Near Zero Front Suspension Droop

[260DET]

With Project S130 it looks like front droop will work out around 10mm. The plan is to use a fairly big front ARB with none on the (Nissan multilink) rear, expected power output will be ~450 horses at the back so power down out of corners could be problematical. Anti squat has been reduced to assist.

 

What else, Torsen type LSD, three way custom Proflex derived dampers, static roll centers fairly low at the front, higher at the rear. Car will be used mainly for circuit sprints (time attack) and a bit of road driving.

 

Question is, what can be expected handling wise on the circuit and what may have to be done to improve it. Possible strengths and weaknesses.

[tube80z]

From my experience running reduced droop you will notice much quicker turn-in and shouldn't really have any ill effects. If you try and run a true zero-droop setup you may get some bouncing that can be hard to tune out.

 

It depends on how much anti-squat you have but that may cause problems when trying to put power down on corner exit. It works well in a straight line but not so well when you are turning.

[260DET]

The anti squat has been reduced, it can be played with more if there is a corner exit power down problem.

 

I'm thinking that the car could be 'interesting' to drive in that it may tend to lift a front wheel on an unevenly surfaced road. The rear suspension is softer than the front and has plenty of droop so perhaps that will help keep all four wheels on the ground most of the time.

[JMortensen]

I thought a soft rear suspension (particularly low roll stiffness) had a higher likelihood of lifting the inside front under power.

[260DET]

Its a bit of a balancing act Jon and I'm not sure its going to work.

 

But what should help is the rear roll center being well above ground level and quite a bit higher than the front. That with the stiffer front springs and fairly big front ARB will hopefully keep body roll under control.

 

If there is a problem then I'll look at using a rear ARB. Make sense?

[tube80z]

But what should help is the rear roll center being well above ground level and quite a bit higher than the front. That with the stiffer front springs and fairly big front ARB will hopefully keep body roll under control.

 

If there is a problem then I'll look at using a rear ARB. Make sense?

 

I've seen a few cars setup this way. They work fine but generally have a problem of abusing the rear tires and the car goes away pretty quickly. Make sure you don't get too out of whack with frequencies front and rear.

 

If you have access to a datalogger look at the roll angle of the chassis mid-corner. What you want to see if more or less an equal roll angle. This indicates the tires are used about the same.

 

Cary

[thehelix112]

Correct me if I'm wrong, the instant one tyre looses touch with the road, is the instant the other tyre is then taking 100% of the load, and providing 100% of the lateral force.

 

This doesn't sound like a good way to set things up to me. You'll have shared load, then as soon as the inside tyre lifts the outside will see a massive spike in load, and I'd be very surprised if it didn't understeer like a pig as soon as that happens.

 

There is an article in this month's RCE where Ortiz talks about this very thing, albeit on the rear of a mustang.

 

Not to even start to talk about the case where you go over a sharp rise which results in the front of the car lifting >10mm.

 

I foresee severe badness, but I am often wrong..

 

Dave

[Clifton]

I lift my inner front when under power. Pretty neutral, just a tad of understeer. If I take any more push out it's too hard to drive fast. The inside doesn't see half the load. I've seen pics of JohnC's car lifting a front too. I wouldn't want to lift a rear though, unless it was front drive.

[tube80z]

You're mistaking suspension droop with tire load. Just because you run reduced droop doesn't mean the tire is being pulled off the road. It's quite the opposite. At least if you're running spring rates to properly support the platform.

 

What happens when a spring is unseated versus still having load on it? Reducing droop in this case keeps a set amount of load on the tire. What happens is you top out the shock and now the tire has to dampen all road irregularities, but it still has load on it.

[thehelix112]

Clifton,

 

I should've been more clear sorry, I wasn't talking about just under power.

 

tube80z,

 

So what I'm picturing is Richard's car rolling, the top of the inside suspension moving both inward (relative to centreline), and upward (relative to road). If the upward component of that movement exceeds 10mm, I don't see how the tyre is still on the road.

 

 

Dave

[260DET]

I've seen a few cars setup this way. They work fine but generally have a problem of abusing the rear tires and the car goes away pretty quickly. Make sure you don't get too out of whack with frequencies front and rear.

 

.................................

 

Cary

 

Yeh, I was reading an interesting old post of yours Cary where you mentioned minimumising torsional loads through the body.

 

The original reason for the lower rear spring rate was to counter the anti squat effect on the rear suspension, so rather than adding a rear ARB it may be better to increase the rear spring rate if the rear tyres show signs of distress.

[tube80z]

Correct me if I'm wrong, the instant one tyre looses touch with the road, is the instant the other tyre is then taking 100% of the load, and providing 100% of the lateral force.

 

Yes, but only for that axle pair. Often before the tire is off the ground it may have almost no net load on it anyway. So even on the ground it wouldn't be contributing.

 

This doesn't sound like a good way to set things up to me. You'll have shared load, then as soon as the inside tyre lifts the outside will see a massive spike in load, and I'd be very surprised if it didn't understeer like a pig as soon as that happens.

 

See above. Well before the tire comes off the ground the most of the load has already transfered to the outer tire. If you pulled a loaded tire off the ground then I could see this happenning.

 

There is an article in this month's RCE where Ortiz talks about this very thing, albeit on the rear of a mustang.

 

I've really been waiting to get this issue because I wanted to see what Ortiz had to write. I've read the article about ten times and I still don't get it. He doesn't really answer the question, talks about dirt cars, and then says something with no real proof.

 

Keep in mind I'm not an expert, I play in parking lots with traffic cones, and occasionally run up mountain roads but here's what I know about reduced droop.

 

If we have a car with 500 pound springs, 500 pounds of corner load, and a motion ration of one, what happens when you set the car on the ground? I think the spring compresses one inch. What happens if I lift the car anything over an inch? I think the spring becomes unseated and there's no load on the tire (I'm not counting the unsprung weight in this case).

 

So in the case of reducing droop to less than an inch I'm keeping a set amount of load on the tire. This is very similar to how the stock suspension works by having a soft rate and a spring the is longer than the available space and it's pre-loaded. I'm just using a lot less space.

 

So here's what I think happens. Droop limiting either via pre-loading the spring on the strut, or using a mechanical stop allows the car to accept lateral loading and roll a specific amount before the inside spring tops out. At that point there is a certain amount of vertical load still left on the inside tire. For any further load transfer (and the necessary increase in lateral loading) the actual roll center moves to the center of the inside tire contact patch. For any further roll (from increased lateral loading) the car now pivots about that point, and lowers itself. That lowering in turn decreases the jacking effect. Roll stiffness from the springs and bars is not changed at all, so the net effect is a decrease in roll stiffness (or at a minimum, no further increase)

 

When I started running stiffer springs (400 to 500 pound range) it was obvious when jacking the car up the springs would unseat with very little droop. I noted the fender gap when this happened. I then looked at pictures I've taken of the car under high lateral loads and saw that sometimes the gap appeared to be bigger than just having the spring unseated. I remembered back to playing with formula cars running droop limiters and figured why not try this on a tin top. My original idea was that it didn't do any good to allow the suspension to droop anymore than the spring becoming unseated, which was a little over an inch (lets say 30 mm).

 

Running the car this way it worked better. I didn't see tires come of the ground and more importantly the rear tire temp were more even. Previously they looked to have too much camber. So then I played with this some more. I tried reducing droop at both ends to see what happened, then one or the other. I found a way to make a killer drift car (not what I wanted) and a way to get quicker turn-in and less roll, which was what I was after.

 

Pictures of the car after it was droop limited have a lower roll angle. I think this is attributed to the car no longer being able to jack the inside up off the spring seat because it now has to overcome a specific load. It's sorta like running a larger swaybar and the car works better. Theory tells you that's wrong but sometimes it works anyway. In this case I think the tires are held flatter to the road.

 

Cary

[260DET]

Yeh I was thinking just the other day that having near zero front droop would be similar to having a very stiff front ARB, in that both strongly limit any variation in droop between the two front wheels. In fact the 260Z I had was for a while fitted with a very stiff front ARB and round a corner near home that cambered left then right, the front wheel would come off the ground. You could feel it and hear a mild thump as it came back down to make contact with the road.

 

Another point is that once an inner wheel reaches its droop limit in a corner, its spring is no longer tending to raise the inside of the car up, so adding to body roll.

 

Anyway the front suspension is now setup for near zero droop so we'll see how it goes, once the rest of the car is finsihed.

[Armand]

Sorry if I'm jacking your thread, but how would you go about limiting droop on a 240z with stock suspension components?

 

This might sound stupid but I used to race on-road r/c cars very competitively for 3 years then switched to off-road. These cars were no more than a foot long and traveled 0-60 in 3 seconds. Everything on the suspension had to be precise. One of the best changes I personally did was limit the car to zero droop on all 4 corners. It might sound stupid to relate an r/c car to a real car, but it really does work. This applies to any part of the suspension. Changing caster affects turn in/out. Toe and Camber makes the same difference as it would on a real car.

 

So if there is a way I can limit droop with the stock suspension that would be great. Every time I jack up the car I cringe at the amount of droop the car has, it's ridiculous. When I drive my brothers 03 Mini Cooper S I know a good part of the reason why its so responsive is because the droop is limited a pretty good amount from the factory. When entering driveways at a slant the inner rear wheel lifts off the ground about a foot.

[tube80z]

Cables between the strut tower and the lower control arm were used. Adjustments were done via turnbuckles. Very crude but it worked.

[johnc]

Konis, Bilstiens, Penskes, Ohlins, etc. can be built with internal droop limiters. Typically its aobut $150 per shock but its less if its done as part of a revalve/rebuild.

 

EDIT: You can accomplish the same thing by shortening your struts more then normally recommended. Instead of doing the math to get the shock in the middle of its stroke at static ride height you do the calcs so that stroke has 75% bump travel and 25% rebound travel at static ride height.

[260DET]

Is that what you suggest John, 75 bump, 25 rebound, for a motorsport orientated car?

 

Anyway I'm thinking more about the effect of the ARB on front droop, particularly as with project S130 its going to need a fairly big one. Which will be firmly mounted, not like those wussy S30 ARB mounts.

[thehelix112]

Cary,

 

I only just noticed you replied, sorry.

 

Thats a very interesting theory, about the roll centre moving to the outside wheel.

 

It seems to me that what we're not considering, is the longitudinal weight transfer. During turn-in, you are going to have more than 500lb sitting on the front inside, so it will take more lateral load, hence roll, before it lifts, because the height its rolling from has been reduced. During exit you will have less than 500lb, so the suspension will be close to full droop anyway, hence you see cars lifting the inside front.

 

I need to do some work, but maybe food for thought. The idea of the RC magically changing is a bit far fetched for me, but I try to remember that RCs, ICs, and all the lines people draw all over their chassis sketches are just there to try and help us understand how things work. They don't actually exist.

 

Dave

[tube80z]

Thats a very interesting theory, about the roll centre moving to the outside wheel.

 

It's inside front, if it says otherwise that's a mistake on my part.

 

It seems to me that what we're not considering, is the longitudinal weight transfer. During turn-in, you are going to have more than 500lb sitting on the front inside, so it will take more lateral load, hence roll, before it lifts, because the height its rolling from has been reduced. During exit you will have less than 500lb, so the suspension will be close to full droop anyway, hence you see cars lifting the inside front.

 

When you droop limit you will have the amount that the spring is pre-loaded. To get the tire off the ground you will need to transfer more than that, which is possible if the car will bicycle. My car won't lift the front tires on acceleration even when I have a true zero-droop setup. I think what you typically see are cars that are fairly soft that allow the chassis to move enough to get a tire off the ground. The weight transfer happens regardless of the roll angle.

 

I need to do some work, but maybe food for thought. The idea of the RC magically changing is a bit far fetched for me, but I try to remember that RCs, ICs, and all the lines people draw all over their chassis sketches are just there to try and help us understand how things work. They don't actually exist.

 

Movement of the geometric and/or forced based roll center standard in most any suspension. If it helps to think of it how do you calculate the RC in a suspension where one wheel is fixed and can't move. That's what we have here when the limiter tops out. And the change in ride height (net up/down) can be measured with a data logger.

 

I don't know if I'm helping or hurting at this point. All I can tell you is this is a trick I use and it works well for me. It's not something everyone should do without understanding why. I have lots of pics of the people I run with and all the cars running stiffer spring packages keep their tires on the ground. All of the softer cars are running around with tires off the ground and many of them have loads of droop.

 

Cary

[johnc]

Is that what you suggest John, 75 bump, 25 rebound, for a motorsport orientated car?

 

No. That was just an idea for a poor man's droop limit setup using shocks as the limiters. I'm still in the camp of allowing 50/50 on bump and rebound travel.