SC/Carrera to RS coversions supension ?

[village idiot]

What setup is everyone running?
Now that I have my car on the road I'm interested in what kind of suspension setup would be good for a primarily street driven RS "impostor".

Currently I have OE boge struts up front(not leaking but worn originals), and Bilstein HD rears(green body). Stock t-bars and sways.

Overall, the car feels good, but the front feels a bit "flighty". I assume this is because of the weight lost due to the conversion, including a fiber glass front hood, bumpers, engine cover and sound deadener, as well as removal of the rear seats and replacement of the front seats with non-reclining race seats.

Looking to make the front feel more "planted".

quick pic for those who have not already seen it:



Thanks!

[Joe Bob]

Well I would consider talking to Chuck at Elephant racing. Consider custom valved inserts, upgraded t-bars front and rear. New bushings, check on your tie rods and general suspension maintenance.

Add a splitter up front. Then a good corner balance/alignment.

[village idiot]

While I had the car apart during the build I replaced the ball joints and outer tie rods. The inners felt fine with no freeplay that I could detect, and the bushing had little to no deformation.

Will I really need bigger t-bars? I figure with the car being lightened as much as it is that the stock bars would be more than adequate.

I do agree on the corner balance though, and that will happen once I have a solid plan as far as mods. There's no track here, so I figure close to stock will provide a good balance between comfort and handling.

[Flieger]

You should get a corner balance and alignment. The ride heights will have changed with the weight loss. Depending on how the weight distribution changed you may want to change torsion bars. Either softer front or firmer rear to balance out what is most likely a rearward shift in weight distribution. Due to the weight loss and even without changing torsion bars, you should have the dampers re-valved.

I recommend Rebel Racing for this.

[village idiot]

Here is a pic when it was in progress:


You can see I had to re-index the rear tb's to drop the car about 2.5" and the front had to drop about 2" .

I'll give Clint a call as well as Chuck and see what they have to say.

Flieger, your car is extensively lightened isn't it? May I ask what size bars your running?

[Flieger]

My car had the suspension upgraded by I think the previous owner (and not prior to that) who was the one who did the motor. I am not sure who converted it to short hood.

Bottom line is that it has 22/27 bars and those should understeer pretty bad but being in college I have not had it out on the track to find out. Just weekends every couple weeks in the canyons. Feels fine there but the front end does tend to feel like it will push but that is usually in a longer turn where I am easing on the power so it could just be the weight transfer.

Not sure if the car was more nose heavy before or if the 27s were the biggest rear bars available then or if someone did not do their homework.

Stock weight distribution means a 22/29 setup is better or 21/27. Lightened would probably be 22/30 or thereabouts. I have to do the formula to figure out but that would be quick. just raise the diameter to the 4th power. You want the ratio of the front to rear to mirror the weight distribution.

My car also has 22mm front and rear anti-sway bars so there is not much roll, to the point that the squat is noticably more prominent, especially in photos.

[Joe Bob]

I'm running 22/28 in my backdated SC cab.....adjustable konis all around set at medium soft up front and soft in the rear. No special valving. I'm happy with it.

[Chuck Moreland]

Tom

Let's start by defining the problem more specifically. You said the car is 'flighty', what exactly is it doing that you don't like (eg. pulls, wanders going straight, shimmy, requires constant correction at wheel etc)? And when is it doing it (speed, straight line, over bumps, mid turn, etc)?

What are the current alignment settings and ride height?

Are there any mods to the suspension now?

[village idiot]

Originally Posted by Chuck Moreland

Tom

Let's start by defining the problem more specifically. You said the car is 'flighty', what exactly is it doing that you don't like (eg. pulls, wanders going straight, shimmy, requires constant correction at wheel etc)? And when is it doing it (speed, straight line, over bumps, mid turn, etc)?

What are the current alignment settings and ride height?

Are there any mods to the suspension now?

Thanks all for the insight guys.

Chuck, right now the car is running stock(as far as I can tell) 78 SC t-bars and sways. It doesn't pull or wander and has no shimmy. Near as I can tell is the caster is probably off (very easy steering compared to my Carrera)and I haven't found anyone I really trust to do the alignment/corner balance and I don't know if anyone locally has scales to do so.

I'm slightly embarrassed to say that I did the initial setup with a tape measure, some very crude home made toe plates and an el cheapo angle finder to get the camber to 1 deg negative.
Obviously not the preferred method. Didn't really touch the rear aside from resetting the t-bars.

Mostly, the feeling occurs in medium to faster corners with some bumps in them. It seems to be fine until there are some sharp-edged bumps then it will tend to understeer. I haven't been anywhere the limit of the tires yet on the street.

I'll take some measurements of it's current ride heights tomorrow and post them here.

Take pity on me, I just got this thing on the road after 5 years of pinching pennies and was extremely happy it didn't fall apart or kill me in the first 10 miles. (if you knew my yearly income you'd understand my user name).

[sc_rufctr]

Honestly Tom... You're a legend in my book. She looks beautiful.

I watched you build your car in your threads here and I'm extremely impressed considering you did all of the work yourself.
I'm sure you'll get good advice for your suspension upgrade so nothing to add there.

Good luck.

[village idiot]

Thanks for the kind words Peter! It was a very long road for me and it isn't even close to being over. I lose sleep thinking about the next step is in this journey.
I know if it weren't for the people on this site I wouldn't have been able to accomplish half of what I did.
This place awes and inspires me to try just about anything.

[porschenut]

Tom, the car looks great! I have a metal long hood on my RS tribute but otherwise my car is just like yours as far as fiberglass bits. I have the stock Carrera sways, but I swapped in 22/28 torsion bars and I have Bilstein HDs on all 4 corners. My alignment settings are slightly aggressive but nothing like a track car. With this setup, my car's handling is almost perfectly neutral which is what I was shooting for. However, the stock torsion bars would have been fine, especially with a lightened car.

I think if I were you I'd keep the stock torsion bars for now, and add Bilstein HD strut inserts for the front. Is there a shop on Oahu who has any experience with 911 suspension setup? Alignment settings and tire pressures can have a big impact on how the car feels in the turns.

[village idiot]

Thanks Dave.

Supposedly, there's a guy here that's familiar w/Porsches but I don't know where he works or if he's got his own shop. I'll have to dig further and see.

[village idiot]

Ok Chuck, here's the car as it sits without me sitting in it
LF=24 in
RF=24 in
LR=24in
RR=23.75 in

Note also that I run a 205/50-16 front tire for clearance reasons. Rears are the usual 245/45-16.

[Chuck Moreland]

Tom

Good stuff. That information is crucial to making informed recommendations.

Start with the diagnosis:

Firstly, your car is too low. With stock torsion bars and that ride height it is hitting the bump stops with regularity.

Specifically you commented about problems in mid to fast corners with bumps in them. You are surely riding the bump stops under these conditions and that is making abrupt changes to the balance of the car. When you compress the bump stop the spring rate on that corner spikes to extremely stiff.

If this happens mid-turn, the under/oversteer balance changes suddenly and requires you to make immediate adjustments at the wheel and throttle. Yeah, that can feel not "planted".

Inherent in the ride height issue is the problem of bump steer. When the car is lowered the already-present bump steer issue is compounded. This exhibits itself as a change in toe when the suspension compresses and rebounds - exactly what is happening when you are hitting bumps mid turn. So when you hit the bumps, you are experiencing a wandering toe setting which makes the car unpredictable. Yeah, that doesn't feel "planted" either.

Secondly, you need a good alignment. While this can certainly be done at home, it does require good deal of study and some specialized tools (which are not that hard to make). You did a quick and dirty alignment so we can assume the settings are not right. And what is right? Depends on what you want the car to do, we can get into that and I'll make some recommendations.

Alignment has a big effect on the feel of these cars.

Thirdly, you need a corner balance. You changed the ride height without scales so if the corner weights are correct, it's just lucky. While this should be done, you haven't said anything that indicates corner balance is the root cause of your problem.

*********

The remedy:

Ride height - you can do several things to resolve the height related problems. Your best path will likely be a combination of these things.

1 - raise the car back up. This is the easiest, but maybe not what you want to do.

2 - go to stiffer torsion bars. Mitigates the problem, but doesn't solve it

3 - raise the spindles up front and install shortened shocks on the rear. This is the best solution and will restore suspension travel and front geometry.

Bump steer - correct this, and the best solution depends on what you do about ride height. In some cases the simple rack spacers are adequate, but for raised spindles or very low cars a tie rod end bump steer kit is a must.

Alignment - Find a shop that is experienced aligning the torsion bar cars. you really need a specialist. Maybe you don't have one on the island. If that is the case, then you need to study up and learn to do this on your own. Build or buy good tools to do it.

Corner weight - Again find an experienced shop. Failing that, see if you can find a local enthusiast with scales to lend. Or buy a scale set. There are several types on the market, with a pretty broad price range. Cheapest is the Speedway load checker - last I checked around $150.

If that isn't in the budget, look up the 'tripod method' which I ginned up and documented on these pages years ago. It is a crude method to tell if you are in the ballpark.

******
Maintenance -

While you clearly have some specific setup issues to be resolved, you should also refresh the wear items on the suspension. Bushings, ball joints, tie rods, shocks. If these items are original, they are done and need replacement.

If you are going to put the time, money and effort into setting the car up correct, address these maintenance items first. Build your house on a solid foundation.

[Flieger]

My car was aligned for maximum negative camber and maximum caster (positve trail) while still keeping both sides equal and without grinding anything or pre-loading the strut brace. It also has a little bit of toe-out.

The steering is amazingly light. I cannot imagine what a car with no caster would be like. When I am turning around on a hill all that caster feels like power steering. I think that is due to the trail rather than the banking the car while turning part of caster.

Early 911s just have light steering since all the weight is out behind the rear axle.

[Flieger]

I was writing my post when Chuck's post went through.

I hear a lot about bump steer but my car has the A-arms level at static ride height so I guess it should have bump steer. I don't know what bump steer feels like but my car feels fine to me so I don't really care if it has bump steer I guess. Maybe the big sway bars are masking the issue?

[village idiot]

Thanks Chuck, that makes a lot of sense. I see what you mean as far as the car hitting the bumpstops. You're right in assuming that I wouldn't want to raise the car as I like the "stance" as it sits now. So I guess I'll be looking for a set of Bilstein strut housings to raise the spindles on.

Also, I don't mind buying tools and the Speedway load checker seems to fit my budget. Plus I like learning this stuff and it'll be a good learning experience for me.
I also have a set of rack spacers that I can try, but I didn't think they were necessary as the tie rods seem to be level with the a-arms. I'll try them, they're easy enough to change.

Looks like I'll be selling off my extra set of 7's and 8's to finance some of this:P

Do you think with raised spindles I can still run the stock t-bars?

[Flieger]

My car has raised rear damper towers so that the car can be lowered while using full length dampers. The front spindles are stock since they are Konis.

[village idiot]

My car is using Boge struts which if I'm not mistaken, do not lend themselves to the raised spindle mod