Zero roll "bump" limit?

[BLS]

I never had a zero roll setup although I bought parts to build one for the Zink before its retirement. I believe I understand it but as I design the new car I realized I am missing something. When both sides are up relative to the chassis (negative camber) there does not appear to be any limit on the cars I have pics of. I understand the shock can act as the limit such that a maximum negative camber is reached when the shock bottoms out. However, if at that point you roll the chassis then all the negative camber could potentially move to one side. What limits this from causing the axles to reach their mechanical limit? Should I have some external limit designed in? Am I missing something?


Thanks,
Barry

[FV80]

Barry,
There have been several threads about ZR rear suspensions .. just search for 'limiter*' (be sure to include the '*') - makes interesting reading .. if you have the time.
As for your question, I don't recall anyone ever being concerned about the NEGATIVE camber travel of the rear (or the front, for that matter)- it's always about the 'positive limit'. The negative limit pretty much takes care of itself in normal operations... the chassis hits the ground or the shock becomes fully compressed. If I had to choose one, I'd take the chassis hitting the ground.

Of course in off course excursions ... or wheel to wheel contact... all bets are off. ONE wheel can go skyward in a BIG WAY and *FAR* exceed it's normal mechanical limits. How to stop that short of putting 1" roll bars around both axles is a real quandry .

Maybe I'm misinterpreting your question ??
Steve, FV80

[BLS]

Steve, thanks for the reply. I think my question might be a bit to poorly worded. I understand the positive camber limit.

I have removed my transaxle in order to make some precise measurements, one of which is just how much actual travel (up/down) there is at the wheels and what the axle angle is at its limits. So, while measuring this and after removing the axle tubes and axles, I started wondering what limits the mechanical movement in the negative direction to prevent the axle from going all the way to whatever its mechanical limit is. I think this is probably the opening where the axle travels out of the gearcase. I am thinking it would not be a good thing for the axle to contact this. So, the total movement (negative) would be limited by the shock becoming fully closed, or bottomed out. In the hypothetical stuation where the shock is bottomed out with equal negative camber on each side there would be no problem (assuming the shock bottoms out before the axle has moved to far in the negative direction). If at the same time this condition exists, if the chassis is then rolled, the total negative camber would then be transferred more to one side than the other (with respect to the chassis and gearbox) causing one axle to get closer to the mechanical negative limit (while the other axle would move further away). Since I had not thought about this I'm concerned that the axle would go to far negative in this situation, perhaps reaching a damaging limit. It sure looks like it can without the axle tube on.

I suppose the axle tube itself provides the mechanical limit, it just doesn't seem like a good one.

I hope this explains my question better. Basically I'm trying to figure out what exactly prevents an axle from going too far negative. I do not see anything that does this in the suspension components. (at least in the pictures I have) Maybe I'm missing something or worried about nothing important. I figured I would ask now. I can see how to do it (by limiting the actuator movement) but I don't see it in any other design. Given that a whole bunch of smarter people than me have designed these things I'm sure I'm missing something. With a conventional setup, the shock bottoms on each side limiting the total negative camber that can develop, but on zero roll there is not the same limit. Perhaps another way to explain it is to think what happens when the shock and spring is removed, what limits the axle movement then?

I'll take a picture and post it to see if it helps understand my question.

Thanks again,
Barry

[BLS]

OK, a couple pics. While taking these I *think* I answered my question. The first pic is the tranny without axle tubes or axles. The second is with the axle tube held in place by hand. I believe we call the round part of the axle tube next to the transaxle the "bell". When the axle tube moves up (neg camber) the bell contacts the tranny before the axle would touch the opening that corresponds to the bell. I guess this is the limit in use on a zero roll car. Nor sure I like it. If I'm wrong please let me know.

Thanks,
Barry

Pic without axle tube:
[ external image ]

Pic with axle tube at extreme neg camber "limit", about 10 degree's:
[ external image ]

[SR Racing]

You are correct, That is the limiting factor. (and you will often see cars that have been in heavy excursions with bent bells, retainers and notched trans cases.) If your beginning camber is right and your spring rate is correct, you aren't going to have a problem on track. Off track, you can bent/break things. Much like any suspension.
Most all of your roll resistance is in the front swaybar. So if you get airborne, etc. a zero roll can take a pretty big offset and you can bend the axle tube bell or worse.

[BLS]

Jim,

Thanks, that is what I was thinking. As you and Steve pointed out "off road excursions" would be the most likely problem. It would seem you could provide a limit before the bell becomes the limit. Might not help much as you'll probably bend something in that case.

Thanks again,
Barry

[hardingfv32-1]

While your concern is correct, the actual chance on the car rolling enough to damage the transaxle are rare for the "average" driver. Fortunately the parts subject to damaged are readily available and easy to replace on your own.

There are more important things to contemplate.

Brian

[BLS]

There are more important things to contemplate.

No doubt about it. Just trying to make sure I understand as much as possible.

Regards,
Barry

[Mystique Racing]

Here is a graph representing dynamic FV chassis roll and rear ride height. This car has a zero roll rear suspension design.

The black trace is chassis roll and the red is rear ride height.

[ external image ]

[BLS]

Scott, thanks. How are you measuring the chassis roll?

[Mystique Racing]

Roll is measured using the front shock potentiometers.

One other note, my rear suspension is probably a bit stiffer then most since I run my car very low.

[BLS]

Scott,

Thank, I should have figured that. A little better than my 30+ year old marked pencils taped to the front beam


Barry

[hardingfv32-1]

Scott

Would you say that the data shows signs of riding up against the droop limiter?

Brian

[Mystique Racing]

Brian,

Yes it does......... Don't they all?

[smsazzy]

When a vee turns in, it is generally on the rear droop limiter by mid corner. This is due to the natural jacking forces of the swing axle.

Whether your car has a droop rod, or just the shock length is limited, that is ultimately where it ends up.

[hardingfv32-1]

Brian, Yes it does......... Don't they all?

Really, I don't see it at all. Can explain where I showed look?

Brian

[hardingfv32-1]

When a vee turns in, it is generally on the rear droop limiter by mid corner. This is due to the natural jacking forces of the swing axle.

Whether your car has a droop rod, or just the shock length is limited, that is ultimately where it ends up.

I had no idea.... So where do you see it on the graphs?

Brian

[problemchild]

Another year ..... deja vu all over again .... for the purposes of this discussion ........

[Mystique Racing]

Really, I don't see it at all. Can explain where I showed look?

Brian,

You know perfectly well where to look. Lay off the champagne my friend.....

[hardingfv32-1]

You know perfectly well where to look. Lay off the champagne my friend.....

I asked a very sound question and now you are blowing me off. I am calling you out. Demonstrate using your graph where the car is hitting the limiter... IF you can.

The one very large downward red spite is a possibility, but that would mean your are way off the limiter on all the other turns. Can you state what you set the droop limit at or is that proprietary?

You have the floor.

Brian

[tiagosantos]

It's the 1st day of 2012 and I refuse to look at that graph for more than 10 seconds, but it appears to me that the downward spikes have nothing to do with this 'discussion'. The spikes are isolated, probably bumps or the effect of turn in, I dunno - I'd have to look at the graph a bit more!

On the valleys of the chassis roll graph though, the ride height graph is pretty stable just a smidge above 0. I'd say the car is riding against the droop limiter on just about every corner.

That said, I should have just asked you to show us where the car ISN'T on the droop limiter

[hardingfv32-1]

It's the 1st day of 2012 and I refuse to look at that graph for more than 10 seconds, but it appears to me that the downward spikes have nothing to do with this 'discussion'. The spikes are isolated, probably bumps or the effect of turn in, I dunno - I'd have to look at the graph a bit more!

On the valleys of the chassis roll graph though, the ride height graph is pretty stable just a smidge above 0. I'd say the car is riding against the droop limiter on just about every corner.

1) Assuming the use of normal sign conventions on the graphs, the red spikes represent droop travel/excursions. With a droop limiter you do not just blow through the limiter and form spikes.... if you are operating on or near the limiter. We should expect some flat valleys when the roll is greatest (black trace). The roll trace (black) in very similar to what you see from a lateral G trace.

I would say the on large downward spike is the car cresting a hill at the exit of a 4th gear turn.

2) Assuming a static setting for the ride height, the red trace should be showing a small amount of squat that falls to zero as you reach the end of the straights.

Brain

[Mystique Racing]

Brian,

Good Job, see I knew you could do it !

BTW, the down ward spices are suspension compressions.

[smsazzy]

When a vee turns in, it is generally on the rear droop limiter by mid corner. This is due to the natural jacking forces of the swing axle.

Whether your car has a droop rod, or just the shock length is limited, that is ultimately where it ends up.

I had no idea.... So where do you see it on the graphs?

Brian

You are reading the graph backwards. The downward spike is the compression leading into turn 9 at Thunderhill. The relatively consistent state that you see on every corner is the droop limit.

[hardingfv32-1]

When a vee turns in, it is generally on the rear droop limiter by mid corner. This is due to the natural jacking forces of the swing axle.

Whether your car has a droop rod, or just the shock length is limited, that is ultimately where it ends up.

1) So this is an exception to your statement? So you think you are compressing going into the turn (uphill and light braking entrance)? Why does the front shock not show any unusual compression?

2) "You are reading the graph backwards." View the scale, I do not think so. Regardless, we really should see two clear range limits for the red trace in you are spending anytime on the limiter. One range limit is when the car is on straights and the other is when it is turns. There only seems to be one in this graph.

Brian