Would a Watts linkage work any better than a Panhard Bar?

I'm not sure I follow the logic here. The diff doesn't seem to move around side to side in the body of the car, it's just the back end of your car is all over the shop, particularly on entry and mid corner. A Panhard won't correct oversteer problems.

You say you were on old tyres. That's probably the best place to start. But other things to look for from CARROLL SMITH’S CAUSE AND EFFECT GUIDE:

Corner Entry Oversteer
• Excessively heavy trail braking
• Excessive rearward brake bias
• Severe rearward ride rate/roll resistance imbalance
• Rear roll centre too high
• Diabolical lack of rear download
• Severely limited rear droop travel
• Broken or non-functioning outside rear damper
• Broken or non-functioning front anti-roll bar

Note: A slight feeling of rear ‘tiptoe’ type hunting on corner entry can be due to excessive rear toe-in or excessive rear damper rebound force.

Mid-corner (mid-phase) oversteer
• Driver threw the car at the corner to get through initial understeer – only cure is to educate the driver and/or decrease understeer
• Excessive rear tyre pressure
• Excessive relative rear ride and/or roll stiffness
• Rear suspension bottoming in roll
• Insufficient rear droop travel (non droop limited cars only)
• Very loose rear anti-roll bar linkage

On the topic of Watts or Panhard. Watts is superior. Panhards are biased to cornering in one direction depending on which side the low end is at. You get slightly different traits in left and rights. Watts linkage is better for that, and allows better adjustment of roll centres as well.

Tom,
I run urethane (polyurethane) in all the suspension bushes, front and rear.
I'll scan the shock dyno and send it though after work.
It seems to me the car doesn't want to settle on turnin. What we can't see from the video is if you are correcting the issue or adding to it via steering input.

On a side note, maybe something to generate some discussion and something Ive been thinking about lately.
In roll, is there a sensible limit to the amount of rear body roll with leaf spring cars? Should total rear roll angle be a key setup value for racing with leaf springs.
If the tyres generate more lateral g (as they do in modern entry racing e.g A032 v A050s), this would change the required rear roll stiffness to limit roll angle. In turn this is balanced with a change in front roll stiffness.

Harry, I'll quote "rear end all over the shop" it does appear to be on a set of casters. As for the diff moving around side to side, it is hard tell but it would appear it doesn't. This begs the question of how much lateral deflection before said hula starts; 10mm 25mm? No way to see that in the video. If the car just simply oversteered then we would not be a having this conversation, notice that trailing the brakes settles it down. It is the oscillation that is the issue; 2/3rds of the way around turn 1 I am on the gas and starting to unwind the wheel and this is where it is at it's worst. The part where the car has a minor wobble is my input, I got on the brakes to avoid contact and had to wind on some lock. I believe it is likely the axle moving but it could turn out to be something else hence my taking it over to my buddies shop............while I have all of the Smith books, I am the first to admit I'Z be driver and ain't no N-ga-near..........My understanding is enough to adjust things but not to fab them. We may put a Watts linkage on it but not until everything is checked.

Bryan as mentioned above; trailing the brakes into the corner the car settles nicely. The middle corner bobble is driver input, my sawing at the wheel and not something I would normally do. It's the uncontrolled Hula which steady with steady throttle and builds as the corner continues and the throttle load is applied. Notice on the 90 degree right after the short shoot I over rotate the car and have to wind on opposite lock coming through the apex but there is an oscillation after the wheels are straight again.

Here are a couple of pics from the event.

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Are all the bump stops the std length? In particular the bump stop directly above the diff centre. Could the car be low enough combined with cornering loads to be pivoting on the centre bump stop. The limit of tyre adhesion occurs and the car slides stc. Looks like the car rolls to a certain angle then locks out and slides, grip is then retained and the cycle starts all over again.

reallly that video is far too short to see what the car's true behaviour is. Start of the race, I assume on cold tyres, and maybe on and off the gas running at close quarters with the other cars in Turn 1. The only real issue seemed to happen in the middle of the 1st turn. Exiting that turn and all through turn 2 was just smooth consistent oversteer. Turn 3 is a non event. Turn 4 you get loose on entry trailing the brake a bit, but I don't see the oscillation after that. But whatever causes the oscillation evident in turn 1, it's not the absence of a panhard bar.

my first question is about your old tyres. just how old are they? You could be chasing a suspension problem that doesn't actually exist.

did it stay the same through the whole race, or get better/worse?

has it always handled like this? or just started displaying this problem?

did you do the full competition suspension manual setup with traction arm and sway bar?

The diff moving laterally under load is not an issue in itself. Any leaf spring setup without lateral location will do that. My old 1200 used to do it noticably. It won't cause this loading and unloading unless something else is now making it bind, like a traction rod or a swaybar clamped to the springs...

I,ll be following this with interest,some very worthy points,if i did,nt know better i,d say you have a failing front bush in a rear leaf spring,Harry,s questions will shed more light,either way it looks a bit scarry

Harry, you are correct, the video really only shows that some of condition exists. I will attempt brief answers but will likely prattle on as usual.

Tires were six months old, I’ve run a cooked/really old set at a track day and these were not that far gone.

There is less oversteer as the tires heat up but the oscillation is still there just not to the same degree and it was pretty consistent throughout the race.

The oscillation has been there but I do not believe to this degree; with the wider track the corner speed is higher. “Displaying” the issue is the operative word. Previous video clips show the car moving around 4-5 inches (more like McGee’s car) but not 10-12 inches. On my admittedly subjective scale of 1-10 with the video being a 5-6 I’d call what it did before a 4.

Rear suspension is Nissan Competition Springs, with traction bars, 5/8 (16mm?) sway bar. The rear bar bushings attach to the frame with the bar ends clamping to the springs. The sway bar has been on the car since day one.

Benny’s question on the bump stops would have been my train of thought but the axle snubber has been removed, the bump rubbers on the shock are not bottomed. As previously mentioned I raced a stock class car which cornered on the bump rubbers and this behaves somewhat similar but not quite the same.

My thought is that there is a cyclical wind up and release like Benny mentioned. If the axle deflects say 10-15mm how much rear steer does this cause and would the axle movement create this cycle? The bushings are Polyurethane except the aluminum ones for the front shackle to frame mounting point.

I want another set of eyes to inspect everything, so it’s going to my buddies shop, it’s obvious the car can be improved.

Bert as for scary part, as someone who used to road race motorcycles and still does some desert and motocross, this rates about .09 on my admittedly skued fright scale.

If you go to minute 6:50 to 7:30 of this video you can kind see how the car responded before.

https://www.youtube.com/watch?v=9NE5zGOs76A


Tom

lol, 6mth old tyres are brand new! I thought you said you had old tyres?

Yes, bump stops binding will do something like this too. You can smear grease on the surface they strike to check if they hit. Are you not using bump stops in the standard location above the diff?

Other things binding suspension movement will cause it too. You could try disconnecting the traction arms to see if they're the cause,but it sounds like it was doing it before they were fitted.

Quote:
got a picture of this? sounds like a pretty unusual setup.

what angle are the rear spring hangers on. The lower end isn't further towards the front of the car than the top is it?

For solid rear axle location, I like the Mumford link the best. It allows you to set your roll center wherever you want. By changing the angle of the outer links, the roll center is where the links would meet in in middle. This could even be below the ground if you wanted.
I have not found it necessary so far. My car has Delrin bushings at the front of the spring, and Urethane on all the other mounts. Koni Single adjustables. It pulled just over 1G at the Streets of Willow Skid pad on Toyo R888's 225/45-13's No noticeable rear axle movement.

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