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Munky King's Avatar
 
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Light weight panels and freeway speed stability

A quick question to those who have gone the route of lightweight 911 panels and car...

I'm thinking of using FRP panels, in place of steel, for my hood, fenders, bumper, small battery in my smugglers box etc, also similar out back with the rear bumper and engine lid.

I'm concerned that if I lighten the front up too much, that freeway speed stability is going to suffer, and when I drive the car to work, it will be unpleasant.

I have had a 356 speedster Intermeccanica replica (an older one) and more recently a high power APS 356 coupe. (I understand that the shorter wheel base wasn't helping much either).

Even with the brand new steering box, good joints etc, it was still a handful at 60mph on a straight road. This was the cause of eventually getting rid of both.

I don't want to recreate a similar issue with my 911 project. I'm also going narrow body with rounded 'R' type bumpers (no chin spoiler) and probably not a ducktail.

Anyone had any experience of a similar car who can help? I'm guessing ride height is going to play it's part too.

Thanks for any advice or insight!
Old 09-10-2017, 01:31 PM
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I had an old IM speedster too. I ran braces from the upright in the frame (just behind the rear door opening) to the pan support and that stiffened the car up considerably but a VW pan based convertible and a 911 are different animals. I don't think you'll compromise stability with FRP in the areas mentioned but I wouldn't recommend a chin and no tail, or vise versa, especially at highway speeds. Lowering the car will help, yes, but I think creating downforce on one end and not the other is asking for an unbalanced car.
I had an SC with a duck and no chin and it would get "floaty" at higher speeds. Fortunately I didn't do a lot of highway driving.
Just my thoughts.
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Old 09-10-2017, 03:44 PM
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First off some fiberglass panels aren't that much lighter then their steal counterparts.

A 911 has a very solid steel unibody tub. Your replica was all fiberglass with no unibody structure. You will probably never notice the difference besides some road noise or if you go with doors. You can take those parts off all together and still have a solid 911.
Old 09-10-2017, 04:29 PM
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Lightness does not compromise stability, all race cars are super light. Stability comes from proper suspension setup and aerodynamics.
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Old 09-10-2017, 04:36 PM
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Porsche did something like this in 1967. They called it the 911R.

Quote:
Four Swiss race car drivers, Jo Siffert, Dieter Spoerry, Rico Steinemann and Charles Vögele, set out to break several endurance world records in Monza. … The record drive took place lasted three days and three nights. After 20,000 km at full throttle, five new world records and 14 international class records had been set – all at an average speed of more than 200 km/h.
I think you'll be all right…
Old 09-10-2017, 05:04 PM
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Thanks for all of the great input and advice
Very valid points and I hadn't considered the structural aspect. More making the front too light so it gets floaty at speed.
As pointed out, suspension, structure and aerodynamics play into this. I hadn't really considered this, I just felt the pain of cross winds in both the coupe and speedster.
I really want this new project to be a keeper, that I can cruise through summer with.
Thanks so much for taking the time to share your great advice
Old 09-10-2017, 05:40 PM
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So... Let's get a few things straight.

Yes, it will make a difference. How much, depends on your current setup.

How? GFRP panels will reduce the weight on the front and rear of the car. This usually means an increase in ride height. Now, if you only take weight off the front the front will rise relative to the rear with allows more air under the car and can make it feel floaty at speed. Additionally, the reduced weight means your front torsion bars will have less weight to support and effectively be stiffer - less jounce per given bump. This shift of effective spring rate balance will also give the car more understeer.

If you change the weight in the rear you will notice the same effects as the front. The difference is that weight sum is much more pronounced up front.

How do you fix this? get some wheel weights before and after the mods. Note your torsion bar size (spring rate) and calculate a wheel weight. Then adjust the spring rates with new torsion bars after the mods to maintain the same relative balance. Or shift the balance to your personal liking. Then tune with shock damping and sway bars.

Yes, I have done this on several cars. Yes, it works. yes, you will have better braking, sharper weight transitions and the cars are really, really fun when you put them on a serious diet. Go drive one-ton or less 911 and report back with your findings...
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Old 09-10-2017, 06:37 PM
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Thanks JP. A great explanation.
I would follow your suggestions to the letter, except there isn't much left on the front of the car right now. And unfortunately I bought it that way!
I do appreciate you explaining this to me and it looks like I will have to tune the suspension without having the greatest start point that you suggest.
It's good to hear that the diet is worth while too!
Munky
Old 09-12-2017, 01:28 PM
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Munky,

You can look up corner balance numbers from other cars and use these for comparison for your own suspension setup.

You can also look up calculated wheel rate (effective spring rate) for the torsion bars. Then scale to match.

Remember your physics class. For a spring: F=kx. F is your force (weight of the car). X is the suspension travel. K is the spring constant. If you want the travel to stay the same, and you reduce F, you also have to reduce k. Linear relationship here in the simplified description.
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Old 09-12-2017, 02:08 PM
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Interesting and pertinent post. I have had a few 996 cars that were floaty at highway speeds with the nose feeling light instead of planted. Not a good feeling driving with your butt cheeks clenched the whole time..
Old 09-12-2017, 05:06 PM
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Thanks JP. Looks like I need to invest in some scales.
I have an AutoX friend with some, but probably should just buy my own.
You have saved me a lot of pain, as I'm at the stage where I was thinking of swapping out the current suspension set up and replacing it once the car is bare metal and then painted.
You certainly have given me a lot to think about!
Munky
Old 09-12-2017, 07:10 PM
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MK,

My 912R weighs 1750 pounds and wears fiberglass wings, hood and lid.

It's planted at 100mph and doesn't get blown around at high speeds w cross winds.

Remember the front end shape of the 911/2 is what is known as an "active shape" so regardless of weight the sloping hood creates downforce.
Old 09-12-2017, 07:16 PM
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Edoyle

That's what my 356 replicas felt like and as much as I loved their looks and town driving manners, I couldn't really live with them through a summer. Too much of a handful and white knuckle ride, while others passed by in beaten up chevy trucks, smoking and drinking with one hand and barely touching the steering wheel with the other!

I don't want to make the same light front end feeling mistake again!

interesting that you have experienced it with something a new as a 996!
Old 09-12-2017, 07:17 PM
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Thanks - that's impressive to say the least! I would be happy with 'stable at 60 - 65 mph'. I think your car must be pretty well dialed in to be stable at 100!
Thanks for the great insight! There's hope for me yet!
Old 09-12-2017, 07:20 PM
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The swing axles on those cars probably contributed to most of the disturbing feel that you fear.
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Old 09-12-2017, 07:26 PM
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You are probably right.

I'm sure that had a significant contribution to the instability. Probably made worse by the difference in weight they were designed for and the lightness of the replicas.

I've owned a good few VW beetles in my time, and while they weren't eaxactly on rails by comparison, they did seem a good bit better.

Then again, I needed a good run up to break 65 mph in them! Especially in my oval!
Old 09-12-2017, 07:37 PM
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FG front and rear lid, fenders, and bumpers won't make a difference in a straight line, although your 911 may feel a little more twitchy under braking. You may also have to adjust your sway bars for neutral cornering.
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Old 09-12-2017, 08:01 PM
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No one mentioned front end alignment. Some of the twitchyness can be tuned out with more toe in.
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Old 09-13-2017, 09:38 AM
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