New Superformance darting L or R at only 40mph.

[Neil]

Years ago I built a small sports racer with a tube chassis, 140 BHP Corvair engine, and a Porsche 914 transaxle. It weighed only about 950 lbs. When I moved out of my rented workspace, I had my wife tow me home in the car. Running it on the road was risking attention from the police so I used about 70 feet of nylon rope to tow me home. At low speed. things were fine but over about 30mph, it exhibited exactly the behavior you described..I had not aligned the suspension- only "eyeballed" it to what looked to be OK. After measuring it later, I found that the front has over 1" of toe-out! Correcting that solved the problem entirely.

 

[Neil]

Minus the fiberglass body...

 

[James Shaw]

Thanks for all the responses. My tire pressures are 25psi fronts and 27 psi rears. When I realigned the car we set it as:
Front toe 0.18" in
Camber 0.6 degree neg
The caster is not adjustable.

Rear toe 0.12" in
Camber 0.1deg neg (this was 0.1 pos on right side and 0.3 neg on left side)
After the realignment I could tell no difference in the handling.
I haven't been able to measure caster on the rear but it seems roughly 6 - 8 degrees pos. I'll try to set up a way to measure it exactly so I can make sure both sides are equal.

As far as bump steer, I have 1/4" toe at full compression, and 0" at full lift with the springs removed - which seemed pretty good to me.
I was also wondering about corner weights. Right now I don't have a set of scales to measure them. I'm only going by the compressed spring length (which I know can still be off).

I’d be amazed if caster isn’t adjustable, but you may have to use washers to move the a-arms fore/aft.

I’d add camber too but what you have shouldn’t be showing this behavior.

time to check everything again!

 

[Jim Kreber]

Ok, so reading everything I can find and listening to you and others, I think what I'll do is just keep dialing in more rear-toe in and driving down the street until/if I feel a difference. The weather here in South Dakota is turning and I won't have much time before snow, but I'd like to try some things. I'll take it into my alignment shop in the spring. The shop owner has been racing for a lot of years, and although he's never worked on this type of car, I trust him to be the best available with the best equipment.
Thanks again everyone!

 

[DaveM]

I’d be amazed if caster isn’t adjustable, but you may have to use washers to move the a-arms fore/aft.

I’d add camber too but what you have shouldn’t be showing this behavior.

time to check everything again!

Just like an original GT40 caster is not adjustable on SPF's. There is no space in the chassis mountings to use washers. I don't think caster is a problem as it is 6-7 degrees on SPF. Unless the car has frame damage I would suspect toe out front or rear.

 

[Lee Patterson]

For what it's worth, since they actually wrote a very detailed assembly manual with specs, ERA specified +6 degrees of front caster and allows for shims to be inserted front and back to achieve that. They spec 0 degrees camber and 1/8" toe in. Tire pressures are 22 front and 25 rear.

 

[Walter]

Years ago I built a small sports racer with a tube chassis, 140 BHP Corvair engine, and a Porsche 914 transaxle. It weighed only about 950 lbs. When I moved out of my rented workspace, I had my wife tow me home in the car. Running it on the road was risking attention from the police so I used about 70 feet of nylon rope to tow me home. At low speed. things were fine but over about 30mph, it exhibited exactly the behavior you described..I had not aligned the suspension- only "eyeballed" it to what looked to be OK. After measuring it later, I found that the front has over 1" of toe-out! Correcting that solved the problem entirely.

Yes, excessive toe-out is where I'd look first. Particularly when the pulling is not to one side only. With excessive toe-out the car wants to steer ``downhill": to the right/left when the road surface slants to the right/left.
This presumes a sound steering rack (mount) and suspension linkages. Note that you need good leverage (eg. via a crow bar, big pliers etc. ) to check for looseness/play. Just using/wiggling with/ your hands won't do the job.
Loose suspension/steering could increase toe-out under driving conditions.
( Personally, I like slight toe-out at the front (instead of toe-in)- I avoid autobahn driving as much as I can and like the improved turn-in behavior on twisty roads or the race track. One gets used to a little bit of darting - keeps up one's attention)

 

[Julian]

This is very reminiscent of my journey with my ERA when I purchased it, almost undriveable initially. It considerably improved once I went through a detailed setup with corner weighting and changed shock spring rates. On my wide body Pantera (with 10" front, 13" rear width tires) I was floored on how much a different brand tire reduced it's prevalence for tramlining.

The SPF platform is well developed, so your issue should not be unique, if everything is dialed in I'd be tempted to try a different tire, especially if the low miles area result of it having previously sat for long periods. Is the steering wheel rigid in your hand or does it react through the steering wheel and a white knuckle ride having to hold on?

 

[Jerry Irwin]

if you have wrong offset on wheels such that the kingpin intersection point to the contact patch of the tire is far too inboard.
perhaps....??

 

[Doug Dyar]

I can't determine from all the comments as to what type of Avons you are using.

I have two sets of wheels and tires for my Superformance MKII. One set has Avon radials, and the other has Goodyear Blue Streak bias ply race tires. On the smooth asphalt of a race track, both behave similarly, although the soft compound of the Goodyears make them the better choice for track duty. But on the street, with rough concrete pavement having various joints and surfaces, the Avon radials are far superior. They drive much better, without them following the pavement joints and ridges. The Goodyears require a firm grip on the wheel and the car will dart back and forth unexpectedly.
Are you by chance running bias ply tires on your car?

 

[Phil G]

PM Sent.

 

[Jim Kreber]

Yes, they are track bias ply tires. And Superformance is certainly a well developed chassis. So I found the right rear (drivers side) tire toed out over 1/2" and left rear toe in 1/4", so 1/4" total toe out. I adjusted the rears as close as I could in my shop to 0.2" total toe in and equal on both sides. It was 35 degrees out yesterday, so I took a spin and it handled WAY better, so at least I know I'm headed in the right direction. As I said earlier, I'll get it to my favorite alignment shop in the spring. Meanwhile if anyone has found the optimal street alignment settings for SPF using 13" rear and 8.5" front wheels, please let me know. The shop manual for my 2006 Ford GT says 0.11 degree toe in on each side.

 

[Lee Patterson]

So then did the rear alignment that you had done come loose? If you search the forum, there are several threads on the SPF alignment specs people have used including a PDF showing where and how to make the adjustments.

 

[Doug Dyar]

One thing I've encountered over the years is that, when doing alignment on a race car, both sides must match. Alignment specs are usually given as a range. Most shops set the alignment to be within the range. On a street car, as long as they're close, this is usually acceptable. The crown of the roadway is also taken into account.

On a race car, I've found that care must be taken to ensure that both sides are not only within the range specification, but equal on each side. Both fronts must match as well as both rears. And the track surface is assumed to be flat, without crown.

 

[Dr. Bob Woods]

I had a similar problem with my Tornado. It turned out that I had some slop or play in my steering system. It came from my removable steering wheel and my double u-joints. There might have been some play int the rack as well. If there is no play in the steering system any ruts in the road can cause the tire to put a force back through the steering system. Normally, the driver holding the steering wheel will prevent the tires from turning to follow the rut. If the steering has some play in it, the tires are free to follow the road and steer its self. This can go back and forth as the tires go in and out of the rut. This happened to me at about 110 mph and was basically terrifying.

Most of my play was from the removable steering wheel. I put a wing screw in the steering wheel to grip the hex mount and that removed most of the problem; however, I haven’t driven that road again at that speed but I have at lower speeds. It is ok now.

Bob Woods
Tornado GT40 in Texas