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Old 09-28-2013, 10:39 PM
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smslavin smslavin is offline
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Default edumacate me on leaf springs

Just doing some digging around and wondering about the performance characteristics of linear vs progressive leaf springs. I'm assuming that for off road use progressive leaf packs may be better. What happens to them under load? Particularly in a truck with a topper, full bed of camping gear, bikes on the hitch, etc?
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Old 09-29-2013, 06:53 AM
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Why would you assume progressive springs are better off road? Do you want the spring rate to increase as the wheel travels over an obstacle? I'd think the right spring would depend on what kind of off road you're doing, mainly if the wheel is traveling relatively fast or slow.

Progressive rate does seem intuitively better when you consider the difference between unloaded and loaded. At least I know that when I setup my suspension for ride quality and height with a camper and outfit it rides more like a prairie schooner with an empty bed and no topper. But it's a lot better than stock, which was either great empty or zero travel with anything more than 250 lbs.

Pickups rear leaf springs usually are more or less linear and institute progressiveness with overload leafs from the factory. IOW, my Hilux had 3 main, full length leafs that did the majority of the work and a big diving board overload for when there was stuff in the bed. There is a small window of payload where the springs are linear. The OME springs I put on also have 3 main leafs (IIRC) but have 3 or 4 decreasingly shorter leafs that progressively do what the one large overload did in the stock springs (and they have a slightly higher overall max capacity). That makes it ride better through a larger window of weight in the bed but empty my truck still rides rougher than stock because even though they are progressive the stack of 7 leafs is still stiffer than 3.

Point being is linear springs seem to me to ride better daily but progressive springs are higher performance over more cases. The ideal situation it would seem is to have linear springs at the correct rate for your payload with a soft (e.g. progressive) bump stop, but then they would be too stiff with less weight and bottom out with more.
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Old 09-29-2013, 06:43 PM
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Originally Posted by DaveInDenver View Post
Why would you assume progressive springs are better off road?
Because the interwebs told me so.

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Combined with properly valved high performance shock absorbers, the ride is enhanced beyond belief.

Deaver Leaf Springs are progressive design springs. They are not suited for carrying weight in the back of your truck or for towing.


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Point being is linear springs seem to me to ride better daily but progressive springs are higher performance over more cases. The ideal situation it would seem is to have linear springs at the correct rate for your payload with a soft (e.g. progressive) bump stop, but then they would be too stiff with less weight and bottom out with more.
Ok, that's what I needed to clear it up. To gain the off road benefits of a progressive spring, could you add air bags to add stiffness under load and/or for daily driving?
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Old 09-29-2013, 07:10 PM
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Ok, that's what I needed to clear it up. To gain the off road benefits of a progressive spring, could you add air bags to add stiffness under load and/or for daily driving?
Air bags are a great way to add instant load capacity while retaining a suspension tuned for a lower load. Commonly done with people who drive their crawlers to camp or tow a trailer.
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Old 09-29-2013, 07:13 PM
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thanks dave. lots to figure out.
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Old 10-02-2013, 05:38 PM
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Air bags are a great way to add instant load capacity while retaining a suspension tuned for a lower load. Commonly done with people who drive their crawlers to camp or tow a trailer.
My predecessor as TLCA President, Alan Loshbaugh put semi-ellipticals on his FJ45 and touted their super-flexiness off road. But, I need my truck to be able to be a work truck too, so what I plan to do is add air bags when I go elliptical on my FJ45.

The picture is with about 1.5 tons of sand bags from when we were sandbagging along Bear Creek a couple weeks ago during the flooding. The stock FJ45 is rated at 1 tons and has a 9 leaf rear spring pack; the 6 normal leaves you get in a 40, plus 3 thick, flat overload leaves underneath. When empty, you can feel every pebble and crack in the pavement, but put a ton or so back there and it smooths right out.
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Old 10-03-2013, 12:26 PM
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Love to see photos of FJ45s that are working - thanks Jeff!
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Old 10-05-2013, 09:40 PM
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Love to see photos of FJ45s that are working - thanks Jeff!
Heh. I had a load of 2 x 4s, sheetrock, bags of mud and assorted construction stuff in there a couple days ago. I have another big trip to Home Depot planned, I should take a picture before that is unloaded because I have calculated that at around a ton and a half.
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