Spring rate, ride height and up vs down travel

[A4L7E3X]

Hey guys, I'm wondering if we can come up with some general guidelines for spring rates vs vehicle weight, and suspension up vs down travel? Not necessarily for hardcore buggies with wheel travel measured in feet, I'm personally more interested in my Rangie, but the same tech will be useful for any coil sprung, live axle 4x4.

I know the trend tends to be to build it low with lots of down travel, but what sort of up-down travel ratio is a good target? I get that this will vary depending on the type of terrain, but maybe we could come up with some general guidelines, like maybe 50:50 for general off-road, 40:60 for high speed / desert, and 30:70 for rock crawling? I just pulled these numbers out of a hat so gentle with the flames.

My Rangie would be in the neighbourhood of 9-10 inches of wheel travel, at a guess I would say the front has about 2-3 inches up and 5-6 inches down, and the rear 4-5 up 6-7 down. I wouldn't mind lifting the front a bit though as I do get into the front bumps a bit. Current springs are OME progressives, but I got them used so I don't know the rates.

I bought some new (used) springs to try out, they are 170 lb front and 300 lb rear. Gut instinct says that 300 lb will be too stiff for the rear so I might stick with my current rear springs. I don't have axle weights yet, again at a guess I would say 1000 kg (2200 lbs) front and 1200 kg (2600 lbs) rear.

PS, a lot of the Rover "tech" on other forums is either grumpy old pricks who think that Land Rovers are perfect from the factory, or 12 inch tall springs that dislocate by 3 feet in the rear... Hopefully we can get some better info together.
Keen to hear from some who have done the math before!

 

[mobil1syn]

under 12" of wheel travel i want to see 50% bias, because trying to get something to ride well with less than 6" of uptravel is a pain. it can be done, but you give up comfort.

over 12" of wheel travel you can get away with 40/60 up/down.

in your case, you need a good shock if you plan on getting it stay off the bumps.

 

[Yotota]

Sprung weight = total weight - unsprung weight (wheels/tires, axles, and half of the driveline and suspension link/leaf weight).

Sprung weight on one spring / spring rate = spring sag at ride height

So a 4000lb rig may have 3000lb of sprung weight, and let's assume 50/50 weight distribution front and rear. With two springs each on the front and rear suspension that's 750lb on each spring.

If you have a 12" travel spring with no preload to consider, and you want to set sag at 6" up and 6" down travel, that would need a 125lb/in spring. (750lb / 6in = 125lb/in)

 

[A4L7E3X]

Follow up for the sake of anyone who is looking for similar info in the future:

I got the Rangie over a weigh bridge and it came to 2320 Kg (5100 Lbs), with a perfect 50/50 split front and rear. 1160 Kg (2550 Lbs) each end. That includes me in the drivers seat, a full tank of fuel, all recovery gear including a chainsaw and all of my regular tools and spare parts. I'm happy with that as an overall number, although technically my front axle is slightly overloaded.

After a big weekend swapping out the transmission... I got to swapping out the front springs for the new 170 lb pair, this gave me a 40mm (1.5") lift in the front. I took it for a test run and it has firmed up a touch but still handles bumps better than most 4x4s I have driven. I don't think I got into the front bumps on my test run, but it wasn't the most serious off-roading either.

The up vs down travel I ended up at was:
Front axle - 100mm (4") of bump stop clearance at ride height, 195mm (8") of bump stop clearance hanging off the hoist.
Rear axle - 65mm (2.5") of bump stop clearance at ride height, 195mm (8") of bump stop clearance hanging off the hoist.

Note that my rear bump stops are extended progressive bump stops, they have two big voids in them that will collapse by about 60-70mm (2.5"). I might swap these back out for the factory solid rubber versions which will give me another 60mm of clearance in the rear, but I haven't decided yet. It does hit the current rear bumps a bit, but as they are progressive it isn't a harsh bump.

If I go back the factory bumps the rear travel will be:
125mm (5") of bump stop clearance at ride height, 255mm (10") of bump stop clearance hanging off the hoist.

So by those numbers I have hit the 50/50 up vs down travel target perfectly, and it does seem to work pretty well. I think its also worth mentioning that the wheels will travel more than the measured 8 inches front or 10 inches rear when the axle is articulating.

I found the deepest rut I could on my short test run and got the axles to flex as much as I could, they weren't deep enough for it to pick up a tyre, but this is with the rear right resting on the bump stop and the rear left as low as the shock absorber would let it go. The front left is still about 30-40 mm from the bump stop. (Range Rover suspension is much more flexible in the rear than the front).

For those who care, those are 285/75R16 tyres (33"), with no contact on the bodywork, even when steering from lock to lock.

Thanks again for the advice