930 handling problem

[Black968]

Anybody out there have any suggestions on how to improve the steering response on my 930. It does not want to turn well on slow tight corners. It wants to plow or go straight. It is a USA model lowered to Euro specs and has been corner balanced. It has new tires. I run the suggested 29 PSI front 36 PSI back as in the manual. Any thoughts would be appreciated.

Cheers

[David]

They understeer pretty bad with the stock suspension. You can replace the torsion and/or sway bars to soften the front and/or stiffen the rear. Or buy adjustable sway bars so you can adjust it to your liking.

Of course this will make it a little hairier on high speed stuff.

[wcc]

EDIT: It doesn't seem like you are running the proper pressure. I'd have to uncover the car to get the owners manual, but it seems like you aren't running the right....

Ok, from my '87 930 manual the Tire Pressure Cold is:

29 PSI Front 205/55
44 PSI Rear 245/45

[Jack Olsen]

Keep in mind that many of the tricks to get the car looser in low speed corners can make it very loose in fast sweepers. If you're setting the car up for autocross and/or are not tracking it, this might not be a problem. But most handling adjustments are about compromises between low and high speed turns.

And as a side note, I can't think of a modern tire that I would run at 44 psi cold.

[wcc]

The MAX PSI posted on the side of my Sumitomo tire is 44PSI. I actually run it a little less around 40-42PSI. But even if I ran it at 44PSI I wouldn't worry about it one bit. The tire company planned it this way. This tire was actually designed at a higher pressure. They are basically saying that 44PSI is the max pressure for legality/warranty reasons. Tires are just like anything else in this world. For example, bridges are posted at, say, 1 ton, but they are designed at 1.5 or 2 tons. Speed limits are 70mph, but most roads are designed for 80mph or faster. So in a sense 44PSI is the posted speed limit or the posted bridge limit. Of course we've never exceeded speed limit so why worry about these tire pressures. I mean, can you imagine if a road or bridge was actually designed to the posted speed limit? We'd have accidents and bridges collapsing everywhere. Just something to think about. I've NEVER had a blow out or any issue. YMMV

Note: My cars are only street driven. If you track your car you have no warranty anyway, or it is VOID, AFAIK, just run it at whatever pressure gives the best traction and makes the tires last the longest. Usually one season if your lucky, so who cares !

125shifter - Had some good ideas with adjusting the stiffness front/rear....

[hatpix]

Ok, I'm no scientist, but I've read that you gain roughly 1PSI for every 10 degrees increase (Fahrenheit). SO if you set your pressure cold to 44PSI, say at 70ºF, then hit a nice hot mountain road, your tires go up to say 165ºF, you've just gained 9.5PSI, and are now running at 53.5PSI.

Even if you didn't deal with max stated pressures, the tire is too hard to get any traction. Why would you do that?

[tctnd]

Does it have an LSD? They can make the car reluctant to turn.
regards,
Phil

[David]

I think your tire pressures are about right for the problem you're having. I wouldn't go much higher or lower than you are.

This is where adjustable sway bars are nice so you can have a different autox and DE setting.

[jluetjen]

What settings is your car aligned to? You can often reduce the front toe-in by a little to reduce corner entry-understeer. This will help the car to point better at turn-in. For autocross purposes, I've heard of people even running toe-out in the front, but this is pretty extreme.

If the car continues to push after you turn in, reducing the rear toe-in slightly can help, but this will make the car looser on high-speed turns, so this is best done judiciously.

Alternatively, depending on how-much camber you are running at the front, you can try increasing the front camber a little. But dialing in too much front camber can also make the car push on turn-in.

I agree with tctnd, if the car has a LSD, then that's a completely different situation.

[hobieboy]

Don't have much to add but would also suggest to check your alignment.

I also run 29/36 or even 34psi in the rear for my street tires. A stiffer rear will help but now we are talking about suspension mods...

[Black968]

Thanks for the feedback. It is an LSD car. I didn't even think of that. The two wheel drive is pushing the car straight. Would a strut brace or a little extra weight in the front help? It maybe something I will have to live with.

Cheers

[javadog]

I don't really like the stock front wheel/tire size on a 930 and I think it is worse on the later cars with the 9 inch rear wheels. You might consider running an 8 inch front wheel width and a 225 section width tire. I prefer the 8x16 from the 944 turbo.

Given that the car will push more with increasing power delivered to the rear wheels, you might experiment a little with when, and how much, you get onto the throttle in a corner.

The car will also feel different with a full fuel tank, than when the tank level is low.

I think the strut brace is largely a waste of time.

Cheers,
JR

[stlrj]

I know that too much negative camber in front will cause your problems.

Do yourself a simple test. Check how much camber your front tires have with a level with your steering wheel at 180 or 270 degrees and then check that against another car that you know that does not plow.

The difference might surprise you!



Cheers,

Joe

[jluetjen]

stlrj;
What you're describing is castor rather then camber. Adding castor can be a means to increase camber in tight corners (and thus reduce understeer), but as a flip-side, it also increase load transfer to the outside wheel which can increase understeer. So the affect will really depend on what else is going on with the car's set-up.

Given the LSD, I would agree that some of it involves learning to drive the car at part-throttle through long turns -- which is something that turbos don't necessarily like if they have turbo-lag. Basically, you need to roll more speed through the entry and middle parts of the corner on part throttle, and then hold off on the WOT until you can have the car pointed at the exit. It's basically like you would drive a kart.

[Richard Burns]

I am reading this as two problems 1 Turn in & 2 Understeer.
Turn in can be adjusted by altering castor.
Underster can be adjusted by suspension or by adjusting the nut on the wheel (steering).
Is it both problems or only one?

[stlrj]

Adding castor can be a means to increase camber in tight corners (and thus reduce understeer),

Sorry, but adding negative camber in tight corners makes understeer worse and this is form personal experience, not from something I read.

Since camber and castor are related, both need to be reduced to reduce understeer.

I have my car set at 2.6 degrees castor and about 1/2 degree positive camber which has totally eliminated my understeer issues.

Cheers,

Joe

[Emission]

Originally posted by wcc
The MAX PSI posted on the side of my Sumitomo tire is 44PSI. I actually run it a little less around 40-42PSI. But even if I ran it at 44PSI I wouldn't worry about it one bit. The tire company planned it this way. This tire was actually designed at a higher pressure. They are basically saying that 44PSI is the max pressure for legality/warranty reasons. Tires are just like anything else in this world. For example, bridges are posted at, say, 1 ton, but they are designed at 1.5 or 2 tons. Speed limits are 70mph, but most roads are designed for 80mph or faster. So in a sense 44PSI is the posted speed limit or the posted bridge limit. Of course we've never exceeded speed limit so why worry about these tire pressures. I mean, can you imagine if a road or bridge was actually designed to the posted speed limit? We'd have accidents and bridges collapsing everywhere. Just something to think about. I've NEVER had a blow out or any issue. YMMV

Note: My cars are only street driven. If you track your car you have no warranty anyway, or it is VOID, AFAIK, just run it at whatever pressure gives the best traction and makes the tires last the longest. Usually one season if your lucky, so who cares !

I think Jack was saying that 44 psi is too hard for a modern tire - not a safety issue. Most modern performance tires work much better (more grip) at lower pressures.

[rlh930]

29 PSI front, 44 PSI rear, cold, is indeed the correct cold inflation pressure for a 930, according to the owner's manual, which I have in front of me. I doubt that tire construction has changed sufficiently since '86 to invalidate this specification, but I could be wrong. You should get less grip in the rear with 44PSI, which may help your understeer.

I also reduced understeer significantly on my 930 by switching from 205's to 225's on the front (SO3's), as someone else suggested earlier. You're still not going to be able to get rid of it entirely without making serious (and potentially dangerous) changes to the suspension. I had better luck altering my driving to more of an in-slow, out fast style. Maybe you're just asking the tires to do too much on corner entry.

[wcc]

All I was trying to get at is these cars were tested and tested and Porsche found that 44PSI was a good all around fit (performance and comfort) for the rear of a 930. I'm sure they tested on tracks, street, mountain roads, and I'm sure they took into account the tires heating up and increasing pressure, etc. So sure, you'll get 'less' traction at 44 then at 40, but still at 44 you are still getting PLENTY of traction for normal or spirited street use, it was tested at that pressure. At 40, would be good for hard street use and maybe DE's, but I think 36 is too low IMO. You can spend money on TB and messing around with alignments, etc. But if you air up your tires to the owners manual you may find that your problem may go away or be minimized greatly for about 1minute of your time and $.50 cents at the local gas station if you don't have a air compressor.

Now, this only applies to an as delivered car with factory sized wheels and tires. If you changed the wheels or changed tire sizes, or lightened up your car a lot, well then you have to adjust as necessary.

Just look at your owners manual, there really is some good info in there.

[F350Lawman]

It does not want to turn well on slow tight corner

I was just messing around in a large 100% vacant lot this afternoon, at 30- 40 or so mph a sharp turn (sort of AX style) will certainly cause my front end to plow badly. Add some throttle and it will turn from a plow into going straight... as in no turn. I suppose I could have gotten the weight more on the front with some brake before I turned in, but I really just was fooling around seeing how the car handles when tossed around. You know ...the weather is warm and I started behaving like a big kid with a new toy! My 16 year old son riding shotgun thought it was fun too, he said he thought the car was going to turn over.

I am running slghtly larger than factory 225s up front and that helped *somewhat. I am about to do a MAJOR suspension upgrade so we will see, I expect it will improve measurably.