this remote but maybe I can give you some hints in which steps you can fit the suspension better to your real vehicle weight.
- First of all don't be stupid and start to fit your suspension to a vehicle weight which you don't really know. You maybe think you can estimate your final vehicle mass sharp enough, but believe me, you would be the first one if you hit it. So first of all finish the whole truck, take it out on a test journey to find all the things that have to be improved.
- Then fill all tanks and all possible cargo and make a test drive: How much do you really have to change to fit the suspension. Consider: You really need a reliable suspension when you are far away from home. And most likely the truck is much more loaded in this situation, than it usually is at home.
Replacing the trapeziod springs by parabolic springs can result in a more progressive and therefore a strong but comfortable suspension. Every parabolic spring leaf can be considered as a independent spring. If they are mounted in a way they will get contact to the bearing one by one with increasing deflection it will result in a more progressive suspension. And this is what you want: A spring that is as strong as the original one, but more soft at low or medium deflection. The curve of the force-to-deflection ratio depends on the strength of every parabolic leaf (not imperative equal) and the deflection it needs to hit the bearing.
Also there is no friction between the parabolic leafs. This brings the need of (stronger) dampers, but it is a big feature: With a trapeziod spring there is friction between the singe leafs, more when it gets loaded, less when it gets unloaded. But friction when a spring gets loaded makes it more stiff and uncomfortabe. You want the friction to consume the energy to avoid oscillations, not to make your suspension more stiff.
Hydraulic dampers can do this: They are way more stiff on extraction then they are when pushed together. So having a suspension without inherit friction brings the benefit to be able to add hydraulic dampers which provide most friction when the suspension is unloaded.
But not all parabolic springs come in a progressive configuration. Often all leafs have direct contact to the bearing so the only benefit from the parabolic setup is (besides less weight) no inherit friction. If you use one of this springs in your truck, you should adjust the gap between the leafs in a way, that the configuration becomes progressive in the right way for your truck. Because most likely you will always find "some" parabolic springs, not "the one" that fits perfect your truck. Same for the dampers. Also you will need to change the bearings because parabolic leafs tend to be much more thick that trapezoid leafs. Normally here is no way to get them into the existing bearings.
So maybe you get a feeling for the effort it takes to create a sophisticated suspension with parabolic springs for your truck. In my opinion this change would be the last step, only to be taken when you really know how to do and configure it. And you also should know if it really is needed.
But roughly estimated you will need to decrease spring force on your rear axle:
- I would not change anything on the front axle: If you hit some obstacle with a little speed in offroad condition you will need every last newton of spring force you can get for your front axle, because it will get a heavy dynamic overload.
- First of all I would grease the existing springs and wait for the weight of the truck to be settled and then check the load on your rear axle fully loaded.
- There can be a nasty situation with the two springs at the rear axle: The suspension becomes really uncomfortable if the top spring of the two springs in the rear 'just' gets contact to the bearings. The load is far away from being enough to make both springs comfortable, but enough to activate the second spring. Unfortunately travelling applications bring a vehicle mass that tend to create exactly this situation. Trucks are build to be empty or fully loaded, "little more then half loaded" is a minor load situation that does not happens so often in a commercial use of a truck. So the customers don't pay the engineers fit the truck to this load situation.
In case you have this problem I would take out some leafs from the top spring or rather just lift the position of the bearings for the top spring. This would keep the benefit of still having the full strength of the top spring, it will just takes more deflection of the lower spring to get the top activated. Your suspension is more progressive instead of more weak.
- If this is not the case or your suspension is still to stiff, you can take out one or two leafs out from the main spring as well. But this can only be done together with moving the bearings of the top spring (or removing the top spring completely), otherwise this would make your spring more weak (because of the missing leafs) but also less progressive and almost the same stiffness, because the top spring will be activated more early.
Caution:
1: Taking out leafs is not without risk: The time to failure is limited by the amount of bending of a spring leaf. Taking out leafs will produce more overall bending in the spring, which is the goal of taking out leafs: More soft means more deflection means more bending. This is no problem within general limits. But taking out leafs also makes the bending less equal along the spring leaf: There is a set of leafs put together in a trapeziod packet with the target to get a almost equal load along the length of the top leaf. The more single leafs are put together the less wide are the steps in length between two leafs and the more equal is the load/bending along the top leaf. A small step between the length of the particular leafs will equalize the load and therefore decrease the probability that one leaf breaks.
So if you take out for instance the 3. leaf from top of a spring, the step in length from the 2. to the 4. leaf will be two times as far as before. And so will be the inequality in bending. As a result you will get a spring that maybe only survives 80% or less of the oszilation amplitude without producing micro cracks and breaking in the end. So there is a limit in unmounting leafs, you can't get the amount of deflection a more soft spring with equal, small steps between the leafs would provide.
So never ever take away two following leafs. In general the first two from topside have to remain. They normally connect the axle to the frame and therefore must not be touched. Than take out every odd leaf. It would make the load more equal if you take an angel grinder and cut the remaining leafs in a way that you get equal steps in length, but then you can't undo the change, so maybe this is not recommended unless you really know what you are doing.
2: Many trucks have a regulator for the breaking force of the rear axle. This regulator is connected to the axle and adjusts the breaking force by measuring the load by sensing the distance between the axle an the frame. So the force-to-deflection ratio of your rear spring defines the amount of force that is delivered to your rear axle. So changing the rear spring can result in a really dangerous distribution of breaking force between your front and rear axle which can cause serious accidents. And it is also a crime (for sure not only in Germany) to drive a truck with a misaligned breaking force regulator. Even tough they will not notice it until something really goes wrong and they start digging...
So you must bring the truck to a workshop where they can recalculate the configuration for your breaking force regulator and create a new rating plate (left door bottom) to make your truck legal again. It hast been a regular job these days trucks where suspended by leaf springs in stet of rubber bags with pressured air, so you likely will find one old guy who still remembers.
Hope this will give an overview.
Aircooled regards,
Felix
Ive bought an ex fire engine 1124 AK with zf autobox with retarder. My problem is i have taken all the fire engine stuff of which weighed around four tons and my intended eventual weght is to be around 10.5 tons gross at the moment she grosses out at 13 tons and the current suspension is to heavy
Especially for AWD-chassis the leaf springs are used until today because they are more robust than the parabolic.
I will visit with my wife when Ive completed this.
) I will post some pictures over the next few days.
these are 12.5 I would like to fit 365/85r20 but don't think they will fit inside the arches.
our intention is to only travel for a month at a time max and mainly stick to Europe east and west. 
I've been very busy with work and now in the new year can look to do more with the interior.
