understanding leaf springs

[bach4660]

what is the difference between having the rear leaf springs overtop of the axle like our trucks, or under the axle like the Dakota? what does this change?

[Merle Coggins]

Ride hight.

[PatS....]

Sprung and unsprung weight.

[Thunder II]

Large/heavy trucks, will have the axle assembly under the spring, so that the "U" bolts, only hold the axle in place, and NOT the weight of the truck. So with smaller trucks, ie: pickups, they can/are built either way. So, Merle pretty much answered it.

[Don Coatney]

Sprung and unsprung weight.

Pat;

Not sure how placing the axle on the top or the bottom of the spring will affect sprung and unsprung weight. If I visualize this correctly the sprung and unsprung weight will remain the same.

[Plymouthy Adams]

this site has some FAQ's that may help denote why one method is used over another...

http://www.alcanspring.com/faqs.htm

[PatS....]

Pat;

Not sure how placing the axle on the top or the bottom of the spring will affect sprung and unsprung weight. If I visualize this correctly the sprung and unsprung weight will remain the same.

Don, if I recall the high school lesson correctly if the axle is below the springs, the axle is not sprung, and if the axle is above the springs, like our cars, it is sprung.

[Don Coatney]

Pat;

Still not clear in my mind. I have been wrong before but as I see it the axle and tires are a unit and connect to the middle of the spring (weather the axle is mounted to the top or bottom) and are therfore unsprung. Everything connected to the ends of the springs is therfore sprung weight. If this is not correct let me know.

[PatS....]

Don, it never was clear in my head. I just remember the distinction was whether the item was supported by the suspension or not.

If you have a rear end with the springs above the axle, the axle isn't "supported" by the spring. If the spring is below the axle the weight would be supported by the spring.

As I said, that's how I remember it being explained to us back then. I know there is controversy over the theory of how this affects the handling of a vehicle. For example, is a driveshaft 50% unsprung when the axle is below the spring? There are folks who say that it is only 50% in the center of the driveshaft and that it is a different percentage at each point either side of center.

Sorry, it gets real complicated for me at that point forward.

There are also changes in the CG when sprung and unsprung are varied.

At that point my head hurts.

[Plymouthy Adams]

try this:

Sprung Weight

Vehicle weight either moves directly with wheel movement -unsprung weight- or indirectly with the wheel as cushioned by the suspension spring -sprung weight. The long and short of this is that the latter is good and the former isn’t. Unsprung weight moves with the wheel. The more unsprung weight, the stiffer the spring/damper assembly needed to absorb the energy imparted when the wheel hits a bump. This both compromises the suspensions ability to keep the wheel on the ground and transmits the upset to the suspended weight.

Various links, control arms, dampers, springs etc. are attached at one end to a sprung member, i.e. the chassis, and at the other to an unsprung component such as a wheel carrier. Accordingly, these are partly sprung and partly unsprung. Put the expensive, light weight rod end on the unsprung end. Low mass wheels have the added advantage of decreasing rotating weight and gyroscopic inputs.

[Merle Coggins]

Sorry Pat,

I have to go with Don and Tim on this one. The axle and wheels are unsprung weight regardless of how it is attached to the springs.

[TodFitch]

Sorry Pat,

I have to go with Don and Tim on this one. The axle and wheels are unsprung weight regardless of how it is attached to the springs.

I too agree with Don, Tim and Merle on what is sprung and unsprung weight.

[Don Coatney]

try this:

Sprung Weight

Vehicle weight either moves directly with wheel movement -unsprung weight- or indirectly with the wheel as cushioned by the suspension spring -sprung weight. The long and short of this is that the latter is good and the former isn’t. Unsprung weight moves with the wheel. The more unsprung weight, the stiffer the spring/damper assembly needed to absorb the energy imparted when the wheel hits a bump. This both compromises the suspensions ability to keep the wheel on the ground and transmits the upset to the suspended weight.

Various links, control arms, dampers, springs etc. are attached at one end to a sprung member, i.e. the chassis, and at the other to an unsprung component such as a wheel carrier. Accordingly, these are partly sprung and partly unsprung. Put the expensive, light weight rod end on the unsprung end. Low mass wheels have the added advantage of decreasing rotating weight and gyroscopic inputs.

So Tim;

Does sprung and unsprung weight change with axle placement above or below the spring?

[Plymouthy Adams]

can't answer that myself...only the effects of the weight is felt different..as I read it...

see above posted link to web site..

[greg g]

The simple answer is "about 3 inches"!!! Truck axles are below the springs for added ground clearance. And the Ubolt thing on HD trucks.

[PatS....]

Sorry Pat,

I have to go with Don and Tim on this one. The axle and wheels are unsprung weight regardless of how it is attached to the springs.

Don't be sorry, as I said I never completely understood it and what I had was from high school. As long as it gets cleared up and straight, I'm happy. I have been wrong before and I'll be wrong again.

Pat

[Merle Coggins]

I once thought I was wrong... but I was mistaken :D

[bach4660]

so, I have the one- ton without shocks, Merle was kind enough to post pictures of how the optional shocks were mounted to the truck. Recently I have used it to haul concrete blocks, and then a load of garbage to the dump. On the second trip it drove beautifully, and felt really nice with 616lbs in the back. Which got me thinking about springs, and shocks. The weight also lowered the back. So what I trying to achieve is a ride equal to carrying 600lbs, and still keep the basic look and stance of the truck (but the 3" off the height looked good). If I put the springs under the axle will this drastically alter the handling?

[greg g]

unsprung mass = the mass of the suspension, wheels or tracks (as applicable), and other components directly connected to them, rather than supported by the suspension