Front end woes.

[Farmer]

While we are discussing varius details on the front end, I am having some trouble getting the right adjustment on my 73 SB.

It is lovered 55 mm in the front & 22 mm. in the rear. I machined my 23 mm Kerscher sway bar so that I got the right caster (2,9 Degree.) I have 6 X 16 fuchs ET 36 w. 5 mm spacer, 205/50X16 Dunlop Sp 9000.
When the suspension sits where it "likes" to be, I have -2 camber. So in order to compensate for that, I adjusted the toe 2 degr. out.
Then the car is terrible at normal speeds, but resonnably steady at 100 mph.
When I squeeze the camper in about to where it should be, (I can only get about +0,5 degree) and adjust the toe to be straight or 0,5 degree toe in, it is somewhat better at normal speeds, but far from perfect. High speed is also reasonable. It seems to be more stedy at 75 mph. than 50 mph. It has a nasty tendency to follow bumps or truck tracks.

I have tried about a dozen different settings, and even changed my tyres. ( Going from V pattern to a type with V in the midlle but with a real shoulder. That actuallky helped some)

The next thing I have been thinking about to try is to raise the inner mount for the trailer arm by 1 inch, to get more possitive angle on that again.
I have also been thinking of trying a stronger steering dampener.

Any other ideas ? And what else could I be overlooking ?

There are so many different factors that could cause this problem. First of all there should be no positive toe at all, this is the (or one of the) cause of the unstable driving conditions. There may also be a "bump-steer" problem since the car is lowered. Lowering the car will change the angle of the tie rods/ends and cause this also. I see you have modified your sway bar to all the correct caster. There are only three settings for the front alignment and that is caster, camber, and toe. It is best to get these as close to factory as possible (after lowering) with the exception of the camber that a slight negative is okay. Now with all that said the other factors that could be of concern here are the condition of the bushings, wheel bearings, ball joints, tie rod ends, steering damper, idler arm, and steering box. All of these things could contribute to these problems. My suggestion is to first check the condition of these items and then have a correct computer alignment done by a professional. After that drive the car and see where you are at that time with these problems.

[Farmer]

Hmm. My "steering train" is OK, that´s one of the reasons this really pussles me. Normally Ì do not have much of a problem to get it right. But my own ! - that´s another story.

When I get the time, I think I will replace my new inner bushings with urethane, to see if there is some unwanted movement in there that I can eliminate. - If that does not help, I will try and raise the inner mount for the trailer arm 1 inch and see where that gets me.

[SprintStar]

Originally posted by Farmer
Hmm. My "steering train" is OK, that´s one of the reasons this really pussles me. Normally Ì do not have much of a problem to get it right. But my own ! - that´s another story.

When I get the time, I think I will replace my new inner bushings with urethane, to see if there is some unwanted movement in there that I can eliminate. - If that does not help, I will try and raise the inner mount for the trailer arm 1 inch and see where that gets me.

Hey Farmer,

You mentioned a stronger damper... Does the wheel feel like it's being wrenched out of your hand when you hit bumps?

What brakes are you using? I may have an idea.... Scrub radius. My front geometry is all ok, except for scrub. I'm still trying to figure it out. I'm supposed to get negative scrub, but all I can see is positve!

I posted a question on it, but no one is answering so far...

Sprint.

[Farmer]

There is a little disturbance at the steering wheel. That is why I have a hunch that my stock inner bushings are not strong enough to hold the 23 mm Sway bar, and thereby causing movement.
But it could also be the slight negative angle on the trailing arms that does it. I´m not sure. - When I get around to getting my new horse installed in the rear I will go deep into it again. I just needed a new angle on the problem.

[SprintStar]

Originally posted by Farmer
There is a little disturbance at the steering wheel. That is why I have a hunch that my stock inner bushings are not strong enough to hold the 23 mm Sway bar, and thereby causing movement.
But it could also be the slight negative angle on the trailing arms that does it. I´m not sure. - When I get around to getting my new horse installed in the rear I will go deep into it again. I just needed a new angle on the problem.

I'm using the PU bushings from TopLine. Good stuff.

A new angle? Seriously, try to figure out your scrub radius.'

You still have not revealed. What brakes are you using?

Sprint.

[JIMP]

Hello farmer,

I think that superman got it right from the begining. I supose that (as you mention the car to be a 73 SB) you use the stock spindles. On these, the outer tie-rods are fitted on them from upside to bottom. When you lower the car you put the tie rods to work on a bigger angle, far from horizontal, fact that is atenuated on every bump, hard braking or hard cornering. I think that if you find a way to fix the outer tie rods from downside to up, you will solve many of the problems you have (maybe to convert on a 944 spindle with the corresponding brakes? -they are from factory for fixing the tie rods from downside)

Good luck

JIMP

[Farmer]

Sprintstar, I tried about a dozen different scrup angles. (That is if you mean toe in/out)
My caster is 2,9 deg. Camber is now +0,5 deg.
The auto centerline is next to perfect. The car is running straight

The car is low, but not THAT low. My steering arms dont go negative unless the springs bottom out. We tried to "wiggle" the car when tools was on it, and the varians is very little when the car is standing still.

I use Sgl. ventilated front discs w. 42 mm brake cylinders.

[SprintStar]

Originally posted by Farmer
Sprintstar, I tried about a dozen different scrup angles. (That is if you mean toe in/out)
My caster is 2,9 deg. Camber is now +0,5 deg.
The auto centerline is next to perfect. The car is running straight

The car is low, but not THAT low. My steering arms dont go negative unless the springs bottom out. We tried to "wiggle" the car when tools was on it, and the varians is very little when the car is standing still.

I use Sgl. ventilated front discs w. 42 mm brake cylinders.

Hello Farmer,

Sgl. ventilated discs? What's that? Is this a non-stock setup?
Does it increase the track of the car? Where did you get the brakes from?

The scrub radius is different from the scrub angle.

Read this 2 links. One explains scrub radius and the other explains the benefits of reduced scrub radius.

http://www.geocities.com/automotive_...rub%20defi.htm

http://www.geocities.com/danielmacmi...ent/angles.htm

I suspect that your car like me has excessive scrub radius.... I'm hoping to correct that soon. But first I need to sort out what's correct for stock and what's not....

Sprint.

[kdanie]

Farmer, Your caster sounds ok but +0.5 camber is not good. Camber should be 0 or 1-2deg negative for better cornering, although it may increase wear on the inside edge of the tires.

Toe should be 1/16" or slightly more in at the front. Some racers use a little toe out but it will make the car twitchy like you discribe.

Sprint star, If you take some measurements of your front suspension mounting points, your spindles inclination angle, wheel offset and draw the layout on paper you can see what your actual scrub radius is. The easy way to correct scrub radius is with different wheel offsets but if you don't know what it actually is you can't really know what to do. A little scrub radius is not really a problem, few cars actually have "0" scrug radius. excessive scrub radius can cause suspension "Jacking". That is where the scrub radius is so large that when the wheel is turned the weight distrubution on the tires is upset, not a good thing at all. For what it's worth, racing Karts use scrub radius to good effect as a tuning aid and changes can dramaicaly effect the handling, good or bad. Remember they don't have any suspension and the rules don't necessairily transfer to a car with suspension.

Get a copy of these two books. Carroll Smiths "Tune to Win" and Herb Adams book on suspension design.

ken

[SprintStar]

Hello Kdanie!

Power! Finally someone understands what I'm trying to ask...

Has anyone here got specs for a stock car? I can get my car's measurements done, but I need to compare it to something stock so as to be able to make corrections.

I know wheel offset can aid adjustment of scrub radius. My problem is I'm already using Boxster rims with 50mm offset compared to the 36mm used on the stock 944s. I should be having negative scrub, but all I see is positive. This is the part that is baffling....

I think it may have to do with the mounting points of the control arms. My car is a early '73 so it came originally with the 3-bolt suspension and in the process of installing the late arms, I had to grind of a bit off the corner of them. Is that correct of something is wrong?

Sprint.

[kdanie]

Sprint star, I'm not much help with 1302/1303, I've never had one.
Reviewing my suspension books this evening, about the only thing you can do on a strut suspension to decrease scrub radius (if your wheels are as inboard as possible) is tilt the top of the strut inboard to increase the kinpin inclination angle but then you get into excessive camber unless you can modify the spindle to compensate for the angle change and reduce the camber (not easy). The strut design is your limiting factor. If you have too much kingpin inclination, say over 9 degrees, your front suspension will feel "floppy" as you turn the tires. Suspension design is all a compromise and never comes out perfect in all situations.

I think the only useable suspension with "0" scrub radius will be a double wishbone suspension with a small spindle tucked well inside the wheel. Due to the strut on a super I don't think the spindle can be placed far enough inside a wheel to achieve "0" scrub radius due to spring interfearance.

You've got me curious, draw it out and let me know what your results are.

If you don't have a lot of scrub radius, I would not loose any sleep over it, minimize it with wheel offset and get on with other things. I would guess that a stock super would have some built in anyway.
ken

[SprintStar]

Originally posted by kdanie
Due to the strut on a super I don't think the spindle can be placed far enough inside a wheel to achieve "0" scrub radius due to spring interfearance.

You've got me curious, draw it out and let me know what your results are.

Hello Ken,

This is the part I'm really curious about. The stock '74 -on Super is supposed to have negative roll radius built-in, so if you don't think "0" is possible, negative is even more impossible, right? Hehe... I'm going to draw it out. Give me a couple of days, ya?

Sprint.

[kdanie]

I was looking through some paperwork I found in a BMW 320is that I just bought for my daughter to drive. I was looking at an alignment print out. It showed amost 8 degrees of caster, that seemed high but it drives very well. You might crank in some more caster, I know it will help the straight line stability. You may have to modify the mounts for a little more (Super, right?).
ken

[SprintStar]

Yup, it's a Super. I'll get some numbers soon and post them...

Thanks in advance!

Sprint.