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For context I have a gen3 swb shogun/pajero which has double a arm front suspension and what looks to be halfway between double a arm and trailing arm rear suspension. It has an open front diff and locking rear and centre.
In the UK our offroad courses tend more towards bumps, soft mud, water etc as opposed to the rock crawling so common in USA.
In offroading circles suspension discussions tend to be limited to how much lift can be achieved as a means to fit bigger tyres and also to achieve more articulation.
The general trend appears to be towards quite soft springs with no anti roll bars for maximum articulation.
There's a lot of available knowledge for suspension setup on race tracks, but how does this translate to offroad?
My gut feel is that my suspension is already too soft which allows the front to bottom out too easily but also that the front anti roll bar does reduce front articulation quite considerably.
So my plan is to take the anti roll bars off and increase the spring/damper rates to take account of this. Are there downsides to this approach?
Many of the high level offroad vehicles eg Dakar or Baja tend to use anti roll bars still, is this simply because their speeds are much higher? Or are there cross axle jacking effects being used?
What sort of bump to droop ratios should you aim for to set ride height?
Really ride height is my biggest issue with another 50mm or so being beneficial.
Would be interested in any thoughts / experience you all may have.
As you said, offroad information, atleast to the caliber track cars, is almost non existent, however, alot of the physics involved in the car setup overall carry over even if the goal is different. For example, offroad may be fairly close to say, an icy track on summer tires, unfortunate for sure, arguably similar. You have little to no traction and will struggle under almost any load. For offroading, the issues may be due to the ground giving way instead of the tires, but the fundamentals should be the same, equally loaded tires will generally preform better.
As for stiff, or softer suspension overall it's a trickier subject. Following my logic from above, a stiffer setup would keep the tires closer to equally loaded, and theoretically improve the car(Digging through some books to see where I goofed up in my logic). However, the stiffer the suspension the more disruptive any individual bump will be. At a potentially microscopic level, the center of your contact patch is airborne as it passes over the apex of a given bump in the road. If you have softer suspension, your wheel will be in contact with the ground longer, resulting in more grip. At some transition from a track that's fairly smooth, to offroading, the bumps will cause more issues than the load transfer on the tires, so softer will benefit you more.
The suspension has no way of knowing the difference between a bump in the road, from an incline. The springs, and dampers are tuned to try and behave properly for a variety of situations. If you go over a speed bump at 1 mph, as the tires go over, the body will rise almost the exact same amount, the bump itself is very low frequency, and the suspension doesn't really compress at all simply transmitting the height increase into the body of the car. If you hit a speed bump at 45 mph, your suspension will compress, and only a portion of the bump will get transfered to the body. This is primarily the realm of dampers, but for a very soft vehicle, the response frequency of the springs may be more influential. The amount of time it takes for the suspension to naturally cycle is directly related to the springs and sprung mass, dampers then reduce and tame the bouncing of the car.
For the less abstract thought process, anti roll bars link one wheel to the other, and when they move different amounts, it applies resistance. For body roll, this is a great thing. If we think about that equal and opposite force concept, a bump hitting one tire, will compress that side, and then try and lift the opposite side. So for a given bump, depending on the ARB stiffness, you'll compromise the opposite wheel. Talking to some rally people, gravel vs dirt seems to be the threshold where they heavily consider their usage of anti roll bars. Offroad is probably even worse than gravel rally, so removing the makes sense. As with everything suspension, it comes at a cost. Stiffening up the springs to compensate for the lack of roll bar will increase the ride frequency, making the spring respond faster over a bump, and affecting the body more. Damping rates should be generally matched to the springs you run, so increasing them similar amounts makes sense, however as you increase the damping coefficient, for the initial "impact" of a bump, you have to overcome both the springs and dampers resistance, so once again, the movement of the vehicle body will be increased.
As a general statement the goals seem to be as much suspension travel as possible, with the body reacting as slowly as possible to try and prevent rollover when crossing an obstacle. Compression and extension should probably follow 2/3rds 1/3rds as suggested, but if you simply don't use some portion of your suspension travel, it's wasted. A ziptie on your damper to see how much gets used is very useful for finding this out. Timed events will tend to head back towards rally or track setups, because the goal is speed as much as it is simply getting through the terrain. A camera looking at your suspension should be able to tell you how often you bottom out, and in what situations.
Looking into damping ratios, ride frequency, and transmissibilty should translate to offroad far easier than looking at load transfer and some of the more track focused concepts. Sorry for the overly complicated post, offroads got an almost infinite amount of possibilities ranging from dirt tracks to rock crawling making it difficult to simplify.
Robbie has some good points there, but I'd suggest some editing - for instance, a SOFTER suspension will more evenly load the suspension, not a firmer one.
Anyway, as you pointed out, there is a LOT of focus on suspension travel and articulation, especially for the extreme rock crawling, and there is as a result a lot of 'wank' factor - AKA 'monkey see, monkey do'. For some applications, this is a big mistake - heck, what you're doing would seem to be much closer to the Formula Offroad type of setup - https://www.youtube.com/watch?v=l3b8WBL9Z4A - but rather less extreme...
More seriously, it seems to be more 'mud plugging, like this local '4x4 trial series - https://www.youtube.com/watch?v=kPq5-J5RoYs - but in more standard vehicles?
Again, working with too little information, but some general comments.
You mention you have an open front diff', this means the torque than can be applied through either front tyre is going to be limited to the level of the lower grip tyre - to maximise the front traction you're going to have to do what you can to load then equally. Unfortunately, this means zero roll resistance, which is impossible without a single centrally mounted spring. Normally this is done by using a very soft front spring and, even better, a soft rear as well, to aid articulation. However, there are other options - if you can use air springs on the front (travel limitations), or all round, and have the left and right connected together on the same axle, in bump they will both absorb the compression but in roll the air will simply move from one side to the other with little resistance. Another option is to use a Z-bar - this is similar to an anti-roll bar but works the opposite way - instead of trying to compress and extend together, it tries to force one wheel into extension as the other is in compression, it's like a positive roll bar. There should be plenty of info on-line on them.
You say 'bumps' - are those short, sharp bumps, larger ones that the vehicle becomes airborne over, or whatever? How hard does the front bottom out? As I think Robbie mentioned, while the springs are technically the 'shock absorbers', the dampers also aid in absorbing some of the forces applied to the chassis through the compression daming, and this may be useful for such higher velocity impacts as they have less affect at low speeds. You may also be able to combine some of that with longer, progressive bump stops - especially as they can be thought of as auxiliary springs for when the suspension is compressed more than usual.
In mud, particularely deep mud, the general thinking is to run the suspension in an extremely raised state, so the chassis/body is clear of the mud and the suction it can apply, and that is good, sound thinking as it also helps, in theory, the tyres to get through to the firmer surface underneath - to a point. Anything clear of the mud is a waste and is introducing a higher centre of mass which will increase the chance of a roll-over or end-over on steep inclines.
There are too many things I don't know about the vehicle, the type of event, your budget, etc, but is you have the springs and/or dampers, it wouldn't hurt - especially if you also disconnect a rear ARB, if fitted, too.
Oh, ARBs are indeed used on some of the DAKAR, and other high speed events, because they reduce body roll and have negligible affect on bump absorbsion - with the trucks, the springs are close coupled on their beam axles and have little roll resistance, and the small trade-off for a little less articulion isn't an issue.
Edited the above, inadvertently linked same video twice when I meant something like this, if a little less extreme and in more production based vehicles - https://www.youtube.com/watch?v=kPq5-J5RoYs
Thanks both for taking the time to respond with some very interesting thoughts, sorry for taking so long to reply.
Coming from track and drift cars where there is actually quite a bit of thought about suspension and what its there to do, the offroad scene has been a bit of a shock (pun not intended!)
What has most caught my eye is how the baja trucks and similar are doing 100mph with a flat body and the suspension working like crazy, its mind blowing.
This seems to be a more extreme version of how the dakar vehicles are setup, not a lot of camber gain on very long travel suspension that is "compliant" but probably not overly soft. i.e they use the full range of movement but only over larger bumps at higher speeds.
I think a lot of the science here is in the dampers. The difference between a set from a company that supplies baja vehicles and the like, vs what is typically fitted in the aftermarket can be seen here.
It really is night and day.
I'm finding recommendations of around 2/3ds bump to 1/3 droop travel which is then limited by hydraulic bump stops for bump and limit straps for droop. The bumps stumps are actually quite long and are really a tuneable 2nd spring to have a higher rate for big loadings.
From the looks of things my next steps are really going to be practical testing.
Thanks for the help :)
Yes, the technology used in some of these vehicles is comparable to any other top-tier motorsport.
However, before you get too involved with chasing down those builds, have a GOOD think about the type of competition you will be doing, as the requirements are very different for desert racing, Icelandic 4x4 competion, or even rock crawling, from what you may need for a more 'mud plugging type of competition.