Handling issue with the '66
 

After some spirited driving on the way to XXX this morning. I've decided something isn't right. The front end feels a little mushy and there's too much body roll. The car is running adjustable Koni's up front, adjusted to the middle range, and Bilstein HD's in the rear. The Bilsteins are new and the Konis are about 7 years old (per the PO). It's got stock T-bars, no rear sway bar, and a stock front sway bar. The tires are inflated to 29/32. I just had an alignment/corner balance done. The ride height is 24 3/8 - 23 3/8.

Any suggestions?

Worn front shock (1 or 2) and a broken front SB component?

Sherwood

Bilsteins seem to last forever, while Konis seem to last a few years. So I'd guess it's time to replace the Konis up front.

Also would be wise to check out the condition of the front sway bar arms. It's not uncommon for the clamping portion to get hogged out, rendering them less effective.

Assuming you have close to the stock 25" diameter tires, your ride height is too low for stock torsion bars.

The low height increases body roll in turns, and you don't have the spring rate to resist it. Hence you feel excessive roll.

Plus you have little compression travel in your suspension so you are bottoming out, again the soft tbars are partly to blame.

And your bump steer curve is worse than stock.

And yes the front shocks are likely ripe for replacement. The konis tend to develop leaks.

Raise the car to Euro height, put in new bilstein hds up front, get a good align / corner balance.

How does lowering the car increase body roll? Does this lower the roll center more than the primary ride height lowering adjustment?

Is it because the effective control arm length is greater due to the cosine of the angle of the A-arm (plan view length) and the greater leverage on the torsion bar?

It's due to the roll center height lowering more than the CG height.

To correct this, you would raise the spindles by the same amount you lower the car, keeping the geometry the same. You then need to reset the bump-steer, because this puts the tie-rod and the lower control arm on even less parallel arcs, which induces even more toe change than stock.

and most 15" tires have wimpy sidewalls.

In the front, I can buy the roll center lowering argument.

But in the rear, the roll center ought not go down faster than the body is lowered.

It's on the line connecting the intersection of the swing arm axis and a vertical plane going through the rear wheel centerline, and the rear tire patch center. More specifically, the intersection of said line with the centerline of the car. Which must drop slower than the car body.

Or did I mis-interpret the books? Entirely possible.

At any rate, I'm not a huge fan of lowering - just my own preference. And "euro height 25.5 / 25.0" is better than 1" lower than spec height / actual "euro height", at least for my 88 ...

Going with what burgermeister says, this should lessen the understeer, though the front may indeed feel softer due to the increased body roll.

Lots of good info. Rather than make a wholesale shock/T-bar/ride height/sway bar change, I'm going to do this in stages to get a better feel for what's going on with my car. I never got to drive the car before I re-did everything, so now is the baseline. Please tell me if I'm AFU and going about this all wrong.

Shocks are the first order of businees and I guess Ill try Bilsteins as I don't want to do this again for a while. Pelican doesn't list a Sport Bilstein for a '66 - will the standard Bilstein be a good choice? The sway bar brackets are in good condition and the bar isn't moving around in the bracket.

The tires are 195/60/15 Michelin Pilots and they're 23.5" in diameter. Is the ride height acceptable with that size tire? I haven't noticed any bottoming out and I was on some pretty wavey, bumpy. crappy roads at some respectable speeds yesterday (before dying the death of 10,000 bicycles). I also didn't notice any bump steer. I just had the car aligned & corner balanced a couple weeks ago. The ride height I quoted was off the alignment specs. I measured this morning and it's a little different - front L/R 24 3/4 & 24 5/8...rear L/R- 23 7/8 & 23 5/8. Some settling must have occured.

Noted.

Rich
Approaching this one thing at a time is smart.
Bilstein Sport would be a bit much for your set up, go with HD like the rear if possible and if your Michelin Pilots are Exaltos, they are H rated and have a fairly flexilble sidewall.

David

In the rear, what you say would be true if it were trailing arm suspension.

But it's not, it's semi-trailing arm suspension. The arm does not pivot parallel with the car centerline, but is at about a 12 degree angle to it. Sort of a compromise between a trailing arm, which has no camber gain, and a swing axle which has too much.

Rich, I would definitely start with the Bilsteins up front. Right now you likely have worn out Konis which are hydraulic, (and notorious for developing dead spots) and have no gas pressure, and good Bilsteins in the rear which are high-pressure gas.

Given the short tires, your ride height isn't too bad.

The HD shocks are the way to go, sports are too much for a stock torsion bar on a street car.

Shocks are an obvious step 1, also check the bushings and linkages on both sway bars.

So where is the roll center of a semi-trailing arm?

This is what I understand:

A trailing arm suspension always has the roll center at the ground. A semi-trailing arm (911) has it somewhere above ground (as noted in my previous post), based on the plan view angle of the trailing arm axis. A swing axle is a large angle semi-trailing arm, and usually has it's roll center near the height of the trailing arm axis (thus the jacking issues).

I also thought that a swing axle has the ideal camber gain - the tire always keeps the same camber relative to ground (which is also a strong point of the solid rear axle, again from what I understand, which may not be a whole lot). Most independent rear suspensions loose negative camber of the tire wrt. the road when the body rolls. The swing axle just has too many other issues to end up a good suspension.

Which ones did I get wrong, and why?

I'd give ball joints and tie rod ends a look. My old firebird got VERY floaty an imprecise feeling when the lower BJ on the RH side got loose... and it was floaty and imprecise feeling on a good day to start with. Also, more pressure in the rear tires might help - should increase understeer, and thus reduce response time, as well as reduce roll a bit.

Bilstein HDs are on order and I'll revisit this once I've got a few miles on them.

Thanks everybody, for the help.

During the past week the handling deteriorated even more to the point where it was scary to drive. This morning I replaced the really worn out Konis with some new Bilstein HDs...problem solved.

When I say, "really worn out Konis", I mean the driver's side insert would fully extend when I turned it upside down and fully compress when I turned it over. No freakin' wonder handling was a little vague. The passenger side wasn't much better. They weren't like that when I re-installed them (they came with the car) 8 months ago.

The past week the old car has really come together. What's the old saying - the definition of stupidity is doing the same thing over and over and expecting a different result? Well, my Weber tuning experience pretty much mirrored that. I read everything, tried everybody's procedure, which were all essentially the same, and got the same result every time...a car that pulled well, but spit and popped and wouldn't idle worth a crap.

So, I tried something different - I took it to John Walker. Overnight, my car has been transformed into a smooth running, smooth idling 911. I drove home from John's in 82 degree Seattle friday rush hour traffic yesterday and the car never missed a beat. And now it handles like it supposed to! Old 911 life is good.

Thanks for the advice on the shocks.

Right on Rich!

Did the HD's effect ride height?

BTW, that's "insanity" not "stupidity", not sure which is worse!

David

What adjustments did Walker make?