Frame flex and type and kind of spring?
- Thread starter Flameffects
- Start date
Following on from what Ski said, there doesn't appear to be any generally agreed rule of thumb for working out spring rates, all are different- some have no idea- just use what fits or is handy- like valve springs!
FWIW, my engineer didn't care about the springs much, just wanted to know the strength of the bolts/nuts used.
Spring failures appear to be very rare, I've only heard of one getting cracked. My spring maker was concerned that my springs will be highly stressed if/when fully compressed, so far so good.
Cheers
FWIW, my engineer didn't care about the springs much, just wanted to know the strength of the bolts/nuts used.
Spring failures appear to be very rare, I've only heard of one getting cracked. My spring maker was concerned that my springs will be highly stressed if/when fully compressed, so far so good.
Cheers
Yes, it seems that just making a best guess and seeing how it works out is what people do. Which I find surprising -- I would've thought there'd at least be some sort of general rule of thumb by now, along the lines of "for every 1k lbs. of weight, add X to your spring compression rate" or something like that. Maybe the result would only be marginally better than a "best guess" approach, but marginally better is still better. I agree with SkiFreak that springs that are too weak could easily be worse than having no spring-mounts at all. Alas. There is an ocean of options out there, but I guess I'll just start somewhere in the middle and go from there.
Agreed, I did heaps of research and found nothing concrete, an engineer can do the maths but the inputs are different each time, I even asked 2 engineers and got different opinions on spring rates, also agreed that stronger is better than weaker.
Just make a decision and move on, lots more things to decide in the rest of the build.
Cheers
Just make a decision and move on, lots more things to decide in the rest of the build.
Cheers
SkiFreak
Crazy Person
javajoe79
Fabricator
Just that they offered more than enough travel than I thought I would need and a spring rate that I thought would be good. The way I built the mounts, I could shorten the shaft that they bolt to and allow more preload or shim it for less. I also remember there being several other rates available in the same size spring if I wanted to change the rate. I painted them with etching primer and a good top coat and they're doing pretty well so far but the truck hasn't seen much action yet.
javajoe79
Fabricator
Pretty much how I feel too. Too many variable to narrow down a spring that would work for everyone.
4x4Truckoverland
New member
Anyone got any suggestions for a FG649 subframe builder in Victoria? I'd like to incorporate tank inot the sub frame too.
I'd look at the AAV subframes that come with tanks etc, or buy their spring mount kit and design up the frame you want for your setup.
4x4Truckoverland
New member
Thanks. Yeah they make a nice one. That is the way I’m leaning at the moment. Then I guess I can get tray and canopy done in Victoria. Recommend any professional tray/canopy builders in ViC?
Hi Guys
I too am going through this very same process at the moment.
I guess fortunately for me I am a motorcycle suspension technician and have my own workshop in Brisbane. I have decided to go with some slightly modified rear shock springs from a motorcycle that fit the bill quite nicely. I will be able to report back my findings when I have tested them. I have already tested the springs on my shock dyno and the results were positive. Now to get them mounted and working is the next task. Next week or two maybe.
I too am going through this very same process at the moment.
I guess fortunately for me I am a motorcycle suspension technician and have my own workshop in Brisbane. I have decided to go with some slightly modified rear shock springs from a motorcycle that fit the bill quite nicely. I will be able to report back my findings when I have tested them. I have already tested the springs on my shock dyno and the results were positive. Now to get them mounted and working is the next task. Next week or two maybe.
BruceAndKrista
Active member
Ok, so now I have a question relating to these spring mounts that everyone seems to be trying to figure out....
My understanding of chassis flex, is that we want the chassis to be able to twist, to allow better articulation of the axles. In looking at the bodybuilder manual for the FG, the stiffest section of the frame is the center, right around the transmission. So... why do we not want to do a ridged mount here, and a spring mount at the BACK, where the most twist is going to be induced by the rear axle?
I envision the frame twisting from axle to axle, with the center moving the least, so, why do we want the “box” to be able to flex at a point where the chassis isn’t?
I have a single cab, and intended to make a sub frame that is rigid mounted from the area just behind the cab, to where the frame drops. Aft of this would be two spring mounts per side, one mid point, and one at the back, thus allowing the frame of the truck to “drop” out from under the subframe on the side which is unloaded...
How wrong am I ?
Edit - I guess my reason for asking this, is... really - the weight of the “box” on the back is supported by the chassis when it is flat, so really the weight of the box is irrelevant (other than to prevent sway) - what the spring should be matching is the flex rate of the chassis, no? If the spring is stiffer than the chassis flex, it serves little purpose, and if it’s softer than the chassis, then the box will likely sway?
My understanding of chassis flex, is that we want the chassis to be able to twist, to allow better articulation of the axles. In looking at the bodybuilder manual for the FG, the stiffest section of the frame is the center, right around the transmission. So... why do we not want to do a ridged mount here, and a spring mount at the BACK, where the most twist is going to be induced by the rear axle?
I envision the frame twisting from axle to axle, with the center moving the least, so, why do we want the “box” to be able to flex at a point where the chassis isn’t?
I have a single cab, and intended to make a sub frame that is rigid mounted from the area just behind the cab, to where the frame drops. Aft of this would be two spring mounts per side, one mid point, and one at the back, thus allowing the frame of the truck to “drop” out from under the subframe on the side which is unloaded...
How wrong am I ?
Edit - I guess my reason for asking this, is... really - the weight of the “box” on the back is supported by the chassis when it is flat, so really the weight of the box is irrelevant (other than to prevent sway) - what the spring should be matching is the flex rate of the chassis, no? If the spring is stiffer than the chassis flex, it serves little purpose, and if it’s softer than the chassis, then the box will likely sway?
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boogie944
New member
I believe main reason to do the springs is to avoid too high torsional stresses on your box. The heavier and longer the chassis the more relevant this point is. The chassis flex may also be an advantage at slow speed on rough terrain to keep wheels on the ground.
To answer your question when springs are too stiff you will get higher torsional load on the box and a front axle will lift on uneven terrain. When they are too loose they only act as end stop in case of chassis torsion. I can only see it will have an effect at high speeds on bad roads, but maybe someone else has more experience with this. I think torsional stiffness of the chassis itself is not so relevant as it is very little compared to whatever box you place on top.
For my NPS build I have not installed any springs and a dumptruck style pivot at the rear. Have not really given it a real test yet. Next build I would want to try to swap that around and make the pivot at about 40% wheelbase from the front axle. Main advantage less movement box vs driver cab.
SkiFreak
Crazy Person
The chassis of an FG is comparable to a wet noodle; it will twist if you look at it sideways...
The primary purpose of any type of kinematic mounting system is not to allow for more articulation, but to reduce (as much as possible) any chassis twisting from being transferred into the body.
Unlike a goods truck, where the body is quite often hard mounted to the chassis, flexing inside the box is not of any major concern, but in a camper, where you have fixed furniture, flexing of the box can cause a significant amount of damage, which is obviously not desirable.
As to which type of kinematic mounting system... this has been discussed to death on this forum over the years.
Our Australian Rural Fire Service use this type of 4x4 truck ( both Fuso and Isuzu) extensively, where they carry heavy dynamic loads, often on rough dirt tracks.
A full spring mount setup is the most common system used by the RFS and the majority of the commercial expedition camper manufacturers in Australia.
It's your truck and your choice, but I cannot count how many times I have encountered people who believe that they are smarter than the professionals and that they can build a better wheel.
Just sayin'...
The primary purpose of any type of kinematic mounting system is not to allow for more articulation, but to reduce (as much as possible) any chassis twisting from being transferred into the body.
Unlike a goods truck, where the body is quite often hard mounted to the chassis, flexing inside the box is not of any major concern, but in a camper, where you have fixed furniture, flexing of the box can cause a significant amount of damage, which is obviously not desirable.
As to which type of kinematic mounting system... this has been discussed to death on this forum over the years.
Our Australian Rural Fire Service use this type of 4x4 truck ( both Fuso and Isuzu) extensively, where they carry heavy dynamic loads, often on rough dirt tracks.
A full spring mount setup is the most common system used by the RFS and the majority of the commercial expedition camper manufacturers in Australia.
It's your truck and your choice, but I cannot count how many times I have encountered people who believe that they are smarter than the professionals and that they can build a better wheel.
Just sayin'...