[Gwagensteve]

Hi all,

if you want to skip the preamble scroll down to the TL,DR in bold.

This topic came out of a thread on FB about drop shackles where I had some thoughts about flex and capability, so rather than them sit in that dusty corner of the internet, I thought they might as well sit in this dusty corner of the internet.

Drop shackles, 3/4 elliptic, SPOA etc are now mostly discredited ideas that have aged badly, and it can seem like we're in a "post flex" era, where people are only half joking when they say "flex doesn't add capability"

For some background, relatively little thought was being given to significantly increasing suspension travel in the general off-road community until the early-mid 1990's. A couple of things happened at that time (from my point of view) David Freiburger put actual 4WDing on the cover of Petersen's 4 wheel & Off-Road magazine, and the Victorville 4-Wheelers started exploring the Johnson Valley OHV park... and started naming the trails after hammers, presumably because of the effect they had on everyone's cars.

Oh, also, Fourwheeler magazine collected, a bunch of mates together at Hollister hills OHV area up near San Francisco and ran the "top truck challenge" competition. The first year was run by Ned Bacon and Tim Hardy was there along with Rick Pewe, Soni Honneger etc, a whole bunch of guys who ended up being (and still are) very influential in the scene. Shannon Campbell won it a couple of years after it kicked off too,

To be fair, these guys had been building high-travel cars for years, but they were a tiny group and all of a sudden the media gave them attention, moving away from the show truck scene of the midwest where 15" of suspension lift and 56 shock absorbers were the go.

Within a couple of years competitive rock crawling was a thing and the aftermarket exploded with parts to add "flex" because flex was the most apparent difference between the "average" 4WD and these crazy rock crawling kids. This was a period where the average jeep might be running a LSD or Detroit locker in the rear, 33" tyres wth stiff springs to keep them out of the guards, and little else. What these rock crawlers could do looked other worldly.

So, the market was lapping these new products. Some of the parts that came out of that period have stayed with us (like high misalignment joints) where others, like hinged Z boxes for the fixed end of the leaf spring, have gone right away.

Drop shackles persist because they are an apparently "bolt on" part that adds travel, and if some travel is good, more travel must be gooder, right?

By now, I think nearly everyone understands that no, more travel isn't better, if it's uncontrolled, (i.e. has no spring rate) or if the geometry is terrible, or if it's only at one end of the car it won't make the car more capable, might even make it less capable, and by now we all recognise that the capability party only starts with twin lockers anyway,

So why does the obsession with travel still exist?

One feasible reason is that posing with your car flexed up while wearing teal slacks will always be cool - The exaggeration of what makes a 4WD a 4WD is the equivalent of stance or oni-kan for the road car scene.

The TL,DR version starts here:

However, I do think more travel can make a more capable, even if it's locked and even if the wheels aren't on the ground.

There are some proviso's here. I'm assuming no obvious flaws - relatively equal travel front and rear, reasonable geometry etc. There's also a big factor - that travel needs to be in droop, not compression.

In simple terms, more droop lowers the car's centre of gravity on the obstacle. This is true even if the wheels aren't on the ground.

I'll explain a little more.

Here's a photo of my car sitting level on uneven ground.



The centre of gravity would normally be assumed to be around gearstick height on a Sierra. That's on level ground and a ride height. However, in this photo you can see how far the drooped wheel has fallen away. In that case, we have at least 1/2 the axle and the weight of the wheel sitting well below their normal position, in effect, pulling the centre of gravity lower, "nesting" the car on the obstacle.

Have a look at this example of Gregc's car on Robertsons. Let's assume the front had enough droop put the driver's side wheel close to the ground. That would be a heap of weight WAY below the car's centre of gravity and that would add stability by itself, but the resulting effect on the compressed side would mean it would be more heavily loaded and generate more traction.



I got thinking about this last time we were at Bill's block, which was pretty sharply eroded in the gullies. Whilst we're all locked and the cars in our little group were running roxxzillas too, the SWB leaf cars just couldn't make headway. Whilst my car has more wheelbase which means it spans obstacles more, It looked to me that the biggest issue was the cars couldn't stay level on the obstacle, so they'd overload the wheels which were on the ground the front ends would go sky high and they'd stop.

Here is a still from a video of my car in a gully. It's fully flexed at this point and the drooped front wheel isn't on the ground, but the body is staying fairly level and the COG is low relative to the compressed wheels, due to the effect of the droop. With maybe half the flex maybe I could get the car up here, but it would be very dramatic. As it is it's hard to tell quite what the wheels are doing unless you mean out and look - the car feels very planted.



The reason it doesn't work the same way for cars with lots of compression is that it takes too much force to compress the spring to get the car to the bump stop. In real world conditions, this leaves the car sitting up high on the obstacle so it doesn't generate the stability. With all the force required to compress the springs, small changes in drive load or angle have big effects on the attitude of the car.

Anyway, just some thoughts.

[mondo2000]

Some well thought out info there.

I thought I'd play devil's advocate to highlight some possible downsides. Droop (or lack of) is one of the tools an engineer has to reduce weight transfer. Consider a hill climb on a steep but smoothish track. Lots of front end droop will allow the front axle to unload and reduce traction. Enter the "winch rope around the front diff" trick to tuck the front end down, reduce droop and gain traction. Another form of motorsport where front droop limiting is common is drag racing. Short shocks or droop limiting chains or straps are used to reduce wheel standing due to weight transfer under acceleration. 4WDs have weight transfer due to incline/gravity when climbing, so front droop limiting will help prevent wheel standing there too. Moving to the rear end, consider a 4wd with lots of rear droop on a steep downhill, this will increase weight transfer to the front, unloading the rear tyres. If the driver makes a sharp turn (either on purpose or accidentally), the lightly loaded rear tyres can lose traction leading to oversteer and possibly an unintended barrel roll down the hill.

These examples may sound extreme but they do happen. Weight transfer can be reduced by lowering the vehicles' fully sprung center of gravity, something every 4wd modifier should keep front of mind. Low cg will offset the potential negatives I outlined above.

[Gwagensteve]

I get your point although I think limiting droop is a bandaid for other issues. In my opinion both the scenarios you describe are attributes of poor suspension geometry (such as stock geometry compromised by lift) and poor weight distribution and/or a high centre of gravity, especially over the axle. (Such as a heavy diesel in a short wheelbase car)

I’m most familiar with suck down winches being used with air shocks to prevent push-off

Roll stiffness, especially in radius arm front suspension, when climbing will tend to take the suspension to near full droop, but again, that’s a problem with the suspension design and limiting droop with a suck down winch is a bandaid. Lowering front roll stiffness, raising rear roll stiffness and reducing weight behind the rear axle improve vehicle behaviour at all times

[mondo2000]

Gwagensteve wrote:

I get your point although I think limiting droop is a bandaid for other issues.

That's true, and why I said it's one of the tools engineers have to reduce weight transfer. Limiting droop is easier for most people than increasing anti-dive/squat by geometry changes, or making large changes to C of G (although most people can and should make small C of G changes which will help.)

Gwagensteve wrote:

Lowering front roll stiffness, raising rear roll stiffness and reducing weight behind the rear axle improve vehicle behaviour at all times

It's dangerous to make blanket statements like that without being explicit about the context. Lowering front to rear roll stiffness in a road going vehicle can lead to the dangerous lift-off > over-steer scenario I described earlier. If someone takes his/her lifted 4wd to a certifying engineer to get mod plated, the engineer will calculate how roll stiffness has changed relative to the stock configuration. The neutral throttle and closed throttle under-steer characteristics must be maintained, otherwise the certifying engineer could potentially be facing some very difficult questions in a coroner's court. Dim-witted motoring journalists are quick to bemoan under-steer, but there is a reason every car has it in spades - it is the safest option.

[Gwagensteve]

You're correct, however, it's also a blanket statement without context to say lowering front roll stiffness by an unspecified amount on an unspecified vehicle can lead to a dangerous oversteer situation. I have lowered front roll stiffness in a number of vehicles and in my opinion the on-road handling has been improved, most markedly in my Gwagen which had a very stiff front swaybar resulting in extreme understeer on wet tarmac and fast dirt and very poor performance when descending off road, to the point I assessed the car as dangerous when descending steeply due to it's likelihood of rolling.

That's also not to forget that both Land Rover and Suzuki added swaybars and/or increased front roll stiffness through geometry because of the "dim witted" public being unable to recognise they were driving an off road vehicle and drive it accordingly. Again, in the case of the Land Rover, swaybars resulted in intrusive and excessive understeer. I owned a 2013 Defender with factory swaybars. I now own a 1993 Discovery which was not equipped with swaybars and it is a much nicer car to drive.

Manufacturers through budgetary compromise, production constraints or very low consumer standards sometimes let terrible compromises through to the market - the SJ80 springs to mind.

So there is some context.

In the case of my Sierra its handling is very good for a vehicle of its type and certainly better than the car in standard form, but we're starting with a very low bar in the case of an SJ40 with drum brakes and bar tread tyres. When someone gets in my car through they are under no pretence will handle like a road car. When someone gets in a 911 GT3RS they are also under no pretence it will handle like a road car.