Suspension limiting straps for autocross?

Has anyone used limiting straps with stock suspension for the road course or autocross? What about with stiffer springs? Does it work or major drawbacks/asking for trouble? For those that may not have heard of these I'm talking about something like a seat belt that bolts to the suspension control arm and only allows it to droop so far. What say ye?

In reply to plain92 :

As a safety device for swing axle cars, yes. However, if/when you reach the end of the strap you get 100% weight transfer to the opposite side, most likely causing it to violently lose traction completely. 

I've run autocross and road race cars with springs that were shorter than the extended length of the shocks without issue. If you're concerned about that happening then just run the springs with tender springs to keep them seated.

If you have a car that understeers excessively, you can use the limiting straps to help rotate (as described above).

I had really thought about that for this year to get the Datsun to turn, but i ended up with bump stop that gave about 1" of downward travel.  they worked extremely well to control dive and roll compared to the setup without them.  The only real issue i could see watching the runs back on video was that it still pulls the nose up like a drag car on soft springs anytime you mash the gas

+1 for tender or helper springs. You don't want limiting straps to actually be doing any limiting unless the car is airborne.

Edit: The reason I say this is that, although it can be used as a way to alter handling, the effect is violent and difficult to control. Longer bump stops might be a better way to do what you want.

What problem(s) are you trying to fix with the limiting straps?

In for a similar answer. Thought about running them after seeing much travel I have on my ford axle before the springs come out of the seats. (Hint- it’s not much. I need taller springs). 

If your springs are unseating when the suspension extends, first you should try tender or helper springs (or even a combination of the two), and only consider limiting straps if those aren't enough. A taller spring of the same rate wouldn't give you any more extension travel at all, but it would take up shock travel. If you just want to take up shock travel, you should consider running spring spacers (outside the spring) instead, or if you have fully adjustable coilovers, you can adjust both the spring seat position and lower mount position upward on the shaft to reduce the available shock travel on extension - although at that point, you're just using the shock shaft as an expensive limiting strap, which accelerates shock wear. If you regularly have your shocks extend fully while driving, that's a case where you should use a limiting strap to preserve the shock.

So how would tender springs work on a solid rear axle? Aren’t the limiting straps supposed to be set just shy of the shock full extension?

Yes the limiting straps should be set just shy of full shock extension. The tender springs work the same on any coil spring regardless of what they're attached to - they hold the spring in alignment with its mounts instead of letting it sit loose and potentially come off the mounts.

I have some on the front of my MGB. It only droops an inch or two iirc. It's like a poor man's swaybar​, only different. One difference is that a stiff sway bar can pick up it's inside wheel while cornering. On a droop limited setup the inside wheel stays on the road because that inside contact patch becomes your roll center.

Yeah I guess what I'm asking is if they'd work instead of helper springs. Not that they'd necessarily be cheaper but adding proper helper springs to a coilover setup adds significantly to the cost. Seems like it could be a cheap way to get a car to stick on a flat track but it might ride too rough or suck in actual practice idk.

In reply to plain92 :

Google "droop limited suspension" and do some reading. I think there was one thread on apexspeed that covered it pretty well. Over on the fsae forum, member "Z" posted some really premo posts about how it works. It's a brain bender to try and understand how it works.

limiting straps are nothing new....  Volvo used them on the 122, and 1800.  I've used them on front suspension as well (mac Pherson strut)

So we're basically talking about two different things here:

1) limiting how much droop the suspension has in order to keep a preload force on the springs at all time, also can be used for keeping parts from over-extending and doing bad things, such as ball joints going past their intended travel.

2) more extreme limiting of droop travel (sometimes so extreme that there is no droop travel at all) to enhance the vehicle's handling.

All of my posts have been in reference to the latter. "Droop limiting [on the front] gives less understeer." I have wondered many times of this would be legal for setup limited street class autocross cars. I've never heard of anybody doing it.

Here are some quotes from Z:

Getting back to the issue of "droop-limiting gives less understeer". Here are two factors missing from the above explanations.

Factor 1.
First the basics. Assume linear rate springs. Assume only just droop-limited, so no preload. Assume typical FF wishbones, so long and almost horizontal virtual swing-arm. Don't think about "roll-centre" (a dodgy concept for independent suspensions). Instead think about line from wheelprint to swing-arm IC - call it the "n-line" (normal to wheelprint up-down movement), or "control-arm-force-line" (as per Mark Ortiz). For the moment forget about ARB's, dampers, and rear suspension.

The droop-limited pair of springs will have twice the roll-stiffness of a normal setup (ie. for given roll moment a normal setup's 2 springs compress/extend "X" and roll angle is 2X/Track radians, while on the droop-limited setup only one spring compresses for roll angle = X/T.) So for given cornering G (and forgetting rear susp.) the droop-limited is stiffer and only rolls half as much. Stiffer should mean more understeer, but half roll angle means less adverse camber change so less understeer. So far this is as noted in above posts.

BUT!!! When the normal setup rolls there is no change in chassis centreline ride height. When the droop-limited setup rolls the centreline ride height DROPS by X/2. This drop in front ride height lowers the slope of the n-lines (= lower RC). Lateral load transfer (LLT) is a combination of spring forces (= elastic LLT), control arm forces (= virtual swing arm, or n-line, or kinematic LLT), and damper forces (= viscous LLT). When the n-lines slope down from the chassis to the wheelprints there is kinematic LLT off the inside wheel and onto the outside wheel. BUT! When n-lines slope up from car centre to the wheelprints (= below ground RC) then kinematic LLT is ONTO the inside wheel and OFF the outside wheel.

So for typical FF wishbones a lower front ride height means LESS kinematic LLT and hence less understeer (from this factor). Lower ride height will also, typically, mean more negative camber on outside wheel, again less understeer. Of course, the car is only interested in the "bottom line". It takes account of all (hundreds+..) factors, their sizes and signs (+ or -), then adds them all to decide whether it is going to under/oversteer. So really you also have to know what the dampers, rear susp., etc. are doing...

Droop-limiting is like fitting falling rate springs (the force/deflection curve bends down to the right). Rising rate springs (curve bends up to right) are great for absorbing bumps, and are often used as a justification for rockers (use linear coils and make rising rate with rocker geometry). Falling rate springs pull the car down during roll, but rising rate lifts the car! A lifted car (higher CG) is a BAD thing, hence most setups that work well are either linear or perhaps falling rate (at least in roll, "third spring" acts only in bounce so can have rising rate). (This is another justification for rockers out the window.)

Factor 2.
A lower front ride during cornering will pull the nose closer to the ground. Formula Fords aren't supposed to be "aero" but a lower nose will probably create some suction under it. Since this is cantilevered out the front of the car it may (?) increase front download AND reduce rear download, hence less understeer.
 

http://www.fsae.com/forums/showthread.php?4047-zero-droop-behaviour