Crosswinds suck

BlueSkyJaunte · 05-12-2002, 05:17 PM

OK, I took a trip down to Tucson this weekend with 3 adults (!) in the 911. I started with a full tank of gas.

I estimate the crosswinds on I10 (traffic flow tends to be 75-85 mph, depending on the time of day) were in the 15-20mph range.

Unfortunately as time (and the miles) went by, the front end got lighter, naturally, and things got very squirrely. I have the typical "rubber lip" (chin spoiler) and a matching Carrera tail. Now, normally I wouldn't have been too bothered about this, but I was all over the lane. Traffic moving around me didn't seem to have much of a problem, though. And the '78-89 Targa that blew by me at 90 mph didn't have a problem either (he had a Carrera tail, and I assume a front lip as well). He was holding a line perfectly.

What's wrong with my set-up??? Before I added the tail (PO had put the lip on) I noticed that the rear end of my car was moving around a lot. When I put the tail on, things smoothed out well--in no-wind situations. Now the front end wanders around like heck in a crosswind. Do I need to be able to adjust the angle of deflection on my tail depending on crosswinds??? Do I need more "downforce" at the front end? Do I need to adjust my tire pressures? Do I need an alignment??? What gives?

Vital stats:
1981 Targa
Front "chin" spoiler (rubber OEM)
Rear Carrera tail (rubber OEM, aftermkt f-glass)
Colgan bra (the full thing incl. the bonnet)
Dunlop SP8000s, 29PSI all around.
Ambient temp about 85-90 degrees.
15-20 mph wind coming from the West as I headed north at 75 mph.

TIA,

Blue

(PS: You CAN do a day trip in a 911--as long as your wife is 5'1" and your guest is 5'4"

)

Wayne 962 · Wayne 962's Garage

Might be what you suggested. I would check with the most obvious culprits:

- Tire pressure
- Alignment (toe in / camber
- Rear alignment too
- Ground clearance (saw a nice 911SC today in MB that looked like it was way too high off of the ground...)

-Wayne

Bill Douglas · 05:51 PM

My '79SC coupe with chin spoiler and no wing is fine in more or less any cross wind - well up to 60 knots, and I go way faster than you were going

But like Wayne suggests the car may have the wrong ride height and/or bad alignment or bushes.

And.... Your rear tyres should be 34psi not 29psi, this may help.

rattlsnak · rattlsnak's Garage

Crosswinds huh?? Try landing a plane when the winds are 35 knots, gusting to 52, 40 degrees off the nose, like they were Friday night going into Toronto!

--Sorry for the OT, but the "crosswinds" title REALLY caught my eye after that adventure!

hoff944 · 05-12-2002, 10:49 PM

You flying RJ's?

dickster · 05-12-2002, 10:57 PM

a long shot but......

what shocks you using/are they tired??

i mention this because i recently changed my knackered boges for beilstein gas.

i only have the carrera tail (no lip) and the motorway/high speed driving has transformed. it used to scare the hell out of me when pushed - now its rock solid and confidence inspiring. wait till i fit the lip!!

the other thing i noticed is how much the braking is improved. i think you tend to forget that shocks affect so much.

hope it helps.

Tony Hersikorn · 05-12-2002, 11:25 PM

I never knew how dangerous crosswinds could be till I drove a '68 beetle about 400 miles to pick up the P-car from the shop. It had really skinny tires on it and there was about a 80-100kph wind coming straight from the side. I probably had the wheel turned a good 270 degrees to compensate. I would pass a farmyard with lots of trees and the car would literally hop into the ditch. When I was in the trees it steered normally so I would relax and straighten the wheel, only to get back on the open prarie and be thrown into oncoming traffic - all in a split second. The worst 7 hours of my life. The 930 had no problems whatsoever on the way back tho.

Eric Coffey · 05-12-2002, 11:59 PM

I think Wayne touched on the main potential culprits. My guess is that the car is in dire need of a complete alignment/corner-balance/ride-height adjustment. A car that has too much negative camber, or too much toe-out can feel like it is "hunting", especially in a decent crosswind. Also, tire pressures are significant in the way the car handles. Bill D offered some good advice on this, although, I tend to go a little higher on the rear (cold) pressures with normal street tires. 30psi on the front, and 36psi in the rear is a good starting point. I have also heard of the same wandering/hunting symptoms produced by worn steering racks, but that is less likely than the previously mentioned usual suspects. Good luck!

911jon · 05-13-2002, 12:50 AM

The 911 body shape is a complex one, subject both to lift (like almost all cars) and sidewind instability under certain circumstances. These are issues the factory has always been aware of, and the f/r ride height is set at the factory to minimise these undesirable characteristics.

The body pitch angle is significant as well as the various aerodynamic addenda available (which you already have). It may be that the additional passenger weight in the car combined with your low-ish rear tyre pressures combined to cause a nose-high attitude, increasing frontal lift and causing the stability problems you describe. If you have an extra passenger AND luggage, make sure the luggage goes tightly into the nose, rather than the other rear seat!

Are you running standard ride height f/r? Your car is 20 years old, how old are the dampers? also, how did you derive 29psi f/r? Sounds low at the back to me. All around suspension alignment would be a good precaution.

N.B. - Richard, just for interest, did you fit HD or Sport Bilsteins?

dickster · 05-13-2002, 05:01 AM

jon,

i asked for sports and got yellow rears and green front. they could be sport rear/hd front, i'll have to check the part numbers again!

it gets confusing with the diff. colors and part numbers.

anyway i believe they are sports - and i can highly recommend them!

wckrause · wckrause's Garage

When I have my skinny snow tires on, the car really gets skiddish in side winds. Feels like my old '68 VW beetle. With my normal summer tires (Dunlop 8000s) it's very solid. I don't have a chin spoiler or wing, but the car is very low.

911jon · 05-13-2002, 05:47 AM

Thanks Richard

Considering this upgrade myself since high-speed handling on my Boges is slightly "nervous", especially at the rear. Some lurid slides have been encountered in the wet even with very restrained, smooth power application.

Some suggest HD's are better for mainly road use, but if you have found "Sport" option ok...

BlueSkyJaunte · 05-13-2002, 10:32 AM

Thanks, all, for the input. Tonight I'll adjust tire pressures (hit the low-hanging fruit, as they say). Then wait for more wind. If that doesn't do it it's alignment time. I may end up going for new struts soon, anyway, depending on how old my current ones are (have to check the records).

Thanks again.

Jim Sims · 05-13-2002, 11:14 AM

Rear engined cars have the lateral center of pressure ahead of the center of mass. This makes for an unstable configuration in crosswinds. There is only so much that one can do about it (tire pressures, stance, etc.) without extensive body modifications or moving mass forward. A third person in the back with a 1/4 full gas tank in a 911 means having to slow down when encountering 50 mph crosswind gusts. These cars were designed for Germany not the steppes of Russia or the North American Great Plains or the current Southwestern US dustbowl! Cheers, Jim

john walker's workshop · 05-13-2002, 11:35 AM

are the sp8000s rated ZR? if not, the sidewalls could be too soft for any control. are they the proper size for the rim? 205/225? 32/36 pressure works better. a 4 way align is probably needed, as mentioned.
push the car sideways repetedly in the front and rear and see how much side slop the tires allow. give each corner the bounce test to check the shocks. good shocks are quite firm, not easy to get much up/down movement going.