Making droop limiters

[JMortensen]

I looked for clearance of all the front suspension components today but particularly the turnbuckles, put the steering rack on and bolted on a wheel and checked for clearance and there is no issue. The brake line will be close, but will run to the outside of the turnbuckle. I was thinking that I may be able to zip tie the brake line to the turnbuckle if it looks like it might rub, but after checking it out I don't think that is going to be necessary.

 

On the downside I did throw my 15x12 wheel on only to find out that it was apparently bent when the shop mounted the tire on it. It's a Hoosier slick too, and my understanding is that they don't have very stiff sidewalls, so I'm not sure what to make of that. It's a bummer because I had it mounted quite a while ago so I don't think I could take it back and ***** about it now. I know these wheels are thin so that they're light, but I hope this is not a recurring issue...

 

So now I have one DRW in 15x10 with 4" backspace, and I'll need to get at least 3, but probably 4 new wheels since I was thinking of running 15x12s in front and 15x14s in rear. Two steps forward, one step back.

[proxlamus©]

excuse my ignorance..

 

but why use a "droop limiter" ?? and what are some of the advantages and disadvantages?

[JMortensen]

It's a race car thing, and even then it is usually a open wheel race car thing. The idea is to limit the roll by limiting how high the inside suspension can lift. This has the effect of increasing front roll stiffness and makes the car react quicker to steering inputs.

 

There is more information on this thread: http://forums.hybridz.org/index.php?/topic/63492-suspension-tech-motion-ratio-unsprung-weight/page__st__60

 

It's been discussed other times too, might search. Here are some discussions from other forums, mostly about single seaters:

http://www.eng-tips.com/viewthread.cfm?qid=207354&page=5

http://www.pistonheads.com/GASSING/topic.asp?h=0&t=781336&mid=0&nmt=Damper%20Movement%20on%20single%20seater%20race%20cars

http://fsae.com/eve/ubb.x/a/tpc/f/125607348/m/18910968321

[proxlamus©]

how cool!

 

thanks for the info jon!

[fast-datsun]

You can buy fabric limiter in different or custom length for that purpose....Several companies sell them for drag cars......

[heavy85]

I wonder if too much droop is the source of my inside tire wear. I even went down to 0 camber and still see more wear on the inside but that was with relatively soft 300/250 springs. This year I'll be running 375/300 which should help some but not totally if I'm dragging the insides around with excess droop. I added the '0' lb/in helper springs just to keep them from rattling around but there's probably an inch or two of droop beyond the main spring being relaxed on the front end. Rear the spring is just tight at full droop with essentially zero preload. This is on a road course - concrete parking lot event will destroy the outside edge with that little camber ... ask me how I know. I dont want to add any preload just thinking about using the limiters to effectively shorten the strut so that when the spring is completely relaxed the suspension stops going down. Thoughts?

 

Cameron

[JMortensen]

I think it's much more likely to be a toe setting than droop causing that problem. If it's not set for a lot of static toe out, I'd look for some dynamic toe out. In the rear that's going to be bushing compliance or control arm rigidity I think.

 

I've been thinking about this a lot recently, and I've come up with a theory on what exactly is the problem with the Z suspension that makes the droop limiters an advantage. Here's the problem with "excessive droop" as I see it: the struts lean inwards. Let's say for example that you have a high roll center, at least above the ground. One would expect that the outside suspension would want to jack up due to the effect of the tire pushing laterally on the inclined control arm. Similarly the inside suspension should want to jack DOWN because the pull on the tire would tend to level the control arm. This also does not happen in practice. Why? The weight transfer of the sprung mass to the outside compresses the outer suspension overriding the jacking effect on the control arm. On the inside the jacking effect is also overrun by the PULL of the sprung mass on the inside strut. I think the sprung mass is effectively sliding up the strut due to the g forces. If the struts were straight up and down, the lateral load on the strut wouldn't cause any jacking at all. The end result of all of this is that the outer tire should wear more on the outside edge. I don't think it would affect inside tire wear tremendously because most of the weight is being transferred to the outside, and that's where the wear is occurring.

 

I came to this realization as to how the suspension is jacking just a couple days ago when I was thinking about these pictures (the grey car was really the one that got my wheels turning):

 

These don't have droop limiters:

 

 

 

This car has droop limiters:

 

 

The fact that the droop limiters have a noticeable effect seems to vindicate my theory. If the suspension didn't extend past the regular droop of the spring rate and sprung weight, the limiter wouldn't do much of anything, as I believe the yellow car has his limiters set to that length. Dan has his front limiters set at 1" which is similarly very close to the natural amount of droop the car would have and came back with a pretty positive experience after trying them.

 

If my theory is correct then I think it's a pretty safe bet to say strut based suspensions with the struts canted inwards would probably benefit from droop limiting. The more traction they have, the more g's they can pull, the more benefit there will be. If you could somehow cant the struts to the outside the suspension would jack down and there would be no need for the limiters. That's my theory anyway. Feel free to poke holes in it.

 

FWIW I did go back and get some much shorter turnbuckles for the front. If I'm lucky I might be able to use the cables I have, maybe with a spacer if necessary, if not I'll make some cables with the wire rope and eye terminals that I have to mount it all up. I'll have to wait to get the car on it's suspension and see what it looks like before I mess with it too much more.

Edited March 19, 2010 by JMortensen

[tube80z]

I think it's much more likely to be a toe setting than droop causing that problem.

 

Actually I had a similar problem and when I started running limiters and it helped to fix this. The pics Jon has shown plus a number of others were showing the car on the inside edges of the inner tires. The limiters helped keep the inner tire flatter to the road (look at the yellow car example, which was the original car wear all this was tried out). Up until that point I was continuing to de-camber chasing tire temps and was seeing similar wear issues. I would at least test this to see if it helps.

 

Cary

[JMortensen]

Actually I had a similar problem and when I started running limiters and it helped to fix this. The pics Jon has shown plus a number of others were showing the car on the inside edges of the inner tires. The limiters helped keep the inner tire flatter to the road (look at the yellow car example, which was the original car wear all this was tried out). Up until that point I was continuing to de-camber chasing tire temps and was seeing similar wear issues. I would at least test this to see if it helps.

Interesting. You can see the inside is the only part contacting in the pictures, I just would have figured that there wasn't that much wear going on there due to the weight transfer to the outside tire...

[heavy85]

Up until that point I was continuing to de-camber chasing tire temps and was seeing similar wear issues.

 

That was me last year. I bought some cable so testing is now in order.

[zredbaron]

You can buy fabric limiter in different or custom length for that purpose....Several companies sell them for drag cars......

 

Which companies? Google isn't turning anything up for the searches I tried.

 

Also, why is it standard to have rigid droop limiters? I understand that instant engagement leads to instant response just like almost every other suspension component, but I wouldn't think that a tow strap for example flexes enough to be detrimental to performance. I would think that an "ever so slight" flex would be somewhat beneficial because it would be smoother engagement, and well, smooth is fast. I'm probably overthinking it?

 

I'm absolutely willing to accept that rigid is better, I'd just like to fully understand why.

[letitsnow]

http://www.polyperformance.com/shop/Synergy-Suspension-Quad-Wrapped-Ultra-Strength-Limit-Strap-p-47.html

http://www.pacificcustoms.com/beard-limiting-straps.html

 

I used the beard straps on a different project, I had no complaints.

[zredbaron]

Thanks for the links! Now I need to decide if I want to bolt in the upper attachment point or weld it in. I noticed Jon bolted his into the side of the shock tower. I wouldn't think that would be strong enough...? Jon, did you put a reinforcement plate in there that I didn't see?

 

Seeing manufactured droop limiters that are straps only confirms my suspicion that *slight* flex is preferred. I'm thinking for ultra-light, open-wheel cars rigid is fine, but for production cars with 500+ lbs. at each corner, *slight* force absorption is in order.

 

From a previous conversation:

 

I don’t think you should attempt to make them [droop limiters] top out in a soft way. It really shouldn’t matter because the spring will already be unloaded, so they’ll be “soft†anyway. If you’re using them to limit droop in an attempt to get the front end to turn faster, then you want them to be a hard stop as well.

 

I'm not sure I follow Jon's logic. If you were to set the droop at zero the spring would never be unloaded, the limiters would still be bearing the weight at that corner of the car once the tire were off the ground, as it would be holding the spring loaded, right? I thought that was the whole point. If that were to happen suddenly, that would be a lot of abrupt structural stress that would also be felt transmitted to the tires. Bad for wear, bad for performance if you're approaching maximum G loading of the tires doing the grunt work, possible including the tire being limited since it may or may not have left the ground.

 

If I'm barking up the wrong tree, please help me understand why!

[JMortensen]

Thanks for the links! Now I need to decide if I want to bolt in the upper attachment point or weld it in. I noticed Jon bolted his into the side of the shock tower. I wouldn't think that would be strong enough...? Jon, did you put a reinforcement plate in there that I didn't see?

Stick your hand up under the tower and you will feel that there is a formed plate that is welded to the sheet metal underneath, which goes down ~1" on the sides. I'd guess it is .100 or so thick. I have mine bolted to that plate area.

 

I'm not sure I follow Jon's logic. If you were to set the droop at zero the spring would never be unloaded, the limiters would still be bearing the weight at that corner of the car once the tire were off the ground, as it would be holding the spring loaded, right? I thought that was the whole point. If that were to happen suddenly, that would be a lot of abrupt structural stress that would also be felt transmitted to the tires. Bad for wear, bad for performance if you're approaching maximum G loading of the tires doing the grunt work, possible including the tire being limited since it may or may not have left the ground.

 

If I'm barking up the wrong tree, please help me understand why!

I think you're forgetting the damper's effect. If you jump a car with droop limiters, it isn't going to "snap" them tight because the damper will slow the movement of the suspension. So they're already going to hit it somewhat softly compared to a non-damped example.

 

If you're setting the droop limiters so that the suspension tops out when the spring is loose on the perch, then it probably doesn't really matter if they're soft or not when they hit the limiter, but if you're trying to use them to actively limit roll, then you're introducing a small amount of extra squish at the end of the travel, and that muddies the tuning that you're doing.

 

EDIT--What you're doing here is limiting the suspension movement and the roll of the chassis, but the tire will still be firmly in contact with the ground, so any additional shock will be at least partially dealt with by the tire as it acts like an air spring.

 

Like most other things race suspension related, having extra compliance in there isn't going to benefit anything at all, and it's only going to make the transition from on the limiter to not slower. It adds abrupt stress to quadruple your spring rates, but I think you've done that compared to stock. It adds abrupt stress to run camber plates with monoballs, or poly bushings as compared to rubber, etc. Making things smooth is not necessarily the goal of racing suspension.

 

The droop limiters on a 4x4 truck perform their task for an entirely different reason. They are there to prevent stress on the ball joints and in some cases maybe to prevent a driveshaft from disconnecting and falling apart.

Edited April 23, 2011 by JMortensen

[zredbaron]

I think you're forgetting the damper's effect. If you jump a car with droop limiters, it isn't going to "snap" them tight because the damper will slow the movement of the suspension. So they're already going to hit it somewhat softly compared to a non-damped example.

 

I think you're right in that I've neglected to consider that it's still under load while it's encountered, and therefore won't have an opportunity to snap (to the degree I was imagining), even if a pothole were encountered.

 

 

The droop limiters on a 4x4 truck perform their task for an entirely different reason. They are there to prevent stress on the ball joints and in some cases maybe to prevent a driveshaft from disconnecting and falling apart.

 

You're right! Both of those sites were offering parts for buggys.

 

Thanks Jon. I think I've made it obvious by now that I tend to overthink things / fixate on a certain aspect...

 

[mark]

What advantage do droop limiters have over a larger anti roll bar? In the pics Jon posted, the grey car looks like it needs stiffer springs or bigger bars. Aren't drop limiters acting like a large ARB? I'm only on my first cup of coffey, be kind.

[zredbaron]

What advantage do droop limiters have over a larger anti roll bar? In the pics Jon posted, the grey car looks like it needs stiffer springs or bigger bars. Aren't drop limiters acting like a large ARB? I'm only on my first cup of coffey, be kind.

 

Hint: see post #23 of this thread:

 

It's a race car thing, and even then it is usually a open wheel race car thing. The idea is to limit the roll by limiting how high the inside suspension can lift. This has the effect of increasing front roll stiffness and makes the car react quicker to steering inputs.

 

There is more information on this thread: http://forums.hybrid...ht/page__st__60

 

It's been discussed other times too, might search. Here are some discussions from other forums, mostly about single seaters:

http://www.eng-tips....d=207354&page=5

http://www.pistonhea...r%20race%20cars

http://fsae.com/eve/...8/m/18910968321

 

The third link talks about about some of the how and whys.

 

Senior members, correct me if I'm wrong, but here's what I've learned from studying up on the topic:

When a spring is compressed on track, an ARB transfers load from one tire to the other, which reduces the tractive capacity of the pair. An ARB renders independent suspension less independent, because the two wheels are tied together via a torsion bar.

 

Achieving anti-roll via droop limiters has neither disadvantage. Droop limiters are used in addition to ARBs, not in lieu of. If you read up on droop itself there are plenty of articles out there that discuss why droop is bad. It's even worse on the S30's antiquated suspension design because our camber changes so much as the wheel travels vertically (in an arc) due to having a single control arm vice dual.

 

Cary shared this with me. Take a look at this car. It's obviously got A LOT of development time in it. I believe class rules require struts (can't see putting that much time into running struts otherwise). Watch how tightly sprung the car is, then watch the suspension closeup video, and take note of how many times that strut is extended so that the helper spring is unsprung. Every time that helper spring extends, that is body roll that could be avoided with droop limiters:

http://paganiproduct...43a5ff1e72cf8a1

Edited April 25, 2011 by zredbaron

[JMortensen]

I'm going to double post this because I saw it in another thread and think the info should be in both.

 

Strut suspension has a tendency to extend the inside suspension more than it otherwise would as the sprung mass slides up the canted strut, causing body roll. Body roll in a strut car is also camber loss for the outside tire. Some other suspensions like SLA don't respond to the angle of the spring and shock, so they aren't negatively affected in the same way. Struts are not the best suspension, and what we're really doing here is working around the limitations of the strut geometry.

 

If the Z was designed for much stiffer springs, Nissan wouldn't have used so much droop in the suspension. So really what we're trying to do first and foremost is get rid of the extra droop that allows the body to roll excessively which is only there because we've upped the spring rate by a huge amount. That means setting the limiter to stop the suspension when the spring comes loose.

 

There is a potential benefit to limiting the droop even further, but this is where you get to the fine tuning bit, and I would start with your limiters set as described above, and then reduce the front droop until you get the turn in you want. I would bet that you find that zero droop doesn't work that great, as it causes its own problems. I drove in a Toyota truck that had the front torsion bars cranked up to zero droop and it was very uncomfortable and the front end would lose contact with the ground a lot. It might be OK for a Formula Ford that drives only on smooth tracks, but not a good idea for a Z car that has to drive on bumpy autox courses or tracks. I wouldn't limit front droop on the front of a street car beyond the loose spring scenario.

Edited April 24, 2011 by JMortensen

[zredbaron]

There is a potential benefit to limiting the droop even further, but this is where you get to the fine tuning bit, and I would start with your limiters set as described above, and then reduce the front droop until you get the turn in you want. I would bet that you find that zero droop doesn't work that great, as it causes its own problems.

 

Very well. Thanks!

 

Now that I'm rethinking it under a different light, I also would think that limiting the rear droop too much would inhibit the ability to put power down while exiting a turn. Right or wrong?

 

EDIT - I wasn't sure which thread this belonged in either, so I went with the one titled "making droop limiters" despite that this post might be more relevant to the most recent posts of the other thread.

Edited April 25, 2011 by zredbaron

[JMortensen]

Too much droop on the inside wouldn't make it have less traction because of weight transfer, but it would make it have less traction because the tire rolls over on the inside edge. It also makes the outside tire roll over on the outer edge. Body roll = bad is basically the idea. You can crank more neg camber in there, but you get to a point where you're hurting straight line accel traction and braking traction as well.

 

Too much droop limiting basically means that if the surface isn't very flat the back end will skitter around on bumps. I haven't tried it, but I hear you can make the car very tail happy with too much droop limiting.

Edited April 25, 2011 by JMortensen