SJB993

Well, I've just spent a lot of money buying Bilstein HDs, having them installed and then getting a full alignment done.

The reason I pushed the button on the bank balance reduction exercise was that the car felt light at the front end when doing 70+ mph on uneven road surfaces. By uneven I don't mean heavily rutted, just the normal undulations we have on our non-motorway roads here in the UK after years and years of digging up and refilling to lay new gas pipes, electric cables, etc. It's not too bad when the road surface is completely smooth (which sadly is seldom the case).

There is a marginal improvement, but I'm still not entirely happy. The 993 (which is a C4) doesn't feel as planted as my VW Golf GTI (daily driver).

One outstanding issue remains though. When the alignment was done by an OPC they said they had been unable to get the caster angle within spec. on one side because the cam-screw adjuster for the ball joint was completely seized on one side. They didn't have the parts in stock, so I'm waiting to get that fixed.

So, I'm looking for ideas on what can still be causing my "light front end".

1) Could the out of spec. caster be the cause?

2) I've got factory fitted M033 springs (same as M030 but softer), height measurement at the front (floor to lip of wheel arch) is 25.50" and 25.72" at the rear. The shocks were fitted by my usual independent, and he used the dimensions in Gert's DIY (Gert supplied the shocks). I got the shocks with the extra thread to match the lower springs. Are these ride heights getting into bump steer territory?

3) I've got 18" Turbo Twists fitted with Bridgestone SO2s - 225/40ZR18 front 265/35ZR18 rear. I use the handbook pressures of 36psi front and 44psi rear. Should I adjust these?

Any comments and/or suggestions would be gratefully received!

TIA

Dr. No

Could be the engine on the GTI is in front and on the 993 is in back.

Seriously, I have exactly the same experience between my 993 and 528, and I'll bet others with front/rear engine drivers have the same.

VNTGSPD

Caster angle definitely affects straightline stability. Get that sorted out first, then evaluate.

Also, when you say the car feels "light", do you mean that the front end "floats" or does the front end "wander" a bit.

On uneven surfaces, I get a bit of "tramlining" where the car wants to follow irregularities or grooves in the pavement but this is due to moderately aggressive camber setting and tires that are worn down to about 3 or4/32nds in the front. Some might consider this "wandering". The car feels very buttoned down, it just moves around a bit.

Wilder

First thing I would do is change tire pressure. I run 36 front, 38 rear. 44 is less than ideal and could be at least a contributing factor.

Alignment will have a lot to do with it. When I got my HD/M030 installed, it took four trips to the alignment shop for the car to be just right for me. Part of the problem was the same lightness in the front you describe. I think you'll find that camber, toe and caster will all have a lot to do with it, especially toe. Rather than going with standard 993 specs, you may want to try RS or RUF specs.

grmnxtc

Also, I remember from the Tony Callas 993 day last year....

The front suspension arm bushings (?) can be shot, and this can also lead to wandering as the lower arm is not held "tight" to the chassis... (at least that's what I remember being the case).

Could be wrong but might be worth looking at? I'm sure someone else who knows more than me will correct me if I'm wrong....

SJB993

Thanks guys.

VNTGSPD - it's a "float" more than a "wander", and it's not the same feeling as tram-lining (which I do sometimes get when running parallel with a rut).

Louis - do you have the same size tryes/wheels as me? I'll give the lower rear pressure a try.

Anyone else got any ideas?

TIA.

geolab

iF it is really 25.50" front and 25.72" rear, you probably found the solution.
The rear should be lower than the front by avg. 16mm (127-144mm)
your measures look 6 mm outside, so the car stands 22mm higher in the front than should be. This could slightly result in a floating sensation.
You do not need to dismantle anything to adjust.
Only the front strut adjustment lowered.
and of course, alignment.

Plenty of docs on the ride height, and where to measure from precisely...

SJB993

Hi Geolab - thanks.

The numbers you quote, namely 144mm and 127mm, are those shown in Gert's instructions, and are the mesaurement from the bottom of the shock's tube to the bottom of the adjustable spring holder. Maybe the metric/Imperial measurement conversion is the confusion 144mm = 5.57" and 127mm = 5.00".

The numbers I quoted are from the floor to the lower lip of the wheel arch - 25.5" (647.70mm) and 27.5" (654.05mm). I was just wondering whether these numbers are getting close to the point where bump steer may be an issue.

geolab

Hi Steve,
The numbers above are the ride height of the car taken from specific points underneath the chassis
to the floor. Search for Jeff in the air DIY and it is thoroughly explained.

These numbers are good and are M033/M030 ROW +/- levels BUT:
.
the 64.7cms/ 25.5"should be the rear, and
the 65.4cms/ 25.7" should be the front

can you verify the rear is lower than the front, and not the opposite?

no bumpsteer for this levels at all

SJB993

Thanks again Geolab.

Yes, the measurement I took suggests that the height at the points I measured means the rear is higher than the front (by 0.25" - 6.35mm). I'll look at Jeff's DIY and make the measurements accordingly.

I'm just feeling a little p*ssed off at the moment. Having posted about my problem before the HDs, and being told that HDs would make a "night and day" difference, I'm feeling a little disappointed that my £1200 (1700 Euro/$2400) investment doesn't seem money well spent - so far.

maybe it will if I get to the real cause of the problem.

Take care.

geolab

do not worry, from berkshire to paris is 4.5 hours.
And you and I will align it to your taste if need so.
What bothers me slightly is that it seems to be sitting a tidbit high on the rear.
Have you driven the car after the alignment for 50 miles or so? or not?
If not, drive it on a paved old road, no need for speed, just to let the springs sit well.
And we will take it from there.
For now, read Jeffs DIY, on how to masure the ride height, and keep me posted after you check, please...
cheers

SJB993

Thanks again Geolab.

I've just read through Jeff's DIT and now I'm confused.

He shows the the measurement being taken between points on the chassis and the ground, whereas the measurements in Gert's DIY are taken between points on the shock itself (i.e. the bottom of its strut tube and the adjustable spring holder). However both sets of dimensions are the same - 144mm and 127mm.

I'm now wondering whether Gert's DIY (which is what the mechanic used to fit the shocks) is wrong?

I'll make the measurements where Jeff says (I note that his diagrams are from the workshop manual, so I guess they must be right).

BTW - Jeff's range of fender heights (25.25" - 25.75" +/- 0.4") means that my heights are about right. He does say the rear should be around 0.25" lower than the front - but given a 0.4" tolerance on the heights, maybe that isn't too important???

geolab

No, not important.
Now that you are aware how the measurements are taken, take the car for a ride, and measure after.
We'll take it from there,
take care

Ritter v4.0

I also have HD Bils, lowered to RoW etc. and love the set-up but I had a similar problem; at 110mph+ the car felt a bit light or unsteady- not to say the answer will be the same neccessarily, but mine was solved by tire pressure - I was running 36/44 and dropped to 40 on the rear and the issue was virtually resolved.

Air is free- try it first.

Jack of Hearts

I would reduce the pressure in the front as well. Play around with pressures in the 32 to 34 psi range. Remember, the car has very little weight on the front end and will never be as inherently stable as a well balanced front-engine rear-drive car.