There seems to be a (small) movement towards the use of drop shackles at the moment. Whilst some of us have been around long enough to remember the first time these weren't a good idea, not everyone has, and that allows them to start to look like a good idea again. They are no better an idea now than they were in the late 90's. Here's a little guide to where they came from and how they work.
What is uncontrolled travel?
Uncontrolled travel is any axle movement that’s unsprung. It’s typically a result of fitting drop shackles or long shocks that allow coil springs to go loose. It could also include Z boxes or other means of having the chassis mounted end of the leaf spring swing down away from its normal position. These are less common though.
A brief history of uncontrolled travel
Range Rover owners have been dislocating their rear springs forever. Coil sprung cars are designed for a certain amount of wheel travel and it’s difficult to get a long enough spring into the car to deal with the stroke of long shocks – the compressed length required means there’s limits on the extended length possible.
When the big flex boom happened in the US in the mid 1990’s, lots of people were trying ideas to increase apparent wheel travel. Teraflex (trading under a different name at the time) released the “Revolver” shackle. These didn’t just fold, they pivoted too. The reasoning being that spring twist is also an impediment to travel.
In the late 90’s there was an explosion of different approaches to the same end, both home made and commercially available. There are also “Z boxes” which appeared around the same time and attached to the front end of the rear spring.
The theory
Obviously, wheel travel is good. Keeping the wheels in contact with the terrain means they can (potentially) drive and steer the car – after all, a wheel in the air can’t. When the spring reaches the end of its travel, rather than dangle the wheel in the air, it can fall away so the tyre stays on the ground. Longer shocks and brake lines are generally required to exploit the extra travel.
The problem
When these drop shackle equipped cars started appearing on the tracks, it became evident they didn’t perform as well as the added travel would have implied. Sure, they looked great parked on an obstacle or ramp, but on off camber terrain, or under power, or on steep descents, they were a hinderance.
Why is that exactly? Surely more travel is better than less travel?
Yep, sure is. But uncontrolled travel isn’t like normal travel. There’s no spring rate associated with the last stage of travel – It’s uncontrolled. Wheel loadings are low, but in a completely different way to a normally sprung wheel –
When a sprung wheel starts to compress, there is always spring rate forcing the wheel down. It won’t start out as very much, but it quickly increases. When a wheel with uncontrolled travel starts to compress, there’s no spring rate, the wheel is sitting basically under its own weight and the weight of ½ the axle. This isn’t very much weight, and therefore doesn’t add much traction.
Surely some traction is better than none though?
Well, maybe…. Or maybe not. Put that lightly loaded, unsprung tyre up against an obstacle and it doesn’t have enough traction to make the tyre climb the obstacle, so the car pivots around the drooped wheel. Get on the power and the drooped wheel now has no spring rate to push it onto the ground, so it can start to bounce up and down, starting a process of flapping and bouncing that’s often called “pig rooting” So while sometimes the small amount of added traction might help, it’s not making much difference to forward motion.
And then there’s the downsides. Put the car on a big compound angle and the uncontrolled travel can’t “pull” on the car to even out a side angle. The weight of the the tyre and ½ the axle isn’t a lot to add traction, but it’s a lot of ballast when it’s helping to keep the body level. Last time I had my car out, I had a shock come loose as I was heading into a side angle. The effect was like having drop shackles – the axle could fall another 4” or so, unsprung. And my car nearly fell over in a spot I’d driven easily earlier in the weekend. The body flopped over.
This was ugly. Sure, I could have driven out of it, but the car didn’t need to be on this spooky angle and with less (and 100% sprung) flex the car didn’t take on the radical lean. It’s the same story when climbing.
Real world example:
Even though my car is coiled and in these photos the springs didn’t go loose at any point, I had poor side angle performance and flaky drivability. So in my last rehash on Piggles, I significantly reduced rear flex.
Here it is in “maximum hektik” mode:
And here it is with reduced travel.
Guess which is better to drive? – Bingo, the “reduced travel” format – the body sits more level and there’s no loss of capability. That’s because the front is being asked to work harder, as the weight of the lifted rear tyre and ½ the axle is levering on the front springs. With the rear flexing harder, the front doesn’t work as hard and so the body takes on more radical angles. Which then encourages really unequal wheel loading.
So, more travel doesn’t automatically mean more capability. It has to be the right sort of travel – sprung travel.
¾ elliptic
¾ elliptic is offered as a better alterative to drop shackles. And it is – the travel is all sprung, the effect is like having a longer, softer leaf. It’s not entirely without problems though – the car can still get up a bounce, and depending on how the ¾ leaf is setup, the car can ride on it rather than the normal springs, kind of sitting the car up and making it feel a little loose.
Could a car be built to work functionally with uncontrolled travel?
Yes. But it’s a huge, huge job, much harder than building a car with conventional travel. Here’s a primer for the curious:
Wide, heavy diffs.
Wide for stability, and heavy to provide traction.
Very narrow bumpstop spacing – the fulcrum effect.
Narrow bumpstop spacing can transfer some of the force being applied to the compressed wheel across to the drooped wheel. This effect is often overstated in relation to drop shackles. The bumpstops (and springs) really need to be a long way inboard to this to be a meaningful effect – Sierra front spring spacing, with the bumpstops under the chassis is a good guide
The axle needs to be controlled
Basically, the axle needs to be linked to prevent the bounce or hop that occurs in the range of uncontrolled motion. A leaf sprung axle with no spring rate has nothing to locate it.
Very limited compression travel.
For the compressed wheel to transfer load to the drooped wheel, it has to be on the bumpstop. No load transfer can occur until this happens. That means the car needs low roll stiffness, and to settle onto the bumpstop very quickly. This isn’t how most people want their cars set up.
For low speed crawling, it might be OK, but get on the power or try and carry some speed, and it’s not the hot ticket. Nonetheless, put all those factors together and you can achieve some incredible things. But I’ll stress, it makes the car more one dimensional than most are willing to put up with.
Here’s another example of car that’s effective with uncontrolled travel, but again, it’s a pretty one dimensional car. It has almost no compression travel, and such heavy and wide diffs the unsprung weight probably goes close to exceeding sprung weight. You’ll also note in this photo the front isn’t flexing. That’s because the rear has lower roll stiffness than the front, so the front can’t work until the rear is out of travel , but at that point, the body has flopped over so far that load on the high side front wheel is negligible and can’t defeat the roll stiffness of the front suspension.
So anyways, thanks for reading. I hope it’s helped to explain some of the stuff that’s going on in relation to uncontrolled travel.
Steve.
It doesn't matter so long as they're monster green.
